Desalination Process Engineering Part I

Transcription

1 Desalination Process Engineering Part I Mark Wilf Ph. D

2 1. RO feed water quality requirements 1.1. Introduction 1.2. Feed water types 1.3. Sparingly soluble constituents 1.4. Particulate matter 1.5. Organic matter 1.6. Biological activity 1.7. Feed water temperature 1.8 Feed water salinity Feed water ph. 2. Feed water supply alternatives 2.1. Introduction 2.2. Brackish wells 2.3. River bank filtration wells 2.4. Beach wells 2.5.Horizontal wells 2.6.Slant wells 2.7.Seawater infiltration galleries Open surface intake Configuration of open intakes Estimation of intake cost Collocation with power plant Design criteria of feed water supply systems and configurations Equipment and hydraulic profile 3. Configuration of feed water pretreatment process 3.1. Conventional pretreatment Configurations and components of conventional pretreatment system 3.2. Membrane pretreatment 3.3. Raw water quality and pretreatment requirements. 4. Pretreatment equipment 4.1. Screening 4.2. Dissolved air flotation (DAF) Example of calculation of operating cost of the DAF unit Coagulation and flocculation 4.4. Granular media filtration Pressure filter Gravity filters Solids management system 2

3 Pretreatment system design method 4.5. Cartridge filtration 4.6.Membrane pretreatment Configurations and components of membrane pretreatment system Settling and screening 4.7. Filtration membranes and membrane unit configuration Fundamentals of the membrane filtration water transport process Membrane material and membrane configurations Membrane filtration process Pressure driven technology Configuration of pressure driven membrane filtration unit Sizing of pressure driven membrane filtration unit Operating cost of pressurized membrane filtration unit Configuration of immersed, vacuum driven, membrane filtration system Sizing of immersed, vacuum driven, membrane filtration unit Operating cost of immersed membrane filtration system Comparison of conventional and membrane filtration technologies as pretreatment for seawater RO desalination systems Offering of commercial membrane filtration technology 5. RO System 5.1.Membrane elements and pressure vessels 5.2.Membrane unit configuration Single stage and multistage Sideport, multiport and center port configuration Two pass, partial two pass, split partial 5.3.Membrane cleaning Configuration of membrane cleaning unit Sequence of operation of cleaning unit 5.4.Membrane flushing unit configuration 5.5.RO membranes unit design criteria in accordance with feed water quality Feedwater quality parameters Membrane fouling Oxidative degradation of membrane performance Colloidal fouling Fouling by organic matter Biofouling Inorganic scale and determination of permeate recovery rate 5.6.Average permeate flux 5.7.Membrane unit design procedure Permeate capacity and permeate quality limits Selection of average permeate flux, recovery rate and array 3

4 Selection of membrane type Membrane train size and configuration Utilizing computer programs in membrane unit design Performance safety margins Configuration of RO membrane unit for high feed salinity operation 100,000 m3/day product water capacity. 6.0.Design for RO high pressure pump and ERD 6.1.Raw water supply and transfer pumps 6.2.High pressure pumps 6.3.Optimized control methods for high pressure pump discharge head and capacity Application of energy recovery devices (ERD) in RO systems Selection of ERD Pelton wheel Turbocharger Pressure exchangers (isobaric devices) Cost and economic benefits of ERD 7.0.Chemical dosing equipment design 7.1.Selection criteria for chemicals used in the RO process 7.2.Procedure for determination of chemicals dosing rate 7.3.Criteria for sizing of chemicals storage equipment 7.4.Selection of chemical dosing pumps capacity and materials of construction 7.5.Example of sizing of chemical dosing system for SWRO plant of permeate capacity of 100,000 m3/day operating at recovery rate of 45%. 8.0.Instrument and control system 8.1.Process control strategy 8.2.Control loops in RO system 8.3.Process control and performance normalization software 8.4.Instrument selection criteria and their location in the RO system 8.5.Frequency of data collection and representative range of operating parameters. 8.6.Methods of control of operation of chemical dosing systems 8.7.Pumps process control in brackish and nanofiltration applications 8.8.Pumps process control in seawater applications Train dedicated configuration Pressure center configuration 9.0.Selection of materials of construction of equipment and components Example of design of 100,000 m3/day brackish RO desalination system Raw water source Product water quality Pretreatment system Equipment description 4

5 10.5. Cleaning in place (CIP) unit Post treatment Equipment list Computer projections for RO membrane unit without blending Computer projections for RO unit with blending Example of design of 100,000 m3/day seawater RO desalination system 5

6 Figure 1.1. RO Technology Mark Wilf Ph. D. Concentration factor of concentrate stream in RO applications Desalination Process Engineering Manual List of Figures 6

7 Figure 1.2. Schematics of spiral wound RO membrane element Figure 1.3. Schematics of configuration feed concentrate channel in RO membrane element. Figure 1.4. Schematic configuration of Silt Density Index (SDI) apparatus. Figure 1.5. SEM picture of clean filter pad, before SDI memasurement. Figure 1.6. SEM picture of clean filter pad, after SDI measurement. SDI = 2.2 Figure 1.7. SEM picture of clean filter pad, after SDI measurement. SDI = 4.8 Figure 1.8 Temperature correction factor for polyamide composite membranes. Figure 1.9 Temperature effect on permeate salinity in brackish RO systems Figure Temperature effect on permeate salinity in seawater RO systems. Feed pressure values are marked as red bars and permeate salinity as gray bars Figure 2.1. Schematic configuration of veridical beach well. Figure 2.2. Schematic diagram of collector well Figure 2.3. Schematics of horizontal directional drilling (HDD) intake Figure 2.4. Model of HDD intake system at 82,000 m3/day seawater RO desalination plant, Alicante, Spain. Courtesy Neodren. Figure 2.5. Infiltration gallery at sweater RO desalination plant, Fukuoka, Japan. Figure 2.6. Schematic configuration of open surface off shore seawater intake. Figure 2.7. Example of off shore intake structure (courtesy Ian Larsen) Figure 2.8. Alternative configuration of intake structure (courtesy Ian Larsen) Figure 2.9. Installation of concrete anchors on intake pipe (courtesy Ian Larsen). Figure Delivery of intake pipe with concrete anchors installed to the plant site (Courtesy Oceana) Figure Intake and discharge system at SWRO desalination plant, Fukuoka, Japan Figure Conveyance of the feed intake and concentrate discharge lines at Fukuoka, Japan Figure Concentrate discharge structure at the SWRO desalination plant, Fukuoka, Japan. Figure Flow diagram of SWRO desalination plant collocated with power plant at Carlsbad, CA. Figure 3.1. Configuration of RO unit operating with well water. Figure 3.2. Configuration of pretreatment unit in desalination plant treating well water. Figure 3.3. RO system treating surface water. Figure 3.4. Schematic configuration of seawater desalination system with membrane pretreatme nt. Figure Dual flow intake band screen (web page Horim Industries Inc., Korea) Figure Automatic disc strainers. (Arkal Filtration Systems) Figure Schematic diagram of DAF system integrated with multi media filtration unit (courte sy UnitedKG) 7

8 Figure Figure Figure Figure Picture Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure RO Technology Mark Wilf Ph. D. Configuration of DAF air saturator Relation between air pressure and concentration of dissolved air in water. Relation between turbidity of DAF effluent and concentration of dissolved air in water. Picture of DAF installation at the Tuas, Singapore, SWRO desalination plant. Transition of Turbidity and Zeta Potential with increasing dose of coagulant (court esy Peter Hillis). Schematic diagram of configuration of coagulation flocculation unit Drawing of a vertical pressure filter (courtesy of Tonka Company) Drawing of horizontal filter (courtesy of Tonka Company) Examples of filter nozzles (web page of FTR, Istanbul) Configuration of effluent flow control in a pressure filter Configuration of vertical pressure filters with valves required for utilization of internal source backwash water. Configuration of horizontal four chambers pressure filter with valves required for utilization of internal source backwash water. Alternative configuration for backwash of pressure filters from the high service line. Configuration of SWRO desalination plant at Carboneras, Spain Schematic configuration of a gravity media filter. Configuration of filtration layers in a gravity filter Schematics of filtration driving head in the gravity filter. Aerial picture of sweater RO desalination plant showing layout of gravity filters (courtesy GES Engineering). Schematic diagram of filtration system including solids management unit. Jar test equipment utilized in estimation of the required dosing rate of alum based coagulant. (Courtesy Peter Hillis). Schematic diagram of a pilot unit for testing of a gravity filtration process. Schematic configuration of cartridge filtration unit. Picture of horizontal housing of cartridge filter. Horizontal cartridge filter housing in open position. Block diagram of submersible membrane filtration system Block diagram of pressure driven membrane filtration system Microstrainer configuration offered by Arkal. Side view of microstrainer assembly. System width 13.9 m Top view of microstrainer assembly. System length 10.1 m Wedged screen strainer configuration Temperature correction factor vs. water temperature 8

9 Figure Separation size range of filtration technologies. Figure Filtrate flow direction in pressure drive capillary membranes: a PDI, b - PDO. Figure Filtration step direct flow mode of operation Figure Backwash step. Figure Integrity test sequence. Figure Configuration of pressure driven membrane filtration unit Figure Pressure driven membrane filtration system (courtesy Simens Water Technologies) Figure a. Valves position during filtration step Figure b. Valves position during backwash step Figure c. Valves position during backwash step Figure d. Valves position during chemical enhanced backwash step Figure e. Valves position during system draining step Figure f. Valves position during air pressurizing step Figure Configuration of immersed, vacuum driven, membrane filtration unit Figure Layout of immersed membrane filtration system (courtesy Siemens Water Technologies) Figure Configuration of conventional pretreatment system for SWRO Figure Configuration of immersed membrane pretreatment system for SWRO Figure Membrane filtration train with Hydracap modules Hydranautics Figure Membrane filtration train with Xiga modules Norit Figure Membrane products offered by Inge Figure Membrane products offered by Pall Asahi Figure Pressurized membrane filtration products (CP) offered by Siemens Figure Immersed membrane filtration products (CS) offered by Siemens Figure Immersed membrane filtration products (ZW-1000) offered by GE (Zenon) Figure mm by 1000 mm spiral wound element Figure mm by 4000 mm spiral wound element Figure Configuration of pressure vessel with membrane elements Figure Flux distribution along the length of pressure vessel Figure Single stage membrane unit configuration Figure Schematic diagram of two stage membrane unit Figure Mechanical drawings of RO membrane train Figure Configuration of a side port pressure vessel Figure Single stage membrane unit with side port pressure vessels Figure Single stage membrane unit with multiport port pressure vessels Figure Two stage membrane unit with multiport port pressure vessels - configuration 1 9

10 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 6.1. Figure 6.2 Figure 6.3. Figure 6.4. Figure 6.5 Figure 6.6 Figure 6.7. Figure 6.8 Figure 6.9. Figure Figure Figure Figure 6.13 Figure 6.14 Figure 6.15 Figure 6.16 RO Technology Mark Wilf Ph. D. Two stage membrane unit with multiport port pressure vessels - configuration 2 Schematic configuration of membrane unit utilizing center port pressure vessels Schematic configuration of a two pass unit Two pass system with second pass concentrate recirculation Schematic configuration of partial two pass processing Permeate salinity distribution along the pressure vessel Split partial two pass configuration Two pass split partial processing Configuration of membrane cleaning unit Alternative configurations of membrane trains in large capacity RO plants Computer projections program water analysis data entry screen Computer projections program process parameters and membrane array entry screen Computer projections program membrane elements look up table Computer projections program screen display of calculation results Printout of calculation results Split partial configuration of a 12,500 m3/day SWRO train Energy usage in RO desalination systems Pressure centers configuration of a large capacity SWRO plant. Pelton Wheel Pelton Wheel electric motor high pressure pump unit Concentrate foaming at the Pelton Wheel outlet Configuration of Hydraulic Turbocharger Brackish RO train with Hydraulic Turbocharger in the interstage position Hydraulic Turbocharger positioned after high pressure pump in seawater RO unit Examples of configurations of seawater (left) and brackish (right) RO units with Hydraulic Turbocharger Example of calculations of pressure boost provided by Hydraulic Turbocharger in seawater and brackish RO membrane unit Hydraulic Turbocharger equipped with electric motor. Schematic configuration of RO membrane unit with isobaric energy recovery device. Configuration of DWEER energy recovery device DWEER isobaric EDR assembly operating in 330,000 m3/day SWRO plant, Ashkelon, Israel. Configuration of PX energy recovery device (ERI) Large assembly of PX ERD s. 10

11 Figure Figure 8.1. Figure 8.2. Figure 8.3 Figure 8.4. Figure 8.5 Figure 8.6. Figure 8.7. Figure 8.8 Figure 8.9 Figure 8.10 Figure 8.11 RO Technology Mark Wilf Ph. D. ISave ERD introduced by Danfoss Basic process control of RO membrane unit Schematic configuration of control system in RO desalination plant Control system configuration no backup control equipment Control system configuration hot backup (PLC only) Control system configuration complete backup (PLC and IO s) Pumping units and RO membrane trains in train dedicated configuration Pumping units and RO membrane trains in pressure centers configuration Pumping unit with Pelton Wheel EFD High pressure pump and hydraulic turbocharger in feed entry position 10 Hydraulic turbocharger in the interstage position High pressure pumping unit utilizing isobaric ERD 11

12 Desalination Process Engineering Manual List of Tables RO Technology Mark Wilf Ph. D. Table 1.1. Table 2.1. Table 2.2. Table 2.3. Table 2.4. Table 2.5 Table 2.6. Table 3.1. Table 3.2. Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Examples of representative compositions of brackish, sea water and secondary wastewater effluent. Listing of large SWRO desalination facilities that utilize beach wells Economic weight of various alternatives of sea water supply to RO desalination plants Intake outfall system cost parameters Cost components of intake outfall system. 300,000 m3/day inflow, 150,000 m3/day outfall Example of values of constants used in Haze Williams and Darcy equations Example of pipe friction loses according to pipe diameter. Representative raw water quality associated with major types of water supply sources Recommended configuration of pretreatment system according to raw water quality Representative design parameters of DAF system Example of energy usage of air saturation unit for a DAF system for the raw water flow of 1000 m3/hr. Design parameters of a DAF unit, 100,000 m3/day effluent capacity Preliminary specifications of coagulation flocculation unit. Nominal flow capacity 100,000 m3/day. Range of specifications parameters of filtration media. Recommended values of filtration media parameters Design parameters of media filtration system utilizing horizontal filters. System effluent capacity 100,000 m3/day. Design parameters of media filtration system utilizing gravity filters. System effluent capacity 100,000 m3/day. Design parameters of solids management unit for a filtration system. Filtration system capacity 100,000 m3/day Example of the permeability results at ex-factory test and during field operation Attributes of PDI and PDO membranes configuration Summary of process parameters of pressure driven and submersible technologies. Air assisted backwash of the pressurized (PDI) membrane filtration system Sequence of operation of pressure driven membrane filtration unit. Water quality parameters of surface water source 12

13 Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table RO Technology Mark Wilf Ph. D. Example of operational parameters and schedule of pressurized membrane filtration system Example of sizing calculations of 200,000 m3/day pressurized membrane filtration system Example of sizing of chemicals dosing equipment in pressurized membrane filtration system Listing of subunits and major equipment in pressurized membrane filtration system Breakdown of energy usage in pressurized membrane filtration system. Filtrate capacity 200,000 m3/day Summary of volumes required for CEB and CIP in pressurized membrane filtration system Summary of chemicals usage in pressurized membrane filtration system. Filtrate capacity 200,000 m3/day. Summary of operating cost of pressurized membrane filtration system Sequence of operation of immerse, vacuum driven, membrane filtration unit. Example of operational parameters and schedule of immersed membrane filtration system Example of sizing calculations of 200,000 m3/day immersed membrane filtration system Listing of subunits and major equipment, immersed membrane filtration system Breakdown of energy usage in immersed membrane filtration system. Filtrate capacity 200,000 m3/day Summary of volumes required for CEB and CIP in immersed membrane filtration system Summary of chemicals usage in immersed membrane filtration system. Filtrate capacity 200,000 m3/day. Summary of operating cost in immersed membrane filtration system Summary of comparison of relative advantages of multimedia and membrane filtration systems for SWRO applications Estimated capital cost of multimedia filtration and membrane filtration units. Filtrate capacity 200,000 m3/day Listing of established suppliers of commercial membrane filtration products Recovery rates of individual elements in pressure vessel according to number of elements. Example of representative dimensions of commercial pressure vessels for RO applications 13

14 Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table 6.1. Table 6.2. Table 6.3. Table 7.1. Table 7.2. Table 7.3. Table 7.4. Table 7.5. Table 7.6. Table 9.1. Table 9.2. Table 9.3. RO Technology Mark Wilf Ph. D. Comparison of side port and center port configurations Comparison of conventional two pass and split partial two pass configuration General specifications of cleaning equipment Summary of membrane fouling categories and their symptoms Summary of membrane fouling processes for various desalination applications and possible cause of fouling Controlling phosphate scaling through ph adjustment Quantity of sulfuric acid required to maintain given ph of the concentrate Ksp and concentration limits for scale forming compounds common to RO Practical limits of saturation values in RO applications Common range of permeate recovery rate in RO applications Process parameters affected by permeate recovery rate Common range of average permeate flux in RO applications Process parameters affected by permeate flux Range of RO design parameters according to application and feed water source Examples of representative membrane elements models according to applications Representative offering of nanofiltration membrane elements Representative offering of brackish membrane elements Representative offering of seawater membrane elements Basic process parameters of a 100,000 m3/day SWRO system Calculation of energy total energy usage in RO systems of permeate capacity of 40,000 m3/day Comparison of operating parameters of pumping unit in SWRO 100,000 m3/day plant. Comparison of cost of pumping energy recovery equipment alternatives for SWRO 100,000 m3/day plant.. Listing of chemicals used in RO and NF applications. Procedures for determination of chemicals dosing rate Representative materials of construction for chemical dosing systems System process information for 100,000 m3/day SWRO system Usage of treatment chemicals in 100,000 m3/day SWRO system Sizing of chemical dosing pumps Selection of piping material according to application Relevant composition and PREN values of alloy steels Recommended flow velocity range in RO applications 14

15 Table Table 10.2 Table Table Table 10.5 Table Table Well water quality Permeate and product water quality Pretreatment system design data System design data CIP unit design data Post treatment unit design data Major equipment list RO Technology Mark Wilf Ph. D. Guidelines for process development and design of brackish water and seawater membrane desalination systems. The guidelines will be based on design approach for a 100,000 m3/day seawater and brackish water desalination units. 3. RO feed water quality requirements 1.9. Introduction Feed water quality requirements for RO and NF applications are results of unique conditions of the RO/NF process and configuration and properties of membrane elements. During the membrane desalination process fraction of feed water is converted to permeate and volume of feed water is reduced. Concentration of all constituents is increased. The concentration of constituents in concentrate stream, leaving the membrane unit is higher than concentration in the feed water according to the concentration factor (CF), which is related to recovery rate (R): CF = 1/(1 R) (1.1) The recovery rate in RO units ranges from 40% - 60% for seawater systems and 75% - 90 % in RO/NF units. As illustrated in Figure 1.1, concentration of all constituents, entering th e membrane unit with the feed stream will be increased significantly in the concentrate stre am. 15

16 Concentration factor Concentration Factor in RO System % 20% 40% 60% 80% 100% Recovery rate RO Technology Mark Wilf Ph. D. Figure 1.1. Concentration factor of concentrate stream in RO applications. Accordingly, concentration of some soluble constituents of the feed water could exceed their saturation concentration during the desalination process, precipitate from solution and deposit on the membrane surface. The other concern is related to presence of suspended and colloidal solids in the feed water. In the spiral configuration of membrane element, shown schematically on Figure 1.2, feed water flows through narrow feed channels, shown schematically on Figure

17 Feed Brine Spacer Concentrate Product Membrane Permeate Carrier Figure 1.2. Schematics of spiral wound RO membrane element Feed channel 0.7 mm (0.031 ) Feed spacer configuration Configurations of feed channel and feed spacer net Figure 1.3. Schematics of configuration feed concentrate channel in RO membrane element. The nominal height of the feed channel is about 0.7 mm and the channel is filled with feed spacer that promotes turbulence. Therefore, the effective channel opening for feed flow is even smaller than 0.7 mm. Any particulate matter, present in feed water, could be trapped in the feed spacer and 17

18 block water flow in the feed channels. For this reason, particulate matter has to be removed from RO feed water in the feed water pretreatment system. Feed water quality parameters are defined mainly in terms of concentration of constituents that could exceed saturation limits and precipitate on the membrane surface and particulate matter that could block passage of the membrane elements feed channels and also deposit on the membrane surface, increasing resistance to water flow Feed water types The composition and quality of water, considered for processing by reverse osmosis is influenced by its origin. For reverse osmosis application water of interest is the one with ions composition that exceeds potable water limits. In brackish water RO applications the ions that commonly exceed potable water limits are calcium, magnesium, sulfates and chlorides. Less common dissolved constituents of brackish water, that may require reduction of concentration, are fluoride and nitrate. Some waters may contain also excessive concentrations of iron, manganese, organic matter, color, hydrogen sulfite and sometimes even radioactive isotopes. Potable water limits are specified by World Health Organization (1) and regional Health Authorities. Accordingly, one may define brackish water as any water of composition of soluble species exceeding potable water limits. The potable water limits, or acceptance of water composition for potable application can vary from country to country, according to local affordability of treatment methods. However, it is commonly accepted that water of salinity exceeding 1000 ppm is considered as brackish and requires treatment for salinity reduction. The upper limit of water salinity that can be effectively treated with brackish RO membranes, in a single pass configuration, is about 10,000 ppm. On the low end of salinity spectrum there are some water sources that have salinity in the potable range but still require membrane treatment. This is usually due to presence of excessive concentration of hardness, iron, organics and/or color. The low salinity water sources are usually treated with loose RO membranes, commonly called as softening or nanofiltration membranes. The composition of brackish water can vary widely. The composition is usually specific for the aquifer it originates from. If brackish water aquifer is very large and/or water is pumped at the rate it is being replenish by natural infiltration, then the composition remains stable. In case of excessive pumping the composition may change. In case of utilization of coastal aquifer there is possibility of seawater intrusion and salinity increase. For other locations there will be influence on composition from adjacent underground bodies of water due to hydrostatic pressure difference. Seawater sources are characterized by high salinity, in the range of 30,000 ppm TDS to 47,000 ppm TDS. The ion composition includes mainly sodium and chloride, about 85% combined. The remaining fraction consists of sulfate (~ 8%), magnesium (~ 4%), calcium, potassium (~1.2% each) and bicarbonate (~0.6%). Boron is one of low concentration constituents. It is present in seawater at concentration of about ppm. However, boron concentration is becoming increasingly important parameter of the process design as its concentration is being specified in RO permeate. Due to relatively low rejection by RO membranes of boron species existing in seawater, stringent boron specifications have significant effect on process design and product water cost. At majority of locations the ions composition of seawater is quite consistent and fluctuates in narrow range. Temperature of seawater usually reflects the seasonal fluctuations of ambient temperatures but could be affected by temperature of local water currents. At some locations, where rivers discharge or rain sur- 18

19 face run off is present, the fluctuations of salinity and concentration of suspended matter could vary in a wide range and require careful consideration during the process design stage. As a part of the design process of RO plant the feed water sources should be evaluated to determine the following: 1. How feed water ions composition and temperature will affect quality of permeate and the required feed pressure? 2. Does water source contain sparingly dissolved species at concentration that could result in membrane scaling at the design recovery rate? 3. Does water source contain particulate matter that could plug feed channels of membrane elements? 4. Does water source contain organic matter at concentration that could adsorb on membrane surfaces and result in significant permeability decline? 5. What is the level of biological activity? Can it result in biofouling of membranes? Preliminary determination of the suitability of given water source for RO processing and requirements of the pretreatment process, is conducted based on results of analysis of water samples from the water source under consideration. During the initial evaluation of water analysis it is important to check if the water analysis report contains values of important water quality parameters and concentration of major ions. The primary group of water composition data includes: ph, temperature, turbidity, electric conductivity and concentrations of Ca, Mg, Na, K, HCO3, SO4, Cl SiO2, Fe, Mn and TOC. Additionally, concentration of any species that their maximum concentration had been defined in permeate, should be also determined in the feed water source, for example concentration of NO3 or B. The analysis should be balanced, i.e. sum of miliequivalents of positively charged ions (cations) should be similar to the sum of equivalents of negatively charged ions (anions). In addition to feed water composition, suitability of raw water for RO processing is defined by the water source. The common sources of feed water for RO/NF applications are: 1. Deep wells. The water originating from deep wells is usually brackish of low or high salinity. In most cases well water from deep wells has very low concentration of suspended and colloidal particles. 2. Shallow wells. This category includes low salinity brackish wells, drilled in the shallow aquifer, river bank filtration wells and wells supplying seawater: beach or collector wells. Water from these wells has also low concentration of particulate matter but sometimes could contain elevated concentration of organics.. 3. Surface water sources usually represent seawater intakes. In majority of cases, concentration of suspended solids fluctuates in a wide range and extensive pretreatment is required for suspended solids reduction. 4. Wastewater, either secondary or tertiary effluent. This water source is characterized by elevated concentration of suspended solids and organics. In cases when wastewater source represents effluent of membrane bioreactor (MBR), then it contains elevated concentration of organics but concentration of suspended solids is quite low. Table 1.1. Examples of representative compositions of brackish, sea water and secondary wastewater effluent. 19

20 Feed constituent Low salinity brackish High salinity brackish Medium salinity Secondary effluent seawater Ca Mg Na K HCO SO Cl F NO B SiO TOC Sparingly soluble constituents During the RO process concentration of all constituents increases due to reduction of the feed water volume. This increase of concentration is function of permeate recovery (Equation 1.1). Some of the constituents presented in natural waters can precipitate if the concentration product of salt forming ions exceeds its solubility product: Ksp. For a given salt of composition CmAn in equilibrium of solid phase salt (S) with dissolved ions, the Ksp is defined as: CmAn(S) = mc +n + na -m (1.2) Ksp = [C +n ] m [A -m ] n (1.3) SI = [C] m [A] n / Ksp (1.4) Where C stands for cation and A for anion, m and n are valency coefficients. Brackets [ ] indicate molar concentration of a given ion in solution. SI is the saturation index, indicating excess concentration of a given salt in comparison to its saturation value. The Ksp is determined through measurement of ions concentrations in solution at saturation conditions (in equilibrium with solid phase). Ksp value is specific for a given salt and it is function of temperature and ionic strength of the solution. In brackish systems, treating natural waters, the salt of concern is mainly calcium carbonate. Less frequently calcium sulfate and silica are at concentrations that my result in scale formation. In very rare cases barium sulfate, ferrous sulfide, and ferrous carbonate could be present at concentrations that may form scale at high recoveries. In RO systems treating municipal effluents calcium phosphate sometimes forms in the tail elements. Calcium carbonate is the most common scaling constituent in brackish waters but also the easiest to control either with ph adjustment or use of scale inhibitor. Discussion on scaling potential of sparingly soluble salts is included in Chapter Particulate matter RO feed water entering membrane elements, should have low concentration of particulate matter. The commonly accepted quality indicators of RO feed water in this respect include: 1. Turbidity 20

21 2. Suspended solids concentration 3. Silt density index (SDI) RO Technology Mark Wilf Ph. D. Turbidity determination, usually expressed as nephelometric turbidity units (NTU) is determined through measurements of intensity of light scattered by suspended particles in water samples. Suspended solids concentration is determined by filtration of measured volume of water sample and weighting of dry residue on the filter. The SDI is determined through measuring the rate of filtration of water sample through the filter (Figure 1.4). Determination of all three above indicators is described in ASTM procedures (3, 4, 5). Out of above three indicators only turbidity can be measured continuously on line. The other two are conducted as discrete measurements on water samples taken periodically. Silt Density index SDI = 100*(1 t 0 /t 15 )/15 t 0 t 15 SDI (15 min) Feed pipe min 500 ml (?) Figure 1.4. Schematic configuration of Silt Density Index (SDI) apparatus. Another water quality parameter that is being used to monitor operation of pretreatment system is number of particles measured with particle counters. This could be applied as on line, semicontinuous measurement. Application of particle counters for RO application is still at very early stages. So far no relations had been reported as being established between particle counter measurements and fouling rate or performance stability of RO membranes. The feed water indicator, most relied on in RO applications, is SDI. It is based on measurement of rate of declining flow, at a constant pressure, of a water sample through a porous filter membrane of nominal 0.45 micron porosity. The filter is placed in a simple circular holder (Figure 4) and connected to feed water line at applied pressure of 2 bar (30 psi). The time measured for filtration of a constant volume (500 ml) at the beginning of the test (t1) and after 15 minutes (t2) is used to calcu- 21

22 late the SDI according to the following equation: RO Technology Mark Wilf Ph. D. SDI = 100% (1-t 1 /t 2 )/15 (1.5) If the filter plugs to fast for meaningful determination of the filtration time, the volume of filtrate being collected or time between measurements can be decreased. However, the SDI values determined for shorter test time or smaller filtrate volume are only indicative of poor water quality and not very useful for the pretreatment system design process. As shown in Figure 1.4 (last entry in the table) it is possible to have long filtration times and still calculate low SDI values. If the reading for filtration time t1 significant exceeds 30 seconds then most likely there is a problem with equipment or test conditions. The SDI method is very sensitive to concentration of foulants but it is not very accurate. No meaningful correlation has been established between values of SDI and turbidity. Attempts to improve accuracy of this method led to introduction of Modified Fouling Index (MFI). The test for MFI is based on measurement of pressure increase required for maintaining of constant filtration rate through well defined membrane filter (6). The MFI results are more reproducible than SDI but the test is difficult to perform manually and automatic equipment is necessary (7) for a routine determination in the field conditions. According to the equation 1.5, the maximum value of SDI (for 15 min measuring interval) can be only The majority of membrane manufacturers specify upper limit for feed water SDI as 5. However, field results show that for stable, long term performance of RO elements, the SDI of feed water should be consistently below the value of 4. Some limited research works (X) demonstrated that in respect of solids concentration the SDI scale is a geometric one. Therefore, for water having SDI = 3 and another water having SDI = 5 the corresponding suspended solids concentration difference is about four times higher. The following figures shows the SEM pictures of clean filter (Figure 1.5) and two filters (Figure 1.6 and 1.7) after being used for SDI determination of seawater feed. Figure 1.6 shows SEM picture of filter pad after SDI determination that resulted in SDI value of 2.2. Figure 1.7 shows corresponding SEM picture for SDI value of 4.8. The tick deposit on the filter, which was used to filtrate water sample with higher SDI, is clearly visible. 22

23 Figure 1.5. SEM picture of clean filter pad, before SDI memasurement. Figure 1.6. SEM picture of clean filter pad, after SDI measurement. SDI =

24 Figure 1.7. SEM picture of clean filter pad, after SDI measurement. SDI = 4.8 Field results have demonstrated that in majority of cases water from deep wells has very low SDI, usually less than 1. RO systems, operating with good quality well water feed, practically do not show any pressure drop increase across the membranes or flux decline. Surface water, after a conventional pretreatment, usually has SDI in the 2 4 range. RO system processing feed water with SDI in the 2 3 range shows stable membrane performance. Membrane cleaning frequency for such feed water does not exceed 1-2 per year. RO systems processing feed water of higher SDI, in the 3-4 range, usually suffer from some degree of membrane fouling and somewhat higher membrane cleaning frequency may be required. Long term operation of RO system with feed water having SDI above 4 is not recommended. As mentioned already, past attempts to correlate turbidity with SDI values were not successful. These two feed water quality indicators correlates to the number and size of suspended particles in a different way. However, usually the feed water with SDI in the 2 3 range has corresponding turbidity below 0.1 NTU, usually at 0.05 NTU range Organic matter Organic matter in RO feed is customary expressed as a total organic carbon (TOC). Surface water, water from shallow wells and municipal secondary effluent usually contains some concentration of dissolved organics. In surface waters the organic matter originates from decomposition of humic substances. In shallow well water the presence of organics could be result of water infiltration through strata containing natural organic matter. In both cases the TOC concentration is usually in the range of ppm, most of the time below 1 ppm. This low concentration of TOC in the fed water has little effect on membrane permeability. In RO feed originated from secondary effluent, the TOC concentration fluctuates in the wide range between 2 10 ppm. The presence of high concentration of organic matter results in flux decline 24

25 due to adsorption on the membrane surface. The initial rate of flux decline, due to organic adsorption is rapid, about 10 30% decrease from the initial permeability value. Afterwards, the permeability usually stabilizes, the decline levels off. However, if colloidal matter is also present in the feed water, the permeability decline is usually more severe, due to formation of thick, mixed foulants layer of low water permeability. Some potential water sources for RO processing are contaminated by oil and grease. Such conditions could exist in industrial wastewater streams or in seawater in the areas of heavy maritime traffic. Oil and grease have to be completely removed from the feed water prior entering the RO membrane elements. Low concentration of oil and grease will not result in structural damage of the membrane element but will cause severe decline of water permeability. Conventional method of removing low concentration of oil and grease includes air flotation and/or media filtration with flocculation using metal based flocculants Biological activity Majority of well water sources have very low biological activity due to low concentration of nutrients. The surface water sources could have at times very high level of biological activity. The increased biological activity shows up during periods of intensive algae blooms, also called red tide. During these periods, turbidity and suspended solids concentration could increase significantly, up to the levels of NTU and ppm of suspended solids. In parallel, level of TOC could increase to the range of 10 ppm TOC and above. During the cycle of algae growing and dying off, the supply of nutrients increases, creating conditions for accelerated grow of bacteria. During the conditions of algae bloom it is very difficult to maintain stable operation of pretreatment and RO unit, unless pretreatment system has been design according to expected high level of biological activity. The secondary effluent, originating from municipal wastewater, also has high level of biological activity, mainly high values of bacteria counts. However, this activity can be effectively controlled prior to RO unit by maintaining 2 4 ppm of chloramines in the feed water. Wastewater reclamation is the only RO application that biological activity can be controlled by applying chloramines. Past attempts of applying this method to control biological activity in brackish or seawater systems always resulted in accelerated membrane damage Feed water temperature Feed water temperature affects both permeate salinity and the required operating pressure. Both water and salt transport follow a similar trend (Equation 1.6 and Figure 1.8). The commercial RO systems are designed to operate at constant permeate capacity (constant permeate flux). Therefore, increase of feed water temperature will result in increased permeate salinity (higher quantity of salt will pass the membrane and will be dissolved in a constant volume of permeate). This increase is about 3% per degree C. The effect is similar for brackish and seawater systems (Figures 1.9 and 1.10). The changes of water permeability with temperature affects the net driving pressure required (Equation 1.7). However, in parallel, temperature also affects osmotic pressure of the feed water (Equation 1.8). With temperature changes net driving pressure and osmotic pressure have opposite effect on feed pressure (Equation 8). An increase of feed water temperature at low temperature range (~ up to about 30 C) enables production of a given permeate flow at reduced feed pressure both in seawater and brackish water systems. At higher temperatures the reduction of feed pressure in seawater systems levels off, mainly due to increase of osmotic pressure of the average feed (Fig- 25

26 ure 1.10). RO Technology Mark Wilf Ph. D. TCF = 1/exp(C (1/(273+t)-1/298)) (1.6) Where TFC is temperature correction factor, t is temperature Centigrades, C is constant, characteristic of membrane barrier material. For polyamide membranes a C values of are being used. 1.8 Feed water salinity Feed water salinity affect the feed pressure required for production of a given quantity of p roduced water per unit membrane area through value of the osmotic pressure. Osmotic pres sure is related to water salinity according to equation 1.7. Posm = R (T + 273) (mi) (1.7) Where P osm is osmotic pressure (in bar), R is universal gas constant (0.082 l atm/mol K), T is the temperature (in C), and (mi) is the sum of molar concentration of all constituents in a solution. 1.9 Net driving pressure The feed pressure is function of osmotic pressure of feed concentrate solution according t o equation 1.8. Pf = NDP + Pos + Pp + 0.5* Pd (- Posp) (1.8) Where: Pf is feed pressure, Pos is average feed osmotic pressure, Pp is permeate pressure, Pd is pressure drop across RO elements and Posp is osmotic pressure of permeate. The NDP is related to specific permeability of the membrane and the design flux rate. 26

27 Permeate salinity, ppm TDS RELATIIVE FLUX VALUE RO Technology Mark Wilf Ph. D PERMEATE FLUX CHANGE WITH TEMPERATURE FEED WATER TEMPERATURE, C Figure 1.8 Temperature correction factor for polyamide composite membranes. 500 Brackish RO, flux 28 l/m 2 -h Feed 6,000 ppm TDS 200 Feed 1500 ppm TDS Feed water temperature, C Figure 1.9. Temperature effect on permeate salinity in brackish RO systems 27

28 Seawater RO system Feed pressure and salinity Feed water temperature, C Figure Temperature effect on permeate salinity in seawater RO systems. Feed pressure values are marked as red bars and permeate salinity as gray bars Feed water ph. Natural sources of brackish and seawater have relatively narrow range of water ph. The brackish waters have ph in the range of The ph of seawater is usually in the range of The ph is result of equilibrium of concentration of dissolved carbon dioxide (CO2) and bicarbonate (HCO3). The concentration of bicarbonate in brackish water vary in a wide range. However, its concentration is usually above level of 250 ppm. Bicarbonate concentration in seawater is much lower, almost always below 200 ppm, in most cases in the range of ppm. Knowledge of correct value of raw water ph is important in brackish and seawater applications. In brackish water applications, prediction of ph of the concentrate stream enables determination of potential for calcium carbonate scaling conditions. In seawater applications, scaling is less of a problem. However, water ph could affect effectiveness of the coagulation flocculation process and also rate of boron rejection. 4. Feed water supply alternatives 5.8. Introduction 28

29 The selection of water source for the desalination system is usually made by the end user according to professional recommendation provided by a hired consultant. In majority of cases, EPC Contractor has to adopt system configuration to raw water quality specified in the project specifications. In isolated cases the EPC contractor has some degree of freedom in selecting location of raw water extraction point. Therefore, it is responsibility of EPC contractor to collect information about raw water quality and its seasonal and diurnal fluctuations. Whenever possible, location of the water source for RO application should be selected to assure good and stable water quality. Once the raw water source is selected and water quality specified, the feed water treatment system has to be configured to produce adequate feed water quality also at the periods of most adverse raw quality Brackish wells Brackish wells are constructed to pump water from underground aquifers. In most cases, salinity of water increases with depth of the water producing aquifer. If the aquifer is confined or semiconfined, then composition of water being pumped will remain stable over time. A confined aquifer is a preferred water source for desalination applications, due to stability of water composition that translates to stable performance of the desalination unit. In majority of cases, water extracted from brackish wells is characterized by low concentration of suspended solids and organic matter. Water turbidity is below 0.1 NTU and SDI below 1. However, if aquifer has been contaminated by infiltration of water from agricultural of industrial activity, it may contain elevated concentrations of fertilizers (high nitrate content), organic matter or even some concentration of toxic constituents. The chemistry of water in an aquifer is controlled by chemical composition of formation where aquifer exists and water movements that occurred over time. The dissolved constituents present in ground water are fairly predictable however, their concentration is highly variable, between different locations. In most cases, wells, located in the same aquifer and pumping water from the same levels, will produce water of very similar composition. Therefore, information of water composition from existing well is a good indicator about water composition from new wells that will be constructed in the same aquifer. When new brackish wells are considered for an RO desalination systems, their configuration and logistic of operation should stability of water quality and supply. Important issues are: 1. Prevent formation of corrosion products. Wells casing, that will be in contact with brackish water should be made of materials that will not corrode. For shallow wells PVC and FRP casing could be used. For deep wells, where structural strength is important, stainless steel, 316 type, could be used. The maximum depth for use of PVC and FRP is casing limited by pipe diameter and wall thickness. For PVC casing, the maximum suitable depth is in the range of m. FRP can be used for deeper wells if FRP piping has 29

30 adequate mechanical strength (collapse pressure), defined by pipe manufacturer. 2. Prevent mixing of water from different production zones. Pumping should be conducted only from the designated layer. Other water bearing zones that well pipe is crossing should be isolated by proper grouting. Mixing of water from different production zones could results in precipitation of some constituents. It is specially critical in cases of pumping water from anaerobic formation. 3. Prevent entrainment of sediments with pumped water. Measures taken to reduce carryover of solids with pumped water includes installation of proper screen, maintaining low flow velocity through the screen pump well column. The screen opening should be in the range of mm. The flow rate through the screen should not exceed 0.03 m3/sec. The flow rate in the well pipe should be below 1.5 m/sec. Wells located in limestone formation may operate without screen with good results. Well pumps motors should have a soft start to avoid creation of sudden water movements and carryover of solids. 4. Prevent environmental effect on water quality. To maintain consistent quality of well water, it should not be exposed to the environment (air). Whenever possible, storage tanks prior to RO processing should be avoided. Well pumps should have foot valves to maintain water level in the pump during system shut down. Decrease of water level will pull in air, that may introduce air born contaminates or change ORP values (in case of anaerobic water). 5. Avoid mixing water from different sources. For RO applications only water from the same aquifer should be combined for use in the given desalination unit. Waters of dissimilar aeration levels, i.e. water from anaerobic and aerobic sources should not be mixed together. Mixing of such sources could result in oxidation of soluble iron or H2S and precipitation. 6. Minimize addition of chemicals to well water. Well water should not be treated with chemicals, prior to RO. Especially, well water of dedicated use for RO unit should not be chlorinated. In the RO unit acid and/or scale inhibitor is used to prevent formation of mineral scale during the RO process. Except for these two chemicals, no other chemicals should be used when treating well water River bank filtration wells River bank filtration (RBF) is applied for extraction of low salinity brackish water from a surface source. The shallow RBF wells are located in aquifer that is hydraulically connected to the surface water body, either river or lake. Because the water flows through aquifer formation it is filtered and contains much lower concentration of colloidal and suspended solids than the surface source. However, in majority of cases, the quality of water obtained from RBF is not as good as water pumped from brackish wells. 30

31 The major issues with RBF water supply are: 1. Hydraulic conductivity of the aquifer. It applies to body of the water bearing formation and also to permeability of the bottom of the surface water source. The water from the surface water source infiltrates through the bottom surface, depositing suspended solids. The RBF acts as a slow filtration rate filter without backwash. At high concentration of suspended solids, deposition and clogging of the infiltration surface could be quite rapid. Rivers flow velocity of over 1m/s is usually sufficient to disperse deposits and maintain filtration rate. 2. Fluctuation of water salinity and water temperature. RBF system is hydraulically connected to the water in the river. Changes of water salinity and water temperature are conveyed to the underlying aquifer and reflected in the water conditions delivered to the desalination plant. 3. Fluctuation of water supply. During a periods of severe draught, flow in the river could decrease affecting quantity infiltrating to the underlying aquifer and also reducing rate of dispersion of deposits on the infiltration surface. 4. Presence of organic matter. The concentration of organic matter in water pumped through RBF system could be at times much higher than water pumped from brackish aquifers. Seasonally, surface water could develop biological activity (algae bloom) leading to high concentration of organics. Also, shallow surface layer could contain high concentration of organic matter form by vegetation deposits. The quality of RBF water is much higher than the hydraulically connected surface water source. However, frequently it is not sufficiently good to send water directly to the RO unit without additional treatment Beach wells Vertical beach wells are considered as effective alternative for supply of good quality seawater to seawater RO desalination plants. The concept and configuration are very similar to the river bank filtration discussed above and shown schematically on Figure 2.1. The differences between RBF and beach wells are related to water salinity and environmental conditions of the body of water connected to the well. Due to high concentration of chlorides in seawater, materials of construction of the beach well column and the well pump have to be corrosion resistant in seawater environment. Because vertical beach wells are shallow wells, PVC and FRP can be used in beach wells casing and screens construction. Pumps have to be made of corrosion resistant alloys (duplexs or equivalent). 31

32 Figure 2.1. Schematic configuration of veridical beach well. In cases that beach wells are located close to the water line, the beach well structure could be exposed to waves and beach erosion. Therefore, a protective containment may be required. In most cases chemical composition of water pumped from a beach well is very similar to composition of seawater water in coastal area. However, at some sites, significant differences have been found. It is more common for beach well to be under influence of low salinity water flowing underground to the ocean. In this case, water from beach well will have lower salinity than sweater. However, opposite conditions, i.e. water from beach well having much higher salinity than seawater has been also found at some locations where beach wells were considered. Another issues of concern are presence in beach well water of constituents that are absent in open seawater but could have harmful effect membrane operation. These include presence of elevated concentration of iron, manganese and/or hydrogen sulfide. For example, seawater pumped from beach wells, at Morro Bay seawater RO desalination facility, contains high concentration of iron (5 15 ppm). Initial attempts of operation of RO unit directly with this water source, resulted in unsustained frequency of replacement of cartridge filters. A green sand iron removal system had to be installed to enable reliable operation of the RO unit. Similar conditions of high concentration of iron and manganese in beach well water are experienced in the 14,500 m3/day seawater desalination plant at Santa Cruz, Mexico. Also here, green sand system is being use as a feed water pretreatment step. Presence of hydrogen sulfite was discovered in beach well water at the seawater RO desalination unit at Avalon, Catalina Island, CA. The hydrogen sulfite was result of anaerobic conditions in formation where beach well has been located. At some locations, beach wells are very effective solution for supply of good quality feed water 32

33 to the RO unit. If the costal formation has good water permeability and do not release excessive quantity of particles during pumping (usual conditions with limestone rock), utilization of beach wells as feed water supply source will result in very simplified pretreatment and reliable operation of the RO unit. Large seawater RO plants that successfully utilize beach wells are located in Malta, Caribbean and Oman. Some of the facilities utilizing beach wells are listed in Table 2.1. Table 2.1. Listing of large SWRO desalination facilities that utilize beach wells Location Number of wells Capacity, m3/day Sur, Oman 33 4,000 10,000 each Pembroke, Malta 54,000 Malorca, Spain 16 5,700 each Ghar Lapsi, Malta 24,000 Pemex, Mexico 3 14,500 each, horizontal wells Fukuoka, Japan 82,900 infiltration gallery Morro Bay, California 5 5, Horizontal wells Horizontal wells, also called Ranney type intake wells, are another approach to provide seawater that is filtrated already by water bearing formation. The collector wells are located on shore, in coastal area. \The Ranney wells are constructed by placing large concrete ring segment on shore. The ring segment has a cutting shoe on the bottom rim. The ring is sunk in place by excavating soil inside the ring. Once the firs ring is in place, the next one is mounted on top of it forming well caisson. When the designed depth is reached a concrete plug is placed in the bottom and horizontal laterals are driven through openings in the bottom segment ring. The laterals have perforations for seawater to infiltrate to the caisson. The caisson could have diameter of 3 6 m and depth of up to 50 m. The laterals could be m long. Schematic diagram of horizontal well is shown on Figure

34 Collector well configuration Caisson Pump Laterals Diameter 3 6 m (10 20 ft) Depth m ( ft) Yield 80 4,000 m3/hr ( MGD) Figure 2.2. Schematic diagram of collector well After completion of wet well construction it is topped with structure that contains pump(s) and electric gears. The collector wells have larger water infiltration area than vertical beach wells. Therefore, on the average, the collector wells will have higher water yields that it would be possible with vertical beach well, constructed at the same location. Seawater produced by collector wells is of a similar quality as would be produced by vertical beach wells at the same location Slant wells The slant wells, also called horizontally directional drilled wells (HDDW), evolved from commercial technology used to lay underground cables and oil and gas pipelines lines. Schematic diagram of slant well is shown on Figure Probably the best known company, providing this technology for seawater RO application, is Neodren, located in Spain. Within the last 10 years Neodren installed number of HDDW systems, mainly in Spain. The slant well consist of high density pipe, entering the soil, below the seabed at degree angle. The pipe diameter is around 450 mm and subsurface length could be m. Seawater enters the HDD pips through wall perforations, sized at about 120 micron size. Configuration of HDD pipe is shown on Figure 2.3. HDD intake structure for a large desalination plant would usually consist of multiple pipes as shown on Figure 2.4, depicting intake structure used at 82,000 m3/day sweater RO plant at Alicante, 34

35 Spain. Seawater obtained from HDD intake is of better quality than produced from an open intake. However, additional pretreatment of feed water is still required to reduce the SDI to the level required in RO plants. Figure 2.3. Schematics of horizontal directional drilling (HDD) intake 35

36 Figure 2.4. Model of HDD intake system at 82,000 m3/day seawater RO desalination plant, Alicante, Spain. Courtesy Neodren Seawater infiltration galleries. Infiltration galleries are similar in operation to HDD intakes. The difference is mainly in method of construction and use of man made filtration layer rather than existing seabed, as it is in case of HDD structures. The infiltration galleries are constructed by removing layer of designated seabed area, placing perforated collector pipes and cover them with a layer of granular filter pack. Schematic diagram of 102,000 m3/day infiltration gallery at the seawater desalination plant at Fukuoka, Japan, is shown on Figure 2.5. The infiltration galleries operate as a slow filters with filtration rate in the range of m3/m2/hr. Like other slow sand filters, backwash is not used for filterability recovery. The filterability is restored by periodically scarping few cm of a lop layer. After few years of such operation the filter bed has to be replaced with new filtration pack. Fukuoka Infiltration Gallery: 102,000 m3/day (27 mgd) 64X313 m (210 X1027 ) Excavation thickness 3 m (10 ) Figure 2.5. Infiltration gallery at sweater RO desalination plant, Fukuoka, Japan. At this point the only commercial seawater plant that utilizes infiltration gallery is 50,000 m3/day RO plant at Fukuoka, Japan Open surface intake 36

37 The open surface intake is the most common method of supplying seawater to RO desalination plants. This configuration is usually the lowest cost water supply alternative, as indicated in Table 2.2. Seawater supplied from an open intake always requires filtration pretreatment prior to entry to membrane unit. The extend of pretreatment will depend on expected seasonal fluctuations of seawater quality. The quality of surface seawater could be affected both by costal discharge, weather conditions and seasonal biological activity (algae bloom). Once the expected range of seawater quality has been determined, a proper pretreatment method could be designed. Table 2.2. Economic weight of various alternatives of sea water supply to RO desalination plants Well type Capacity Development cost Vertical well 400 2,000 m3/day ( MGD) Slant well 800 8,000 m3/day ( MGD) Collector well 2, ,000 m3/day ( MGD) $200K - $500K $1M - $1.5 M $3M - $5M Beach filtration galleries Very high output Very high cost Conventional intakes Very high output Moderate cost Configuration of open intakes. Open intakes configuration will depend to some extend on system size. Intakes for small capacity desalination systems may consist of pump attached to a dock or suspended from a floating platform and connected by short pipe segment to the on shore raw water storage tank. Large desalination system could utilize on shore intake located at the end of a channel protruding into shore. Another, more common configuration is off shore velocity cap structure connected by pipe to on shore wet well (Figure 2.6) 37

38 Figure 2.6. Schematic configuration of open surface off shore seawater intake. The off shore intake configuration, usually more expensive than the channel intake, enables system designer some freedom in selection of intake location with somewhat improved seawater quality. If possible, the location of off shore intake should be selected at depth that is below the depth of storm base waves according to the annual lowest tide. The intake should be located at the depth that will not interfere with marine traffic and location of low biological activity. From the aspect of sweater quality it is desirable to collect seawater at a depth of over 10 m. However, such a depth may not be available at a reasonable distance from shore. If a required seabed depth is located at distance larger than 500 m off shore, it is more cost effective to select location at smaller immersion depth and shorter distance. An example of intake structure is shown on Figure 2.7. The top of intake structure is 4 m below low tide level. The water intake opening is 1 m above seabed level. 38

39 Figure 2.7. Example of off shore intake structure (courtesy Ian Larsen) As shown on the above drawing, the intake structure has provision for chlorination. The chlorination is only applied intermittently, as shock chlorination, to control grow of marine organism inside the intake structure. Another measure to reduce biological activity is concrete cover on the top of the intake structure that prevents direct light to reach inside the intake. Another configuration of intake structure is shown on Figure 2.8. This structure is being used in 54,000 m3/day RO seawater desalination plant in Mediterranean area. The structure has inlet openings in the lower part of the intake head. In this configuration also there is no direct access of light to the internal surfaces of the intake structure. The grow of marine life inside the intake is controlled by application of intermittent shock chlorination (5 10 ppm of free chlorine) and mechanical removal of incrustations from intake walls. The mechanical cleaning procedure involves applying a hydraulic trust to plastic ball(s) and sending them through intake pipe to the off shore structure. The intake structure has to have built in an access (man hole) for recovery of cleaning balls. This pipe cleaning procedure is called pigging. During application of pigging, Ro plant is out of operation. The pigging procedure could take 4 8 hr. 39

40 Figure 2.8. Alternative configuration of intake structure (courtesy Ian Larsen) The important issues in designing of intake structures are reduction of impingement and entrainment marine life during operation of desalination plant. Low rate of removal of marine life with seawater is achieved by maintain low inflow velocity of seawater into the intake structure The open inlet area should be designed to maintain inlet flow below 0.2 m/sec. The intake structure is connected to the on shore located wet well by a single or multiple pipes. The connecting pipes are laid in a trench or anchored to the seabed utilizing concrete blocks (shown on Figure 2.9 and 2.10). To assure longevity of intake structure and connecting piping, it has to be constructed from materials resistant to seawater corrosion. Usually the intake structure is made of concrete and connecting piping from high density polyethylene (HDPE) or FRP. 40

41 Figure 2.9. Installation of concrete anchors on intake pipe (courtesy Ian Larsen). RO sweater desalination plant converts fraction of seawater into product water and discharges concentrate, corresponding to a flow of 40 50% of the feed water flow, back to the sea. The concentrate outfall line, starting at the discharge of energy recovery devices, extends to the ocean, to the discharge point. The terminal point of outfall line has to be at location that satisfies the following conditions: 1. Provides rapid dispersion and salinity reduction. 2. Will not results in short circuit of discharged concentrate with the intake inlet. 3. Will be located at a depth that protect discharge structure from storm waves, marine traffic, etc.. (similar to considerations for intake structure). Figures 2.11 and show configurations of intake and concentrate discharge arrangements at the Fukuoka desalination plant. The drawing shows that the top if intake structure is about 7 m and concentrate structure about 11 m below the seawater level. Both lines are extending about 200 m into the ocean, with the terminal points about 60 m apart. As indicated on Figure 2.12, the diameter of the Intake line is 1.2 m and of concentrate line 0.7 m. These differences of pipes diameter reflects the difference of flow rates of feed and concentrate during normal operation of the plant. Some plants design both feed and concentrate lines of the same diameter. This is to enable operation of pretreatment system at full capacity, also during the time when the RO membrane units are not in operation (commissioning period) or when they are operating at partial capacity. 41

42 Figure Delivery of intake pipe with concrete anchors installed to the plant site (Courtesy Oceana) 42

43 Intake tow RO Technology Mark Wilf Ph. D. 1.2m dia. x 220 m in length Maintenance man-hole Intake pipe system 0.7m dia. x 230 m length Maintenanc hole Diffuser Equ ment Discharging pipe system Figure Intake and discharge system at SWRO desalination plant, Fukuoka, Japan 43

44 Figure Conveyance of the feed intake and concentrate discharge lines at Fukuoka, Japan Effective dispersion of the concentrate is an important issue in seawater desalination plants. Usually, plant permitting process includes submittal evaluation of concentrate dispersion based on modeling. The method of disposal and dispersion of concentrate applied at the Fukuoka desalination plant is demonstrated on Figure According to this model of dispersion process, the background salinity of the ocean is reached already at the distance of 12 from the discharge point. This is archived by configuring the concentrate discharge structure with set of nozzles that increase concentrate discharge velocity and create turbulence. Pressure drop losses on discharge nozzles have to be included in development of hydraulic profile for the outfall line. 44

45 Effluent volume: 67,000m 3 /d Discharging velocity: 6.0m/sec Concentrated seawater: 5.8% Raw seawater: 3.5% l l l Distance (m) l 5.8 l 3.6 l 3.54 Concentration(%) Diffuser Equipment for Discharged of Concentrate, Figure Concentrate discharge structure at the SWRO desalination plant, Fukuoka, Japan Estimation of intake cost The estimation of cost of intake and outfall system is based on system configuration, local conditions and equipment and material used in intake construction. It includes the following cost items: 1. Cost of connecting pipe, based on pie weight, material cost and pipe length. 2. Cost of pipe components 3. Cost of pipe placement 4. Other components and equipment, derived based on reference cost and scale up fator. 5. Contingency factor Example of estimation of intake outfall system cost Table 2.3. Intake outfall system cost parameters Cost component Intake flow Intake pipe length Outfall flow Intake outfall length Pipe material Pipe diameter Parameter and cost 300,000 m3/day 1,500 m 150,000 m3/day 1,000 m HDPE 1,600 mm 45

46 Pipe weight Pipe and assembly cost Pipe placement cost RO Technology Mark Wilf Ph. D. 300 kg/m $3/kg ($900/m) $1/kg ($300/m) Table 2.4. Cost components of intake outfall system. 300,000 m3/day inflow, 150,000 m3/day outfall Intake pipe ($1,950/m) $2,925,000 Outfall pipe (1, 950/m) $1,950,000 Subsurface structure $1,000,000 Beach crossing $2,000,000 Siphon structure $200,000 Clearwell pumps $2000,000 Total $10,675,000 Contingency (30%) $3,200,000 Total $13,875, Collocation with power plant The intake cost could contribute 5 20% of the overall cost of the desalination system. At some locations permitting of intake and concentrate discharge is a very lengthy process, creating significant delay of implementation of desalination projects. Collocating of desalination plant at the site of power plant that utilizes significant volume of seawater for cooling is economically attractive alternative to stand alone desalination system. Electric power plants in coastal areas, utilize large volume of seawater for cooling of steam condensers. After the cooling process, seawater is discharged back to the ocean. The temperature of discharged seawater is 5 10 C higher than the temperature of inlet water. RO seawater plant can be incorporated into water circuit of the power plant by taping to the seawater line on condenser discharge. After RO processing, the concentrate is return to the same discharge line, downstream of the feed connection point. Schematic diagram of the seawater flow in RO plant collocated with power plant is shown on Figure

47 Power plant co-sitting configuration Figure Flow diagram of SWRO desalination plant collocated with power plant at Carlsbad, CA. According to Figure 2.13, the power plant pumps 2.3 Mm3/day (600 MGD) of seawater for condenser cooling. After passing through condenser, 380,000 m3/day of the 2.3 Mm3/day flow would be diverted to the SWRO facility. The SWRO plant will operate at 50%recovery rate. Therefore, 190,000 m3/day of concentrate would be returned to the power plant discharge stream. The salinity of SWRO concentrate will be about 68,000 ppm TDS. After mixing with 1.9 Mm3/day of condenser discharge, the resulting salinity will be about 37,000 ppm TDS, only 9% above normal seawater salinity. The collocation alternative seems is beneficial, both improving process economics and addressing issues of permitting of seawater withdrawal and discharge. However, evaluation of feasibility of collocation for a specific project should consider the following issues: 1. Quality of seawater source. Power plant quality requirements of cooling water are significantly less stringent than required in RO applications. Therefore, concentration of suspended solids, including biological debris, in discharged cooling water could be significantly higher than in seawater produced by well designed SW RO dedicated intake. 2. Some power plant condensers are constructed from metals that undergo some level of corrosion and add metal contaminants to the discharged stream. Concentration of metals should be carefully evaluated in respect of membrane fouling. 3. It is common for power plant to conduct continuous or intermittent chlorination of seawater 47

48 inflow. Usually, heat treatment procedure is applied periodically to remove incrustation from the condenser surfaces. During heat treatment procedure RO plant will have to be shut down during that time to avoid presence of sharp edged shell fragments in the feed stream. 4. The increase of seawater temperature during the cooling cycle could be beneficial if the seawater original temperature is low, below 20C. At such conditions, increased seawater temperature will result in lower feed pressure and lower energy requirement of the SWRO system. However, during the periods of high seawater temperature (> ~ C), the additional temperature increase contributed by the condenser of the power plant, could result in less beneficial operating conditions of the SWRO system. In the higher temperature range, above 30 C, the reduction of feed pressure with temperature is marginal. On the other hand there will be significant increase of permeate salinity at higher temperature. Therefore, to compensate for permeate salinity increase a larger second pass processing will have to be included in system configuration. At such conditions, the additional equipment cost and higher operating cost could be higher that savings realized by not including dedicated intake and concentrate discharge in scope of the desalination project. The present commercial RO membrane have maximum temperature limit of 40C. Therefore, the design of using condenser discharge as RO feed water, should have provision not to exceed this maximum feed water temperature, otherwise, plant operation will be outside membrane warranty terms set up by membrane manufacturer Design criteria of feed water supply systems and configurations The design criteria of feed water supply for RO units have to follow sound engineering principle and experience gained by the desalination industry in design of similar systems. Some of the design considerations are listed below: 1. Saline waters and seawater are highly corrosive, therefore, raw water supply systems components should be constructed from corrosion resistant materials. 2. Raw water supply system should not introduce additional constituents to the water that may adversely affect RO membranes or other equipment. - Exposure of raw water to light should be minimized as light exposure my provide energy for biological growth. - Chlorination of raw water should be minimized as presence of oxidants could result in development of nutrients for biological activity in the RO system. - Stagnant area in the water supply system should be avoided as stagnant water supports biological activity. - Raw water should not be aerated beyond what is required by the treatment process (DAF, biological filters). Excessive concentration of oxygen could result in elevated corrosion rate of metal components. 5. Raw water supply lines should be short as possible to reduce system cost and holdup volume of water in the water supply system. 6. Number of pumping steps in the water delivery and concentrate outfall systems should be 48

49 reduced to an absolute minimum. Each pumping step increases system cost and energy requirement of the process Equipment and hydraulic profile As mentioned already above, all components of water supply system should be made from corrosion resistant materials. For piping, the materials of choice are high density polyethylene HDPE and FRP. The HDPE is manufactured according to standardized sizes (diameter and wall thickness). Dimensions of FRP piping and components could vary between different manufacturers. For seawater applications, wetted pars of valves should be made of duplex steels (disk) and cast iron with EPDM liner (valve body). Seawater storage tanks could be made of concrete with protective coating, FRP or glass lined stainless steel. Transfer pumps should be constructed from duplex steel. Due to high cost of this construction material, whenever practical, a horizontal split case pumps, should be specified in place of vertical pumps. In the seawater applications the recovery rate is about 50%. Accordingly, any energy expanded in the water supply system contributes twice its value to the final energy usage per unit of product water produced. Therefore, it is important to select intake pumps with high hydraulic efficiency. The design of raw water supply and outfall system should follow a hydraulic profile developed for specific site conditions and elevation differences. As shown schematically in Figure 2.6, the water level in the wet well, located on shore, will dpend on the level of seawater: the high and low tide. The depth of the wet well and immersion of the pump has to be designed according to the lowest tide (specified in the project documents or listed in reference information) and the friction loses in the connecting piping. The friction loses of the piping connecting off shore intake structure with the wet well are calculated at conditions of maximum flow. The friction loses are combined loses of friction head in the pipe and form loses. The pipe friction head is the pressure loss in straight segments of the connecting pipe. The form loses are friction loses in valve and fittings. The form loses are calculated for individual components and are combined together in one value. The pipe loses can be calculated using Haze Williams formula (Equation 2.1) ( H L ) = (Q C )1.852 D 4.87 (2.1) Where: H is head loss over length L, Q is flow rate, D is pipe diameter and C is a roughness constant. The roughens constant depends on the condition of the internal pipe walls. The rougher the wall surface, the lower the value of C, resulting in higher head loses. The form loses, H, can be calculated according to Darcy Formula, listed as equations 2.2 and

50 ( H L ) = K D h V 2 /2g (2.2) H = K V 2 /2g (2.3) In the above equations, K is a Darcy constant, V flow velocity and g is the gravity constant. Examples of C and K values that are applied in the Hazen Williams and Darcy equations are listed in Table 2.5 Table 2.5 Example of values of constants used in Haze Williams and Darcy equations Component C K Plastic pipe Steel pipe Rounded inlet 0.25 Rounded outlet 1.00 Gate valve 0.20 Butterfly valve 0.20 Globe valve Elbow 45 deg 0.30 Elbow 90 deg 1.10 Example of calculation result of friction loses in straight HDPE pipe, applying Hazen Williams equation, is provided in Table 2.6. It is evident that with small diameter pipe, large friction loses could be created if pumping over long distances is necessary. Table 2.6. Example of pipe friction loses according to pipe diameter. Flow rate: 200,000 m3/day, Q = 2.36 m3/sec. Roughness constant, C = 140 Pipe diameter, m Flow velocity, m/sec Hydraulic gradient, m/100m

51 The hydraulic profile is the basis for selection of proper equipment and process optimization. Based on hydraulic profile, the designer is able to specify required discharge head of pumping equipment. Process optimization is conducted to reduce overall energy usage. This is achieved through selection of pipe diameter, system configuration and elevations for location of equipment and tanks. As mentioned before, the objective is to reduce number of pumping steps and exposure of water to outside environment (air and light). 6. Configuration of feed water pretreatment process 6.1. Conventional pretreatment The term conventional pretreatment usually refers to use of granular media filtration as opposed to membrane filtration. However, this differentiation is being blurred today as it is common to design pretreatment systems that integrate granular media separation or clarification through settling with the membrane filtration step Configurations and components of conventional pretreatment system The configuration of desalination unit and the pretreatment system depends on source and quality of the raw water. In case of desalination system operating with raw water pumped from dedicated brackish wells or seawater beach wells, the pretreatment is very minimal, as shown on Figure

52 Figure 3.1. Configuration of RO unit operating with well water. The configuration of pretreatment unit is limited to sand separator, addition of acid and/or scale inhibitor and cartridge filtration, as shown on Figure

53 Figure 3.2. Configuration of pretreatment unit in desalination plant treating well water. Some systems, that operate with shallow wells, that occasionally produce water of elevated turbidity, utilize high filtration rate pressure filters prior to the cartridge filters. In case of presence of media filtration, sand separation equipment is not required. In isolated cases additional pretreatment is applied to the well water to adsorb and remove organic industrial microcontaminants utilizing activated carbon filters. Some systems utilize green sand filters for removal iron and/or manganese from the feed water if present in high concentrations. Desalination systems that treat surface water require some form of filtration prior to cartridge filtration. If the concentration of suspended solids is moderate, than utilizing granular media filtration could be sufficient. Granular media filter could be configured as pressure filters, ether vertical or horizontal, as shown schematically on figure 3.3. Figure 3.3. RO system treating surface water. Pretreatment configuration shown on Figure 3.3 includes chlorination, acidification, followed by addition of ferric coagulant. The coagulated water is filtrated with vertical pressure filters. The assumption is that effluent from the media filters will have quality sufficient for introduction to membrane unit. The cartridge filter located after the media filtration step is to protect pumps and membrane elements from sudden influx of particulate matter, for example due to granular filter media break through. 53

54 Chlorination of raw water should be only applied as a intermittent measure. Continuous chlorination could result in conversion of TOC to assimilable organic carbon (AOC). Increased AOC could be utilized by bacteria that survived chlorination and promote bacterial grow in the membrane unit. Whenever, chlorine is added to the feed water, it has to be reduced, preferably using sodium bisulfate according to the following reaction: NaHSO3 + HOCl = NaHSO4 + HCl (3.1) Three parts of sodium bisulfate are required for one part of free chlorine. The reaction is very rapid, proceeding to a complete dechlorination in seconds. ORP analyzer is used to monitor dechlorination reaction. The target ORP value for dechlorination is below 200 mv. The granular pressure filters, either in vertical or horizontal configurations, are usually used in small and medium size systems. In very large desalination systems gravity media filters are being used. However, in Spain, pressure filters are being used in majority of seawater desalination systems of all sizes. Majority of pretreatment systems that utilize granular media filtration, incorporate coagulation and flocculation to improve removal of colloidal particles. Coagulats of choice are iron based salts. The usual dosing rate of metal coagulants is in the range of 1 20 ppm. Sometimes organic polymers are also added in the coagulation process to increase flock strength. The usual dosing range of filtration polymers is in the range of ppm. Utilization of coagulants requires incorporation of solids management system to treat backwash stream produced by granular filters. The objective of solids management system is to concentrate and dewater backwash effluent to a solids concentration above 20%. At this level of solids concentration, the backwash residual can be transport off site and dispose to land fill. Granular media filtration systems that do not utilize coagulants usually are able to discharge backwash effluent together with concentrate stream back to the ocean. At some locations surface seawater may seasonally experience high concentration of suspended and colloidal solids. The rough indicator of raw water quality is turbidity. At turbidity levels above NTU a clarification step could be required prior to media filtration. The selection of clarification technology will depend on nature of suspended solids. If solids are manly of inorganic nature (silt) than most likely settling type clarifiers would be more effective. However, if suspended solids are of organic nature (algae), which are buoyant and may not settle well, than dissolved air flotation (DAF), would be more effective technology to apply Membrane pretreatment 54

55 Membrane filtration, both microfiltration and ultrafiltration, is pretreatment technology of choice for wastewater reclamation systems. In these applications, membrane filtration replaced almost completely conventional pretreatment methods used previously. Similar trend is observed in seawater desalination systems treating seawater from open intakes. The configuration of seawater desalination system that utilizes membrane pretreatment is similar to system with conventional pretreatment, with media filtration replaced with membrane unit, as shown on Figure 3.4. The immersed (vacuum driven) membrane filtration units are immersed in membrane tanks, similarly to granular media filters. In pressure driven units the membrane modules are connected in parallel and configured as membrane trains. The membrane backwash unit plays similar role as filtration media backwash unit used in granular media filters. One of the differences is use chemicals, mainly NaOCl, during chemical enhanced (CEB) backwash Number of seawater desalination systems that utilize membrane filtration is growing. Still the majority of seawater RO plants in operation and new systems being built utilize the conventional filtration technology. Figure 3.4. Schematic configuration of seawater desalination system with membrane pretreatment. 55

56 Compared to the membrane filtration systems, the pretreatment systems based on granular media filtration are in most cases less expensive to construct and operate. Evaluation of configuration and economics of pretreatment systems based on membrane filtration and granual media filtration is provided in Chapter 4.6. Another disadvantage of membrane filtration pretreatment technology is extended use of chlorination for membrane performance recovery. As mentioned before, use of chlorine in the pretreatment process could lead to biofouling of RO membranes. In addition, the hydraulic conditions of operation of membrane filtration systems lead to breaking of microorganism present in the seawater and release of cell fluids. This matter could be metabolize by other bacteria for cell growth and formation of biofilms in membrane unit. Utilization of coagulation improves stability of operation of membrane filtration systems, improving stability of membrane permeability. Addition of metal coagulant at the level of ppm could result in significant improvement of performance. However, utilization of coagulat will require inclusion of solids management system as a part of pretreatment unit. Therefore, the tendency is not to include coagulation in configuration of membrane pretreatment systems and relay of backwash procedures to maintain a sufficient level of membrane permeability. The membrane filtration technology consists of wide range of configurations and some membrane modules configurations are more robust in respect of treatment of streams containing high concentration of suspended solid, for example membrane elements utilized in membrane bioreactor (MBR) applications. Usually the MBR membrane modules have low membrane area packing density and would be quite expensive for use as pretreatment in RO desalination systems. In most cases, the membrane modules utilized in RO applications are the same as being used in potable applications. The tolerance of membrane filtration technology to high concentration of suspended solids in raw water is similar to granular media filtration. Also here if the raw water turbidity exceeds NTU range, an initial clarification step may be necessary to maintain stable performance of membrane filtration system. The addition of initial clarification step is necessary if raw water experiences high turbidity levels for extended periods of time. If the increase of turbidity is only during small fraction of the annual operating time, maintaining lower output capacity of the pretreatment system or even temporary discontinue of its operation could be considered as more cost effective solution rather than addition of clarification equipment that will be idle most of the time. The schematic diagram on Figure 3.4 shows cartridge filtration as part of the pretreatment system. The usual approach is to include cartridge filtration if membrane filtrate is store in an intermittent tank prior to membrane unit. If membrane filtrate flows directly from filtration system to the suction of RO feed pumps, cartridge filtration equipment could be omitted from the feed water pretreatment process. 56

57 6.3. Raw water quality and pretreatment requirements. RO Technology Mark Wilf Ph. D. The quality of raw water is closely related to water source. The configuration of pretreatment systems for RO applications has evolved around specific water sources. For each RO application: brackish, seawater and wastewater reclamation pretreatment system configurations have been developed that can effectively produce feed water of adequate quality. Table 3.1 includes water quality parameters for the three major water types of water supply sources. The last row of the table lists configuration of pretreatment systems that in majority of circumstances will be sufficient to produce feed water of adequate quality. Table 3.1. Representative raw water quality associated with major types of water supply sources Quality parameter Well water Surface water Secondary effluent Turbidity < 1 NTU < 5 NTU 2-10 NTU SDI < 1 < 5 not measurable Suspended solids < 1 ppm < 5 ppm < 20 TOC < 1 ppm < 5 ppm < 20 SiO2 < 25 ppm n. a. n. a. Common configuration of pretreatment system Acidification Scale inhibitor Cartridge filtration Acidification Flocculation Media filtration Cartridge filtration Membrane filtration Acidification Scale inhibitor Cartridge filtration However, at some locations, quality of raw water could fluctuate outside range listed in Table 3.1. In such cases the pretreatment system has to be augmented by additional treatment steps. Table 3.2 provides recommended configurations of pretreatment system according to water source and range of water quality parameters. Table 3.2. Recommended configuration of pretreatment system according to raw water quality Water source Brackish well water Brackish well water Water quality parameters Turbidity < 0.2 NTU TSS < 2 SDI < 1.0 Turbidity > 0.2 NTU TSS > 2 SDI >1.0 Configuration of pretreatment system Acidification Scale inhibitor Cartridge filtration Sand filtration Acidification Scale inhibitor Comments 57

58 Brackish well water Seawater beach well Seawater beach well Seawater open intake Seawater open intake Seawater open intake Seawater open intake Turbidity < 0.2 NTU TSS < 2 SDI < 1.0 Presence of dissolved Fe & Mn Turbidity < 0.2 NTU TSS < 2 SDI < 1.0 Turbidity >0.2 NTU TSS >2 SDI >1.0 Turbidity < 5 NTU TSS < 5 TOC < 2 Turbidity < 5 NTU TSS < 5 TOC < 2 Turbidity 5-20 NTU TSS > 5 TOC > 2 Turbidity 5-20 NTU TSS > 5 TOC > 2 Seawater open intake Turbidity > NTU TSS > 5 TOC > 2 Cartridge filtration Acidification Scale inhibitor Cartridge filtration Cartridge filtration Sand filtration Cartridge filtration Acidification Coagulation + flocculation Single stage granular dual media filtration Membrane filtration Acidification Coagulation + flocculation Two stage granular dual media filtration Acidification Coagulation + flocculation Membrane filtration Settling clarification Coagulation + flocculation Single stage granular dual media filtration RO Technology Mark Wilf Ph. D. Maintain feed water at anaerobic conditions If seawater is under influence of brackish water, acidification and scale inhibitor may be required If seawater is under influence of brackish water, acidification and scale inhibitor may be required Short excursion of turbidity up to 20 NTU is possible for few days in year Short excursion of turbidity up to 20 NTU is possible for few days in year Short excursion of turbidity up to 30 NTU is possible for few days in year Short excursion of turbidity up to 30 NTU are possible for few days in year Suspended solids mainly inorganic particles(silt) Seawater open intake Turbidity > Settling clarification Suspended solids 58

59 NTU TSS > 5 TOC > 2 Seawater open intake Turbidity > NTU TSS > 5 TOC > 2 Seawater open intake Turbidity > NTU TSS > 5 TOC > 2 Wastewater secondary effluent Wastewater MBR effluent Turbidity 2 20 NTU TSS < 20 TOC < 20 Turbidity <1 NTU TSS < 2 TOC < 20 Coagulation + flocculation Membrane filtration DAF Coagulation + flocculation Single stage granular dual media filtration DAF Coagulation + flocculation Membrane filtration Membrane filtration Acidification Scale inhibitor Cartridge filtration Acidification Scale inhibitor Cartridge filtration RO Technology Mark Wilf Ph. D. mainly inorganic particles(silt) Suspended solids mainly organic and biological particles(algae) Suspended solids mainly organic and biological particles(algae) Chloramine concentration of 2 4 ppm maintained in the RO unit Chloramine concentration of 2 4 ppm maintained in the RO unit 7. Pretreatment equipment 7.1. Screening The extend of screening of raw water will depend on application and type of the pretreatment process. For brackish RO system, operating with well water, usually no screening is applied in the pretreatment system. Only in case that well releases sand particles, sand trap screen should be applied. In majority of cases, the quantity if sand being released from well is very small and manually cleaned sand screen will be adequate. Usually, the sand screen is configured as a wedge wire type cylindrical shape screen installed in flow through vessel that is part of the raw water supply piping (schematically shown on Figure 3.2). Raw water coming from an open intake structure could contain large objects that could damage pumping equipment. The initial screening is conducted by the bar screen installed at the entry openings to the intake structure. In submersed structures the opening bar spacing will be in the range of 3 10 mm. This is to maintain entrance form velocity not to exceed 20 cm/sec. In channel configuration intakes the opening bar spacing if the trash rack would usually be larger in the range of mm. The large spacing trash racks would be followed by finer screens. Large capacity installations could utilize automatically washed traveling band screens as shown on 59

60 Figure 3.5. RO Technology Mark Wilf Ph. D. Figure Dual flow intake band screen (web page Horim Industries Inc., Korea) RO seawater systems that utilize multimedia filtration in the pretreatment unit would not require any additional screening beyond 3 10 mm range screens. Any debris that will enter the pretreatment system will be stopped on the surface of filtration media layer and will be discharged from the system during the filter backwash step. RO systems that utilize membrane filtration in the pretreatment would require micron range screens ahead of membrane filtration system. The rating of screens is specified by membrane manufacturers, usually in the range of microns. Variety of screening equipment is available on the market. The important issues in selection of screening equipment are: 1. Material of construction to be compatible with seawater environment. The equipment operates at low pressure, therefore, plastic materials of construction are preferred. 2. Low energy requirement for operation (low pressure loses) 3. Low pressure requirement for backwash (otherwise dedicated pumping equipment for backwash may be required) 4. Low equipment cost. Example of strainer equipment being used in seawater application is shown on Figure

61 Figure Automatic disc strainers. (Arkal Filtration Systems) 7.2. Dissolved air flotation (DAF) DAF is considered as very effective process for removal of light particles from water. It could be applied as initial treatment step to remove algae from seawater feed, if present in large concentrations. DAF process involves number of process steps: 1. Destabilization of colloidal particles by coagulation and flocculation. 2. Saturation under pressure of fraction of raw water with air. 3. Injection of air saturated water into the stream of treated raw water and release of air micro bubbles. 61

62 4. Attachment of destabilized colloidal particles to the air micro bubbles and their rise to the surface. 5. Hydraulic or mechanic collection of floating solids and discharge from the system. 6. Collection of subnatent clear water and its direction to the storage or filtration unit. Schematic diagram of DAF system, integrated with media filtration unit, is shown on Figure Raw water entering the system is acidified followed by addition of coagulant. After a rapid mixing step, water flows into two stages flocculation unit. From flocculation unit water enters the DAF section. At the entrance to the DAF (contact zone), the influent is combined with the pressurized recirculation stream, saturated with air. The air saturated recirculating stream flow rate is about 10% of the total water flow in the DAF unit. Typical DAFF Plant Schematic AIR ACID COAG POLY AIR SATURATOR RECYCLE PUMPS FLOAT TO SEA RAW WATER INLET RAPID MIX STAGE 1 FLOCCULATOR STAGE 2 FLOCCULATOR TO TREATED WATER STORAGE DAFF TANK UnitedKG (AU) Figure Schematic diagram of DAF system integrated with multi media filtration unit (courtesy UnitedKG) The sudden pressure drop at the mixing point of two streams results in release of air, which forms large quantity of micro bubbles: micron size. The bubbles with attached flock particles rise to the surface, forming dense layer of captured particles (float). The float layer flows to the collection and discharge channel and leaves the DAF unit. The clarified water body passes through the multimedia filtration layer and flows to the storage tank. In the DAF configuration without media filtration unit, the clarified water is collected at the bottom of the filtration unit and overflows to the clear well. The flock particles, lifted to the surface by attached air bubbles, form a dense layer of floating solids, known as float. The float is removed through a mechanical skimming unit (mechanical removal) or by solids overflow to the collection through (hydraulic removal). Mechanical removal results in 62

63 waste stream with solids concentration of 2 3%. The hydraulic removal produce wastewater with lower solids concentration in the rate of 0.5 1%. A 10% fraction of the clarified water effluent flow is pumped to the air saturators. The air saturators are configured as pressure tanks filled with plastic spheres filling, as shown on Figure The saturator vessel is configured based on the water flow rate of m3/m2/hr and air pressure of bar. The objective of operation of saturator is to achieve dissolved air concentration level of about 100 ppm. The representative design parameters of DAF system are listed in Table 4.1. The solubility of air in water is governed by Henry s law. The solubility is function of temperature as shown on Figure The target of 100 ppm of air concentration in the recirculation stream is selected to create a dissolved air excess concentration of about 10 ppm in DAF influent, after mixing both streams together. It has been shown that turbidity of DAF effluent reaches plateau at air concentration of about 10 ppm, as shown on Figure Therefore, concentrations higher than 10 ppm would only increase energy consumption without additional reduction of concentration of colloidal particles. The relation between mass concentration of air released at the entry of the DAF cell (CDAF), concentration of air in the recirculating stream (CR) and concentration of dissolved air in the raw water influent (CIN) is given by equation 4.1. CDAF = (CR CIN)*r/(1+ r) (4.1) Where: r is recirculation fraction of DAF influent flow Accordingly, required concentration of air in the recirculation flow is given by equation 4.2. CR = CIN + CDAF*(1+r)/r (4.2) CIN of the surface water is usually close to 24 g/m3. Pressure required in the air saturation units is determined based on target CDAF, recirculation ratio air solubility vs. pressure shown on Figure Picture of DAF system operating at Tuas, Singapore, SWRO desalination plant is shown on Figure The picture shows DAF-multimedia filters basins and air saturator tanks. 63

64 DAF air saturator Pressurized water inlet, m/h (24.5 gpm/ft2) Pressurized air inlet, bar (65 80 psi) Air transfer packing m (3 5 ft) Saturated water outlet Figure Configuration of DAF air saturator 200 Dissolced air concentration, ppm P = KC 10 C 30 C Air pressure, bar Figure Relation between air pressure and concentration of dissolved air in water. 64

65 DAF effluent turbidity, NTU Air concentration, ppm Figure Relation between turbidity of DAF effluent and concentration of dissolved air in water. DAF system UnitedKG (AU) Figure Picture of DAF installation at the Tuas, Singapore, SWRO desalination plant. 65

66 Although DAF is well known water treatment technology, number of SWRO desalination plants that utilize DAF technology is limited. DAF by itself can not produce effluent with quality sufficient for RO applications. It has to be followed by additional filtration equipment: multimedia filtration or membrane filtration. Therefore, utilization of DAF is only considered when raw water source has high frequency of presence of algae or similar nature particles. Operation of DAF is associated with additional energy usage, in the range of 1 3 KWh/1000 m3. Table Representative design parameters of DAF system Design parameter Value or range Hydraulic loading rate, m3/m2/hr DAF tank length, m < 11 DAF tank with to length ration < 1 DAF tank surface area, m Maximum DAF tank capacity, m3/hr 1,000 2,000 Basin depth, m Contac zone detention time, sec Recycle ratio, % 6 10 Recycle system pressure, bar 4 6 Saturator hydraulic loading rate, m3/m2/hr Saturator packing depth, m Air bubble size, micron Float (sludge) concentration, % Example of calculation of operating cost of the DAF unit. Operating expenses of the DAF unit includes: 1. Usage of acid for acidification of seawater 2. Usage of ferric coagulant 3. Energy of pumping water to the saturator 4. Energy for pumping air to the saturator 5. Maintenance parts Dosage of acid will depend on ph required for effective flocculation. Dosing rate will depend on required ph, water alkalinity and temperature. For the propose of preliminary cost estimation a dosing rate of 20 ppm of sulfuric acid can be used. Dosing rate of ferric coagulant will depend on results of Jar Test or pilot unit operation. For preliminary cost estimation a dosing rate of 10 ppm of ferric sulfate can be used. 66

67 Energy (kwhr) required for pumping is calculated based on flow rate of water pumped to the saturator and pressure: E = * Qs*Ps/( M* P) (4.3) Where: Qs is water flow to the saturator (m3/hr), Ps is the saturator pressure (bar), M is efficiency of the motor, P is efficiency of the pump Energy required by air blower to saturate the reciculation stream with air to the designed air concentration level is given by equation 4.4. E = 0.133* QS*AS/(1000* A)*(((Ps +1) ) 1 )/( M* B* VFD) (4.4) Where: QS is the water flow through the saturator, AS is air saturation concentration in water at the outlet from the saturator (g/m3), usually the same as CR equation 4.1, A is air density ( C), Ps is the saturator pressure (bar), M is efficiency of the motor, B is efficiency of the blower and VFD is efficiency of the VFD. Table Example of energy usage of air saturation unit for a DAF system for the raw water flow of 1000 m3/hr. Process Parameter Value Water flow to Saturator, m3/hr Water pressure, bar 5.0 Air saturation concentration, ppm Air density, kg/m Pump efficiency 0.75 Motor efficiency 0.90 Blower efficiency 0.55 VFD efficiency 0.98 Energy for water pumping, kwhr Energy for air pumping, kwhr 1.50 Total energy, kwhr Table Design parameters of a DAF unit, 100,000 m3/day effluent capacity Process Parameter Units Value Effluent capacity m3/day 100,000 Suspended solids concentration ppm 10 Water loses with float % 2 Designed hydraulic loading m3/m2/hr 20 67

68 Cell area m2 60 Cell length m 8 Cell with m 7.5 System area m Calculated number of cells 3.5 Actual number of cells 4 Actual hydraulic loading m3/m2/hr 17.7 Recycle ratio % 10.0 Recycle flow m3/hr Saturator loading rate m3/m2/hr 70.0 Total saturator cross section m2 6.1 Number of saturators 4.0 Saturator tank diameter m 1.4 Air conc. in raw water g/m Designed air concentration in the DAF unit g/m Air concentration in recirculation Stream g/m Required pressure in the saturator bar 6.0 Energy of pumping water to saturator kwhr 99.4 Energy of pumping air to saturator kwhr 9.5 Energy usage KkWhr/m Sulfuric acid dosing rate ppm Sulfuric usage t/day 2.23 Ferric dosing rate ppm 10.0 Ferric usage t/day 1.02 Solids to concentration t/day Coagulation and flocculation Coagulation and flocculation is a combined process of destabilization and conglomeration of colloidal particles to facilitate more effective removal in media filtration process. Colloidal particles in the water stream are negatively charged and electrostatic repulsion helps to maintain them in suspension. Coagulants are positively charged hydrolyzed metal salts that neutralize negative charges of suspended colloids and help to aggregate them into larger, heavier, more filterable solids. For coagulation mainly ferric or alum salts are used. During hydrolysis of these salts a complex polynuclear, positively charged species are formed in a matter of seconds. The solubility of these species is low and they form dense, suspended flock. The action of coagulants is threefold: they adsorb colloidal particles on the flock surface, neutralize negative charges that surround colloidal particles and also enmesh suspended particles in the body of the flock formed. The effective quantity of coagulant required is specific to water composition, type of colloidal particles, water ph and temperature. 68

69 An excessive quantity of coagulant could have the undesirable effect of increasing the stability of colloidal particles. Excessively high concentration of coagulant may increase dispersion of colloids, due to reversal of surface charges: formation of high density, positive charges on the colloids surface and mutual electrostatic repulsion. The transition of Zeta Potential and Turbidity with increasing dose of coagulant is illustrated on Figure 4.5. As shown on Figure 4.3.1, the preferred range of zeta potential to achieve low turbidity of the effluent is between 0 to - 20 mv. Zeta Potential values outside this range will result in increased stability of colloidal matter and higher turbidity. The initial estimation of the required dosing rate of coagulant and optimum ph range for the process is determined by conducting a jar test (described in chapter ). The dosing rate of inorganic coagulant is usually in the range of 1 30 ppm and ph in the range of 6-8. Following the results of the jar test the adjustment of coagulation process parameters is conducted during the initial stages of commercial system operation. For RO applications ferric salts are preferred over aluminum due to the lower tendency of forming deposits in membrane elements. The solubility of hydrolyzed species of aluminum and iron depends turbidity (NTU) zeta potential (mv) Picture Transition of Turbidity and Zeta Potential with increasing dose of coagulant (courtesy Peter Hillis). on ph. The solubility is at a minimum at about ph 6 for aluminum hydroxide and at about ph 8 for ferric hydroxide. The solubility of hydrolyzed ferric compounds is much lower than of correspond- 69

70 ing aluminum species. The minimum solubility for Fe is about 10-9 mol/l compared to 10-6 mol/l for Al. Therefore, if metal salt coagulation is applied, the pretreatment system effluent, and subsequently RO feed water will have lower concentration of ferric ions then the potential concentration of aluminum ions at the corresponding conditions. Accordingly, in case of ferric coagulant there will be lower potential for precipitation as a result of feed water ph changes and/or due to the increase in concentration of dissolved species that occurs in the RO process. As mentioned already coagulation is a very rapid process requiring just a few second to complete. However, effective coagulation required intensive mixing to bring the coagulant in contact with a large number of colloidal particles. In majority of applications this is usually achieved by incorporating coagulation tanks with mechanical mixers, either vertical rotating blades or horizontal paddle mixers. In RO systems in line coagulation, using static mixers positioned downstream of the coagulant injection point, is also being used. The configuration of the coagulation flocculation system will depend on configuration of filtration unit downstream. Systems that utilize pressure filters usually relay on in line coagulation - flocculation, using static mixers. Use of static mixers and in line flocculation, avoids reduction of pressure of the pumped stream to the atmospheric pressure, which would require repumping. Static mixers are compact devices that can be incorporated into feed line piping. The mixing efficiency of static mixers is depended on the flow rate through the mixer. Therefore, the dispersion of coagulant could be lower at partial flow. Use of static mixer introduces head loss of 0.5 1m. For efficient flocculation downstream of static mixer, a straight segment of pipe is required, about 20 pipe diameters long. The gravity filtration systems are configured to provide gravity flow through the whole system, after initial pumping boost, through a cascade of overflowing steps. In gravity filtration systems, the initial coagulation step is accomplished either in static mixers or in tanks equipped with rapid mixers. The flocculation is usually conducted in tanks open to atmosphere with slow mixing. Mechanical mixing consists of coagulation tank with a mixer that can create velocity gradient (G) of about 300 sec -1. The tank size is designed for a retention time of min. The power required for mechanical mixer is in the range of HP/1000 m3/day. The velocity gradient G, is related to coagulation or flocculation tank volume V, power rating of the mixer motor P and water viscosity, according to equation

71 G = [P/( * V)] 0.5 (4.5) RO Technology Mark Wilf Ph. D. For the coagulation process the G value is selected in the range of s -1. For the flucculation process the G values are lower, usually below 150 s -1. During coagulation, metal salts hydrolyze and dissociate. The conversion of metal coagulants to the hydrolyzed form consumes alkalinity in the water. Therefore, the raw water ph is reduced ( ph units), in proportion to the coagulant dosing and alkalinity present, according to the following equations: FeCl3 + 3HCO3 - = Fe(OH)3 + 3Cl - +3CO2 (4.6) Fe2(SO4)3 + 6HCO3 - = 2Fe(OH)3 + 3SO4 = +6CO2 (4.7) AlCl3 + 3HCO3 - = Al(OH)3 + 3Cl - + 3CO2 (4.8) Coagulation can be also conducted using long chain synthetic organic polymers, which could be of nonionic, anionic or cationic types. The nonionic and anionic polymers destabilize colloids by bridging particles together. The cationic type polymers have a dual action of bridging and neutralization the negative surface charges of the colloids. Cationic organic polymers can be used as primary coagulants. However, in RO pretreatment systems polymers are usually used as additives to enhance the effectiveness of metal based coagulants by binding flock particles together. In most cases, polymers are applied at low dosing rage, below 1 ppm, directly injected to the feed water downstream of the dosing point of the metal coagulant, at the location where hydrolyzed metal flock has been already formed. If polymers are used at a high dosing rate, and a carryover from the sand filters occurs, cationic polymers may react with anionic scale inhibitors and form a fouling layer on the membrane surface. Flocculation, which follows coagulation, is a process of flock formation during gentle mixing. Flocculation is a slower process than coagulation and takes number of minutes to complete. During flocculation, colloidal particles and some fraction of dissolved organics are being attached to the flock body, and are eventually retained on the filtration layer in the granular media filters. Flocculation in conducted in tanks equipped with mixers that are able to create velocity gradient of sec -1. The rotating velocity of the mixers is slow, in the range of rpm. The depth of 71

72 flocculation tanks is m and the working volume such that the retention time during flocculation will be min. The schematic configuration of coagulation flocculation unit is shown on Figure Static mixer Media filters Flocculators Ferric dosing Acid dosing Figure Schematic diagram of configuration of coagulation flocculation unit According to diagram shown in Figure 4.3.2, there is in line coagulation, utilizing static mixer, followed by flocculation in two flocculation chambers equipped with blade mixers. Preliminary specification of coagulation flocculation systems are provided in Table The unit flow capacity is designated for a filtration system of nominal effluent capacity of 100,000 m3/day. Such a system requires about 104,000 m3/day effluent to produce sufficient excess of filtrate for filters backwash. Table preliminary specifications of coagulation flocculation unit. Nominal flow capacity 100,000 m3/day. Net effluent system capacity m3/day 100,000 Influent flow rate m3/hr 4,335 Common manifold diameter mm 864 Flow velocity m/sec 2.05 Static mixer diameter mm 864 Number of flocculation lines 4 72

73 Number of flocculators in series in each line 2 Floccculator cell length m2 6.0 Flocculator cell with m 4.3 Flocculator cell depth m 6.00 Operational volume of flocculator cell m Lotal volume of flocculators m Flocculation time min 15.4 Coagulant dosing rate ppm as Fe2(SO4)3 10 Coagualnt usage t/day (100%) 1.04 Accid dosing rate ppm as H2SO Acid usage t/day (96%) Granular media filtration In the granular media filtration process, suspended solids are removed through attachment to the filtration media particles and through blockage/capture by the filtration cake. The preferred process of filtration is capture of suspended solids with significant bed penetration as opposed to surface filtration, since the latter results in faster increase of pressure loss and therefore shorter filter runs. In a single medium filtration bed, after number of backwash runs, fine size filtration media particles are aggregated at the top of the bed. This reduces penetration of suspended solids and therefore, mainly results in surface bed filtration. A graduation of the filtration bed from coarse to fine particles of the filtration bed can be achieved in dual media configuration by placing fine, high specific gravity, filtration media as the lower filtration layer and coarse, low specific gravity, filtration media as a top layer. Filtration media selection that provides coarse to fine filtration bed configuration, in- 73

74 cludes anthracite (specific density t/m3, effective size around 1.5 mm. as a top layer and silica sand (specific density: t/m3, effective size around 0.6 mm, as a bottom filtration layer. The value of effective size (ES) means that size of 90% of filter media particles in the given lot is larger than the value indicated. Another important parameter of filtration media is uniformity coefficient (UC). The uniformity coefficient is expressed as a ratio of ES value corresponding to 60% passage over ES value corresponding to 10% passage: UC = ES60/ES10 (4.9) The lower the values of UC for a given lot, the closer together are sizes of particles forming the lot. The practical values of UC specifications for filtration media are in the range of Example of filtration media (anthracite and sand) specifications is provided in Table Table Range of specifications parameters of filtration media. Parameter Designation Anthracite Filtration sand Effective size ES = D Uniformity coefficient UC = d60/d Specific gravity SG Hardness Moh scale The operation of sequence of operation of media filters is based on capture of suspended solids in media layer and formation of filtration cake on the surface. Increasing load of solid particles in media results in increase of pressure drop across the media. At the end of filtration cycle, the flow of water through the filter is reversed. Usually using filtrate, the flow through the filter is from the outlet port to the inlet port, expanding the filtration layer and flushing out the solids particles captured. The duration of filtration cycle is usually determined based on the operating time, the standard length being 24 hr. The backwash process usually includes sequence of steps, which are finalized rinsing filter in forward direction to create initial coating on filtration particles and improve solids capture efficiency of the filter. Once the quality of filter effluent reaches the required level, the filter 74

75 is connected back to the common effluent piping manifold. RO Technology Mark Wilf Ph. D. Effective backwash requires that the filter layer will be fluidized, which corresponds to the filtration bed expansion of about 30%. The bed expansion occurs as the drag forces of backwash flow increases of gravity forces. The backwash flow rate required for bed expansion will depend on water viscosity. The required flow rate will be higher at increased water temperature (reduced water viscosity and lower drag forces). At the temperature range of 20 C, the backwash flow rate for bed expansion will be about 40 m/hr for silica sand and about 50 m/hr for anthracite. The backwash flow rate (VB) required for a given rate of bed expansion can be calculated according to the equations below: VB = *Re/( w*d) (4.10) Where: is dynamic viscosity of water, Re is Reynolds number, w is density of water an d is representative media particle size. Re = V (1 )/2 l + 1/(2 l) [ 2 V(1 )2 + 4 l 1/2 (4.11) Where: V is head loss coefficient due to viscous forces, dimensionless, l is head loss coe fficient due to inertial forces, dimensionless, is porosity, dimensionless and is backwash calculation factor, dimensionless. The recommended range of the above parameters is provided in Table Table Recommended values of filtration media parameters Filtration medium V l Sand Anthracite = g w( p w)d 3 3 / 2 (4.12) 75

76 Where: g is gravity acceleration (9.81m/sec) and p particle density RO Technology Mark Wilf Ph. D. Example of calculation of required backwash flow rate (VB) Filter media anthracite Bed depth 1.0 m Required expansion 30% Media particle size 1.5 mm Media porosity 0.50 Media density 1700 kg/m3 Water temperature 15 C V 230 l 4.4 expanded bed (9.81*1000)*( )*(0.0015) 3 *(0.615) 3 ]/( ) 2 = 3405 Re = -230( )/(2*4.4) + 1/(2*4.4)*[(230) 2 ( ) 2 + 4*(4.4*3405)]0.5 = 19.3 VB = *19.3*3600/(1000*0.0015) = 52 m/hr Pressure filter There is a variety of media filtration equipment configurations used in potable and waste water filtration. In RO applications the frequently used filter types are pressure or gravity down-flow filters in single or two stage configuration. The pressure filters are cylindrical pressure vessels filled with a layer(s) of filtration media. The filters could be configured for horizontal or vertical operation (Figure and 4.4.2). Maximum diameter of filter is limited to about 3 4 m, due to logistic of transportation of large pressure vessels from the manufacturer to the project site. The shell length of horizontal pressure filters usually does not exceed 12 m. The important features of the pressure filters configurations are the top distributors for the uniform entry of influent to the filter and bottom collectors of the filtrate. The influent distributors are usually configures as a grid of perforated laterals. For the filtrate collection laterals could be used or false 76

77 bottom with nozzles could be utilized. Examples of filter nozzles are shown on Figure Pressure filters are equipped with air relief valves, installed at the highest point of the filter shell. In horizontal filters, which are divided into chambers, each chamber should be equipped with a separate air relief valve. The filters that utilize air scouring to enhance media backwash, could have separate port and internal manifold for air distribution for this purpose. In systems utilizing false bottom and nozzles, the air is injected into the filtration layer through the filtrate collection nozzles. The filter shell should have man port for loading the media and filter maintenance and a smaller viewing port to enable observation of the condition of the surface of the media during normal operation and during the backwash. Filters that utilize bottom laterals for filtrate collection should have the volume below laterals filled with concrete. This is to eliminate presence of areas of stagnant water, below the bottom collectors. In the filtration step, raw water enters the filter shell through the inlet port and, flows through distributors over the surface of filtration layer. Water infiltrates through the media and is collected as a filtrate by the bottom laterals or nozzles (in the false bottom configuration). In the backwash mode the flow directions are being reversed. The filtrate collector serves as the entry port for the filtrate used as backwash water. The backwash water eventually exits the filter through the top, influent port, and is directed to the backwash water storage tank or to the outfall. 77

78 Figure Drawing of a vertical pressure filter (courtesy of Tonka Company) Additional step that follows the backwash and produces water stream for disposal is rinsing of the media. The rinse flow is applied in the same direction as the flow direction during regular filtration step, with the exception that filtrate produced flows to waste and not to the filtrate storage tank. Accordingly the valves arrangement at the influent port should allow entry of raw water during the filtration step and discharge of backwash water to waste during filter backwash. The valves arrangement at the bottom effluent port should allow exit of filtrate during filtration step, entry of filtrate during backwash step and discharge off speck filtrate to waste during filter media rinse. Media rinse that follows the backwash step, is conducting at a normal filtration rate till the turbidity of filtrate declines to the required level. 78

79 Figure Drawing of horizontal filter (courtesy of Tonka Company) Figure Examples of filter nozzles (web page of FTR, Istanbul) In RO application the filtration system operates at constant output. The control of the filtration rate is accomplished by adjusting the throttling of the valve located on the effluent line. As shown on figure 4.4.4, the differential head (DH) is a sum of head losses (HL) in the filtration bed and flow 79

80 resistance of the outlet valve. RO Technology Mark Wilf Ph. D. Figure Configuration of effluent flow control in a pressure filter Initially, when the filtration bed resistance is low, the valve is only partially open. As the head loses increases during the filtration run, the valve is gradually open to maintain constant output. When the fitter reaches the point that valve position is close to being fully open, filter enters the backwash step to remove collected matter from the media and reduce flow resistance to the original value. Otherwise, filter output will start to decline. The filtrate for the backwash can be provided from a storage tank (clear well) or can be generated internally from filters that are in filtration mode. Such configuration for backwash with internal filtrate supply is shown in Figure During the filtration step the effluent valve is open and the valves to the waste are closed. To start the backwash, the effluent valve of the filtration unit composed of filters 1 4 will close. Then for the filter entering the backwash, the inlet valve will close and valve to waste will open. In configuration shown on Figure 4.4.5, if filter # 1 enters to backwash, filters 2 4 will produce filtrate that will flow through the filtrate collecting manifold back to filter 1 in a reverse direction. When the backwash is completed, the valve to waste will close, effluent valve of the filtration unit will open. The inlet valve of filter # 1 will open also, returning filter to normal flow direction. Most likely the 80

81 initial filtrate will be send to waste till turbidity of the effluent will reach the level compatible with requirements of the RO feed. Figure Configuration of vertical pressure filters with valves required for utilization of internal source backwash water. Figure Configuration of horizontal four chambers pressure filter with valves required for uti- 81

82 lization of internal source backwash water. RO Technology Mark Wilf Ph. D. The backwash described above can only be conducted if the backwash flow requirement of the filter being backwashed does not exceed the combined filtrate production rate of the filters remaining in operation. Another alternative configuration for backwash of pressure filters, utilizing water supply from high service line is shown on Figure In this configuration the backwash starts with closing Influent valve and opening the valve directing flow to waste for a set time. The backwash water from the distribution line will enter the filter in reverse flow, backwashing the filter bed. Influent Effluent to distributiion Waste Figure Alternative configuration for backwash of pressure filters from the high service line. The operating inlet pressure of the pressure filter is equal to driving head plus the pressure required by the equipment operating downstream of filter outlet. The design inlet pressure is usually 20 30% higher. For example a pressure filter having driving head of 5 m (0.5 bar) and required discharge 82

83 pressure of 20 m (2 bar), will have designed inlet pressure of 3 bar (2.5 * 1.2). The design of length of filter shell for vertical filters and shell diameter of horizontal filters is based on the height of filtration layers in the filter. For vertical filter, starting from the bottom, gravel media will be loaded to cover the filtrate collection laterals up to the level of about 20 cm above the laterals. The filtration layer will be about 80 cm to 1 m deep. Above it, there should be allowance for filtration bed expansion during backwash of about 30% (25 cm), additional 30 cm freeboard. To this combined length of 160 cm ( ) one should add the shell length required for installation of inlet water distributor, additional 25 cm. The same length of 25 cm will be added for the bottom collector. This result in a total length of the strait filter shell of 200 cm. Additional heights will be added to include bottom and top elliptical heads, 40 cm each, making total height of filter of 280 cm. The actual filter structure will be higher than this due to filter foundations and filter supporting frame. The horizontal media filters are sized in a similar way. The sizing of the filtration system is based on the nominal filtration rate and the backwash rate. The horizontal filters are usually divided into four compartments, each being backwashed separately. The system is divided into number of filters that would provide sufficient operational flexibility during filters backwash and filters maintenance. Example of results of sizing of filtration system utilizing horizontal media filters is included in Table Table Design parameters of media filtration system utilizing horizontal filters. System effluent capacity 100,000 m3/day. Net effluent system capacity m3/day 100,000 Influent flow rate m3/hr 4,408 Suspended solids concentration ppm 5 Nominal filtration rate m/hr 12.0 Actual filtration rate m/hr 12.4 Required filtration area m

84 Number of filters in operation 10 Number of filters installed 11 Nominal filtrate flow per filter m3/hr 417 Actual filtrate flow per filter m3/hr 441 Filter diameter m 3 Filter shell length m 12 Inlet/outlet pipe diameter m 0.25 Gravel layer depth m 0.3 Sand layer depth m 0.3 Anthracite layer depth m 0.9 Media expansion allowance m 0.4 Free board m 0.4 Width of media surface m 3.0 Filtration area per filter m Filtration compartments per filter 4.0 Filtration area per chamber m2 8.9 Backwash rate m/hr 45.0 Backwash flow per chamber m3/hr Backwash duration min Backwash volume m3/backwash Daily backwash volume m3/day Filtration interval hr 23.5 Filtrate volume per filtration cycle m Nominal recovery rate %

85 Gravel volume m3/filter 7.2 Sand volume m3/filter 9.0 Anthracite volume m3/filter 31.7 Gravel, total volume m Sand, total volume m Anthracite, total volume m In RO applications pressure filters are applied in small in medium size systems, seldom exceeding capacity of few thousands m3/day. However, in Spain it is common to use pressure filters also in large seawater RO desalination systems. Example of one such installation is the SWRO desalination plants at Carboneras, Spain. The plant has permeate water capacity of 120,000 m3/day. Configuration of this plant, including location of horizontal pressure filters is shown on Figure The filters are located on both sides of main RO building, feeding the symmetrically divided two lines of RO trains. Carboneras, Spain 120,000 m3/day Figure Configuration of SWRO desalination plant at Carboneras, Spain 85

86 Gravity filters The gravity filters have the configuration of rectangular tanks, usually made of concrete. The tank has connections for entry of influent and exit of filtrated effluent. In addition there is a connection for air supply used air scouring of filtration media during backwash step. In the same manner as with pressure filters, during the backwash step the flow direction is reversed, supplying the backwash flow through the effluent exit port. An example of a gravity filter configuration is shown on Figure In this diagram the influent water to the filter is supply through a side channel. Filtrate leaves the filter through the effluent outlet located at the bottom of the filter. The filter bottom is covered with blocks with slits that provide support to the filtration media and also collects filtrate. The exit port and the filtrate collecting blocks serve as an entrance of the backwash water. In some filter configurations, the blocks are replaced with a raised concrete floor with evenly spaced filter nozzles (shown on figure 4.4.9) Backwash operation is sometimes augmented by air. The air port is a separate port for injecting the compressed air to the underdrain. The backwash water is collected by the wash water troughs, located above the level that the media expands during the backwash. Some gravity filters utilize washing of media surface using water jets. 86

87 Wash water troughs Surface wash, not shown Air backwash Header Backwash supply & filter effluent Dual filter media Underdrain blocks & media retaining plate Influent & backwash water channel Figure Schematic configuration of a gravity media filter. The dimensions of gravity filters cells are determined by the surface area which is in the range of 25m 100 m2 per filter cell. The number of filtration cells for a given system is determined by the logistic of operation, the filtration and backwash steps. The preffered configuration is not increase filtration rate of filters in operation by more that 10 15%, while one filter is off line due to backwash. Another consideration is logistic operation of the RO membrane trains. In very large systems it is sometimes convenient to have media filter cells divided into two groups, so the desalination system can have flexibility to operate effectively at 50% of production capacity. The usual range of length to with ratio of filter cell dimensions is in the range of 2 4. The depth of the filter cell should be such to accommodate design depth of filtration layer, provide required driving head and safety free board. In dual media gravity filter the combined depth of filtration layers usually does not exceed 2 m. The depth of sand layer is in the range of m and the depth of anthracite layer in the range of m. Usually, the depth of top anthracite layer is larger than the depths of underlying sand filtration layer. Depending on type and configuration of filtrate collectors, the media layer could be supported by layer of gravel, m thick. The dimension range of filtration layers in gravity dual media filter is shown on Figure

88 Figure Configuration of filtration layers in a gravity filter Figure shows schematically the inlet outlet connections of the gravity filter. Raw water enters the filter through inlet port of channel above the filtration media. The filtrate exits the filter at the bottom effluent port. Through this port the filtrate can be directed to the storage clear well or send back to the head of the system in case that quality of filtrate is not within the limits. This port also serves as an inlet port for the backwash water. The backwash water is usually collected through backwash water troughs and sent through the dedicated outlet port to the backwash water storage tank. In gravity filters the filtration process is driven by hydrostatic level difference between water level in the filter and water level in the clear well. These conditions are illustrated schematically in Figure If the level in the clear well is the same or higher than the level of the top surface of filter media than the filtration driving head is regarded as positive driving head. If the level in the clear well is below the level of the surface of top filtration layer, this difference of levels is designated as negative driving head. It is advisable that the positive driving head be as large as possible, as the presence of negative driving head could result if formation of vacuum and release of air in the filtration 88

89 bed. Some filtration systems are configured to relay only on a positive driving head for filter operation. Feed Driving head Filtrate Clearwell Figure Schematics of filtration driving head in the gravity filter. The driving head determines the duration of the filtration run. It should be sufficiently high to compensate for the flow resistance of clean filtration bed and additional friction loses due to solids accumulation. In desalination applications, the gravity filters operate at constant flow rate during the filtration cycle (the same way as pressure filters). The filtration rate is controlled by opening of the valve installed on the effluent line. The valve is initially partially closed and its opening is increased with operating time. At the end of filtration cycle, determined by the filtration time, the filter is taken out of operation, into a backwash step. In gravity filters the backwash step is usually longer than in pressure filters. It is composed of number of steps and lasts min. The backwash steps include, partial draining of the filter, repeated segments of reverse filtrate flow, air scouring, removal of backwash water and rinsing of the filter media. After quality of filtrate returns to the specified values, the filter cell is returned to the regular operation cycle. 89

90 Example of design parameters of a gravity filter of nominal filtrate capacity of 100,000 m3/day is provided in table Table Design parameters of media filtration system utilizing gravity filters. System effluent capacity 100,000 m3/day. Net effluent system capacity m3/day 100,000 Number of filtration cells 10 Influent flow rate m3/hr 4,335 Suspended solids concentration ppm 5 Coagulant dosing rate ppm as Fe2(SO4)3 10 Nominal filtration rate m/hr 12.0 Total filtration area m Actual filtration rate m/hr 12.1 Filtration rate during backwash step m/hr 13.5 Filter cell with m 4.3 Filter cell length m 8.3 Filtration area per filter cell m Filter cell height m 6.0 Inlet/outlet pipe diameter m 0.23 Underdrain height m 0.40 Gravel layer depth m 0.25 Sand layer depth m 0.80 Anthracite layer depth m 1.20 Media expansion allowance m 0.68 Free board m

91 Media level relative to filter floor m 2.65 Filtration driving head m 3.35 Filter cell draining time min Backwash # 1,duration min 6.0 Backwash # 1,flux m/hr 50.0 Backwash # 2,duration min 2.0 Backwash # 2,flux m/hr 50.0 Backwash # 3,duration min 2.0 Backwash # 3,flux m/hr 50.0 Air scouring, duration min 2.0 Air flow rate m/hr 50.0 Air volume per backwash m Media settling time min 10.0 Forward rinse min 15.0 Rinse water volume m3/backwash Rinse volume per filtration cycle m Total off line time min 60.0 Backwash water volume m3/backwash Daily backwash volume m3/day Filtration interval hr 23.0 Filtrate volume per filtration cycle m Nominal recovery rate % 96.1 Total gravel volume m Total Sand volume m

92 Anthracite volume m Coagulant usage t/day (100%) 1.04 The gravity filters usually require extensive site preparation, sometimes including soil stabilization and excavation to built concrete filter cells. An aerial picture of layout of gravity filters in seawater desalination plant is shown on Figure The relative level of filters and the clear well is very important as the clear well is the threshold point of hydraulic profile of the pretreatment system. The relative level of clear well will affect the energy required for transfer of raw water from intake to the media filters and energy of pumping of filtrate to the suction of high pressure pumps. Access to the clear well will determine configuration of transfer pumps that could be used: vertical or horizontal. The horizontal pumps are usually less expensive but require side wall access to the clear well. Palmahim 90,000 m3/day RO seawater plant Figure Aerial picture of sweater RO desalination plant showing layout of gravity filters (courtesy GES Engineering) Solids management system 92

93 Almost all media filtration systems utilize coagulation process to improve capture efficiency of suspended solids. The solids together with coagulant are flushed from the filters during the backwash step. Due to presence of metal coagulant (usually ferric hydroxide), the backwash stream can not be disposed directly to the ocean. The backwash stream represents 3 10% of the volume of the influent to the desalination system. The backwash stream contains about % concentration of solids. Prior to disposal, dispersed solids have to be concentrated into sludge with solids concentration above 20%. The concentration process includes solids thickening in a clarifier and dewatering in a filter press. Figure shows schematically the concentration and dewatering process of the backwash stream. Pumps to RO Backwash pumps Filtrate Clear well Filters backwash water storage Lamella clarifiers Belt press Polymer dosing Polymer dosing Figure Schematic diagram of filtration system including solids management unit. The backwash water is initially stored in the backwash water storage tank. From the storage tank it is transfer to a clarifier. If necessary a polymer is added to this stream to improve settling of the solids. The upper clarified, fraction (overflow stream) from the clarifier is transfer back to the head of the pretreatment system. The lower fraction, which is sludge of solids concentration of 0.5% 1.5% is transfer to the filter press. The press reduces water content in the sludge to less than 80%. The 20% solids sludge can be disposed to the landfill. The effluent produced during press dewatering is retuned to the backwash water storage tank. 93

94 The solids management system is essential part of modern pretreatment system based on media filtration. It has to be design and sized to enable uninterrupted operation of the pretreatment system. The sizing of the equipment also includes sufficient capacity of storage and transportation to the landfill to account for periods when landfill is not operational (weekends and holidays). An example of design parameters of solids management unit is provided in Table Table Design parameters of solids management unit for a filtration system. Filtration system capacity 100,000 m3/day Net effluent filtration system capacity m3/day 100,000 Filtration system influent flow rate m3/hr 4,335 Suspended solids concentration in raw water ppm 5 Filter effluent coagulant dosing rate ppm as Fe2(SO4) Daily filter solids loading kg/day 1,076.8 Total filtration area m Backwash flux m/hr 50 Total duration of backwash flow min 10 Daily filter backwash volume m3 2,975 Solids concentration in the backwash flow % 0.036% Operational volume of the filter backwash m3 446 water equalization tank Operational capacity of the clarifier m3/hr 248 Clarifier sludge flow (@1% solids) m3/hr 4.5 Clarified effluent flow m3/hr Daily sludge production (@20% solids) t/day 5.4 Filter press discharge liquid flow m3/hr 4.3 Filter press polymer dosing rate ppm 20 Filter press polymer usage kg/d (100%) Pretreatment system design method The pretreatment system design methods starts with evaluation of quality of raw water, follows by 94

95 the evaluation of effective pretreatment alternatives, bench scale testing and field operation of a pilot unit. The evaluation of raw water quality should address seasonal fluctuation of salinity, water temperature and the physical water quality parameters, such as suspended solids and turbidity. Important quality indicators include indicators of biological activity, such as TOC, COD, bacterial count and presence of algae. The relevant information includes complete as possible chemical composition of raw water. More detailed discussion on quality of raw water for RO application is included in chapters The selection of pretreatment technology is based on raw water source and quality, along the guidelines listed in Table 3.2. Bench testing and pilot operation is recommended only in selective application cases. These include very variable and unpredicted raw water quality or very stringent requirements on quality of product water. One of the more common bench scale testing is the Jar Test a procedure enables preliminary optimization of the coagulation and flocculation process. Jar test equipment is shown on Figure It includes number of beakers equipped with stirrers. The optimization of coagulant dosing rate is conducted by adding sequentially increasing quantity of coagulant to each beaker containing raw water sample. Following addition of coagulant the stirrers start to operate for min at high speed, to simulate coagulation. After this initial period the stirring velocity is decreased to simulate gentle mixing during flocculation. 95

96 Figure Jar test equipment utilized in estimation of the required dosing rate of alum based coagulant. (Courtesy Peter Hillis). After a defined period of 5 20 min, the stirrers are stopped, allowing flock to settle. Appearance of flock and clarity of water in the beakers are indicators of effective flocculation and adequate dosing rates. In the example shown on Figure the effective dosing rates are in the range of 2 5 mg/l. The dosing rate of 1 mg/l is not sufficient to develop noticeable flock particles. At dosing rates higher than 5 mg/l, the flock is small and most likely the excess of metal ions in solution increases stability of colloidal particles (effect of charge reversing on colloidal particle surfaces). The Jar Test apparatus is useful tool to evaluate condition of coagulation flocculation, both during the process of the pretreatment system design and also later on during the system operation, to optimize performance of the commercial unit. Some types of Jar Test equipment are configured to test also effect of diffused air flotation (DAF) in addition to testing of the coagulation process. Operation of pilot unit provides more complete representation of treatment process as the operation 96

97 is conducted under real site conditions. However, field test pilot program is significantly more expensive than the bench testing. In addition to the cost of equipment there is cost of site preparation, providing water supply, utilities, permitting and manpower for operation and equipment maintenance. The pilot unit to test the pretreatment process could be configured as to mimic configuration of commercial unit and test operation of the complete desalination system. Such pilot unit would include pretreatment and RO unit. Sometimes, sufficient results could be obtained just by operation of the pretreatment equipment only. Schematic configuration of pilot unit for evaluation pretreatment process based on gravity media filtration is shown on Figure Vacuum pump Spent backwash tank Filtrate storage Media filter Flocculator Static mixer M To RO unit Ferric Acid dosing dosing Spent backwash Filtrated water Flocculator Media filter Figure Schematic diagram of a pilot unit for testing of a gravity filtration process. The pilot unit configuration shown on the above figure contains all components of the commercial pretreatment system with the exception of the solids management equipment. The pilot equipment could be of any size as long as the size of equipment used will not affect efficiency of treatment process or results. In the pilot unit that includes an RO unit, the size of pretreatment unit will be dictated by the water demand of the RO unit downstream. Otherwise, pre- 97

98 treatment pilot unit could be quite small. Small size pilot unit have advantage of being not only less expensive to purchase but also more convenient to operate. It is advisable to operate pilot unit though a complete cycle of raw water quality fluctuation to test the selected pretreatment process at most adverse conditions at the future site of the commercial system. The results of bench testing, and possibly pilot testing, are good source of information for pretreatment system design. In case that commercial desalination system operated in vicinity of the future system site, their configuration and operating parameters are good reference for the design process. Also vendors of pretreatment equipment are good source reference information on configuration and process parameters. The number of possible pretreatment process alternatives has to be reduced to 2 3 options, which should be evaluated in respect of required results of effluent quality, references, design difficulties and capital and operating cost. The evaluation process includes development of preliminary configuration, process and flow diagram including process parameters (mass and flow balance) and general specification of major equipment. Once the pretreatment process has been selected, it is important to have alternative selection of major equipment in order to arrive to a competitive prices of final system Cartridge filtration The role of cartridge filters in RO system is mainly to protect equipment located downstream (pumps and membrane elements) from sudden appearance of particulate matter in feed water. Such conditions could be sometime experienced due to sudden sand or silt release from wells or from sand filters (in system utilizing sand filters). Use of cartridge filters as a feed water filtration step for colloidal matter removal is usually prohibitively expensive in respect of the cartridge replacement cost. Extensive field experience shows that RO systems treating well water, with cartridge filtration as the only filtration step, operated successfully over the years. In some isolated cases that the plants of this configuration experienced release of silt and/or sand from wells, and cartridge filters do not operated properly, the particulate matter eventually ended up reaching membrane elements. In almost 98

99 all reported cases of such events, the solids intrusion and accumulation was limited to lead elements only. This condition was eventually rectified by flushing of lead elements (in reverse flow direction, after turning them around and moving to the end of the system) and replacing some of them. Cartridge filters for RO applications have nominal porosity in the range of 5 15 micron. The preferred porosity rating of filtration cartridges is 5 micron. Feed water flow through cartridge filters should not exceed 1 m3/hr per 25 cm of cartridge length. The schematic configuration of cartridge filter housing is shown in Figure CARTRIDGE FILTER CONFIGURATION Max flow, m3/hr Number of 25 cm cartridges Diameter, cm Height, cm In-out diameter, cm Weight, kg Figure Schematic configuration of cartridge filtration unit. In the above configuration the filtration cartridges are mounted in vertical position. In large capacity plants horizontal cartridge filters (Figure ) are sometimes being used. 99

100 Figure Picture of horizontal housing of cartridge filter. As shown in the above picture, the entry of the feed water is at the housing vessel wall and the exit of filtrated water at the end of the vessel. The manual valves on the exit and entry are always in completely open position, except for the events of cartridge replacement. The lid of the cartridge vessel is mounted on hinge. This arrangement enables system operator to open the lid without need of a lifting device, as it is necessary with vertical cartridge housing. Horizontal cartridge housing in a process of cartridge replacement is shown on Figure

101 Figure Horizontal cartridge filter housing in open position. The important part of cartridge filter housing is the baffle, which protects cartridges from direct impingement of suspended particles. Filtration cartridges are usually made of soft polymeric materials. Exposure to direct impingement of hard particles could result in abrasion of cartridges and eventually loss of integrity. One big advantage of pretreatment filtration configuration, limited to cartridge filters only, is reduction of exposure of feed water to outside environment, which is very convenient in treatment of anaerobic water sources. Anaerobic water sources (for example water from deep Floridian aquifer) may contain variable quantity of hydrogen sulfide and usually sulfate reducing bacteria are present as well. Had this water been exposed to air there would be high probability of hydrogen sulfide being partially oxidized to elemental sulfur according to the following reaction: 2H2S + O2 = 2H2O + 2S (4.13) Elemental sulfur has very limited solubility in water or water based solutions and once deposited in feed channel of RO elements can not be removed. At the early stages of RO technology development attempts were made to oxidize hydrogen sulfide presented in feed water with strong oxidants, prior to RO. This process configuration almost always ended up in either fouling of membrane ele- 101

102 ments with elemental sulfur or in oxidative damage of membrane barrier. The design approach that provides stable system performance is to maintain anaerobic conditions of the water through the RO system. After the RO unit, hydrogen sulfide is removed from permeate (and sometimes from concentrate as well) either by aeration or oxidation. If location of the RO system is close to urban centers the degasifiers can not vent off gasses to the air. The common solution in case of hydrogen sulfide aeration is to follow degasifiers with an absorption system. In such a system hydrogen sulfide is absorbed on an iron based catalyst and eventually disposed as a solid waste. Recently some system designer introducing alternative equipment in place of cartridge filtration as a safety filters. The new safety filters are 20 micron strainers of the same configuration as are being used in protecting membrane filtration systems (Figure 4.1.2, Chapter 4.1). This equipment is somewhat more expensive than the conventional cartridge filters, however, no cartridge replacement is necessary. Therefore in systems that experience high rate of cartridge replacement, use of this type of equipment could be cost effective Membrane pretreatment Utilization of membrane pretreatment for RO applications is increasingly growing. In the RO systems, designed for salinity reduction of secondary treated municipal wastewater effluent, membrane filtration is the most commonly used pretreatment technology. Also in systems treating waters for industrial applications, either treatment of wastewater or production of process makeup water, membrane filtration is quite frequently being applied. In RO systems treating surface water for potable applications, either brackish or seawater feed, membrane filtration is considered as an emerging technology, with great potential but still being usually more expensive than the conventional pretreatment. The obvious benefit of membrane filtration is the existence of membrane barrier that is preventing suspended particulate to pass through, regardless of the quality of the raw water. However, membrane pretreatment systems are more complex than the conventional pretreatment equipment and require more energy to operate. In RO system treating secondary effluent, past application of conventional pretreatment resulted in unacceptably high fouling rates of RO membranes. Application of membrane pretreatment, that practically removed all colloidal matter from the feed water, improved dramatically stability of operation of RO membranes in wastewater reclamation systems. In RO systems treating surface water, well design conventional pretreatment is usually capable of producing RO feed water of acceptable quality. However, in cases when quality of raw water is poor 102

103 with high degree of seasonal fluctuations, use of membrane pretreatment may improve stability of performance of RO membranes and also be cost effective. The membrane filtration technology that is used as pretreatment process in RO applications has the following attributes: 1. The filtration process is conducted at low pressure, usually not exceeding 1 bar. Energy requirement of this process is in the range of kwhr/m3. 2. The filtration process is conducted through the membrane barrier that rejects all suspended particles in the feed water. 3. Membranes are exclusively in a hollow fiber configuration. 4. The operation consists of sequence of steps. Filtration step, conducted at 100% recovery rate (direct filtration), lasted min. During the direct filtration step foulant layer is built on the membrane surfaces, resulting in permeability decline. 5. The foulant deposits are removed and membrane permeability restored by reversing water flow, i.e. pushing filtrate through the membrane in reverse direction for a short duration of min. 6. This backwash step periodically includes addition of cleaning chemicals, mainly hypochlorite, in a process called chemical enhanced backwash (CEB). 7. At variable frequency of once every few weeks or months, permeability restoration is conducted by applying cleaning in place (CIP) Configurations and components of membrane pretreatment system The membrane filtration system could be either pressure driven or operating under vacuum. The membrane unit in pressure driven system consists of encapsulated membrane modules and water is driven through the membrane by feed pressure, developed by a feed pump or hydraulic head. In vacuum driven membrane filtration system, membrane bundles, that form membrane modules, are immersed in tanks. Water is driven through the membranes by negative pressure created by suction of filtrate pumps. Block diagram of submersible membrane filtration system is shown in Figure The difference of configuration of pressure driven membrane filtration system is replacement of membrane tanks with membrane racks and filtrate pumps with feed pumps as indicated in Figure

104 Dosing systems 2 Cleaning and neutralization unit Heat exchanger Dosing systems 1 Compress air Vacuum pumps Feed pumps Strainer Static mixer Membrane tanks Filtrate pumps Filtrate clearwell Electric system Control system Solids handling Blowers Dosing systems 3 Backpulse pumps Backpulse tank Figure Block diagram of submersible membrane filtration system Dosing systems 2 Cleaning and neutralization unit Heat exchanger Dosing systems 1 Compress air Feed pumps Strainer Static mixer Membrane racks Filtrate clearwell Electric system Control system Solids handling Blowers Dosing systems 3 Backwash pumps Backwash tank Figure Block diagram of pressure driven membrane filtration system System components that are common to both membrane configurations are: - Membrane units (racks or submersible trains) - Micro strainers - Cleaning chemicals storage and dosing units - Feed pumps or filtrate pumps - Blowers and/or compressors - Membrane backwash unit 104

105 - Chemical cleaning and neutralization unit - Filtrate storage and distribution unit. - Solids management unit (if coagulant is being used) - Electric power supply unit - Process control unit RO Technology Mark Wilf Ph. D Settling and screening Membrane filtration systems are very robust in respect of solids concentration in the feed water. The quality of filtrate is very little affected by the variation of quality of the feed water. However, with increased concentration of solids in the feed water the membrane system has to operate at lower filtrate flux rate and the filtration time between backwash steps is being reduced. These conditions result in reduced filtrate flow rate and reduced recovery rate. Therefore, if raw water turbity exceeds NTU for significant period of time, it is more effective to include settling (clarification) unit ahead of membrane filtration unit. As indicated previously, if suspended solids are of biological nature (algae) light particles, then use of DAF would be a more effective solution than clarification through settling. All filtration systems treating surface water include screens as safety measure to prevent large size particles to reach membrane elements. For applications involving pressure drive membrane filtration technology, the screening devices are rated at mm. The immersed, vacuum driven, technology can tolerate larger particle sizes than the pressure driven systems. The opening size of screens used in submersible system could be as large as mm, depending on configuration of membrane module. The sequence of operation of membrane filtration systems is fully automatic, controlled by PLC. Therefore, the screening units used in membrane filtration systems are self cleaning, automatically operated. An example of screening device, used frequently in membrane filtration system is automatic filter manufactured by Arcal, shown on Figure The filtration device consists of set of grooved discs held together by a clamp. Water is passing through grooves between the discs. For membrane filtration applications the grooves opening size is 80 or 100 m. During filtration the trapped solids are accumulating in grooves between discs, increasing flow resistance and the differential pressure. Once the differential pressure reaches the preset limit, usually in the range of 0.3 bar, the filtration unit is disconnected from the effluent manifold, clamping device holding the discs opens and the accumulated solid matter is flushed from the filtration unit. 105

106 Figure Microstrainer configuration offered by Arkal. Configuration of a microstariner unit, based on Arkal equipment, that corresponds to system capacity of 200,000 m3/day is shown on Figures and Unit specifications are listed in Table Table Specifications of microstariner unit. Output capacity 200,000 m3/day Number of modules 4 Number of filters in module 12 Unit footprint 13.9 m X 10.1 m = m2 Pressure loss during operation bar Backwash flow rate 960 m3/hr Backwash duration 20 second Backwash volume 43 m3/filtration cuycle 106

107 Number of filtration cycles per day System recovery rate 99.7% % Backwash pressure required 3.5 bar RO Technology Mark Wilf Ph. D. Figure Side view of microstrainer assembly. System width 13.9 m 107

108 Figure Top view of microstrainer assembly. System length 10.1 m In addition to strainer configuration described above there are commercial strainers that utilize screens in form of baskets or cylindrical cartridges. One example of such a filter is strainer offered by Boll Filter Corporation. Schematics of system with boll strainer is shown on Figure The filter contains tubular filtration elements made from wedged metal wires. Backwash is conducted by rotating arm that moves sequentially between filtration elements, disconnecting them from the effluent manifold and connecting to backwash discharge line. Majority of components of disc filter is made of polypropylene. The material of construction of screen filters is mainly metal, which could be a reason of concern in operation in highly corrosive environment. 108

109 Figure Wedged screen strainer configuration In membrane filtration system, microstrainers operate as one unit producing screened water to the membrane filtration system Filtration membranes and membrane unit configuration Fundamentals of the membrane filtration water transport process 109

110 The water is driven through the membrane by difference of pressure between feed and permeate sides of the filtration membrane. The driving force is defined as trans membrane pressure (TMP) and it is calculated through the following equation: TMP = (Pf + Pc)/2 Pp (4.7.1) Where Pf is feed pressure Pc is concentrate pressure Pp is permeate pressure Accordingly, specific permeability, SP is calculated according to the following equation: SP = Q/(Am * TMP) (4.7.2) Where Q is instantaneous filtrate flow Am is total membrane area in the system Example of calculation of TMP and permeability for filtration membrane module is included in Table As can be seen from the results displayed there is significant difference of TMP and SP values for new membrane (ex-factory results) and for the membrane in operation at field conditions. At field conditions the TMP is higher and water permeability significantly lower. The membrane filtration devices operate at conditions of 100% recover rate (dead end operation). Therefore, during operation at field conditions the membrane surface is always fouled by deposits, which reduces the water permeability rate. The water permeability rate is affected by temperature. The effect of temperature on water permeability is result of changes of water viscosity and changes of pore size of filtration membrane. The effect of temperature on water permeability is expressed as temperature correction factor (TCF). It is calculated by the following equation: TCF = exp(-0.031*(t 20)) Where T is water temperature expressed in C. The reference temperature for calculation of TCF is 20 C. Table Example of the permeability results at ex-factory test and during field operation 110

111 Test parameter New membrane Field conditions Pp, bar (psi) 0.25 (3.6) 0.70 (10.1) Pc, bar (psi) 0.15 (2.2) 0.60 (8.5) Pp, bar (psi) 0.10 (1.5) 0.15 (2.2) TMP, bar (psi) 0.10 (1.5) 0.50 (7.2) RO Technology Mark Wilf Ph. D. Q, l/hr (gpd) 3,500 (22,000) 5,100 (32,300) Am, m 2 (ft2) 46.5 (500) 46.5 (500) SP, l/m 2 -hr (gfd/psi) 750 (29) 219 (8.9) In graphic form changes of TCF are provided in Figure Temperature correction factor TMP Correction Factor Feed water temperature Figure Temperature correction factor vs. water temperature 111

112 The membrane filtration represents almost absolute barrier to suspended particles. The filtration spectrum, shown on Figure indicates particles separation capability of different filtration technologies. The figure shows distinct difference of projected particle separation sizes by utrafiltration (UF) and membrane filtration (MF). This is due to difference of pore sizes range of UF and MF membranes. However, after short exposure to feed water a dynamic membrane is formed on the membrane surfaces, making separation properties of UF and MF membranes quite similar. THE FILTRATION SPECTRUM um A MOLECULAR WEIGHT ,000 20, , ,000 Aqueous salts Carbon black Paint pigment RELATIVE SIZE OF COMMON MATERIALS Metal ions Pyrogens Virus Colloidal silica Yeast cells Bacteria Pollens Beach sand Sugars Albumin protein Milled flour FILTRATION TECHNO- LOGY Reverse Osmosis and NF Ultrafiltration Microfiltration Particle filtration Figure Separation size range of filtration technologies Membrane material and membrane configurations The sequence of operation of UF and MF membranes require high durability of membranes and membrane modules. The important properties of membrane material are: - Narrow pore distribution or sharp molecular weight cut off (MWCO) - High polymer strength: elongation, high burst and collapse pressure - Good polymer flexibility - Permanent hydrophilic character - Wide range of ph stability - Good tolerance to strong oxidants - Acceptable cost 112

113 There is no single membrane material that has all of the above properties. However, number of membrane materials provide most of the listed properties and are preferred in commercial applications. These are: - PVDF (polyvinylidiene fluoride) - PS (polysulfone) - PES (polyether sulfone) - PAN (polyacrilonitrile) Majority of commercial membranes for filtration applications are made from the above listed polymers. Majority of membranes are configured as capillary devices. Very limited number of filtration membranes is configured as flat sheet, plate and frame, or spiral modules. The capillary membranes can operate as pressure driven or submersible, vacuum driven units. In the pressure driven category there are two possible flow directions: 1. Feed water can be pumped through the lumen and filtrate collected outside the capillary: inside out operation (Figure a), also called pressure driven inside (PDI). 2. Feed water can be applied under pressure outside the capillary and filtrate collected through the capillary lumen: outside in operation (Figure b) also called pressure driven outside (PDO). a b Figure Filtrate flow direction in pressure drive capillary membranes: a PDI, b - PDO. According to flow directions through the membrane, the membrane elements are configured differently and operating parameters are different. The main difference are related to different size of active membrane area per fiber length, difference of backwash flux rate and utilization of air scouring for foulants removal. Both PDI and PDO configurations are commercially viable technologies that provide comparable 113

114 performance at field conditions at similar process economics. The selection of a given filtration technology configuration for specific application is usually based on the designer preferences. The differences between membrane modules in these two flow configurations are listed in Table Table Attributes of PDI and PDO membranes configuration Filtration direction Inside out Outside in Designation PDI PDO Membrane barrier On the lumen surface Outside fiber surface Backwash flux rate 3 4 times forward flux times forward flux Air scouring No Yes Advantages Barrier protected inside fiber High membrane area per module Advantages Sufficient backwash with filtrate only Reduced backwash volume required Advantages Small volume of cleaning solution Small volume of cleaning solution The other significant version of membrane filtration technology is the submersible, vacuum driven process. In the submersible process the only practical water flow direction alternative is outside in configuration. In the past, the submersible technology has been used preferentially for high turbidity waters and for large capacity systems. Today both the pressure driven and submersible, membrane filtration technologies have converged in respect of performances and economics and they are competing for the same markets and applications. Summary of process attributes of pressure driven and submersible technologies are provided in Table Table Summary of process parameters of pressure driven and submersible technologies. Pressurized Treats low to medium feed turbidity Ability to compensate feed temperature fluctuation in wide range Immersed Treats medium to high feed turbidity Ability to compensate feed temperature fluctuation in narrow range 114

115 Can operate at high flux rates Limited ability to treat high turbidity water Small footprint Cost effective in small to medium capacity systems Small volume of cleaning solution Suitable for prefabricated systems RO Technology Mark Wilf Ph. D. Flux rate limited by available TMP Limited ability to treat high turbidity water Larger footprint Cost effective in medium to large capacity systems Larger volume of cleaning solution Can retrofit gravity filters Membrane filtration process The membrane filtration process operates in a stepwise manner. The sequence of operation depends somewhat on type of technology: pressure driven or immersed Pressure driven technology. The sequence of operation of pressure driven technology includes: - Filtration - Forward flush - Backwash - Chemical enhanced backwash (CEB) - Air scouring (for PDO only) - Cleaning in place (CIP) The operation sequence starts with a filtration step, usually conducted in a direct flow mode (also called dead end filtration), i.e. operation at 100% recovery. This step is shown schematically on Figure

116 Figure Filtration step direct flow mode of operation The filtration step lasts between min. During the direct flow step, membrane surface is being covered with particles. The water permeability declines, requiring increase of the feed pressure in order to maintain the design filtrate flow rate. The next step that follows is restoration of permeability of the membrane. This is accomplished through removal of foulants that accumulated on the membrane surfaces by applying backwash of the membrane module. In the backwash step, filtrate is pushed in reverse direction, through the membrane to dislodge foulant form membrane surfaces and remove them from the membrane filtration modules. The backwash step may include application of air scouring or addition of chemicals to increase efficiency of foulants removal. The chemical most frequently used during chemical enhanced backwash (CEB) step is hypochlorite added together filtrate backwash flow. Depending on quality of feed water, usually the CEB is conducted once to few times per day. Schematics of backwash step, including listing of process parameters is shown on Figure

117 Figure Backwash step. The forward flush, applied in addition to backwash for permeability restoration, involves rapid opening of the concentrate valve for number of seconds to flush solids that has accumulated in the membrane module. The forward flush is usually applied during the periods of very high turbidity of the feed water. CIP also utilized for recovery of water permeability is applied at much lower frequency than the backwash step, usually once every few months. In differentiation to CEB, which is applied on the filtrate side of the membrane, the CIP is applied from the feed side. The cleaning solutions used during the CIP step include solutions of citric acid, mineral acids, NaOH and hypochlorite. Membrane filtration units that operate for potable water production have to verify integrity of membrane barrier. The verification of membrane barrier integrity is described in ASTM procedure (ASTM D ) The frequency of application of integrity test is specified in the operational permit of the membrane filtration system. The unusual frequency of integrity testing is once per day. The integrity test procedure in commercial systems is based on measuring decay profile of air pressure applied on the feed side of the membrane filtration system. The sequence of integrity test procedure is shown schematically in Figure

118 Figure Integrity test sequence. The integrity is calculated through determination of pressure decay rate (PDR) according to equation below PDR = (Pi-Pf)/t Where Pi initial pressure Pf final pressure t time interval PDR = PDR (measured) rate of diffusion Vacuum decay rate (VDR) VDR = VDR (measured) rate of diffusion During determination of PDR, the rate of pressure decline due to diffusion of pressurized air through water filled pores should be subtracted from the total measured PDR. The pressure decline due to air diffusion is function of membrane surface. Therefore, the number of membrane modules that could be tested for integrity as a one unit is limited by this phenomenon. In majority of commercial systems the integrity test is conducted automatically, controlled by PLC, which also automatically evaluates the results and displays warnings if the test results are outside the range of sufficient membrane integrity Configuration of pressure driven membrane filtration unit 118

119 The schematics configuration of pressure driven membrane filtration unit is shown in Figure and system layout shown in Figure The basic components include: - Raw water supply source - Feed water pump - Optional dosing systems of oxidant (sodium hypochlorite) and coagulant - Micro strainer - Membrane unit(s) - Filtrate storage tank - Backwash system - Storage and dosing systems for cleaning chemicals - Filtrate pumping system Figure Configuration of pressure driven membrane filtration unit In addition, the filtration system will include cleaning system, control system and power supply unit. Pressure driven system with modules operating in outside in configuration (PDO), will also include air compressor for conduction membrane air scouring step. During the filtration step, feed pump pumps raw water through the micro strainer to the membrane filtration units. The membrane unit will operate with concentrate valve closed (100% conversion). The filtrate is directed to the filtrate clear well. Filtrate pumping system transfers filtrate to distribution or to the next processing step. As the membrane surfaces being fouled and water permeability decreases, the feed pump develops higher feed pressure to maintain the design filtrate flow. At presented time interval, after min of operation in filtration step, membrane units enter sequentially into backwash step. In the backwash step, set of valves isolates the membrane unit from the 119

120 pressurized feed line, connect filtrate line to the backwash system and connects feed or concentrate line to discharge. The backwash is initiated by starting the backwash pump and pumping filtrate in reverse direction for a period of seconds. The filtrate dislodges foulants form the membrane Figure Pressure driven membrane filtration system (courtesy Simens Water Technologies) surfaces and they are discharged from the membrane unit through feed or concentrate connections. The configuration and status of valves during filtration and backwash steps is shown on Figures a f. 120

121 Figure a. Valves position during filtration step Valves During Backwash - Top Figure b. Valves position during backwash step 121

122 Valves During Backwash Bottom Figure c. Valves position during backwash step Valves During CEB Figure d. Valves position during chemical enhanced backwash step 122

123 Valves During Water Draining Figure e. Valves position during system draining step Valves During Air pressurizing Figure f. Valves position during air pressurizing step 123

124 The air pressurizing step is utilized by one of the manufacturer of pressure driven membrane modules (Hydranautics PDI configuration) to increase effectiveness of filtrate backwash step. It is called air enhanced backwash (AEB). During the AEB air is introduced on the feed side of the membrane to drive off feed water from the module. At this conditions, the filtrate backwash that follows, flows against empty feed volume, therefore at higher flow velocity resulting in better removal of deposits from membrane surface. Another manufacturer of pressure driven membrane modules (Siemens PDO configuration) applies both air scouring and compressed air to drive filtrate back through the membranes. The process steps during the backwash follows the schedule listed in Table Table Air assisted backwash of the pressurized (PDI) membrane filtration system Step 1 Normal filtration The unit is in filtration, the feed control valve is open. Step 2 Drain to backwash Filtration stops. The unit drain valve is opened and water is level drained from the unit until the backwash level is reached. Step 3 Aeration and liquid backwash Low pressure air enters the shell side to provide air scouring to remove solids from the membrane fibers. The flow meter on the discharge of the blower is used to monitor the backwash air scour flow. Compressed air pushes the filtrate in the lumen side to the shell side of the fibers, thus achieving liquid backwash. Step 4 Post aeration Air scouring continues after the liquid backwash ends. The lumen side remains pressurized with the compressed air. Step 5 Shell draining The top shell side of the fibers is vented, while the lower backwash discharge valve is opened. The backwash waste drains under the influence of gravity out of the array. Site hydraulics should be such that shell draindown can be achieved in 45 seconds. Step 6 Shell fill - flush Feed enters the shell side via the lower feed manifold and exits the unit via top backwash discharge valves. Step 7 Resume to filtration ready status Feed continues to enter the shell side via the lower feed manifold, however the top backwash discharge valve is closed and the filtrate exhaust valve is opened. This forces water into the fibers. Once the unit is refilled with feed water, the unit returns to filtration (or enters STANDBY) Sizing of pressure driven membrane filtration unit The sizing of membrane filtration unit consist of: 124

125 - Determination of process parameters (filtration flux, backwash flux) - Defining duration of operating steps. - Calculation of time allocation for each process step - Calculation of gross filtrate capacity and membrane area required - Defining membrane train size and number of membrane trains An example of operational sequence in pressure driven membrane unit is provided in Table The values of duration and frequency of operational steps are provided as a range. The actual value selected for design will depend on quality of raw water or previous experience of similar operation. Table Sequence of operation of pressure driven membrane filtration unit. Process step Objective Duration Frequency Forward flow Permeate production min Continuous Backwash Foulants removals sec Every min Chemicals enhanced backwash (CEB) Foulanlts removal 1 15 min Once twice a day Cleaning in place Foulants removal 2 4 hr Every 1 6 months Integrity test Verification of membrane integrity 20 min Every 1 7 days Example of water quality parameters of surface water source is provided in Table Table Water quality parameters of surface water source Parameter Value Turbidity, NTU Total suspended solids, ppm 2 5 Water temperature, C 2-25 An example of filtrate flux and schedule of operating sequence for membrane filtration system treating surface water of quality parameters listed in Table is provided in Table Defining of operational schedule allows calculation what fraction of the operating time the membrane system is on line, producing filtrate. It is evident that the filtrate capacity of the unit should be planned to provide design output and to produce sufficient filtrate for backwash and cleaning. Table Example of operational parameters and schedule of pressurized membrane filtration 125

126 system Operating step Parameter value RO Technology Mark Wilf Ph. D. Design filtrate flux, l/m2/hr 85 Design backwash flux, l/m2/hr 300 Backwash frequency, min 30 Backwash duration, min 1.0 Off line time due to backwash, min 1.5 Integrity test frequency per day 1 Integrity test duration, min 20 CEB frequency per day 1 CEB duration, min 10 Total off line time per day, min 97 (integrity + CEB + backwash) Backwash time per day, min ( )/( ) = 45 Backwash off line time, min 1.5*45 = 67 Off line time due to monthly cleaning,% 1 Membrane trains on line time fraction (1440 ( *1440))/1440 = 0.92 According to the values listed in Table 4.7.7, the scheduled off line time represents close to 8% of the available time. This off line time requires compensation of filtrate production capacity in sizing of the membrane filtration unit. The sizing of the membrane filtration unit is based on schedule of the process steps, configuration of membrane elements selected and the designed filtrate flux rate. Example of sizing of membrane filtration system of filtrate capacity of 200,000 m3/day according to parameters listed in Table is provided in Table Table Example of sizing calculations of 200,000 m3/day pressurized membrane filtration system Design process parameter Value Filtrate capacity required for backwash, % 45*300/[85*( )] = 11.8 Capacity to compensate off line time, % 97/( ) = 7.2 Time for membrane cleanings, % 1 Contingency capacity, % 5 Total additional capacity, % 25 ( ) 126

127 Installed filtration capacity, m3/d 200,000* 1.25 = 250,000 Membrane area required, m2 250,000,000/(24*85) = 122,550 Number of membrane modules (46m2/module) 122,550/46 = 2,664 Rack size, membrane modules per rack 192 Number of racks required 14 Number of racks installed Instantaneous feed flow per rack, m3/day 192*46*85*24/1000 = 18,020 Backwash volume, m3/day/rack 192*46*300*45/(60*1000) = 1,990 CIP interval, days 20 CIP frequency per day 14/20 = 0.7 CIP volume per day, m3/day (CIP tank = 20 m3) 0.7*20 = 14 Maintenance wash (MW) interval, days 5 MW frequency per day 14/5 = 2.8 MW volume per day, m3 (CIP = 20 m3) 2.8*20 = 56 Total waste volume, m3/day 14*( ) = 28,840 System recovery rate, % 100* (1 28,840/(14*192*46*85*24*0.92/1000) = 88 As expected the recovery rate is function of filtration intervals (time between backwash events), filtrate flux and backwash flux. Increase of filtration intervals and filtrate flux and reduction of backwash flux will increase recovery rate. The rate of usage of chemicals determines the size of storage tanks and capacity of dosing equipment. Example of sizing of chemical dosing equipment is listed in Table Table Example of sizing of chemicals dosing equipment in pressurized membrane filtration system Chemical reagent Quantity Sodium hypochlorite for CIP Sodium hypochlorite concentration, % 12.5 Sodium hypochlorite specific gravity 1.2 Dosing concentration, ppm 1,000 Daily volume of CIP solution, m3 14 Daily volume of hypochlorite for CIP, l 14*1000/(1000*1.2*0.125) = 93.3 Sodium hypochlorite for MW Dosing concentration, ppm 200 Daily volume of MW solution, m

128 Daily volume of hypochlorite for MW, l 56*200/(1000*1.2*0.125) = 75.7 Total volume of hypochlorite, l/day 169 Storage volume of hypochlorite for 30 days, m3 ~ 5.0 Citric acid for CIP Citric acid concentration, % 50 Citric acid specific gravity 1.25 Dosing concentration, ppm 20,000 Daily volume of citric acid, l 14*20000/(1000*1.25*0.5) = 448 Storage volume of citric acid for 30 days, m3 ~ 14.0 Sodium hydroxide for CIP Sodium hydroxide concentration, % 40 Sodium hydroxide specific gravity 1.2 Dosing concentration, ppm 1,000 Daily volume of sodium hydroxide, l 14*1000/(1000*1.2*0.4) = 29 Storage volume of citric acid for 30 days, m3 ~ 1.0 The system will be composed of subunits providing functional operational capability according to listing in Table Table Listing of subunits and major equipment in pressurized membrane filtration system Equipment Quantity Sizes/comments Feed water system Centrifugal, end suction pumps m3/hr, 3 bar, VFD controlled motor Automatic backwashing strainer m3/hr Instrumentation Lot Turbiditymeter, pressure indicators and transmitters, temperature transmitter Valves Lot Manual, butterfly Membrane unit Trains frame and piping 16 Corrosion protection coating Membrane modules 3, m2/element Set of automatic valves 16 lots Inlet, outlet, waste discharge, air release Instrumentation 16 lots Magnetic flow meters, turbiditymeter, pressure indicators and transmitters Chemical cleaning system 128

129 CIP/MW tank 2 20 m3 Immersion heated kw Centrifugal, end suction pump m3/hr, 3 bar, stainless steel wetted parts Instruments 2 lot Magnetic flow meter, ORP sensor/transmitter, CIP tank level transmitter, CIP temperature transmitter, pressure gauges Valves 2 lot Valves: isolation, flow control, air release, pressure release, chemical injection isolating valve Chemical dosing systems Sodium hypochlorite Transfer pumps l/hr Storage tank 1 5 m3 Calibration column 2 Valves and piping 1 lot Citric acid Transfer pumps l/hr Storage tank 1 14 m3 Calibration column 1 Valves and piping 1 lot Sodium hydroxide Transfer pumps l/hr Storage tank 1 1 m3 Calibration column 1 Valves and piping 1 lot Operating cost of pressurized membrane filtration unit The operating cost includes the following cost items: - Power - Chemicals - Labor - Maintenance - Membrane replacement The combined power usage of the filtration system is based on power usage of individual equip- 129

130 ment according to the scheduled operation time. Example of energy usage in membrane filtration system of filtrate flow capacity of 200,000 m3/day is provided in Table As expected the major energy user is feed pump that delivers the feed water and provides sufficient pressure to drove water through the membranes. Table Breakdown of energy usage in pressurized membrane filtration system. Filtrate capacity 200,000 m3/day Power component Efficiency M (P) Operating pressure, bar Feed pump 0.95 (0.80) *750 = 10,500 Flow, m3/hr Active, % Power, kw BW pump 0.95 (0.80) 2.4 2, Cleaning pumps Cleaning tank heater 0.95 (0.80) Compressor Valve actuators Instrumentation &PLC Total Annual usage 6,626,808 kwhr/y Specific energy usage kwhr/m3 Another significant contributor to the water operating cost is cost of chemicals used in the process for recovery of membrane permeability. These are chemicals used in the course of CEB and CIP. The calculation of quantity of chemicals used is based on volume CEB and CIP and concentration of chemicals used. An example of calculation of volumes required is provided in Table Summary of chemicals usage in a 200,000 m3/day membrane filtration system is provided in Table The generic chemicals used in permeability recovery procedure include sodium hypochlorite, sodium hydroxide, sulfuric acid and citric acid. If coagulation flocculation process is utilized in the filtration process, then quantity and cost of coagulant (usually ferric salts) should be included in calculation of water cost. Table Summary of volumes required for CEB and CIP in pressurized membrane filtration system 130

131 Procedure Volume used, m3/day/train Number of treatments in system per year Total volume used, m3/year CEB1(1/4 backwash) 1990/4 = *365*0.95 = 4,950 2,475,500 CEB2 (1/40 backwash) 1990/50 = 50 4, ,550 CEB3 (1/40 backwash) 1990/50 = 50 4, ,550 CIP *14*(365/20) = 102 2,040 CIP *14*(365/20) = 102 2,040 CIP *14*(365/20) = 51 1,020 Table Summary of chemicals usage in pressurized membrane filtration system. Filtrate capacity 200,000 m3/day. Process step Chemical Dosage, ppm t/year (as 100%) $/year CEB1 NaOCl ,800 CEB2 H2SO4 1, ,140 CEB3 NaOH 1, ,600 CIP1 Citric acid 20, ,600 CIP2 NaOH 3, ,100 CIP3 NaOCl 1, ,000 Total 497,240 Chemicals cost Summary of total operating cost is provided in Table Table Summary of operating cost of pressurized membrane filtration system Cost component $/year $/m3 Power@C10kWhr 662, Chemicals 497, Operation and maintenance Membrane replacement Total 480,000 (8 operators@$60,000/y) 658,285 (16 trains,7 years membrane life) 2,298, $0.007/m Configuration of immersed, vacuum driven, membrane filtration system 131

132 The schematic configuration of immersed, vacuum driven, membrane filtration unit is shown in Figure and a layout of immersed unit is shown on Figure The basic components include: - Raw water supply source - Optional dosing systems of oxidant (sodium hypochlorite) and coagulant - Micro strainer - Membrane tanks - Membrane unit(s) - Filtrate pump - Backwash unit - Air scouring unit - Storage and dosing systems for cleaning chemicals - Filtrate storage tank - Filtrate pumping system In addition, the filtration system will include cleaning system, control system and power supply unit. During the filtration step, feed pump pumps raw water through the micro strainer to the membrane filtration units. The membrane unit will operate with concentrate valve closed (100% conversion). Air blower Strainer 100 m Vacuum pump Filtrate storage & backwash tank Backwas h pump Filtrate pump CIP1 CIP2 CEB1 CEB2 CEB3 Cleaning chemicals Backwash chemicals Figure Configuration of immersed, vacuum driven, membrane filtration unit The filtrate is directed to the filtrate clear well. Filtrate pumping system transfers filtrate to distribution or to the next processing step. As the membrane surfaces being fouled and water permeability decreases, the filtrate pump develops higher suction pressure to maintain the design filtrate flow. At 132

133 presented time interval, after min of operation in filtration step, membrane units enter sequentially into backwash step. In the backwash step, set of valves isolates the membrane unit from the feed line, connecting filtrate line to the backwash system. The backwash is initiated by reducing water level in the membrane tank and then starting the backwash pump and pumping filtrate in reverse direction for a period of seconds. The reverse filtrate flow dislodges foulants form the membrane surfaces to the tank volume. At the end of backwash step, water in the membrane tank is drained (deconcentration step) and then the tank is refilled again with the feed water. ` The layout drawing on Figure is of submersible system with 10 tanks, each containing racks with connected membrane elements in configuration of clovers of four elements connected in parallel. The equipment shown on the layout includes chemical storage tanks in containments, located at the lower right corner. In clockwise direction there are two rows of five membrane tanks with the manifolds and filtrate pumps located between the tanks. Each row of tanks is equipped with one membrane lifting device. The compressed air system is located at the upper right corner. Low pressure blowers are located to the right of the air compressors. Cleaning tanks and CIP recirculation pumps are located between blowers and chemicals storage area. Figure Layout of immersed membrane filtration system (courtesy Siemens Water Technologies) In system configuration shown in Figure all the equipment including membrane storage tank is located above the ground, mounted on a concrete slab. In alternate configuration the membrane tanks and filtrate pumps could be located below the ground surface level. The total system area is about 1200 m2. 133

134 Sizing of immersed, vacuum driven, membrane filtration unit. The sizing of immersed membrane filtration unit is conducted in similar way as described in Chapter for the vase of pressurized membrane filtration unit. The process operational steps of the immersed unit are listed in Table The only significant difference of operational steps is the draining of tanks (deconcentration step) conducted at the end of the backwash sequence. Table Sequence of operation of immerse, vacuum driven, membrane filtration unit. Process step Objective Duration Frequency Permeation Permeate production min Continuous Backwash & tank deconcentration Chemicals enhanced backwash (CEB) Foulants removals sec Every min Foulanlts removal 1 15 min Twice a day once per week Cleaning in place Foulants removal 2 5 hr Every 1 6 months Integrity test Verification of membrane integrity 20 min Every 1 7 days An example of filtrate flux and schedule of operating sequence for membrane filtration system treating surface water of quality parameters listed in Table is provided in Table Defining of operational schedule allows calculation what fraction of the operating time the membrane system is on line, producing filtrate. It is evident that the filtrate capacity of the unit should be planned to provide design output and to produce sufficient filtrate for backwash and cleaning. Table Example of operational parameters and schedule of immersed membrane filtration system Operating step Parameter value Design filtrate flux, l/m2/hr 40 Design backwash flux, l/m2/hr 160 Backwash frequency, min 30 Backwash duration, min 1.0 Off line time due to backwash, min 3.5 Integrity test frequency per day 1 134

135 Integrity test duration, min 20 CEB frequency per day 1 CEB duration, min 10 Total off line time per day, min 177 (integrity + CEB + backwash) Backwash time per day, min ( )/( ) = 42 Backwash off line time, min 3.5*42 = 147 Off line time due to monthly cleaning,% 1 Membrane trains on line time fraction (1440 ( *1440))/1440 = 0.87 According to the values listed in Table , the scheduled off line time represents close to 13% of the available time. This off line time requires compensation of filtrate production capacity in sizing of the membrane filtration unit. The sizing of the membrane filtration unit is based on schedule of the process steps, configuration of membrane elements selected and the designed filtrate flux rate. Example of sizing of membrane filtration system of filtrate capacity of 200,000 m3/day according to parameters listed in Table is provided in Table Table Example of sizing calculations of 200,000 m3/day immersed membrane filtration system Design process parameter Value Filtrate capacity required for backwash, % 45*160/[40*( )] = 14.2 Capacity to compensate off line time, % 177/( ) = 14.0 Time for membrane cleanings, % 1 Contingency capacity, % 3 Total additional capacity, % 31 ( ) Installed filtration capacity, m3/d 200,000* 1.31 = 262,000 Membrane area required, m2 262,000,000/(24*40) = 272,916 Number of membrane modules (28m2/module) 272,916/28 = 9,747 train size, membrane modules per tank (1 train per tank) Number of tanks required 14 Number of tanks installed Instantaneous feed flow per tank, m3/day 720*28*40*24/1000 = 19,350 Backwash volume, m3/day/rack 720*28*160*42/(60*1000) = 2,

136 CIP interval, days 20 CIP frequency per day 14/20 = 0.7 CIP volume per day, m3/day (CIP tank = 20 m3) 0.7*20 = 14 Maintenance wash (MW) interval, days 5 MW frequency per day 14/5 = 2.8 MW volume per day, m3 (CIP = 20 m3) 2.8*20 = 56 Total waste volume, m3/day 14*( ) = 32,620 System recovery rate, % 100* (1 32,620/(14*720*28*40*24*0.87/1000) = 86 As in the previous example the recovery rate is function of filtration intervals (time between backwash events), filtrate flux and backwash flux. Increase of filtration intervals and filtrate flux and reduction of backwash flux will increase recovery rate. The rate of usage of chemicals determines the size of storage tanks and capacity of dosing equipment. Example of sizing of chemical units was provided previously and is included in Table The system will be composed of subunits providing functional operational capability according to listing provide in Table Table Listing of subunits and major equipment, immersed membrane filtration system Equipment Quantity Sizes/comments Feed water system Automatic backwashing strainer ,800 m3/hr Instrumentation Lot Turbiditymeter, pressure indicators and transmitters, temperature transmitter Valves Lot Manual, butterfly Membrane unit Membrane tanks and piping 16 Made of FRP or concrete with ckorrosion protection coating Membrane modules 11, m2/element Centrifugal, end suction filtrate pumps m3/hr, negative suction head 1 bar, discharge head 2 bar, VFD controlled motor Set of automatic valves 16 lots Inlet, outlet, waste discharge, air release 136

137 Instrumentation 16 lots Magnetic flow meters, turbiditymeter, pressure indicators and transmitters Chemical cleaning system CIP/MW tank 2 20 m3 Immersion heated kw Centrifugal, end suction pump m3/hr, 3 bar, stainless steel wetted parts Instruments 2 lot Magnetic flow meter, ORP sensor/transmitter, CIP tank level transmitter, CIP temperature transmitter, pressure gauges Valves 2 lot Valves: isolation, flow control, air release, pressure release, chemical injection isolating valve Chemical dosing systems Sodium hypochlorite Transfer pumps l/hr Storage tank 1 4 m3 Calibration column 2 Valves and piping 1 lot Citric acid Transfer pumps l/hr Storage tank 1 4 m3 Calibration column 1 Valves and piping 1 lot Sodium hydroxide Transfer pumps l/hr Storage tank 1 1 m3 Calibration column 1 Valves and piping 1 lot Operating cost of immersed membrane filtration system The operating cost includes the following cost items: - Power - Chemicals - Labor 137

138 - Maintenance - Membrane replacement RO Technology Mark Wilf Ph. D. The combined power usage of the filtration system is based on power usage of individual equipment according to the scheduled operation time. Example of energy usage in membrane filtration system of filtrate flow capacity of 200,000 m3/day is provided in Table As expected the major energy user is feed pump that delivers the feed water and provides sufficient pressure to drove water through the membranes. Table Breakdown of energy usage in immersed membrane filtration system. Filtrate capacity 200,000 m3/day Power component Efficiency M (P) Operating pressure, bar Feed pump 0.95 (0.80) *810 = 11,340 Flow, m3/hr Active, % Power, kw BW pump 0.95 (0.80) 2.4 3, Cleaning pumps Cleaning tank heater 0.95 (0.80) Compressor Air blower Valve actuators Instrumentation &PLC Total Annual usage 705,056 kwhr/y Specific energy usage kwhr/m3 Another significant contributor to the water operating cost is cost of chemicals used in the process for recovery of membrane permeability. These are chemicals used in the course of CEB and CIP. The calculation of quantity of chemicals used is based on volume CEB and CIP and concentration of chemicals used. An example of calculation of volumes required is provided in Table Summary of chemicals usage in a 200,000 m3/day membrane filtration system is provided in Table The generic chemicals used in permeability recovery procedure include sodium hypochlorite, sodium hydroxide, sulfuric acid and citric acid. 138

139 If coagulation flocculation process is utilized in the filtration process, then quantity and cost of coagulant (usually ferric salts) should be included in calculation of water cost. Table Summary of volumes required for CEB and CIP in immersed membrane filtration system Procedure Volume used, m3/day/train Number of treatments in system per year Total volume used, m3/year CEB1(1/4 backwash) 2260/4 = *365*0.95 = 4,950 2,970,500 CEB2 (1/40 backwash) 2260/50 = 50 4, ,550 CEB3 (1/40 backwash) 2260/50 = 50 4, ,550 CIP *14*(365/20) = 102 2,040 CIP *14*(365/20) = 102 2,040 CIP *14*(365/20) = 51 1,020 Table Summary of chemicals usage in immersed membrane filtration system. Filtrate capacity 200,000 m3/day. Process step Chemical Dosage, ppm t/year (as 100%) $/year CEB1 NaOCl ,500 CEB2 H2SO4 1, ,140 CEB3 NaOH 1, ,600 CIP1 Citric acid 20, ,600 CIP2 NaOH 3, ,100 CIP3 NaOCl 1, ,000 Total 522,940 Chemicals cost Summary of total operating cost is provided in Table Table Summary of operating cost in immersed membrane filtration system Cost component $/year $/m3 Power@C10kWhr 705, Chemicals 522, Operation and maintenance Membrane replacement 480,000 (8 operators@$60,000/y) 822,857 (16 trains,7 years membrane life) Total 2,530, $0.0075/m3 139

140 The above examples of calculations of power consumption, chemicals consumption and operating cost are not specific for any commercial membrane filtration technology. The present calculations are based on generic process parameters and just provided here to demonstrate the approach to this type of calculations. In calculations conducted for a commercial project the specific membrane module configuration, filtration process sequence and operating parameters specified by equipment provider will have to be applied. The results will have to be confirmed with provider of membrane filtration equipment to receive their approval and eventually warranty for system performance and operating cost, once the project is secured Comparison of conventional and membrane filtration technologies as pretreatment for seawater RO desalination systems The conventional pretreatment technology is based on suspended solids removal through coagulation, flocculation and media filtration. The process is well established and in majority of cases is capable to produce RO feed water of the required quality in respect of SDI and turbidity. The conventional process is very effective as pretreatment. However the quality of the effluent could be significantly affected by the quality of the raw water. The membrane filtration process is more equipment intensive than the conventional pretreatment process. Therefore maintenance requirements are much higher than of the conventional system. As in the case of conventional system, deterioration of raw water quality will affect operation of the filtration system. However, due to presence of the membrane barrier, filtrate quality will be much less affected by the quality of raw water, as compared to operation of the conventional system at the same conditions. Schematic configuration of a conventional pretreatment system is shown on Figure The pretreatment system components include coarse strainer, storage and dosing systems for chemicals (hypochlorite, acid, coagulant and polymer), static mixer, flocculation system followed by gravity filters. The filtration effluent is collected in the clear well. The filter backwash unit consists of backwash pumps and air blower (for air scouring). The backwash stream is treated in solids management unit. The auxiliary units include electric supply unit and process control system. 140

141 Dosing systems 1 Dosing systems 2 Dosing systems 3 Seawater source Intake pumps Strainer Static mixer Flocculation Media filters Filtrate clearwell Dosing systems 4 Blowers Solids handling Backwash pumps Electric system Control system Figure Configuration of conventional pretreatment system for SWRO Dosing systems 2 Cleaning and neutralization unit Heat exchanger Dosing systems 1 Compress air Vacuum pumps Feed pumps Strainer Static mixer Membrane tanks Filtrate pumps Filtrate clearwell Electric system Control system Solids handling Blowers Dosing systems 3 Backpulse pumps Backpulse tank Figure Configuration of immersed membrane pretreatment system for SWRO The configuration of membrane filtration pretreatment system is shown on Figure The pretreatment system components include fine strainer, storage and dosing systems for hypochlorite, static mixer, flocculation system followed by membrane tanks with membrane modules. The filtration effluent pumped through the membrane by filtrate pumps is collected in the clear well. The fil- 141

142 trate line is equipped with air receivers and vacuum pumps. The membrane backwash unit consists of filtrate backwash storage tank, backwash pumps and air blower (for membranes air scouring) and storage and chemicals dosing system(s) for CEB s. The membrane filtration system also includes cleaning (CIP) unit with circulation pump, heat exchanger and cleaning solution neutralization system. The auxiliary units include electric supply unit and process control system. Addition of coagulant to feed water improves stability of permeability in membrane filtration system. However, there is tendency not to use coagulation in RO seawater applications to avoid necessity of inclusion of solids management system, which is necessary in the conventional pretreatment systems. The relative advantages of conventional filtration and membrane filtration systems are summarized in Table Table Summary of comparison of relative advantages of multimedia and membrane filtration systems for SWRO applications. Selected equipment and cost component Conventional pretreatment Microstrainers No Yes Flocculators Yes Yes/No (1) Filtration media Anthracite + sand Membranes Filtration basins 100% 50 60% Solids handling Yes Yes/No (1, 2) Cartridge filters Yes Yes/No (3) Footprint 100% 50 60% Additional energy use Chemicals usage % Membrane replacement Membrane filtration pretreatment kwhr/m c/m3 Equipment maintenance cost + 100% (1) Frequently membrane filtration pretreatment systems operate without addition of coagulant. (2) Solids handling unit only required if coagulant is being used (3) In RO system utilizing membrane filtration pretreatment, cartridge filtration equipment is frequently omitted from the treatment process. Comparison of estimated capital cost of two pretreatment systems, one including multimedia filtration and the second including membrane filtration, based on filtrate capacity of 200,000 m3/day is provided in Table

143 Table Estimated capital cost of multimedia filtration and membrane filtration units. Filtrate capacity 200,000 m3/day. System cost component Conventional pretreatment technology Membrane filtration pretreatment technology Cost component, Cost component, Cost component, Cost compo- $M % $M nent, % Microstrainers Filter media Static mixer Flocculators Filter system Filter piping Membrane equipment All equipment installation Concrete tanks Civil works Chemical systems Electric and instrumentation Solids handling Subtotal equipment Engineering (5%) Construction management (8) Commissioning (1%) Subtotal indirect Total $/m3/day The results of estimation included in Table indicate capital cost of membrane filtration pretreatment to be about 20 25% higher than the conventional pretreatment. Higher capital cost of membrane pretreatment system is expected as it includes more expensive equipment components compared to single stage multimedia filtration. However, the above evaluation did not accounted for much larger footprint of multimedia filters and therefore an additional land cost, which could be significant at some sites Offering of commercial membrane filtration technology Offering of commercial membrane filtration products includes both membrane elements and complete systems. Some suppliers offer just membrane elements or systems, some offer both. The fol- 143

144 lowing list of suppliers, included in Table , illustrates variability of approach to market place by different suppliers Table Listing of established suppliers of commercial membrane filtration products. Provider Market approach type terial figuration Membrane Membrane ma- Module con- Well established Aquasource Systems UF CA PDI + Dow/Omexell Elements & systems MF PVDF PDO Hydranautics Elements UF PES PDI + Inge Elements UF PES PDI + Koch Systems UF PS PDI + Norit Elements & systems UF PES PDI + Pall/Asahi Systems MF PVDF PDO + Polymem Systems UF PS PDO Siemens (Memcor) Systems MF PVDF SUB Siemens (Memcor) Systems MF PVDF PDO Toray Elements MF PVDF PDO Zenon Systems MF PVDF SUB + The important information included in Table is related to membrane material. Majority of manufacturers provide membrane made of PVDF, followed by PES and PS. There is large variety of membrane modules configurations. Larger number of manufacturers offers pressurized products, while immersed products are only offered by two. One of the major providers, Siemens, offers both, the pressurized and the immersed products. Unlikely the offering of membrane elements for RO applications the offering of membrane filtration products is not standardized in dimensions and configurations. The variability of membrane filtration products is illustrated in the following pictures of products offers by various providers

145 Figure Membrane filtration train with Hydracap modules Hydranautics Figure Membrane filtration train with Xiga modules Norit 145

146 Figure Membrane products offered by Inge Figure Membrane products offered by Pall Asahi. 146

147 Figure Pressurized membrane filtration products (CP) offered by Siemens. 147

148 Figure Immersed membrane filtration products (CS) offered by Siemens. 148

149 Figure Immersed membrane filtration products (ZW-1000) offered by GE (Zenon) 149

150 Figure Immersed membrane filtration products (ZW-500) offered by GE (Zenon) Membrane filtration membranes made of ceramic materials has been manly used for specialty application that could justify high cost membranes and membrane modules. In the last few years the cost of ceramic membranes has declined and in parallel membrane elements of increased membrane area per elements has been offered. The ceramic membranes has unique properties that make it very attractive for filtration applications. The membrane is permanently hydrophilic It has practical ability to operate at high permeate flux (~200 lmh) at field conditions The membrane can be preserved in dry condition The membrane can be clean and sanitized with hot water, high concentration oxidant solution, extreme range of ph It has long useful membrane life, significantly longer than polymeric membranes The disadvantages are Lack of standardization of membrane configurations The membrane is mechanically strong but brittle High cost of membrane 150

151 Figure Representative summary of configuration of ceramic membrane modules 151