MODELLING AND OPTIMIZATION OF URBAN WATER SYSTEMS

Transcription

1 MODELLING AND OPTIMIZATION OF URBAN WATER SYSTEMS Gbenga Akinbola 25 TH September, 2009 Supervised by: Professor Nilay Shah A thesis submitted to Imperial College London in partial fulfilment of the requirements for the degree of Master of Science in Chemical Engineering with Process Systems Engineering and for the Diploma of Imperial College Department of Chemical Engineering and Chemical Technology Imperial College London London SW7 2AZ, UK

2 ACKNOWLEDGEMENT My sincere appreciation goes to my supervisor, Professor Nilay Shah, for his support, guidance and ideas all through the duration of this work. I also appreciate his painstaking effort to proof read the final version of the dissertation. Also, I like to thank my sponsors, Nigeria Agip Exploration, for the postgraduate scholarship award granted me to undertake my Masters at Imperial College. My gratitude also goes to my parents, siblings, friends and colleagues for their moral support and contributions toward ensuring the success of this work. Above all, I give all glory to God almighty for the good health, inspiration and divine grace I enjoyed during the course of this research and all through the entire Masters programme. ii

3 ABSTRACT This work is based on the development of a total water system which guarantees optimal water allocation in terms of quantity and quality for the various water-using activities within a typical urban water system. Mathematical optimisation strategies are employed via the development of a superstructure model which resulted in a non-linear programming problem. Engineering, as well as physical insights, based on the understanding of the system being considered complements the optimisation strategy to reduce the complexity of the superstructure model developed. The optimisation was carried out on the GAMS platform. Water-use within a residential household with 4 occupants was considered. A minimum fresh water flow rate of 240L/day was obtained. This represents a fresh water savings of about 57.36%. The annualised cost for the proposed water system was about $3119. This cost comprises capital cost of treatment units, piping cost and fresh water supply cost. A design such as this is necessary in order to address the wanton wastage characteristic of water use within urban systems. It is unique in that unlike other methods employed to save water within such systems, it is the only strategy that establishes a base minimum for fresh water use. i

4 TABLE OF CONTENTS CHAPTER 1 INTRODUCTION BACKGROUND NEED FOR WATER CONSERVATION RESEARCH OBJECTIVE RESEARCH LAYOUT... 4 CHAPTER 2 LITERATURE REVIEW BACKGROUND URBAN APPROACH TOWARDS WATER CONSERVATION: USE OF WATER SAVING GADGETS: RAIN WATER HARVESTING: GREY WATER REUSE: INDUSTRIAL APPROACH WATER PINCH ANALYSIS (WPA) FRAMEWORK: FLOW RATE TARGETING TECHNIQUE: MULTIPLE UTILITY CONSIDERATIONS: NETWORK DESIGN LATEST DEVELOPMENTS RESEARCH GAP RESEARCH METHOD CHAPTER 3 WASTEWATER CHARACTERISATION INTRODUCTION WASTEWATER CONSTITUENTS PHYSICAL CHARACTERISTICS CHEMICAL CHARACTERISTICS BIOLOGICAL CHARACTERISTICS iii

5 CHAPTER 4 WASTEWATER TREATMENT TECHNOLOGY INTRODUCTION PRELIMINARY & PRIMARY TREATMENT SCREENING SEDIMENTATION FLOTATION SECONDARY TREATMENT CHEMICAL PROCESSES BIOLOGICAL PROCESS ACTIVATED SLUDGE PROCESS TRICKLING FILTER BIOLOGICAL NUTRIENT REMOVAL (BNR) PROCESSES TERTIARY TREATMENT CHAPTER 5 MODEL DEVELOPMENT INTRODUCTION DOMESTIC HOUSEHOLD WATER-USE TREATMENT UNITS PROBLEM FORMULATION EQUATIONS AND CONSTRAINTS FOR WATER USING OPERATIONS : TREATMENT UNIT EQUATIONS OPERATIONAL CONSIDERATIONS DATA PRESENTATION OBJECTIVE FUNCTION SOLUTION STRATEGY CHAPTER 6 ANALYSIS OF RESULT FRESH WATER NETWORK DESIGN : MINIMUM COST DESIGN CHAPTER 7 CONCLUSION & RECOMMENDATIONS...69 REFERENCES...71 iv

6 APPENDIX A. FRESH WATER QUALITY REPORT 76 B. CAPITAL COST DATA FOR TREATMENT UNITS C. GAMS FILE..79 v

7 LIST OF FIGURES 1.1: Distribution of Earth s water 1 2.1: Urban water cycle 6 2.2: Household water use 8 2.3: Water composite curve : Algorithm for water source diagram : SDDC Plot : Water cascade table for flow rate targeting : Conversion of fixed flow rate operations into fictitious operations : Algorithm for NAD : Water Management Hierarchy (WMH) framework : Wastewater characterisation : Wastewater treatment methods : Cross-section of a rectangular basin with horizontal flow : Typical flotation unit : Flocculation and neutralisation chemical process : Flow diagram for Activated Sludge process : Trickling filter cross-section (a): A/O process (b): Phostrip process : Particle size separation comparison chart 46 vi

8 List of Figures 5.1: Superstructure for treatment unit : Schematic representation for water-using operations : Fresh water requirements for household water-using activity : Fresh water savings per activity : Proposed Network : Fresh water savings for minimum cost design : Variation of annualised cost with number of household : Variation of cost per household with number of household 68 vii

9 LIST OF TABLES 2.1: Flow reduction devices in urban systems 7 2.2: Highlights of some research works : Research gaps to be addressed : Physical characteristics of wastewater : Inorganic constituents of wastewater : Organic constituents of wastewater : Indicator organisms for various water uses : Different screen types : Application of chemical unit processes : Typical membrane process characteristics for wastewater treatment (a): Microfiltration performance data (b): Reverse osmosis performance data : Per capita domestic water usage : Per capita pollution loading : Potable water standard : Average removal ratio for microfiltration and reverse osmosis : Flow rate data for a 4-occupant household : Contaminant concentration per appliance : Fresh water and prescribed concentration for regenerated water : Operating data for water using activity within the home 60 viii

10 List of Tables 6.1: Fresh water savings within each operation : Results obtained for minimum cost design : Sensitivity analysis result 67 ix

11 CHAPTER 1 INTRODUCTION 1.1 BACKGROUND Water is hugely abundant on the earth s surface, occupying over 70% of it. However, only about 3% of this quantity is fresh water with the remaining 97% comprising saline or brackish water found in oceans, rivers and lagoons. Most of the available fresh water is inaccessible for direct use as about 68% is stored in form of ice caps and glaciers. The remaining 32% is in liquid form and available as ground water or surface water. These are the two basic sources of water supply to meet human needs. Fresh water distribution on earth is depicted in Figure 1.1. It is worthy to note that its quality depends largely on the source and varies greatly. Figure 1.1: Distribution of Earth s Water (Source: USGS) The municipal water network incorporates various processes governing water utilisation to meet daily needs within the urban environment. Conventionally, such a network consists of 6 components: water source, transportation from source, treatment process, demand by end users, wastewater collection and treatment; and finally, discharge back into the environment. The major challenge facing the municipal water distribution is source preservation, which guarantees quality water supply; and demand management, to ensure efficient use. The problem of source preservation has been efficiently managed by ensuring that waste water is adequately treated before being discharged into the environment. This has been made 1

12 Introduction possible by advancements in wastewater treatment technologies over the years. As a result, the integrity of existing water bodies which may later accommodate such treated wastewater is protected. Presently, demand management is receiving more attention. The problem has been to determine water quality standards to meet the respective water demands. Water is primarily used for three purposes within the urban environment: domestic (urban), industrial and agricultural. By far, the most water using activity is agriculture where a large volume of water is used for irrigation purposes. This is followed by industrial water demand where water is used in cooling towers, gas scrubbing and several industrial processing activities; and as a raw material. Domestic demand encompasses water-using activities in the home, schools and other commercial establishments. As far back as 1958, the United Nations Economic and Social Council gave a directive that prohibits using water of high quality to meet water demands that can tolerate water of lower grade (USEPA, 2004). This directive ensures that the quality of water used to meet a demand should be fit for purpose. This directive has formed the fulcrum of water conservation strategies being canvassed today. Consequently, agricultural water demand is met more with reclaimed water than fresh water. Municipal wastewater or surface water can be treated to a quality sufficient to meet water standards set for irrigation. A similar approach has been adopted towards meeting industrial water demands. However, in this case, no specified standard exists because the treatment quality is determined by the nature of the industrial activity and the particular water-using operation involved. This strategy is yet to be fully adapted to meeting urban demands. Potable water is currently being supplied to homes, schools and other premises to satisfy all water using operations. This should not be the case. Toilet flushing, car washing, lawn watering and many such activities do not require potable water to be executed. Hence, this work focuses on developing a technique that shows how efficient systems-based water use strategies can be developed within the urban sector. 1.2 NEED FOR WATER CONSERVATION A careful analysis of prevailing water-use trend shows a steady increase in water demand. This observed increase has been caused by the population boom and rapid urbanisation being 2

13 Introduction experienced in the world; especially in the developing countries. Fresh water withdrawal is predicted to increase by 50% by 2025 in the developing nations and 18% in the developed countries (UN- Water statistics). Evidently, the major problem being envisaged is that available water sources may not be sufficient to cope with increasing water demands. It is estimated that by 2025, over 1.4 billion people will be living in areas where water use exceeds minimum recharge levels and another 1.8 billion people will live in areas of acute scarcity (UN Water statistics). In fact, Middle Eastern countries no longer rely on conventional water supply sources and have resorted to the costly technique of desalination to augment their water supply (Manan et al 2006). Desalination is prominent in the Gulf States where little fresh water is available and it involves the demineralisation of seawater. Thus, the impending fresh water scarcity can only be averted through adequate water conservation techniques. On a comparative basis, industrial water conservation strategies have been more effective than conservation techniques practiced within urban systems. While the industrial approach is based on proven analytical techniques, water conservation in urban systems has been based on the design of water saving gadgets and individual attitudes towards water use. It is estimated that industrial water use in England and Wales has decreased by approximately 900 million cubic meters since 1998 (Manan et al 2006). This demonstrates the efficiency of the techniques adopted. Presently, in Malaysia, the domestic sector contributes about 60% in water use compared with other sectors (Manan et al 2006). Therefore, there is evident need for improvement in water use efficiency within urban systems. 1.3 RESEARCH OBJECTIVE The research objective is to develop a systematic framework for analysing water use within urban systems. This framework will ensure that the quality of water used to satisfy demands are fit for purpose; thus ensuring efficient utilisation of water resources. The proposed framework seeks to achieve the following goals: Design of a total water network for an urban system Establish minimum fresh water requirement for such designed network Efficient water distribution in terms of quality and quantity 3

14 Introduction The urban system considered is a residential household. The method is based on adapting current water conservation strategies used in industry to analyse water use within the home. More importantly, the framework developed provides an alternative means of analysing water use in the home and other domestic units. 1.4 RESEARCH LAYOUT This work progresses with a critical review of existing conservation efforts in both urban and industrial sectors in chapter 2. Most especially, research trends in the industrial sector with respect to the Water Pinch Framework are carefully analysed. Chapter 3 deals with the characterisation of municipal wastewater. This provides insight into the nature of municipal wastewater constituents and their average concentration. Chapter 4 builds on the achievements of the previous chapter. It discusses the available treatment technologies used to treat municipal wastewater. The engineering principle behind each technology is reviewed and its limitations highlighted. Chapter 5 deals with data gathering and analysis as well as the implementation of knowledge gathered in previous chapters. The focus is on developing a water-use model for a residential household. The results obtained in chapter 5 are analysed in chapter 6 while conclusions and recommendations are made in chapter 7. 4

15 CHAPTER 2 LITERATURE REVIEW 2.1 BACKGROUND Water is primarily used for industrial, agricultural and domestic purposes. Although these applications are distinct, they compete with one another and are interdependent. All three activities deplete the quantity of available fresh water and generate wastewater streams. The wastewater streams associated with each application is unique with respect to the pollutant loading and associated contaminants. Hence, careful analysis is required before treatment options are proposed. More importantly, interactions between these tripartite water applications have to be adequately managed to guarantee sustainable fresh water supply and prevent environmental degradation. For example, in domestic or urban applications, the strategy being canvassed is the development of an Integrated Water Resource Management (IWRM) approach. According to Thomas (2003) the IWRM framework would include the development of alternative water resource, preservation of the quality and quantity of existing water resource and implementation of demand management options. In order to be effective, the IWRM must embrace a holistic view of the problem and engage all users, planners and policy makers. In agriculture, the quality of water to be used for irrigation has been a subject of much debate. This has led to various water quality standards to be adopted by different countries. Emphasis in industry has been based on optimum water consumption and efficient treatment solutions that can enhance the reuse of spent water. This section focuses on the various methods adopted by both urban and industrial systems towards fresh water conservation and minimum wastewater generation. It will be observed that the method employed in both sectors is independent and unique to the nature of the water using operation. Also, an extensive review of literature has been carried out to highlight the research trend over the years. Wastewater characterisation and treatment options are considered in chapters 3 and 4 respectively. 5

16 Literature review Figure 2.1 Urban water cycle (Angelakis, 2008) 2.2 URBAN APPROACH TOWARDS WATER CONSERVATION: Water conservation techniques adopted within urban systems (domestic and public buildings) have tended towards saving fresh water through efficient consumption and reuse /recycle of wastewater. Emerging awareness on dwindling fresh water supply has dictated a collective participation between consumers and suppliers for embracing sound water management practices. Water companies have introduced stricter water metering systems, improved maintenance of water distribution networks and prompt response to pipe rupture/ vandalism to forestall massive leakage. Laudable as these strategies may seem, they have no direct impact on reducing excessive water usage. Thus, the bulk of water reduction measures lie with the end consumers of the commodity. Water is said to be efficiently consumed, when the quantity and quality of water used is fit for purpose. Hence, the use of potable water for toilet flushing amounts to wastage of a valuable resource. To ensure judicious water consumption within urban systems, Cheng (2003), has proposed the use of a Water Conservation Index (WCI). Cheng (2003) defined the WCI as the ratio of actual water consumption within a building to the average water consumption in general. The WCI is a quantitative tool to determine the water saving rate within a building. The index takes into consideration water saving efficiency within bathrooms, kitchen, water 6

17 Literature review taps, as well as recycling of rain and grey water. Water conservation strategies being employed in urban systems are discussed below Use of Water Saving Gadgets: This involves the installation of water efficient devices and low plumbing fixtures (Flowers, 2006). Examples of such devices include automatic flushing systems, faucet aerators, water saving shower heads, efficient water-using washing machines and dish washers. These devices when installed in buildings ultimately help reduce water consumption. For instance, the faucet aerator mixes air with water which serves to reduce the flow. This alone can cut water flow rate by 50 % (Flowers, 2006). Recent studies conducted in the Netherlands have shown that overall water consumption per person declined by 4 litres per day when these devices are installed within domestic buildings (Flowers 2006). In general, 20% reduction is achievable in community water consumption when these devices are installed (Flowers 2006). Table 2.1 lists some of these devices and their percentage reduction over conventional counterparts. Table 2.1: Flow reduction devices in urban systems (Qasim, 1999) Flow reduction device Description % reduction in water use over conventional systems Pressure Reducing PRVs are used to make home supply valves (PRV) pressure lower than the city s water supply pressure. This eliminates unnecessary waste due to spurts from Showers : Limiting flow valves(lfv) and Flow limiting shower head (FLSH) Washing machine: Level controller or water efficient washer Dishwasher: New designs Toilet: Dual-cycle toilets designs (DCD), Reduced flush device (RFD), Vacuum flush toilet systems(vts) faucet, leaks and drips LFV restricts flow to a fixed rate, independent of supply pressure. FLSH reduces water consumption by restricting and concentrating water jets Level settings uses water in accordance with wash load; new designs of washing machines are water efficient LFV: 8 10 FLSH : Water efficient designs 4 10 DCDs have two flush cycles, one for urine and the other for faecal transport; RFDs are devices inserted into the toilet tank to reduce the flush volume; VTS mixes air with small amount of water or foam to transport waste. Mostly used in aircrafts. DCD: RFD: VTS:

18 Literature review Rain Water Harvesting: Rain water harvesting is the collection and storage of rain water for future use. This strategy can be employed by both individual households and municipalities to reduce fresh water demand. Households typically require a collection system like a storage tank where rain water from the roof top can be stored. Municipalities on the other hand, will require dams to trap large volumes of rain water. The collected water is then channelled to treatment units before being distributed. Apart from reducing fresh water demand, rain water harvesting mitigates risk of flooding, helps in ground water recharge, increases soil moisture levels and prevents erosion of topsoil (Flowers, 2006). Rainwater is of relatively high quality and can be used without treatment for washing, toilet flushing, lawn-watering and bathing. However, treatment may be required to make it safe for drinking Grey Water Reuse: Grey water is defined as effluent water collected from sewage flows from washing machines, bathtubs, showers and sinks (Al-jayyousi, 2003). It excludes discharges from kitchen sinks, dishwashers and toilets. Grey water reuse can lead to huge water savings in the home. About 50-70% of water discharges from residential buildings are grey water (Al-jayyousi, 2003). A research study in Arizona showed that the average household generates about 30,000-40,000 gallons of grey water per year (Al-jayyousi, 2003). This result illuminates the potential for water savings that can be made through reuse of grey water. In addition to reducing potable water consumption, grey water helps to reduce sewage flows and pollutant concentration in the environment. Figure 2.2: Household water use (Data source: Thames water company) 8

19 Literature review The first step in grey water reuse is its separation from black water. Most recycle systems involve a diverter valve which does the separation. The grey water stream is then passed through a sand filter to remove organic matter. The resulting stream can then be used as it is or passed to treatment units. Popular treatment methods include disinfection using iodine or chlorine, biological treatment and membranes for separation. Untreated grey water is primarily used for outdoor purposes where contact with humans is minimal. It is recommended that grey water be used immediately and not stored. This is because it could serve as a breeding site for bacteria and other pathogens. Also, it could give out foul odours when stored for a long period. The United States, Japan and Australia have the highest profile in grey water reuse; although, grey water reuse is limited to external uses in Australia. Because of the quality concerns, its use is prohibited in the home. Other countries involved in active grey water research include Canada, United Kingdom, Germany and Sweden (Al-jayyousi, 2003). 2.3 INDUSTRIAL APPROACH Water is used in industry for different purposes. These include, cooling in heat exchangers, absorption process to remove impurities from a gas stream, cleaning of process equipment, and more importantly, as a raw material. Manan et al (2004) proposed a classification of industrial water-use based on the nature of the operation. These are: Mass Transfer Based (MTB): Here, water is employed as a mass separating agent. It involves the transfer of contaminants or desired species from the rich stream to a lean stream (water). Such processes are assumed to have a fixed contaminant load. An example is the use of water in a petroleum refinery as a stripping agent to remove suspended solids, hydrogen sulphides and ammonia from crude oil. For MTB operations, the water flow rate is fixed; that is, the input and output streams have equal flow rate since no water loss occurs within the process. Processes such as these are otherwise termed fixed flow rate problems. Non Mass Transfer Based Operation: In this class of operation, water does not function as a mass separating agent. This includes water used as coolants, water produced from reactions, water employed as raw materials, product or by-products from operations. The flow rate is very important in this instance and it is not fixed. The process is also referred to as a fixed contaminant problem. 9

20 Literature review Industrial approaches to water conservation have been a subject of extensive research. Unlike the aforementioned urban strategies, industrial techniques not only guarantee minimum water use for the whole process, it also ensures that water-using units within the overall process are served with its optimum water quantity. The water conservation problem has been regarded as a mass integration problem which is a part of ongoing research in the field of process integration. Two distinct approaches to the problem are discussed here Water Pinch Analysis (WPA) Framework: Water Pinch Analysis is a systematic technique for implementing strategies to maximise water reuse and recycling through integration of water-using activities or processes (Manan and Alwi 2007). WPA framework owes its development to the success recorded in the Thermodynamic Pinch Analysis for Heat Exchanger Network design in process Integration. Most of the methods used in WPA are adaptations of similar techniques employed for heat analysis. Examples are Water Composite Curve of Wang and Smith (1994) and Load Problem Table of Aly et al (2005) which are adaptations of Grand Composite Curve of Linnhoff and Hindmarsh (1983) and Problem Table Algorithm (Linnhoff and Flower,1978) respectively used in Heat Pinch Analysis. The goal of the WPA framework is to ensure optimum fresh water consumption within a process. As a consequence, wastewater generation is also minimised. This is achieved by maximising water reuse within the process by considering options for reuse, recycle and regeneration. WPA also provides guidelines for efficient water allocation between sources and demands through stream mixing, splitting and bypass. Mann and Liu (1999) defined some of the terminologies associated with WPA thus: Water reuse: this refers to the direct use of wastewater from one operation to satisfy the water requirement of another operation within the process provided the level of contamination does not interfere with the subsequent process. Reuse excludes returning the water through units where it has previously been used to prevent the accumulation of trace contaminants which have been ignored in the analysis. Regeneration: This refers to the partial treatment of wastewater to reduce its contaminant load and make it fit for reuse. Depending on the nature of the contaminants, regeneration is done through filtration, stream stripping, adsorption etc. 10

21 Literature review Recycle: This refers to the complete treatment of wastewater by returning it to its original state. All contaminants are removed and the water can be reused in units where it emanated from. WPA is a powerful tool because it allows fresh water and wastewater flow rates to be determined before the actual network is designed. The developments in the targeting and network design methodologies will be analysed independently in this work Flow rate targeting technique: Targeting involves determining the minimum fresh water requirement and the wastewater generated by a process before the water network is actually synthesised. It provides a basis for the actual design of a minimum water network. Targeting has been done using graphical or numerical methods Graphical Methods Graphical techniques were the earliest methods to be implemented in addressing the problem of water targeting and network design. Methods such as Water Composite Curves (WCC), Water Source Diagram (WSD), Source Demand Composite Curves (SDCC) are all insightbased approaches. They are very easy to implement and provide conceptual insights into the problem. Researchers have employed various kind of graphical analysis to achieve the targets. Pioneering work in this regard was carried out by Wang and Smith (1994). They developed the Water pinch concept and proposed a graphical framework for targeting fresh water consumption of an MTB operation. Since the flow rate for these operations are fixed, minimum fresh water targets results in minimum wastewater generation. Wang and Smith (1994) introduced the concept of the limiting water profile which can be obtained by specifying the maximum possible outlet and inlet concentration such that the flow rate required is minimised. The pinch point is determined graphically with the aid of the Water Composite Curve (WCC) which essentially is an adaptation of the Grand Composite Curve in a heat exchanger network design. The WCC method was successfully applied to single and multiple contaminant systems. 11

22 Literature review Figure 2.3: Water Composite Curve (source: Manan and Alwi, 2007) One major limitation with this method is that it only considers MTB operations. The method fails when applied to NMTB systems. In order to address this setback, Hallale (2002) presented a methodology for obtaining water targets for global water operations (systems having both MTB and NMTB processes). Hallale (2002) introduced a new graphical technique, Water Surplus Diagram (WSD), which is a modification of Water Composite curve of Wang and Smith (1994). Water targets obtained with the WSD are unique and are independent of any assumed stream mixing strategy. Hallale (2002) also considered the possibility of fresh water reduction through process modification and regeneration techniques for water reuse or recycle. The WSD algorithm is given below Figure 2.4 : Algorithm for Water Source Diagram(Source: Manan et al, 2004 ) 12

23 Literature review Another method capable of handling global water operations effectively is the Source Demand Composite Curve (SDCC) introduced by Prakash and Shenoy (2005a). SDCC is also a graphical tool like the WCC and WSD; it is obtained by plotting separate source and demand composite curves. The flow rate is plotted on the horizontal axis while the contaminant load is represented on the vertical axis. The key difference between this approach and the other two before it (WCC and WSD) lies in the definition of the vertical axis. The other methods plot the concentration of the streams on the vertical axis. However, using the contaminant load as the vertical axis allows all mixing possibilities to be considered and eliminates several iterative steps. However, a major problem with the SDCC is that it only considers the case of single contaminants which is rarely encountered in industry. Figure 2.5: SDDC Plot: (Source: Prakash and Shenoy, 2005a) A method capable of handling multiple contaminants was developed by El-Halwagi et al (2003). They employed the principle of dynamic programming to derive the mathematical conditions for optimally targeting streams with single and multiple contaminants. This mathematical formulation incorporates all options for stream segregation, mixing, and recycles. These conditions are then analysed graphically to obtain the targets that yield minimum fresh water usage. This 2-step approach combines the benefits of mathematical formulation and graphical insights. The dynamic programming step is non-iterative and easy to apply while the graphical representation provides the designer with conceptual insights into the problem by determining the pinch point. 13

24 Literature review Numerical Techniques: Unlike the graphical methods considered above, numerical techniques have the advantage of being compact, easily implemented and rigorous. It is also non iterative and time saving. More importantly, numerical techniques are capable of handling complex problems, (especially when handling multiple contaminants), effectively, accurately and optimally. Often times, it is combined with the graphical approach to yield adequate understanding of a problem through visualization. The first method to be considered is the Evolutionary Table Analysis introduced by Sorin and Bedard (1999). This method is applicable to single contaminant problems and is capable of obtaining fresh water and wastewater targets prior to actual network design. Manan et al (2004) introduced the Water Cascade Analysis (WCA) which is a resilient numerical targeting and design tool. The Water Cascade Analysis is applicable to both MTB and NMTB operations basically because it identifies both concentration and flow rate as driving forces for water reuse. Water Cascade Analysis is a two-step approach which involves the use of Water Cascade Table (WCT) and Water Cascade Diagram (WCD) to determine water targets. WCT is first employed to partition the water streams within the process into various purity intervals and determine the net source or demand within each interval. WCD is then implemented by cascading the water stream from a high purity interval to lower purity intervals below it. This yields the cumulative net water source and demand for the process. The fresh and waste water targets are determined by considering both the water flow rate balance and concentration driving force. A worthwhile benefit of this method is that it displays both the fresh water and wastewater targets directly on the table (Manan et al, 2004). This is unlike the Water Source Diagram(WSD) where only the minimum fresh water target can be read directly from the graph while the wastewater target needs to be calculated. Besides, the WCA approach gives valuable insights for the determination of the pinch - causing stream thereby enhancing water allocation techniques above and below the pinch (Manan et al, 2004). WCA can also be used to retrofit existing network to achieve optimal targets and cope effectively with problems with multiple pinch points as identified by Hallale (2002). Applying the water cascade analysis to determine water targets is very direct. In fact, the benefit of its numerical feature led to the development of computer software called the Heat-Matrix (Manan et al, 2003) which has further reduced the computational time. Figure 2.6 is an illustration of the calculation of the water cascade table within an interval K, contaminant load C, and stream flow rate F. 14

25 Literature review Figure 2.6: Water Cascade Table for flow rate targeting (Foo, 2007) Another notable method was introduced by Liu et al (2007) known as the Modified Concentration Interval method (MCI). This method was specifically applied to NMTB operations to determine fresh water and wastewater targets although they demonstrated that it can also be adapted to MTB operations. The first step is the introduction of fictitious operations to convert the NMTB operations to equivalent MTB processes. The MCI technique is then applied to yield the desired targets. Liu et al (2007) identified two types of fictitious operations. Type one operations are units where the inlet flow rate of the NMTB process has the same concentration constraints as an MTB process. The other type is any operation where the quantity of water lost or generated is assumed to represent the limiting flow rate. The lost/generated water is then assumed to have a limiting inlet concentration equal to the specified outlet concentration of the original unit; and also, a limiting outlet concentration equal to the maximum concentration of the water-using system. This is illustrated in the figure below: 15

26 Literature review Figure 2.7: Illustration of the conversion of fixed flow rate operation into fictitious operations (Liu et al 2007) Although this method is numerical and can quickly yield targets and determine the pinch point in a design, it is only applicable to single contaminant problems and therefore cannot handle industrial scale problems. Aly et al (2005) also introduced the Load Problem Table (LPT) which is also effective only for single contaminant processes. The LPT is an adaptation of the problem table analysis techniques used in specifying utility targets in Heat Exchanger Network design. A more versatile approach capable of handling multiple contaminant operations was given by Tanaka et al (1980). The mathematical programming formulation resulted in an NLP which was solved using the complex method. A major limitation of this technique is the fact that the solution obtained for the targets might not be the optimum solution. This is due to the nature of the optimisation problem. Hence, in order to overcome this problem, Doyle and Smith (1997), proposed another optimisation technique for optimum targeting of fresh water and wastewater utilisation in a water network. 16

27 Literature review The unique feature of Doyle and Smith s (1997) method is that the mathematical programming formulation uses both Linear Programming and Non Linear Programming approach to achieve optimum solution. Doyle and Smith (1997) modelled the mass transfer operation in two different ways. The first approach was to consider the process as a fixed mass load problem which resulted in an NLP optimisation problem. The second approach considered the mass transfer operation in terms of a fixed outlet concentration. This yielded a linear programming problem. Doyle and Smith (1997) went further to show that for multiple contaminant cases, each contaminant can be modelled according to its behaviour in the process. Hence, multiple contaminant problems often results in an NLP formulation. To overcome the problem of Tanaka et al (1980), Doyle and Smith (1997) showed that local optimal solution can be avoided if the LP formulation is used as an initialisation for the overall NLP problem. This provides a good initial guess for the problem and reduces computation time. Also, unlike the method of Tanaka et al (1980), this approach is capable of handling large scale industrial problems. It is worth noting that Tanaka et al (1981), overcame this problem associated with their previous methodology by proposing a strictly LP approach to achieve the desired targets. Due to the linearity of this new method, multi dimensional and large scale problems can be effectively handled. The method consists of a two level approach. The first is to use a set of heuristic rules to simplify the problem structure and the other is to employ a LP algorithm to solve the linearized problem. Tanaka et al (1981) demonstrated the effectiveness of this technique by applying it to an industrial system consisting of 15 subsystems and 5 pollutants. A problem of this magnitude would have been impossible to solve using the earlier method proposed by Tanaka et al (1980) Multiple Utility Considerations: All the research works reviewed so far have dealt with fresh water as a single utility source. Intrinsically, the authors have assumed 100% purity (zero contaminants) for the fresh water source used in the process. An implicit assumption made is that potable water is employed. The targets obtained for such networks though minimum may not be prudent with respect to cost. Foo (2007) addressed this problem by exploring the possibility of using an impure fresh water source. The procedure was further extended to consider the targeting of multiple impure fresh water sources. Examples of impure water sources include rainwater, water from 17

28 Literature review rivers and ponds and snow. Foo (2007) modified the WCA technique of Manan et al (2004) to solve the problem. It was observed that the original WCA worked very well for the case when the impure fresh water source has the lowest contaminant concentration among all the process water demand and source streams. However, the modified WCA technique is employed when the fresh water source has significant contaminant concentration higher than at least one of the process water demand or source streams. For instances such as this, the WCT is developed in a way such that the fresh water source is located at its appropriate purity interval to ensure adequate flow rate targeting. This is unlike the original WCT where the fresh water source is located at the highest purity interval. Foo (2007) further proposed guidelines to locate minimum fresh water flow rate for the case of multiple impure sources. He suggested that water of lowest quality is first employed and its flow rate determined. Subsequently, higher quality fresh water source can then be used and the flow rate calculated. More importantly, Foo (2007) remarked that there exists the possibility of substituting the higher quality source with the lower quality source by adjusting the flow rate of the latter. The underlying assumption is that maximum use of the lower quality sources must be ensured. This assumption has cost of supply as its basis. This framework guarantees the realization of an economic water target which is crucial for regions where fresh water is expensive NETWORK DESIGN Once water targets have been obtained, the actual network can then be designed to achieve this objective. Network design involves the systematic matching, splitting and mixing of the water streams within the process. It entails a feasible allocation of the various water sources to satisfy specific water demands. Although, the primary objective is to satisfy the set targets, sometimes this might not be possible. Often times, some penalty is incurred in terms of fresh water use, in order to synthesize a cost optimal network. Other constraints such as forbidden stream pairing, space, geographical location, process limitations etc might lead to designs which violate the set targets. Like the targeting phase, the methodology employed for network synthesis has progressed in a similar way. Graphical analysis and mathematical programming models have been used. 18

29 Literature review Both methods are complementary and can be used singly or jointly to design optimum water networks Conceptual Approach for Network Design This method involves determining the pinch point of the design problem. The pinch point represents the most constrained part of the design. It partitions the problem into two distinct regions: above the pinch and below the pinch. As a design rule, in order to achieve the targets, only fresh water should be used above the pinch and wastewater generated below the pinch. Furthermore, there should not be any water transfer across the pinch. This implies that water demands above the pinch can only be met with fresh water or other water sources located in that region. Similarly, demands below the pinch are satisfied solely with water sources in that vicinity. The WCC introduced by Wang and Smith (1994), is one of the earliest graphical approaches to solving the network design problem. Wang and Smith (1994) also introduced the water grid diagram which was used alongside the WCC and the Limiting Composite Curve (LCC) to generate a network satisfying the minimum water (fresh water and wastewater) flow rate targets for the case study considered. Two separate designs satisfying the set targets were obtained. The first design was based on maximising the concentration difference while the other sought to achieve a minimum number of stream matches through bypassing and mixing. This method is applicable to single and multiple contaminant systems, MTB operations and considers opportunity for water regeneration or recycling. Moreover, using the LCC allows various process constraints such as solubility, fouling, and minimum mass transfer driving force to be incorporated in the design. A faster method for water network was introduced by Prakash and Shenoy (2005a). They introduced the Nearest Neighbour Algorithm (NNA). NNA simply implies allocating a source stream to its nearest demand in terms of contaminant concentration. The quantity to be allocated is determined by material balances. For cases when the nearest neighbour of a source is incapable of satisfying the demand within its concentration interval, that source is completely used before using the next neighbour source to satisfy the demand. The NNA approach is capable of handling both NMTB and MTB operations unlike Wang and Smith (1994) suitable only for MTB. 19

30 Literature review Prakash and Shenoy (2005b) improved on their previous work on NNA algorithm. It was observed that the NNA algorithm can only generate a single design to achieve the targets obtained. This eliminates other possible options that should be considered. Prakash and Shenoy (2005b) developed the Source shifting technique to overcome the limitation of the NNA approach. Source shift guarantees that all possible design alternatives that can satisfy the pre-determined targets are analysed. The final network is selected based on other considerations like simplicity, operability and flexibility set by the designer. To implement the source shift concept, Prakash and Shenoy (2005b) introduced the Equivalent Source Relation. This simply states that a water source can be expressed as a linear combination of two other sources. This would be possible when both sources are mixed in a particular ratio and provided all three sources have the same flow rate and contaminant concentration. Prakash and Shenoy (2005b) went further to develop a matrix with sources represented as rows and demands as columns. On the matrix, both sources and demands are arranged in order of increasing contaminants. The matrix is used for stream matching and provides a compact representation of all the possible designs that can be generated. Like the NNA, the source shift technique is applicable to both NMTB and MTB operations and multiple contaminant systems. However, a shortcoming is that the method is slow and requires a large number of calculations to determine equivalent source relations for multiple contaminant systems. Therefore, except the numerical feature of this technique is exploited by coding it in computer software, like the WCA technique of Manan et al (2003), it would be unsuitable for industrial problems handling large number of contaminants. Another notable approach for synthesising water networks is the use of Network Allocation Diagram (NAD) introduced by Alwi and Manan (2008). NAD is a graphical technique for simultaneous targeting for minimum water use and network design to satisfy the predetermined target. Essentially, NAD is a combination of the Source Demand Composite Curve (SDCC) and the Source Demand Allocation Diagram (SDAD) methodologies introduced by Prakash and Shenoy (2005b). SDDC is used to obtain the targets and SDAC implemented to generate network configuration. NAD is simply a visualisation tool employed at the design stage to address complex design problems when the SDAC method fails especially for multiple pinch problems. The key steps to implement NAD are shown below: 20

31 Literature review Figure 2.8 : Algorithm for NAD (Source: Wan Alwi and Manan, 2008) Mathematical Programming Approach: This technique is increasingly being employed primarily because of its ability to handle problems of high dimensionality and the ease of implementation. Besides, it allows the designer to incorporate cost consideration (investment and operational cost) in the design. Although the objective function is associated with cost minimisation, the targets obtained are incorporated in the constraints during problem formulation. This ensures that the water network generated is cost optimal and satisfies the targets. Tanaka et al(1980) proposed an NLP formulation for designing water allocation problems typical of refineries. The objective function was formulated in terms of the total annual cost which is the sum of the Investment cost, operating cost of wastewater treatment systems and the cost of fresh water. This resulted in a non linear objective function. Tanaka et al (1981) was able to linearise the cost function to achievee a LP problem as alll the other process constraints were linear. The initial NLP problem was reconstructed using partitioning programming techniques. 21

32 Literature review Alva-Aragaez et al(1998) pointed out that Tanaka et al(1980) ignored interaction factors between subsystems within the water network. Hence, they proposed an integrated approach to solving the problem. In addition, other constraints not considered in Tanaka et al (1981) were included. Examples are safety, control and network topology for reuse, regeneration and recycle. A decomposition strategy was used to solve the resulting superstructure. An MINLP model was obtained which was solved using a sequence of MILP formulations. Alva-Aragaez et al(1998) made the following assumptions during the problem formulation: known number of water system units, isothermal and isobaric operation of water network, limiting water profile is known and removal ratio for contaminants in each treatment unit is independent of the inlet concentration to that unit. This approach is applicable to global water operations and allows for several other practical constraints to be incorporated satisfying design specifications. Also, like Tanaka et al(1980), the objective was to minimise the investment and operating cost of the resulting network. Bagajewicz and Savelski (2001) pointed out that total cost alone should not be the basis for the design. Bagajewicz and Savelski (2001) showed that a trade-off should be made between cost and operability of the design. Several other network configurations satisfying water targets can be obtained when a different objective function is considered. Examples of other objective functions include: minimum number of inter connection of streams, minimum fixed cost and forbidden / compulsory matches. They proposed a LP formulation to obtain the desired targets and a MILP problem to design networks that satisfies this target. The approach also considered cases involving water reuse, water regeneration and water recycle. The method is very fast and allows a designer to quickly explore all possible alternatives before that which meets his objective is selected. Savelski and Bagajewicz (2000) gave the necessary conditions for optimality of water networks in industrial processes with single contaminants. The necessary conditions guarantee optimal water allocation for networks which incorporate water reuse in the design. The objective here is to minimise fresh water intake. They introduced the concept of concentration monotonicity. Concentration monotonicity states that the outlet concentration from a process unit must be greater than the combination of the combined wastewater being reused as inlet stream into the unit. This is the first necessary condition to guarantee optimal water allocation. The second condition proposed by Savelski and Bagajewicz (2000) states that the network is optimal when all fresh water-using processes reach their maximum 22

33 Literature review possible outlet concentration. A summary of the scope and research techniques employed are highlighted in table 2.2 below. Table 2.2: Highlights of some research works. OPERATIO N MTB TECHNIQUE SCOPE AUTHOR WPA (Water composite curve and Concentration interval diagram) Targeting and Network Design MTB WPA ( Water source diagram) Targeting and Network design NMTB WPA ( Mass concentration interval) Global Mathematical programming (LP formulation) Global WPA ( Source demand composite curve) Global Mathematical programming ( LP and MILP formulation) Wang,Y.P and Smith,R (1994) Gomes et al (2007) Targeting Liu et al (2004) Targeting and network design Takama et al (1980) Targeting Dhole et al (1996) Targeting and Bagajewicz & Network design Savelski (2001) Global WPA (Water cascade analysis) Targeting Manan,Z and Foo,D (2004) Global WPA ( Load problem table) Targeting and Network design Aly et al (2005) Global WPA (SHARPS) Targeting, network design and Cost trade-off Wan Alwi and Manan (2007) LATEST DEVELOPMENTS This section considers the current trend in water minimisation research. It focuses on methods aimed at designing water networks that achieve true minimum fresh water consumption and also probes the possibility of totally eliminating wastewater generation HOLISTIC NETWORK DESIGN: So far, all the research strategies and methods considered have sought to reduce both fresh water consumption and wastewater generation of water-using processes. Minimum flow rate targets have been obtained through water reuse, regeneration and recycle. However, according to Manan et al (2008), these targets obtained are not the true minimum. Manan et al (2008) claims that the absolute minimum target for fresh and wastewater can only be 23

34 Literature review realised when all options for water reduction have been considered. These options include process modification strategies such as replacing water with air in a cooling process, source elimination techniques, source reduction etc. These steps are significant in the design and should be explored before reuse, regeneration and recycle options are considered. In order to realize this new minimum targets, Manan et al (2008), proposed the first holistic framework for simultaneous targeting and design of Minimum Water Networks (MWN). Another important feature of this framework is that it considers the impact of any process modification step on the overall process efficiency and economics. Manan et al (2008) introduced two strategies for screening: Water Management Hierarchy (WMH) and Systematic Hierarchical Approach for Resilient Process Screening (SHARPS). WMH is used to determine the most viable process modification option to adopt by considering the effects of all probable options on the design. SHARPS, on the other hand, is a cost screening tool for the WMH options generated. The framework as given by Manan et al (2008) is shown below: Figure 2.9: WMH framework by Manan et al (2008) ZERO DISCHARGE NETWORKS The method of Manan et al (2008) has been successfully applied to industrial systems and significant reduction in fresh water consumption and wastewater generation has been recorded. However, due to stricter environmental regulations, it is becoming necessary to eliminate wastewater effluent through complete regeneration. This is the underlying basis for Zero Discharge Networks. The concept of Zero Liquid Discharge (ZLD) has been considered 24

35 Literature review rigorously by researchers. According to Bagajeweicz (2000), zero discharge applies either to the complete elimination of the hazardous contaminants in the wastewater streams or the total elimination of liquid discharge. The latter would result in a closed loop network where waste is fully regenerated and reused. Koppol et al (2003), pioneered efforts to explore the economic feasibility of zero discharge networks. They employed a mathematical programming technique to explore the possibility of zero discharge networks in different industries such as paper mill and refineries. For each industrial case study considered, the mathematical framework incorporated various regeneration technique using different treatment technologies unique to the particular industry to explore the possibility of zero discharge. Koppol et al (2003) showed that the critical factors that determine the feasibility of such networks are regeneration costs, fresh water costs and discharge concentration of the wastewater streams. According to Bagajewicz (2000) and Koppol et al (2003), the benefits of developing ZLD networks are immense. Apart from minimising fresh water consumption and eliminating the problem of wastewater disposal, industries can cope effectively in locations where fresh water is scarce or expensive because the water within the network can always be reused. Moreover, Bagajewicz (2000) and Koppol et al (2003) further pointed out that ZLD guarantees the attainment of green water utilisation in process industry and shows sensitivity to the environment. In terms of cost, money can be saved from the location of new facility since location near suitable receiving water-way would be unnecessary. In fact the solid waste generated can be sold to generate additional revenue which also serves to solve solid waste disposal problems. However, major problems associated with the implementation of this design were also highlighted by Bagajewicz (2000) and Koppol et al (2003). These are higher operating costs incurred for water treatment to high quality standards capable of reuse, higher capital cost for retrofit projects due to large scale restructuring and procurement of new treatment equipment. 2.4 RESEARCH GAP From the foregoing discussion, it is quite evident that industrial systems have evolved a more robust approach towards efficient water management than urban systems. This situation has been created and sustained by researchers focussing strictly on industrial water utilisation. A plausible explanation for the observed research trend can be ascribed to the fact that the quantity and cost of water used in industrial processes is far greater than that consumed 25

36 Literature review within urban systems. Hence, optimum water utilisation is imperative within industrial systems in order to reduce operational costs. However, this should not be the case. Inefficient water allocation and utilisation within urban sectors not only leads to wastage but is also capable of undermining industrial activity by reducing the quantity and quality of fresh water needed to sustain such processes. Pioneering efforts into water utilisation management in urban sectors based on the techniques developed for industrial water systems was carried out by Manan et al (2006). They were able to propose a retrofit design to the existing water network in the Sultan Ismail Mosque at the University of Technology in Malaysia. The Water Pinch Analysis concept (WPA) was successfully applied using the Water Cascade Analysis technique (WCA) earlier developed by Manan and Foo (2004). This technique predicted reduction of about 65.1 % for fresh water and 51.5% for wastewater generation considering reuse options only (Manan et al, 2006). With possibility of water regeneration via treatment, freshwater savings increased to 85.5% and wastewater generation reduced to 67.7% (Manan et al, 2006). Earlier, the Institute of Environmental and Water Resource Management (IEWRM) on the UTM campus also proposed a retrofit design for the same mosque using conventional water conservation strategy. The IEWRM design could only yield 25% fresh water savings and 19.8% wastewater reduction (Manan et al, 2006). A quick comparison of results obtained by Manan et al (2006) and the IEWRM reveals the efficacy of the WPA method over existing conventional methods. It is worthy to note that only Manan et al (2006) have adapted the WPA technique to an urban system based on my extensive review of existing published literature at the time of my research. Table 2.3 shows the present research gaps in this field. Table 2.3: Research gaps SCOPE INDUSTRY URBAN Minimum fresh water targeting X X Network design X Wastewater allocation problem to treatment units X Single contaminant consideration X X Multiple contaminant consideration X Single utility X X Multiple utility X Simultaneous Fresh water and waste water treatment X allocation problem Note: x denotes areas already covered in literature 26

37 Literature review 2.5 RESEARCH METHOD The research method to be employed would be the mathematical optimization method. This method is preferred to water pinch analysis because of the complexity involved in the problem being considered. Also, optimisation strategy avoids the iterative and graphical difficulties associated with Pinch analysis. Mathematical optimisation is a flexible approach which considers all possible options for water minimisation and wastewater allocation to treatment units via the development of a superstructure model. However, the results obtained are subject to the constraints imposed on the system. This work seeks to extend the achievement of Manan et al (2006) to domestic household water use. Also, it adopts a more rigorous approach of multiple contaminant consideration as the limiting factor for water reuse unlike Manan et al (2006) which was based on single contaminant formulations. Wastewater allocation problem to treatment units were simultaneously considered with the fresh water minimization problem. 27

38 CHAPTER 3 WASTEWATER CHARACTERISATION 3.1 INTRODUCTION Wastewater can be defined as effluent water leaving a process (in case of an industrial activity) or the end-state of water used for a specific activity such as washing, bathing, cleaning etc. Therefore, on a larger scale, municipal wastewater refers to water leaving residences, commercial buildings and industrial establishment via the sewer network to waste treatment plants. Municipal wastewater also includes storm water runoff resulting from rain water or snowmelt. Wastewater is commonly referred to as Sewage. The constituents of wastewater depend greatly on the polluting source. Hence, an in-depth understanding of the water application is necessary in order to predict adequately possible contaminants of the resulting wastewater. In addition, this knowledge guides in the selection of the most effective water treatment technology. Another important consideration in wastewater analysis is its flow rate. As stated earlier, municipal wastewater originates from different sources. Consequently, its flow rate fluctuates rapidly and depends on several factors like water usage, extent of water conservation, time of day, level of industrialization, weather and cost of fresh water. For example, it has been observed that water discharges from the home are highest early in the morning and late in the evening (Metcalf and Eddy, 2004). Industrial discharges on the other hand are at their peak during the day. Wastewater flow rate plays an important role in the design of appropriate treatment plant. In order to protect the environment, stringent discharge guidelines have been issued for wastewater treatment. Wastewater treatment facilities are required by law to meet these requirements in order to avert problems associated with improper wastewater discharges. Some of these problems as highlighted by Rowe and Abdul-Magid (1995) include the following: Wastewater contains disease carrying-vectors which may endanger public health. Improper discharge may affect the quality of other water sources through the production of tastes, odours and malodorous gases such as methane, hydrogen sulphide and ammonia. Wastewater contains nutrients such as nitrates and phosphorous which enhances the growth of plants over water bodies. 28

39 Wastewater characteristics Wastewater contains solids which settles at the bottom of water bodies or along their banks and contributes to the physical, chemical and biological degradation of natural water courses. As a consequence of the above reasons, wastewater needs to be adequately characterised, its constituents identified and an effective treatment utilised in order to mitigate the risk associated with its discharge into the environment. The figure below shows how wastewater can be characterised based on its constituents Wastewater Characterisation Physical Characteristics Chemical Characteristics Biological characteristics Solids Turbidity Colour Organic Inorganic Bacteria Viruses Figure 3.1 Wastewater characterisation 3.2 WASTEWATER CONSTITUENTS Constituent in this case refers to individual components, elements or biological entities present in wastewater (Metcalf and Eddy, 2004). The nature of these constituents provides the basis for the classification of wastewater based on its physical, chemical and biological composition. It is worth noting that these properties could be somewhat interrelated. For example, temperature, a physical property, determines the solubility of gases (chemical) and microbial activity (biological) in the wastewater (Metcalf and Eddy, 2004). Characterisation based on the physical, chemical and biological considerations will be considered. 29

40 Wastewater characteristics PHYSICAL CHARACTERISTICS The physical parameters often used to characterise wastewater include temperature, turbidity, colour, odour and its solid content. The most important is the solid content which comprises suspended solids, dissolved matter and floating solids. Other parameters which are often considered include conductivity, transmittance, density and specific weight. Table 3.1 below gives a brief description of some of these parameters. Table 3.1 Physical characteristics of wastewater Parameter Description Units Solids A variety of solid materials abound in wastewater. mg/l This includes rags, coarse materials and colloidal particles. A wide range of classification exists depending on the nature or state of the solids within the water. Examples include Total Suspended Solids (TSS), Total Volatile Solids (TVS), Total Dissolved Solids (TDS) etc Temperature Often, wastewater temperature is higher than that of Celsius or water supply and varies with the season. Fahrenheit Temperature is a very important parameter as it affects microbial activity and gas solubility. In case of gas solubility, this is particularly important with respect to the quantity of oxygen available for aquatic life when wastewater is discharged into large water bodies. Turbidity This is a measure of the cloudiness of the wastewater. It is caused by the presence of a wide variety of colloidal and residual solids. It is often measured by determining the light transmitting properties of the water sample. Colour Colour is a function of condition of wastewater. Condition is a term which refers to the age of the wastewater and how long it has spent in the collection system. Fresh wastewater is often light gray in colour. As time passes and due to anaerobic conditions the colour is observed to change from gray to dark gray and finally black. When it turns black, the wastewater is often said to be septic. Density An important characteristic because of the possibility of the formation of density currents in sedimentation tanks (Metcalf and Eddy, 2004). Density of domestic wastewater is approximately equal to that of fresh water at the same temperature. Nephelometric Turbidity Unit (NTU) kg/m 3 30

41 Wastewater characteristics CHEMICAL CHARACTERISTICS From Figure 3.1, it is seen that the chemical quality of wastewater is expressed in terms of its organic and inorganic constituents. The inorganic components are further subdivided into metallic and non metallic components. Sources of these components in wastewater are due in part to original additions in water supply and also additions resulting from domestic detergents and industrial chemicals. Examples of metallic constituents present include Arsenic, Cadmium, Zinc, Lead, Copper etc. Similarly, organic constituents have also been further classified as individual and aggregate (Metcalf and Eddy, 2004). Aggregate components consist of a number of compounds that are not individually distinguishable. Both individual and aggregate components are of immense importance in wastewater treatment for reuse or disposal. Examples of aggregate components include Biological Oxygen Demand, commonly referred to as BOD, Chemical Oxygen Demand (COD), Soluble Chemical Oxygen Demand (SCOD), Total Organic Carbon (TOC), Dissolved Organic Carbon (DTOC), Fats, Oil and Grease (FOG) and surfactants. Individual Organic compounds have been classified under priority pollutants by the U.S Environmental Protection Agency (USEPA) due to their suspected high toxicity and carcinogenicity. Examples include Phenols, Aldehydes, Halo acetic acids (HAA) etc. Table 3.2 gives a brief description of Inorganic constituents and Table 3.3 describes organic constituents Table 3.2: Inorganic constituents of wastewater. (Source Qasim, 1999) Parameter Description Concentration Concentration(Typical) (range) ph Indicates the acidity or basicity of wastewater Hardness(Ca 2+, Mg 2+ ) Chloride Inorganic Phosphorous Hardness is due to the presence of Calcium and Magnesium ions. Wastewater hardness depends on hardness of water supply Sources of Chloride include water supply, human waste, and leaching of soils in which the wastewater comes in contact Inorganic phosphorus exists as ortho-phosphate and polyphosphate mg/l 240 mg/l mg/l 50 mg/l 3 6 mg/l 4 mg/l Nitrogen As ammonia, nitrates, nitrites mg/l 20 mg/l 31

42 Wastewater characteristics Table 3.3: Organic Constituents of wastewater (Source Qasim, 1999) Parameter Description Concentration (Range) mg/l Concentration (Typical) mg/l BOD 5 It measures the amount of dissolved oxygen required by microorganisms to stabilize organic matter within the wastewater in 5 days. BOD deals with the biodegradable portion of the organic matter COD COD measures the amount of oxygen required to oxidize the organic matter using strong oxidizing chemicals such as potassium dichromate under acidic conditions TOC Like BOD and COD, TOC is also a measure of organic matter. TOC measures the total organic carbon present in an aqueous sample. This is determined by converting organic carbon into CO 2. The resulting gas can be measured using an infrared analyser or any other means. FOG Fats, oil and grease are commonly added from fats and oils used in foods in domestic wastewater; and kerosene, lubricating, and road oils from industries Organic Phosphorus Found in proteins and amino acids BIOLOGICAL CHARACTERISTICS Biological characteristics deal with the identification and activity of microorganisms present in the wastewater. Identification of these organisms is important for the prevention and control of diseases which could be detrimental to public health. However, not all of these organisms are pathogenic. Some are of immense benefit in the decomposition and stabilization of organic matter in biological wastewater treatment plants and in nature. The major group of microbes in wastewater treatment include fungi, protozoan, bacteria, viruses, algae and helminths. Due to the small number of pathogenic organisms and the associated difficulty in isolating and identifying these microorganisms, other more numerous and easily detected microbes are 32

43 Wastewater characteristics used as indicator organisms (Metcalf and Eddy, 2004). These indicator organisms when present in wastewater shows the presence of particular specie of pathogenic microbes in the wastewater sample. For example, coliform bacteria when detected in water samples reveal the possibility of faecal contamination. This shows that pathogenic bacteria associated with human and animal faecal waste would also be present. According to Qasim (1999), ideal indicator organisms should have the following characteristics: Easily detectable and the process should be reproducible The number should correlate with the degree of pollution Indicator organisms chosen should have at least equal survival time with pathogens The organism must be harmless to man and not grow in nature. It must reside in the intestinal tract of warm blooded animals. Table 3.4 below shows the various indicator organisms used to establish the quality of water for various uses. Table 3.4: Indicator organism for various water use (Source: Metcalf&Eddy,2004) Water use Drinking water Fresh water for recreation Saltwater recreation Agricultural irrigation Wastewater effluent Disinfection Indicator organism Total coliform Faecal coliform and E. coli Total coliform, faecal coliform, Enterococci Total coliform Total coliform Total coliform, faecal coliform, MS2 coliphage 33

44 CHAPTER 4 WASTEWATER TREATMENT TECHNOLOGY 4.1 INTRODUCTION Generally, municipal wastewater arriving at treatment plants contains approximately 99% water. The remaining fraction is made up of solids (dissolved and suspended), organic matter and inorganic compounds. The selection of a treatment process primarily depends on the characteristics of the wastewater such as its concentration and the nature of contaminants to be removed. Other factors that could affect this decision include desired effluent quality, available land space and treatment cost (Ramalho, 1983). Wastewater treatment technology has developed over the years. Initially, only physical methods such as settling and screening were employed to remove suspended particles. As understanding of wastewater composition improved, other chemical and biological processes have been incorporated into the treatment process. Moreover, these techniques have been of immense benefit as both (chemical and biological) have greatly increased the quality of effluent water that can be achieved. Wastewater treatment methods have been broadly classified into three: Physical, Chemical and Biological. This classification is based on the operational principle underlying the unit operation or process associated with each method. For instance, physical treatment methods are achieved through physical forces such as gravity whereas chemical treatment involves chemical reactions such as ion exchange. Figure 4.1 gives a summary of some unit operations associated with each treatment method. 34

45 Wastewater treatment technology Figure 4.1: Wastewater treatment methods (Source United Nations publication, ESCWA 2003: p.5) In order to achieve a desired effluent quality, these unit operations are often combined. The combination must be such that the liquid and (or) solid portion of the wastewater stream are effectively processed. Also, the quantity / flow rate of wastewater to be handled determines the type of treatment units to be included in the combination. Based on the combination of the unit operations within the treatment plant and level of treatment achieved, the treatment stages can be classified into four: Preliminary, Primary, Secondary, Advanced/Tertiary treatment. Each of these treatment stages and its associated unit operations will be considered in this chapter. 4.2 PRELIMINARY & PRIMARY TREATMENT Both of these stages are quite similar as they employ physical separation means to achieve the required treatment. They are the first stages in the treatment process and both help to ensure that the influent going to other treatment equipment downstream is of good quality. This helps to prevent damage to downstream process equipment and reduce maintenance cost. 35

46 Wastewater treatment technology Preliminary treatment deals with the removal of large particles, coarse solids, rags and many more obnoxious items. Grit chambers, comminutors and screens are usually employed at this stage. Immediately following the preliminary treatment is the primary stage where suspended solids and organic matter are partially removed. Flotation, sedimentation and pre-aeration (for odour removal) are frequently used in this stage. Sometimes, chemicals can be added to enhance precipitation of colloidal particles within the wastewater stream. Most of the solids are removed here as sludge and can be further processed economically before being ultimately disposed. Some of the unit operations in both stages are briefly discussed SCREENING Screening is one of the earliest physical methods of separation employed. Screens help to remove objectionably large matter from the wastewater stream and also help protect pumps from damage. The size of object retained on the screen depends on its mesh size. Screens can be parallel rods, bars or wire gauzes. Screens are cleaned either manually or mechanically depending on their location within the treatment unit. Screens are of two types: fine screens and coarse screens. Fine screens have smaller openings (mesh size) than coarse screens; thus, they remove more suspended matter than coarse screens. However, due to rapid clogging, coarse screens are preferred. Smaller particles passing through coarse screens are removed by sedimentation. The typical screen types are compared below in Table 4.1 Table 4.1 Different screen types (Source: United Nations publication, ESCWA 2003:p.6) 36

47 Wastewater treatment technology SEDIMENTATION Sedimentation is an integral process in wastewater treatment. It takes place in a settling tank commonly known as Clarifier. Sedimentation is based on the difference between the specific gravity of the solids and the water stream. It occurs under the influence of gravity. Sedimentation is usually utilised in one or more steps of a treatment sequence: grit chamber, where heavy particles like sand, suspended matter are removed; units preceding the biological reactor, where organic matter are removed ; and units immediately after the biological reactors, where biological sludge is removed from effluent. Three designs are common, namely: horizontal flow, inclined surface and solids contact. Figure 4.2 below shows a sedimentation tank with horizontal flow. Figure 4.2 Cross-section of a rectangular basin with horizontal flow (Source: United Nations publication, ESCWA 2003:p.8) FLOTATION Flotation as a unit operation involves passing air bubbles or fine gas upwards through a liquid. The gas molecules attach itself to suspended particles in the liquid and consequently, help the particles to rise to the surface. It is a useful means of separation for fine particles which are suspended within the liquid. Compared with sedimentation, it is faster and less expensive for removing suspended solids. At the surface, the particles are skimmed out. In wastewater treatment, air is the only flotation agent currently used. Figure 4.3 shows a typical flotation unit. 37

48 Wastewater treatment technology Figure 4.3 Typical Flotation Unit (Source: United Nations publication, ESCWA 2003:p.9) 4.3 SECONDARY TREATMENT Secondary treatment immediately follows the primary treatment stage. It is included to further improve on the quality of the effluent from the treatment plant. This is especially true when a desired water treatment quality is to be achieved. Secondary treatment is often made up of a combination of chemical and biological unit processes. It should be noted also that certain chemical unit processes can be used in conjunction with primary treatment to enhance its performance. An example is chemical precipitation where certain chemicals are added to precipitate solids out of solution. Examples of unit processes associated with chemical and biological operations are discussed below CHEMICAL PROCESSES Chemical unit processes involve the addition of chemicals to the water stream. This is done to achieve one or more of the following: precipitation, coagulation, oxidation, neutralization, scale control, disinfection, ion exchange and stabilization. Although an effective process in wastewater treatment, chemical operations are additive processes. This could be a disadvantage because adding chemicals to the wastewater streams increases the dissolved solid content of the stream. This is a factor if the water is to be reused and not discharged (Metcalf and Eddy, 2004). For example, in chemical precipitation, although the removal efficiency of the sedimentation process is enhanced, the total dissolved solid content (TDS) of the clarified liquid also increases. Another disadvantage of chemical processes is the cost of some of the chemicals used. Table 4.2 shows the various applications of some chemical 38

49 Wastewater treatment technology unit processes in wastewater treatment and figure 4.4 shows the flow diagram for a chemical treatment system. Table 4.2 Application of chemical unit processes (Source: Metcalf and Eddy, 2004) Process Application Advanced oxidation process Removal of refractory organic compounds Chemical coagulation Destabilization of particles to bring about their aggregation during flocculation Chemical disinfection Often done with chlorine, ozone and chlorine compounds. Control of slime growth, odours and destruction of pathogenic microbes. Chemical neutralization ph control. Chemical Oxidation BOD, grease, NH 3 removal. Also destroys microbes and removes resistant organic substances. Chemical precipitation Removal of suspended solids, Phosphorous and heavy metals Chemical Scale control Scale prevention due to calcium or magnesium carbonates. 39

50 Wastewater treatment technology Figure 4.4 Flocculation and Neutralization chemical process (Source: United Nations publication, ESCWA 2003:p.11) BIOLOGICAL PROCESS Given the right treatment conditions, biological treatment can be applied to all wastewater containing biodegradable components. Cultured micro-organisms mostly bacteria, are carefully introduced into the wastewater stream to convert dissolved organic matter into precipitated solids which can be removed by sedimentation or any other appropriate process. Unlike the physical and chemical processes, biological process requires adequate monitoring and close supervision to ensure that conducive conditions are provided for the organisms to thrive. For domestic wastewater, biological processes can be used to oxidize dissolved biodegradable components, convert colloidal solids into biofilm or floc, remove nitrogen, phosphorus and other nutrients, and finally, remove specific organic compounds (Metcalf and Eddy, 2004). In the case of industrial wastewater, pre-treatment before biological processes are applied is usually necessary. This is imperative because of the presence of certain toxic components in industrial wastewater. Such components compromise the biological process as the micro-organisms are either killed or rendered inactive. Generally, biological techniques are used for two specific objectives: reduce organic content of wastewater (mostly depicted in terms of its BOD, TOC, and COD concentration) and nutrient removal (especially phosphorus and nitrogen). These dual objectives can be achieved through different metabolic pathways. Based on metabolic functions and according to Metcalf and Eddy (2004), biological processes can be classified thus 40

51 Wastewater treatment technology Aerobic processes: Treatment taking place in the presence of oxygen. Anaerobic processes: Treatment occurring without oxygen. Anoxic processes: Conversion of nitrate nitrogen to nitrogen gas in the absence of oxygen. Facultative processes: Here, micro-organisms function in the presence or absence of molecular oxygen. Combined processes: Various combinations of anaerobic, aerobic and anoxic processes to achieve a specific treatment objective. Biological processes have also been classified based on the treatment process with respect to the position or location of the micro-organisms. Metcalf and Eddy (2004) highlighted four (4) categories based on this method of classification. These are briefly discussed below: a) Suspended growth process: In this process, the organisms are maintained in liquid suspension through adequate mixing. Many suspended growth processes are usually carried out under aerobic conditions; although, anaerobic applications do exist especially for industrial wastewater with high organic concentration. For municipal wastewater treatment, the most common application of suspended growth treatment is in the activated- sludge process. b) Attached growth processes: Unlike the suspended growth process, the microorganisms in this process remain attached or confined in an inert packing material. The packing materials used could be rocks, gravel, slag, plastics and other range of materials. The packing can be totally or partially submerged with the wastewater stream. Attached growth processes can also be operated as aerobic or anaerobic. The trickling filter process is based on this technique. c) Combined growth processes: A system where suspended and attached growth processes are combined. In municipal wastewater treatment, biological treatment techniques are usually applied in the following processes: activated sludge, trickling filters, aerated lagoons, rotating biological contactors, stabilization ponds and biological nutrient removal units. Some of these treatment units are briefly discussed Activated sludge process The activated sludge process was developed based on the observation that organic content in wastewater is reduced when the water stream is aerated over a period of time. Also, 41

52 Wastewater treatment technology flocculent sludge was observed to form on the water surface, which upon analysis was discovered to be a heterogeneous population of microscopic organisms. The organisms present are mostly bacteria and protozoan. This flocculent sludge was later described as activated sludge (because it comprises active microorganisms) and found to be responsible for stabilizing the wastewater stream under aerobic conditions. The process can be operated under continuous flow conditions. Clarified wastewater influent stream is delivered into aerated basins where it is mixed with active microorganisms. These organisms then degrade the organic contents of the wastewater stream into carbon dioxide and other products. The water stream is then clarified once more to remove the suspended microorganisms as biomass. This active biomass can then be reused in the aeration tank to continue the process for other influent wastewater stream. The flow process is illustrated below Figure 4.5: Flow diagram for activated-sludge process (Source: United Nations publication, ESCWA 2003:p.15) Trickling Filter This is the most common example of an attached growth process. In this process, wastewater is distributed over a packed media covered with biological slime (Ramalho, 1983). The waste water is uniformly distributed around the media and percolates downwards. The effluent is then collected at the bottom. Usually, the packed media has a fraction of void space such that air can be circulated to provide oxygen for the microorganisms (Metcalf and Eddy,2004). A cross section of the trickling filter is shown below. 42

53 Wastewater treatment technology Figure 4.6: Trickling filter cross section (Source: United Nations publication, ESCWA 2003:P.16) Biological Nutrient Removal (BNR) Processes Biological nutrient removal units are integral parts of wastewater treatment plants. They can be set up as distinct units or added on to other units within the plant. Typical nutrients which are removed are nitrogen and phosphorus. These nutrients have been discovered to cause eutrophication in water bodies. They encourage the growth of seaweeds and algae which makes water reuse impossible and create unsightly scene on water bodies where wastewater is discharged. In addition, significant concentration of these elements depletes the quantity of oxygen available in receiving waters, reduces the efficiency of chlorine disinfection and is toxic to aquatic animals. Although many chemical methods can be employed to remove these two nutrients, biological removal is more beneficial. Some of the benefits associated with BNR processes were highlighted by Qasim (1999). Qasim (1999) stated that BNR units have relatively lower operational costs, produce less sludge and that organic matter can be simultaneously removed. For nitrogen removal, two stages are involved. These are nitrification and denitrification. Nitrification is the first step and it involves the conversion of ammonia to nitrates. This is a two-stage oxidation reaction process carried out by nitrifying bacteria. The first stage is the oxidation of ammonia to nitrites by nitrosomonas bacteria then followed by the conversion of nitrites to nitrates by nitrobacter. The nitrification step is temperature sensitive and at very low temperatures, the activity of these nitrifying bacteria are adversely affected (Qasim, 1999). The next stage in nitrogen removal is the denitrification step where nitrates are then converted to nitrogen gas. Unlike nitrification, denitrification proceeds through a series of reduction reactions. Denitrification is an anoxic process usually facilitated by facultative 43

54 Wastewater treatment technology heterotrophic bacteria known as denitrifiers (Qasim, 1999). Temperature is also a factor because the growth rate of denitrifiers decreases with decrease in temperature. Phosphorous exists as orthophosphates, polyphosphate (major inorganic form) and associated element in fats, oils and proteins (organic forms). Microorganisms require phosphorus for cell synthesis and energy (Qasim, 1999). Therefore, each organism takes up phosphorus in an amount required for its biosynthesis. Acineobacter are the organisms mostly used for phosphorus removal. The process is arranged such that a sequence of anaerobic and aerobic zones alternates (Metcalf and Eddy, 2004). The biomass under anaerobic conditions releases inorganic orthophosphates. This is accompanied by extra phosphorus uptake in aerobic conditions (Qasim, 1999). Typical processes used for phosphorus removal are the A/O process, PhoStrip process and sequential batch reactors (SBR). The A/O and PhoStrip processes are illustrated below. Figure 4.7a A/O process (Source: United Nations publication, ESCWA 2003:p.21) 44

55 Wastewater treatment technology Figure 4.7b PhoStrip Process (Source: United Nations publication, ESCWA 2003:p.21) 4.4 TERTIARY TREATMENT Tertiary treatment is often referred to as advanced treatment. It deals with treatment options initiated to remove residual components from the wastewater stream after secondary treatment has been applied. This is becoming increasingly necessary due to stringent effluent quality demands imposed by government on wastewater treatment companies. More importantly, advanced treatment methods can also be used to target the removal of specific residual toxic components in the water stream to ensure that their concentration is reduced to the absolute minimum. With ongoing research into the environmental impact of residual components found in secondary treatment effluents, methods hitherto classified as advanced are expected to be conventional in the next 10 years (Metcalf and Eddy, 2004). Presently, this expectation is being fulfilled as water treatment facilities are being retrofitted to incorporate tertiary treatment options. An example is the San Diego potable reuse project where after the secondary treatment train, multimedia filtration, cartridge filters and reverse osmosis units have been added (USEPA, 2004). Other locations include Tampa, Florida; Denver Colorado; and Windhoek, Namibia (USEPA, 2004). Advanced wastewater treatment systems are classified based on the type of unit operation or process involved. Examples include advanced filtration (consisting of depth, surface and membrane filtration techniques), adsorption, gas stripping, ion exchange and advanced oxidation processes. Emphasis in this work is given to advanced filtration techniques 45

56 Wastewater treatment technology particularly membrane filtration systems due to their efficiency in handling a wide range of contaminants present in wastewater. Membrane processes are highly suitable for domestic wastewater treatment as they cope effectively with the removal of suspended solids, dissolved matter and microorganisms. The major problems with their utilisation has been the high capital and operational treatment cost; and operational constraint of rapid fouling. Fouling has been overcome by implementation of pre-treatment techniques using lime clarification and multi-media filtration prior to treatment (USEPA, 2004). Examples of membrane modules are Cellulose esters, Polyacrylonitrile and Polysulphides. Figure 4.8 below shows the particle size distribution chart for each membrane separation technique. Figure 4.8: Particle size separation comparison chart (Source: USEPA, 2004: p.109). From Fig 4.8, microfiltration (MF) systems are able to reject particles with size greater than 0.1µm, ultrafiltration (UF) effectively removes particles above 0.01 µm and reverse osmosis (RO) removes particles in the µm range. The general characteristics of these membrane processes as well as the performance data for MF and RO membranes are highlighted in the following tables below: 46

57 Wastewater treatment technology Table 4.3: Typical membrane process characteristics for wastewater treatment application ( Metcalf and Eddy, 2004) Membra ne process Driving force Pressure range (kpa) Pore size Separatio n mechanis m Typical flux rate; L/m 2.d Membrane type Micro filtration Ultra filtration Nano filtration Reverse osmosis Hydrostatic pressure difference or vacuum in open vessels Hydrostatic pressure difference Hydrostatic pressure difference Hydrostatic pressure difference Macro pores (>50nm) Mesopores (2 50nm) Micropore s (< 2nm) Dense (< 2nm) Sieve Sieve Sieve+ solution/ diffusion + exclusion Solution/ diffusion + exclusion Polypropylene, acrylonitrile,nylo n and polytetrafluoroet hylene Cellulose acetate, aromatic polyamides Cellulose acetate, aromatic polyamides Cellulose acetate, aromatic polyamides 47

58 Wastewater treatment technology Table 4.4a MF performance data based on flux rate of 1600L/m 2.day (Source USEPA, 2004:p.112) Table 4.4b RO performance data based on flux rate of 348L/m 2.day (Source USEPA, 2004:p.112) 48

59 Wastewater treatment technology The treatment system to be proposed in this work will be a combination of microfiltration and reverse osmosis units. Both have been selected based on the performance data obtained from literature. A two-unit treatment system is preferred to ensure efficient constituent removal and guarantee the delivery of high quality treated water that meets the domestic reuse standard. 49

60 CHAPTER 5 MODEL DEVELOPMENT 5.1 INTRODUCTION This section deals with the development of a superstructure model via mathematical optimisation to design a total water system for a typical household with four occupants. A total water system as defined by Gunaratnam et al (2005) consists of all water-using operations and water treating systems within a process. According to Gunaratnam et al (2005), the water using operations most often require different quantity and quality of fresh water to satisfy their respective process needs. Based on this concept, water use within the house can be grouped in terms of each water-using activity such as washing, bathing, flushing, kitchen requirements etc. However due to the sensitive nature of water use within the home, it is assumed that the reuse standard for these activities are similar; although the quantity of water required may differ. The total water system being proposed considers simultaneously the optimum distribution of water within the home to satisfy each of these activities and guarantees optimum treatment of waste water to meet reuse standard. Two implicit assumptions are made in this work. First, water use within the home is modelled as a continuous process. This is not the actual case in reality because we all use water in discrete amounts and at different times of the day. This, upon strict consideration is an example of a batch process. However, due to the fixed, regular and repeated pattern of water use which is complemented by the availability of storage tanks to hold treated and spent water separately, the system can be approximated a continuous process. Thus, it is safe to regard household water use as a repeated batch process which closely approaches a continuous system. The second assumption is that the mass loading of pollutants in each activity is constant and that no water loss occurs in the process either due to spillage during the activity or leakage in pipes during transportation to treatment units. The problem is intrinsically being considered as a mass transfer based operation. As a result, the mass loading of contaminants associated with each water-using activity is assumed to be constant irrespective of the age or sex of the individual involved and period of day when it is used. It is acknowledged that domestic wastewater flow rate varies with the period of day being considered reaching its peak in the early hours of the morning. This observation was taken into account by using average daily flow from each activity within the home. Operations like drinking, car-washing and lawn 50

61 Model Development watering have been excluded from consideration because there is no possible way of recovering the spent water for reuse or treatment. Also, toilet wastewater is excluded from treatment due to its level of contamination and the nature of treatment unit proposed. 5.2 DOMESTIC HOUSEHOLD WATER-USE As mentioned earlier, water use in the home has been grouped into different categories based on the appliance/activity involved. For the purpose of this work, five distinct groups have been identified: kitchen sink, bath and shower, wash basin, washing machine and toilet/water closet. Each of these operations requires different quantity of fresh water. More importantly, the effluent wastewater discharged had to be characterised at the source of discharge to enable proper analysis to determine the pollutant loading contribution of each activity. Few research efforts have been directed to determine residential wastewater characterisation by appliance discharge. The bulk of data available in literature gives a general residential wastewater composition just before inlet into sewer network. The data used in this work are based on the works of Siegrist et al (1976) and Laak (1974) as reported by Butler et al (1995). Both studies were carried out in the USA. The flow rate has been reported as Litres per head per day which makes it suitable for analysis. The data are reproduced below: Table 5.1 : Per Capita domestic water usage (source: Butler et al, 1995) Appliance Flow rate in litres/head/day Percentage of total daily usage (%) Seigrist et al(1976) Laak (1976) Seigrist et al(1976) Laak (1976) Water closet Kitchen sink Wash basin Bath and shower Washing machine

62 Model Development Table 5.2 : Per capita pollution load (mg/c/day) (Source: Butler et al,1995) Appliance Water Kitche Wash Bath and Washing Contaminant closet n sink basin * shower machine BOD COD TOC TSS TOTAL-P NH 3 -N NO 3 -N Washbasin*: given by Laak (1976); All other data as obtained by Seigrist et al(1976) + : No available data from either source. Assumed to be similar with bath and shower The contaminants listed in table 5.2 are only some of the few constituents of domestic wastewater. They represent the common and frequently analysed constituents of residential wastewater flows. They have been selected based on their importance in wastewater treatment and their possibility to limit water reuse. One notable contaminant not included in this list is the total coliform count or faecal coliform composition. This is an important parameter as it reveals the presence of pathogenic organisms which would endanger human health if contact through reuse occurs. Coliform content has been excluded in this analysis because of the assumption that they are totally removed within the treatment unit. Moreso, toilet effluent which has the highest contamination of faecal coliform, is not considered suitable for reuse in this work. Hence, all pathogens present in effluent streams of appliances considered for reuse are efficiently eliminated in the treatment unit; thus making the regenerated water safe for reuse. Fresh water supplied to residential households from water utility companies is assumed to be of potable standard. This potable water contains some concentration of dissolved nutrients such as phosphorous and nitrates. Phosphorus is usually added to prevent the dissolution of lead while water is being transported in the pipes. The water quality report used was obtained from Thames water website. A summary of potable water quality as supplied by Thames water is given below. The full quality report is located in the appendix A. 52

63 Model Development Table 5.3: Potable water standard (source: Thames water company, extracted 13/02/09; Abbey wood supply zone.) Parameter Units Prescribed conc value (PCV) Coliform bacteria Minimum concentration Mean concentration Maximum concentration No/100ml Number of samples Phosphate has a maximum dose of 3mg/l after which a steady concentration of mg/l is maintained in drinking water to effectively retain its corrosion inhibition property (US EPA, 2004). 5.3 TREATMENT UNITS The treatment units proposed for water regeneration within the home are microfiltration and reverse osmosis units. Both are membrane systems which are capable of handling contaminants present in residential wastewater. More importantly, regeneration concentration well above the non-potable reuse standards can be achieved. It is assumed that only the kitchen unit requires potable water supply. Hence, only fresh water is directed to this unit. All other activities within the household can be served with regenerated water. The removal ratios for both units with respect to some of the contaminants present in domestic wastewater are given below: Table 5.4: Average Removal ratio for MF and RO units (USEPA, 2004) Nitrate-NO 3 mg/l Nitrites-NO 2 mg/l 0.5 <0.01 < Ammonium- mg/l 0.5 <0.05 < NH 4 TOC mg/l PROCESS BOD COD TOC TSS TOTAL- P AMMONIA (NH 3 ) NITRATE (NO 3 ) MF RO The distributed effluent system technique was adopted for the development of the treatment unit superstructure. This method involves sending the effluent stream from each activity to 53

64 Model Development either of the treatment units. This ensures that no single unit is overburdened. Also it helps reduce treatment cost, as often times treatment cost is directly proportional to the flow rate handled. These are the benefits obtained over the centralized approach where all the effluent streams are combined and sent sequentially to each of the treatment units. With the centralized method, effluent streams that could otherwise be handled by a single treatment unit are treated twice and therefore not cost efficient. The superstructure adopted is given below: Effluent stream 1 MICROFILTRATION Effluent stream 2 REVERSE OSMOSIS Figure 5.1 : Superstructure for treatment unit 5.4 PROBLEM FORMULATION Defining the following sets: I = K = J = T = { i :i is a water using operation} ; i = 1,2,3,4,5 { k : k is any water using activity} ; k = { j : jis any contaminant present in the water} ; { t : t is any treatment unit} ; t = MF, RO 1,2,3,4,5; k i; j = A, B, C, D 54

65 Model Development Decision variables: F C C C C Y i W X X X i t, t : Fresh water flow from mains into operation in i, j in t, j : Waste i, k out i, j treat j t i R tr i fr i t water flow from operation i : Direct reuse water flow rate from operation k into i : Flow rate from operation i into treatment unit : Flow from treatment unit into operation i :Concentration of contaminant j out of operation i :Concentration of contaminant j into operation i :Concentration of contaminant j into treatment unit t : Concentration of : Flow rate leaving treatment unit t contaminant j leaving treatment system L : Flow rate into treatment unit t from operation i : Flow from one treatment unit into the other Parameters: M C C C C r max in i, j max out i, j fresh j reg j t, j j : Mass load of contaminant : Proposed : Removal ratio of contaminant j in treatment unit t j : Maximum inlet water concentration of contaminant j into operation i : Maximum outlet water concentration of contaminant j from operation i :Concentration of contaminant j present in fresh water stream non - potable reuse regeneration concentration from treatment i Equations and constraints for water-using operations X i,k W i F i Tank Operation i Tank X tr i X fr i X k,i Figure 5.2: Schematic representation of water using operation process 55

66 Model Development Equation 1: Overall water balance: k X FR TR i, k Fi + X i Wi X i X k, i = k + Equation 2: Mass Balance on operation i C + 0 out fresh FR treat ( X i, kck, j ) + FiC j + X i C j M i, j out TR i, j X k, i Wi + X i = k k Equation 3: Inlet concentration constraint: C X C + F C + X i, k out k, j i fresh j FR i in k i, j = FR X i, k + Fi + X i k C max in i, j Simplifying, k X max in out max in fresh FR max in treat ( C C ) + F ( C C ) + X ( C C ) 0 i, k i, j k, j i i, j j i i, j j Equation 4: Outlet concentration constraint C M out in i, j i, j = Ci, j + FR X i, k + Fi + X i k C max out i, j Substituting for C, using equation 3; and simplifying in i j k X max out out max out fresh FR max out treat ( Ci, j Ck, j ) + Fi ( Ci, j C j ) + X i ( Ci, j C j ) M i, j i, k Equation (2) ensures that water of adequate quantity is supplied to the process to ensure the removal of the fixed mass of contaminant. Equation (3) guarantees that the quality of the water supplied to the operation is adequate. Equation (4) ensures that the outlet concentration approaches the maximum allowable. Although from equation (2), the fixed mass of contaminant j is completely removed; its concentration may not be at the maximum in the effluent stream due to increased flow rate needed to cope effectively with the removal of other contaminants. Hence there is some fresh water penalty associated with Equation (4). 56

67 Model Development 5.4.2: Treatment Unit Equations Equation 5: Flow rate balance on effluent stream TR X i = L MF, i + L RO, i Equation 6: Flow rate balance across each treatment unit MF: + Y = Y + i L MF, i MF, RO RO, MF R MF RO: + Y = Y + i L RO, i RO, MF MF, RO R RO Equation 7: Regenerated water balance: i X = R + FR i MF R RO Equation 8: Treatment Unit inlet concentration balance in out in in MF: L MF, i ( C MF, j C i, j ) + Y MF, RO ( C MF, j ( 1 rro, j ) C RO, j ) = 0 i in out in in RO: L RO, i ( C RO, j C i, j ) + YRO, MF ( C RO, j ( 1 rmf, j ) C MF, j ) = 0 i Equation 9: Outlet treatment concentration balance R MF in treat in treat ( 1 r ) C C ) + R ((1 r ) C C ) 0 ( MF, j MF, j j RO RO, j RO, j j = Equation 10: Outlet concentration constraint R MF in reg in reg ( 1 r ) C C ) + R ((1 r ) C C ) 0 ( MF, j MF, j j RO RO, j RO, j j Equation (10) ensures that the treatment units meet the concentration requirements for nonpotable reuse. Equation (7) shows the water balance across each treatment unit and Equation (9) deals with the inlet concentration balance for contaminants at the treatment units. 57

68 Model Development Operational Considerations Due to the sensitive nature of the process being considered the following additional constraints have been imposed on the design: Equation 11: No direct reuse that is, X i, k = 0. This ensures that all effluent streams are treated before being reused. This is imperative for hygienic reasons. k Equation 12: Toilet wastewater is not to be treated and no fresh water for flushing. TR X 5 = 0 ; F 0 5 = Equation 13: Minimum wastewater bleed-off W i FR 0.25 ( Fi + X + X, i k i k ) Equation 13 is very important because of the possibility of having a recycle stream. This consideration becomes more important when a particular water-using activity is being serviced by only a recycled stream. By bleeding-off some of the spent water, we can control the build-up of trace contaminant in the process Data presentation Scaling up the data obtained by Seigrist et al (1976) as presented in table 5.1 and table 5.2 to represent average flow rate and contaminant concentration for a residential dwelling with 4 occupants, the following result is obtained. Table 5.5: Flow rate data for a 4-occupant household APPLIANCE WATER USAGE (L/DAY) WATER USAGE (te/day) Water closet (WC) Kitchen sink Wash basin* Bath and shower Washing machine Washbasin*: Based on Laak (1976) 58

69 Model Development Table 5.6: Contaminant Concentration (mg/l) per appliance Water closet Kitchen sink Wash basin Bath and shower Washing Machine APPLIANCE CONTAMINANT BOD TSS Total-P* NO 3 * NOTE: BOD has been selected as the representative parameter for organic material present in the water stream. COD and TOC are related measurements for the organic constituent. NH 3 has been excluded because it is a major component of toilet wastewater which is not being considered for reuse or treatment in this work. Total-P*, NO 3 *: Phosphate and Nitrate concentration have been adjusted to incorporate residual composition of both components in potable water as supplied by water utility companies. Table 5.7: Fresh water and prescribed concentration for regenerated water Contaminant Fresh water C FRESH Prescribed regenerated water concentration, C regen BOD TSS TOTAL-P NO

70 Model Development Table 5.8: Operating data for the water using activity within the home OPERA TION APPLIANCE CONTAMI NANT 1 KITCHEN SINK* BOD TSS Total-P NO3 2 BATH BOD TSS TOTAL-P NO3 3 WASHBASIN BOD TSS TOTAL-P NO3 4 WASHING MACHINE BOD TSS TOTAL-P NO3 C IN MAX C OUT MAX F (L/D AY) MASS (g) Toilet/WC BOD TSS TOTAL-P NO Kitchen sink*: Only kitchen is allowed to use potable water. Other appliances can use a mixture of potable and regenerated water Objective function Two designs have been undertaken in this work. These are minimum fresh water network design and minimum annualized cost design. Each has a unique objective function. a) Minimum fresh water network: The objective is to determine the minimum flow rate per day of fresh water that can be supplied to satisfy the various water demands. The objective function is given below: obj : min F tot s.t F = tot F i i and equation (1) - (14) b) Minimum annualised cost network: Here, emphasis is on the total cost of the network. The total cost considers cost associated with fresh water supply, wastewater 60

71 Model Development treatment unit and piping cost to link the treatment unit with wastewater tank. Existing piping in the house linking fresh water storage tank to kitchen, bath, washing machine and toilet are excluded from analysis. The problem is considered as a retrofitting exercise. Annualised cost function (AC) = FC + CRO + CMF + CPL Fresh water cost (FC): This is expressed as follows FC ($/year) = a * F tot (where a: based on prevailing water cost in the UK) Treatment cost: Wastewater treatment cost was based on the assumption that its cost is directly proportional to the effluent flow rate entering a particular treatment unit. Cost was computed using the one-sixth rule by comparing cost of existing plants. The six-tenth rule is stated below: Cost new = Cost existing Capacity Capacity new existing n Cost of Reverse Osmosis (CRO) : Cost estimation was done based on existing units for desalination. The data used was obtained from Rosaler (2002). ( F ) Cost RO = * RO Cost of Microfiltration unit (CMF): Cost for the micro filtration unit was assumed to be similar with that obtained for Ultra filtration unit as given by Water Research Commission (2009): ( F ) Cost MF = * MF Piping cost (CPL): The cost for pipe network was based on the cross-sectional area of the pipe and flow velocity through it. Flow velocity was assumed to be 1m/s. Cost = pipe b ( F ) 0. 5 * pipe b= (Carbon steel pipe based on Chemical engineering equipment index, CE, 1997 = 389.1). All cost in US dollars. 61

72 Model Development The objective function can be stated thus: obj : min C cos t Subject to: cos C t = FC + CRO + CPL F tot = F i i And equation (1) (14) Solution strategy The minimum freshwater design has a linear objective function which is subject to a set of linear and non-linear constraints. Similarly, the minimum cost design is also subject to these linear and non-linear constraints but with a non-linear objective function. In both cases, the optimisation problem developed is a non-linear programming (NLP) formulation. The optimisation was done using the GAMS (General Algebraic Modelling System) platform. The NLP problem was solved using SNOPT solver. SNOPT was chosen because of its ability to handle large-scale NLP problems with linearly and smooth nonlinearly constrained problem, as present in this work, using the sequential quadratic programming (SQP) method. Appropriate initial conditions were specified based on physical insights into the problem to provide a good initialisation point. 62

73 CHAPTER 6 RESULT ANALYSIS 6.1 FRESH WATER NETWORK DESIGN The minimum fresh water flow rate required to satisfy the water demands in the house was determined as 238 L/day. This represents about 57.79% total fresh water savings. Also, it was observed that only 3 operations require fresh water supply. These operations are kitchen activities, bathing and shower, and washing machine. However, unlike the kitchen and bathing activities that use only fresh water, regenerated water from the treatment unit is mixed with fresh water to meet the washing machine water demand. Also, water requirements in the wash basin and toilet are satisfied completely with regenerated water. In addition, since no constraint violation is observed especially with respect to contaminant concentration, it implies that the specified flow rate is sufficient to cope with the contaminant mass load of each appliance. A summary of the results obtained are shown below: Table 6.1: Fresh water savings within each operation Operation Activity Observed Fresh water flow rate (L/day) Minimum Fresh water flow rate (L/day) Regener ated water flow rate (L/day) Waste water generate d (L/day) Fresh water saving within the operation 1 Kitchen Bath and % shower 3 Wash basin % 4 Washing % machine 5 Toilet % 63

74 Analysis of Result Fresh water requirements of each activity kitchen bath wash basin washing machine toilet Observed Fresh water flow rate Minimum fresh water Figure 6.1: Fresh water requirements of each activity 120 Fresh water savings (percentage) Fresh water savings 20 0 kitchen bath wash basin washing machine toilet Figure 6.2: Fresh water savings per activity in percentage The results above were obtained based on the requirements that at least 25% of the spent water generated is bled-off as wastewater to control the accumulation of trace contaminants within each activity. Without this consideration, a minimum total fresh water requirement of about 139 L/day is achievable. This represents a total savings of 75.32%. However, the build-up of trace contaminants not removed in the treatment unit may limit water reuse in the long run. 64

75 Analysis of Result Waste water to sewer Kitchen MF Waste water Fresh water Tank Waste water Bath Regenerated water Treatment Unit RO Tank Washing hi Toilet Waste water Tank Wash basin Waste water to sewer Figure 6.3: Proposed Network 6.2: MINIMUM COST DESIGN The minimum annual cost for the network design was obtained as $3119. This cost incorporates capital cost for the treatment units, piping cost and annual cost of fresh water supply. This network will require fresh water flow rate of 240 L/day. This represents fresh water savings of about 57.36%. This represents about 1% increase in fresh water flow rate when compared with the result obtained for minimum fresh water design. A summary of the result obtained is tabulated below 65

76 Analysis of Result Table 6.2 Result obtained for minimumm cost design Operation Activity Observed Fresh water flow rate (L/day) Minimum Fresh water flow rate (L/day) Regener ated water flow rate (L/day) Waste water generated (L/day) Fresh water saving within the operation Kitchen 72.1 Bath and 152 shower Wash basin 32 Washing 164 machine Toilet % 55.31% 87.31% 100% Fresh water savings( percentage) Fresh water savings kitchen bath wash basin washing machine toilet Figure 6.4: Fresh water savings for Minimumm cost design Comparing figure 6.3 with figure 6.2, we observe a decrease in fresh water savings in wash basin activity and an increasee in savingss in washing machine operation. Fresh water savings in other activities remained constant. Sensitivity analysis was performed on the minimumm cost design to determine the relationship between annual cost and number of households connected to the treatment unit. Here, it was assumed that each house has the same number of occupants (4), similar daily water use per activity/appliance and within the same distance from the treatment unit. The sensitivity analysiss result is given below: 66

77 Analysis of Result Table 6.3: Sensitivity analysis result Number of Total fresh water flow rate Annualised cost of Cost per Household Household (L/day) network ($) ($/house) From the results presented in table 6.3, the average fresh water flow rate per household is 240 L/day. This implies that constant freshwater savings of 57.36% can be achieved in all the households connected to the treatment unit. It can also be observed that the annualised network cost increases linearly with the number of household considered but the cost per household decreases. This shows that it is more economical to have as many houses as is feasible to be connected to the treatment system. However, the feasible number of houses will be limited by the capacity of the treatment unit. 67

78 Analysis of Result Annualised cost COst ($) Annualised cost Number of household Figure 6.5: Variation of annualised cost with number of households Cost per household 3500 Cost per house ($/House) Cost per household Number of household Figure 6.6: Variation of cost per household with number of households 68

79 CHAPTER 7 CONCLUSION AND RECOMMENDATIONS A total water network has been developed for a typical residential dwelling using a robust strategy. The optimisation technique employed ensures that water-using operations and treatment units are efficiently integrated. Water distribution between these systems is simultaneously optimised to fully consider fresh water trade-offs within the system. The optimisation problem was an NLP formulation and was solved using GAMS platform. Two design scenarios were considered: minimum fresh water design and minimum annualised cost design. For both cases, a fresh water savings of about 57% was achieved. The annualised cost for the proposed network was obtained as $3119 for a single household. This cost comprises all cost associated with treatment, piping and fresh water supply. The network cost per household was found to decrease as more households are connected to the treatment system. The cost per house was reduced to as low as $1726 for ten houses connected to the treatment system. However, the fresh water requirement per house remained constant at about 240 L/day. Thus, in order to fully exploit the benefit of the total network system, it is recommended that the capacity of the treatment unit be such that can handle effluent water from as many households as possible. Also, within each water-using operation in the home, fresh water savings ranged from 12.3% for bath and shower to 100% for toilet flushing. This suggests that toilet water demands can be safely met with regenerated water. This goes to show that potable water as currently being supplied by water utility companies is not required to satisfy water demands of some household activities. It is acknowledged that being a non linear programming formulation the result obtained might not be the global optimum. However, the strategy employed shows the possibility for water savings within the home and highlights areas for improvements in potable water distribution. Besides, it can be extended to explore water savings in other urban establishments such as schools, offices and other commercial edifice. This will shape our attitude towards water usage and ensure sustainable fresh water supply. 69

80 Conclusion and recommendations Another major problem associated with the total water network design being proposed is its implementation. This problem arises because the conventional water network within the home handles only potable water supply. A retrofitting exercise must be done to incorporate the regenerated water stream into the existing pipe network. Also, the retrofitted network must ensure the delivery of the right water quality for every water-using operation within the home. For instance, it has been demonstrated earlier that while certain activities solely require potable water to meet its demands, others require the blending of potable and regenerated water or only regenerated water to satisfy their water needs. This represents one of the few issues that must be considered during the retrofitting exercise. It is acknowledged that the proposed design will be easily implemented for new houses than existing ones. For effective implementation, the following strategies can be adopted as safety measures to track flow of regenerated water stream used for non potable applications within the home: Use of separate storage tanks for holding potable water and regenerated water Pipe network for distributing regenerated water must be clearly distinguishable from potable water supply lines. This can be achieved by using pipes of a particular colour to distribute regenerated water. At junctions where blending or mixing occurs, a check valve can be used to prevent backflow of regenerated water into potable supply lines. Wastewater must be sent immediately for treatment and should not be stored. This is necessary for health reasons. After treatment, some degree of disinfection may be necessary during storage of regenerated water to control growth of microorganisms. It is recommended that regenerated water should not be stored for too long. Future research efforts could be directed on incorporating alternative water sources such as rain water into the water network proposed. This will help reduce the proposed minimum fresh water target obtained in this work. More importantly, rain water is of high quality and can be used directly without treatment to satisfy some of the water demands within the home. As a result, the quantity of regenerated water required will be reduced thereby decreasing the associated cost of treatment and overall network cost. However, seasonal variation of rain water quantity is a design constraint that should be considered. 70

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86 APPENDIX Appendix A: Fresh water quality report 76

87 77