Desalination Plant in Singapore

Transcription

1 Faculty of Engineering and Physical Sciences Desalination Plant in Singapore Final Report Students Academics Sara-Louise Bent Eric Do Ali Niazi Elena Rusu Irene Usiomoifo Marco Volino Prof. Gerry Parke Prof. Nigel Seaton Prof. Carl Sofield 14/01/2011 Multi Disciplinary Design Project 2010/11

2 Executive Summary Water is one of the most fundamental resources on the planet, essential for life as we know it. Over 70% of the earth s surface is covered in water but only a small fraction of it is safe for human consumption. The scarcity of clean drinking water is one of the major global issues facing humanity today. Singapore, a small country in the south of Asia, is one of the busiest shipping ports in the world and this has given rise to an increasing population and growing industry. With no natural resources of its own Singapore relies on its neighbours exports. Singapore, like most other countries, is facing a water shortfall that is being driven by population growth and industrial activity. Currently Singapore relies on imported water from Malaysia (for approximately 40% of its demand) for which it has two agreements, one set to expire in 2011 and one in Singapore has set up the Public Utilities Board (PUB) who are in charge of providing clean drinking water for Singaporeans. The PUB have set ambitious targets for 2020 which involve educating citizens about using less water, collecting rainwater, recycling waste wastewater for industrial use and producing more water from desalination. By 2060 the PUB have set the target to be fully self sufficient in terms of water. The Singaporean government has invested substantial amounts of money into research and development into desalination technologies with a strong focus on reverse osmosis. This report details the theory behind and the application of Seawater Reverse Osmosis at a proposed desalination plant in Tuas, southwest Singapore. The conventional pre-treatment methodology was used but nanofiltration was also incorporated as part of the pre-treatment; an option which has not been used by many desalination plants. The use of nanofiltration has allowed the reverse osmosis unit to run at a higher recovery rate of 55% as the feed to the reverse osmosis train is of a better quality: A result of nanofiltration which removes up to 68% of total hardness ions (Ca 2+ Mg 2+ ) and 30-60% of the salt ions. The solution to this water shortage crisis is to build a desalination plant using the best available technology. The proposed plant consists of pre-screening, Dissolved Air Floatation (DAF) units, sand gravity filters, nanofiltration, reverse osmosis and post treatment. The processes which have been selected are similar to the existing SingSpring plant in Tuas, Singapore with the inclusion of nanofiltration as a pre-treatment option. The production capacity is 250,000m 3 /h which is about twice the capacity of the current desalination plant in Singapore. i Group effort

3 The estimated cost of the plant is $ 215 million and is expected to become operational by The cost of the water is initially expected to be $0.54 /m 3 with the expected selling price to be $0.81/m 3. This would allow the plant to break even within seven years. If the sale price of water was raised to $1/m 3 the plant would break even in five years. As the proposal includes powering the plant from the national grid, the price of water will vary with energy costs. It is estimated that 40% of the cost of the water is made up from energy costs. This report begins by defining the boundaries of the study; including the composition of the seawater to be treated, and Singaporean drinking water standards to be met by the product. The theory behind seawater desalination processes is expounded upon and the processes involved discussed in depth. These are split into three key zones; pre-treatment to improve the quality of the influent water (essential for longevity of the sensitive membranes), treatment using a membrane assembly, and post treatment to return essential (nutritional) ions to the desalinated water. A plant is then designed, considering Health and Safety, and social, economic and environmental sustainability. A financial analysis is conducted and a series of conclusions and recommendations made (such as the necessity for stringent testing and pilot scale plants before final implementation). A project plan is then suggested, including a Gantt chart showing the actual project schedule. ii Group effort

4 Contents Page Executive Summary... i Contents Page... iii List of Figures... viii List of Tables... xi 1. Introduction Brief Objectives Learning Outcomes Report Overview Assumptions Singapore Overview Public Utilities Board Water Targets Water Demand Projection Politics Water Agreement with Malaysia Drinking Water Standards Coastal Water Quality Applications and Theory Intake & Pre-Screening Types of intakes Comparison of different intakes Impact of Intake Type on SWRO Pre-treatment Main components of direct sub-surface intake Marine Pipeline selection Environmental Consideration Screening methods Passive Screens iii

5 Pumping Station Pre-treatment Process Dissolved Air Floatation Process Fundamental Theories Coagulation and Flocculation Design Parameters Practical Implementation Rapid gravity Sand Filter Filtration Methods Rapid Filtration Rapid filtration hydraulics Process mechanisms Rapid Filter Classification Filter Backwashing Choice of rapid filtration system Membrane Processes Nanofiltration Mechanism of transport Reason for choosing NF as pre-treatment Reason for chosen recovery rate Reverse Osmosis Reason for choosing RO Thermodynamics of RO system Post Treatment Disinfection Chlorination (Cl 2 ) Chlorine Dioxide (ClO 2 ) Ozone (O 3 ) Remineralisation Power Distribution Design Considerations Planning Guide for Electrical Distribution iv

6 4. Proposed Plant Introduction Site Location Site Layout External facilities Internal facilities Main administration building Process overview Intake Dissolved Air Flotation Equipment Selection Cost Operation Coagulant Type and Quantity Rapid Sand Filter Performance criteria Process design criteria Major process components Pilot plant Cartridge Filter Membrane processes Modelling RO/NF design using ROSA Software Nanofiltration Configuration design ROSA results Reliability checks Process description NF waste disposal NF membrane cleaning Reverse Osmosis Reverse osmosis system parameters ROSA results v

7 Reliability checks Process description Energy consumption and Energy recovery device (ERD) Membrane cleaning UV Disinfection Clearwell Power Engineering Plant Energy Requirements Sources of Energy National Grid Renewable Options Energy Options based on Proposed Location Design Procedure Load Survey Demand Voltage Utility Service Systems Substation Layout Distribution Arrangement Generation Equipment Locations One-Line Diagram Power Factor and Power Factor Improvement Waste and Brine Disposal Other disposal methods Most feasible option Control Systems Instrument control scheme Computer based control scheme Health and Safety General working precautions vi

8 5.2. Handling Chemicals Sustainability Environmental Construction phase Operational phase Social Economic Project Finance Project delivery method Project Evaluation Conclusions and Recommendations Conclusions Recommendations References Appendices... I Appendix 1: Water Demand Projection... I Appendix 2: World Water Works DAF Unit Range and Selection... I Appendix 3: ROSA Results for Reverse Osmosis... II Appendix 4: Design Calculations... VIII Appendix 5: Financial Evaluation... IX Appendix 6: Gantt Chart... X vii

9 List of Figures Figure 1 : Map of Singapore (Larabancroft, 2011)... 4 Figure 2: The Water Loop (Public Utilities Board, 2008)... 5 Figure 3: Projected Water Demand... 6 Figure 4: Different types of sea water intake for desalination (Le Roux, 2010) Figure 5: Basic Illustration of Direct intakes via channel and pipeline (Raynolds, 2008) Figure 6: Basic illustration of indirect intake (Raynolds, 2008: p 10) Figure 7: Schematic illustration for S-lay and J-lay (Le Roux, 2010: p 28-29) Figure 8: Intake with a velocity cap headwork s (Le Roux, 2010: p 19) Figure 9: S-shaped and L-shaped passive screen (Ovivo, 2010: p 4) Figure 10: Dry pumping station - top and lateral view (Moretti, 2005) Figure 11: Dissolved Air Flotation Process Diagram Adapted from (Ross and Smith, 2000: p 3) Figure 12: DAF Unit with labelled Zones (Peleka and Matis, 2008: p 3) Figure 13: Typical diameters of colloids in meters (Koohestanian, Hosseini and Abbasian, 2008) Figure 14: Rectangular DAF Unit (Ross and Smith, 2000: p 2) Figure 15: Circular DAF unit (Krofta Engineering, n.d.) Figure 16: A vertical pressure filter (The Water Treatments, 2010) Figure 17: Cross section of a dual media rapid gravity sand filter (Crittenden et al., 2005) Figure 18: A typical filter cycle (American Water Works Association and American Society of Civil Engineers, 2005) Figure 19: Ranges of Reynolds number for different granular filtration technologies (Crittenden et al., 2005) Figure 20: Particle transfer mechanisms in fundamental depth theory with each theory given a different label-a) Interception B) Sedimentation C) Diffusion (Crittenden et al., 2005) Figure 21: Arrangement of elements in a pressure vessel (Voutchkov, 2008) Figure 22: Diagram of a two stage membrane system (American Water Works Association, 1999: p 27) Figure 23: First-pass SWRO trains (Stover, n.d.: p 2) Figure 24: Construction of spiral-wound membrane element (Critttenden et al., 2005: p 911) Figure 25: Donnan exclusion mechanism (Diawara, 2008) Figure 26: Limiting factors to membrane desalination by reverse osmosis (Fritzmann et al., 2007: p 19) Figure 27: Membrane process salt rejection mechanism (Critttenden et al., 2005: p 23) Figure 28: Chlorine residuals for a water with a constant ammonia concentration Taken and edited from Principle of Water Quality Control (Tebbutt, 1992) viii

10 Figure 29: Basic Ozone generation configuration Figure 30: Reaction pathway of ozone in water Figure 31: Electromagnet spectrum with detailed section of UV Taken and edited from Water Treatment Plant Design (Water Treatment Plant Design, 2005) Figure 32: The comparison of wavelength emitted from low pressure lamp and medium pressure lamps Taken and edited from Water Treatment Principles & Design (Crittenden et al., 2005) Figure 33: Proposed location for the desalination plant Figure 34: Artist impression of the proposed water treatment plant Figure 35: Floor plan for the main administration building Figure 36: Process Diagram including Input and Recovery Rates Figure 37: Basic illustration of custom builds method horse and procedure for installation (Amiantit Group, 2009) Figure 38: Passive screens arrangement for the proposed plant showing dual intake (StaticOrb Screens, 2008) Figure 39: World Water Works DAF Unit Figure 40: Comparison of coagulants on the removal of turbidity (Mohammed and Adbul, 1998) Figure 41: Flow chart for creating a ROSA system design (The Dow Chemical Company, n.d.) Figure 42: Feed water data tab in the control panel Figure 43: System configuration tab in the control panel Figure 44: Block diagram of the nanofiltration system Figure 45: Flow diagram for RO system case Figure 46: Flow diagram for RO system case Figure 48: Flow diagram of the reverse osmosis unit Figure 48: Energy consumption for desalination process stages (Fritzmann et al., 2007: p 66) Figure 49: Pelton wheel energy recovery device (Avlonitis, Kouroumbas and Vlachakis, 2003: p 4) Figure 50: Turbo charger energy recovery device (Avlonitis, Kouroumbas and Vlachakis, 2003: p 5) Figure 51: Pressure exchanger energy recovery device (Avlonitis, Kouroumbas and Vlachakis, 2003: p 5) Figure 52: Schematic diagram for SWRO train with PX ERD (Stover, n.d.: p2) Figure 53: The dimensions of Trojan UV Swift Figure 54: The cross section of a pipe showing the proposed equally distributed distance between UV lamps Figure 55: The inverse square proportional relationship between the dispersion number and the log reduction (Crittenden et al., 2005) Figure 56: Fuel Cell Energy Scalable Plant (Fuel Cell Energy, 2010) ix

11 Figure 57: Load Survey with Site Map Figure 58: Substation Layout using double bus double breaker (Jcmiras, 2006) Figure 59: Distribution Arrangements (Boal and Taylor, 1966: p 2) Figure 60: Basic Distribution System Adapted from (IEEE, 1976: p 196) Figure 61: Outfall pipe with diffuser at the end (Voutchkov, 2010: p 12) Figure 62: Concentrate discharge using an existing outfall of a power plant (Voutchkov, 2010: p 12) Figure 63: Distance between the Tuas Power Station and the proposed site location for the desalination plant (Google, n.d.) x

12 List of Tables Table 1: Water targets for 2020 and Table 2: Summary of Drinking Water Standards (Government of Singapore, 2008)... 7 Table 3: Summary of the Composition of Singapore Coastal Water... 8 Table 4: The composition of the main ions in seawater (Public Health and the Environment World Health Organization, 2007: p 6)... 9 Table 5: Allowable particle size for different desalination process (Gille, 2003) Table 6: Impact of Intake type on SWRO pre-treatment (Christopher et al., 2010) Table 7: Material selection for intake pipes (Mostert, 2009) Table 8: Advantages and disadvantages of inorganic coagulants (U.S. Engineering Corporation, 2001) Table 9: Comparison of granular filtration technologies for water treatment (Crittenden et al., 2005) 30 Table 10: Properties of filter media (adpated from Crittenden et al., 2005) Table 11: Typical backwash conditions for different filter media size (Parsons and Jefferson, 2006) 44 Table 12: Characteristics of nanofiltration unit for each plant (Gaid et al., 1998) Table 13: Comparison of the mercury vapour lamp types Adapted from Water Treatment Plant Design (American Water Works Association, 2005) Table 14: Mineral requirements for drinking water WHO Guidelines for drinking water Table 9. (2-4,56) Table 15: Countries with water fluoridation schemes covering their populations WHO Guidelines for drinking water Table 9. (2-4,56) Table 16: Specification of the passive screen used for the proposed plant (Ovivo, 2010) Table 17: Specifications for nanofiltration membranes (The Dow Chemical Company, n.d.) Table 18: Composition of feed water to the nanofiltration unit Table 19: Specifications for the 2 pumps being considered (Springer Pumps LLC, 2009) Table 20: Potential scaling factors (Al-Amoudi and Lovitt, 2007: p 10) Table 21: Natural organic matter fouling factors (Al-Amoudi and Lovitt, 2007: p 10) Table 22: Cleaning solutions for nanofiltration membranes (The Dow Chemical Company, n.d.) Table 23: Membrane element specification Table 24: Number of stages of a seawater system (The Dow Chemical Company, n.d.) Table 25: ROSA result for case Table 26: ROSA result for case 1 (energy consumption) Table 27: ROSA result for case Table 28: ROSA result for case 2 (energy consumption) Table 29: Major ion composition of normal seawater (mg/litre) (Al-Mutaz, 2000) xi

13 Table 30: RO pump specifications Table 31: Composition of water Table 32: Pros and Cons of Renewable Energy Technologies Table 33: Power Demand for each process Table 34: Singapore Voltage Standards Table 35: Required Input Voltage (SP Powergrid, 2010) Table 36: Advantages and Disadvantages of different substation layouts Taken and adapted from (Nack, 2005: p 9) Table 37: Comparison of Distribution Arrangements Adapted from (Boal and Taylor, 1966) Table 38: Typical instruments part of the control system for a reverse osmosis plant (American Water Works Association, 1999: p 34) Table 39: cost analysis for SWRO desalination plant xii

14 Chapter 1: Introduction 1. Introduction 1.1. Brief Water is the most essential part of life. The ever increasing population growth is outgrowing the total availability of fresh water supplies. This suggests that water scarcity continues to be an important global issue. Despite 70% of the earth s surface being covered by water, only a very small percentage is fresh water which can be safely consumed. The water in the ocean contains a high concentration of salt, which if consumed, is unsafe and can be harmful to human health. The sea, however, has great potential to become a rich source of fresh water. Seawater still contains salt however the salt concentration can be removed by desalination technology. Singapore is thoroughly developing new desalination techniques to aid its limited supply of fresh water. With the population of Singapore gradually growing annually, new water sources must be in place in order to meet this demand. It is proposed that by 2060, Singapore will be completely self-efficient with desalination providing a significant amount of the water demand. The project is to design a desalination plant in Singapore which will generate the required fresh water for the Singaporean population Objectives The aim of the project is to design a desalination plant which will help Singapore meet the proposed 2020 water targets whilst producing water at a low cost. The Singaporean government s target is to become self-efficient thereby not relying on the water resources from Malaysia. To achieve this, it is proposed that Singapore will meet its water demand by supplying 25% of the water through desalination plants. The water treatment process should be cost efficient, sustainable, and socially acceptable. In order to meet this objective, the following issues must be considered in detail: Total water capacity of the plant The most appropriate water treatment process The total energy usage of the entire water treatment plant Financial impact in relation to the capital and operating costs Environmental impacts 1 Group effort

15 Chapter 1: Introduction 1.3. Learning Outcomes The expected learning outcomes from this project are to: Be aware of the complexity in terms of technical, managerial and financial, of multi-disciplinary design activities. Be able to undertake a complex design and integrate it with design components from other disciplines. Understand the components required to complete the project and how each individual design component is integrated with the others. Be able to discuss complex specifications and implement a detailed technical design Be able to contribute properly to such projects. Understand the differing information requirements of engineers from different disciplines. Be able to communicate efficiently with different engineering disciplines 1.4. Report Overview This report will provide a detail analysis of the possible water treatment process which will be incorporated into the proposed desalination plant. Chapter 2 discusses all aspects of the project which relate to Singapore, this ranges from the current water crisis and the government targets for water conservation to the composition of costal water and the national standards for drinking water. Chapter 3 presents the theory and applications of selected water treatment processes and power engineering. Chapter 4 shows the technical design work for the proposed plant with each treatment process designed in detail and a basic electrical distribution system presented. Chapter 5 discusses the health and safety with respect to building and operating a desalination plant. In chapter 6 sustainability issues are addressed from an environmental, social and economic point of view. Chapter 7 discusses the financial aspects of the project and from this it is possible to calculate the cost and retail price of the desalinated water, there is also a discussion regarding the project financing options and break even analysis. Chapter 8 shows the planning aspects of the project with a proposed and actual schedule and a break-down of the work between the team members. Conclusions and recommendations from each section in the report are presented in Chapter 9. 2 Group effort

16 Chapter 1: Introduction 1.5. Assumptions Throughout the project, a number of assumptions have been implemented, which are listed below: All the calculation parameters were designed for the maximum capacity of 250,000 m 3 /day When determining the pipe diameters in each section, an average volumetric velocity of 0.75 m/s was used based on the fact that 1 m/s is too fast and 0.5 m/s would be too slow. The composition of the seawater from Singapore was assumed to be that of normal seawater and remains constant throughout the year. Singapore has busy seaport traffic so it was assumed petrochemicals would contaminate the seawater so DAF units are required to remove these contaminants. The total organic content (TOC) is negligible because seawater is extracted at a certain depth where the TOC level is low. The site for the proposed desalination plant has already been purchased by the client and planning permission has been granted. 3 Group effort

17 Chapter 2: Singapore 2. Singapore This chapter presents information about Singapore, the water crisis, the Public Utilities Board (PUB) and the targets which have been set to ensure that Singapore is able to meet the water demands of the future Overview Singapore is a small island country located off the southern tip of Malaysia. With a population of approximately 5 million and a total area of 710 km 2, it is one of the most densely populated areas. It is home to one of the busiest sea ports in the world and has become a hub for many different types of industrial activity. Despite the densely populated island and the industrial activity Singapore has no natural resource and is full dependant on neighbouring countries for everything from liquefied natural gas and coal to water. Figure 1 shows a map of Singapore.. Figure 1 : Map of Singapore (Larabancroft, 2011) 2.2. Public Utilities Board The Public Utilities Board (PUB) is Singapore s national water agency. The PUB is responsible for helping identify ways of meeting the national demand for water now and for the future. This is being achieved using the four national taps. The first national tap is imported water, this is raw untreated water that is imported from Johor region of Malaysia, currently this is how Singapore obtains 40% of their national demand of water. The second national tap comes from catchment water collected from rain, rivers, and streams and stored in reservoirs. The third national tap 4 Group effort

18 Chapter 2: Singapore comes from NEWater which is wastewater which has been treated; this water is used predominantly for industrial purposes. The final of the four national taps is desalination. These national taps can all be seen in Figure 2 which shows the lifecycle of water. Figure 2: The Water Loop (Public Utilities Board, 2008) Water Targets The PUB has set targets which for years 2020 and They are represented in Table 1. Table 1: Water targets for 2020 and Cut Daily per Capita consumption to 147 Aim to be self sufficient litres 25% of demand met by desalination 30% of demand coming from desalination 40% of water demand met by NEWater 50% coming from NEWater 8.2 km 2 of reservoirs and 90 km of Water demand will be 3,460,000 m 3 /day waterways used for recreation double the current demand R&D focused on energy efficiencies, reducing evaporative losses 5 Group effort

19 Cubic Meters per Day Chapter 2: Singapore 2.3. Water Demand Projection The 2060 targets are based on the projection that the national water demand will double from 1,730,000 m 3 /day to 3,460,000 m 3 /day. Assuming that the rise in the demand is linear it is possible to estimate the 2020 and 2040 water demands as 2,076,000 m 3 /day and 2,768,000 m 3 /day respectively. Figure 3 shows the rise in water demand for the next 50 years. This also assumes that the percentage of the water demand from desalination increases from 25% in 2020 up to 30% in 2060 linearly. The full list of data from the graph is available in Appendix A. 3,500,000 Projected Water Demand 3,000,000 2,500,000 2,000,000 1,500,000 Projected Water Demand 1,000, , Year Figure 3: Projected Water Demand 2.4. Politics Water Agreement with Malaysia Based on the bilateral relations of Singapore with Malaysia, Singapore draws up to 1.3 m 3 /day of raw water from the state of Johor (Lee, 2003) under the two agreements between these two nations. The first agreement, signed in 1961 gave Singapore the rights to extract 380,000 m 3 /day from the state of Johor and in return 12% of the treated water was sent back to state of Johor (Chew, 2009). The 1961 agreement is due to expire in The second agreement signed in 1962, gave Singapore exclusive rights to draw up to 980,000 m 3 /day of raw water and 2% of that treated water is provided back to Malaysia. This agreement is will expire in These two agreements involve some other aspects such as rent for land use, review of price changes, extension of agreements, and price disputes between these two countries. Political instability has grown between these two countries over water agreements as Malaysia thinks Singapore is paying too little for the water. In the last 50 years, there have been 37 violent 6 Group effort

20 Chapter 2: Singapore conflicts between states involving water rights and access to water, according to the United Nations (Mauzy, n.d.). The bilateral relations between these countries have deteriorated over the water agreement. Singapore has stated clearly that it doesn't intend to renew the contracts expiring in 2011 and Singapore wants to meet these demands by improving its catchment system and by building five desalination plants by In summary, Singapore doesn t want to rely on Malaysia or any neighbouring state to meet their water demands in the future. It doesn t want to jeopardize its relation with Malaysia as it has already deteriorated over water agreements. Political instability could cause further issues in the on-going agreement of Royal Singapore Air Force and Malaysia which gives right to Singapore's air force to use Malaysian air space (Channel news Asia, n.d.) Drinking Water Standards The composition of drinking water produced by the proposed desalination plant has to meet certain standards in order to be distributed to the human population. Two plans have to be formulated, a water safety plan and a water sampling plan, to ensure that the drinking water meets these standards (Government of Singapore, 2008). Before they can be implemented, they have to be approved by the Government. Once they have been approved, they cannot be changed unless the Government is notified, otherwise the plant could incur a fine. Both plans have to be reviewed every few years to ensure the quality of the drinking water is still up to standard and the results obtained must be sent to the Government for approval. The drinking water standards in Singapore are based upon the parameters set by the World Health Organisation (WHO) which are well established. These can be seen in Table 2. Table 2: Summary of Drinking Water Standards (Government of Singapore, 2008) Property Value Unit ph Turbidity Less than 5 Nephelometric Turbidity Units Colour Less than 15 True Colour Units Boron 0.5 mg/litre Chlorine 5 mg/litre The amount of boron in the drinking water must be below 0.5 mg/lit as it can affect fertility in women, and harm foetuses in the womb. Any ph values outside this range would not be 7 Group effort

21 Chapter 2: Singapore beneficial to the human health i.e. acidic water leads to corrosion of teeth while alkaline water could neutralize the acid in the stomach Coastal Water Quality The composition of the feed water plays a very important role in the design of the proposed desalination plant. The pre, main and post-treatment, all have to be customised according to the seawater properties. Table 3 provides a summary of some of the important properties of the costal water surrounding Singapore. Table 3: Summary of the Composition of Singapore Coastal Water Property Value Unit ph 8 Temperature 28 o C Total Dissolved Solids - mg/litre Boron 5 gg/litre Total Suspended Solids 22.5 mg/litre Oil Content 10 mg/litre Silt Density Index 7 ph measures how acidic or alkaline the seawater is. Membranes operate at an optimum ph so certain chemicals will be added such as coagulants and sodium hydroxide, to either increase or reduce the ph, depending on the desired output. The Total Dissolved Solids (TDS) is a measurement of the organic and inorganic matter present in seawater which is so small that it can pass through sieves. The main contributors are the salt ions, Cl - and Na + and the hardness ions, Ca 2+ and Mg 2+. The TDS of the seawater in Singapore is not known and so it was assumed to be that of normal seawater as shown in Table 4. The Total Suspended Solids (TSS) content of the seawater around the island of Singapore is highly variable. Gin et al (2002) conducted a study of the estimate the chlorophyll content of the coastal waters of Singapore. In the paper, it was reported that during the period of study, the TSS recorded in three separate locations around the island varied from between 2.9 mg/litre up to 72 mg/litre. The average of the annual averages of the three locations was calculated to be 22.5 mg/litre so this is the value which will be used as a basis for the TSS (Gin et al., 2002). 8 Group effort

22 Chapter 2: Singapore Oil content: Singapore is one of Asia s busiest seaports, with approximately one third of all cargo ship passing through the Singapore Strait. This large shipping industry can introduce oil and grease into the sea water from emissions and spillages. In a discussion, between the employees from Black & Veatch who built the existing Tuas desalination plant, the value of the oil content was discovered to be as high as 10 mg/litre (Web4water, 2005). The Silt Density Index (SDI) is a way to quantify the fouling potential of suspended particles. The SDI value for sea water was also confirmed by the discussion by Black & Veatch (Web4water, 2005). To avoid fouling the membranes, the SDI for the feed water must be less than 3 so pre-treatment is implemented to bring down the SDI. One of the most important assumptions was to take the composition of seawater from Singapore to be the same as normal seawater. The pre, main and post-treatment were designed around this composition which is shown in the table below: Table 4: The composition of the main ions in seawater (Public Health and the Environment World Health Organization, 2007: p 6) Constituent Normal seawater (mg/l) Chloride (Cl -1 ) 18,890 Sodium (Na +1 ) 10,556 Sulfate (SO 4-2 ) 2,649 Magnesium (Mg +2 ) 1,262 Calcium (Ca +2 ) 400 Potassium (K+1) 380 Bicarbonate (HCO 3-1 ) 140 Strontium (Sr +2 ) 13 Bromide (Br -1 ) 65 Boric Acid 26 Fluoride (F -1 ) 1 Silicate (SiO 3-2 ) 1 Iodide (I -1 ) <1 Other 1 Total Dissolved Solids 34,483 9 Group effort

23 Chapter 3: Applications and Theory 3. Applications and Theory 3.1. Intake & Pre-Screening Water intake is a vital design parameter of the desalination plant as the amount and the quality of water depends on the type and location of the water intake. A good intake design should provide the desired amount of good quality feed with minimum ecological and economic impact. Physical characteristics, meteorological and oceanographic data, marine biology, and the potential effects of fouling, pollution, and navigation must be evaluated, and an appropriate intake design employed (Panktratz, 2006: p 1). Sea water intake is a fundamental part of every desalination plant. An intake design must assure that clean water at a constant flow rate is available throughout the plant life. Before designing the intake facility, it is important to know the type of process being implemented in the desalination plant. The first step would be identifying the needs of the process. Each process has different allowable particle size requirements in the feed that it can sustain. Table 5 shows a comparison for the maximum allowable particle size in the feed. Table 5: Allowable particle size for different desalination process (Gille, 2003) Desalination technology MSF (multi-stage flash) MED (multi-effect desalination) + MED vapour compression Maximum particle size 1/3 of tube inner diameter e.g. 5 mm for 15 mm tubes 1/3 of inner diameter of condenser tubes e.g. 8 mm for 24 mm tubes plus 0.5 mm for spray nozzle RO (reverse osmosis) 5-20 µm + SDI value <3.5 It can be seen that the RO process needs good quality water to assure that sea water feed doesn t affect the performance of fragile membranes. Other parameters like SDI are also maintained for sea water entering membranes Therefore, sea water intake design should incorporate very fine screens and bars either at the end of pipes or at shore to make sure an excellent quality of sea water is fed to the proposed desalination plant. The material of intake pipes for offshore intake plays an important role as well. The growth of marine life in pipe lines could cause biological fouling. Therefore, when choosing the type of material factors should be considered i.e. cost, reliability, resistance to bio fouling, weight, ease of installation etc. The most common types of materials used for marine pipelines around the world 10 Ali Niazi

24 Chapter 3: Applications and Theory for major desalination plants are HDPE (high density poly-ethylene), GRP (glass reinforced plastic), FRP (fibre reinforced plastic), PP (polypropylene), steel, ductile iron pipe and concrete. Each one of them has their own characteristics in terms of physical, economical, sustainable factors which are discussed thoroughly at a later point in this section Types of intakes Sea water intake is generally categorised in two main types: direct and indirect intake. Figure 4 shows a flowchart with the various intakes options for a desalination plant. Figure 4: Different types of sea water intake for desalination (Le Roux, 2010) Direct Intake: Direct intake (often referred to as open intake) structures transfer the sea water directly straight from the sea to the plant. Direct surface refers to the intake of sea water via channel as shown in Figure 5. Direct sub-surface intake refers to the construction (pipelines) below the surface directly towards the sea with a bar/screen at the end as shown in Figure 5. These two types of direct intake are usually used for large scale desalination plants with production capacity more than 40,000 m 3 /day (Panktratz, 2006). Screens are added to the direct intake structures to reduce impingement and entrainment of marine life and to also minimise the amount of debris entering the RO plant. Over the last century the screens have evolved dramatically. Earlier methods included fixed trash rack, made of fixed bars preventing large debris from entering the plant. Travelling screens use rotating screens and are washed intermittently with high pressure water from nozzles. Other technologies for screens include velocity cap, passive screens (large cylindrical wedge wire screens), fine mesh screens, Ristroph screen and filter net barrier (Panktratz, 2006). 11 Ali Niazi

25 Chapter 3: Applications and Theory As mentioned earlier both direct surface intake and direct sub-surface are the only alternative for large scale desalination. Generally, direct surface intake has a lower capital cost but has a higher operational cost due to excessive pre-treatment. Whereas direct sub-surface has a higher capital cost compared to open surface intake but it compensates by reducing the operational cost as the feed sea water is generally cleaner than the one obtained from the surface. Figure 5: Basic Illustration of Direct intakes via channel and pipeline (Raynolds, 2008) Indirect intake: It is also known as sub-surface intake. In sub-surface intake, sea water is collected below the sea bed via horizontal or vertical wells. Other alternative methods include beach filtration and horizontal directional drains. The sub-surface intake has the main advantage of taking the use of natural filtration caused by sedimentation. This minimises the amount of total organic carbon and total suspended particles present in sea water thus reducing the pre-treatment required. Figure 6 shows an example of sub-surface intake. Figure 6: Basic illustration of indirect intake (Raynolds, 2008: p 10) 12 Ali Niazi

26 Chapter 3: Applications and Theory Generally sub-surface intake design is adopted for small/medium scale desalination plants. Experience show that seawater intakes with a requirement below 20,000 m 3 /h are mainly equipped with submerged seawater intakes to protect the sea water intake pumps (Gille, 2003) Comparison of different intakes Indirect Advantages No preliminary treatment only minor primary treatment of feed water required After construction, minor impact on coastline Not affected by wave action and turbidity No entrainment or entrapment of marine organisms Disadvantages Limited feed water extraction volumes High impact on coastline during construction Direct: Surface Advantages Extract large volumes of feed water Maintenance operations more simple and accessible Disadvantages Major impact on natural coastline Mitigation measures required against entrainment and entrapment of marine organisms Intake requires protection against waves and turbidity and therefore expensive construction costs (breakwaters) Subject to oil pollution (oil spills) Direct: Sub-surface Advantages Extract larger volumes of feed water than indirect intakes, but less than direct surface intakes Minor impact on the coastline during construction and none after construction Disadvantages Mitigation measures required against entrainment and entrapment of marine organisms Marine pipelines require protection (or buried) in surf zones Expensive maintenance operations 13 Ali Niazi

27 Chapter 3: Applications and Theory Impact of Intake Type on SWRO Pre-treatment The type of intake also greatly influences the type of pre-treatment used for the SWRO plant. Therefore, it gives the opportunity to choose between various pre-treatment procedures available. It may be that some pre-treatment is more expensive or not viable at a specific location. Table 6 shows the impact of intake type it may have on the SWRO plant. Table 6: Impact of Intake type on SWRO pre-treatment (Christopher et al., 2010) Unit Treatment Process On Shore Open sea intake Off Shore Subsurface Intake Screened well collector Neodren Tube Screening Medium on shore Coarse off shore + Medium on shore Screen integral to design Not required Flocculation Required with clarification Possibly with clarification otherwise unlikely Unlikely Not required Clarification Very likely Possible Unlikely Not required Filtration Yes Yes Yes Yes, higher rates may be possible Pre-treatment chemical/sludge production Decreasing from left to right Operational risk of compromised intake capacity Increasing from left to right 14 Ali Niazi

28 Chapter 3: Applications and Theory Main components of direct sub-surface intake Intake headworks/structure: It is generally located at the end of the pipeline in the sea. The function of the headworks is to make sure that it transports the desired amount of seawater to the desalination plant. The type of intake works will depend on the required abstraction rate, site specific physical and environmental conditions as well as construction costs (Le Roux, 2010: p 8). Intake pipelines or extraction: Sea water has to be transported deep from the sea to the desalination plant using either pipelines or extraction tunnels. The materials of the pipelines are chosen by taking into consideration various factors. Primary treatment (part of extraction system): The function of this is to reduce the coarse particles, marine life and debris entering the main pipeline with the help of either bar or wedge wire (passive screen) located at the end of the pipeline in the sea. Normally it is recommended to have some sort of off shore screen to further remove suspended particles to make sure pumps are not damaged. Pump stations: This structure houses the intake pumps and is located on the shore to make sure it provides sufficient head to overcome frictional losses and pump sea water to the desalination plant. The intake pipelines from sea need to be carefully divided accordingly to make sure all the pumps extract water at a constant flow rate Marine Pipeline selection The intake pipelines along with the materials are an important aspect of a desalination plant. Intake pipes can have a major impact on the hydraulics and the operation of the plant. The materials for pipes which are more resilient to biological and marine growth and less prone to corrosion should be considered such as GRP and HDPE pipes. But there are many other factors which could determine the choice of specific material for intake pipes such as (Mostert, 2009): Design life of pipe line Financial aspects related to: (i) Cost of pipe (ii) availability of local support Importance of required inside diameter i.e. the design flow, hydraulics losses and frictional losses Installation of pipeline such as handling of pipes, cost of installation, installation above or below sea bed and special installation arrangement due to the weight of material Availability of components i.e. length of pipes, availability of pipes, construction times, flexibility for fittings and bends International acceptance among major desalination plants 15 Ali Niazi

29 Chapter 3: Applications and Theory Maintenance Testing procedures Support to local industry Table 7 summarises the advantages and disadvantages for a variety of materials for intake pipelines. Based on this comparison a choice of material can be made. Each type of material for intake pipes has pros and cons and the best choice is made by looking at the factors which suit the proposed RO desalination plant. Table 7: Material selection for intake pipes (Mostert, 2009) Pipe Material Maximum Dia(m) c* (m/s) ks** (mm) Advantages Disadvantages HDPE 1.6 Approx to Corrosion & Chemical resistance Abrasion resistance (thick walls) Relatively light weight Flexible No stress cracking Ability to install seamless pipeline Proven technology Non toxic Lack of UV resistance Not suited for suspended applications prone to sag/stretch/shrink Thick walls fusion/weld/melt conditions much more difficult and critical Fittings and connections could be difficult to install underwater Required weighting PP Approx 600 Ability to withstand high temperatures Corrosion and chemical resistance Abrasion resistance Relatively light weight Flexible No stress cracking Ability to install seamless pipeline Limited range of sizes Limited expertise manufacture/installation Limited range of jointing options Inadequate specifications Inadequate training for installers Imported for Special Applications GRP 4 Approx Lightweight (especially large diameter) Corrosion resistance Ease of handling and installation Easy jointing Ease of fabrication Aggressive environments/media Higher pressures (up to PN32) Low co-efficient of friction Low wave celerity Long pipes (up to 18m) Positively buoyant in water Not resistant to negative pressures without trench preparation Require careful handling Compacted sand backfill around pipe is required for support the pipe from deforming and cracking when negative pressures occur inside or from overburden pressures Cannot use as a subsea pipe where there are waves Cannot be laid directly on bedrock without specific bedding preparation. 16 Ali Niazi

30 Chapter 3: Applications and Theory Steel Ductile Iron Pipe to to 1200 Concrete 0.15 * Wave celerity for 6 to 16 Bar working pressure ** Roughness (new pipe) High strength ( MPa) Can be laid on bedrock No specific and inexpensive bedding and backfilling Many skilled labour available for welding pipes Can be installed as a continuous pipe length High tensile strength High impact strength High pressure High ring bending strength High beam strength High circumferential tensile strength High design safety factors Increased pressure headroom Every pipe pressure tested Strength increase with time Soil conditions, bedding and load not problematic Constant internal shape and size as well as rugged abrasion resistant surface Protection against corrosion (need to keep cathodic protection operational for lifespan of pipe) High cost in comparison with other pipe materials Temporary works and construction of steel marine pipelines are more expensive than GRP and HDPE pipe materials Heavy to handle, especially in Imported Prone to exchange rate fluctuations Long lead times Not labour intensive friendly Only standard range of fittings use for steel fabrication Corrosion of steel reinforcement when used in marine environments Heavy to handle, especially when used as a subsea pipe Locally available pipes would need to be redesigned if used as a subsea pipe Cannot be laid directly on bedrock in a trench or in the sea without specific bedding preparation. Cannot be used as a subsea pipe where there are waves as installation requires flat water conditions Subsea concrete pipe needs to be buried for stability Bedding and backfilling construction costs in a subsea application are more than that of a continuous jointed pipe such as steel or HDPE making it not feasible as a subsea pipe Installation methodology of marine pipes There are various installation methodologies for offshore intake pipes. The choice of location, onsite specific condition (e.g. exposed coastline, rocky terrain), the location of the desalination plant and the material of intake pipe affect the installation methodology. The methods commonly used for the intake pipeline installation are Lay-barge (conventional), S-lay and J-lay, Surface Tow, Near Bottom Tow, S-Curve Float and Sink and Bottom Tow (Le Roux, 2010: p 26-27). 17 Ali Niazi

31 Chapter 3: Applications and Theory S-lay and J-lay: The S-lay and J-lay names are derived from their gentle shape form the end of lay vessel and the bottom of the sea bed. S-lay method has been used to install intake pipes in the sea at depths from 10 m to 500 m (Le Roux, 2010). In S-lay, the pipe tension is required for the installation where as in J-lay pipe tension is not required. Figure 7 shows the basis schematic illustration for S-lay and J-lay. Figure 7: Schematic illustration for S-lay and J-lay (Le Roux, 2010: p 28-29) Environmental Consideration The intake seawater and the outfall of the brine from the desalination plant have been considered to be the greatest ecological impediment threat to marine life. All the necessary precautions must be taken in order to minimise or eradicate this threat. Previously, it was considered that the marine life was at much greater risk from the outfall brine discharge until the study carried on by California s Department of Water Resources (CDWR) in It was concluded that, with proper design and location of outfall, concentrate disposal may not be a major impediment to desalination (Panktratz, 2006: p 3) and it carried on to say that the intake of a desalination plant is much more of a concern for the marine life and harder to quantify. Impingement occurs when marine organisms get trapped into the intake screens due to velocity and force of water. The mortality rate of the marine organisms depends on the size of fish, size of screen, age, location and water condition. Some of the tough species can still survive after impingement and returned to the sea. But for most of cases, species with less than 24-hour survival rate or species less robust like jelly fish have a chance of 15% of surviving after impingement (Panktratz, 2006). The Environmental Protection Agency (EPA) recommends that in 18 Ali Niazi

32 Chapter 3: Applications and Theory order to reduce the impingement and reduce mortality rate of marine organisms, seawater velocity at the intake screen should be 0.15 m/s or less (Le Roux, 2010). Entrainment occurs when very small microscopic organisms (e.g. Phytoplankton, zooplankton, eggs and larva) in sea water pass through an intake screen. Small marine organisms that enter the screens generally have a mortality rate of 100% (Panktratz, 2006). The impacts of entrainment and impingement depend on a variety of factors such as extraction flow, local sea condition and site of the intake facility. The following technologies should be taken into consideration when designing the intake facility for desalination (EPA, 2001): Intake velocity Location Type of screens Velocity caps Physical barriers Screening methods For direct intake, screening is a compulsory option. There are various alternatives available for screening depending on different applications. Screens could be either placed on the shore e.g. travelling screens, fish net and fine mesh or off shore at the end of pipeline e.g. passive screens and velocity cap. Others methods include louvers, angled traveling installation and: Travelling Water Screens consist of revolving mesh wire screens with slot width of 6 mm to 9.5 mm openings. It is one the oldest and widely used screening method. This method always needs to work in conjunction with other methods like fine mesh wire to further reduce the debris load. This is only suitable for direct surface intake therefore, not suitable for the proposed plant. Velocity Cap consists of a horizontal structure at the beginning of the intake pipe. The screens are located at the top of the structure in order to reduce the debris and fish passing through. As shown in Figure 8, the cross-sectional flow gives more chance to marine life to escape the screen. Studies have shown reduction of 80-90% in fish impingement at two California power plants (Panktratz, 2006). The only downside is that offshore screening is still needed to further filter seawater. The other downside is that maintenance is performed manually by the sea divers. These screening methods can be used for direct sub-surface intake a shown in Figure Ali Niazi

33 Chapter 3: Applications and Theory Figure 8: Intake with a velocity cap headwork s (Le Roux, 2010: p 19) Passive Screens Passive screens are contrasted of trapezoidal-shaped wedge wire with screens opening from a range of 0.5 mm to 10 mm. These cylindrical shaped screens come in two types known as S- shaped and L-shaped as shown in Figure 9. S-shaped passive screens are used for small extraction of seawater whereas L-shaped passive screens are used for much large intake due to their large surface area. The diameter of these passive screens ranges from 0.4 m to 2.1 m and the length ranges from 0.5 m to 8 m. Selection of an optimum passive screen depends on the arrangement of passive screens, number of intake pipes and the intake volumetric flow rate. Passive screen are generally best suited for the area where an ambient cross-flow current is present (Panktratz, 2006: p 6). Figure 9: S-shaped and L-shaped passive screen (Ovivo, 2010: p 4) Passive screens are constructed with larger pipes than the intake pipes and for that reason inlet velocity is kept at 0.15 m/s or even less. Due to low inlet velocity of seawater, impingement and entrainment of marine organisms is reduced by 80%. These screens are cleaned by automated air backwash (hydrobrust) system. 20 Ali Niazi

34 Chapter 3: Applications and Theory Maintenance: As mentioned earlier, the cleaning is performed by air backwash system also known as hydrobrust. There are four main components of the air back wash which are listed below (Johnson Screens, 2010): 1. A compressor is used to recharge the air in the receiver tank 2. A receiver tank which then stores the compressed air supplied by the compressor 3. A distributer system, required for multi-screen application i.e. air-actuated valves and piping for the air to flow from the tank to the desired passive screen 4. A control panel for either manual or automated service to carry out the cleaning process This short burst of compressed air is released after every few hours to repel debris and zebra mussels from the screens. Also the screens slots are designed in a V-shaped wire specifically to prevent clogging as the wire allows only two-point particle contact Pumping Station Pumping station is one the most important part of the intake structure located next to the shore line. The pumping requires an underground pit usually m deep with special arrangement of sheet piling or concrete wall piling and dewatering. The geology of the vicinity must be evaluated to make sure that the underground pit is not constructed on the hard rock (Tsiourtis, 2008). The construction of the pumping station not only depends on the intake type but also the type of coastline. The three most common types are given below (Moretti, 2005): Sandy coast with low gradient Rocky coast Natural or artificial enclosure There are two types of pumps which can be built: A dry pumping station consists of a room in watertight room which contains set of pumps in open air A flooded or wet pumping station consists of submersible or vertical pumps installed in a reservoir Dry Pumping station: Dry pumping station is being used for the proposed plant as this is more suited for sea water intake with pipelines and it can also cope with high volumetric flow intake. This type of pumping station is generally equipped with horizontal centrifugal pumps but vertical axial pumps are also used. These pumps should be located above the highest level of sea to prevent back flow. The Figure 10 shows the top view and side view of the pumping station. 21 Ali Niazi

35 Chapter 3: Applications and Theory Figure 10: Dry pumping station - top and lateral view (Moretti, 2005) Choice of number of pump sets: There are different options available to deliver a non-stop variable flow to deliver the maximum desired flow (Moretti, 2005): 1. The minimum number of pumps required for installation is ten pumps, each one delivering the total maximum flow (Q max ). This is the minimum required for security purposes. This is the cheapest option in terms of capital investment but in terms of operational cost it s not very economical as each pump would need to operate for 12 hours alternatively. This would mean the pumps would not operate at full capacity since each pump can work up to 16 hours thus making it inefficient. 2. Three pumps can be installed, each delivering Q u = 0.5 Q max, giving a total flow of 1.5Qmax. This requires more capital investment but at any given point two pumps would be running continuously to give Q max. If this configuration is in mode for 48 hours then each pump will have been in operation for 16 hours. Therefore, at any given point the intake of seawater by two pumps (0.5 Q max Q max = Q max ) doesn t fall below the Qmax. 3. Moreover, four sets of pumps can be installed, two pumps giving Qu= 0.5 Qmax and the other two pumps delivering Qu = 0.25 Qmax, giving a total flow of 1.5 Qmax. Running cost for this set up would be much more variable. 22 Ali Niazi

36 Chapter 3: Applications and Theory These are the options available for the pumping station A detailed study should be performed and the annual cost for each configuration should be included which would help in choosing the best suited option for the desired intake. Pump station protection: Generally a dyke made of rocks is built in front of the pump station in order to protect it during sea storms. The design is based on the Hudson formula which helps to determine the appropriate rock cover. The formula if given below (Moretti, 2005): P = ( ) ( ) Where: P = the weight of the rocks in tonnes KΔ = a coefficient (equal to 3.2 for rocks) d = the specific density of the rocks, in t/m 3 d o = the water specific density or about 1 t/m 3 a H c = the angle with the horizontal of the external wall of the dyke = the amplitude of the waves breaking on the structure 3.2. Pre-treatment Process The pre-treatment process has been determined based on the composition of the seawater intake. Its function is to reduce the SDI from 7 to 3 by removing visible solid particles and suspended materials. This is to reduce the risk of membrane fouling in the main treatment process Dissolved Air Floatation Process DAF is a water treatment process used to remove oil, grease and suspended matter from raw water. DAF, like traditional sedimentation processes requires particles to settle however, DAF uses the buoyancy added to particles by bubbles instead of the force of gravity. DAF first uses coagulation and flocculation to group the suspended matter into floc particles this is then introduced to the main DAF basin. Some of the effluent water is taken into a separate tank where it is put under a pressure of typically 4 8 bar, this is then saturated with air. This water is then 23 Ali Niazi

37 Chapter 3: Applications and Theory reintroduced close to the water inlet area in the main basin which is at atmospheric pressure. The change in pressure causes air tiny bubbles which are typically µm, to form in the water. As the bubbles rise they attach to the floc particles and can even form within the particles. This gives the floc particles a greater buoyancy causing them to rise to the surface. At the surface any floc particles, now known as float, are removed using a mechanical scraper. The advantages of using a DAF system as opposed to a traditional gravity settlement system are that DAF has a higher flow rate, less of a retention time and therefore requires much less space when compared to a traditional gravity settlement technique. ACID CAUSTIC INFLUENT COAGULANT POLYMER TANK SCREEN ph CONTROLLER DAF UNIT EFFLUENT SCREENING EQUALISATION TANK FLOC TUBE BOTTOMS TO COLLECTION BOTTOMS SKIMMINGs SKIMMING DECANT SKIMMING Figure 11: Dissolved Air Flotation Process Diagram Adapted from (Ross and Smith, 2000: p 3) Fundamental Theories The process of DAF is based upon the laws of fluid mechanics and gas Henrys Law At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. This law describes how much air can be saturated into a volume of water at a particular pressure. 24 Ali Niazi

38 Chapter 3: Applications and Theory Stokes Law Stokes law describes the dynamics of a particle when in a viscous fluid. ( ) v s is the particles settling velocity (m/s) g is the gravitational acceleration (m/s 2 ) p p is the mass density of the particle p f is the mass density of the fluid (kg/m 3 ) (kg/m 3 ) R is the radius of the particle (m) is the fluids viscosity (kg m -1 s -1 ) Coagulation and Flocculation Coagulation and flocculation is performed prior to the raw water being added to the DAF basin. It is used to clump together the colloidal and suspended matter into larger chunks. The coagulation and flocculation process starts with a chemical, called a coagulant, being added to the water this encourages the particles to join. The water is then mixed first very quickly in what is called a rapid or flash mix; this uniformly distributes the coagulant chemicals in the raw water. This is then mixed progressively slower which allows the floc particles to join together. Figure 12: DAF Unit with labelled Zones (Peleka and Matis, 2008: p 3) Almost all of the particles in water carry a negative charge. As like charges repel the particles are pushed apart by electrostatic forces. Coagulant chemicals are generally positively charged and the addition of a coagulant neutralises the charge of the suspended and colloidal particles. Colloids 25 Ali Niazi

39 Chapter 3: Applications and Theory are very fine particles with diameter in a range of 10 nm to 10 um. Figure 13 shows typical diameter of colloids. Figure 13: Typical diameters of colloids in meters (Koohestanian, Hosseini and Abbasian, Types of Coagulants 2008) The commons types of coagulants are slum, ferric sulphate, ferric chloride and sodium aluminate. The first three decrease the alkalinity and ph of the solution, whereas sodium aluminate increases the alkalinity and ph of the solution. A comparison is needed to determine the best choice of coagulant. Table 8 shows the comparison between various coagulants. Table 8: Advantages and disadvantages of inorganic coagulants (U.S. Engineering Corporation, 2001) Coagulant Advantages Disadvantages Alum (Aluminium Sulphate) Al 2 (SO 4 ) 3.18H 2 O Sodium aluminate Na 2 Al 2 O 4 Ferric sulphate Fe(SO 4 ) 3 Ferric Chloride FeCl 3.6H 2 O Lime Ca(OH) 2 Easy to handle and apply; most commonly used; produces less sludge than lime; most affective between ph 6.5 and 7.5 Effective in hard water; small dosage usually needed Effective between ph 4-6 and Effective between ph 4 and 11; cheaper than alum; requires small dosage compared to alum Commonly used; very effective; may not add salts to Adds dissolved solids (salts) to water; effective over a limited ph range Often used with alum; high costs; ineffective in soft waters Adds dissolved solids(salts) to water; usually need to add alkalinity Adds dissolved solids(salts) to water; consumes twice as much alkalinity at alum Very ph dependent; produces large quantities of sludge; 26 Ali Niazi

40 Chapter 3: Applications and Theory effluent overdose can result poor effluent quality. The three most widely used coagulants in water treatment are ferric chloride, alum and ferric sulphate. The typical basic reactions are given below (Hamidi et al., n.d.): Alum Al 2 (SO 4 ) 3 + 3Ca(HCO 3 ) 2 2Al(OH) 3 + 3CaSO 4 + 6CO 2 Alum is combined with the calcium bicarbonate which is already in the water will produce three new species including aluminium hydroxide, calcium sulphate, and carbon dioxide. Ferric Chloride 2FeCl 3 + 3Ca(HCO 3 ) 2 2Fe(OH) 3 + 3CaCl 2 + 6CO 2 Ferric chloride, 2FeCl 3, is reacted with calcium bicarbonate producing Ferric hydroxide, calcium chloride, and carbon dioxide. Ferric sulphate FeSo 4 + 2Ca(HCO 3 ) 2 Fe(OH) 2 + CaSO 4 + 2CO 2 Ferric sulphate is reacted with calcium bicarbonate forming Ferric hydroxide, Calcium sulphate, and carbon dioxide. Coagulant Aid: The coagulation process is sometimes optimised by the addition of coagulant aid to produce larger flocs. Primary coagulant is added in water to produce large flocs. However, sometimes, even the addition of excessive dosage of coagulant is not enough to produce desired flocs. Therefore, coagulant aids can be added to produce denser flocs and also reduce the reaction time. Coagulant aids also make the primary coagulant effective over a broader range of ph. The most commonly used coagulant aids are slightly anionic polyacrylamides with very high molecular weight. They help to create polymer bridging between the smaller flocs particles to create long chained flocs due to electrostatic forces i.e. anionic and cationic charge (Binnie, Kimber and Smethurst, 2003). Coagulant Dosage: In order to choose the right type and optimum dosage in mg/l of seawater, a jar test should be carried out using the seawater to be treated. The optimum dosage depends on various factors given below e.g. ph of seawater, type of coagulant, type of coagulant aid, and 27 Ali Niazi

41 Chapter 3: Applications and Theory turbidity of seawater, temperature and stirrer speed. Every desalination plant carries out a jar test to choose an optimum dosage in order to avoid over dosing and also reduce the chemical costs. Jar test: This simple test can be carried out in the laboratory to determine an optimum dosage. The price of this setup can be around $2000 (Satterfield, 2005). This test consists of four to six beakers with each 1 litre of volume. Each beaker is filled with identical volume of raw water and is treated with different dosage of coagulants. Different parameters are altered e.g. coagulant type, mixing speed, filtration time and coagulant aid type and the test is repeated several times with various combinations. After the addition of coagulants and coagulant aids, the paddles inside the beaker are rotated at high speed for a short time and then at low speed for up to 30 minutes. After that it is finally stopped and left to settle for 15 minutes. To determine the optimum dosage all the beakers are observed for flocs. A hazy/cloudy solution indicates poor coagulation and a clear solution with flocs settled at the bottom of the solution is considered properly coagulated. Further analysis is needed to check turbidity of each beaker using a turbidometer. A solution which gives the desired turbidity i.e. below the minimum is considered the optimum dosage. This test is repeated several times with the addition of coagulant aid until an optimum dosage of coagulant + coagulant is obtained Design Parameters There are several key parameters that must be considered when designing a DAF unit. Each of the parameters is described below. Air-to-solids ratio The air to solids ratio describes the ratio between the volume, or weight, of air which is to be dissolved into the water to the amount of TSS. This generally varies between applications however, the typical values range from to 0.06 millilitres of air per milligram of solids to be removed. Hydraulic Loading Rate The Hydraulic Loading rate of a DAF unit is the total amount of effluent per unit surface area per unit of time. It gives a measure of the amount of water produced per unit of surface area. Recycle Ratio The recycle ratio is the percentage of the effluent which is saturated with air and returned back to the main basin. 28 Ali Niazi

42 Chapter 3: Applications and Theory Practical Implementation There are two different approaches to implementing a DAF system, these are using a rectangular basin and a circular basin both of which are shown in Figure 14 and Figure 15 respectively. Figure 14: Rectangular DAF Unit (Ross and Smith, 2000: p 2) Figure 15: Circular DAF unit (Krofta Engineering, n.d.) Rapid gravity Sand Filter Filtration Methods Currently two types of filtration are used in Seawater Reverse Osmosis (SWRO) desalination pretreatment: 1. Conventional granular filtration 2. Membrane filtration The most commonly used of the two methods, conventional filtration, involves passing the raw water at a continuous rate through a bed of granular media, typically sand. Conventional filtration operates using both straining and biological process mechanisms to remove both suspended solids and pathogens- slow sand filtration- or a physico-chemical process preceded by a coagulation and 29 Marco Volino

43 Chapter 3: Applications and Theory flocculation stage where solids are removed due to the physical and chemical interactions between the granular media and the particles- rapid filtration. Table 9 compares the three different conventional granular filtration methods; slow sand filtration, precoat filtration and rapid filtration. Slow sand filtration and precoat filtration have lower filtration rates than rapid filtration and generally longer filter run lengths. Due to these factors and the low turbidity required for source water by these two filtration methods, rapid filtration is the only form of conventional granular filtration used in SWRO desalination. Table 9: Comparison of granular filtration technologies for water treatment (Crittenden et al., 2005) Process Characteristic Slow Sand Filtration Rapid Filtration Precoat Filtration Filtration rate m/h 5-25 m/h m/h Media diameter mm mm 4-30 μm Bed depth m m 2-5 m Required head m m 6-30 mm Run length 1-6 months 1-4 days 6 h- 30 days Ripening period Several days 15 min- 2 h None Pre-treatment None required Coagulation None required Dominant mechanism filtration Straining, activity biological Depth filtration Straining Regeneration method Scraping Backwashing Bed replacement Maximum turbidity raw-water NTU Unlimited with proper pre-treatment 10 NTU Membrane filtration is a pressure driven process, whereby a pressure differential is used to pass raw water through a semi permeable membrane (Twort, Ratnayaka Don and Brandt, 2000). These membranes only allow particles and suspended solids of a certain diameter, typically less than 0.1μm to pass through (Crittenden et al., 2005) and rejecting all other particles. The two types of membrane filtration are: Microfiltration (MF) and Ultrafiltration (UF). In the last ten or so years, there has been an increase in the number of desalination plants opting to use membrane filtration instead of conventional granular filtration, with 24 large scale SWRO plants using membrane filtration worldwide (Voutchkov, 2010). For the new SWRO desalination plant in Singapore, it has been decided to utilise conventional rapid filtration. This decision is based on the following reasons: 1. It has been assumed that the seawater has a high petrochemical content, which can be effectively removed by conventional chemical pre-treatment and Dissolved Air Filtration 30 Sara-Louise Bent

44 Chapter 3: Applications and Theory (DAF); therefore rapid filtration is required to remove the larger floc particles and suspended solids from the processed seawater. Using membrane filtration after DAF would cause frequent fouling of the semi permeable membranes 2. The higher water recovery rates using rapid filtration compared to membrane (95-98% compared to 88%) (Voutchkov, 2010) 3. The overall costs of membrane filtration, including chemical cleaning and membrane replacement for the quality of seawater in Singapore that has been assumed Rapid Filtration As previously mentioned, rapid filtration involves passing raw water through a granular media at filtration or loading rates of 5-15 m 3 /m 2.h (or m/h). This rate is a measurement of the volumetric flow rate of water per unit surface area of filter. With improvements in rapid filtration design, it is possible to have filtration velocities up to 33m/h; however one problem associated with increasing filtration rates is the increased likelihood of filter compromise causing deterioration in effluent water turbidity and microbiological content. Rapid filters can be either pressure or gravity, depending on the whether the filtration process operates because of a head due to pressure or gravity. Pressure filters consist of a sealed cylindrical vessel made from a corrosion resistant material such as stainless steel or glass reinforced plastic, with an internal configuration similar to that shown in Error! Reference source not found. (The Water Treatments, 2010). These filters can either be horizontal or vertical with typical diameters of m (Parsons and Jefferson, 2006) (Twort, Ratnayaka Don and Brandt, 2000). These filters operate at pressures of at least 2 bar (Prihasto, Qi- Feng and Seung-Hyun, 2009: 310). Figure 16: A vertical pressure filter (The Water Treatments, 2010) Due to the high capital costs of pressure filters and the high energy requirements to produce pressures that can feed water through the filters, pressure filters are generally used for filtration at small and medium SWRO desalination plants with a production capacity of less than 31 Sara-Louise Bent

45 Chapter 3: Applications and Theory 20,000m 3 /day, however according to Voutchkov (2010, p 358), the majority of plants in Spain use pressure filters. A SWRO desalination plant in Barcelona, Spain, produces 200,000m 3 /day of potable water using a two-stage filtration system comprising a dual-media gravel filter followed by a dual-media pressure filter (Voutchkov, 2010). Rapid gravity filters comprise of an open-top rectangular or square reinforced concrete tank, containing typically 2-3m head of raw water, 0.6m -1.8m of filter media, 0.5m of support gravel (see Table 9) and an underdrain system that supports the filter media and provides a channel for both the effluent and influent filtered water and backwashing. Rapid gravity filters also have wash water troughs that collect waste water from filter backwashing and a filter to waster pipe to collect the high turbidity water removed from the filter during filter ripening. Error! Reference source not found. shows the main components of a dual-media rapid gravity filter. Figure 17: Cross section of a dual media rapid gravity sand filter (Crittenden et al., 2005) For rapid gravity filter design, the typical filter lengths are 4-12m, whilst filter widths are m. Common surface areas of rapid filters range from m 2. As a general design rule of thumb, the ratio of filter width to length should be between 1:1.25 and 1:1.33 (Indian Institute of Technology Madras, n.d.). Error! Reference source not found. shows the typical stages in a filter cycle. Water produced during the function of influent stage is used in potable water production. Water produced in the other stages are either returned to the front of the water pretreatment system i.e. before coagulation or sent to waste to be treated. Filter run lengths for rapid filters last between 1-4 days. 32 Sara-Louise Bent

46 Chapter 3: Applications and Theory Figure 18: A typical filter cycle (American Water Works Association and American Society of Civil Engineers, 2005) Uses of Rapid Filtration There are currently four different uses for rapid sand filters in potable water production. This is dependent on the level of pre-treatment required. 1. Direct filtration- for this form of filtration, the rapid sand filters follow coagulation and flocculation stages, however there is no clarification or sedimentation stage (Crittenden et al., 2005). Chemicals such as oxidants and granular activated carbon (GAC) are added to the influent water to remove unwanted particulates, metals and organic matter (New Zealand Ministry of Health, 2001). This level of filtration is suitable when using raw water from lakes and reservoirs or where the source water does not have a high or variable turbidity i.e. <15 NTU (Crittenden et al., 2005). 2. In-line or contact filtration-typically used in source water with turbidity less than 10 NTU, contact filtration involves coagulation and incidental flocculation. In this type of pretreatment, the particulates in the raw water are destabilised, however very little floc growth occurs. The destabilised particles in the influent raw water then attach to the filter grains and other existing solids within the filter media (Crittenden et al., 2005). 3. Single-stage filtration- also known as conventional filtration or polishing filtration. The most common use for rapid sand filters, the filtration stage follows the conventional pre-treatment processes of coagulation, flocculation and sedimentation or dissolved air flotation. This form of filtration can be used with all types of source water especially those with high turbidity up to 1000 NTU. A benefit of this form of filtration is that it responds very well to variability in source water quality and changes to filtration rates (Crittenden et al., 2005). 4. Two-stage filtration- in this form of filtration, the rapid filters act as a first stage roughing filter that removes large particles such as floc and suspended solids from the influent water 33 Sara-Louise Bent

47 Chapter 3: Applications and Theory (Crittenden et al., 2005). The filtered water is then passed through a second-stage finer filter e.g. a cartridge filter or sand filter with finer media to remove finer particles and pathogens. Two-stage filtration is generally preceded by coagulation-flocculation and can be used for source water with turbidity up to 100 NTU. This type of rapid filtration is beneficial for source waters with a high algal or pico-plankton content or where there is a propensity for solids/turbidity spikes as described by Voutchkov (2010, p ).. Rapid sand filters are used as both single and two-stage filtration for SWRO desalination plants. Ashkelon in Israel, the largest SWRO plant in the world, produces 325,000m 3 /day of water using single-stage rapid gravity sand filters (Voutchkov, 2009), whilst a desalination plant in El Coloso, Chile uses Dissolved Air Flotation followed by a two-stage horizontal pressure filter Rapid filtration hydraulics In rapid filtration design, understanding the hydraulics of the flow through the granular media is important, as it used to determine the head loss through the filter during operation and the fluidisation or expansion of the filter bed during backwash (Crittenden et al., 2005: p 887). For simplicity, it is assumed that filter media grains are spherical (not true in nature), so the first step in understanding hydraulics in flow through granular media is to understand the flow regime around these spherical grains. To do this, the Reynolds number (Re) has to be determined. Equation 1 Due to the presence of the filter media, the flow of water through a filter can be generally described as laminar (Re < 2x10 5 ), however turbulent flow is experienced in some filters, yet the transition from laminar to turbulent flows does not occur as quickly as it would in pipes (Crittenden et al., 2005: p 887). The flow of water can be described by four different flow regimes (Crittenden et al., 2005): 1. Darcy or creeping flow- this occurs at the lower end of the scale for flow with Re less than Forchheimer flow- this occurs at Re between 1 and 100. This type of flow is influenced by both viscous and inertial forces and can be described as steady laminar flow. 3. Transition zone- for flows with Re between 100 and Fully turbulent flow The Reynolds number for different granular media filters is shown in Figure 19. From this figure, it can be seen that the Re for typical rapid filters ranges between 0.5 and 5. The flow regime in 34 Sara-Louise Bent

48 Chapter 3: Applications and Theory filters with lower filtration rates i.e. around 5 m/h can be described as Darcy flow, whilst the flow regime in all other rapid filters can be described as Forchheimer flow. The flow regime during backwashing is also Forchheimer flow -Re between 3 and 25 (Crittenden et al., 2005). Head loss Figure 19: Ranges of Reynolds number for different granular filtration technologies (Crittenden et al., 2005) Head loss in filters is a function of the media grain size and this increases as the number of particles deposited in a filter accumulates. To determine head loss in filters with the Darcy flow regime, Darcy first suggested the following equation (Crittenden et al., 2005): Equation 2 where v superficial velocity (filtration rate), m/s - hydraulic permeability, m/s h L- head loss across media bed, m L- depth of granular media bed This was then developed further by Kozeny in 1927, to include terms that took into consideration filter media properties. For Forchheimer flow, Equation 3 was initially proposed to determine clean bed head loss through the filter media (Crittenden et al., 2005): 35 Sara-Louise Bent

49 Chapter 3: Applications and Theory Equation 3 where k 1 = permeability coefficient for linear term (s/m) k 2 = permeability coefficient for square term (s/m 2 ) However, like Equation 2, this formula never considered the properties of the filtration media, therefore other researchers worked to improve the formula, and develop one that included these properties. In 1952, Ergun proposed Equation 4, and this is used to determine the clean bed head loss in filters with Forchheimer flow regime. ( ) ( ) Equation 4 where: ɛ- media porosity, (%) - head loss coefficient due to viscous forces, dimensionless - head loss coefficient due to inertial forces, dimensionless According to Crittenden et al. (2005, p 891), because the flow regime in rapid filters can be either the Darcy regime or Forchheimer regime, and the transition between Darcy flow and Forchheimer flow is gradual, Equation 4 can be used to determine head loss for both flow regimes. Backwashing hydraulics The hydraulics of backwashing is slightly more complex than the hydraulics of clean bed head loss, and as such the majority of this theory falls outside of the scope of this report. For further reading into this, consult the literature referred to in this chapter such as (Crittenden et al., 2005). In filter backwashing, deposited particles within the filter media are removed by the process of fluidisation, where the fluid causes the filter media bed depth to expand in size between 5-30% (Parsons and Jefferson, 2006). The velocities required to cause bed fluidisation and expansion are dependent on the following parameters: media grain size, media density, media voidage and packing and temperature. To determine the bed expansion at the point of fluidisation based on media porosity and water velocity, the following equation is used (Parsons and Jefferson, 2006: p 84): 36 Sara-Louise Bent

50 Chapter 3: Applications and Theory According to Parsons and Jefferson (2006,p 84), to achieve the same bed fluidisation at 5 C as at 15 C requires an increase in flow rate of about 50%. As the temperature increases, the density and viscosity of water both decrease and inversely as temperature decreases, the density and viscosity of increases, so at the lower temperatures, more energy is required to cause the more viscous water to push the accumulated particles out of the filter media voids. Energy is proportional to velocity squared, therefore if more energy is required for bed fluidisation at lower temperatures, faster fluidisation velocities are required Process mechanisms Rapid filtration is able to remove suspended solids with diameters of greater than 0.1μm (Parsons and Jefferson, 2006), by a range of physical and chemical processes grouped into three main mechanisms; straining, transport and attachment. 1. Straining Straining is the process by which particles with diameters greater than the void spaces between the filter media is removed from the influent water (Crittenden et al., 2005). As the particles are larger than the void space they are trapped on the filter media, forming a cake layer, which increases in thickness as the number of particles deposited builds up. In rapid filtration, typically particles with diameters between 30-80μm are removed by straining, but this is dependent on the diameter, shape and arrangement of the filter media (Crittenden et al., 2005). The build of the cake layer on the top of the filter media bed is one of the reasons why rapid filters are designed to minimise straining (Crittenden et al., 2005: p 904)as a process mechanism. This build up causes head loss within the filter, thus reducing filter run lengths and filter effectiveness as only the top of the media bed is used to filter water (Crittenden et al., 2005). Instead of using straining to remove particles, mechanisms that encourage depth filtration-particle removal throughout the entire filter bed are used (transport and attachment). Research on depth filtration in rapid filters, has shown that the rate of particle removal in filter is related to the concentration of particles in the filter, such that (Crittenden et al., 2005): where: λ- filter coefficient, m -1, Equation 5 C- mass or number concentration of particles, mg/l z- depth of filter, m 37 Sara-Louise Bent

51 Chapter 3: Applications and Theory 2. Transport Mechanisms Error! Reference source not found. shows the transport of a particle in influent water near a filter media grain for the three different transport mechanisms; interception, sedimentation and diffusion. Interception- This form of particle transport occurs when a particle is moving on a fluid streamline that is within half the particle diameter from the filter (collector) media grain. The grain is then able to intercept and remove the particle out of the influent water. According to Crittenden et al. (2005, p 910), interception efficiency increases as the ratio between the diameter of the collector grain and the particle increases, i.e. it is easier to intercept larger particles. This efficiency is described by the following equation develop by Yao in 1971 (Crittenden et al., 2005: p 910): ( ) Equation 6 Where transport efficiency due to interception, dimensionless - diameter of particle, m - diameter of collector (filter media) grain, m Figure 20: Particle transfer mechanisms in fundamental depth theory with each theory given a different label-a) Interception B) Sedimentation C) Diffusion (Crittenden et al., 2005) Sedimentation- Sedimentation occurs when the density of the particle in the influent water is greater than the density of water. Due to the effects of gravity, the particle leaves the fluid streamline, deviating towards the collector grain (see Error! Reference source not found.). As 38 Sara-Louise Bent

52 Chapter 3: Applications and Theory with interception, it is possible to measure the efficiency of particle transportation by sedimentation, such that: ( ) where: - transport efficentcy due to gravity, dimensionless Stokes settling velocity, m/s - filtration rate (superficial velocity), m/s ρ- density ( of p-particle or w water ), kg/m 3 μ- dynamic viscosity of fluid, kg/ms Diffusion- In this transport mechanism, particles deviate from the influent water streamlines due to the effects of Brownian motion, which is the random motion of small particles suspended in a fluid (Crittenden et al., 2005: p 911). Diffusion is most efficient for particles with a diameter less than 1μm. The efficiency of this mechanism is described by (Crittenden et al., 2005): where: Equation 7 Equation 8 Where K b- Boltzmann constant, J/K T- absolute temperature, K (273 + C) The transport mechanisms described are based on the Yao theory for depth filtration (Crittenden et al., 2005). According to this theory, the total transport efficiency (η) of a filtration is the sum of all the transport efficiencies, such that: 3. Attachment Equation 9 39 Sara-Louise Bent

53 Chapter 3: Applications and Theory Attachment is the process by which particles in water within a filter collide with and stick to collector grains. This occurs when the particles pass the surface of the collector grains and shortrange surface forces begin to influence particle dynamics (Crittenden et al., 2005: p 911).These forces include; London-van der Waals, electrostatic forces and hydrophobic interactions. If the forces between the filter media grain and the particles are attractive, then the particle attaches to the media, however if the forces are repulsive, no attachment occurs. Ensuring sufficient particle destabilisation during coagulation prior to filtration is the most important factor in will improving the attachment efficiency between the filter media and particles within the influent water (Crittenden et al., 2005). Attachment efficiency ranges from 0 for no attachment after the collision of particles and media grains through to 1 for attachment after every collision between the two Rapid Filter Classification There are different types of rapid filters and these can be classified based on the following three categories: 1. Filter media 2. Operation method- (pressure or gravity) 3. Backwash method Filter media In water pre-treatment, the properties of granular media affect the performance of rapid filters, as particulate and pathogen removal occurs within the filter media, therefore the selection of granular media is one of the most important considerations in rapid filtration design. For filtration design it is important to consider the following; filter media type, uniformity, size (diameter) and depth of media bed. Media type In terms of media type, rapid filters can either be mono, deep mono and dual or tri-media filters. Monomedia filters comprise of one layer of either coarse or fine sand up to a depth of m (Crittenden et al., 2005). Originally rapid filters were designed as monomedia; however the stratification of the media bed during backwash created a need for other filter designs that do not cause this phenomenon. This lead to the development of filters with deeper media or more than one layer of media; deep monomedia or dual and tri-media filters. 40 Sara-Louise Bent

54 Chapter 3: Applications and Theory Deep monomedia filters are used to produce filters with a longer filter run and improved filtration capacity compared to monomedia filters. A deeper bed, typically m deep of anthracite (hard, black coal) or granular activated carbon (GAC) is used. More commonly dual media filters are being used in water treatment and SWRO desalination. This type of filter consists of two layers of filtration media, traditionally m of anthracite overlying m of denser sand (Crittenden et al., 2005). The benefit of dual media filters over mono or deep monomedia ones is the particulate removal and hence the turbidity removal of the filters and the robustness of the system. Also, including anthracite as the filter media reduces the headloss development in the filter, increasing the filter run length compared to a monomedia filter (Twort, Ratnayaka Don and Brandt, 2000). When the source water has a high organic content, i.e. petrochemical content from seawater, then the anthracite layer can be substituted with granular activated carbon (GAC). Where there is the need for enhanced removal of soluble organics from the seawater by biofiltration and/or to handle seawater with high pico-plankton content (Voutchkov, 2010), SWRO desalination plants commonly use deep bed dual-media filters, where the depth of the anthracite bed is increased to m. In seawaters that are affected by high algal blooms or high levels of silt, tri-media or mixed media filters are used (Voutchkov, 2010). These filters are similar in configuration to dual-media filters, except there is an additional bottom layer of garnet-a reddish or pinkish mineral that is often used as a gemstone- or ilmenite (Crittenden et al., 2005) a black iron titanium oxide (FeTiO 3 ). Garnet and ilmenite are both denser than sand and anthracite, (sg = 3.8 and 4.3 compared to 2.65 and 1.5 respectively), but have much smaller effective grain sizes (see Table 10) and are therefore able to trap the silt particles and algal cells in the influent water. Media properties For rapid filtration, it is beneficial to use filter media with a narrow size (diameter) range. This is to prevent stratification in the media beds, with the finer media grains moving to the top of the bed during backwash and the larger grains settling to the bottom (Crittenden et al., 2005). The process of stratification detrimental to the performance of a rapid filter, because in the filter bed causes excessive head loss (Crittenden et al., 2005: 902); reducing the filters effectiveness and making it harder to fluidise the is it encourages head loss development and reduces filter run lengths as mentioned previously. In order to maintain this narrow size distribution when selecting media for filters, the following two parameters have to be determined: the Effective Size (ES) and the Uniformity coefficient (UC). 41 Sara-Louise Bent

55 Chapter 3: Applications and Theory Introduced by Hazen in the late 19 th century (Crittenden et al., 2005), the effective size (ES) or d 10 describes the sieve size (in mm) for which 10% of media grains by weight are smaller, whilst the Uniformity coefficient is the ratio between the sieve size of which 60% of media grain by weight are smaller (d 60 ) and the effective size (ES) such that: Equation 10 This coefficient determines how uniform the sizes of media grains are. When rapid filters are cleaned, the larger the media grain size, the further down the bed the grain will settle, with the smallest particles settling at the top of the bed.. Therefore in rapid filters, it is desirable to have grains with a low UC value, ensuring that the sizes of grains are similar, thus preventing the problem previously described. Table 10 shows the typical ES and UC values for the most commonly used filter media. The uniformity coefficient range is generally the same for all filter media, except GAC, but the effective size increases as the media moves up the filter column. It is now recommended to use media with UC less than 1.4 for rapid filters (Crittenden et al., 2005). Other media properties that should be considered are when selecting granular filter media are porosity and void bed ratio (Prihasto, Qi-Feng and Seung-Hyun, 2009). Table 10: Properties of filter media (adpated from Crittenden et al., 2005) Property Unit Garnet Ilmenite Sand Anthracite GAC Effective Size (ES) mm Uniformity coefficient, UC Specific Gravity, sg Porosity,e % N/A N/A Media bed depth The depth of each filter layer is also important in rapid filtration design because depth filtration is one of the most important particulate removal processes in this form of filtration. If the filter bed depth is not sufficient, then there will be inadequate particulate and pathogen removal during filtration, causing breakthrough where the turbidity and microbiological content requirements of the effluent water will not meet standards. A sufficient lack of media depth will also promote rapid head loss development and decrease the filter run length times. A common design rule to determine a suitable media bed depth is the ratio of bed depth (l) to effective media grain size 42 Sara-Louise Bent

56 Chapter 3: Applications and Theory (de). Most published literature agrees such that ES=1.1mm:, therefore for anthracite grain with Equation 11 so the anthracite bed should be at least 1.1m deep. Other more specific values have been suggested e.g. Parsons and Jefferson (2006) suggest for mono and dual media beds, and 1250 for tri-media and coarse mono-media beds, whilst Voutchkov (Considerations for selection of seawater filtration preatreatment system, 2010: 357) states a ratio of 1500 for all beds. This ratio is not valid for selecting a suitable bed depth for a granular activated carbon layer. According to Voutchkov (Considerations for selection of seawater filtration preatreatment system, 2010), the depth of the GAC layer is determined by filter loading rate and the average contact time between the influent water and the media, which is typically minutes. Therefore: Equation 12 where; - bed depth (m), - loading rate (m/h), t- time as a ratio of one hour (per hour) Filter Backwashing As filter runs progress, deposited particulates and suspended solids accumulate within the filter bed and as this occurs, there is a headloss increase within the filter and its ability to remove particles and pathogens from the influent water decreases. When this headloss reaches the maximum acceptable level or there is an increase in effluent turbidity, the filter cycle is moving out of the effective filtration stage (end of the acceptable operation period), into the breakthrough phase. When this occurs, the filter has to be cleaned or backwashed. Backwashing is the process by which traditionally water is pumped upwards, back through the under drain system at a rate greater than the filtration rate, through the filter media bed, to remove these accumulated particulates and pathogens. To commence filter backwashing, the flow of inlet is stopped, allowing the water within the filter to fall to a level below the wash water troughs. The filter water and filter to waste water pipes are then closed, and the valves for the wash wastewater and backwash pipes are opened. The backwash pumps are gradually turned on to prevent sudden media disturbance and filtered water is then pumped through the underdrain system for approximately minutes until the wash water is sufficiently clear (turbidity of approximately 10NTU) (Parsons and Jefferson, 2006). The backwash pump is then turned off, and the valve on the backwash pipe is closed. At this point, the 43 Sara-Louise Bent

57 Chapter 3: Applications and Theory filtration cycle and the flow of influent water are restarted. There is still backwash water in the filter, but this is washed out of the system during filter ripening cycle (see section ) Backwashing rates range from m 3 /m 2/ h (Parsons and Jefferson, 2006) and generally 2-5% of filtered water is used for backwashing, giving a recovery rate of 95-98% for rapid gravity filters (Voutchkov, 2010). Traditionally, only water was used to backwash monomedia filters up to 0.9m depth, however this system is no longer in use, due to the ineffective removal of deposited particulates and pathogens in deeper filter beds (Crittenden et al., 2005). Now, the backwash technique of choice for rapid filters is combined air scour and water. For fine media grains, an air scour followed by fluidising water wash is used, whilst for coarse grains, a simultaneous air scour and water wash followed by a fluidising water wash (Parsons and Jefferson, 2006: 88) is used. The inclusion of air in the backwash system dislodges particles and suspended solids within the media beds, making it easier for them to be removed totally by the backwash water. Typical rates for air scour and water backwashing for fine and coarse sand and anthracite are presented in Table 11. Table 11: Typical backwash conditions for different filter media size (Parsons and Jefferson, 2006) Filter medium d 10 (mm) Sequence Air rate (m/h) Water rate (m/h) Fine sand 0.5 Air then water Fine anthracite 1.0 Air then water Coarse sand 1.0 Air + water then water Coarse sand 2.0 Air + water then water Coarse anthracite 1.5 Air + water then water Insufficient backwashing of a filter will lead to the formation of mud balls, which occurs when mud on top of the filter bed sinks down into the bed coating the media grains and particles within the filter bed (Indian Institute of Technology Madras, n.d.). There are currently three different delivery systems used in rapid filter backwashing: 1. Backwash pumps- Centrifugal pumps and air pumps (when using combined air scour and water) take filtered water from a either an effluent channel or a storage clear well and pump it directly through the underdrain system up into the filter media. The most commonly used 44 Sara-Louise Bent

58 Chapter 3: Applications and Theory delivery method for large scale water treatment, pumps provide the operators with the most amount of control over backwash rates compared to the other methods, however large pumps are required to pump the stored filtered water through each filter at the required filtration rate. (Crittenden et al., 2005) 2. Elevated tank- This delivery system, also requires pumps, except that these are smaller and are used to pump filtered water to an elevated tank, where the minimum water level is typically 9-12 m above the filter media (Crittenden et al., 2005: 939). With such a large head of water, this system is able to initially provide the required backwash rates, however as the stored backwash water level declines during backwash, the water flow can also decline and therefore backwashing may not be as effective as using the previous delivery method. (Crittenden et al., 2005) 3. Effluent channel (self-backwashing filter) For this delivery system, filtered water from all filters is collected in a common effluent channel located above the filters. The water level in this channel is controlled by a weir. Backwashing occurs by reducing the water level in the filters that need to be backwashed, to provide a sufficient head of water in the effluent tank and ensure design backwash rates are met. This system is relatively simple to construct as no pumps are used, however unlike the other two systems there is less control over backwash rates, which will impact the effectiveness of the backwashing process (Crittenden et al., 2005: 939) Choice of rapid filtration system Based on the information presented in this section, two-stage dual-media (sand-anthracite) rapid gravity filters will be used for the rapid filtration process at the new desalination plant in Singapore. After rapid filtration, 1-5 micron cartridge filters will be used to remove those particles that are not removed by to prevent biofouling of the Nanonfiltration and Reverse Osmosis membranes. For backwash, combined air scour and water backwashing will be delivered using centrifugal pumps to provide the air and water. The reasons for this choice of rapid filtration system are: 1. The improved particulate and pathogen removal of a dual-media filter compared to a monomedia filter. From the assumptions made about the quality of the influent seawater, there is no need to utilise a tri-media filter. 2. The capital and operating costs of a gravity filter is less than a pressure filter. Pressure filters are also only typically used for small desalination plants, and with the amount of potable water that the desalination has been designed to produce each day (250,000 m 3 ), it is not economical to use pressure filters. 45 Sara-Louise Bent

59 Chapter 3: Applications and Theory 3. Using a combined air scour and water backwash is common practice in water treatment and desalination, and as mentioned previously, despite the large pumps required, utilising a backwash pump delivery system will provide operators at the plant with greater control over backwash rates Membrane Processes Over the past few years, more and more desalination plants and water treatment systems use membrane technology to remove dissolved solute (salt) and non-salinity contaminants (bacteria and viruses) from water. The main membrane technologies used for seawater/brackish water desalination and wastewater treatment are: electrodialysis (ED), nanofiltration (NF) and reverse osmosis (RO). ED is described as an electrically-driven membrane process while RO and NF are pressuredriven membrane (Critttenden et al., 2005: p2) processes in which pure water from pressurized seawater is separated from dissolved solute (salt) by forcing it through a semipermeable barrier. The process involves the use of a semipermeable membrane; a material which only allows certain molecules (usually pure water) to pass through while the solute is rejected. The two main membrane technologies used in the proposed desalination plant design are NF and RO. Although RO and NF are both membrane processes, they are different in terms of the applied pressure and the type of material rejected by the membrane. NF membranes remove divalent cations as well as a small percentage of monovalent ions from the feed water while RO membranes are specifically used for the removal of monovalent ions. In membrane processes, the pressure applied must be greater than the osmotic pressure of the feed water in order to overcome the phenomena of osmosis which is the natural tendency for water to diffuse through (Porteous, 1983: P 205) a semipermeable barrier from a lower concentration to a higher concentration until the system reaches equilibrium osmotic pressure. When a pressure higher than the osmotic pressure is applied to the feed side of the membrane, the flow is reversed. Water is forced to diffuse through the membrane barrier from the high concentration to the lower concentration side. This process is termed Reverse Osmosis. The pure water is called permeate due to the fact that it has been permeated or passed through the membrane barrier leaving the solute behind. The feed water from which water molecules have been removed with the contaminants remaining is called the concentrate or brine. 46 Group effort

60 Chapter 3: Applications and Theory The smallest physical unit of production capacity (Critttenden et al., 2005: p 7) in a membrane process is called a membrane element. There are different types available and every element has its own specification in terms of flow rate, salt rejection and active membrane area. Membrane elements are enclosed in pressure vessels made from different materials (such as PVC, stainless steel and fibreglass- reinforced plastic), mounted on a skid with piping connections for the feed, permeate and brine. Pressure vessels can house different numbers of membrane elements but a common type is a six element pressure vessel. The elements are arranged in series in a pressure vessel as shown in Figure 21. The elements are connected by interconnectors which seal the internal permeate tube of each element. A pressure vessel has end caps at each end which seal the ends in order to generate high pressure. Feed water enters and permeate and concentrate leave the pressure vessel through ports in the end cap. Figure 21: Arrangement of elements in a pressure vessel (Voutchkov, 2008) The choice of pressure vessel material used depends on certain factors such as salinity of the feed water, operating pressure, corrosion resistance and cost. A group of pressure vessels arranged in parallel is called a stage (Critttenden et al., 2005: p7). Some reverse osmosis systems are made up of two-stage design where the concentrate of the firststage becomes the feed of the second-stage. This type of design configuration helps to increase water recovery. A typical schematic of a two stage system is shown in the figures below. 47 Group effort

61 Chapter 3: Applications and Theory Pressure vessel Elements Qf Qc Qp Figure 22: Diagram of a two stage membrane system (American Water Works Association, 1999: p 27) A unit of production capacity, which may contain one or more stages, is called a train or an array (Critttenden et al., 2005: p 8). The diagram below shows the first-pass of a typical RO desalination plant and how the pressure vessels are arranged to form an array. Figure 23: First-pass SWRO trains (Stover, n.d.: p 2) Membrane material In the design of the RO system, it is important to consider the type of membrane material because the performance of the membrane is strongly influenced by the chemical and physical properties of the material used. An ideal membrane material would have the following qualities: high permeability and selectivity, resistant to chemicals, inexpensive, temperature stability, high durability, non-biodegradable and resistant to clogging or fouling (Roberts, 2007: p96). The 48 Group effort

62 Chapter 3: Applications and Theory most commonly used RO membrane materials are cellulose acetate and polyamide derivatives (Critttenden et al., 2005: p14). Cellulose acetate (CA) membranes Cellulose acetate is the oldest membrane material and it is normally of asymmetric construction. The material is hydrophilic which helps to reduce fouling or clogging in the membrane while maintaining a high flux. However, this membrane material has several drawbacks that tend to limits its use in RO membrane system. Seawater contains 4.5 mg/l boron which has to be reduced to a certain level before the water can meet the drinking standard set by the World Health Organisation. High boron levels in drinking water can harm developing foetus and it can also reduce fertility. Boron removal is normally achieved at a ph value greater than 9.4 and as a result of this, the cellulose acetate material cannot be used for such application because the material tends to hydrolyse when the ph is above 8. The poor temperature and chemical stability of cellulose acetate membrane is also a major problem. The material cannot withstand temperatures greater than 30 C and it tends to degrade in the presence of free chlorine concentration higher than 1 mg/l (Critttenden et al., 2005: p14) Polyamide membranes (PA) Polyamide membranes are normally of thin-film construction and they are more stable and immune to chemical and bacterial degradation compared to the cellulose acetate membranes. The membrane possesses certain properties which makes it suitable for boron removal. It can withstand a higher ph between 3 and 11 and does not hydrolyse in water. Currently, thin-film composite is the most commonly used membrane material for seawater desalination. This material comprises of a micro-porous polysulfone support layer with a thin semi-permeable polyamide layer. The support structure as well as the physical strength of this type of membrane allows it to withstand the high pressures required in RO/NF desalination. It also has a high salt rejection capacity and can produce higher flux due to the fact that it is hydrophobic in nature. Thin-film composite membranes materials have good performance over a wide range of temperature and ph, high salt rejection, can maintain high productivity for a long time and they are not degradable by bacteria. This is the reason for choosing this membrane material for the proposed RO and NF systems in the desalination plant. 49 Group effort

63 Chapter 3: Applications and Theory Membrane configuration Membrane module configuration plays a vital role in membrane performance and application. There are four main membrane configurations: tubular, plate and frame, hollow fibre and spiral wound. The spiral-wound configuration is the most widely used membrane configuration for reverse osmosis process because it requires less space, it is cheaper, has a large membrane surface area per volume and a high mass transfer rate as a result of the spacers. Despite this advantage, the use of this configuration in seawater desalination is limited because of the fouling problem. However, this problem can be easily reduced with the use of adequate pre-treatment steps in the design to eliminate suspended particles from the water before it enters the membrane. The spiral wound type membrane ranges in size from cm in diameter and 3.05 to over 10 cm in length. The common size of an element in seawater desalination is 2.03 cm diameter by cm in length. Figure 24: Construction of spiral-wound membrane element (Critttenden et al., 2005: p 911) Nanofiltration Mechanism of transport Nanofiltration is a pressure driven membrane process said to be an in-between reverse osmosis and ultrafiltration. The membrane pore size ranges from 1 nm to a few nm. The membrane is more permeable than in reverse osmosis and a higher percentage of ions can pass through which means a lower pressure is needed, typically bar (Yacubowicz and Yacubowicz, 2007). Nanofiltration differs from reverse osmosis because only monovalent ions i.e. Na + and Cl - are allowed through while most of the divalent (Ca 2+, Mg 2+ ) and multivalent ions are retained. Therefore, it is not very effective for drinking water production through desalination as the main membrane process but can be used as part of the pre-treatment process because it is very efficient in eliminating naturally occurring colour (more than 90%) and most of the turbidity in the inlet water and DBP (Disinfection By-Products) precursors. DBP precursors are also known as 50 Elena Rusu

64 Chapter 3: Applications and Theory disinfection by-products which form when natural organic matter reacts with the different chemicals needed for the disinfection such as chlorine. DBP precursors include trihalomethanes and haloacetic acids which are only allowed be present in drinking water in really small quantities. Nanofiltration has been around since the 1980s when the main use was for softening water as it can remove up to 80% of hardness ions (American Water Works Association, 1999). Over the years new applications have been discovered in the pharmaceutical, food, chemical processing and drinking water industry for removing pesticides, arsenic, certain pathogens and organic material (Van der Bruggen, Manttari and Nystrom, 2008). The demand for high quality drinking water has played an important role in the discovery of nanofiltration as pre-treatment for a lot of processes including brackish and seawater desalination. As with every process, there are some drawbacks associated with nanofiltration, the main one being the lack of understanding the fundamentals behind the separation process. An overview of all the findings concerning the basics is described below. Mechanisms of transport and rejections are quite complex and research is currently being undertaken to develop new models. So far it has been found that nanofiltration works by two principles: uncharged molecules are separated by size and charged molecules are rejected due to electrostatic interactions between the ions and the membrane (Garba et al., 1999). Uncharged molecules are transported by convection due to a pressure difference and by diffusion due to a concentration gradient across the membrane (Teixeira, Rosa and Nystrom, 2005). Steric hindrance also plays an important role in their rejection which occurs when the shape of the molecule, caused by an arrangement of atoms, prevents the molecule from diffusing through the membrane. The main parameter involved in the separation of charged molecules is the membrane charge. When the membrane comes into contact with an aqueous solution, it obtains an electric charge by a number of mechanisms such as: dissociation of surface functional groups, adsorption of ions from the solutions, adsorption of polyelectrolytes, ionic surfactants and macromolecules (Teixeira, Rosa and Nystrom, 2005). The ions are separated due to the electrostatic interactions between the membrane charge and the ions which is based on the Donnan exclusion mechanism. In this mechanism, the ions with the exact charge as the membrane are rejected while the rest of ions can pass through the membrane, to maintain the electroneutrality condition. A schematic of this mechanism can be seen in Figure 25. The charge on the membrane varies with ph as different ph protonates and deprotonates the functional groups of the membrane. At neutral ph about 6, nanofiltration membranes are negatively charged and they are positively charged at a lower ph of about Elena Rusu

65 Chapter 3: Applications and Theory Figure 25: Donnan exclusion mechanism (Diawara, 2008) However, the Donnan exclusion theory cannot predict the rejection mechanism of divalent ions and as a result a new theory has been considered, dielectric exclusion. This mechanism occurs because of the difference between the polymeric matrix of the membrane and the dielectric constant of the aqueous solvent (India Study Channel, 2010). The charges between the two different media are induced by the ions in the aqueous solution. These induced charges have the same sign as the ions that generated them and as a result the rest of the ions with a different sign are rejected. The rejected ions get repelled out of the pore because of electrostatic repulsion. Another reason for dielectric exclusion is the variation of membrane dielectric constant inside the membrane with respect to the bulk dielectric constant (India Study Channel, 2010) Reason for choosing NF as pre-treatment The membrane processes considered for pre-treatment were microfiltration, ultrafiltration and nanofiltration. Microfiltration removes bacteria, large colloids, precipitates and coagulants. Ultrafiltration removes everything that microfiltration can plus viruses, high molecular weight proteins and organics. Nanofiltration is very efficient in removing everything mentioned above plus most divalent ions, some monovalent ions, colour and odour (American Water Works Association, 1999). The first time nanofiltration was used as pre-treatment for seawater reverse osmosis was by Hassan in a pilot plant (Hilal et al., 2004). He discovered that at a pressure of 22 bar, nanofiltration removed total hardness ions by 93.3% while also reducing monovalent ions (Cl -, Na + ) by 40.3%, resulting in an overall TDS reduction of 57.7%. This meant that the feed to the reverse osmosis unit was of better quality than the intake seawater, which allowed the unit to run at a lower pressure and a higher recovery rate, of up to 70%. After these findings, a plant in Umm Luji, Saudi Arabia, used nanofiltration as pre-treatment to reverse osmosis. The results obtained confirmed the outcomes of the pilot plant experiment. The nanofiltration unit processed 8640 m 3 /day at 25 bar with a 65% permeate recovery. It removed 38% of TDS and 97% of total 52 Elena Rusu

66 Chapter 3: Applications and Theory hardness (Hilal et al., 2004). This made it possible for the reverse osmosis unit to run at a higher recovery rate of 55%. The removal of monovalent ions varies from 30-60% depending on the type of membrane, composition of feed water and operating conditions. In this case, nanofiltration will remove 30% as the type of membrane chosen has a high reduction in hardness ions primarily. One of the most important parameters influencing this decision was the composition of the feed seawater, which contains a lot of hardness ions that would precipitate on the reverse osmosis membrane if they are not removed prior. As a result, the membranes would foul leading to an increase in capital cost. Based on the points mentioned above, nanofiltration was chosen as part of the pre-treatment. It is known that the main problem associated with membrane processes is fouling so to avoid that, conventional pre-treatment has been implemented. This is taking out suspended solids, oil particles, sand and any other organic material that is likely to foul the nanofiltration membrane Reason for chosen recovery rate The typical recovery rate for a nanofiltration system is between 40-90%, with groundwater having higher recoveries than seawater (Asano, 2006). This is a very wide range so to narrow it down the information from the performance of nanofiltration plants over 3 years described in the journal by Gaid et al (1998) was used. The details of the 4 plants are shown in the table below. Table 12: Characteristics of nanofiltration unit for each plant (Gaid et al., 1998) Mery sur Oise (France) Debden Road (Great Britain) Jarny & Soirion (France) Feed water Surface water contaminated with pesticides and organic matter Groundwater contaminated with pesticides and hardness Groundwater contaminated with sulphates and hardness Type of membrane Recovery rate (max) NF 70 85% NF % NF 70 75% These plants have high recovery rates because their feed source, groundwater, has a lower TDS than seawater, 435 mg/l compared to 35,000 mg/l (Gaid et al., 1998). This means that groundwater is more pure and has less contaminants than seawater so membranes can run at higher recoveries without the risk of fouling. The feed water to the plant in Jarny has a high sulphate content of 1,675 mg/l and total hardness of 1,307 mg/l and is able to run at 75% recovery. The seawater feed to the nanofiltration unit has a sulphate content of 2,649 mg/l and a total hardness of 1,802 mg/l which is much higher than the values for the Jarny plant so a lower 53 Elena Rusu

67 Chapter 3: Applications and Theory recovery is needed to avoid fouling the membrane. Taking into account that the seawater also contains a high level of Cl - and Na + ions, of which 30% are removed, it is recommended to have a recovery rate of 65%. A more accurate method for choosing the recovery rate would be to calculate the concentration at which the salts in the concentrate stream precipitate out on the membrane and set a recovery rate below that value as to avoid scaling. However this is very difficult to predict as there is not enough information available related to this subject Reverse Osmosis Reason for choosing RO The different technologies available for seawater desalination were examined in the inception report of this project. To help decide on the most economic/environmental friendly technology for seawater desalination, factors such as total cost of water production, land scarcity and sustainability issues were considered. Based on the research carried out, it was decided to design a reverse osmosis desalination plant because of the following reasons: RO process consumes the lowest energy out of all the various technologies available for seawater desalination. RO plants occupy less space than distillation plants; hence RO desalination plant is a better option in countries like Singapore where there is land scarcity. The concentrate (brine) rejected is used to generate energy to drive the high pressure pumps which cuts down power requirement by almost 50%. The feed water to the RO system does not need heating so the thermal impacts of discharge are lower (The water treatments, 2010) compared to other desalination methods. RO systems are easy to design and quite environmental friendly because the rejected brine can be put back into the sea without causing any harm to aquatic organisms. Reverse osmosis is a continuous separation process (Critttenden et al., 2005: p 7), so it does not need backwash cycle which makes the process easy to operate. The use of reverse osmosis for seawater desalination has some drawbacks that tend to limits its use in large scale desalination process. Operating cost and performance of RO membranes are significantly influenced by certain factors such as concentration polarization, fouling and scaling, feed salinity and temperature. The figure below shows some of the limiting factors of reverse osmosis process. 54 Irene Usiomoifo

68 Chapter 3: Applications and Theory Limiting Factors Membrane deterioration Blocking Decreasing performance Fouling Scaling Hydrolysis (CA) Suspended CaSO 4 Osmotic pressure Acids, bases material CaCO 3 Viscosity Free chlorine Metallic oxide CaF 2 Free oxygen Colloids BaSO 4 Organic solvent Biological matter SiO 2 Free ozone SrSO 4 Figure 26: Limiting factors to membrane desalination by reverse osmosis (Fritzmann et al., 2007: p 19) Fouling and scaling are the major reverse osmosis limiting factors. Scaling occurs at the surface of the RO membrane due to increase salt concentration caused by concentration polarization. Seawater contains substances (such as CaSO 4, CaCO 3, Silica and BaSO 4.) that can cause scaling thereby reducing permeate flux. Scaling is common in the part of the RO membrane where the feed concentration is highest (Downstream stage). Membrane fouling occurs when materials are trapped at the membrane surface causing permeate flux to decrease. These two limiting factors can affect the cost and performance of the RO system Thermodynamics of RO system Reverse osmosis is a membrane flow process normally analysed in terms of the thermodynamics of irreversible processes (Dresner and Johnson, 1980: p 413). RO membranes have been described using different types of mathematical models, this is important because the performance of membrane plays a vital role in the design of reverse osmosis systems. Most of the models that have been used to analyse reverse osmosis process assume an equilibrium condition (or near equilibrium) in the membrane. These models are grouped into three main categories: irreversible thermodynamics model, pore models and nonporous models. The irreversible model (such as Spiegler-Kedem model) and nonporous model (such as solution-diffusion) are commonly used to explain the theory behind the thermodynamics of reverse osmosis. 55 Irene Usiomoifo

69 Chapter 3: Applications and Theory 1. Irreversible thermodynamics model This model assumes that the flux is not far from equilibrium condition hence the solute and solvent flux can be described by the phenomenological relationships (Williams, 2003: p 2) shown below: Flux of solvent ( ) Where ΔP- trans-membrane pressure (bar), Δπ- osmotic pressure difference (bar), L w - solvent membrane permeability (m 3 /m 2 /h/bar) and σ( reflection coefficient) ( P/ π) jw=0. Flux of solute ( ) (C m )avg is the logarithmic mean solute concentration in the membrane and σ represents coupling of solute and solvent flux through the membrane (Williams, 2003: p 1). The main disadvantage of this model is that the coefficients in the equations depend on concentration of the solute. To prevent this concentration dependence on the transport parameters (Williams, 2003: p 2), Spiegler and Kedem (1966) proposed a model with a defined solute (L s ) and local water (L w ) permeabilities and reflection coefficient to describe the solute and solvent flux: ( ) ( ) The solute flux can be simplified as shown below: ( ) Where C s is the concentration of the solute driving force across the membrane and is the mean concentration on either side of the membrane (Kirkby, 2009: p 1). Irreversible thermodynamics model is used to describe reverse osmosis transport process and is based on the assumption that the driving forces (differences or gradient of chemical potentials) in the flux equations are given by equilibrium thermodynamics. However, this model does not provide detailed understanding of the membrane transport mechanisms so it cannot be used for separation process based on membrane properties and structure. 56 Irene Usiomoifo

70 Chapter 3: Applications and Theory 2. Solution-diffusion (SD) model The solution-diffusion model is mostly used to analyse transport in membrane processes as it is relatively simple and requires only two parameters to fully describe RO membrane systems. The model is used to describe diffusion of solvent (water) and solute (salt) across a dense membrane (Williams, 2003: p 3). SD model is based on the following assumptions: (1) The active layer of the RO membrane is homogenous (nonporous) implying that transport occurs between the interstitial spaces of the polymer module usually by diffusion (Williams, 2003: p 1); (2) the solute and solvents both dissolve and diffuse across this layer; (3) diffusion process is uncoupled implying that the flux of solute and solvent are completely independent of each other as result of the chemical potential gradient across the membrane. Separation in membrane process occurs when the flux of the solvent is different from the solute flux. Diffusion takes place by the movement of solute and solvent molecule in the down-gradient direction of the driving force (Critttenden et al., 2005: p 22). In solution-diffusion model, the driving force for the flow of each component is grouped into two terms: pressure differences and concentration differences between the feed water and permeate exiting the membrane. Water flux across the membrane is due to the applied pressure difference while the solute flux is due to concentration difference. Figure 27: Membrane process salt rejection mechanism (Critttenden et al., 2005: p 23) 57 Irene Usiomoifo

71 Chapter 3: Applications and Theory In solution-diffusion model, the solvent and water flux for membrane process are expressed by Fick s law shown by the equation below: Solvent flux: ( ) ( ) Where A is the water permeability coefficient (Williams, 2003: p 5;Fritzmann et al., 2007) Solute flux: ( ) ( ) The solute flux equation is based on the assumption that the driving force is due to the differences in concentration because pressure chemical potential is negligible. Salt rejection is used instead of solute flux for RO plant design. RO percentage rejection is the ability of the membrane to reject dissolved solids present in the feed water and is expressed as shown below: 3.4. Post Treatment The post-treatment process is generally the last stage before the final water product is used for the community. Post-treatment is generally a complicated stage because of the high chances of spoiling the treated water. Careful procedures must be considered as any errors will negate all the efforts of the pre and main treatments. The post-treatment may consist of many different stages including disinfection, adding chemicals to soften the water, and to re-mineralise the water. The final product after the treatment must be high in quality, within the WHO water standards, and safe to be used and consumed Disinfection Disinfection Untreated water taken from any natural source will contain harmful micro-organisms. Some may cause minor discomfort and some will be fatal. It is essential that all potable water is free from active micro-organism and fulfils the water standards from the WHO guidelines. It is important to note that throughout the water treatment process, the system cannot guarantee complete removal of all micro-organisms, but can only substantially reduce their numbers. Active micro-organisms will still be present in the treated water. The process of disinfection is essential in any water 58 Irene Usiomoifo

72 Chapter 3: Applications and Theory treatment plant as this is to ensure that any active micro-organisms that are still present at the latter stages of the water treatment are removed. Disinfection has been used for over a hundred years, when it was first regularly used to kill germs in the 1900 s. Chlorination was first used in London in 1905 and ozone was first used in Nice in From there, disinfection developed further and was widely accepted as a method to treat water. Chlorination is the most widely used method with ozone slowly becoming more popular. Chlorine dioxide is also another disinfectant that is readily used in the water treatment industry. UV disinfection is a relatively new disinfection method even though it was also discovered approximately at the same time as the other techniques. The technology, unfortunately, only started to fully develop fifty years later to a stage where it would be feasible to be used in major water treatment systems (Water Treatment Plant Design, 2005). Only the gaseous disinfectants can also be used as oxidising agents suggesting that killing microorganisms is not their only usage. The removal of taste and odours, turbidity, iron, manganese, and organic matter is a welcoming addition. Theory of disinfection The rate of change in the number of viable organisms is the product of the reaction rate constant for particular disinfectants and the number of viable organisms (Tebbutt, 1992). It is expressed as: Where k = rate constant for particular disinfectants N = the number of viable organisms. This expression is also defined as Chick s Law and is the first-order reaction of a uni-molecular reaction (Crittenden et al., 2005). Integrating the formula and changing to base 10 gives the formula: Where N t = Number of micro-organisms at time t N 0 = Number of micro-organisms initially k = K Rearranging the formula to determine the time required for the adequate disinfection is expressed as: 59 Eric Do

73 Chapter 3: Applications and Theory Since the number of micro-organisms can never be zero no matter how much disinfected the water is, the percentage of removing micro-organisms can never be 100%. However, majority of the water treatment plants must be able to remove or inactivate 99.9% (3-log) of Giardia cysts and 99.99% (4-log) of viruses (Critttenden et al., 2005). Some micro-organisms are more vulnerable and have less resistance to certain disinfectants therefore only 90% (1-log) reduction is required. For effective disinfection there are numerous parameters that are required for killing or inactivating micro-organisms. This includes the residual concentration, the time at which the water is in contact with the disinfectant, the water temperature, the ph balance, and the type of disinfection used. If the concentration of disinfectant, C, measured in milligrams per litre (mg/l), and the contact time, T, measured in minutes, are multiplied together it will produce a value known as the CT value. This value is required to produce adequate parameters in order to achieve the required log requirements. Different micro-organisms will require different CT values therefore, in general, the contact time and the concentration of the disinfectant must be able to remove the most resistant of the microorganisms. It is important for the water system to have an adequate CT value so that the water is treated properly before being distributed Chlorination (Cl 2) Chlorine is the most widely used water disinfection method. Chlorine kills the micro-organisms by immobilising their metabolism rendering them harmless. Chlorine is a slow stable reaction process, which obeys Chick s law. It has many advantages over the other gaseous disinfectants. All three gases can be compressed but ozone and chlorine dioxide must be generated on site, whereas chlorine is readily available in gaseous, liquid, or powder form. It is mainly available in pressurised vessels in both gaseous and liquid form, as sodium hypochlorite, and as calcium hypochlorite. Many facilities use the gaseous form of chlorine as it is cheap to purchase. However, the delivery and transportation of pressurised chlorine can be relatively expensive and there are hazardous risks associated with accidents. With the risks involved, many have switched to hypochlorite to reduce the likely hazards in densely populated areas. Excluding the hazardous disadvantage, gaseous chlorine can be toxic and has a likelihood of increasing taste and odour. 60 Eric Do

74 Chapter 3: Applications and Theory The main advantage of chlorine is the formation of residuals which remain in the water for longer periods of time protecting the system from bacterial contamination (Parsons and Jefferson, 2006) (World Health Organisation, 2003). Total chlorine residuals are described as the combination of chlorine and ammonia or other organic compounds. An example is if chlorine is mixed in with pure water then all the chlorine in the solution would be total free-chlorine residuals as there are no compounds to react with. If ammonia was in the water solution, the residual chlorine would react with the ammonia and any excess residual chlorine would be free-chlorine residuals. When gaseous chlorine is added to water, it behaves differently and does not obey Henry s Law like other gaseous elements would do in water. Chlorine reactions actively and rapidly form hydrochloric and hypochlorous acid: Cl 2 (g) + H 2 O HCl + HClO Hydrochloric acid, HCl, quickly dissociates into hydrogen and chlorine ions. HCl H + Cl - Hypochlorous acid, HClO, is a weak yet effective disinfectant and is only ionised in an alkaline solution. It slowly disassociates further forming another less effective disinfectant, chlorite ion, ClO -. HClO H + + ClO - It is important that hypochlorous acid is the primary source of free chlorine as this will be reacting with the micro-organisms. With the dissociated ions in the solution, the ph of the solution will vary. In alkaline conditions, chlorine is more effective and in acid conditions, the hypochlorous acid is more effective. Therefore it is recommended that the typical ph for disinfection should be less than 8 and if chlorine is used, then a lower ph is better. Chloramine, which can be formed during a chlorine and ammonia chemical reaction, is a combined residual chlorine and is also an useful disinfectant. 61 Eric Do

75 Chlorine residual (mg/l) Chapter 3: Applications and Theory Combined residual Free residual Break point Chlorine dose (mg/l) 8 10 Figure 28: Chlorine residuals for a water with a constant ammonia concentration Taken and edited from Principle of Water Quality Control (Tebbutt, 1992) However, the chlorine and ammonia ratio must be carefully considered as it will influence the rate and the products of the reaction. Monochloramine is formed when the ratio does not exceed four and is the primary chloramine to be used for disinfection. The ratio of three or four is recommended as this is to minimise the concentration of free ammonia in the water. An increase of free ammonia will decrease the quality of the water because it changes the taste and odour. As the chlorine is increased, it reacts with ammonia until the mole ratio is 1 (a weight ratio of 5.07). Beyond the ratio point, additional chlorine will oxidise the chloramines and further increasing the concentration of chlorine will promote the formation of dichloramines and nitrogen, which has an unpleasant taste and odour and should be avoided. The chloramines and chlorine oxidisation equation is represented as: 2NH 2 Cl + Cl 2 N 2 + 4HCl Break point occurs once all the ammonia is reacted and the combined residual chlorine is converted back to free chlorine (Figure 28). Increasing the chlorine concentration also increases the free-chlorine residual, however increasing it beyond the break point is counterproductive due to the production of nitrogen and nitrogen trichloride, chemicals that have a strong unpleasant odour. The break point is a good indicator when to supply sufficient amounts of chlorine to remove any excess ammonia without the risk of producing less welcoming products including dichloramine and nitrogen trichloride. Some plants may primarily use chlorine to only meet the chlorine demand and any excess amount of chlorine can be used for disinfection. Chlorine demand is the 62 Eric Do

76 Chapter 3: Applications and Theory total amount of chlorine that is to be reacted with organic material, ammonia, and with other compounds including manganese and iron (World Health Organisation, n.d.). Using monochloramine does not produce as much byproducts as chlorine. However, excess ammonia may promote nitrification within the water system (Xie, 2004)). The only method to reduce nitrification is to use free chlorine, thereby reducing the amount of ammonia, until biological activity is inactivated. Some water treatment plants alternate between using free chlorine and combined chlorine in scheduled intervals thereby reducing the chances of nitrification. Furthermore, if 0.2 mg of ammonia per litre is reacted with chlorine, then approximately 68% of the chlorine used for disinfection will be unavailable (World Health Organisation, 2003) Chlorine Dioxide (ClO 2) Chlorine dioxide is an excellent gaseous disinfectant and has a quicker reaction rate compared to that of chlorine in high alkalinity conditions. It has the same biological reaction mechanism as chlorine which is metabolically inactivating microorganisms. Although, it is much more expensive and highly unstable than chlorine, it is still in use in Europe especially in areas of low water quality (Critttenden et al., 2005) as it does not produce as many harmful by-products. However, there is still a 0.80 mg/l limit to how much chlorine dioxide is used as its by-products, chlorine dioxide produces chlorite ion (ClO - 2 ) and chlorate ion (ClO - 3 ), have been noted to increase the risk of developing methaemoglobin (Water Treatment Plant Design, 2005), which is the form of haemoglobin with the inability to carry oxygen in the bloodstream. Chlorine dioxide must be produced on site by reacting chlorine or an acid with sodium chlorite. Its chemical equation is represented as: 2NaClO 2 + Cl 2 2ClO 2 + 2NaCl 5NaClO 2 + 4HCl 4ClO 2 + 5NaCl + 2H 2 O Sodium chlorite produces high levels of chlorine dioxide so it must be under strict control measures to prevent hazardous and even catastrophic events. Aqueous sodium chlorite may undergo crystallisation at low temperatures or at high concentrations therefore the equipment is prone to blockage which will obstruct the water flow. Solid sodium chlorite, an orange coloured powder, is not stable and when dry is a fire hazard. Storage units should be fire resistant and should be designed with explosive potential in mind. Ignited sodium chlorite releases mass amounts of heat energy that can evaporate the water rapidly. 63 Eric Do

77 High AC potential difference Chapter 3: Applications and Theory Ozone (O 3) Ozone, derived from the Greek word ozein meaning to smell, is a powerful yet unstable oxidising agent that is used in the disinfection of water. It is an allotrope of oxygen formed by three oxygen atoms. Highly concentrated ozone can be very toxic and has a distinguished pungent odour in the air. It is produced by passing regular dry oxygen, O 2, through a high electrical charge between two electrodes separated by a dielectric material (Figure 29). Heat Electrode Dielectric O 2 O 3 Electrode Heat Figure 29: Basic Ozone generation configuration Taken and edited from Water Treatment Principles & Design (Crittenden et al., 2005) Typical voltages range between 5000 to 20,000V at a frequency of 50 to 500 Hz (Tebbutt, 1992). The covalent bond of the regular oxygen molecules are broken forming highly reactive singular elemental oxygen, O. This will actively react and rejoin with other oxygen molecules to form O 3. This electrical process is relatively expensive to operate compared with other disinfection processes; however it is effective in the control of turbidity, odour and taste. Unlike chlorine disinfection, ozone must be generated on site due to its natural instability nature. Once ozone is in contact with the solution, it undergoes first-order rapid decay, like chlorine, into two different types of reaction. This includes direct oxidation and auto-decomposition. Direct oxidation is less common and is a slow reacting process involving the direct reaction between the ozone modules with selective micro-organisms (Critttenden et al., 2005). Auto-decomposition is the formation of hydroxyl radical from the presence of organic radicals, hydrogen peroxide, light or high concentrations of hydroxide ion. Auto-decomposition is the most common of ozone reaction and it is a rapid process in oxidisation. However, in order for auto-decomposition to initiate, ozone must react with hydroxide ions to form superoxide radials (O - 2 ) and peroxide ions (HO - 2 ). Additional oxygen molecules continue the reaction process and enter a cycle where the ozone promotes itself to accelerate the reaction (Figure 30). Hence, this is the main reason to why 64 Eric Do

78 Chapter 3: Applications and Theory ozone decays rapidly and the reason to not having any residuals in the solution. Therefore it can be assumed ozone systems are prone to biological growth. The ozone molecule also reacts with carbonate and bicarbonate ions to form the radicals of each of the ions respectively. Acidic conditions promote the slow direct oxidation whereas alkalinity conditions promote the auto-decomposition route. However, increasing ph levels will favour the formation of carbonate and bicarbonate radicals which in turn reduces the rate of autodecomposition by reaction with the hydroxyl radicals (Water Treatment Plant Design, 2005). O Direct oxidation Products O 2 HO 4 Reaction with OH Oxidation of H + HO 2 OH - O 2 bicarbonate ion O 3 HO 3 - Bicarbonate radical Carbonate radical O 2 - O 3 - Organic radical O 2 Products Figure 30: Reaction pathway of ozone in water Adapted from Water Treatment Plant Design (American Water Works Association, 2005) The rate can be expressed as (Crittenden et al., 2005): Where C t = Ozone concentration at time t C 0 = Ozone concentration initially k = decay rate constant (l/min) t = time (min) Further effects of decomposition include UV light, temperature, and the ozone concentration. The ozone can be increased by applying a greater frequency of applied current, greater potential difference, and increasing the gap between the electrode and the dielectric. 65 Eric Do

79 Chapter 3: Applications and Theory UV disinfection Of all disinfectants, UV disinfection is the only non-oxidising process that has been accepted to effectively treat water. The technology was discovered in 1877 and the first UV lamp was constructed in It was found to be not as successful as chlorine and was set aside, with the main reasons being including high costs and unpredictable reliability issues in that era. UV disinfection was not a recommended technique to be used. The technology and research gradually grew and the first potable water treatment plant in Switzerland opened in From there, there was a large surge of interest of UV disinfection as it was noted that it does not contribute to any toxicity nor have any measureable by-products in the finished water product (Critttenden et al., 2005). The medium-pressured UV lamp was demonstrated to work effectively in inactivating Cryptosporidium parvum and Giardia lamblia with ease compared to the required effort from chlorine. As of 1996, there are more than 2000 plants in Europe alone using the UV disinfection technology UV light is within the electromagnetic spectrum, lying between visible and X-ray light. It has typical wavelengths ranging from 100 nm to 400 nm, and within this region it is further separated into four sections included UV-A, UV-B, UV-C, and vacuum UV (Figure 31). The nearly entire wavelength range is capable of disinfecting however the most effective wavelengths are from nm regions and is referred to as the germicidal range. As the wavelength decreases, the effectiveness of the UV light is distorted by the barrier of water and therefore has limited absorption. Wavelengths of less than 200nm i.e. Vacuum UV, cannot penetrate the barriers of water. 100 nm 400 nm Gamma Rays X-ray UV Visible Infrared 254 nm 315 nm Vacuum UV UV-C UV-B UV-A 280 nm 100 nm 200 nm 300 nm 400 nm Figure 31: Electromagnet spectrum with detailed section of UV Taken and edited from Water Treatment Plant Design (Water Treatment Plant Design, 2005) UV light behaves differently as it interacts with different types of materials and is either absorbed, reflected, refracted, or scattered. It is most commonly absorbed and scattered in water. If electromagnetic radiation is suggested to be packs of photon (light) energy, and then it can be 66 Eric Do

80 Chapter 3: Applications and Theory stated that the photons are related to the wavelength of the radiation (Critttenden et al., 2005). The relationship can be repressed in a mathematical formula as: Where E = Energy of each photon (J) h = Planck s constant ( J.s) c = speed of light ( m/s) λ = wavelength of radiation (m) Energy is inversely proportional to the wavelength of the radiation, therefore as the wavelength increases the energy decreases. Consequently, light and infrared radiation are harmful to organisms whereas UV-A and UV-B can produce a tan by activating melanocytes in skin. Longer exposure to UV-B causes sunburn and, most important in terms of disinfection, UV-C radiation is absorbed by Deoxyribonucleic acids (DNA) in cells causing cancerous cells. Therefore, it was established that UV radiation can penetrate and corrupt the DNA or RNA of micro-organism, disabling their ability to replicate. The micro-organism is then inactivated and is harmless, however the damage is not fatal as their metabolism is not altered and there is a possibility of reactivation and becoming infective again. Light radiation with wavelength between 310 and 490 nm can repair the damage (photoreactivation) however micro-organisms can still repair themselves in the absence of light (Water Treatment Plant Design, 2005). When atoms are excited by an electric discharge, they enter a higher orbital state. When they return back to their original state they emit light. Following the Plank-Einstein equation, if the energy difference levels are appropriate, then UV radiation will be emitted. It was found that Mercury is able to emit such wavelength and is widely used in UV lamps (Water Treatment Plant Design, 2005). Furthermore, due to its high tolerance to heat, mercury can be incorporated in many different types of lamp technology. Lamps contain filler gas, mainly argon, which aids the promotion of ionisation of mercury (Water Treatment Plant Design, 2005). Treatment plants employ three different types of lamps: low pressure, low intensity lamps; low pressure, high intensity lamps (also referred to as low pressure, high output); and finally medium pressure, high intensity lamps. The pressure in the lamp is the vapour pressure of mercury inside of the lamp. The most common lamps are low pressure and low intensity as they have longer life span and require lower energy cost compared with the other lamps. The low pressure lamps emit monochromic wavelength of only 254 nm at vapour pressures near to vacuum pressures (0.14 to 14.0 Pascals) whereas medium pressure lamps operate with high vapour pressures and temperatures, emitting a range of wavelengths at high intensity (Figure 32). 67 Eric Do

81 Relative lamp output Relative lamp output Chapter 3: Applications and Theory Low-pressure UV Medium-pressure UV Wavelength (nm) Wavelength (nm) Figure 32: The comparison of wavelength emitted from low pressure lamp and medium pressure lamps Taken and edited from Water Treatment Principles & Design (Crittenden et al., 2005) The characteristic of each lamp types are displayed in Table 13. Low pressure lamps are generally the most common however, large water treatment plants are opting to use low pressure high intensity lamp and medium pumps due to the large requirements of disinfection. Table 13: Comparison of the mercury vapour lamp types Adapted from Water Treatment Plant Design (American Water Works Association, 2005) Parameters Units LP lamps LPHO lamps MP lamps Germicidal UV light 254 nm 254 nm 200 to 300 nm Operation temperature ( C) N o of relative lamps required High Intermediate Low Lifetime (per lamp) hr Power (per lamp) W Germicidal output/input % Vapour pressure in lamp Torr Sleeve life yr Ballast life yr Germicidal UV output W/cm Maximum radius cm LP low pressure lamps, LPHO low pressure high output (low pressure high intensity), MP medium pressure When UV light comes into contact with water, the most common interaction is absorption and scattering. The absorption rate is affected by the turbidity, dissolved organics and suspended 68 Eric Do

82 Chapter 3: Applications and Theory particles. The absorbance of light in an aqueous solution is explained by Beer-Lambert Law (Critttenden et al., 2005): ( ) ( ) Where I = light intensity at distance x from the light source (mw/cm 2 ) I 0 = light intensity at light source (mw/cm 2 ) C = concentration of light-absorbing solute (mol/l) x = light path length (cm) ε(λ) = molar absorption of light-absorbing solute at wavelength λ (l/mol.cm) Absorptivity, k(λ), is the absorbance to a path of 1 cm and is an important value in the design of a UV reactor chamber. In general, the greater the absorptivity of water the more energy is required for an equivalent disinfection (Critttenden et al., 2005). It can be expressed as: ( ) ( ) Where A = absorbance = ( ) Another method of measuring the degree of UV absorbance is the UV transmittance (UVT). It is measured as a percentage of the wavelength passing through the water at the specific path length (generally 1cm). It can be expressed as (Critttenden et al., 2005): Where %UVT A x = UV transmittance at specified wavelength x and specific path of 1cm = absorbance of wavelength x Absorbance of UV light is an important parameter as the absorbed photons initiate the chemical reaction i.e. inactivation of the nucleic acid in the micro-organism (Water Treatment Plant Design, 2005). The Ct value required for disinfecting microorganisms can also work for UV disinfection. It is referred to as the UV dosage which is the product of the UV intensity (mw/cm) and the exposure time. It is typically measured in mj/cm Eric Do

83 Chapter 3: Applications and Theory Remineralisation Desalination of seawater is an effective method of removing salt; however, the process also removes many of the minerals and organic matter that are normally found in drinking water. Desalinised water is aggressive and may corrode the pipe works in the distribution system and therefore it is important to re-harden water. It has little benefit to health as there are minimal amount of minerals present in the water and there are possible consequences of constant consumption of low mineral water. This includes high calcium and magnesium intake from water and prepared food which will minimise heart disease risks. It is important that the process of remineralisation places the mineral back into drinking water which will benefit human health. The minimal mineral requirements for all drinking water are listed in Table 14. Table 14: Mineral requirements for drinking water WHO Guidelines for drinking water Table 9. (2-4,56) Requirement Requirement Mineral Mineral (mg/l) (mg/l) Iron 0.3 Magnesium 10 Zinc 3.0 Calcium 30 Copper 2.0 Potassium Not specified Fluoride 1.5 Phosphorus Not specified Sodium 200 Iodine Not specified Chloride 250 It is noted that calcium, magnesium, and zinc are very low in Asian countries and the potable water is very soft. However it is highly concentrated with fluorine which has dental benefits (Lewis, 2011). Country Table 15: Countries with water fluoridation schemes covering their populations WHO Guidelines for drinking water Table 9. (2-4,56) Population (millions) Adjusted fluoride Population covered (millions) Australia Israel Malaysia Singapore UK USA Population covered (%) 70 Eric Do

84 Chapter 3: Applications and Theory 3.5. Power Distribution This section explores some of the aspects involved in designing a power distribution system Design Considerations There are several basic design considerations which are applicable when planning any electrical distribution system. These considerations are well established and have been set out by the IEEE (IEEE, 1976: p 29). A description of each is given below. Safety to Life (Highest Priority) When designing an electrical distribution system, safety to life must not be compromised under any circumstances. Following established codes in the selection of materials is imperative. Reliability Another consideration is the required level of reliability of the electrical supply. This will ultimately be determined by the type of plant. Some plants can accept interruption where as it may be extremely important that the plant has an uninterrupted supply. The system can be designed to isolate faults with minimum disruption if it is at a justifiable cost. Simplicity of Operation Simplicity of operation is an important design consideration as it allows reliable operation and simple maintenance. The operation must be as simple as possible to meet the systems requirements Voltage Regulation Poor voltage regulation has a negative effect to the life and operation of electric equipment. The utilisation voltage must also remain within the equipment tolerance limits under all load conditions. Maintenance The accessibility and availability for inspection and repair are important considerations in selecting equipment. Space must be provided for inspection, adjustment and repair in a clean, well-lighted and temperature controlled area. Flexibility Distribution systems must be flexible, this means in terms of the adaptability to development and expansion and to meet the varied requirements during the life of the plant. Also considerations of the plant voltages, equipment ratings and space for additional equipment and capacity for increased load must be given serious study. 71 Marco Volino

85 Chapter 3: Applications and Theory Cost (Lowest Priority) Although cost is an important factor, all of the above must be considered to be higher priority Planning Guide for Electrical Distribution The IEEE has outlined a planning guide for distribution systems (IEEE, 1976: p 30). Each step is explained below. Load Survey In a load survey the general layout of the plant is used and all major loads are marked out on the relevant locations. Demand Summing up the electrical rating of all pieces of equipment gives a maximum connected load. Voltage Several voltages must be selected for the distribution system. This starts with the incoming supply voltage from the utility company. Other key voltages are the utilities voltage which will be set standard levels and vary from country to country. Utility Service When appropriate, the supply of electricity must be discussed with a utility company. Generation Determine whether the plant requires parallel generation to supplement the existing supply. Other options for generation are emergency backup which would require the capacity to run the plant for a temporary period, should the main supply develop a fault. Systems An investigation into the various available systems is required to find the best solution suitable for the proposed plant. Equipment Locations Selecting appropriate locations on the site for the equipment is important. One-Line Diagram A complete one line diagram should be produced accompanied by a physical layout of the plant. Typical symbols which are included in a one line diagram are power sources, short circuit current. Short Circuit Analysis It is necessary to calculate the fault voltage for all pieces of equipment. Protection A protective system must be designed as it is an integral part of final system. 72 Marco Volino

86 Chapter 3: Applications and Theory Communications An important aspect of protection is to provide a reliable communication system. This can be achieved by either a self-contained or self-maintained system of phones and alarms. Maintenance Aspects which are important for maintenance o Cleanliness o Moisture Control o Adequate ventilation o Reduced corrosion o Maintenance of Conductors o Regular inspections o Regular testing o Adequate records Codes and Standards Throughout the design any national or local laws and standards must be adhered to. 73 Marco Volino

87 Chapter 4: Proposed Plant 4. Proposed Plant 4.1. Introduction The maximum outlet capacity of the proposed desalination plant is 250,000 m 3 /day. This is based upon the water demand projection presented in Chapter 2. The current desalination plant located in the south Tuas area of the island has an output capacity of 136,000 m 3 /day. Aside from having different capacities, the two plants differ in the water treatment technology. The Tuas plant only uses conventional pre-treatment whereas the proposed plant will combine conventional with nanofiltration, which will increase the life span of the reverse osmosis membranes Site Location The site of the proposed plant will be located in the industrial sector in the western part of Singapore. Tuas, being situated for away for residential areas, is an ideal location for a desalination plant to be constructed. The proposed site is situated at Tuas South, which is newly reclaimed land. There are major industrial facilities including Tuas incinerators and the Tuas Singspring desalination plant all in the area and are all within close proximity. The specific site will be at Tuas South Avenue 5 and is very close to the sea borderline, which is ideal to the readily available raw water and the short distance for it to be treated on site. Figure 33: Proposed location for the desalination plant 74 Eric Do

88 Chapter 4: Proposed Plant 4.3. Site Layout The proposed treatment plant will be structured adjacent to the road Tuas South Avenue 5 on an empty lot. The site consists of external and internal facilities which will supply the 250,000 m 3 /day water demand. The total site area site covers approximately 80,000 m 2. Figure 34 shows an artist impression of what the proposed site will look like. Figure 34: Artist impression of the proposed water treatment plant External facilities Pumping station The main intake pump covers approximate 100 m 2, and is responsible for the constant supply of raw water. Off-site pumping station The off-site pumping station is first stage of the water treatment system. The station pre-screens the water that is extracted from the 10 m below water surface using a deep sea extraction technique. Dissolved air floatation The dissolved air floatation units are commonly placed outside but can also have a covering or roof. It removes oils, greases, suspended particles by attracting bubbles and floating them to the top. Each unit covers an area of 36.3 m 2 therefore with 35 units being used the total area is m Eric Do

89 Chapter 4: Proposed Plant Gravity sand filters The rapid gravity sand filter covers approximately 2400 m 2 and it filters the water through three layers of different grading of sand. It is a passive process driven by the force of gravity. Car park The 800 m 2 staff car parking lot has the capability of accommodating fifty family-sized vehicles with five of the spaces reserved for disability permitted drivers. The car park has also allocated a designated bicycle rack area for cyclist. Substation The substation is used to step down the voltage supplied from the power station and bring it down to a useable level. The substation will distribute appropriate electrical current throughout the site. The substation requires a 30 by 20 metre field which would be available space for future expansion. A proposed hydrogen fuel cell is to be installed within the site perimeters Internal facilities Nanofiltration & Reverse Osmosis The membrane process removes salt, ions, and suspended particles from the water using a high pressure pumps to push water through a membrane. The membrane filtration pumps and equipment are housed inside a 40 metre by 30 metre concrete structure. The building is a single story 10 metre tall which enables the stacks of membrane vessels to be fitted inside of the building. There will still unused floor area readily available for future expansion options. Post treatment The post treatment process is to disinfection and to condition the water before it is distributed. The processes are separated into two buildings. The first building, with the UV disinfection chambers, is a 18 m by 40 m concrete building containing four 800 mm diameter pipes and has more than enough room to fit a large array of control, monitor network system, and maintenance. The second building consists of a clearwell with its main function of chlorine disinfection and remineralisation. The clearwell is situated in a 25m by 40 m building with a height of 10 metres. A ventilation system is installed in the facility to prevent accumulation of chlorine gases. Control room The main control room is situated at the centre of the site and monitors the entire process system. The control room is able to change parameters at point maintaining a smooth and effective treatment 76 Eric Do

90 Chapter 4: Proposed Plant system. The 30 m by 40 m building is spacious enough to contain a network of large monitoring systems. Security The security front desk is situated next to the main administration building is the first building to be greeted by visitors. The security personnel are responsible of the safety of the entire site and also decide the incoming and outgoing traffic. The CCTV control system is located inside the security desk records and oversees the site throughout the day and night. Chemical storage The chemical storage warehouse is situated to the south of the site with its main entrance facing the centre of the site. The road leading towards the storage warehouse is designed to have a large turning radius allowing long delivery vehicles to enter and exit the warehouse with ease. This warehouse will store the main chemicals used in the pre-treatment such as coagulant, coagulant aid, sodium hydroxide Main administration building The main administration building is the main focal building for visitors, guests, VIPs, as well as staff to use. The building is separated into four main selections including the laboratory, office block, staff facilities, and the learning resource & conference selection. The detail floor plan is shown at Figure 30. Floor flat slab design The floor for the building is a concrete flat slab and spans in one direction. The slab is 300 mm in depth can be simply supported. The calculation (Appendix 4 FS/1) shows the largest slab size in the building. It is reinforced using H10 bars at 50 mm centres and secondary reinforcement bars of H8 bars at 125 mm centres. Steel beam and column design There will be four columns spans through the centre of the lobby and exhibition area. The steel columns are designed to be 305 UC 118 and is encased in concrete. Therefore the total width of the column is 425 mm width and has a compressive resistance of 4870 kn. The full set of calculation is found at Appendix 4 CO/1 and designed according to EN :2004. The steel beams will span from the column and resting on the internal load bearing walls. The 254 UC 167 steel beams will span 10 metres in length and will be supporting the loading from the roof. It has designed using BS 5950 and the full side of calculations are shown in Appendix 4 BD/1. 77 Eric Do

91 Chapter 4: Proposed Plant Substructure Design The foundations of the main building at the desalination plant have been designed according to EN :2004. In total 95 no. 600 mm diameter CFA piles of 7 and 25 m length will be used,. There will be 35 piles of 7m length arranged in a 7x5 pile group around the central columns, whilst at each corner of the building; there will be 15 piles of 25m length arranged in a 5x3 pile group. There will be a 2 m spacing between each pile. For the detailed calculations and description of site stratigraphy, see Appendix 4. Figure 35: Floor plan for the main administration building 78 Eric Do

92 Chapter 4: Proposed Plant 4.4. Process overview The water treatment process is separated into three categories. This includes pre-treatment, where suspended particles are removed; main treatment, where the water is desalted; and post-treatment, where the water is disinfected and conditioned before being distributed. A flow diagram of the proposed water treatment process is shown in Figure 36. An intake of approximately 830,000 m 3 of raw water is capable of producing the required maximum output capacity. The detailed analysis of each stage of the treatment process is discussed in the upcoming chapters. Input 826,632 m 3 /day Pre-Screening 826,632 m 3 /day 99.9% Dissolved Air Flotation 825,805 m 3 /day 100% Pre-treatment Rapid Sand Filter 825,805 m 3 /day 97% Cartridge Filter 801,031 m 3 /day 97% Nano Filtration 777,000 m 3 /day 65% Main Treatment Reverse Osmosis - First Pass 505,050 m 3 /day 55% Reverse Osmosis - Second Pass 277,778 m 3 /day 90% UV Disinfection 250,000 m 3 /day 100% Post-treatment Chlorination 250,000 m 3 /day 100% Remineralisation 250,000 m 3 /day 100% Output 250,000 m 3 /day Figure 36: Process Diagram including Input and Recovery Rates 79 Group effort

93 Chapter 4: Proposed Plant 4.5. Intake Since the proposed plant requires a maximum intake of 826,000 m3/day, indirect intake is not suitable to cope with this large volumetric flow rate. The remaining two options, direct open intake and direct sub-surface are two viable options for the proposed plant. It was decided to use direct sub-surface for intake due to the fact that the sea water obtained from deep sea has a small amount of debris load by at least 20 times (Gille, 2003: p 250) compared to open direct intake. Also the seawater obtained from deep sea is cooler, thus it increases the overall efficiency of the desalination process (Gille, 2003). Seawater obtained from direct sub-surface requires less pre-treatment and has a small environmental impact on coastline as opposed to direct surface intake. Marine of Pipeline selection From the comparison of different materials, HDPE and GRP seem very attractive and they are also widely used around the world for direct sub-surface intakes. For the proposed plant, HDPE seemed the better option but because the maximum available diameter is quite small it s not practical for large extraction of sea water. For instance, intake pipes for Shoaiba (Saudi Arabia), Frujarah (UAE) and Perth (Australia) desalination plants are made of GRP due to their large amount of sea water intake requirement (Le Roux, 2010: p 63). GRP pipe are the longest pipes of up to 18 m thus reducing the installation time and reducing the number of joints needed for a long intake pipes. Therefore, it was decided to use GRP pipeline to meet the requirement of intake for the proposed plant. Pipe material = GRP Number of intake pipes = 2 Total length of each pipe = 1000 m (500 m off shore m on land) Diameter of each pipe = 2.85 m (see Appendix IT/1) Pipe chosen = GRP SN 5000 (Lianzhong, 2010: 9) (via ) Using the brochure provided by Lianzhong (GRP manufacturer) For d = 2.85 m, thickness of pipe, t= 35 mm Weight, W= 650 kg/m Total pipe to be laid off shore (in sea) = 2 x 500m x 650 kg/m = 650 tonnes Total pipe to be laid on land = 2 x 500m x 650 kg/m = 650 tonnes 80 Ali Niazi

94 Chapter 4: Proposed Plant GRP intake pipe installation Since the proposed plant is using GRP intake pipes, a method suitable for this type of pipe has to be adopted. This type of installation is performed by contractors who are experts in this industry. Flowtite is a company which is renowned for installing GRP marine pipes. They have performed extremely well in projects involving Shoaiba in Saudi Arabia and Fujarah in UAE. The type of methodology for installation of intake pipes which should be used is described below. This methodology of installation is also performed in Saudi Arabia and UAE for GRP intake pipelines for their desalination plants. Two of the 18m long GRP pipes are assembled together with the help of the thermal fusion forming a joint as strong as the piece itself. Then they are loaded and attached to the horse on the slow moving lay vessel. The actual installation is accomplished by a custom build structure known as horse as shown in Figure 1. Two of the 36 m long segments are installed per day in a trench where the pipes are buried (Amiantit Group, 2009). Figure 1 shows the side and front view of the installation procedure carried out by Flowtite. Figure 37: Basic illustration of custom builds method horse and procedure for installation (Amiantit Group, 2009) Divers use the hydraulics controls on the horse to join the pipe segments. Finally the trench created for the pipes is filled back with anti-scour stone to protect it from the waves. Biological fouling: This causes serious problems to the intake pipe. Marine life growth in the intake pipe could hinder the process performance. There is no type of material available for pipes that ensure 100% eradication of marine life growth in pipelines. It is suggested to install a 100 mm pipe diameter alongside the intake pipe (Amiantit Group, 2009). The chlorination pipelines are used to bring chlorine to the head of the intake pipe where it is dispersed into the sea to make sure bio-logical fouling doesn t occur in the pipelines. Screening type 81 Ali Niazi

95 Chapter 4: Proposed Plant For the proposed plant, passive screens should be used for the screening process as this eliminates the need of screening off-shore. These screens are self-maintained by a cleaning process called air backwash hydrobrust. Installation of passive screens There are various ways in which passive screens can be arranged. More than one passive screen can be attached to one intake pipe (multi-screen). Since the proposed plant has two inlet pipes it would be more than sensible to attach two medium sized screens to each pipe than one on each. This is due to the fact if there is a mechanical failure in one of the screen, the inlet pipe would still work at half the inlet capacity and using three screens on each pipe would increase the installation cost. This arrangement for the proposed plant is shown in Figure 38 and is justified in appendix IT/1. Figure 38: Passive screens arrangement for the proposed plant showing dual intake (StaticOrb Screens, 2008) Since four passive screens are being used, it must be made sure that each one has a capacity of roughly ¼ the overall maximum capacity (826,000 m 3 /day). Passive screen specifications for the proposed plant are given the table below: Table 16: Specification of the passive screen used for the proposed plant (Ovivo, 2010) Screen Overall Outlet Flange Airburst Approx. Flow/unit Model diameter Length Diameter Connection Weight M3/h D (mm) l (mm) D (mm) Size d (mm) (kg) T72-UC Note: Capacity given based on 3mm slot width, 0.15m/sec slot velocity and width of wire 3mm. 82 Ali Niazi

96 Chapter 4: Proposed Plant Four of these screens are being used, two on each inlet pipe as shown in Figure 38, giving a maximum flow rate of 872,544 m 3 /day. This is slightly more than the maximum required but it is always more sensible to overestimate than underestimate from an engineering and economical prospective. Materials & Costs: Passive screen should be made of a material which is resistant to corrosion and bio-fouling. Z-Alloy is a widely used material and it is both bio-foul and corrosion resistant. According to the Johnson Screens, leading worldwide screen manufacturers, T60-HC which is similar to the desired passive screen, made of Z-alloy would cost $ 118,400 (Watson, 2007). Therefore, total cost of four passive screens = $ 473,600 Pumping station For the proposed plant option 2 (See chapter 3.19 Choice of number of pump sets) is chosen as this configuration reduces the operational cost and gives more flexibility without putting too much burden on the capital cost. For this set of pumps, three pumps need a maximum pumping capacity of 1.5 Qmax as mentioned in the section before. Qmax = 826,000 m3/day and 1.5 Qmax = 1,239,000 m3/day Each pump for this configuration needs to have a capacity of about Qu =0.5 Qmax = 413,000 m 3 /day Horizontal Centrifugal are normally used for seawater intake. Qu needed is around 17,000 m 3 /day Pump chosen for intake = horizontal centrifugal pump (3 pumps) Model = OTS series double-suction Axially Split Volute Casing Pumps are our newly developed products of advanced technology. It has the following technical advantages-impeller (Alibaba, 2011). Specification Capacity: 50~18000m3/h Head: 10~220m Motor Rating: 15~2500kw Size: 125~1000mm Speed: 590~2950r/min Three pumps would be in operation and one on stand-by as stated in option 2 of the choice of number of pump set section. Therefore, the total cost for 4 pumps would be $ 360,000. The specification falls within the criteria for the requirement. Each pump would be pumping at 413,000 m 3 /day, which is 0.5 Qmax as desired in option Ali Niazi

97 Chapter 4: Proposed Plant Pump power Total energy required is calculated from the following equation (Sinnott, 2003: p 205): g = acceleration due to gravity, m2/s =difference in elevation (z1-z2), m = difference in system pressure (P 1 -P 2 ), N/m 2 = pressure drop due to friction including miscellaneous losses, etc., N/m 2 = liquid density, kg/m 3 W = work done, J/kg Pressure drop due to frictional losses is calculated first as shown in the appendix IT/1. First Reynolds number is calculated (appendix IT/1) then using pipe roughness of GRP pipe(appendix) and using Moody chart, friction factor is obtained, f = Using friction factor, pressure drop is calculated as shown in appendix IT/2 and IT/3. Using the pressure drop, calculated. (appendix IT/3) = kpa and the equation of energy balance above, work done (W) is Finally using work done and the equation of power for pump, power is calculated for each pump (Sinnott, 2003: p 205). m = mass flow rate in kg/s = efficiency of a pump Power required for each pump = 650 kw Even though power required for each pump is 650 kw, it is more practical to choose a pump which has a higher power rating than the minimum required to be on the safe side in terms of flexibility. Pump chosen with power rating of motor = 1000 kw Using the power of pump, the cost is calculated using cost against power graph (Gerrad, 2000): 84 Ali Niazi

98 Chapter 4: Proposed Plant 120,000 x 1.55 = $ 186,000 Cost of 4 intake pumps (3 in operation and one stand by) = $ 744, Dissolved Air Flotation Equipment Selection Many different companies design and manufacture DAF units. World Water Works is one such company which has a good reputation within the portable water industry. Appendix 2 shows the range of DAF units provided by World Water Works and compares the required number of DAF units and the energy requirements to treat 825,805 m 3 /day of raw water. From this it can be concluded that using 30 units of the RSP-13L requires the least amount of power when compared to any other model. Each DAF unit consists of four pumps and a mechanical rake and requires 91.8kW of power. The dimensions of each unit is 3.66 wide, 9.91m long and 4.42 m high each unit weights 8.5 tonnes when empty and 70 tones when full Cost Although an exact cost for the selected DAF system could not be found, a worst case estimate would be approximately $100,000 per unit. This estimate is made by pricing the components of the system individually. As it is a fairly simple system made using simple components Operation The calculation of worked out that a minimum of 30 DAF units would be required should one unit breakdown or a fault develop then the required amount of water would not be treated. In this case it would be beneficial to have some back up DAF units. This would also be useful as it would be required to periodically put some of the units offline for scheduled maintenance. With this in mind it would be. 35 DAF will be required. Figure 39: World Water Works DAF Unit 85 Marco Volino

99 Chapter 4: Proposed Plant The DAF process produces large amounts of sludge which consists of oil and biological matter which is removed from the raw water. The conventional way of disposing of it is to send it to land fill, however land in Singapore comes at a premium and will not be many landfill sites. As mentioned the plant location is close to a incineration plant which could provide a way of disposing of the sludge in a sustainable way Coagulant Type and Quantity Figure 40 shows the typical results obtained using sea water. It shows the residual turbidity (NTU) using different coagulants and coagulant aids. It can be seen that ferric chloride (coagulant) with anionic polymer (coagulant aid) produces the best result. Based on these results and the advantages mentioned earlier, it was decided to use ferric chloride as primary coagulant and anionic polymer as coagulant aid to further enhance the coagulant process. Figure 40: Comparison of coagulants on the removal of turbidity (Mohammed and Adbul, 1998) Based on the Figure 40and the study that took place in SWRO desalination plant, Al Jubail(Saudi Arabia), the coagulant dosage and coagulant aid dosage to be used for the proposed plant are given below, assuming the sea water quality to be used for proposed plant is similar: Coagulant (ferric chloride) = 0.8 mg/l as Fe Coagulant aid (anionic polymer) = mg/l As mentioned earlier, these are approximate values as the accurate coagulant can only be obtained by the jar test. Total primary coagulant = 3 26,000m / day l / day 8 86 Ali Niazi

100 Chapter 4: Proposed Plant (Ferric chloride) 8 5 = mg/ day = mg/day = kg/day or 0.66 tonnes/day Total coagulant aid (anionic polymer) 8 5 = mg/ day = kg/day or kg/hr Cost of Coagulant = $300/tonne to $340/tonne (ICB Americas, 2001) Using the average value of $320/tonne, cost of coagulant/day = $ 211.0/day Cost of coagulant aid = $89.0/day 4.7. Rapid Sand Filter Crittenden at al (Water Treatment- Principles and Design, 2005) lists criteria to be considered in the preliminary design of a Rapid Filter. These criteria have been used for the initial design of the dualmedia rapid gravity filters. (See Appendix A for in depth calculations) Performance criteria Effluent turbidity- 0.3 NTU Filter run length- 36 hours Recovery- Assumed recovery rate 3% Unit Filter Run Volume (UFRV) ( ) ( ) The rapid filter will be designed to operate at a filtration rate of 12 or (m/h) therefore: ( ) Process design criteria Required level of pre-treatment- The dual media rapid gravity sand filters will be used for single stage filtration (see section 3.x) and the required pre-treatment as detailed in section 3 will be: Coagulation, flocculation and Dissolved Air Flotation. 87 Sara-Louise Bent

101 Chapter 4: Proposed Plant Filter media type- As previously mentioned the filter media will consist of sand and anthracite of the follow grain size and depth: (see Appendix A for full filter media size calculations) Sand- (ρ= 2650 kg/m 3 ), ES=0.5mm, UC=1.3 Depth of filter media required, assuming l/le =1000 Anthracite (ρ= 1500kg/m 3 ), ES= 1.0mm, UC=1.3 Depth of filter media required, assuming l/le = 1500 A general design rule is to include an additional 30-50mm of media when first filling filters to account for gradual removal during the backwashing process (Voutchkov, 2010). Therefore an additional 50mm of media will be added to each media layer, so: There will also be a gravel support layer in the filters beneath the sand layer. This will be 0.5m thick of graded gravel with a maximum media size of 2.3mm. Size and depth- (see appendix A for detailed calculations) In order to produce 250,000m 3 /day of potable water from desalination, approximately 825,806 m 3 /day of seawater needs to be filtered, which is equivalent to 344,508.6 m 3 /h of water. The surface area of filter required to do this is: ( ) ( ) ( ) ( ) ( ) ( ) Assuming that 36 no. filters will be used and a filter width: length ratio of 1.3, the size of filters required is: 7.9m width x 10.2m length. The total depth of the filter and support media within the rapid filters is: 88 Sara-Louise Bent

102 Chapter 4: Proposed Plant Assuming that there is 2.5m head of water above the top of the anthracite bed and an additional 0.5m of filter wall above the top of the water, the total depth of the rapid filters is: Available head (see appendix A for detailed calculations) Using Ergun s equation for headloss in the filter (Equation 4), it has been calculated that there will be a 0.8m headloss in every filter, during each filtration cycle, so the available head in the filter during operation will be: Major process components Backwashing system- Combined air scour with water will be used, with two pumps; a centrifugal pump and air pump providing the water and air to each filter. Backwashing will occur for 15 minutes every filter cycle and each filter will be backwashed at a rate of 40m/h when using water. Underdrains- The underdrain system for the dual-media rapid gravity filters will comprise of Tetra LP Blocks supplied by Severn Trent Services, a supplier of water and wastewater treatment products. The Tetra LP Block is a dual parallel lateral underdrain system that works well with combined air scour and water backwashing. This system was chosen due to the ease of installation, lack of maintenance required and its long product life compared to other systems as well as it being able to be used with the chosen backwash system. The dimensions of the underdrain blocks will be 200m (height) by 430mm (width) (Unsupported source type (DocumentFromInternetSite) for source Sev11.); in total approximately 24 no. blocks will be used in each filter. ( ) ( ) ( ) Wash troughs- There a lack of information about wash water troughs; so it has been assumed that 5 no. plastic (HDPE) wash water troughs of 1m depth will be used for the filters. Prior to commencing further design of the project beyond feasibility stage, the cost and dimensions of the troughs will need to be determined. Process piping- based on the assumption that the velocity of water in the influent and effluent pipes is 0.75m/s, the diameter of pipe required is 700mm. Super-duplex stainless steel pipes will be used. 89 Sara-Louise Bent

103 Chapter 4: Proposed Plant Pilot plant Due to the uncertainty of the quality of seawater in Singapore, it is advisable for a small scale pilot plant to be built to ensure that the depths of filter media specified provide adequate turbidity and pathogen removal in the effluent filter water Cartridge Filter It is assumed that there will be a separate set of 5 micron cartridge filters for each pipe coming from the rapid gravity filters. It is also assumed that the cartridges will be assembled like a membrane pressure vessel, with a special housing containing 6no. cartridge filters. Therefore the number of cartridge filters used will be: For Seawater Reverse Osmosis Desalination, it has been assumed that the cartridges have a 2 month lifetime before they need to be replaced. Therefore over the proposed 20 year lifetime of the desalination plant the number of cartridge filters required is: ( ) 4.9. Membrane processes Modelling RO/NF design using ROSA Software The RO and NF units will be analysed using the Reverse Osmosis System Analysis (ROSA) software, which is a computer program used for the design of RO/NF membrane processes. First the system configuration has to be determined and then inserted in the software. The input to the model includes: feed water compositions, flow rates, ph, temperature, system recovery rate, membrane configuration (number of pressure vessels in each pass), and the type of membrane element used. This program is used to determine membrane performance as well as energy requirement for RO desalination systems (Gilau and Small, 2007: p 619). When the input parameters are specified, the software calculates the feed pressure, the power required and the quantity of water produced based on the set recovery, membrane area needed. The energy needed to produce potable water is then determined by running the program multiple times (Gilau and Small, 2007: p 618) varying the number and types of membrane element. This is done in order to achieve the best design option in terms of energy consumption and cost. Booster 90 Group effort

104 Chapter 4: Proposed Plant pumps and energy recovery device are incorporated in the software to reduce energy consumption. The flow chart on how to create a ROSA RO/NF design is shown below: 1) Product info tab: fill in the appropriate information 2) Feed tab: select the water classification type 3) Feed tab: enter feed TDS, or check Specify individual solutes (recommended option) and complete the feed analysis 6) Scaling tab: choose one of the 3 scaling calculation options (If TDS was chosen in the Feed tab, scaling calculation is not possible) 5) Feed tab: enter feed ph 4) Feed tab: enter feed temperature 7) Scaling tab: choose No chemicals added if no ph adjustment needed 7) Scaling tab: User adjusted ph values if ph adjustment is needed. Enter ph value or concentrate LSI/ S&DSI value 7) Scaling tab: choose IX softening if feed is softened with ion exchange resins 8) Scaling tab: enter % recovery and operating temperature 9) Configuration tab: enter # of pass(es) 10) Configuration tab: enter # of stages in pass 11) Configuration tab: enter fouling factor 12) Configuration tab: choose a stage in select a stage in the pass 16) Configuration tab: Select element type (product model) 15) Configuration tab: enter # of elements in each vessel 14) Configuration tab: enter # of pressure vessels in stage 13) Configuration tab: enter back pressure (if any) 17) Configuration tab: enter permeate flow and % recovery 18) Configuration tab: check recirculation if needed, and enter flow 19) Configuration tab: repeat steps 12 through 19 until all stages 20) Configuration tab: check blending permeate if needed, and enter the flow 24) Configuration tab: Click perform calculation 23) Configuration tab: Adjust ph of second pass feed: 1) use Adjusted ph and dosing chemical ; or 2) select Pct carbon removal or CO2 pressure 22) Configuration tab: repeat steps 10 through 21 21) Configuration tab: if this is a two pass system design, then select Pass 2 in Current pass Figure 41: Flow chart for creating a ROSA system design (The Dow Chemical Company, n.d.) 91 Group effort

105 Chapter 4: Proposed Plant Nanofiltration Configuration design The steps in designing a nanofiltration or reverse osmosis system according to Dow Water & Process Solutions are as follows (The Dow Chemical Company, n.d.): 1. Consider feed source, feed quality and required product quality The feed water to the nanofiltration unit is seawater that has undergone conventional pre-treatment so it will be low in turbidity, oil and any other suspended solids. The composition of the seawater is the one for normal seawater according to the WHO organization as shown in Section Select flow configuration and number of passes A single pass nanofiltration system is needed because all of the contaminants can be removed in one step. The advantages of having a single pass over the double pass is that energy will be saved which means money is also saved as well as space. The type of flow configuration chosen is plug flow where the feed is passed through the system only once. Recirculating the concentrate is an alternative but it would not be a suitable flow configuration as a fraction of the concentrate is mixed with the feed and put back through the membrane which will increase the overall pressure leading to more pumps and higher operating costs. This is also used when there are not enough elements to achieve a high recovery, which is not the case in this instance (The Dow Chemical Company, n.d.). 3. Select membrane and element type There were a number of membranes being considered. The specifications are shown in the table below: Table 17: Specifications for nanofiltration membranes (The Dow Chemical Company, n.d.) Compounds Stabilised salt passage (%) Test conditions NF NaCl; MgSO ; > mg/l NaCl, 2000 mg/l MgSO 4, 15% recovery NF CaCl 2 ;MgSO ; <3 500 mg/l CaCl 2, 2000 mg/l MgSO 4, 15% recovery NF CaCl 2 ;MgSO ; mg/l CaCl 2, 2000 mg/l MgSO 4, 15% recovery NF will be used to simulate the nanofiltration system as it has a high rejection of hardness ions but also a high rejection of salt ions compared to the other types of membranes. The maximum 92 Elena Rusu

106 Chapter 4: Proposed Plant recovery per element is 15% according to the test performed and it will be taken as that when simulating the unit. 4. Select average membrane flux The manufacturer does not provide a specific flux for the membrane chosen but The Dow Chemical Company specify a flux for seawater with an SDI<5, between l/m 2 h (The Dow Chemical Company, n.d.). A flux of 12 l/m 2 h has been chosen because the flux decreases with decreasing pressure and nanofiltration operates at a lower pressure than reverse osmosis hence a smaller flux is required. 5. Calculate the number of elements needed The number of elements is determined as follows: Number of elements = Design permateflow Design flux Activemembranesurface area of seletedelement = 12 ( ) = The surface area of the selected element is provided by the manufacturer, FilmTec membranes. 6. Calculate the number of pressure vessels needed The number of pressure vessels required varies on how many elements can fit inside. The number of elements depends on the capacity of the systems, ranging from 1 to 8. The typical configuration for large systems is 6 elements in a pressure vessel and this will be used in the simulation. The number of pressure vessels required is: Number of PV = = T otalnumber of elements Number of elementsin a PV = 7899 PV 7. Select the number of stages based on the recovery desired The number of stages depends on the feed water quality, the desired recovery rate and the number of elements in a pressure vessel. If the feed water quality is poor and the recovery is high then more stages are required to achieve the desired drinking water quality. In each stage the pressure vessels 93 Elena Rusu

107 Chapter 4: Proposed Plant operate in parallel and the feed passes through each stage in series. For a recovery rate of 50% the number of stages needed is 1, for 55-75% 2 stages are required and 3 stages are used for systems with recoveries greater than 75% (American Water Works Association, 1999). The nanofiltration system has a recovery of 65% so the number of stages required is 2. According to The Dow Chemical Company (n.d.), for a two stage seawater system with 6 elements in a pressure vessel, the staging ration is 3:2. So the first stage will contain 4739 while the second stage will contain 3160 pressure vessels ROSA results ROSA72 software was used to simulate the nanofiltration system and the steps involved can be seen in Section Once the composition of the feed water was determined, the values were entered in one of the tabs available in the control panel, as shown in Figure 42. The type of feed water chosen was open intake seawater with an SDI (Silt Density Index) less than 5 because by the time it reaches the nanofiltration unit, it would have less contaminants. The reason for using open intake is explained in more detail in section xxx. The next step was to input the system configuration, determined in the previous section, such as the number of stages and pressure vessels in each stage, number of elements in a pressure vessel and the type of membrane used (NF90-400). This can be seen in Figure 43. Temperature and ph were inserted manually. Figure 42: Feed water data tab in the control panel 94 Elena Rusu

108 Chapter 4: Proposed Plant To insert the configuration for stage 2, select stage 2 from the stage in pass. The recovery of the system was inputted along with the feed flow. The software calculated the permeate flow based on these parameters. Figure 43: System configuration tab in the control panel After all the information has been inserted in the software, the report tab will generate a new window containing the results as it can be seen below: System Details Feed Flow to Stage m³/h Pass 1 Permeate Flow m³/h Osmotic Pressure: Raw Water Flow to System m³/h Pass 1 Recovery % Feed bar Feed Pressure bar Feed Temperature 30.0 C Concentrate bar Flow Factor 0.85 Feed TDS mg/l Average bar Chem. Dose None Number of Elements Average NDP 3.07 bar Total Active Area M² Average Pass 1 Flux lmh Power kw Water Classification: Seawater (open intake) SDI < 3 Stage Element #PV #Ele Feed Flow (m³/h) Feed Press (bar) Recirc Flow (m³/h) Conc Flow (m³/h) Conc Press (bar) Perm Flow (m³/h) Specific Energy Avg Flux (lmh) Perm Press (bar) Boost Press (bar) 1.15 kwh/m³ Perm TDS (mg/l) 1 NF NF Pass Streams (mg/l as Ion) Name Feed Adjusted Concentrate Permeate Feed Stage 1 Stage 2 Stage 1 Stage 2 Total NH K Na Mg Ca Sr Elena Rusu

109 Chapter 4: Proposed Plant Ba CO HCO NO Cl F SO SiO Boron CO TDS ph Design Warnings <None> Solubility Warnings Langelier Saturation Index > 0 Stiff & Davis Stability Index > 0 CaSO4 (% Saturation) > 100% CaF2 (% Saturation) > 100% Antiscalants may be required. Consult your antiscalant manufacturer for dosing and maximum allowable system recovery. Stage Details Stage 1 Element Recovery Perm Flow (m³/h) Perm TDS (mg/l) Feed Flow (m³/h) Feed TDS (mg/l) Feed Press (bar) Stage 2 Element Recovery Perm Flow (m³/h) Perm TDS (mg/l) Feed Flow (m³/h) Feed TDS (mg/l) Feed Press (bar) To balance: mg/l Cl added to feed Reliability checks Some checks must be performed in order to ensure the software provides accurate results. The osmotic pressure required for the system will be calculated and then compared with the value given 96 Elena Rusu

110 Chapter 4: Proposed Plant by the ROSA software. The method used to determine the osmotic pressure is described below but the actual calculation is in Appendix 4 (Lachish, 1998): 1. Determine the composition of seawater going into the nanofiltration membrane In this case we are using a membrane that will remove approximately 30% of the salt ions. So 30% of the initial Cl - and Na + ions is 5694 and mg/l respectively. However the membrane removes around 30% of these ions so the composition in the feed is: Table 18: Composition of feed water to the nanofiltration unit Ion Amount (mg/l) Chloride (Cl - ) Sodium (Na + ) Sulphate (SO 2-4 ) 2649 Magnesium (Mg 2+ ) 1262 Calcium (Ca 2+ ) 400 Bicarbonate (HCO - 3 ) The osmotic pressure (π) is given by the Van t Hoff formula: Where c = molar concentration of the ions R = universal gas constant ( L.bar/moles.K) T = temperature (Kelvin) The temperature of the seawater in Singapore is known to be C, the average of 30 C is taken for this calculation. 3. Determine the molar concentration of the ions present in the feed First the main salts that can form are defined as being Ca(HCO 3 ) 2, CaSO 4, MgSO 4 and NaCl. The number of moles present in seawater for each salt is determined on a molecular weight basis and the amount of each ion present in the feed. Ca (HCO 3 ) 2 is made up of 140 mg/l of bicarbonate and 45.9 mg/l of calcium. CaSO 4 is made up of mg/l of calcium and mg/l of sulphate. MgSO 4 is made up of 1262 mg/l of magnesium and the remaining sulphate of mg/l. NaCl is made up of mg/l of sodium and mg/l of chloride. Now the number of moles of each salt in seawater is calculated using the following formula: 97 Elena Rusu

111 Chapter 4: Proposed Plant Amount of ion x amount of ion y Molecular weight of salt made up of x and y When the salts dissolve in the water, they dissociate into the respective ions so the molar concentration of the salt ions is the number of dissociated ions times the number of moles of each salt in the feed water. For example Ca(HCO 3 ) 2 dissociates into one calcium ion and two bicarbonate ions while the rest of the salts dissociate into 2 ions each. 4. Determine the osmotic pressure of the feed The molar concentration used is the sum of all the molar concentrations for each salt. The osmotic pressure calculated is bar. The osmotic pressure given by the ROSA software is bar. This is similar to the value calculated but it is slightly bigger because in the hand calculation only the main salts were considered and a few ions were omitted like potassium, silicate and fluoride. Based on the similarity between the two values it can be concluded that the ROSA software is reliable and is able to provide accurate results Process description PI NF1A HPP Permeate water to RO unit concentrate NF1B Concentrate valve High pressure concentrate to energy recovery device in RO unit Figure 44: Block diagram of the nanofiltration system The input to the nanofiltration unit is the output from the cartridge filters and the rate at which is being pumped in is 777,000 m 3 /day. The system has a recovery of 65% so the amount of permeate generated is 505,050 m 3 /day while the rest of 271,950 m 3 /day is the concentrate. It is designed as a two stage single pass system. The feed enters the first stage (NF1A), made up 33 trains that contain 12 by 12 pressure vessels. The concentrate from the first stage is then sent to the second stage (NF1B), 98 Elena Rusu

112 Chapter 4: Proposed Plant made up of 22 trains, each with 12 by 12 pressure vessels. The concentrate from the second stage is at a very high pressure, similar to that of the feed so it is sent to the energy recovery device in the reverse osmosis unit and after that it is mixed with the brine from the reverse osmosis and disposed of. The permeate from both stages is mixed and sent to the reverse osmosis unit to be further purified. The pressure vessels are the housing for the membrane so a material strong enough to withstand high pressures and corrosive resistant is required. There are 3 typical materials used for pressure vessels. PVC (poly vinyl chloride) vessels have a maximum operating pressure of 14 bar, stainless steel 316 can withstand pressures of up to 20 bar while fiberglass vessels can go up to 68 bar and can also withstand temperature between C. The pressure of the feed going into the pressure vessels for nanofiltration is 21 bar so the material that can cope with this pressure is fiberglass. High pressure pump used High pressure pumps (HPP) are the heart of a desalination plant because without them, the high pressure required for the pure water to move through the membranes could not be generated and as a result no drinking water would be produced. When choosing a HPP it must be made of a material that can withstand the corrosion of seawater, must be able to operate without much maintenance, should not take up a lot of space and it needs to provide the high operating pressures for the nanofiltration and reverse osmosis systems. Nanofiltration operates at half the pressure of the reverse osmosis unit (21 bar), so an initial assumption can be made that a different HPP should be used with a lower maximum operating pressure. To determine which HPP should be used a cost comparison was performed between the HPP used for the reverse osmosis unit and an HPP with a lower power. The specification of the two pumps can be seen in the table below: Table 19: Specifications for the 2 pumps being considered (Springer Pumps LLC, 2009) High Pressure Roto-Jet pump KBS HPP Max. operating pressure (bar) Max. flow rate (m 3 /h) Max. power (kw) Material *Hastelloy C Duplex stainless steel Number required Price per pump ($) 68, ,000 Total price ($) 22,506,00 22,506,000 *Hastelloy C is a nickel-molybdenum-chromium super alloy with an addition of tungsten The number of pumps was determined based on the flow rate of water per train. There are 33 trains in the first stage and the total flow rate is 777,000 m 3 /day so the flow rate per train is 981 m 3 /h. Based on this 10 roto-jet pumps are needed to power one train while only 2 KSB pumps are required for one 99 Elena Rusu

113 Chapter 4: Proposed Plant train. The price for each pump could not be found on any company s website and so an approximation was made using the cost curves from the IChemE s Guide to Capital Cost Estimating (2000). The prices that were read from the cost curve were in pounds sterling so they were timed by an inflation rate of 1.55 to obtain US dollars. Another issue was that these prices were from 2000 so they were multiplied again by a different inflation rate of 1.1 (Trading Economics, 2010) to obtain 2010 prices. As it can be seen in Table 19, it would cost the same amount to use either of the pumps so the next step is to look at how much power each pump requires. The roto-jet pump would require 99,000 kw for all 330 pumps operating at full capacity while the KSB pump would only need 74,118 kw for full capacity. The maximum operating pressure for the KSB pump is 100 bar and the pressure required for the nanofiltration unit is 21 bar so in fact the power consumption would be closer to 20,000 kw as the KSB pump would not need to work near full capacity. The maximum operating pressure of the rotojet pump is close to the feed pressure of the nanofiltration unit so the pumps would need to operate near full capacity so the power would not change much. Based on this, it was decided to use the KSB pumps. Another reason for this choice is also the fact that 66 pumps would take up less space than 330 pumps and would also require less maintenance. Outlet pipe design The diameter of the outlet pipe is determined based on the flow rate of permeate generated by the nanofiltration unit, 505,050 m 3 /day, and the velocity of water which is taken to be 0.75 m/s: 505, d p 3. 15m 0.75 This is a very large pipe diameter and it would cost a lot of money as it would have to be custom manufactured. The maximum standard pipe diameter is 2.2 so instead of having one 3.15 m pipe, two pipes will be used each with a diameter of 1.8 m. material NF waste disposal There are many options available for disposal of the nanofiltration concentrate depending on what contaminants it contains. If antiscalants were used then they would be present in the concentrate stream and would add to the total phosphate content. This compound was found to promote the growth of algae and as a result the concentrate would need to be treated before disposal. A study on concentrate disposal for 6 nanofiltration plants in Netherlands determined different options of treating the waste before disposal (Thorsen and Flogstad, 2006): Concentrate can be treated by rapid gravity sand filtration 100 Elena Rusu

114 Chapter 4: Proposed Plant Select a nanofiltration membrane that removes a smaller amount of hardness ions so that antiscalants don t need to be used Change to a feed water that contains less contaminants Mix the concentrate from the nanofiltration unit with the effluent from a wastewater plant Dispose of the concentrate in a large water body, a few mile away from the point of discharge One of the uses could be for irrigation but the waste stream is very concentrated in hardness ions and also contains a high salt content so that would destroy the crops and damage the land, preventing it from being used again. Pilot studies should be conducted to determine whether the concentrate requires further treatment along with samples of concentrate being checked for toxicity. These will determine the disposal method for the concentrate. In this case no antiscalant is used so the concentrate can be mixed with the brine stream from the reverse osmosis unit and put through an energy recovery device. This is because the concentrate from the nanofiltration system is at a very high pressure similar to the feed pressure and it would be more economical to recover that energy and use it to run the high pressure pumps NF membrane cleaning Membrane fouling is one of the biggest problems associated with membrane processes. After a certain amount of time, a decline in membrane performance can be observed due to fouling which leads to a decrease in permeate quality due to an increase in salt passage. This will also increase the cost as a higher energy demand is required along with physical or chemical cleaning and additional labour for maintenance. If cleaning is delayed then the foulants become more difficult to remove leading to a membrane replacement. This must be avoided as the cost of a membrane unit is about 20-25% of the total capital cost as mentioned by Al-Amoudi and Lovitt (2007). A way to overcome this problem is to have a really good pre-treatment which will take out the main particles that are likely to foul the membrane. The first step is identifying the main types of fouling likely to occur based on the quality of the feed water. The main types of nanofiltration fouling that can occur are (Al-Amoudi and Lovitt, 2007: p 8): 1. Inorganic fouling as a result of inorganic scales such as CaSO 4, CaCO 3 depositing on the membrane surface 2. Organic fouling NOM (natural organic material) present in the feed water 3. Biofouling as a result of viruses and bacteria collecting on the membrane surface, they can grow and multiply in the presence of certain nutrients that deposit on the surface Inorganic fouling, also known as scaling, occurs when certain salts reach their solubility limits so they precipitate out on the surface of the membrane. A way to avoid this problem is to run the nanofiltration unit at a recovery rate lower than the critical solubility limit. However this is very 101 Elena Rusu

115 Chapter 4: Proposed Plant difficult to predict as it is a complex problem and no reliable way has been found. Antiscalants can be used to decrease the solubility of the salts but again specific treatments are hard to predict. Formation of scale can cause physical damage to the membrane and due to irreversible pore plugging it can be very difficult to restore the performance of the membrane. There are various parameters that can affect the precipitation of the salts and some are shown in Table 20: Table 20: Potential scaling factors (Al-Amoudi and Lovitt, 2007: p 10) Value Precipitation Cause Ionic strength High Increased Solubility and supersaturating CP (concentration polarization) High Increased Solubility and supersaturating Co-precipitation Presence Increased Changing structure of precipitate ph Higher Increased Solubility decreased Pressure Higher Increased Increasing CP and osmotic pressure at membrane surface Velocity (flow rate) Higher Decreased Higher wall shear rate Temperature higher Increased Solubility decreased Organic fouling is caused by natural organic matter formed of primarily humic substances that contain aromatic components with phenolic and carboxylic functional groups. Due to their composition, the humic substances have a negative charge at a neutral ph. The performance of the membrane can be partially or fully restored with the help of chemical cleaning. NOM fouling can be influenced by a number of factors, some of which are mentioned below: Ionic strength concentration ph Table 21: Natural organic matter fouling factors (Al-Amoudi and Lovitt, 2007: p 10) Value NOM fouling rate Cause Increased Increased Electrostatic repulsion High Low Increased Increased Hydrophobic forces Divalent cations Presence Increased Electrostatic repulsion and bridging between NOM and membrane surface Membrane charge High charge Increase Electrostatic repulsion CP High Increased Permeate flux (high recovery) Higher Increased Hydrophobicity Pressure Higher Increased Compaction Biofouling tends to occur often because the membrane s surface provides a very good location for bacterial growth as nutrients deposit there. When viruses and bacteria multiply, they can lead to a 102 Elena Rusu

116 Chapter 4: Proposed Plant formation of biofilms which increase in size and cover the surface of the membrane. Biofilms can also favour inorganic scaling as they enhance the nucleation and crystallization kinetics. According to Al- Amoudi and Lovitt (2007), biofouling can be minimised by removing certain components from the feed water that tend to degrade, using very pure chemicals for dosing and performing effective cleaning procedures. Before any cleaning can be performed it is vital to understand the point of origin of the fouling. Fouling occurs in the first element in the pressure vessel or the last element. This is because particles tend to become trapped in the first element and by the time the water reaches the last element it is very concentrated in salts. Metal oxides, colloids, organic and biofouling tend to deposit in the first element while scaling occurs in the last element. As mentioned previously, fouling can be minimized by using an effective pre-treatment such as coagulation, filtration/sedimentation, membrane processes (MF, UF) and scale inhibitors. All but one pre-treatment method are used before nanofiltration so the chances of fouling have decreased dramatically but after a period of time the membrane will require cleaning. Cleaning is defined as a process where material is relieved of a substance, which is not an integral part of the materials (Al-Amoudi and Lovitt, 2007: p 15) and is a vital step in preserving the membrane and its performance. There are different cleaning methods available such as chemical, physical and a mixture of both, but it has been found that chemical cleaning is the most effective for nanofiltration and reverse osmosis membranes. Membranes should be cleaned when the permeate flow drops by 10% or the feed pressure increases by 10% or the pressure difference between the feed and concentrate increases by 15% (Al-Amoudi and Lovitt, 2007). However, there are a number of factors that could cause these changes such as malfunction in pre-treatment or in the pressure control or a very high recovery rate. So these should be considered and checked first before making a decision on cleaning. The cleaning frequency depends on how often these parameters change but on average, for nanofiltration systems using seawater as the feed, it falls in the range of 4 to 6 months. The frequency of cleaning is an essential part in maintaining the performance of the membrane (Al-Amoudi and Lovitt, 2007). If the membrane is cleaned too often it could lead to deterioration of the cross-linking network of the membrane surface properties by making the chemical interactions between the chemical solution molecules and surface functional groups of the membrane stronger (Al-Amoudi and Lovitt, 2007: p 24). Therefore the cleaning frequency is a function of the duration of cleaning and the chemicals used during cleaning. Al-Amoudi and Lovitt (2007) suggest that the cleaning duration has been discussed in only a few journals and the conclusions that could be drawn were that more frequent cleaning but longer cycles are most effective as the fouling layers over time become more difficult to remove because they are more compact. 103 Elena Rusu

117 Chapter 4: Proposed Plant Having said all this, it is very difficult to predict the type of chemical cleaning for the nanofiltration system. Specific cleaning is determined after several years of tests on pilot plants. This is done by first determining the type of foulants (The Dow Chemical Company, n.d.): Evaluate the available data on the plant performance Examine the composition of the feed water as potential foulants could be visible there Analyse the results of previous cleanings Asses the deposits on the cartridge filter Examine the inside of the feed pipe as particular foulants have a specific colour i.e. presence of iron materials means the pipe is reddish-brown while if the inside is gelatinous then it is biofouling or organic material Cleaning will be most effective when it is tailored to the specific type of fouling so it may be that every time a membrane is cleaned, it requires different chemicals. If the wrong cleaning chemicals are used then they would damage the membrane resulting in higher operating costs as it has to be replaced. It is also important to operate the membrane at a reduced flow and pressure, after cleaning, to remove the chemicals used during cleaning from the elements, before normal operation. The Dow Chemical Company (n.d.) advises that acid and alkaline cleaners are the standard chemicals used. Acid cleaners are most effective at removing inorganic foulants while alkaline cleaners are best for removal of organic fouling and biofouling. They also provide a list with some cleaning solutions which can be used with NF90 membranes and they are a good starting point: Table 22: Cleaning solutions for nanofiltration membranes (The Dow Chemical Company, n.d.) Foulant Dosing chemical Inorganic salts 0.2 wt% HCl at 25 C and ph 1-2 Biofilms and organic 0.1 wt% NaOH at 35 C and ph 12 or Na-DSS at 35 C and ph 12 Na-DSS stands for sodium salt of dodecylsulfate. It is also stated that when cleaning NF90 elements, the maximum temperature must be 95 C at a ph 1-12, because the materials cannot withstand really high temperatures. In order to evaluate whether the cleaning procedure worked effectively, a number of methods could be used to assess that such as flux measurements and surface analysis tests Reverse Osmosis Reverse osmosis system parameters Reverses osmosis systems are usually designed for a specific permeate flow rate (m 3 /day) and recovery. Plant capacity and feed water source (seawater or brackish water) are important RO design performance criteria required in order to determine the membrane array configuration (number of 104 Elena Rusu

118 Chapter 4: Proposed Plant passes, stages, membrane elements and pressure vessels), feed osmotic pressure and operating pressure of the RO system. Most SWRO desalination plants operate at a recovery rate of 40-50% (Othmer, 2007: p 76). However with the development of a new system known as the brine conversion two-stage system (BCS), the recovery rate can be increased up to 60% depending on the feed water salinity. According to Norman et al (2008, p 8), the two-stage brine conversion system is the most effective way to save energy while still keeping a low operating cost. According to Crittenden et al (2005, p 9), the operating pressure for seawater desalination ranges from 55 to 85 bars depending on the feed water salt content and the membrane configuration. The use of nanofiltration as part of the pre-treatment reduces the feed water salinity as it removes 30% of monovalent ions from the feed entering the RO membrane. This implies that the reverses osmosis plant can be operated at a much lower pressure. System flux According to the Dow Chemical company, the recommended average system flux for seawater RO desalination plant with spiral wound membrane element and SDI <5 ranges between 11 and 17 L/m 2 h. A high system flux increases the energy consumption due to a higher system pressure which would ultimately lead to higher operating costs of the plant. For this reason, an average flux of 14 L/m 2 h was chosen for this design. Membrane element selection and configuration Table 23: Membrane element specification 1 st pass 2 nd pass Element type FILMTEC SW30HR-380 FILMTEC BW Design permeate flow rate (m 3 /d) Active membrane area (m 2 ) Maximum operating pressure (bar) Designed salt rejection (%) FILMTEC SW30HR-380: This element has a high productivity, active area and the minimum rejection is 99.7%. It also has the ability to remove about 70% of boron from the feed water. FILMTEC BW30-400: This element is designed to treat large quantity of water at low pressure thereby allowing low operating cost. It also has a high resistance to fouling. 105 Irene Usiomoifo

119 Chapter 4: Proposed Plant The number of elements and pressure vessels were calculated with same method used in the nanofiltration configuration determination which can be seen in Section Determination of the number of stages Table 24: Number of stages of a seawater system (The Dow Chemical Company, n.d.) System recovery (%) # of serial element position # of stages 6-element vessel # of stages 7-element vessel # of stages 8-element vessel RO systems with recovery rate greater than 50% are usually configured as two-stage. This configuration improves the quality of the final product. For two-stage seawater system with 6 membrane elements per pressure vessel, the standard staging ratio is 3:2. In a two-stage system, the first-stage concentrate is the feed to the second-stage. As the feed water passes from one stage to the other, the flow rate is reduced due to the extraction of permeate from the feed water. Two-stage RO systems are normally designed with decreasing number of membrane elements in each succeeding stages as to maintain sufficient cross flow velocity and also to prevent accumulation of solute on the membrane surface. A booster pump is used in the second stage to increase permeate flow and improve the quality of water. In terms of potable water production, energy recovery and quality of the product, two-stage RO designs are preferred for desalination plant. membrane elements See Appendix MF/1 for membrane configuration calculation. After examining different design alternatives, two-pass design was chosen for the RO desalination process. In a two-pass system, permeate from the first-pass becomes the feed for the second-pass, this helps to improve solute removal from the seawater hence a better permeate quality can be obtained. Two-pass design option enables the required water quality to be achieved while minimising energy consumption and cost. The first-pass removes the dissolved solids from the feed water while the second-pass is used to decrease the level of boron in the treated water in order to meet the World Health Organisation limit of 0.5 mg/l. 106 Irene Usiomoifo

120 Chapter 4: Proposed Plant The recovery rate of the first-pass is limited by the scaling potential of the feed water so the first-pass of the RO system is normally operated at a lower recovery. The feed water to the second-pass (i.e. first-pass permeate) is of a much better quality therefore the second pass is designed with a higher recovery rate than the first-pass with fewer membrane elements. The recoveries for the first and second-pass of the RO system are 55% and 90% respectively ROSA results In order to assess the RO system performance in terms of energy consumption and permeate quality, two different design alternatives were considered: Case 1: RO process without concentrate recycle Case 2: RO process with concentrate recycle Case 1 Figure 45: Flow diagram for RO system case 1 Table 25: ROSA result for case 1 Pass 1 Pass 2 Steam # Flow Pressure TDS Flow Pressure TDS Steam # (m 3 /h) (bar) (mg/l) (m 3 /h) (bar) (mg/l) A A A A /1 % recovery A/1A % recovery Irene Usiomoifo

121 Chapter 4: Proposed Plant Table 26: ROSA result for case 1 (energy consumption) Pass # Pass 1 Pass 2 Stage # Element type SW30HR-380 SW30HR-380 BW BW Pressure Vessel per Stage Elements per Pressure Vessel Total Number of Elements Pass Average Flux lmh lmh Stage Average Flue lmh 6.38 lmh lmh lmh Permeate Back Pressure 0.00 bar 0.00 bar 0.00 bar 0.00 bar Booster Pressure 0.00 bar 0.00 bar 0.00 bar 0.00 bar Chemical Dose - 100% NaOH 7.33 mg/l Energy Consumption 2.69 kwh/m kwh/m 3 Specific energy usage for the RO plant without the concentrate recycle was calculated to be kW (2.86kWh/m 3 ). The ROSA software gave similar value of kW (2.88kWh/m 3 ) with an overall RO system recovery of 49.50%. The product water TDS is 13.02mg/L and boron level is 0.47mg/L. See appendix 4 EC/1 for energy consumption calculation. Case 2 Figure 46: Flow diagram for RO system case Irene Usiomoifo

122 Chapter 4: Proposed Plant Table 27: ROSA result for case 2 Pass 1 Pass 2 Steam # Flow Pressure TDS Flow Pressure TDS Steam # (m 3 /h) (bar) (mg/l) (m 3 /h) (bar) (mg/l) A A A A A /2 % recovery R A/1A % recovery Table 28: ROSA result for case 2 (energy consumption) Pass # Pass 1 Pass 2 Stage # Element type SW30HR-380 SW30HR-380 BW BW Pressure Vessel per Stage Elements per Pressure Vessel Total Number of Elements Pass Average Flux lmh lmh Stage Average Flue lmh lmh lmh lmh Permeate Back Pressure 0.00 bar 0.00 bar 0.00 bar 0.00 bar Booster Pressure 0.00 bar 0.00 bar 0.00 bar 0.00 bar Chemical Dose - 100% NaOH 7.33 mg/l Energy Consumption 2.62 kwh/m kwh/m 3 Recycling the concentrate from the second- pass of the RO system with the use of booster pumps and ERD, specific energy usage is reduced to 2.81kWh/m 3 and the overall system recovery increased to 52.38%. The reason for the increase in recovery rate is because the concentrate from the second pass in the second stage has a better quality than the fresh seawater so it mixes with the seawater reducing the salinity level. This causes a decrease in the osmotic pressure therefore a lower operating pressure is required leading to a reduced energy consumption. As a result this is a better design alternative to case Irene Usiomoifo

123 Chapter 4: Proposed Plant Reliability checks Some checks must be performed in order to find out if the results generated by the ROSA software are reliable. Two checks will be made on the osmotic pressure and the energy required producing 1 m 3 of water. Osmotic pressure: The osmotic pressure of the feed coming from the nanofiltration unit is calculated and then compared to the value from the ROSA software. Seawater contains a variety of salt but the calculation of the osmotic pressure is based on the assumption that all the salt present is sodium chloride (NaCl solution). See appendix 4 OP/1 for osmotic pressure calculation. Table 29: Major ion composition of normal seawater (mg/litre) (Al-Mutaz, 2000) 30% ion removed by Constituent Normal seawater Nanofiltration Chloride (Cl - ) 18,980 mg/litre (0.7*18,980)=13286 mg/litre Sodium (Na + ) 10,556 mg/litre (0.7*10,556)= mg/litre Total dissolved Solids (TDS) 29,536 mg/litre 20,675.2 mg/litre Nanofiltration removes 30% monovalent ion from seawater and this reduces the osmotic pressure of the feed water hence lower operating pressure are needed for the RO system. Osmotic pressure π is given by Van t Hoff formula: Where C = molar concentration of the salt ions R = universal gas constant: (Litre.bar/mole.K) T = the temperature (Kelvin) Seawater temperature is taken as 30 C (303.15K). From the table above, the amount of salt in seawater is 20,675.2 mg/litre. Atomic weight of sodium and chlorine are 23 g and 35.5 g respectively. When sodium chloride dissolves in water, it dissolves into Na + and Cl - ions. There are 2 ions per salt molecule so the ion concentration is twice the molecule s concentration. Ionic concentration, c = 2 number of NaCl moles Osmotic pressure for normal seawater is bar implying that the system operating pressure will be about 60 bar. However using nanofiltration as part of the pre-treatment, the osmotic pressure of sea water was calculated to be bar meaning that the RO system can be operated at a much lower 110 Irene Usiomoifo

124 Chapter 4: Proposed Plant pressure hence a lower cost. Osmotic pressure from the ROSA software is bar which is similar to the value calculated. Seawater reverse osmosis membranes are operated at pressures about 2.5 times higher than the osmotic pressure of the feed water, so the RO system pressure should be about 44 bar (ROSA software calculated the operating pressure to be bar). Energy consumption for RO system: Energy consumption is one of the major factors that must be considered in the design of an RO desalination plant. Power consumption includes power for the low pressure and the high pressure pump and ERD. The high pressure pumps consume the highest power because the feed water pressure must be greater than the osmotic pressure of the seawater to push the pure water across the RO membrane. The energy requirement varies with the system condition (system flux, recovery and temperature) and the feed water quality. The specific energy usage for RO desalination plants ranges between 3-7 kwh/m 3 with energy recovery systems and booster pumps. The full calculation can be seen in Appendix 4 EC/1. The equation below is used to calculate the total power required for RO systems (Gilau and Small, 2007: p 619): p wf Q f P E n rf Where P wf = power consumed by the system (1 st and 2 nd pass) (kw) Q f = feed water flow rate (m 3 /s) P rf = Feed pressure (kpa) E n = pump efficiency (85%) The energy requirement for the first pass for case 1 (without recirculation) was calculated to be 2.62 kwh/m 3, while the value for the second pass was determined to be 0.18 kwh/m 3. The values generated by ROSA were 2.69 and 0.19 kwh/m 3 respectively, which are almost identical to the ones determined independently. This is yet more proof that ROSA can be trusted with the results it generates. 111 Irene Usiomoifo

125 Filtered Feed Water HPP Booster Pump ERD Two stage RO unit 1st pass R01A R01B 1st pass low pressure concentrate to waste Interstage booster Concrete value pump Figure 47: Flow diagram of the reverse osmosis unit NaOH 2nd pass concentrate Two stage RO unit 2nd pass R02A R02B Concrete value Permeate water Chapter 4: Proposed Plant Process description 112 Irene Usiomoifo

126 Chapter 4: Proposed Plant The RO section of the desalination plant consists of high pressure pumps, RO membrane modules (membrane element + pressure vessel), booster pumps, concentrate valves, piping and energy recovery system (pressure exchanger). The first pass of the RO system has 27 trains, each with a production capacity of approximately 21,044 m 3 /day. There are 27 high pressure pumps and 27 energy recovery devices in the first-pass of the RO system (i.e. one HPP/ERD per RO train). Permeate from the nanofiltration system is transferred to the high pressure feed pump where the high feed pressure is created for the RO saline feed water. The pressurized saline water enters RO1A, where the flow is distributed to each of the membrane modules consisting of pressure vessels with 6 elements in each pressure vessel. Due to the pressure differential between the feed water and permeate side of the membrane module, a certain amount of pure water goes through the membrane causing the flow to split into a high saline product (concentrate) and a low saline permeate. Permeate which has passed through RO1A is collected from a tube located at the centre of the pressure vessels and the concentrate enters RO1B where more solute is removed. The feed water becomes more and more concentrated as it enters the next element in the membrane module due to loss of pure water and eventually leaves the last membrane element to the concentrate valve where the pressure is released. Permeate leaves the RO membrane at nearly atmospheric pressure and the TDS is reduced to about mg/l. The concentration of the brine goes up due to loss of pure water and leaves the RO pressure vessels at nearly the same pressure as the feed water. The pressurized concentrate from R01B is transferred to the energy recovery device and used to generate energy required for the high pressure pumps before it is finally disposed of. To further reduce the salinity and the level of boron in the final product water, permeate from RO1A and RO1B enters the second-pass consisting of 23 low-pressure RO trains and this pass is operated with low pressure pumps. Sodium hydroxide is injected into the permeate from the firstpass to increase the ph to about 9.5 as this is necessary for boron removal. High quality permeate (drinking water) is then collected from both stages of the second-pass and transferred to the posttreatment section of the plant. To increase the overall system recovery, the concentrate from RO2B is recycled back to the feed stream of the first-pass. The second-pass is operated at a high recovery rate in order to help maximise the permeate quality and reduce the system cost. From the ROSA energy consumption calculation, the high pressure pump requires kw to produce 277,776 m 3 /day of permeate. The specific energy consumption for the first-pass of the 113 Irene Usiomoifo

127 Chapter 4: Proposed Plant RO system is 2.62 kwh/m 3. The low pressure pump (booster) consumes kw to produce 249,984 m 3 /day of drinking water. The specific energy consumption for the second pass is 0.19 kwh/m 3. Therefore the overall specific energy performance of the RO system is kW (2.81kWh/m 3 ) and permeate TDS and boron level are 11.27mg/L, 0.45mg/L respectively. Reverse osmosis pumps The most commonly used high pressure pumps in seawater desalination are: piston, centrifugal and positive displacement pumps. The decision on which type of pump to use depends on factors such as the capital and maintenance cost, operating pressure, pumping efficiency and plant capacity. Positive displacement pumps are mostly used in desalination process because of its high efficiency (85-95%), however, its use is highly limited by capital and maintenance cost. Type Table 30: RO pump specifications HGM-RO HPP RPH-RO booster pump Energy recovery device (DT-PX) Manufacturer KSB KSB KSB Pumping medium Seawater First-pass permeate First-pass concentrate Maximum flow rate (m 3 /h) Maximum pressure (bar) Efficiency (%) Power (kw) Material Duplex stainless steel Duplex stainless steel Duplex stainless steel Numbers required Outlet pipe design calculation The diameter of the RO outlet pipe is calculated based on the total permeate flow rate (527,777.7 m 3 /day) and the velocity of water which is assumed to be 0.75 m/s d p 2. 22m 0.75 Because the permeate exits the RO system at atmospheric pressure (low pressure) and it is not corrosive, the choice of material for the outlet pipe can be either carbon steel or plastic (PVC). This material is cheaper compared to GRP. 114 Irene Usiomoifo

128 Chapter 4: Proposed Plant Concentrate stream As more permeate goes through the RO semipermeable membrane, the feed water solute concentration increases, this leads to the accumulation of solute on the surface of the RO membrane and this could results in fouling of the membrane if not controlled. To prevent membrane fouling, the concentrate is normally discharged through the concentrate line so as to allow more fresh seawater into RO system. The concentrate (brine) from the first-pass of the RO system is still at a high pressure as it exits the final membrane element. Unlike the cross-flow process, this concentrate is not recycled back to the feed water stream but is regarded as waste. This high pressure concentrate is dissipated through the control valve leading to the loss of pressure energy in the system. However, most RO system utilizes energy recovery devices in order to minimise energy loss through the concentrate stream Energy consumption and Energy recovery device (ERD) Energy consumption is a vital contributor to the total operating cost of a reverses osmosis desalination plant. Power usage in RO desalination processes is represented by the specific energy consumption (SEC), which is defined as the amount of energy required to produce a unit volume of permeate (Sharif, 2009: p 112) from saline water expressed in kwh/m 3. The major energy consumption in a reverse osmosis desalination process is at the high pressure pumps (Sharif, 2009: p 112) where the saline water is pressurized so as to overcome the osmotic pressure. The other parts of the RO plant such as pre/post-treatment units consume about 10% of the total energy consumption (Sharif, 2009: p 112). The figure below shows the energy consumption breakdown for desalination processes. Figure 48: Energy consumption for desalination process stages (Fritzmann et al., 2007: p 66) 115 Irene Usiomoifo

129 Chapter 4: Proposed Plant Reverse osmosis desalination is an energy intensive process due to the high operating pressure and the low recovery rates. The use of energy recovery system has helped to recover as much energy as possible from the rejected brine. The high pressure concentrate from NF membrane process and first-pass of the RO unit has potential energy that can be transferred to the feed stream of the RO unit using ERD. This helps to reduce energy cost by about 40%. There are three major types of energy recovery devices commonly used in seawater reverse osmosis desalination process: 1. Pelton wheel turbine (PWT): This type of ERD operates by converting the potential energy in the concentrate into rotational energy. This energy is then delivered in the form of mechanical shaft power (Avlonitis, Kouroumbas and Vlachakis, 2003: p 4) and it provides the energy required to drive the motor of the high pressure pumps. The efficiency of Pelton turbines ranges from 40 to 60% depending on the plant recovery rate, pressure and plant capacity. For large scale desalination plants, energy consumption associated with the use of PWT can be quite high due to the low efficiency of the device. Figure 49: Pelton wheel energy recovery device (Avlonitis, Kouroumbas and Vlachakis, 2003: p 4) 2. Hydraulic turbo charger (HTC): Turbo charger is another kind of energy recovery device placed between the high pressure pump and the RO membrane replacing concentrate control valve. The HTC captures the potential energy of the concentrate with a turbine connected to the high pressure pump and this concentrate energy is returned back to the feed stream as pressure boost. Energy is saved using this device because the high pressure pump is operated at a very low pressure. 116 Irene Usiomoifo

130 Chapter 4: Proposed Plant Turbo charges are flexible, easy to operate and the efficiency ranges between 50-65% implying that this is a better energy recovery device than the Pelton wheel turbine. Figure 50: Turbo charger energy recovery device (Avlonitis, Kouroumbas and Vlachakis, 2003: p 5) 3. Pressure exchanger (PX): This type of ERD transfers the pressure energy in the rejected concentrate to the low pressure feed water by using a positive displacement system. The efficiency of pressure exchanger devices is about 98% and the energy consumption is very small compared to the pelton and turbo charger energy recovery devices because the PX does not use electrical power. For this reasons, pressure exchanger ERD are commonly used in SWRO desalination process to recover energy from the concentrate. Figure 51: Pressure exchanger energy recovery device (Avlonitis, Kouroumbas and Vlachakis, 2003: p 5) Pressure exchanger device capture energy from the high pressure concentrate in the first pass of the RO system and transfers this energy to the low pressure feed. The high pressure feed water from the pressure exchanger device combines with the pressurized water from the high pressure pump and this water is sent to the RO membrane element. The use of the PX device results in a 117 Irene Usiomoifo

131 Chapter 4: Proposed Plant decrease in the amount of water pressurized by the high pressure pump. The figure below shows a typical membrane and pressure exchanger configuration for RO membrane processes. Figure 52: Schematic diagram for SWRO train with PX ERD (Stover, n.d.: p2) Considering the different types of energy recovery devices available, the most feasible option for the proposed desalination plant is the pressure exchanger device because it recovers almost all the energy in the brine and also reduces the load on the high pressure pumps Membrane cleaning Although the use of improve membrane elements and feed water pre-treatment process has helped to reduce the effects of fouling in membrane processes, this problem cannot be completely eliminated due to the deposition of fouling layers on the membrane surface. There are five different types of membrane fouling: colloidal fouling, scaling, fouling by metal oxides, plugging and bio-fouling. Due to the use of nanofiltration pre-treatment option, the possible causes of RO membrane fouling is narrowed down to colloidal and bio-fouling which results in increased pressure drop along the membrane, low permeate flow, decreased flux and decreased salt rejection rate. In order to prevent membrane deterioration and eventually damage, membranes are cleaned when changes such as: 10% decrease in permeate flowrate, 15% increase in feed channel pressure loss and 10% increase in salt rejection (Fritzmann et al., 2007: p 25) are noticed in the process within 48hours of plant operation. To ensure continuous operation of the plant, half of the membrane elements are cleaned at a time. The type of cleaning solution used depends on the severity and cause of fouling. Bio-fouling cleaning solutions include (The Dow Chemical Company, n.d.): 0.1 wt% NaOH, ph 13 and 35 C max-this is the preferred membrane cleaning solution 0.1 wt% NaOH, 0.1wt% Na 4 EDTA(2-ethyl-acetate), ph 13 and 35 C max 118 Irene Usiomoifo

132 Chapter 4: Proposed Plant 0.1 wt% NaOH, wt% Na-DDS (Sodium-lauryl-sulphate), ph 13 and 35 C max. Hydrodynamic cleaning procedure is commonly used to clean membrane elements. This procedure involves cleaning the elements in another pressure vessel, reversing the feed flow with the same chemical, but with higher feed flow (Avlonitis, Kouroumbas and Vlachakis, 2003: p 6). This cleaning procedure results in better permeate quality and flow rate. The use of chemicals solutions in membrane cleaning has a major drawback because chemicals used may have an effect the membrane material. This is common when cellulose acetate membranes are used due to its poor chemical resistance. Polyamide materials have a better chemical resistance; so cleaning with chemical has no effect on the membrane material. Another membrane cleaning procedure is direct osmosis. Cleaning RO membrane by direct osmosis involves the use of high salinity solution which is introduced into the RO train with an osmotic pressure of 200 bar. The high osmotic pressure overcomes the feed pump gauge pressure shifting reverse osmosis to direct osmosis (Fritzmann et al., 2007: p 26). This results in permeate backwash through the RO membrane. Cleaning RO membrane helps to reduce membrane replacement cost hence great reduction in plant operating cost UV Disinfection UV Design The UV disinfection system required for the proposed water treatment plant must have: (1) UV lamps contained in transparent quartz sleeves (2) Sleeve cleaning system (3) Within a closed vessel structure (4) Control/monitoring system (5) Ballast system It is assumed that the volumetric velocity of water is 0.75 m/s throughout the disinfection system. The maximum flow rate through process that needs to be achieved is 250,000 m 3 /day therefore approximately 10,500 m 3 /h of water is needed. Determining the UV system The system to be used for UV disinfection will be Trojan UV Swift 24 which has a maximum capacity of 3950 m 3 /h each. It can house up to eight lamps which is equipped with an automatic mechanic cleaning system. The chamber is constructed out of 316L stainless steel and can withstand pressures up to 10 bar. The UV transmittance ranges from 70-98% indicating a highly adequate finished water product. The product dimensions are shown in Figure Eric Do

133 Chapter 4: Proposed Plant A E flow flow C Space B D Dimension of the elements (mm): A = 864, B = 1372, C = 813, D = 610, E = 889 Figure 53: The dimensions of Trojan UV Swift 24 Determining number of chambers From Trojan UV Swift brochure, a reaction chamber of 3950 m 3 /h capacity can be readily purchased of the market. The number of pipes corresponding to the number of chambers required is: Therefore the minimum number of UV chambers required is 3. In order for the system to be fully running during maintenance and breakdowns, an additional chamber will be included. The total number of chamber reactors needed is 4 which are more than enough. Determining the UV dosage Taking into account one chamber and assuming internal diameter of approximately 800 mm. With 0.75 m/s volumetric velocity, the maximum exposure time (contact time) for disinfection is approximately 1.06 seconds. UV dose (mj/cm 2 ) = Intensity (W/cm 2 ) contact time (s) = Intensity of one lamp = Germicidal UV output (W/cm) Maximum radius (cm) Using medium pressure lamps, the parameters for disinfection can be taken as Germicidal UV output of 20 W/cm and maximum exposure radius of 100 cm. 120 Eric Do

134 Chapter 4: Proposed Plant 20 W/cm = 100cm = = J/cm 2 UV dose (mj/cm 2 ) = 212 mj/cm 2 The proposed UV dose is sufficient to achieve log-4 reduction. For a log-4 reduction it is seen that Bacillus subtilis ATCC6633, the most resist spore, requires a UV dose of 81 mj/cm 2 ; Escherichia coli bacteria O157:H7 requires a dose of 6 mj/cm 2 ; Cryptosporidium parvum requires a 7.9 mj/cm 2 dose; and the most resist virus, Rotavirus requires a dose of 200 mj/cm 2 (Cairns et al., 2006). The UV dose also assumes a 10% reduction of intensity as the lamps age; however, it is still capable of removing the majority of micro-organisms. Determining the number of UV lamps The UV dose has to be distributed evenly throughout the cross section. Therefore the number of UV lamps must be able to equally distribute through the cross section. Since medium pressure lamps are used and have an intensity diameter of 100 cm, Figure 54 shows a feasible calculation. 100 mm 100 mm 100 mm 100 mm 100 mm 100 mm 100 mm 100 mm Figure 54: The cross section of a pipe showing the proposed equally distributed distance between UV lamps The intensity of UV obeys the inverse square law. It is stated that for a given energy source, the strength is inversely proportional to the square of the distance away from the point source. This means at a point source, an object will receive a certain amount of energy whereas an object of the same size but two the distance away from the point source will receive half the amount of energy 121 Eric Do

135 Chapter 4: Proposed Plant at the same time. However, in the design, a UV lamp is not a point source but a linear source of energy. Therefore, the energy is the intensity over the distance. So, by placing the UV lamps at the same distance as their maximum intensity radius, the intensity strength will overlap creating the same intensity throughout the cross section. For the 800 mm diameter pipe, it is suggested that the UV chamber will require 7 UV lamps. Total quantity of equipment The proposed treatment plant will require: 4 Trojan UV Swift 24 chamber reactors, a minimum of 28 medium pressure UV lamps, minimum of 28 quartz sleeves with fitted automatic wipes, a control panel with intensity sensors and a UV transmittance monitor. All the equipment is readily available in the market from Trojan UV Clearwell A clearwell is the final stage of the process design before the water enters the water distributed system. Clearwell tanks are used for storage of the production water but other features, including disinfection, ph buffering, and remineralisation, can be integrated with the system. Chlorine chamber design The basic design for using chlorine disinfection in the proposed treatment plant includes: (1) Pipe chamber (2) Chlorine-to-water mass transfer (Injection system) (3) Mixing process (4) Control/monitoring system (5) Storage for liquid chlorine, Sodium Hypochlorite, and for aqueous ammonia It is assumed that the volumetric velocity of the water is 0.75 m/s throughout the disinfection system. Determining the length of the chamber With chlorine being the secondary disinfection process after the water was been treated by UV, the number of micro-organisms in the water would have decreased. Maximum chloramine concentration is 3 mg/l as Cl 2. (World Health Organisation, 2006) 122 Eric Do

136 Chapter 4: Proposed Plant Figure 55: The inverse square proportional relationship between the dispersion number and the log reduction (Crittenden et al., 2005) The dispersion number is proportional to the inverse of the velocity of the flow and the length of the pipe. The dispersion is the degree of how uniform the water velocity in a given time. In the pipe, water will travel are different velocities which can affect the log reduction as shown. The information plotted can be used to approximate calculate the length of the chlorine chamber required for disinfection. The chlorine chamber is calculated to be m (Refer to Appendix 4 CL/1). Determining the contact time The contact time inside the chamber can be calculated assuming the volumetric velocity, v is 0.75 Determining the maximum Ct value t = m/s = s With the maximum contact time of seconds and using the maximum chloramines concentration permissible, the maximum Ct value can be calculated to be: Ct value = C t Ct value = 16.5 = 3 mg/l 200.8/60 (for log-1 reduction 90% removal) The maximum Ct value for the chlorine is 16.5 which for primary disinfection are not sufficient and not a recommended. Since UV disinfection is capable to supply log-4 dosage, which can 123 Eric Do

137 Chapter 4: Proposed Plant remove Rotavirus, then the majority of the micro-organisms can be deactivated with ease. The chlorine residual will be in the product to ensure the micro-organisms do not reactivate and that their metabolism is immobilised permanently, therefore a log-1 should be sufficient enough for this. Remineralisation and ph rebalance design After the reverse osmosis stage, the water feed is passed on to the UV process stage. Once the water has been disinfected by UV it is then fed onto the final stage of the clearwell. This is where the minerals will be placed back into the water. Minerals that are placed back in the water according to the minimum requirements of World Health Organisation. Table 31 will list the composition of the water product the proposed treatment plant will have. Table 31: Composition of water Mineral (mg/l) Mineral (mg/l) Iron 0.3 Magnesium 10 Zinc 3.0 Calcium 30 Copper 2.0 Potassium 0.8 Fluoride 1.5 Phosphorus 0.07 Sodium 200 Sulphur 3.38 Chloride 250 Boron 0.45 Bicarbonate 0.03 Iodine Trace ph value 7.5 The proposed pipe required is approximately m however it will require three U-bends. Chlorine and ammonia injectors are required at the start of the clearwell to safely feed sodium hydrochlorite and aqueous ammonia together for the chlorine disinfection to proceed. The injectors are connected a static mixer pipeline that can be readily purchased off the market. The proposed plant will use a 316L stainless steel Komax 96 inch Hi-Pass Static Mixer which can rapidly mix the chemicals together. The injectors will also require to pump the chemicals listed in Table 4 after the disinfection process before it is pumped into the water reverse tanks. Cylinders containing 1000 kg the chemicals can be readily purchased and delivered to site. 124 Eric Do

138 Chapter 4: Proposed Plant Power Engineering This section presents an assessment of the energy requirements of the plant. It is first necessary to define the type of energy that the plant requires which then leads to identifying and comparing the possible sources of energy taking into account the proposed location of the plant. Once this has been determined a proposed source of the energy for the plant and an implementation can be concluded using the IEEE electrical distribution planning procedure Plant Energy Requirements The type of energy the plant requires depends on the processes that are used, for example a Multi- Stage Flash plant requires thermal energy which can be generated or can be a by-product of energy generation. In this case the desalination plant would ideally be located close to a power station to take advantage of the waste thermal energy. As described previously the main process of the plant will be reverse osmosis. Reverse osmosis is a pressure driven process that requires high-pressure pumps that require electricity to operate. This makes electricity the primary source of energy for the plant and all pre and post-treatment process also require electrical energy as the primary source Sources of Energy This section discusses the possible sources of energy for the proposed plant. This includes an assessment of the power infrastructure and the options regarding renewable technologies in Singapore National Grid The national grid of Singapore is well developed and provides a reliable service. As mentioned previously, Singapore has no resources and relies on the imports of the raw materials for power generation in the form of liquefied natural gas and coal. Singapore has the capacity to produce more than the current national demand of electricity. There is no import or export of electricity. Currently Singapore has a 12,300 MW licensed capacity. Singapore has recently changed the energy sector from a regulated system to an unregulated system. This could be advantageous as an unregulated energy sector should promote competition between the energy companies and lead to lower prices for the customers. An area of risk associated with purchasing electricity from the national grid is that if political issues arise between Singapore and the countries it relies on for power generation resources, it could lead to a shortage of power leading to the proposed plant being unable to operate. 125 Marco Volino

139 Chapter 4: Proposed Plant Renewable Options A major consideration in any modern plant is if it can be powered using renewable energy. In Singapore the National Climate Change Committee (NCCC) has been charged with identifying suitable renewable energy options suitable. Table 32 summarises the conclusions of the NCCC regarding various types of renewable energy technologies. Table 32: Pros and Cons of Renewable Energy Technologies Technology Solar Wind Hydro Fuel Cells Details Solar Power requires direct sunlight in order to function. The major disadvantage associated with using solar power in Singapore is that it would require a large solar array and land in Singapore is expensive. Singapore does not have wind to justify the use of wind power. Hydro plants require hills or mountains and an abundance of water of which Singapore has neither. Fuel Cells are a clean source of energy with the only waste products being h 2 o and heat. The fuel cell can run in excess of 600 C. Based on the recommendations from the NCCC, the preferred source of renewable energy would be to use a fuel cell plant. A company called Fuel Cell Energy, designs and manufactures fuel cell plants, one of which is scalable up to 50MW, as shown in Figure 56. This is an option that can be considered for future expandability. Figure 56: Fuel Cell Energy Scalable Plant (Fuel Cell Energy, 2010) 126 Marco Volino

140 Chapter 4: Proposed Plant Energy Options based on Proposed Location The proposed site location is in the south Tuas area of Singapore which is an industrialised area of the island. The site is located between two power plants 1. Tuas South Incineration Plant which has a capacity of 80MW 2. Tuas Power Station which has a capacity of 2,670 MW Also the site which has been selected is large enough for it to incorporate a hydrogen fuel cell plant at a later date should this become a viable option Design Procedure This section goes through some of the design procedure for distribution systems which was described previously with the aim of producing a preliminary design for the power system for the plant Load Survey Figure 57 shows a rough site map with the major loads for each process being identified. Substation UV Disinfection 688kW Remineralisation General Reverse Osmosis kw Rapid Gravity Sand Filter Nano filtration kw DAF 3213 kw Inlet Station kw Figure 57: Load Survey with Site Map 127 Marco Volino

141 Chapter 4: Proposed Plant Demand In this step, the demand for each process is assessed and a maximum power rating for the plant can be estimated. Process Table 33: Power Demand for each process Equipment Quantity per Unit Number of Units Ratting kw Total Power kw Inlet Inlet Pump DAF RO + Nano UV 118, Voltage M80FP DAF Pump Mechanical Rake High Pressure Pumps Booster Pump UV Bulb Reactor Total This aim of this step is to establish the input voltage expected from the utilities supplier and the utilisation voltages. Table 34 shows the standard voltages for transmission, distribution and utilisation in Singapore. The utilisation voltages are 230V and 400V. Table 34: Singapore Voltage Standards Use Voltage Tension Frequency 400 kv Ultra High 230 kv Tension(UHT) Transmission Extra High 66 kv Tension (EHT) Distribution 22 kv 50Hz High Tension 11 kv (HT) 6.6 kv Utilisation 400 V Low Tension 230 V (LT) To get the required input voltage from the utilities company, it is necessary to refer back to the total power demand which was found in the previous step and check the recommendation from a utility company. Table 35 shows the recommended voltage line based upon the power requirements, this was supplied by a Singapore based utility company (SP Powergrid, 2010). 128 Marco Volino

142 Main and Transfer Bus Sectionalized Bus Single Bus Chapter 4: Proposed Plant Table 35: Required Input Voltage (SP Powergrid, 2010) Required Transmission Line HT with 1 or 2 Feeders HT with 3 or more Feeders EHT UHT Minimum Power Rating 1,700 kw 12,750kW 25,500kW 85,000kW From this it can be seen that the required input voltage should be a minimum of 230kV Utility Service The first stage of applying for an electrical connection is to have a consultation with the company. During this, engineers from the power company will advise on the necessary equipment required for safe use of the electricity. The estimated time scale for installing a 230kV connection to an industrial plant with a new substation is 26 months. The customer is expected to hand over full control of the substation 8 months before the target date of the service connection. There are two requirements that are given regarding the system (SP Powergrid, 2010): For 230kV the company advises a solidly earthed system. For 230kV a 63kA (1 sec) switchgear is required Systems In this section the available substation layout are assessed based upon their advantages, disadvantages and price Substation Layout Table 36: Advantages and Disadvantages of different substation layouts Taken and adapted from (Nack, 2005: p 9) Type Advantages Disadvantages Price Lowest cost, Smallest Area required, Easy to Expand, Simple operation, Relatively simple for the application of protective relaying Flexible operation, Isolation of bus sections for maintenance, Loss of only part of the substation for a breaker failure or bus fault lowest reliability, Failure of a circuit breaker or a bus fault causes loss of substation, Maintenance switching can complicate and disable some of the protection schemes and overall relay coordination Additional circuit breakers needed for sectionalizing, thus higher cost, Sectionalizing may cause interruption of non-faulted circuits 100% 122% Maintain service and protection during circuit breaker maintenance, Reasonable in cost, Fairly small land area, Easily expandable Additional circuit breaker needed for bus tie Protection and relaying may become complicated, Bus fault causes loss of the entire substation 143% 129 Marco Volino

143 Double Breaker- Double Bus Breaker-and-a-Half Ring Bus Chapter 4: Proposed Plant Flexible operation, High reliability, Double feed to each circuit, No main buses, Expandable to breaker-and-a-half configuration, Isolation of bus sections and circuit breakers for maintenance without circuit disruption Flexible operation and high reliability, Isolation of either bus without service disruption, Isolation of any breaker for maintenance without service disruption Double feed to each circuit, Bus fault does not interrupt service to any circuits, All switching is done with circuit breakers During fault, splitting of the ring may leave undesirable circuit combinations, Each circuit has to have its own potential source for relaying, Usually limited to 4 circuit positions, although larger sizes up to 10 are in service. 6 is usually the maximum terminals for a ring bus One-and-a-half breakers needed for each circuit, More complicated relaying as the centre breaker has to act on faults for either of the 2 114% 158% Flexible operation and very high reliability Isolation of either bus, or any breaker without disrupting service Double feed to each circuit No interruption of service to any circuit from a bus fault Loss of one circuit per breaker failure All switching with circuit breakers Very high cost 2 breakers per circuit 214% Based on the advantages and disadvantages of the different substation layout, it has been decided to use the double bus double breaker as it is reliable and well protected against faults. Figure 58 shows an example of a double bus double breaker layout. The only disadvantage of this system is the cost as every circuit which is added requires two circuit breaker, but due to the nature of the plant the extra cost is justified. Figure 58: Substation Layout using double bus double breaker (Jcmiras, 2006) 130 Marco Volino

144 Chapter 4: Proposed Plant Distribution Arrangement There are several different arrangements which can be used for distributing electrical power. Radial Parallel Ring Interconnected Closely Interconnected Figure 59: Distribution Arrangements (Boal and Taylor, 1966: p 2) Table 37 presents a comparison of the different types of distribution arrangements. Table 37: Comparison of Distribution Arrangements Adapted from (Boal and Taylor, 1966) Type Radial Parallel Ring Interconnected Closely Interconnected Details Cheap to implement but the if a fault occurs there is no alternative supply Improved reliability of supply over radial particularly if the circuits follows different routes Gives an alternative supply should one fail A common development of the simple ring circuit Interconnects all circuits in a heavily loaded area Generation Due to the nature of the plant it is important that there is an uninterrupted supply of electricity, regarding generation it has already been pointed out that a fuel cell plant could be used to supplement the mains supply. Another consideration is whether or not to have a backup generator Equipment Locations As previously mentioned the load survey, Figure 57, shows the position of the major loads on the site. The substation is also shown in the diagram, located closer to the most energy intensive processes, namely the nanofiltration and reverse osmosis modules. 131 Marco Volino

145 Chapter 4: Proposed Plant One-Line Diagram Figure 60 shows a basic distribution system with the power required by each section being supplied using three phases at a predefined utilization voltage. 230kV 6.6kV 400V 25A 400V 7.835A 400V 400V 261A 400V 649A 400V 0.42A Inlet 10000kW DAF 3134kW Sand Filter NANO kW RO kW UV 168kW Figure 60: Basic Distribution System Adapted from (IEEE, 1976: p 196) Power Factor and Power Factor Improvement The power factor is a ratio of the real power to the apparent power, it is dimensionless and is usually expressed as a percentage as it ranges from 0 to Marco Volino

146 Chapter 4: Proposed Plant Almost all useful loads are inductive and in the case of a desalination plant, this will be the case due to the use of many high pressure pumps. In order to improve the power factor, the capacitive load is place in parallel to the main load. One of the consequences of having a low power factor is that the system must carry a higher current. The utility companies measure and charge for the reactive power which makes it cost effective investing in power factor improvement Waste and Brine Disposal This section discusses the alternatives available for brine disposal from seawater desalination plants, the key environmental aspects of each option and the best possible option for this situation. The three most used methods for brine disposal from seawater desalination plants are discussed in more detail below (Voutchkov, 2010). 1. Direct discharge into the sea through a new outfall Over 90% of the largest desalination plants in the world design and build a new outfall into the sea to dispose of the brine. Some example of seawater reverse osmosis desalination plants include the already existing plant in Singapore, Tuas that produces 136,000 m 3 /day of drinking water, plant in Ashkelon with a capacity of 330,000 m 3 /day and 64,000 m 3 /day plant in Larnaka, Cyprus. The main aim of this direct discharge is to dispose of the concentrate in an environmentally friendly way but at the same time keeping the area of discharge to a minimum as to not harm any sea life. There are two methods of speeding up the mixing process of the brine in the ocean. One is to rely on the mixing capacity of the tidal zone or discharge the brine beyond the tidal zone and use diffusers at the end of the outfall pipe to aid the mixing. An example of an outfall system with diffuser can be seen in Figure 61. When the concentrate is released from the ports in the diffuser they have to be at a very high velocity, typical 5-8 m/s, so that adequate mixing is achieved (Voutchkov, 2010). The tidal zones generally provide better mixing as there is a lot of turbulent energy available but the capacity they can dissipate is limited. It is also more economical than building a new outfall pipe with a diffuser on the end. However, if the amount of brine disposed is over the threshold of the tidal zone s salinity load transport capacity, then the salt excess would start to deposit and would lead to an increase in the salinity level beyond the limit of which sea life can tolerate. Therefore, it is advised to discharge the concentrate in the tidal zone if it has the capacity to transport and mix the amount disposed of. The mixing and transport threshold of the tidal zone can be determined using hydrodynamic modelling which is outside the scope of this project. The specifications of the outfall pipe and diffuser can also be determined using hydrodynamic modelling which is not going to be included in this report. 133 Elena Rusu

147 Chapter 4: Proposed Plant Figure 61: Outfall pipe with diffuser at the end (Voutchkov, 2010: p 12) There are some challenges associated with this method of disposal, mainly choosing the best location for the new outfall structure (Voutchkov, 2010): The area must not contain endangered sea life Strong currents are needed to quickly mix and dilute the concentrate Areas with a lot of naval ships traffic should be avoided as this could disrupt the dilution patterns and damage the structure of the outfall The area must be far off into the ocean to ensure good mixing but close enough to the shore to minimise the construction costs Another major disadvantage is the cost associated with designing and building a new outfall, which is typically in the range of 10-30% of the total capital cost for a desalination plant. A lot of engineering and environmental studies are also required to implement this option in the existing surroundings. Some of the benefits of having a new discharge outfall are that this option can be used by almost any size of desalination plants and that there is more freedom when choosing the location compared with the other options where the location and capacity depend on where the existing outfall systems are situated. 2. Discharge through existing wastewater treatment plant outfall This option can only work if there is an existing wastewater treatment plant near the desalination plant and it must have a big enough outfall to accommodate for the extra concentrate. Direct discharge through an existing wastewater treatment plant outfall has not been used many times because of its limited application. The largest plant practicing this method is the SWRO Barcelona plant in Spain, which produces 200,000 m 3 /day of drinking water. One of the main problems encountered is the toxicity effect of blending the wastewater effluent with the 134 Elena Rusu

148 Chapter 4: Proposed Plant concentrate. The wastewater effluent originates from fresh water while the concentrate has seawater origin which means the fresh water has different ion ratios to the seawater and blending the two could lead to an ion imbalance that can have a toxic effect on sea life when discharged into the ocean. Therefore, these effects caused by the ion imbalance must be investigated to make sure that the organisms near the discharge area are not harmed. Because of the potential poisonous effect of blending the two effluents, the amount of concentrate discharged may be limited to very small flows. Another limitation could be that if the effluent from the wastewater treatment plant is usually used for irrigation then this option would not be viable due to the high salinity content of the concentrate that would damage the crops. One other problem that could arise from blending the two effluents is that the high salinity content of the concentrate may cause some particles and contaminants in the wastewater to aggregate and form different size floccs. As a result, sedimentation would increase along with certain metals present that can have a negative impact on the specific organisms in the discharge area. One concern that should be considered when evaluating the feasibility of blending concentrate with wastewater effluent is the possibility that the existing outfall may need to be modified to cope with the buoyancy changes of the mixed plume. The concentrate is much heavier than the wastewater effluent so it will reduce the overall buoyancy and the mixing energy provided by the existing diffuser may not be enough so the structure may need modifying. To see whether the existing outfall can generate the required dispersal, hydrodynamic modelling should be used. There are some benefits associated with this option such as it is a simple procedure and there is no need to construct a new outfall which saves money. Blending the two effluents promotes rapid mixing and the concentrate is diluted by the wastewater effluent. 3. Disposal with cooling water from existing power plant collocation This option is only feasible if there is a coastal power plant near the desalination plant. The feed to the power plant is seawater and is screened before being pumped to the condensers. The screening system is usually made up of fine screens and bar racks which are similar to the ones used for the seawater intake for desalination plants. This means there is no need to build a new intake structure with pipeline and screening equipment, which would save on capital cost as the construction cost for a new intake system is around 5-30% of the overall cost of building a desalination plant (Voutchkov, 2010). An environmental benefit of sharing intake structures is that less sea and shore areas are used along with less entrapment of sea life in the screens as the same volume of seawater is utilised by 2 plants. Usually, the waste from the screens is either sent to a landfill or is put back into the sea but if it is disposed in the cooling water stream, which is the feed to the desalination plant, then there is a need for screens which would cost money but it still wouldn t be as expensive as building a new intake system. 135 Elena Rusu

149 Chapter 4: Proposed Plant Mixing the concentrate from the desalination plant with the cooling water from the power plant will reduce the salinity content of the total discharge by dilution which removes the need for a complex and expensive diffuser structure as seen in Figure 62. The waste cooling water from the power plant is warmer than the concentrate and therefore is much lighter and will float on the surface of the sea. The heavier concentrate will pull the cooling water downwards and therefore the whole sea water column will participate in the dissipation process. The result is that it takes less time for the discharge to disperse and a smaller area is needed. Figure 62: Concentrate discharge using an existing outfall of a power plant (Voutchkov, 2010: p 12) The feed water to the desalination plant is the waste cooling water from the power plant which is warmer than ambient seawater by about 5-10 C. This has a significant effect on the power demand for the desalination plant because the feed pressure to a reverse osmosis membrane decreases 6-8% for every 10 C increase in the feed water (Voutchkov, 2010). This will reduce the energy required for the process and the costs associated with water production as power costs make up 30-40% of the overall costs. The main concern of using the cooling water from the power plant as the feed to the desalination plant is the quality of that water. If the cooling water contains iron, nickel or copper ions in a quantity higher than those present in seawater, then it cannot be used as feed water as the metals present can cause irreversible fouling of the membranes. Collocation of the seawater desalination plant with the power plant could further reduce power costs as it can be connected to the electricity source for the power plant which would save the need for using the power grid. The power plant can also benefit from this as they are gaining a new customer that is reliable and has a continuous demand for high power which generates a profit. For this option to be viable and cost effective, the cooling water from the power plant must 136 Elena Rusu

150 Chapter 4: Proposed Plant to be several times bigger than the production capacity of the desalination plant. This is to ensure there is enough cooling water to be used as feed water and as discharge to dilute the concentrate Other disposal methods There are some other disposal options but they are not as widely used as the ones mentioned above. Deep well injection is one of them and it involves injecting the concentrate thousands of feet below the surface, into an injection zone. This zone is naturally high in TDS levels so the high salinity content of the concentrate would not harm the ground. The injected concentrate does not migrate to the surface because the injection zone contains many thick and impermeable layers of rock which permanently store the waste. Some of the issues associated with this method of disposal are risk of leakage from the well, risk of contaminating the ground water if the area of the well is not separated from the water supply, high costs for the construction and monitoring of the well and additional storage for the concentrate when the well requires maintenance. The location is limited to areas that have the conditions necessary to store the concentrate and cannot be placed in areas with high seismic activity as an earthquake could cause the well to leak into the groundwater. Evaporation ponds are artificial and rely on the natural evaporation of the concentrate (Public Health and the Environment World Health Organization, 2007). This method of disposal is suitable for only small desalination plants because the evaporation process is dependent on sunlight and it usually takes a long time. There is a risk of the concentrate contaminating the fresh water nearby so the ponds must be lined to avoid this problem. While the concentrate evaporates, holding ponds are needed to store the on-going concentrate produced by the plant. A lot of land is also required for the ponds and since Singapore does not have a lot of land available this method is not viable. Irrigation is another disposal method that is not widely used because of the limitations associated with it. There are not a lot of crops that can survive the high salt content of the water. The frequency of irrigation depends on the climate of the country and the land available so this option is restricted to small desalination plants. When irrigation is not needed, some other way of concentrate disposal must be used. There are long term negative effects of adding salt to the soil such as contamination of the groundwater, reduced aeration due to less water moving into the soil and reduced life of vegetation. Singapore has a monsoon season so during that time irrigation is not required. Also the desalination plant has a large capacity and irrigation would not be able to cope with the concentrate generated and as a result it is not feasible. Discharge of concentrate to the sanitary sewer is very popular with small brackish and seawater desalination plants. This method can only dispose of a small volume of concentrate due to the 137 Elena Rusu

151 Chapter 4: Proposed Plant negative effects of the high salt content in the concentrate on the wastewater treatment plant s operation. According to Voutchkov (2010), the biological treatment of the wastewater plant is obstructed when the TDS concentration of the effluent is greater than 3000 mg/l. Therefore to ensure this option is feasible, the effect of increasing the salinity of the effluent on the biological treatment of the wastewater plant must be investigated beforehand. The typical TDS level in the concentrate from seawater desalination plants is around 65,000 mg/l so the capacity of the wastewater treatment plant must be at least 35 times greater than the daily intake of concentrate so that the TDS concentration in the wastewater effluent does not exceed 3000 mg/l. This option along with discharge through existing wastewater treatment plant outfall are not viable in Singapore as there aren t any wastewater treatment plants near the location of the proposed desalination plant Most feasible option It can be concluded that the best method of brine disposal for the proposed desalination plant is direct discharge into the sea through a new outfall. This is because Singapore does not have a lot of land available which limits the disposal options. This method can cope with the large volumes of concentrate generated by the proposed desalination plant, 271,950 m 3 /day from nanofiltration plus 227,272 m 3 /day from reverse osmosis. It is very widely used by more than 90% of the large desalination plants so it can be seen that it is very successful. As mentioned previously in this section, there are two ways to dissipate the brine, one is to rely on the tidal waves and the second one is to build a diffuser which releases the brine at high velocity promoting the dissipation process. The average height of the high tides in January and February 2011 is around 3 m in the morning and around 1 m in the afternoon for low tides (Singapore Government, 2008). The mixing capacity of the tides must be determined in order to know if the tides can cope with the high salinity brine. If a 3 m tide has the capacity to dissipate the brine then the diffuser would only need to be used when the tide is low, i.e. 1 m, which would reduce the operating costs. The specifications and structure of the diffuser can be determined using hydrodynamic modelling which is outside of this scope. A good starting point is the specifications of the outfall design for the 130,000 m 3 /day Perth desalination plant. Voutchkov (2010) mentions that the diameter of the outfall for the Perth plant is 1.2 m with a 160 m long, 40 port diffuser that is 470 m from the shore, at a depth of 10 m and a discharge angle of 60. The ports have a diameter of 0.22 m and they are spaced every 5 m. The diameter of the outfall pipe is determined based on the total flow rate of brine to be disposed of, 499,222 m 3 /day, and the velocity of water which is taken to be 0.75 m/s: 138 Elena Rusu

152 Chapter 4: Proposed Plant This is greater than the one for the Perth plant because the proposed plant has a higher capacity. The best material for the outfall pipe is GRP (glass reinforced plastic) which is the same for the intake pipes Control Systems The control system is a vital area in the design of a desalination plant. For health and safety reasons, it is important to control the plant and ensure all the products are of the right quality. The main control schemes used in desalination plants are: instrument and computer based control scheme Instrument control scheme The basic plant parameters to be controlled are: pressure drop across the reverse osmosis and nanofiltration membranes, concentrate flow velocity, ph, feed temperature, system pressure and permeate quality. Generally, SWRO plants are controlled by measuring the flow rates of the permeate and the concentrate. This will indicate whether the feed pressure needs to be increased or decreased in order to maintain a constant recovery. The feed pressure is controlled by a valve on the concentrate line so for example if the quality of the permeate declines then more pressure must be applied to the feed to increase the recovery to the initial starting point. This valve is a needle valve and it generates the high pressures needed for the system to operate at high efficiencies. A ph value above 3 and below 8 must be maintained at all times (Ulrich, 1966: p 209) for the membranes. This is done with a ph indicator; an instrument used to monitor the ph of the permeate. If at any point the reading is either lower or greater than the recommended ph, the permeate is treated with lime in the post treatment section of the plant. Before the membrane units can be controlled, checks must be taken to ensure the system is working efficiently by monitoring certain parameters such as flow, pressure and temperature. Flow rate must be monitored to ensure that the salts in the concentrate stream do not exceed their saturation point as they would scale and foul the membrane. This can be performed by installing flow meters on the feed, permeate and concentrate line and taking weekly readings (American Water Works Association, 1999). Permeate quality is monitored by taking samples on a regular basis and testing it in the laboratory. 139 Group effort

153 Chapter 4: Proposed Plant The right concentrate flow is required in the system to prevent the accumulation of salt with subsequent poor water quality and/or precipitation in the membrane modules. The higher the temperature of the feed, the lower the water viscosity which decreases the pressure required for the water to move through the membrane. However at high temperatures, above 45 C, the membranes stop working because the materials cannot withstand those temperatures. For this reason, it is important to control the temperature of the feed. Some typical instruments used in the control of reverse osmosis plants are listed in Table 38:. Table 38: Typical instruments part of the control system for a reverse osmosis plant (American Water Works Association, 1999: p 34) Stage Parameter Instrument Pre-treatment Feed water temperature Feed water conductivity Feed water turbidity Feed water ph Scale inhibitor flow Indicator Indicator Indicator Indicator Controller RO train Post-treatment Inlet pressure Permeate flow Concentrate flow Permeate conductivity Concentrate conductivity Membrane pressure pump (motor) Permeate conductivity Permeate ph Concentrate ph Indicator Indicator Indicator Indicator Indicator Hour meter, event counter, indicator Indicator Indicator Indicator Computer based control scheme This involves the use of low cost PC based SCADA and Lab View systems. The SCADA system is a computer based simulation which monitors data hourly. It is used for data acquisition, logging and control of the plant variables such as temperature, ph, and pressure (Torky, Elamvazuthi and Hazrin, 2009). The three most important control variables for a reverse osmosis plant are flow, pressure and conductivity. Flow is extremely important because it will change if membrane foul or tear due to chemical or microbiological attack (Torky, Elamvazuthi and Hazrin, 2009: p 444). The plant is monitored with electrical devices and sensors like level control switches, level probes and thermocouples. These electrical devices are connected to a DAQ card (Data Acquisition) which sends Input /Output signals to a computer. This way, all the results and graphs are displayed in the lab so that further analysis can be carried out. SCADA works in conjunction with a system called Lab View. Lab VIEW contains a comprehensive set of tools for acquiring 140 Group effort

154 Chapter 4: Proposed Plant analyzing, displaying, and storing data, as well as tools to help you troubleshoot the code (Torky, Elamvazuthi and Hazrin, 2009: p 443). Lab View consists of two main components; controls and indicators. Controls include knobs, push buttons, dial and other miscellaneous buttons while indicators contains all the LED, live data graph and other displays. The SCADA and Lab View systems monitor all the variables throughout the plant from the control room. To ensure that accurate results are obtained with computer based scheme, samples are taken on a regular basis by workers and analysed in the lab. 141 Group effort

155 Chapter 5: Health and Safety 5. Health and Safety 5.1. General working precautions All confined spaces throughout the plant should be identified. A confined space is an area large enough for an employee to fit inside but small enough to make it difficult to exit. The confined spaces have been identified as risk hazards due to the following reasons (American Water Works Association and American Society of Civil Engineers, 2005): 1. There is a chance an oxygen deficient environment is created due to restricted ventilation. 2. An employee can get trapped and even suffocate in a tank. 3. The space has converging walls that could trap an employee. Safety equipment must be used by the workers in the plant and it must be easily cleaned and disinfected. A safety team should perform regular checks in order to maintain the quality of the safety equipment. Typical safety equipment includes gloves, safety goggles, safety shoes, hearing protection, face protection and helmets. The workers should be given adequate training to operate all the NF and RO equipment properly. It must be made sure no one is incompetent in handling any sort of machinery (American Water Works Association and American Society of Civil Engineers, 2005). Respirators or masks must also be provided to all the staff and it must be made sure that it fits properly. The type of mask depends on the type of dust particles. Masks are generally used for occasional dust exposure. It is vital that these face masks fit properly on the worker's face and annual tests must be carried out to ensure that correct size masks are being used. Safety showers and eye washes must be located all around the plant in case of accidents. They should be specific for each type of chemical and must also provide facility for full body wash as well. Safety screens can also prevent staff injuries by indicating areas of plant which possess threat to the workers. These locations are identified as a joint collaboration between the plant facility staff and design team. The plant must also have first aid equipment and staff with first aid training present in the plant. The proposed plant should have all the necessary equipment to extinguish fires. Fire exits should never be blocked and different types of fire extinguishers should be located throughout the plant. The plant must also have adequate ventilation for the confined spaces. The contaminated areas must also be isolated by exhaust system from the rest of the plant. An evacuation plan, in case of an emergency, should be in place and should be rehearsed regularly (American Water Works Association, 1999). 142 Group effort

156 Chapter 5: Health and Safety 5.2. Handling Chemicals A variety of chemicals are being used in the water treatment plant including sodium hypchloride, lime, and chlorine. These chemicals are hazardous and extra precaution must be taken to handle these chemicals carefully whilst wearing Personal Protective Equipment. The manufacturers should provide data sheets with each chemical provided to the plant. Generally, it is a good idea to have storage for chemicals for 7-30 days on-site but chlorine is seldom stored on-site as it is extremely toxic and dangerous to store in large amount. Instead, it s transported from shipping container. A pressure differential must be maintained between the shipping container and the storage container during transfer (American Water Works Association and American Society of Civil Engineers, 2005: 15.3). Chlorine is commercially available in 4 types of containers but it doesn't have one exact standard size. These containers are filled with 85% of the volume chlorine and the rest of 15% allowance leave room for the expansion of the gas. Chlorine tanks must be placed away from source of heat as it would expand, causing tremendous hydrostatic pressure which may cause rapture. As a safety precaution, cylinder outlet valves are equipped with a small fusible plug that melts at 70 C (American Water Works Association and American Society of Civil Engineers, 2005: 15.4). This releases some liquid chlorine which cools the container before serious damage occurs. It is to very important to follow the health & safety guideline strictly during chemical disposable procedure. It must be made sure that chemical disposable procedures are followed properly and the manufacturers should be contacted for extra recommendations when issues occur. All protective gear must be worn when handling with acids and caustics. Emergency and containment procedures for spillages should be determined and practised. 143 Group effort

157 Chapter 6: Sustainability 6. Sustainability 6.1. Environmental Construction phase When designing a desalination plant certain measures have to be incorporated to minimise the impacts on the environment and the human population. The next step after design is building the plant which will involve a lot of noise, dust and many trucks carrying pieces of equipment which could disturb the landscape and will contribute to air pollution. These impacts will only last for a short period of time but they still have to be taken into consideration. This can however be neglected as the proposed location is situated in an industrial area in the South Tuas area of Singapore away from residential areas Operational phase The impacts that tend to last for longer are associated with the plant operation but they can be avoided if the plant is well designed, well-constructed, and properly maintained and operated (Water Treatment Plant Design, 2005). The main issues are related to the intake of feed water and waste disposal. There are a number of problems that can be caused by the withdrawal of water such as entrainment of fish, change in depth due to a reduction in water quantity which would result in a temperature change that can harm sea life and conflicts associated with other consumers using the same source. Most of the impacts do not apply to the proposed desalination plant because the feed water is taken from the sea and the amount removed does not make a difference to the sea level. To reduce entrapment, a slower intake velocity is used which will allow the fish enough time to get away. Typical problems associated with the disposal of brine into the ocean are (Voutchkov, 2010): Need to make sure the salt level in the discharge area does not exceed the threshold that the sea life can tolerate Make sure any chemicals used in the pre-treatment would not harm the aquatic organisms Ensure the outfall structure and installation does not damage the aquatic life on the ocean bed Check the concentration of metals ions is not at a level where it can harm the sea life Make sure the brine disposed of is compatible with the water in the ocean so that the ion ratios are not disturbed which can lead to toxicity Tests must be performed to check the toxicity effect of the brine on the sea life. Different types of aquatic organisms will be affected differently depending on the level of salt, the exposure time and how far away they are from the discharge area. The most sensitive and endangered species should be 144 Group effort

158 Chapter 6: Sustainability tested as they are the most likely to be harmed. Voutchkov (2010) recommends performing tests on a minimum of 3 species: a type of fish, a type of invertebrate and a type of algae. These species are usually chosen by a marine biologist who is a specialist in the aquatic life in the discharge area. These tests should be completed at different salt levels including the highest level that may form in the worst-case scenarios. The results of the tests will indicate whether open ocean disposal is a viable and environmentally friendly option Social Social aspects involve educating the population in how to preserve water resources. At the moment, there are many TV programs such as cartoons and publications available to the public in Singapore. One of the lifestyles magazines entitled PURE, gives information about conserving water, with young adults and teenagers as the main target audience. Wally Water, the official PUB mascot, delivers information about saving and re-using water through a cartoon series Economic The proposed plant not only provides jobs in the local region but also helps the local market. During the construction phase of the proposed plant, which is labour intensive, a lot of jobs would be created for the local community. There is a great opportunity for the local market to provide the essential equipment and chemicals required. The proposed plant would also contribute to the economy of Singapore by paying taxes on the profit generated and also by indirectly providing fresh water to the growing industry which is water intensive. The industry in Singapore would also benefit from this. 145 Group effort

159 Chapter 7: Project Finance 7. Project Finance 7.1. Project delivery method The proposed plant delivery method is Design-Build-Own-Operate and transfer (DBOOT). The major risk associated with this project delivery method is the constant change in inflation rate. Predicted inflation rate (P.I.R) increases with increase in inflation and this is classified as risk because the interest rate will change as a result of increase in inflation. The cost of a desalination plant is broken down into operating and capital cost. Capital cost includes purchase cost of equipment, auxiliary cost, land and installation cost while the annual operating cost is the cost of owning and running the plant Project Evaluation Table 39: cost analysis for SWRO desalination plant Fixed capital $ , Working capital cost $ Fixed cost $3,227,710 Direct cost $33,131, Indirect cost $8,050,821 Total annual expense $49,805,163 Cost of water $0.5458/m 3 Sales price $0.82/m 3 The fixed capital cost is estimated to be $189,865, and the working capital is 15% of the fixed capital cost (Gerrard, 2000: p.39) which is $28,479, This two are added together to give the capital investment. The money required for this project will be borrowed from the bank at a certain interest rate. Interest rate is calculated using goal seek by setting the value to the NPV to zero. 146 Irene Usiomoifo

160 Chapter 7: Project Finance The fixed cost, direct, indirect and the other costs that makes up the annual operating cost were estimated with the cost percentages, for example plant insurance is (0.7% of fixed capital cost) (Holmes, 2010: p.11). (See spread sheet in appendix 5) The annual operating cost of the plant was calculated to be $49,805,163. From this the cost of water produced is calculated as shown below: ( ) ( ) It was assumed that the plant operates continuously even during maintenance (only half of the membranes are cleaned at a time), therefore the plant availability is taken as 100%. The selling price of the potable water was estimated based on the assumption that $50 profit is made per day of plant operation; see spreadsheet for details calculation Potable water sales cost= ( ) ( ) ( ) ( ) It takes approximately 9 years to breakeven; a point when the NPV value becomes positive. Net present value for two different plant operation period; 10 and 20 years was calculated (see spread sheet and graphs in appendix 5). 147 Irene Usiomoifo

161 Chapter 7: Project Finance From the result obtained; NPV for 20 years period is $1.12.Since this value is greater than zero, which means that profit is made. The interest rate is 6% For the 10 years period, the NPV is 0 and the interest rate is -1%. It is therefore not advisable to invest in this project or 10 years. Since the plant operating life is 20 years and the NPV is greater than zero, it is economically feasible to invest in this project because it makes profit. The project is transferred to the government when the contract ends. Assumptions The economic evaluation of this project was based on the following assumptions: 13% of membrane module cost Inflation and cost of finance remind fixed Cost of maintenance is constant Plant availability is 100% 148 Irene Usiomoifo

162 Chapter 8: Conclusions and Recommendations 8. Conclusions and Recommendations 8.1. Conclusions The total water output capacity The proposed water plant has been designed to produce a maximum of 250,000 m 3 /day of potable water. If this production rate is maintained then it would be able to meet the predicted water demands in 2033 with ease. Intake For the proposed plant it was decided to use direct sub-surface intake based on the comparison mentioned in Section 3. Two parallel inlet pipes are used, each with a diameter of 2.85m. If maintenance is taking place on one of the inlet marine pipe, the other one would be in operation thus giving more flexibility to the intake facility. At 500m away from the shoreline, the quality of water is better than available from the open surface intake because it contains a lower proportion of total dissolved solids; this reduces the cost of the pre-treatment. The preferred choice of material for the intake pipes is glass reinforced plastic because this material is suitable for the corrosive high salinity feed. The type of screening proposed for the plant is passive screens. This type of screening helps to reduce the impingement and entrainment of marine life by 80% due to the relatively small intake velocity from passive screens. These passive screens are self-maintained so there is no requirement for manual cleaning. Two passive screens were attached to the inlet of each pipe to meet the maximum flow intake (826,000m 3 /day). Also three horizontal centrifugal pumps (and one extra pump on stand-by) arrangement was chosen from the three configurations options available. The design ensured that the inlet seawater was extracted from 500m away from the shoreline and pumped to the storage tanks at minimal cost without negative environmental impact on local marine life. Nanofiltration Nanofiltration has only been used as a form of pre-treatment to reverse osmosis in a few pilot plants. The results obtained indicated that the use of nanofiltration reduces the hardness ions by 93.3% while also removing monovalent ions by 40.3%. This resulted in an overall TDS reduction of 57.7%. As a result the feed to the reverse osmosis unit was of better quality than the intake seawater which made it possible for the unit to run at a higher recovery rate of 55%. Because the hardness ions are removed there is a lower chance of scaling occurring which saves on the operating costs as the membranes are cleaned less often. 149 Group effort

163 Chapter 8: Conclusions and Recommendations The nanofiltration unit was simulated using the ROSA72 software. The required operating pressure was found to be 21 bar which is half the pressure for the reverse osmosis, so less power is needed to operate the high pressure pumps; 24,200 kw. A two stage single pass configuration was selected to obtain the desired recovery rate of 65%. The first stage is split into 33 trains, each made up of 12 by 12 pressure vessels (see Section 3.3) while the second stage is made up of 27 trains. The feed water enters the first stage and the concentrate leaving is sent to the second stage where it is further purified. The permeate from both stages is then sent to the reverse osmosis unit. The overall concentrate generated from both stages is mixed with the brine from the reverse osmosis unit and disposed of into the sea through a new outfall structure. Membrane fouling was found to be the biggest problem associated with nanofiltration. Cleaning is required when the flow of permeate is reduced by 10% or the pressure of the feed increases by 15%. There are 3 different types of fouling that can form: inorganic, organic and biofouling. They all require different cleaning procedures and the optimum type is determined after several years of testing on pilot plants. Care must be taken not to use the wrong chemicals as that can damage the membranes adding to the operating costs. Reverse Osmosis The performance of the reverse osmosis system was examined by considering two different design alternatives. The result of the analysis showed the importance of energy recovery devices and booster pumps in increasing water productivity and decreasing the amount of energy required. The use of energy recovery systems and booster pump reduces energy consumption from about 7.87kWh/m 3 to 2.81kWh/m 3. Based on the permeate quality and overall recovery, the preferred choice of design is a two-pass system with concentrate recycle because the permeate that is recycled has a better quality than the feed water, hence the operating cost is reduced due to decrease in the operating pressure. The potable water produced meets the drinking water standard in Singapore in terms of boron content (0.45mg/L) and the permeate contains fewer total dissolved solids. This was achieved with the use of a brackish water membrane (BW30-400) operated at a high ph and high recovery rate. The total cost of water produced by reverse osmosis technology is usually affected by two factors; energy consumption and membrane replacement cost. However, the use of more efficient energy recovery systems and improved membrane cleaning techniques have made the reverse osmosis desalination process the most widely used technology for production of less expensive potable water. 150 Group effort

164 Chapter 8: Conclusions and Recommendations Post treatment The post treatment process is separated into two categories, including disinfection and remineralisation. It was decided that the disinfection method would be include UV disinfection as the primary disinfection technique and chlorination as the secondary. The result shows the flow rate through each pipe would be 3950 m 3 /hr. At a velocity of 0.75 m/s, seven medium pressure UV lamps are required to achieve log-4 reduction of micro-organisms in an 800 mm diameter pipe. The UV dose is 212 mj/cm 2 which is capable of removing rotavirus. Each lamp has a high power rating of 6000 W. The secondary disinfection treatment uses sodium hypochlorite with a Ct value of 16.5, as a precaution against any reactivation of micro-organisms that were disinfected in the UV disinfection. The minerals that will be added back into the water will be within the WHO drinking water guidelines and the minerals will be beneficial to human health i.e. fluorine for dental benefits. The total costing The total project cost for the proposed desalination plant is $215,000,000. The operational cost is $49,000,000/annum; meaning that the cost of the fresh water generated by the plant is is $0.54 /m 3. However, the plant will be expecting to sell the water at a price of $0.81 /m 3 therefore; the total profit per year is approximately 25 million. Moreover, it will take approximately 7 years in order for the plant to reach its break-even point. This is a good investment, as the plant will start to make a profit approximately half way into the plant s design life. The proposed plant is therefore economically feasible. Technical Uncertainties The technical uncertainties include the inflation rate. As the inflation rate increases, the interest rate increases proportionally, this in turn reduces the profit margin. In order to maintain a constant profit margin the selling price of water may need to be increased. The insurance of the water treatment plant is unknown. 151 Group effort

165 Chapter 8: Conclusions and Recommendations 8.2. Recommendations Dissolved Air Floatation Regarding Dissolved Air Flotation the recommendations are as follows: The equipment which has been selected is to be provided by World Water Works. Based upon the required flow rate it was found that using 30 of the RSP-13L modules would treat the correct volume of water and consumer the least energy. It is also recommended that an extra five units be installed at the plant as spares; this will allow periodic maintenance of units and provide contingency in case of faults and breakdowns. Nanofiltration The recovery of the nanofiltration unit is 65% for the proposed seawater desalination plant. This is not as high as the recoveries for the brackish water desalination plants, which are around 80%. The reason for this is that seawater contains a high level of salts that can precipitate onto the membrane once they reach their solubility limit so a lower recovery rate reduces the risks of scaling. This recovery rate has a big impact on the total cost of the plant so a recommendation would be to increase this value. This can be achieved by using certain antiscalants which can give a recovery rate as high as 75% (Al- Amoudi et al., 2004). Antiscalants are surface active materials that restrict the precipitation process so scaling is decreased. It was found that with the use of antiscalants, the total recovery of a pilot desalination plant was increased from 32.5 to 38.5% which would save a lot of money and would generate more permeate. Further research should be undertaken into the different types of antiscalant, the optimal dosage, the cost and what effect they would have on the concentrate disposal. Reverse osmosis Reverse osmosis desalination technology has experienced some drawbacks in over the last few years due to the energy intensive nature of the process as well as the problem with brine disposal. However, this problem has been recently addressed by the invention of a new desalination method known as manipulated osmosis desalination MOD (Shariff A., 2010: p.2). The process combines two-stage forward osmosis with reverse osmosis, the forward osmosis section is operated at a recovery rate of 35% and is used to dilute the seawater before it enters the reverse osmosis membrane system. This process is recommended instead of reverse osmosis alone because the arrangement enables the feed to be diluted before entering the membrane; this reduces energy consumption by up to 20% (Shariff A., 2010: p.2). Another major advantage of combining forward osmosis with reverses osmosis is that the FO membranes have high fouling and chemical resistance hence the feed water to the high pressure membrane is free of particles that can clog the membrane. 152 Group effort

166 Chapter 8: Conclusions and Recommendations The forward membrane is operated at a very low pressure (about 2 bar); this uses approximately 50% less energy than the seawater reverse osmosis membrane and produces a high permeate quality. This compensates for the water lost by evaporation thereby conserving water for other use (Shariff, 2010: p.3). Intake and Brine disposal The brine disposal method for the proposed plant is direct discharge into the sea through a new outfall. This is the best option for large desalination plants such as the proposed plant and it does not have as many limitations as the other available methods. However, it represents a large part of the total cost of the plant, 10-30%, which is associated with designing and building a new outfall structure. A recommendation is to co-locate the proposed plant with the Tuas Power Station which is only 965 m from the proposed site location as shown in Figure 63. If the power station uses seawater as cooling water then there would be no need to build a new intake structure for the desalination plant as the one for the power station could be used. The cooling water must be disposed of after it has been used; thus it is suggested the brine from the desalination plant could be combined with the waste cooling water, which would dilute the salt content and would eliminate the need for a new outtake structure with a diffuser pipe and the end. This would save a lot of money and would make the desalination plant more environmentally friendly as no extra structures need to be built which would otherwise disturb the sea life. Post-treatment Figure 63: Distance between the Tuas Power Station and the proposed site location for the desalination plant (Google, n.d.) Nanofiltration is capable of filtering and removing virus during its process, however the specific types of viruses are still unidentified and further research is required. Once the research of nanofiltration, or any other treatment process, expands to the capability of identifying the specific removal of virus, 153 Group effort

167 Chapter 8: Conclusions and Recommendations then the UV dosage may decrease therefore improvement the overall costing, maintenance and sustainability of the post-treatment process. Further recommendations of UV disinfection include the usage of a new type of UV lamp. The UV lamp, designed and sold by Trojan UV, is capable of providing high output with low energy requires.i.e. taking the best of both of the low pressure lamps and the medium pressure lamps (Trojan UV, 2010). The new lamp, called Trojan UV Solo lamp, uses approximately a third of the medium pressure lamp thereby reducing the energy consumption, the overall costing, and increasing the durability of the UV disinfection process. The technology of the Solo lamp is relatively new which was only unveiled in June 2010 (Trojan UV, 2010). Further research is required to identify its advantageous and disadvantageous before it can be used in large water treatment facilities. Power engineering It is recommended that the plant is supplied with electricity from the national grid; this was concluded from the Singapore government s assessment of renewable technologies. The government identified fuel cell plants as the most viable technology due to other technologies requiring large areas or the lack of natural features required for the technology. The site which has been selected has the space to incorporate a fuel cell plant should it be required in the future. Following the IEEE electrical distribution planning guide and SP PowerGrid s recommendation it was possible to develop a basic distribution system for the plant. With reliability and equipment protection a high priority when selecting the substation layout a double bus double breaker was chosen. 154 Group effort

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177 Appendices I Appendices Appendix 1: Water Demand Projection Shortfall Total from desalination Percentage from desalination Demand Year Shortfall Total From Desalination Targeted % from desalination Water Demand Year

178 Model # RSP-15MS RSP-30MS RSP-4S RSP-11S RSP-17S RSP-3L RSP-6L RSP-10L RSP-13L RSP-16L RSP-20L Flow m 3 /d Dimensions (m) W L H Pipe Diameters (m) Influent Effluent Sludge Weight kg Empty Model # M32NPD-15 M40NP M50SP-1 M65SP M80SP M80FP M80FP M80FP M80FP M80FP M80FP Nikuni Dissolved Air Quantity # of DAF units Table Adapted from Source: and converted to metric. Full Power kw rake kw Total kw Total Power kw Appendices Appendix 2: World Water Works DAF Unit Range and Selection This shows the range of DAF units from World Water Works and justifies the number of units required and size selection based upon requiring to treating 825,805 m 3 /day. -I-

179 Appendices Appendix 3: ROSA Results for Reverse Osmosis Case 1 -II-

180 Appendices Project Information: Case-specific: Two-pass RO system without concentrate recycle (case 1) System Details -- Pass 1 Feed Flow to Stage 1 Raw Water Flow to System m³/h Pass 1 Permeate Flow m³/h Osmotic Pressure: m³/h Pass 1 Recovery % Feed bar Feed Pressure bar Feed Temperature 30.0 C Concentrate bar Flow Factor 0.85 Feed TDS mg/l Average bar Chem. Dose None Total Active Area M² Water Classification: Seawater (Open Intake) SDI < 5 System Recovery % Stage Element #PV #Ele Feed Flow (m³/h) Number Elements of Average Pass 1 Flux Feed Recirc Flow Conc Flow Conc Press Press (m³/h) (m³/h) (bar) (bar) Average NDP bar lmh Power kw Perm Flow (m³/h) Specific Energy Conc. Flow from Pass 2 Avg Perm Flux Press (lmh) (bar) Boost Press (bar) 2.69 kwh/m³ 0.00 m³/h Perm TDS (mg/l) 1 SW30HR SW30HR Pass Streams (mg/l as Ion) Name Feed Adjusted Feed Concentrate Permeate Stage 1 Stage 2 Stage 1 Stage 2 Total NH K Na Mg Ca Sr Ba CO HCO NO Cl F SO SiO Boron CO TDS ph III-

181 Appendices Project Information: Case-specific: Two-pass RO system without concentrate recycle (case 1) System Details -- Pass 2 Feed Flow to Stage m³/h Pass 2 Permeate Flow m³/h Osmotic Pressure: Raw Water Flow to System Feed Pressure m³/h 5.22 bar Pass 2 Recovery Feed Temperature % Feed 0.21 bar 30.0 C Concentrate 1.96 bar Flow Factor 0.85 Feed TDS mg/l Average 1.09 bar Chem. Dose (100% NaOH) 7.33 mg/l Total Active Area M² Number of Elements Average Pass 2 Flux Average NDP 3.61 bar lmh Power kw Water Classification: RO Permeate SDI < 1 Specific Energy 0.19 kwh/m³ System Recovery % Stage Element #PV #Ele Feed Flow (m³/h) Feed Press (bar) Recirc Flow (m³/h) Conc Flow (m³/h) Conc Press (bar) Perm Flow (m³/h) Avg Flux (lmh) Perm Press (bar) Boost Press (bar) Perm TDS (mg/l) 1 BW BW Pass Streams (mg/l as Ion) Name Feed Adjusted Feed Concentrate Permeate Stage 1 Stage 2 Stage 1 Stage 2 Total NH K Na Mg Ca Sr Ba CO HCO NO Cl F SO SiO Boron CO TDS ph IV-

182 Appendices Case 2 Project Information: Case-specific: Two-pass RO system with concentrate recycle and boosters (case 2) System Details -- Pass 1 Feed Flow to Stage 1 Raw Water Flow to System m³/h Pass 1 Permeate Flow m³/h Osmotic Pressure: m³/h Pass 1 Recovery % Feed bar Feed Pressure bar Feed Temperature 30.0 C Concentrate bar Flow Factor 0.85 Feed TDS mg/l Average bar Chem. Dose None Number of Elements Average NDP bar Total Active Area M² Average Pass 1 Flux lmh Power kw Water Classification: Seawater (Open Intake) SDI < 5 System Recovery % Stage Element #PV #Ele Feed Flow (m³/h) Feed Press (bar) Recirc Flow (m³/h) Conc Flow (m³/h) Conc Press (bar) Perm Flow (m³/h) Specific Energy Conc. Flow from Pass 2 Avg Flux (lmh) Perm Press (bar) Boost Press (bar) 2.62 kwh/m³ m³/h Perm TDS (mg/l) 1 SW30HR SW30HR Pass Streams -V-