Desalination Process Engineering Part II

Transcription

1 Desalination Process Engineering Part II Mark Wilf Ph. D

2 5. RO System 5.1. Membrane elements and pressure vessels The basic blocks of RO membrane unit are membrane elements. The membrane elements are highly standardized. They have the same outside dimensions and in most cases identical dimensions of the connecting permeate ports. They are interchangeable. Regardless of type of elements they look practically the same. The dimensions of commercial elements are 200 mm in diameter and 1000 mm long as shown on Figure Membrane area of such element is 37 m2 41 m2. Depending of application and element type elements of the same membrane area will produce different quantity of permeate in field conditions as indicated by permeate capacity numbers in Figure mm, m2 (400 ft2) Brackish application: ~24.2 m3/d (~6,400 gpd) 1 m, 40 Wastewater reclamation: ~18.2 m3/d (~4,800 gpd) Seawater application: ~12.8 m3/d (~3,400 gpd) Second pass RO: ~30.3 m3/d (~8,000 gpd) Figure mm by 1000 mm spiral wound element and corresponding product capacity values in various applications. In addition to 100 mm and 200 mm diameter elements, larger, 400 mm diameter elements are being introduced recently. The relative size and corresponding permeate capacity is shown on Figure

3 8 and 16 diameter elements 8 element Membrane area 40m2 (430 ft2) Nominal flow 45 m3/day (12,000 gpd) Avg. field flow 24 m3/day (6,500 gpd) 16 element Membrane area 160 m2 (1,700 ft2) Nominal flow 180 m3/day (47,000 gpd) Avg. field flow 95 m3/day (25,000 gpd) Nominal salt rejection 99.8% Nominal salt rejection 99.8% Figure mm and 400 mm diameter, 1000 long spiral wound membrane elements. In order to be able to apply feed pressure to feed solution and continently separate and collect permeate and concentrate streams, the elements have to be enclosed in pressure vessels. The basic configuration of elements in pressure vessel is shown on Figure Figure Configuration of pressure vessel with membrane elements. 140

4 Membrane units in commercial desalination systems are configured as having between 6 8 elements per vessel. The number of elements per vessel is determined based on number of concentrate stages and pressure drop along the membrane unit. In majority of cases, larger number of elements per vessel will result in lass expensive membrane unit and smaller footprint. The distribution of recovery rate in seawater membrane unit having 6, 7 and 8 elements per vessel is illustrated in Table Table Recovery rates of individual elements in pressure vessel according to number of elements per vessel. Element position 6 elements/vessel Recovery rate, % 7 elements/vessel Recovery rate, % 8 elements/vessel Recovery rate, % The results listed in Table were calculated for a membrane unit operated at recovery rate of 50%. It is evident from the above results that larger number of elements per vessel will results in more gradual reduction of recovery rate along the pressure vessel. As in each case the total system permeate capacity and the total membrane area in the system are the same, the average permeate flux in the system will be the same regardless number of elements per vessel. The systems with different number of elements per vessel are designed to operate at the same feed salinity and recovery rate. The feed pressure and concentrate pressure will be very similar in each case. Therefore, the net driving pressure (NDP) and permeate flux distribution in the pressure vessel will be the same, as shown on Figure Large capacity seawater RO desalination systems in configuration of eight elements per vessel have been operating successfully. Some of the systems have close to 10 years on line operating record. 141

5 Average element flux, l/m2-hr Average flux distribution in RO seawater system Mediterranean seawater, 50% recovery, avg flux 14.3 l/m2-hr /PV 7/PV 6/PV Figure Flux distribution along the length of pressure vessel for 6,7, &8 elements per vessel membrane unit configurations.. In the same way as membrane elements, internal dimensions of pressure vessels are highly standardized and the outside dimensions and configurations are similar. The dimensions of pressure vessels for 100 mm and 200 mm diameter membrane elements are summarized in Table Table Example of representative dimensions of commercial pressure vessels for RO application. Element type - elements number/pv Fraction of pressure vessel length Inside diameter, mm (in) Outside diameter, mm (in) Length, mm (in) (8.02) 259 (10.2) 1478 (58.2) (8.02) 259 (10.2) 2494 (98.2) 142

6 (8.02) 259 (10.2) 6558 (258.2) (8.02) 259 (10.2) 7574 (298.2) (8.02) 259 (10.2) 8590 (338.2) (4.13) 127 (5.0) 1194 (47) (4.13) 127 (5.0) 2210 (87) (4.13) 127 (5.0) 4242 (167) 5.2.Membrane unit configuration In membrane unit pressure vessels are arranged in array of parallel units, connected together through the corresponding ports to feed, permeate and concentrate manifolds Single stage and multistage RO membrane systems are configured as a single stage unit in seawater applications due to low recovery rate, usually not exceeding 50%. In a single stage unit all pressure vessels form a single parallel grid, connecting feed and concentrate manifolds, as shown on Figure Accordingly feed water passes only once through the array of pressure vessels. After the passage through the array of pressure vessels, feed water becomes concentrate and it is discharged form the system, usually back to the body of water where the feed water was pumped from (ocean). The permeate is collected from individual pressure vessels into a single manifold connected to the product water system. 143

7 Permeate Permeate Concentrate Concentrate Feed Figure Single stage membrane unit configuration (Courtesy CH2M Hill) The recovery rate of a single stage unit configuration is usually do not exceed value of about 65% value due to limitation of a minimum concentrate flow. For this reason brackish and nanofiltration systems, that operate at higher recovery rates are configured as two, three or even four concentrate stages units. A two stage unit configuration is shown schematically on Figure The pressure vessels are arranged in array of 4:2. The 2:1 ratio of pressure vessels between consecutive stages is related to the objectives of maintaining similar feed cross flow rate in pressure vessels along the system. Approximately in each stage about 50% of feed water is converted to permeate. Therefore, only half of the original feed flow rate is available for the next stage. Accordingly, the number of pressure vessels should be half of the number of pressure vessel in the proceeding stage. As shown on Figure the concentrate from pressure vessels in the first stage is collected in common concentrate manifold and flows according to hydraulic pressure gradient as a feed to the second stage. There are no flow or pressure regulating valves between the stages. Mechanical drawing of RO membrane train is shown on Figure This drawing corresponds to pressure vessel array of 36:18. The drawing includes two view of membrane unit. 144

8 Figure Schematic diagram of a two stage membrane unit. Pressure vessel array 4:2. 145

9 Figure Mechanical drawings of RO membrane train. Pressure vessels array 36:18. The feed side of the membrane unit is shown as View A A. The concentrate side of the membrane unit is shown as View B B. The B B view illustrates the common approach to the design of interstage piping connections when no iterstage booster pump is being used. The concentrate collecting manifolds of the first stage are connected directly to the feed manifold of the second stage Sideport, multiport and center port configuration The membrane unit shown on Figure utilizes side port type pressure vessel, shown on Figure This type of pressure vessel have only two high pressure inlet outlet ports: one for feed and the second for concentrate. In a never pressure vessels design (called multiport configuration) the number of high pressure ports is increased to four: two feed ports and two concentrate ports. The two ports (feed or concentrate) are on opposite sides of pressure vessels 180 degree apart. The multiport configuration enables connecting number of pressure vessels together through the 146

10 corresponding ports, therefore reducing number of central distributor piping. Comparable configurations of membrane units using side port and multiport pressure vessels for single stage and two stage units, is shown schematically in Figures The number of pressure vessels that can be connected together is determined by the port size and limit of pressure drop in the feed/concentrate ports or limit of maximum flow velocity through the ports Figure Configuration of a side port pressure vessel. 147

11 Figure Single stage membrane unit with sideport pressure vessels (85") 2050 (81") Figure Single stage membrane unit with multiport pressure vessels. 148

12 980 (38.5") 2850 (112") 360 (14") 2050 (81") Figure Two stage membrane unit with sideport pressure vessels. 149

13 Figure Two stage membrane unit with multiport pressure vessels. The connection of pressure vessels can be in any direction, both horizontal and vertical. An example of a vertical connection of multiport pressure vessels is shown on Figure Figure Unit with multiport pressure vessel in vertical flow configuration (courtesy PUB, Singapore) The limits and corresponding ports diameter is provided by the manufacturers of pressure vessels. The usual limit is maximum pressure drop of up to 0.2 bar and/or maximum flow velocity of 3 m/sec. The general approach to assure good flow distribution is that the pressure drop along the pressure vessel should be significantly higher than the pressure drop in the ports. The feed and concentrate connection to the grid of parallel connected array of multiport pressure vessels could be configured in a number of ways. The proffered connection is the Z configuration, however the U configuration (feed and concentrate connected to the same side of the array) is also used quite frequently. In this configuration the feed enters at one corner of the array and the concentrate is collected at the opposite corner. When designing and construction a membrane unit that utilizes multiport pressure vessels it is important to remember that the pressure vessels should not have unconnected ports. Unconnected ports will create pockets of stagnant water that eventually could become centers of development of biological activity. 150

14 Center port pressure vessel concept has been developed in Netherlands by Vitens Company. The center port configuration, shown on Figure allows entry of feed water through two ports at the end of pressure vessel and collection of concentrate through one port in the center of pressure vessel. Figure Schematic configuration of membrane unit utilizing center port pressure vessels. The benefit of center port configuration is lower pressure drop in the membrane unit as illustrated in Table According to the comparison included in Table the center port configuration can provide about 15% reduction of feed pressure, which translates directly to saving of pumping energy. Table Comparison of side port and center port configuration Configuration Side port Center port Number of stages 2 2 Pressure vessels array 40:20 34:18 Elements/vessel 7 8 Total number of elements Recovery rate 85% 85% Average flux rate 24.5 l/m2/hr 24.5 l/m2/hr Feed pressure 9.0 bar 7.8 bar Feed pressure reduction 15% 151

15 The reduction of feed pressure is related to reduction of pressure drop due to reduction of average feed flow rate by half and the same rate of reduction of the path length. Application of center port configuration increases significantly concentration polarization as the stage recovery rate is achieved in a path length of 4 elements in place of 6 or 7 in the conventional configuration. So far the center port configuration has been applied in number of small nanofiltrtaion unit is Netherland. However, more recently a 55,000 m3/day nanofiltrtaion plant with center port pressure vessels has started operation at the town of Jupiter, Florida Two pass, partial two pass, split partial Second pass processing of permeate from the first pass unit is implemented when permeate salinity, or specific constituent(s), produced by the first pass unit exceed the specified values. Schematic configuration of a two pass system is shown on Figure Feed PG Pressure vessel, 1 st pass Pressure vessel, 1 st pass Pressure vessel, 1 st pass Permeate pass one Pressure vessel, 1 st pass Concentrate pass two FI Pressure vessel, 2 nd pass Pressure vessel, 2 ND pass FI Permeate pass two PG Two Pass RO System FI Concentrate pass one Figure Schematic configuration of a two pass unit. As shown in the above diagram, first pass permeate is collected and pumped, using a high pressure pump, as feed water to the second pass unit. The concentrate from the second pass unit is usually returned to the suction of the first pass high pressure pump. This configuration reduces the need for additional feed water and additional pretreatment equipment. 152

16 Application of two pass system requires additional RO equipment and also reduces overall system recovery rate, as illustrated on Figure The necessary additional equipment includes equipment of the second pass unit and also equipment to increase permeate production capacity of the first pass unit that will be lost with the second pass concentrate. The required increase of the first pass unit permeate capacity is direct function the recovery rate of the second pass unit. Frequently, the second pass processing of the full flow from the first pass is not required and partial two pass processing is implemented. Schematic configuration of a partial two pass system is show on Figure Two Pass RO System R1 =? 100*50/100 = 50% R2 =? 100*45/50 = 90% Rt =? 100*45/100 (w/o recirculation) = 45.0% Rt =? 100*45/95 (w recircirculation) = 47.4% Figure Two pass system with second pass concentrate recirculation 153

17 Combined permeate 1 st Pass RO 2 nd Pass RO Figure Schematic configuration of a partial two pass processing The fraction of first pass that will be processed is calculated based on salinities of the first pass permeate, salinity of the second pass permeate and the required salinity of the product water (after blending). Due to possible fluctuations of feed water temperature, salinity and membrane condition, the permeate salinities from the first and the second pass units may fluctuate as well. Accordingly, the first and second pass units are sized based on the most extreme conditions of feed water, as defined in project specifications. The partial two pass processing is effective way of improving product water quality, requiring smaller additional equipment and lower operating cost than the full two pass system. The partial two pass processing can be optimized further, applying modification of this process called split partial two pass processing. The split partial processing takes advantage of internal permeate salinity distribution along the pressure vessel, shown schematically on Figure

18 Combined salinity in pressure vessel Salinity ppm TDS Element position Element position in pressure vessel Figure Permeate salinity distribution along the pressure vessel. The permeate salinity distribution, shown on Figure , is result of two processes: - Increase of feed salinity along the pressure vessel, therefore increased salinity gradient. - Increase of feed osmotic pressure, therefore lower NDP and lower permeate flux along the pressure vessel. The split partial design utilizes this phenomenon in a configuration shown in Figure In the split partial configuration, high salinity permeate, collected from the concentrate end of the membrane unit, is processed with the second pass system. The low salinity permeate, collected from the feed end of the membrane unit is used for blending. The split partial design results in significan reduction of the second pass processing required. 155

19 Combined permeate Low salinity permeate High salinity permeate 1 st Pass RO 2 nd Pass RO Figure Split partial two pass configuration. An example of potential saving possible with split partial configuration, compared to a conventional partial two pass system is illustrated in Table Table Comparison of conventional partial two pass and split partial two pass configuration Configuration Conventional partial two pass Split partial two pass First pass capacity (recovery rate) m3/day (50%) m3/day (50%) Second pass capacity (recovery rate) 6700 m3/day (85%) 3000 m3/day (85%) TDS (boron), ppm 111 (0.75) 118 (0.77) First pass array 129 PV ( 8 M) 120 PV (8 M) Second pass array 18:9 PV (8 M) 8:4 PV (8 M) Power consumption, kwhr/m (-8%) Total number of elements (-15%) Another illustration of benefits of split partial configuration compared to conventional partial two pass design is shown on Figure The plots on Figure provides fraction of second pass processing required for a given salinity of the blended product. For example, for a combined permeate salinity of 300 ppm, the conventional partial two pass system would require processing of about 24% of the first pass 156

20 permeate. In case of split partial configuration, only 5% processing of the first pass permeate would be required. Partial two pass processing Combined permeate salinity, ppm TDS Fraction of 1st pass processed, % Conventional Split partial Figure Comparison of second pass processing required for conventional partial two pass and split partial two configurations. The relative effectiveness of the split partial configuration increases with decrease of the fraction of the first pass that has to be process again. The closer that two pas system is to the full two pass processing the less attractive the split partial processing is. The split partial system operates without any obstruction of valve regulation of the relative flow from both ends of the membrane unit. The flow is regulated by the pump of the second pass. The higher the flow rate of the feed pump of the second pass unit the higher fraction of the first pass unit is taken from the concentrate end. The split partial unit operates without buffer tank. Utilization of buffer tank would reduce flexibility of operation and effectiveness of the process Membrane cleaning Membrane cleaning is conducted to recover membrane performance by removing deposits from the membrane surfaces of membrane elements. The decision to conduct membrane cleaning is triggered by reduction of membrane water permeability, increased pressure drop and/or increase of salt passage. The decision about timing, frequency and type of cleaning procedure applied is usually based on 157

21 the relevant experience gained during operation of the membrane system and analysis of the foulant deposit. The limiting parameters of cleaning procedures, such as maximum temperature of cleaning solution, maximum and minimum ph, average flow rate and pressure, is specified by membrane manufacturers Configuration of membrane cleaning unit The cleaning of membrane elements in an RO train is conducted using the cleaning unit. The configuration of cleaning unit is shown in Figure The cleaning unit consists of cleaning tank, heater, recirculation pump, cartridge filter and connecting piping. It is recommended to have an air gap between cleaning tank and piping that returns cleaning solution from the product manifold of the membrane unit. This is to prevent contamination of the product side of the membranes with cleaning solutions. Larger cleaning units also include separate tank for dissolving and mixing of cleaning solutions. Depending on the size of the system and frequency of cleaning, some systems may require a dedicated tank and dosing units for neutralization of cleaning solution, prior to disposal. Materials of construction of the cleaning unit should be selected to withstand low and high ph cleaning solutions (ph 2 11) at temperatures up to 50 C. The size of cleaning tank and capacity of cleaning pump is determined by the number of pressure vessels that will be cleaned at one time. During cleaning operation the flow rate of cleaning solution per vessel should be close to 7-9 m 3 /hr (~ gpm). The cleaning tank volume should hold enough cleaning solution volume to provide at least 3-5 min of pump capacity. If, for example, the RO membrane train is a two stage unit with pressure vessels array of 64:32, then the maximum number of pressure vessels that will be cleaned at one time will be 64. Accordingly, the maximum flow rate of the cleaning solution to the membrane unit will be 64 * 8 m3/hr = 512 m3/hr. The operational volume of the cleaning tank, equivalent to 5 min of pump operation, will be 42 m3. The quantity of chemicals required for cleaning is not based only on operating volume of cleaning tank but consideration must be given to the total empty volume of the RO unit, which includes, pressure vessels, manifolds and connecting piping. Estimation of empty volume of membrane unit for cleaning is illustrated in Example In large RO systems, the connecting piping from the cleaning unit is permanently attached to all trains. Valves or removable piping segments are used to connect/disconnect given train or train segment to the cleaning unit. The connecting manifold should be configured to enable to transfer of majority of spent cleaning solution after the cleaning to the neutralization tank. 158

22 Cleaning solution storage tank Heater CF Cleaning solution neutralization tank Figure Configuration of membrane cleaning unit that includes neutralization tank. In the configuration of cleaning system shown on Figure the cleaning solution neutralization tank is shown as of similar size as the cleaning tank. The actual size will depend on the mode and frequency of conducting cleaning neutralization operations. Table General specification of cleaning equipment. Equipment type Description Cleaning solution flow 7 9 m3/hr/pressure vessel in the train (or stage) being cleaned Cleaning solution inlet pressure 2 4 bar Materials of construction - Metallic components Stainless steel 316/316L - Plastic components PVS, FRP and HDPE - Elastomers Teflon (PTFE) or EPDM Inlet outlet flanges class ANSI Class 150 Tank immersion heater power output 1kW/m3/hr of recirculation flow. Cartridge filters flow sizing 1 m3/hr/25cm cartridge length Cartridge rating 20 micron 159

23 Cleaning pump configuration Main instrumentation - Level gauges - ph analyzer Pressure gauges Corrosion resistant, weatherproof Magnetic type Provide discrete high and low ph alarms Low pressure gauges Sequence of operation of cleaning unit. Cleaning operation sequence includes: 1. Flushing RO train with permeate water. 2. Connecting train or train segment to the cleaning unit. 3. Preparing cleaning solution in the cleaning unit. 4. Recirculating cleaning solution for 1 4 hr through the RO train. 6. Draining cleaning solution to the neutralization tank. 7. Flushing cleaning solution with permeate (after high ph cleaning) or with feed water. 8. Repeating steps 2 5 with next cleaning formulation or reconnecting cleaned train to high pressure pump and restoring normal operation. Membrane cleaning, like any other dispersive process, is more effective when conducted at elevated temperature. Cleaning should be conducted at temperature of cleaning solution in the range of C. Cleaning solutions can be purchased from specialized suppliers or generic cleaning formulations could be used. Composition of generic cleaning formulations can be obtained from all major membrane manufacturers. One of the generic low ph cleaning formulation, frequently used, is 2% solution of citric acid. ph of such solution is about 2.5. Citric acid cleaning solution is very effective in removal of deposits of metal hydroxides and dissolving of carbonate scale. If it has been established that fouling deposit contains mainly calcium carbonate or metal hydroxides, temporary operation with feed water acidified to low ph (ph = 4.5 5) with mineral acid (H2SO4 or HCl), may be sufficient to restore membrane performance. Cleaning, through operation of membrane unit at low feed ph, is only possible if discharge of low ph concentrate is allowed by local regulation at a given site. The generic high ph cleaning formulations consist of solutions of NaOH in combination with EDTA or SDBS (surfactant). The caustic cleaning solutions have ph of and are effective in removal deposits of organic matter from membrane surface. It has been found (14) that EDTA or surfactants are essential components of high ph cleaning solutions and their presence contributes to improved removal of surface deposits that contain Ca ions imbedded in the organic fouling layer. In majority of cases, fouling layer is of a mixed nature, it contains a mixture of inorganic and organic matter. The effective cleaning sequence is to 160

24 apply low ph cleaning followed by application of high ph formulation. Prior to use of cleaning solution containing surfactant, a small scale test should be conducted to check if designed concentration of surfactant will not result in excessive foaming. RO systems treating well water feed very seldom required membrane cleaning. Cleaning frequency is usually less than one cleaning per 2 3 years of operation. RO systems treating surface water or wastewater feed, require more frequent membrane cleaning. For the purpose of operating cost estimation, budget for cleaning operation is usually based on two four cleaning events per year. If more frequent cleanings are required, then it is an indication of inadequate pretreatment process. Frequently, only segments of membranes in a train require cleaning. For example membrane elements in the last stage my require cleaning at higher frequency then the rest of the system. Connections between cleaning unit and membrane train should be configured to enable cleaning of either the whole train or single stage. Example Calculation of annual cleaning cost. System permeate capacity: 100,000 m3/day (26.4 mgd), RO unit configuration: 8 trains, 96 PV per train, 7 elements per vessel, 64:32 array Train segment size for a single cleaning: 98 pressure vessels. Annual cleaning frequency: 2 Cleaning procedure: low ph cleaning followed by high ph Free volume of pressure vessels: 98 X 7 X 0.025m 3 = 17 m 3 (4,500 gallons) Volume of manifolds (10% of PV): 0.1 x 17 m 3 = 1.7 m 3 (450 gallons) Volume of connecting piping (50% of PV): 0.5 x 17 m 3 = 8.5 m 3 (2250 gallons) 98 PV X 8 m3/hr X 3min/60 min = 39.2 m3 (10,300 gallons) Total empty volume of membrane unit for cleaning: ( ) m m3 = 66.4 m 3 (17,500 gallons) Chemicals quantity for annual cleaning operation: Solution 1 citric acid 2% Citric acid: 0.02 x 66.4 = t/cleaning 8 train X 2 cleanings/year X 1.328= 21.2 t Solution 2 NaOH + SDBS 0.2% NaOH: x 66.4 = t/cleaning 8 train X 2 cleanings/year X = 2.1 t 0.2% SDBS: x 66.4 = t/cleaning 8 train X 2 cleanings/year X = 2.1 t Annual cost of cleaning chemicals Citric acid: 21.2t X $3500/t = $74,200 SDBS: 2.1t X $3500/t = $7,350 NaOH: 2.1t X $350/t = $

25 Total cost of cleaning chemicals per year: $82,285 Cost per water produced: $0.0024/m 3 ($0.009/kgallon) As indicated by above example cost of generic cleaning chemicals is not significant if cleaning frequency is limited to two cleanings per year. Additional cost, associated with cleaning operation, that should be considered is loss of production capacity. The system off line time required for cleaning is in the range of days, which is corresponds to about 0.4% of availability The major expense related to cleaning, at some locations, could be the disposal cost of spent cleaning solutions. In majority of cases, cleaning operation is capable to restore some of the lost permeability and reduce pressure drop. Very seldom salt rejection is improved. Usually, it remains the same or can even decline after cleaning. This is because foulant layer plugs imperfections and damaged areas in membrane barrier and effective cleaning opens them again to salt passage. If cleaning attempts do not result in sufficient performance improvement, membrane element replacement is the only practical solution available for additional performance correction. As it will be discussed in section X.X1 usually a considerable fraction of elements in the system have to be replaced to achieve noticeable performance improvement. Number of elements that require replacement could be reduced, if elements with worst performance can be identified in the RO system. Discussion of such approach is included in chapter X.X Membrane flushing unit configuration Flushing of membrane unit is required every time the unit shuts down for planed stoppage of system operation, shut down due to equipment failure or power outage. Without flushing of membrane unit on shut down, there is possibility of scale formation, especially in brackish RO unit or bacterial grow, especially in seawater RO. Flushing of membrane unit is conducted with permeate water. If permeate water is not available, the membrane unit can be flushed with clean feed water. Point of supply of permeate for flushing should be selected to prevent of flow of chlorinated permeate water to the membrane unit. If possible, the flushing should be configured so the membrane units will be flushed also, when shut down is caused by power outage. The standard recommendations of membrane manufacturers are to flush membrane unit on shut down with permeate water and then repeat flushing every 5 days if the membrane unit remains idle. Flushing involves replacing 1 2 empty volumes of the membrane unit, including manifolds and interconnecting piping. Empty volume for flushing is estimated as illustrated in Example 162

26 The general specifications of flushing equipment are the same as listed in Table for the cleaning equipment RO membrane unit design criteria in accordance with water quality The design of RO system starts with the ultimate project objective: consistent production of a design flow of product water at a required quality. The selection of process parameters is influenced by type of feed raw and design range of water quality parameters: dissolved species composition and concentration, fluctuation of water temperature and turbidity. The direct design parameters of the RO membrane unit, such as an average permeate flux rate and system recovery rate, are based on type of raw water source and projected feed water quality. The optimization of values of design process parameters, within the recommended ranges, requires understanding the relation between system operating conditions and stability of membrane performance. The average permeate flux rate should be within the range that would not accelerate membrane fouling. The recovery rate should be below the value that would result in excessive saturation limit of the sparingly soluble salts in the concentrate. The design process begins with evaluation of feed water quality. Based on feed water quality and composition the designer decides on selection of membrane element type, average flux rate (total membrane area in the system) and maximum recovery rate Feed water quality parameters The primary indicators of fed water quality for RO applications are Silt Density Index (SDI) and turbidity. Membrane manufacturers usually define values of these two indicators as part of membrane warranty terms. Secondary indicators, such as concentration of suspended particles, TOC and concentration of sparingly soluble salts, guide system designer to define process parameters: average permeate flux and system recovery rate. Some of the feed water quality indicators, including SDI have been discussed briefly in Chapter 1. SDI is very sensitive indicator of presence of suspended solids in water. However, the SDI readings could vary significantly with type and size of suspended particles present in the water and type of material of the filter pad used for SDI determination. The required equipment for determination of SDI is very simple and procedure easy to conduct at field conditions. However, both the accuracy and reproducibly of results are not very satisfactory. In spite of the above deficiencies, SDI is universally adopted as a primary indicator of feed water quality in RO 163

27 systems. Determination of SDI takes about min. The secondary feed water quality indicator, used in RO applications is turbidity. Turbidity results, usually expressed as nefelometric turbidity units (NTU), are determined through measurements of intensity of light scattered by suspended particles in a water sample. Turbidity sensor could provide continuous measurement of feed water turbidity. Turbidity results are sensitive to quantity of colloidal particles, their size and shape. Past attempts to correlate between SDI and turbidity results, even for the same site, demonstrate very weak relations. These two water quality indicators correlate to the number and size od suspended particles differently. However, the general rule is that feed water with SDI in the range of 2 3 has a corresponding turbidity below 0.1 NTU, usually at 0.05 NTU range Membrane fouling During the membrane fouling process, performance of membrane elements changes due to formation of deposits on membrane surface or inside of feed brine channel. Fouling affects membrane elements performance. The symptoms of fouling are decrease or increase of water permeability, increase or decrease of salt rejection and increase of pressure drop across the RO unit. Later stages of uncontrolled fouling could result in structural damage of membrane or membrane element. List of fouling factors encountered in RO applications is included in Table Table Summary of membrane fouling categories and their symptoms Fouling factor Exposure to strong oxidants Initial fouling stage effect Some permeability and salt passage decline. Initially in the lead element(s) Colloidal matter Some increase of pressure drop. Initially in the lead element(s). Dissolved natural organic matter Some permeability and salt passage Advanced fouling stage effect Increase of permeability and significant increase of salt passage Significant increase of pressure drop, some decline of permeability and increase of salt passage Moderate decline of permeability, same salt Potential membrane damage Irreversible damage of membrane barrier Element telescoping and extrusion of brine spacer. Membrane barrier damage None 164

28 (NOM) decline. passage decline Biological matter Some increase of pressure drop. Some permeability and salt passage decline. Initially in the lead element(s). Inorganic scale Some increase of pressure drop. Some permeability decline. Initially in the tail element(s). Petroleum products, also oil and grease Composite foulants (organics + colloids) Sharp edged, micron size particles in feed water Prolonged exposure to very high ph (> 12) Prolonged exposure to very low ph (< 2) Low concentration of organic solvents Significant permeability decline in the lead element(s). Small effect on salt passage Some increase of pressure drop. Some water permeability decline. Initially in the lead element(s). Gradual salt passage increase Increase of water permeability and salt passage Initial decrease of water permeability and salt passage Gradual decrease of water permeability, increase of salt passage Significant increase of pressure drop. Some permeability and salt passage decline. Severe increase of pressure drop. Some permeability decline and salt passage increase. Severe decrease of permeability. Small effect on salt passage Significant increase of pressure drop. Significant permeability decline and some salt passage increase. Significant passage increase salt Significant increase of water permeability and salt passage Significant decrease of water permeability Significant decrease of water permeability, increase of salt passage Element telescoping and extrusion of brine spacer. Membrane barrier damage Severe blockage of feed channels None at low concentration. At high concentrations barrier integrity damage Significant blockage of feed channels. Element telescoping. Membrane barrier damage Cuts and holes in membrane barrier Chemical damage of membrane barrier material Chemical damage of membrane barrier material Swelling of membrane barrier and eventually chemical damage 165

29 The specific membrane fouling process that could be of concern in desalination system will depend on application type and composition of feed water. Summary of more frequent potential fouling processes, grouped by application type, is provided in Table Table Summary of fouling processes for various desalination applications and possible cause of the fouling processes. Application type Fouling process Likely culprit Desalination of seawater Particulate fouling High concentration of particulate matter in seawater (algae bloom), carryover of coagulant from the pretreatment system Biofouling High concentration of biological activity in seawater, use of continuous chlorination Desalination of ground water Reclamation of industrial streams Reclamation of municipal wastewater Membrane scaling High concentration of silica, calcium sulfate, calcium carbonate, barium sulfate in the concentrate Particle induced cuts of Presence of mineral membrane barrier particles in feed water. Any of the fouling Potentially any industrial processes listed in Table waste constituent present in the feed water Scaling High concentration of calcium phosphate, calcium carbonate in the concentrate Blocking of feed channels High concentration of colloidal matter in the feed water. Adsorption of organics Present of high concentration of NOM in the feed water Biofouling Insufficient concentration of chloramines in the feed water 166

30 Oxidation of membrane barrier Presence of free chlorine (insufficient ammonia concentration) in the feed water Fouling can be prevented or mitigated by reduction of concentrate of the foulant in the pretreatment step, adjustment of feed water ph, addition of scale inhibitor, operation at average flux rate and recovery that would not result in high fouling rate Oxidative degradation of membrane performance The sensitivity of membrane performance to presence of strong oxidants was known at the very onset of development composite polyamide membrane. The early tests of exposure of polyamide membrane to free chlorine, indicated limited initial stability followed up by a significant decline of salt rejection. Feed water to RO membrane unit has to be completely dechlorinated. The better solution is to totally avoid introduction of free chlorine to RO feed water. The only exception is formation and maintaining of chloramine in wastewater reclamation systems. RO membrane operating in municipal wastewater reclamation systems are exposed to high concentration of organics in the feed water that rapidly adsorb on membrane surface during the initial stage of operation. This condition enables use of chloramines to prevent membrane biofouling. With the presence of chloramines in the feed water in the concentration range of 2 4 ppm, salt passage increases at a rate of about 100% within 3 5 years of operation. This is an acceptable rate for majority of applications. Use of chloramines for other RO application, almost always results in very rapid salt passage increase. The possible reason for slower effect of chloramines on salt rejection change in wastewater application is presence of organic layer on membrane surface. Most likely, this organic layer reacts with chloramines, decreasing its concentration at the membrane surface. In addition, the organic matter present in the feed, plugs pinholes and other defects in membrane barrier, effectively reducing the salt transport. Presently, the use of chloramines in RO systems desalting tertiary municipal effluent is a common operational procedure. The operating conditions are designed to maintain chloramines level at 2 4 ppm in the feed to the membrane unit. If level of ammonia in feed water is too low to produce sufficient concentration of chloramines, ammonia could be added, usually in the form of ammonium chloride, adjacent to chlorine (or hypochlorite) injection point Colloidal fouling Ground water sources, even these contaminated by agricultural or industrial effluents, usually have very low concentration of colloidal matter. Water undergoes natural filtration during 167

31 passage through a sand layer in the underground aquifer. Brackish water RO systems very experience colloidal fouling. Presence of colloidal fouling in seawater systems will depend on process selection and operation of seawater pretreatment unit. In seawater systems utilizing conventional pretreatment, colloidal fouling is possible. In seawater systems utilizing membrane pretreatment (MF or UF) with good membrane integrity, the colloidal fouling is not very likely. Colloidal fouling is not a common phenomenon in membrane wastewater reclamation systems. Membrane filtration of the secondary municipal effluent, used as pretreatment, prior to RO, practically removes all colloidal matter. The other municipal source, effluent from membrane bioreactor (MBR), is practically of the same quality as tertiary effluent after MF or UF filtration. Presence of colloidal fouling, in RO units operating in municipal wastewater reclamation systems, is usually a result of lack of integrity of membrane barrier in the pretreatment system. In some cases, colloidal particles could form as a result of precipitation from a saturation solution of scale forming salts. Another possible source of colloidal particles could be coagulant used in the membrane filtration system. The coagulant particles could pass through micropores of MF membrane or form in the RO system as the feed volume is reduced and concentration of soluble metal ions increases into a saturation range. Presence of colloidal matter in RO feed could result in very rapid permeability decline. Colloidal matter, mixed with organics, forms on the membrane surface a very dense layer of low permeability. Colloidal particles could also become a crystallization centers for sparingly soluble salts that are at saturation, and induce formation of scale. Use of membrane filtration as a pretreatment step, prior to RO, would prevent in most case colloidal fouling of RO membranes Fouling by organic matter Membranes in both brackish and seawater RO desalination system could experience fouling by dissolved organics. However, the origin of organic matter is usually different in both cases. Majority of brackish water sources have very low concentration of dissolved organics. However, some low salinity well waters, treated by nanofiltrtaion system may have high concentration of dissolved organic matters, usually in the form of humic substances. Treatment of these water sources require use of special, low fouling, membrane elements. Clean seawater contains very low concentration of organic matter, usually below 1 ppm of TOC. However, seawater in the area of heavy marine traffic or in the areas containing oil fields, seawater could be contaminated with petroleum products. Exposure of RO membranes to feed water containing petroleum product will result in rapid decline of water permeability. Petroleum products in seawater have to be removed completely, below the detection limit, prior to introduction of feed water to membrane unit. The municipal and industrial wastewater contains high and variable concentrations of organics. Some fraction of the organics is in a form of suspended colloidal particles. Majority of 168

32 these will be removed during the pretreatment step of membrane filtration. The reduction of concentration of dissolved organics by MF or UF filtration is not significant. Some reduction of TOC, up to about 30% could be achieved if coagulation is applied prior to membrane filtration. The reduction mechanism is through adsorption of organic compounds on coagulant particles and subsequent removal by membrane filtration. In majority of cases, the effect of dissolved organics on membrane performance is reduction of water permeability, associated with some decrease of salt passage (improvement of salt rejection). The extend of water permeability decline and the decline rate depends on composition of organic mixture in the feed, characteristics of membrane surface and composition and ionic strength of feed water. In RO reclamation units with well functioning pretreatment, the long term water permeability decline is expected to be in the range of 30% - 40%. Usually, the decline irate is high at the startup, reaching 10% - 20% and then levels off with time. Accordingly, the feed pumping system has to be designed to provide sufficient feed pressure to compensate for the above decline of permeability over time. Except for permeability decline, fouling of RO membranes by dissolved organics, usually does not result in other adverse effect on membrane performance parameters (build up of feed channel pressure drop or increase of salt passage). In some isolated cases organic scale inhibitor, added to the feed to prevent scaling, could react with coagulant and foul the membrane. Some dissolved industrial organics may cause membrane swelling. Membrane manufacturers should be consulted regarding potential compatibility issue with industrial type feed constituents Biofouling Biofouling, which is the phenomena of bacterial film formation in RO systems, could represent a serious operation problem. Biuofouling manifest itself in increase of feed channel pressure losses and productivity decline. Systems contaminated with established bacterial grow are very difficult to clean and restore to the original performance level. The assumption is that every body of water contains microorganisms in equilibrium with the local nutrients supply. The water born microorganisms easily attached themselves to the surfaces in the RO systems and form colonies. The attachment to the surfaces is through excretion of extra cellular polymeric substance (EPS), composed mainly of polysaccharides. On wetted surfaces, the microbiological cells and surrounding EPS form biofilm, which can grow at rapid rate if sufficient nutrients and energy are available. The structures of biofilms are not uniform, depending on type of microorganisms and environmental conditions: ph, temperature, flow velocity, age and variety of other parameters. The biofouling occurrence is limited mostly to RO systems processing surface water (including seawater) or ground water from shallow wells. Majority of RO systems processing well water experience little or no biofouling. Number of procedures have been developed to identify 169

33 condition of availability of nutrient and to monitor biofilm grow. RO treatment of municipal effluents represents unique situation of high biofouling potential of RO feed water and development of effective way of controlling microorganisms population. In the past, municipal effluents were processed using cellulose acetate membranes. Therefore, it was a common practice to chlorinate RO feed water to prevent microorganisms grow. Due to presence of ammonia and high concentration of organics in the effluent the free chlorine was converted to chloramines. Today, almost all RO systems treating municipal effluents use composite polyamide membranes. Based on field experience it has been established that in wastewater applications salt rejection of composite membranes, in presence of chloramines concentration of 2 4 ppm, is sufficiently stable to provide 3 5 years effective membrane life. The above range of chloramines concentration is also sufficient to effectively mitigate membrane biofouling in RO unit Inorganic scale and determination of permeate recovery rate. The concerns for potential of scale formation in RO systems are related to the very nature of the RO process: removal of water from the feed stream and increase of concentration of the ions present in the feed water. During the RO process concentration of all constituents increases due to reduction of the feed water volume. This increase of concentration, expressed by concentration factor (CF) is function of permeate recovery. CF = 1/(1 R) (1.1) There is possibility for some of the dissolved constituents to nucleate and 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 170

34 conditions (in equilibrium with a solid phase of a given salt). Ksp has a specific value for each salt and it is function of temperature and ionic strength of the solution. In RO seawater desalination systems concentration of scaling constituents is very low and the solubility is significantly increased due to high ionic strength. Therefore, scaling in seawater RO membrane units is very unlikely. Brackish water sources could have significant concentration ions that could form scaling compounds. Usually, the recovery rate of brackish RO systems is limited by the saturation limits of sparingly soluble salts. The salts of concern are calcium carbonate, calcium sulfate, strontium sulfate, barium sulfate and silica. In wastewater reclamation systems, the salt of concern is mainly calcium carbonate, less frequently calcium phosphate. In some isolated cases, potential for precipitation of barium sulfate has to be considered. Calcium carbonate is the most common scaling constituent in natural occurring waters and also in wastewater. However, it is also the easiest to control either with ph adjustment or use of scale inhibitor. In solution, calcium ions are in equilibrium with bicarbonate and carbonate species as shown in the following equations: H2CO3 = H + + HCO3 - (5.1) HCO3 - = H + + CO3-2 (5.2) Ca +2 + CO3-2 = CaCO3 (5.3) At sufficiently high concentrations of Ca +2 and CO3-2, crystallites of CaCO3 could nucleate and form a scale. The calcium carbonate system is quite complex. Saturation conditions are not just function of concentrations of Ca, CO3 and HCO3 ions but also influenced by concentration of hydrogen ion (ph) and other ions that contribute to water alkalinity. Attempts to define relations for saturation conditions in potable water networks lead to development of a number of saturation indexes. The calcium carbonate saturation index developed by Langelier for potable water networks has been adopted by RO industry as an indicator of saturation conditions in concentrate stream of brackish water RO systems. The Langelier Saturation Index (LSI) is calculated according to relations: LSI = ph - phs (5.4) Where ph I is the actual ph of the water and phs is ph that corresponds to saturation concentrations of ions forming calcium carbonate. 171

35 K2 = [H + ] X [CO3-2 ]/[ HCO3 - ] (5.5) Ksp = [Ca +2 ] X [CO3-2 ] = [Ca +2 ] X[HCO3 - ] X K2/ Hs + (5.6) Hs + = [Ca +2 ] X [HCO3 - ] X K2/ Ksp (5.7) - log[hs + ] = phs = - log[ca +2 ] - log[hco3 - ] + log [Ksp/ K2] (5.8) LSI = ph - phs = ph - pca - palk +pk (5.9) Where K2 is second dissociation constant to carbonic acid (H2CO3), Ksp solubility constant of calcium carbonate at given ph and temperature. Other parameters represent molar concentrations of relevant species in the solution. Few years after introduction by Langelier the saturation index was modified to account for ionic strength in a form of correction factor proportional to water salinity. phs = (9.3 + A + B) - (C + D) (5.10) where: A = (Log10 [TDS] - 1) / 10 (5.11) B = Log10 ( C + 273) (5.12) C = Log10 [Ca +2 as CaCO3] (5.13) D = Log10 [alkalinity as CaCO3 ] (5.14) The parameter A is related to ionic strength of the solution. The value of A increases with increase of salinity. Parameter B reflects the changes of calcium carbonate solubility and changes of equilibrium of carbonic acid dissociation with temperature. The value of B decreases with temperature increase. In practical applications the LSI is either calculated using computer programs or monograms based on ph and composition of concentrate stream. Water solution has potential for CaCO3 scaling at LSI > 0 and it is assumed that saturation prediction using LSI is reliable up to salinity of about 5000 ppm TDS. For very high water salinity and seawater applications the LSI was modified to account for increased ionic strength by Stiff and Davis. The Stiff and Davis saturation index (SDSI) introduces empirical constant K in calculations phs to account for high ionic strength of seawater 172

36 concentrates. Stiff and Davis determined value of K experimentally in the high range of ionic strength, that covers salinities encountered in RO seawater applications. SDSI = ph (9.3 + K pca palk) (5.15) Where ph has the same meaning as in LSI equation and K is a constant found in monograms. For high salinity solutions (seawater concentrates) the SDSI value is about units lower then calculated according to the LSI relations. In low salinity applications the values of LSI and SDSI are similar. Example Calculation of Langelier saturation index Brackish water system is design to operate at recovery ratio of 80%. Feed water feed has TDS = 1700 ppm, ionic strength 0.04 and the following concentrations of the relevant ions: Ca = 300 ppm, HCO3 = 250, ph = 7.3, temp = 25 C. After acidification and ph adjustment to 6.5, HCO3 = 166 ppm, CO2 = 78 ppm Concentration factor for 80% recovery rate, CF = 1/(1-R) = 5 Approximate concentrations in the concentrate: TDS = 8500 ppm, ionic strength 0.18, Ca = 1500 ppm, HCO3 = 830 ppm, CO2 = 78 ppm Calculation of concentrate ph. ph = pk1+ log ([HCO3]/[CO2] K1 is the equilibrium constant of the carbonic acid dissociation reaction. For water temperature of 25 C and dilute solutions K1 value is 4.2X10-7, pk1 = Then the Calculation of phs and LSI Following equations : phs (feed) = = 7.48 LSI (feed) = = phs (concentrate) = = 5.97 LSI (concentrate) = = 1.29 The LSI provides qualitative indication about saturation condition of solution that contains calcium and carbonate ions. The LSI does not enable prediction about quantity of CaCO3 that could potentially precipitate from solution. The saturation index that provides indication about quantity of CaCO3 that could precipitate is the calcium carbonate precipitation potential (CCPP). The calculations of CCPP are more laborious than calculations of LSI. The calculations of CCPP 173

37 are usually conducted using specialized computer programs. The saturation relation of calcium sulfate is simpler to calculate than the one for calcium carbonate. For RO calculations it is assumed that its solubility depends only on concentrations of calcium and sulfate ions, temperature and ionic strength. The effect of ionic strength on solubility of calcium sulfate is quite significant as shown in the following example of calculations of saturation indexes (SI) of CaSO4 for water salinities corresponding to brackish and seawater RO systems: Example Calculation of calcium sulfate saturations For CaSO4; Ksp = (temp/25) X1.8X10-3 X IS 0.75 Brackish water system R = 75%, TDS of concentrate = 4000 ppm, IS = 0.07 Ca = 900 ppm, SO4 = 2400 (concentrations in concentrate) Ksp = 2.5X10-4 SI = ([900/40000] [2400/96000])/2.5X10-4 = 2.25 Seawater system R = 50%, TDS of concentrate = 80,000 ppm, IS = 1.60 Ca = 900 ppm, SO4 = 6000 ppm (concentrations in concentrate) Ksp = 2.6X10-3 SI = ([900/40000] [6000/96000])/2.6X10-3 = 0.54 In the example above the product of calcium and sulfate ions concentrations in solution is much higher in concentrate stream of the seawater system as compared to the brackish system. However, due to the differences of ionic strength, the calcium sulfate is above saturation level in the brackish system and below saturation in the seawater system. In wastewater reclamation systems calcium sulfate practically never reaches a saturation potential due to limits of calcium and sulfate ions concentrations in potable water supply. In some isolated cases barium sulfate could be a recovery rate limiting constituents. Solubility of barium sulfate is five orders of magnitude lower than solubility of calcium sulfate. Another constituent that sometimes can present problem in RO applications is silica. Silica concentration in wastewater effluents will depend on its concentration in potable water, which is low at majority of locations. Silica can be present both in colloidal and reactive (soluble) form. In the past, the safe limit of silica concentration in concentrate was considered as being about ppm (as SiO2). In the last decade a new scale inhibitors were introduced that are effective in maintaining much higher concentration of silica in solution. Some suppliers of these 174

38 specialty inhibitors claim safe limits for silica concentration as high as 300 ppm. When treating brackish sources with significant silica concentration an extreme caution should be exercised with maintaining the designed recovery rate as silica scale is very difficult to remove from membrane elements. Beside calcium carbonate, the major scaling compound of concern in RO wastewater reclamation systems is calcium phosphate. Calcium phosphate presence is almost uniquely associated with household or industrial effluents at various concentrations. In majority of RO system treating municipal effluents concentration of phosphates is low and does not result in membrane scaling. The chemistry of calcium phosphate compounds is quite complex and precipitants of calcium and phosphates can be formed as number of chemical formulations of different solubility. The prevailing assumption is that most likely scale forming compounds are: calcium phosphate dihydrate (DCPD-Brushite) and Tricalcium phosphate (TCP). At locations where phosphates concentration is high, formation of scale during RO operation could be prevented by using scale inhibitors or acidification. The effectiveness of scale inhibitor in prevention of formation of calcium phosphate scale is not well documented. Results of one experimental work of evaluation of commercial scale inhibitors indicate very low effect on rate of scale formation. Reduction of feed ph is very effective in prevention of phosphate scaling. The solubility limits for DCPD were developed experimentally and follows the equation that provides maximum value for Ca and P product in solution according to concentration of hydrogen ion (ph): Ca X P = X [H + ] (5.17) For TCP an empirical relation for calculation of saturation index was developed by BETZ Company: log Ca log( PO4 ) 2 log( t) Index ph (5.18) 0.65 Where ph Ca PO4 t - ph of the concentrate - Calcium concentration in the concentrate, ppm CaCO3 - Phosphate concentration in the concentrate, ppm - Water temperature, C Positive value of the phosphate saturation index indicates that scaling could occur. Scaling can be controlled by use of scale inhibitor or acidification of feed water. Table provides values of concentrate ph required for maintaining phosphate saturation 175

39 index below the threshold value of 1 at the recovery rate range of 75% - 85% for the concentration of calcium and phosphate in the feed water and concentrate as listed below. Table Controlling phosphate scaling through ph adjustment Parameter Feed water R = 75% R = 80% R = 85% ph Ca, ppm P, ppm PO4, ppm Phosphate Saturation Index (Eq ) However, acidification of RO feed, required to prevent formation of calcium phosphate scale, could be very expensive if significant quantity of acid would be required due to high alkalinity concentration. Example Calculation of quantity of acid required to prevent calcium phosphate scaling. For concentration of bicarbonate ion [HCO3 - ] in feed water is 300 ppm as CaCO3, calculate quantity of sulfuric acid required: a) To reduce feed ph from 8.1 to 7. b) To bring ph of concentrate from feed ph of 8.1 to the ph values listed in Table 3. Feed water temperature 25 C. Values of K1, K2 and KW are: 4.45X10-7, 4.42X10-11 and 9.17X10-15 Calculate concentration of [CO2], [CO3 = ], [H + ] and [OH - ] in feed according to the following equations: H CO 2 HCO 3 (5.19) K 2 K2 3 H ph H 1 HCO3 CO (5.20) 10 (5.21) 176

40 ( ph pkw) OH 10 (5.22) According to the above relations, the concentrations (mol/l) are: [H + ] = 7.94X10-9 [OH - ] = 1.15X10-6 [HCO3 - ] = 6.00X10-3 [CO2] = 1.07X10-4 [CO3 = ] = 3.34X10-5 The concentration of total alkalinity (AT) and total carbonate species (CT) are calculated according to equations 5.23 and The assumption is made that contribution of phosphate species to alkalinity can be neglected. AT = [OH - ] + [HCO3 - ] + 2[CO3-2 ] [H + ] (5.23) CT = [HCO3 - ] + [CO3-2 ] + [CO2] (5.24) Accordingly AT = 6.07X10-3 CT = 6.14X10-3 Acidification to adjust ph has no effect on CT. Therefore, new concentration of CO2 can be calculated according the equation [ CO 2 C T ] K1 K1K 1 [ H ] [ H ] 2 2 (5.25) [CO2] = 1.13X10-3 mol/l (49.7 ppm) Concentrations of other components of the alkalinity are calculated according to equations The total feed alkalinity and total inorganic carbon at ph = 7 are listed below: AT1 = 5.02X

41 CT1 = 6.14X10-3 Quantity of sulfuric acid required to acidify feed water from ph of 8.1 to ph of 7.0 is (AT AT1)X eqv H2SO4 =(6.07X X10-3 ) X49000 = 51 mg/l (as 100%) The next step is to estimate what should be the feed ph required for a required ph of the concentrate. An approximate value can be derived by calculation of concentrate ph for a designed recovery rate. The calculations of alkalinity species and ph of the concentrate are based on the assumption that CO2 is not rejected by the membrane, i.e. concentration of CO2 in feed and concentrate are the same, and the other dissolved species are rejected completely. Accordingly concentration of carbonate species in concentrate, CTC will be: CTC = (CT1 [CO2])/(1-R) + [CO2] (5.26) For a concentration of carbonate species at feed water ph of 7.0 and recovery rate R = 80% CTC = (6.14X X10-3 )/(1-0.80) X10-4 = 3.03X10-2 mol/l Substituting the value of CTC and [CO2] in equation 5.25, concentration of [H+] and phc of the concentrate can be calculated. phc = 7.8 Accordingly, for concentrate ph, listed in Table the required feed ph should be about 0.8 units lower for the determined ph of the concentrate.. Following the calculations of alkalinity and quantity of sulfuric acid for feed acidification, shown in the first part of this example, the quantity of sulfuric for required ph of the concentrate, starting with feed ph of 8.1, is given in Table Table Quantity of sulfuric acid required to maintain concentrate ph below the threshold value of saturation index of calcium phosphate. Recovery rate, % Required concentrate ph

42 Required feed ph Dosing rate of H2SO4, ppm (100%) Contribution to product water cost, $/m3 at sulfuric acid cost of $500/t The results listed in Table indicate that cost of acidification, to prevent calcium phosphate scaling, could be quite significant and should be consider in the early stages of evaluation of project feasibility. The solubility of sparingly soluble salts is affected by ionic strength. The Ksp values (at 25 C and for low ionic strength solutions) or concentration limit of common salts that could form scale in RO systems are listed in Table Table Ksp or concentrations limit of scale forming compounds common to RO Compound Formula Ksp or concentration limit, (ppm) Calcium sulfate CaSO4 2.5X10-5 Barium Sulfate BaSO4 2.0X10-10 Tricalcium Phosphate Ca3(PO4)2 2.8X10-30 Calcium Phosphate Dihydrate CaHPO4 2H2O 2.2X10-7 Calcium Carbonate CaCO3 LSI < 0, S&DSI < 0 Reactive Silica H4SiO4 ( ppm) The potential for formation of calcium sulfate scale and blocking of feed channels in the spiral wound element is demonstrated in the following example: Example Calculation of calcium sulfate scaling potential in the RO concentrate. For the following concentration of Ca and SO4 in the concentrate calculate scaling potential of calcium sulfate: [Ca] = 1000 ppm = mol [SO4] = 2400 ppm = mol Ksp = 2.25x10-4 Saturation [Ca] = [SO4] = (2.25x10-4)^1/2 = mol Excess [Ca] = [SO4] = mol 179

43 MW CaSO4 = 40,000+96,000 = 136,000mg/l Excess [CaSO4] = mol = 0.010X136,000 = 1,360 mg/l The RO system operates at 80% recovery rate. At average flux of 26l/m 2 -hr, 7 elements per vessel the concentrate flow in the last element is: 26X36.8X7X(1-0.80)/0.80 = 1674 l/hr Potential CaSO4 deposit = 1674X1.36 = 2276 g/hr = 2.28 kg/hr Assuming specific density of CaSO4 = 4g/cm3, volume of excess CaSO4 that could precipitate is RO element is (2.28 X1000)/4 = 570 cm3/hr. Free volume of feed channels in SW element is: 100 cm X100 cm X0.075 cm X 20 = cm 3 (about 50%) The above calculations shows that at the saturation conditions scale could lead in a short time to complete blockage of tail elements. Potential for scale formation is extremely important issue in brackish and wastewater reclamation applications. At some locations, it may determine the maximum recovery rate for the RO systems. Due to variability of water compositions and limited level of understanding of the relevant salt solutions systems at saturation in RO conditions, it is quite difficult to make accurate predictions about scaling. RO industry adopted limits for individual salts based literature data and some field experience (Table Conservative limits). Due to lack of accurate analytical models, developed for RO applications, these limits include significant safety margins. Manufacturers and suppliers of scale inhibitors are continuously introducing new scale inhibitors that enable operation at higher levels of concentrations then those initially proposed by membrane manufacturers (Table Possible limits). So far, the experience with the subsequent products introduced over the years was quite positive. Seldom could any problems of scale formation be related to malfunction of scale inhibitor if applied according to manufacturer specifications. Scale inhibitors prevent scale formation by retardation of the nucleation process of scale forming crystals. The mechanism of prevention of crystal grow is either through threshold effect, crystal structure distortion, dispersion or sequestration. The dosing rate of scale inhibitor is determined by supplier of chemicals based on feed water composition and recovery rate. Feed water analysis should include information on concentration of iron. High concentration of iron containing compounds in concentrate may reduce effectiveness of some scale inhibitors. Required concentration of scale inhibitor seldom exceeds 10 ppm of active ingredient in the concentrate stream. Table Practical limits of saturation values in RO applications 180

44 Compound Formula Ksp Conservative limits Possible limits (*) Calcium sulfate CaSO4 2.5X X Ksp 4 X Ksp Barium Sulfate BaSO4 2.0X X Ksp 100 X Ksp Reactive Silica H4SiO4 Not defined 160 ppm 250 ppm Calcium Carbonate CaCO3 LSI < 0 S&DSI < 0 LSI < 1.8 S&DSI < 0.5 LSI < 2.5 S&DSI < 1.5 Calcium Phosphate Ca3PO4 CPSI < 1.0 < 1.0 (*) Saturation limits recommended by suppliers of scale inhibitors In brackish and wastewater reclamation RO units the designed recovery rate is determined based on composition of feed water and saturation limits of sparingly soluble salts, as listed in Table For brackish applications, the common approach is to run computer projection program of one of major manufacturers of RO membrane elements, which will flag the saturation limits. The next step is to contact number of suppliers of scale inhibitors to get their opinion about possible maximum recovery rate with a given feed water composition. The ultimate decision is made by the system designer based on his confidence level, but the common practice is to follow recommendations of supplier of scale inhibitors. In seawater applications the recovery rate is limited by the osmotic pressure of the concentrate, the feed pressure and required permeate salinity. The common practice is to design seawater RO units for feed pressure not to exceed 82 bar (1200 psi). The preferred feed pressure limit is 70 bar (1000 psi). Up to 82 bar a standard pressure vessels made of polymeric materials are available. Above 82 bar feed pressure, special pressure vessels made of stainless steel tubing are required, which in majority of cases would result in significant increase of system cost. The common range of permeate recovery rate in RO application is listed in Table Table Common ranges of permeate recovery rate in RO applications. Membrane type Feed water source Common range of permeate recovery rate Nanofiltration membranes Low salinity wells 85% - 90% Brackish membranes Brackish wells 75% - 85% Brackish membranes Tertiary effluent 80% - 85% Brackish membranes Second pass (RO permeate feed) 85% - 90% Seawater membranes Atlantic Pacific Ocean 50% - 55% 181

45 Seawater membranes Mediterranean 45% - 50% Seawater membranes Arabian Gulf 40% - 45% Effect of recovery rate on system operating parameters is summarized in Table Table Process parameters affected by permeate recovery rate Permeate salinity Increased recovery rate results in higher salinity of permeate Feed pressure Higher recovery rate results in higher feed pressure System size Higher recovery rate results in smaller size of flow related equipment (feed water supply, pumps and piping). Has no effect on number of membrane elements. Membrane fouling rate Higher recovery rate may result in higher rate of fouling including scaling Frequency of membrane cleaning Higher recovery rate may result in higher frequency of membrane cleaning due to higher rate of fouling 5.6. Average permeate flux Similarly to the permeate recovery rate, permeate flux is very important operational parameters that determines stability of membrane unit operation and the economics of the desalination process. The RO membrane units are designed based average permeate flux, which is the unit permeate capacity divided by total membrane area installed in the system. The selected permeate flux range should result in stable operation of the membrane unit. The range of average permeate flux is part of membrane manufacturer design recommendations. The average permeate flux range is selected by the system designer based on feed water source which is related to the expected feed water quality. The common ranges of average permeate flux according to water source is listed in Table Table Common ranges of average permeate flux rate in RO applications. Membrane type Feed water source Common range of average permeate flux, l/m2/hr Nanofiltration membranes Low salinity wells or surface water with membrane pretreatment Nanofiltration Surface water conventional membranes pretreatment Brackish Brackish wells membranes Brackish membranes Tertiary effluent

46 Brackish membranes Seawater membranes Seawater membranes Seawater membranes Second pass (RO permeate feed) Open intake conventional pretreatment Open intake membrane pretreatment Beach wells The value of permeate flux affects performance of the membranes and economics of the desalination process. Process parameters affected by the value of permeate flux are listed in table Table Process parameters affected by permeate flux Permeate salinity High flux rate results in lower salinity of permeate Feed pressure High flux rate results in higher feed pressure System size High flux rate results in smaller size of membrane unit due to lover number of membrane elements and pressure vessels Membrane fouling rate High flux rate results in higher rate of fouling Frequency of membrane cleaning High flux rate results in higher frequency of membrane cleaning due to higher rate of fouling 5.7.Membrane unit design procedure The objective of membrane unit design is to design a system that will produce required quantity and quality of product water in the whole range of feed water quality and temperature as listed in project specifications. The designed have to consider the most extreme conditions of raw water quality, salinity and temperature in defining membrane unit configuration and the operating parameters. The basic design parameters are recovery rate, average permeate flux, membrane type and array, train sizes, number of trains and configuration, selection of major equipment (pumps, instrumentations, etc..) and unit layout Permeate capacity and permeate quality limits The membrane unit is sized according to specification of system capacity, taking into 183

47 consideration plant load factor and possible schedule of product water flow delivery fluctuation. The load factor is based on planed system down time (membrane cleaning, equipment maintenance, etc..) and additional time set up for emergencies (safety margin). Usually down time imposed by the client and power outages are excluded from the above considerations. Well operated system could have load factor as high as 95%, however, majority of the desalination systems have load factor in the range of 90% - 95% of the available operating time. Permeate quality limits are defined by specification of the final product water quality. In defining the limits of permeate quality, considerations must be given to the following conditions: - Fluctuation of raw water quality - Fluctuation of raw water temperature - Aging of membrane elements and expected increase of salt passage - Increase of product water salinity due to addition of chemicals during the permeate stabilization process. - Safety margin applied to mitigate project risk factor. Permeate quality is calculated utilizing computer programs provided by membrane manufacturers. The programs calculate required feed pressure and permeate quality. System designer has to clarify with membrane manufacturer if the values calculated by the computer program will be accepted as the membrane manufacturer warranty limits or additional safety margin will be applied Selection of average permeate flux, recovery rate and array The average permeate flux and recovery rate are determined according to the application type, source of raw water and type of pretreatment applied. The array of pressure vessels is function of number of stages and system size. The representative ranges of average flux rate, recovery rate, number of stages and number of elements per vessel for typical RO applications are listed in Table The average flux rate is determined based on water source and pretreatment type, which stipulates expected feed water quality. The recovery rate of nanofiltrtation, brackish and wastewater applications is determined based scaling potential of feed water. In seawater applications the recovery rate is in the range of 40 55%, increasing with decreased salinity of seawater source. The number of stages is function of recovery rate and number of elements per vessel. Some nanofiltrtaion membrane units that operate at recovery rate of 90% could have up to 4 stages. With number of stages of 3 to 4, the recommended number of elements per 184

48 pressure vessel is 6. This is in order to maintain acceptable pressure drop along the membrane unit. However, membrane unit includes interstage booster pump, than the number of elements per vessel can be increased to 7. Table Range of RO design parameters according to application and feed water source Application Feed water source Nanofiltration Low salinity wells Brackish Brackish wells Wastewater Secondary reclamation effluent with membrane filtration Second pass Second pass RO (RO permeate feed) Seawater Surface intake Seawater Beach wells or intake with membrane filtration Average flux range, l/m2/hr Common range of permeate recovery rate Number of stages Number of elements per vessel % - 90% % - 85% % - 85% % - 90% % - 55% % - 50% Membrane units in brackish systems that operate at recovery rate up to 85% are configured as 2 3 stage units. At recovery rate higher than 85%, usually a 4 th stage is added, frequently with a dedicated booster pump. For configuration of 2 3 stages the number of elements per vessel could be 6 7, usually 7 element configuration for a 2 stage unit. Wastewater reclamation membrane units are configured in similar way as in brackish applications. Membrane units operating as a second pass RO, treat very clean feed water (RO permeate). Therefore, the common limitations of average flux rate and recovery are not applicable here. In most cases the second pass membrane units are configured as a two stage units with 7 8 elements per vessel. Presently, majority of seawater RO membrane units are configured as a single stage units with number of elements per vessel

49 Selection of membrane type Considerations for selection of membrane elements include type of application, parameters of feed water and required performance in respect of permeate quality and feed pressure. Membrane are categorized according to applications they are effective: Nanofiltration for color removal Nanofiltration for sulfate reduction Nanofiltration for hardness reduction Low pressure brackish RO Low fouling brackish High rejection brackish RO Low pressure seawater RO High rejection seawater RO Most of the membrane manufacturers offer membrane elements in all above categories with very similar performance. The membrane elements from different manufacturers have the same dimensions and are interchangeable in pressure vessels. Therefore, the EPC or end user have significant flexibility with selection of membrane supplier among major membrane manufacturers. Table includes examples of correlation between membrane elements and applications. Except for very specific nanofiltrtaion applications, in other application there is number of membrane elements type that can be selected. The listing provided in Table is only representative. Additional commercial membrane elements are available from major manufacturers. Table Examples of representative membrane elements models according to applications Application Membrane selection Representative membrane Reduction of color and organics Loose nanofiltration (tight UF) HYRACoRe Reduction of sulfates Loose nanofiltration SR

50 Reduction of hardness, sulfates, iron and organics Desalting low salinity brackish water Wastewater reclamation Desalting high salinity brackish, nitrate reduction Desalting low salinity, low temperature seawater Desalting high salinity, high temperature seawater Regular nanofiltration, Low fouling nanofiltration Low pressure brackish RO Low pressure brackish RO, Low fouling RO High rejection brackish RO Low pressure seawater RO High rejection seawater RO NF90, NF-270, ESNA-LF, SU620F ESPA4+, ESAP2+, BW30XLE-440, TMG ESPA2+, LFC3, BW30-400FR, TMG BW30-400, ESPA2, CPA3, SW30XLE-400, SWC5 SW30HR-380, SWC4+ Table Representative offering of nanofiltration membrane elements Element model Hydracore ESNA-LF SU620F NF-90 NF-270 Membrane area, m2 (ft2) 37.1 (400) 37.1 (400) 37.1 (400) 37.1 (400) 37.1 (400) Permeate flow, m3/d (gpd) 31.0 (8,200) 29.5 (7,800) 21.9 (5,800) 37.9 (10,000) 47.3 (12,500) Salt rejection, Test flux rate, l/m2- hr (gfd) Permeability, l/m2- hr-bar (gfd/psi) 34.8 (20.5) 7.7 (0.31) 33.2 (19.5) 24.7 (14.5) 42.5 (25.0) 55.9 (32.9) 7.2 (0.29) 8.7 (0.35) 11.9 (0.48) 15.7 (0.63) 187

51 Relative salt transport: salt passage*flux rate 17.4 (10.2) 6.6 (3.9) 11.1 (6.5) 1.3 (0.8) 1.7 (1.0) Table Representative offering of brackish membrane elements (*) Element model ESPA2+ ESPA4+ TMG BW30- XLE440 BW30 LE- 440 Membrane area, m2 (ft2) 40.0 (430) 40.0 (430) 40.0 (430) 40.9 (440) 40.9 (440) Permeate flow, m3/d (gpd) 41.6 (11,000) 49.2 (13,000) 41.6 (11,000) 48.1 (12,700) 48.1 (12,700) Salt rejection, Test flux rate, l/m2-hr (gfd) Permeability, l/m2-hr-bar (gfd/psi) (25.6) 51.3 (30.2) 43.5 (25.6) 49.1 (28.9) 49.1 (28.9) 5.0 (0.20) 8.2 (0.33) 6.2 (0.25) 7.7 (0.31) 6.0 (0.24) Relative salt transport: salt passage*flux rate (0.153) (0.181) (0.128) (0.289) (*) Brackish membrane elements are also used for wastewater reclamation applications (0.202) Except for nanofiltration elements offering, where the range of element performance is relatively wide, in the brackish and seawater groups of element offering the performances of elements belonging to the same group are very similar. 188

52 The element nominal performances are defined through limited number of parameters. These are: membrane area, permeate flow and salt rejection. The nominal permeate flow and nominal salt rejection values depend not only on membrane property but also on nominal test conditions. The nominal test conditions applied in testing of seawater elements are similar. However, the testing conditions for brackish elements vary significantly in respect of feed salinity and feed pressure. For this reason, comparison of membrane elements in a given category cannot be based nominal performance alone. Good indicator for comparison of various membrane elements belonging to the same category (i.e. brackish, seawater..) are parameters of water permeability (also called specific flux) and relative salt transport. Table Representative offering of seawater membrane elements Element model Membrane area, m2 (ft2) Permeate flow, m3/d (gpd) SWC4+ SWC5 TM SW30HR-LE SW30HR- XLE 37.1 (400) 37.1 (400) 37.1 (400) 37.1 (400) 37.1 (400) 24.6 (6,500) 34.1 (9,000) 24.6 (6,500) 26.5 (7,000) 34.1(9,000) Salt rejection, Test flux rate, l/m2-hr (gfd) Permeability, l/m2-hr-bar (gfd/psi) Relative salt transport: salt passage*flux rate 27.6 (16.3) 38.2 (22.5) 27.6 (16.3) 31.3 (18.4) 38.2 (22.5) 1.0 (0.04) 1.5 (0.06) 1.0 (0.04) 1.2 (0.05) 1.5 (0.06) (0.032) (0.045) (0.041) (0.046) (0.067) The water permeability (WP) is calculated by dividing element nominal permeate flux by the net driving pressure (NDP). The units of WP are l/m2/hr/bar. 189

53 Nominal permeate flux (NFLUX) is the nominal permeate flow (NPF) divided by the membrane area of the element (MA). NFLUX = NPF/MA (5.27) NDP = Pf Pos 0.5* DP (5.28) WP = NFLUX/NDP (5.29) Where Pf is nominal test feed pressure Pos is osmotic pressure of average feed solution during the test DP is pressure drop along the element during test, usually 0.2 bar. The relative salt transport (RST) is calculated as a product of nominal flux and salt passage (SP). RST = NFLUX * SP Example 5.1. Calculation of water permeability and relative salt tratnsport Nominal element performance Permeate flow 48 m3/day Salt rejection 99.5% Membrane area 40 m2 Nominal test conditions Feed salinity 2,000 ppm NaCl Feed pressure 15.5 bar Feed temperature 25 C Recovery rate 15% NFLUX = 48 * 1000/(24 * 40) = 50 l/m2/hr AFS = 2000 * (1 + 1/(1 0.15))/2 = 2,176 ppm NaCl Pos = *2176 = 1.65 bar NDP = = bar WP = 50/13.75 = 3.63 l/m2/hr/bar RST = 50 * ( ) = 25 Values of WP and RST are indicative of membrane performance at field conditions. RO system utilizing membrane elements with higher value of water permeability will require lower feed pressure. RO system utilizing membrane elements with higher value of relative salt transport will produce permeate of higher salinity. 190

54 The values of WP and RST are related. Higher value of WP is usually associated with higher value of RST Membrane train size and configuration Size of RO membrane train is determined based on system permeate capacity, desired flexibility of variability of output capacity and convenience of system maintenance (membrane cleaning). Very small systems up to few thousands m3/day capacity are usually designed as a single train units. The contingency capacity is realized by maintaining sufficient supply of spare equipment, either installed or stored in the plant warehouse. Larger systems are built as a multi train configurations. Depending on size the number of membrane trains can vary from 2 to In principle, larger number of trains results in increasing system cost. The additional cost results from pumping equipment, valves, instrumentation and control equipment associated with each train. In addition to consideration of flexibility of water production, the train size is also determined by the logistic of membrane cleaning. Membrane train with large number of pressure vessels will require large size cleaning systems to conduct membrane cleaning. This is one of the reason that the train size is limited to about 200 pressure vessels in a very large RO systems. In large capacity brackish and nanofiltration systems two hydraulic configurations of membrane trains are possible, as shown on Figure

55 Stag 1 Stage 1 Stag 2 Stag 3 Stage 2 Stage 1 Stage 1 Stage 3 Stage 3 Stage 1 Stage 2 Stage 1 Stage 3 Satge 3 Stage 3 Stage 3 Stage 2 Stage 3 Stage 2 Stage 1 Stage 1 Stage 2 Stage 3 Stage 2 Figure Alternative configuration of membrane trains in large capacity RO plants. Accordingly, the subsequent desalination stages (1 3) can be aggregated together and form integrated membrane trains (left configuration). Alternatively, the last stages can be grouped together forming a separate train(s) that process the combine concentrate from trains consisting of stages 1 and 2. The second alternative reduces operational flexibility of water production but provides some saving of the system capital cost. Regarding the type of pressure vessels used, it is current tendency to utilize more multiport pressure vessels over traditional side port configuration in order to reduce cost of RO trains as it is discussed in section Utilizing computer programs in membrane unit design The calculations of projected performance of RO membrane unit is conducted utilizing computer programs developed by membrane manufacturers. The information on membrane products and computer programs for projection of membrane unit performance are available, free of charge, 192

56 form all commercial RO membrane manufacturers through their web pages. Various performance projection programs are quite similar in functionality, design of user interface, input values required and output format. The computer program provides projected values of feed pressure and composition of permeate and concentrate streams based on system design parameters and according to nominal performances of selected membrane element. The input to computer program includes the following information: - Feed water analysis including water ph and temperature - Type of water source - Required permeate flow - Designed recovery rate - Selection element type - Membrane array (number of stages, number of pressure vessels per stage, number of elements per vessel) - Fouling factor or membrane age - Feed water ph - Feed water temperature Figure Computer projections program water analysis data entry screen Composition of feed water is entered through water analysis screen (Figure ). The program converts concentration units, if necessary, and enables adjustment of positive negative ions balance. Following the entry to the water analysis screen, the next step is to entry process parameters, select membrane model type and membrane array in the membrane unit design screen (Figure 5.7.3). 193

57 Figure Computer projection program process parameters and membrane array entry screen After entering the required permeate flow and recovery rate, the membrane element type is selected from the look up window (figure 7.7.4) and the array is being adjusted till the average permeate flux value is within the range recommended for the type of feed water being processed. Partial results of calculations are displayed on the screen (Figure 5.7.5) and complete results are provided through a printout. In addition to the calculated process parameters, both the screen display of results and the printout include warnings if some of the process parameters are outside limits recommended by the membrane manufacturer. If the warnings are displayed the design of the membrane has to be modified. In case if warnings pertain to excessive saturation limits than the usual approach is to reduce feed water ph (if calcium carbonate scaling is of concern) or reduce recovery rate if other constituents (CaSO4, SiO2,..) has high potential for precipitation from the concentrate stream. 194

58 Figure Computer projection program membrane elements look up table 195

59 Figure Computer projections program screen display of calculation results The performance projection printout (Figure 5.7.6) provides detail information on the projected performances that include pressures, chemical composition and hydraulic parameters of the feed, permeate and concentrate stream. The printout also includes information on the dosing rate of acid or caustic designed to be used for ph adjustment of the feed stream. Some of the computer programs provide option to calculate energy requirement of the high pressure feed pumps and energy recovery devices. Addition calculations option included in some program provides capability to calculate chemical dosage in the post treatment process required to stabilize permeate to prevent corrosion of product water distribution piping system. 196

60 BASIC DESIGN RO program licensed to: Calculation created by: Project name: Low salinity Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: 10.6 bar Permeate recovery: 85.0 % Feedwater Temperature: 25.0 C(77F) Feed water ph: 7.0 Element age: 3.0 years Chem dose, ppm (100%): 0.0 H2SO4 Flux decline % per year: 7.0 Fouling factor: 0.80 Salt passage increase, %/yr: 10.0 Average flux rate: 24.8 lm2hr Feed type: Well Water Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar LEWABRANE x7 01S LEWABRANE1 01S 70 10x7 Raw water Feed water Permeate Concentrate Ion mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 3% 3% 33% SrSO4 / Ksp * 100: 0% 0% 0% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 17% 17% 113% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 1.1 bar 1.1 bar 7.1 bar 197

61 BASIC DESIGN RO program licensed to: Calculation created by: Project name: Low salinity Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: 10.6 bar Permeate recovery: 85.0 % Feedwater Temperature: 25.0 C(77F) Feed water ph: 7.0 Element age: 3.0 years Chem dose, ppm (100%): 0.0 H2SO4 Flux decline % per year: 7.0 Fouling factor: 0.80 Salt passage increase, %/yr: 10.0 Average flux rate: 24.8 lm2hr Feed type: Well Water Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar LEWABRANE x7 01S LEWABRANE1 01S 70 10x7 Stg Elem Feed Pres Perm Perm Beta Perm Conc Concentrate saturation levels no. pres drop flow Flux sal osm CaSO4 SrSO4 BaSO4 SiO2 Lang. bar bar m3/hr lm2hr TDS pres Stag e NDP bar

62 BASIC DESIGN RO program licensed to: Calculation created by: Project name: Low salinity Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: 10.6 bar Permeate recovery: 85.0 % Feedwater Temperature: 25.0 C(77F) Feed water ph: 7.0 Element age: 3.0 years Chem dose, ppm (100%): 0.0 H2SO4 Flux decline % per year: 7.0 Fouling factor: 0.80 Salt passage increase, %/yr: 10.0 Average flux rate: 24.8 lm2hr Feed type: Well Water ********************************************************************* **** THE FOLLOWING PARAMETERS EXCEED RECOMMENDED DESIGN LIMITS: *** ********************************************************************* Pass 1-1: Conc. polarization factor (beta) too high (1.25). Concentrate saturation of SiO2 too high (113%) Concentrate Langelier Saturation Index too high (2.07) The following are recommended general guidelines for designing a reverse osmosis system using Hydranautics membrane elements. Please consult Hydranautics for specific recommendations for operation beyond the specified guidelines. Feed and Concentrate flow rate limits Element diameter Maximum feed flow rate Minimum concentrate rate 8.0 inches 75 gpm (283.9 lpm) 12 gpm (45.4 lpm) 8.0 inches(full Fit) 75 gpm (283.9 lpm) 30 gpm (113.6 lpm) Concentrate polarization factor (beta) should not exceed 1.2 for standard elements Saturation limits for sparingly soluble salts in concentrate Soluble salt Saturation BaSO4 6000% CaSO4 230% SrSO4 800% SiO2 100% Langelier Saturation Index for concentrate should not exceed 1.8 The above saturation limits only apply when using effective scale inhibitor. Without scale inhibitor, concentrate saturation should not exceed 100%. Figure Printout of calculation results System performance safety margins 199

63 The results of computer calculations are basis for RO system design and terms of membrane performance warranty. It is customary for membrane manufacturers to provide system performance warranty with some safety margins, applied to the results calculated by their programs. The usual range is 10 20% below calculated permeate salinity and 5 10% above calculated feed pressure. The designer should conduct system performance calculations at two extreme conditions: - Highest feed water salinity and the lowest feed water temperature to determine the maximum feed pressure that will be required to produce the designed permeate output. - Highest feed water salinity and the highest feed water temperature to determine the maximum permeate salinity Additional safety margin should be applied by the designer of RO system to account for increase of salinity as a result of post treatment. In brackish water systems the salinity increase due to post treatment is usually small. In sweater treatment the increase of permeate salinity in post treatment can be significant, in the range of ppm Configuration of RO membrane unit for high feed salinity operation 100,000 m3/day product water capacity. Table Basic process parameters of a 100,000 m3/day SWRO system Parameter Value Product capacity, m3/day 100,000 Feed Salinity, ppm TDS 45,000 Feed water temperature, C Required permeate salinity, ppm TDS < 400 Required permeate boron concentration, ppm < 0.75 The RO membrane system will be configured as eight trains, partial two pass in a split partial configuration. The first pass permeate trains will produce 104,000 m3/day. It will operate at recovery rate of 45%. The second pass RO units will process about 23% of the first pass permeate. The second pass trains will produce 20,000 m3/day permeate and operate at recovery rate of 85%. Feed ph to the second pass RO will be increased to ph up to 10.5 to archive required level of boron in the blended product water. The system will be composed of eight first pass trains. The number of second pass trains usually is smaller than the number of first pass trains. In systems where only part of first pass permeate is processed by the second pass RO, The number of second pass trains will be half or less of the number of trains in the first pass. The schematic configuration of the RO trains arrangement, in the spilt partial configuration, is shown on Figure For the simplicity of presentation the trains are shown as one first pass train with the corresponding second pass train. In actuality, there will be two or more first pass trains feeding one second pass train. Details regarding feed water composition, membrane trains configuration, selection of membrane 200

64 elements and projected system performances are provided in the following computer projections. Split partial configuration 9 Pass 1 Pass Temp 20C Tag Flow, m3/hr TDS, PPM Pressure, bar Temp 36C Tag Flow, m3/hr TDS, PPM Pressure, bar Figure Split partial configuration of a 12,500 m3/day SWRO train 201

65 SPLIT PARTIAL TWO PASS & Permeate THROTTLING(1ST STAGE) WITH Pressure/Work Exchanger RO program licensed to: Calculation created by: Blended flow: m3/d Project name: Kuwait Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: bar Permeate recovery: % Feedwater Temperature: 20.0 C(68F) Total system recovery: 44.1 % Feed water ph: Element age: 3.0 years Chem dose, ppm, ppm Flux decline % per year: Fouling factor: Salt passage increase, %/yr: Average flux rate: lm2hr Feed type: Seawater - open intake Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar SWC4B MAX x ESPA2 MAX 64 8x ESPA2 MAX 24 3x8 Raw water Adjusted Water Feed water Permeate Concentrate ERD Reject Ion mg/l mg/l mg/l mg/l mg/l mg/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 30% 31% 65% SrSO4 / Ksp * 100: 44% 45% 94% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 1% 1% 2% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 32.3 bar 32.7 bar 59.3 bar H.P. Differential of Pressure/Work Exchanger: 1.0 ba Pressure/Work Exchanger Pump Boost Pressure: 2.9 bar r Pressure/Work Exchanger Leakage: 1 % Volumetric Mixing: 6 % 202

66 SPLIT PARTIAL TWO PASS & Permeate THROTTLING(1ST STAGE) PASS 1 WITH Pressure/Work Exchanger RO program licensed to: Calculation created by: Project name: Kuwait Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: 76.2 bar Permeate recovery ratio: 45.0 % Feedwater Temperature: 20.0 C(68F) Feed water ph: 7.5 Element age: 3.0 years Chem dose,ppm (100%) 28.2 H2SO4 Flux decline % per year: 7.0 % Fouling factor: 0.80 Salt passage increase, %/yr: 10.0 Average flux rate: 13.8 lm2hr Feed type: Seawater - open intake Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar SWC4B MAX x8 Raw water 1 Feed water 1 Permeate 1 Concentrate 1 Ion mg/l meq/l mg/l meq/l Back Front mg/l meq/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 30% 31% 65% SrSO4 / Ksp * 100: 44% 45% 94% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 1% 1% 2% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 32.3 bar 32.7 bar 59.3 bar 203

67 SPLIT PARTIAL TWO PASS WITH Pressure/Work Exchanger & Permeate THROTTLING(1ST STAGE) PASS 2 RO program licensed to: Calculation created by: Project name: Kuwait Permeate flow: m3/d Feed pressure: 11.6 bar Permeate recovery ratio: 85.0 % Feedwater Temperature: 20.0 C(68F) Feed water ph: 10.5 Element age: 3.0 years Chem dose, ppm (100%) 15.6 NaOH Flux decline % per year: 5.0 % Fouling factor: 1.00 Salt passage increase, %/yr: 5.0 Average flux rate: 29.0 lm2hr Feed type: Seawater - open intake Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar ESPA2 MAX 64 8x ESPA2 MAX 24 3x8 Raw water 2 Feed water 2 Permeate 2 Concentrate 2 Ion mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 0% 0% 0% SrSO4 / Ksp * 100: 0% 0% 0% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 0% 0% 0% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 0.3 bar 0.3 bar 2.2 bar 204

68 SPLIT PARTIAL TWO PASS & Permeate THROTTLING(1ST STAGE) WITH Pressure/Work Exchanger RO program licensed to: Calculation created by: Blended flow: m3/d Project name: Kuwait Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: bar Permeate recovery: % Feedwater Temperature: 36.0 C(97F) Total system recovery: 44.1 % Feed water ph: Element age: 3.0 years Chem dose, ppm, ppm Flux decline % per year: Fouling factor: Salt passage increase, %/yr: Average flux rate: lm2hr Feed type: Seawater - open intake Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar SWC4B MAX x ESPA2 MAX 64 8x ESPA2 MAX 24 3x8 Raw water Adjusted Water Feed water Permeate Concentrate ERD Reject Ion mg/l mg/l mg/l mg/l mg/l mg/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 27% 28% 59% SrSO4 / Ksp * 100: 40% 41% 86% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 1% 1% 1% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 34.0 bar 34.5 bar 62.5 bar H.P. Differential of Pressure/Work Exchanger: 1.0 ba Pressure/Work Exchanger Pump Boost Pressure: 2.8 bar r Pressure/Work Exchanger Leakage: 1 % Volumetric Mixing: 6 % 205

69 SPLIT PARTIAL TWO PASS & Permeate THROTTLING(1ST STAGE) PASS 1 WITH Pressure/Work Exchanger RO program licensed to: Calculation created by: Project name: Kuwait Permeate flow: m3/d HP Pump flow: m3/hr Raw water flow: m3/d Feed pressure: 72.0 bar Permeate recovery ratio: 45.0 % Feedwater Temperature: 36.0 C(97F) Feed water ph: 7.5 Element age: 3.0 years Chem dose,ppm (100%) 22.6 H2SO4 Flux decline % per year: 7.0 % Fouling factor: 0.80 Salt passage increase, %/yr: 10.0 Average flux rate: 13.8 lm2hr Feed type: Seawater - open intake Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar SWC4B MAX x8 Raw water 1 Feed water 1 Permeate 1 Concentrate 1 Ion mg/l meq/l mg/l meq/l Back Front mg/l meq/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 27% 28% 59% SrSO4 / Ksp * 100: 40% 41% 86% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 1% 1% 1% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 34.0 bar 34.5 bar 62.5 bar 206

70 SPLIT PARTIAL TWO PASS WITH Pressure/Work Exchanger & Permeate THROTTLING(1ST STAGE) PASS 2 RO program licensed to: Calculation created by: Project name: Kuwait Permeate flow: m3/d Feed pressure: 9.8 bar Permeate recovery ratio: 85.0 % Feedwater Temperature: 36.0 C(97F) Feed water ph: 10.5 Element age: 3.0 years Chem dose, ppm (100%) 35.8 NaOH Flux decline % per year: 5.0 % Fouling factor: 1.00 Salt passage increase, %/yr: 5.0 Average flux rate: 29.0 lm2hr Feed type: Seawater - open intake Stage Perm. Flow/Vessel Flux Beta Conc.&Throt. Element Elem. Array Flow Feed Conc Pressures Type No. m3/hr m3/hr m3/hr l/m2-hr bar bar ESPA2 MAX 64 8x ESPA2 MAX 24 3x8 Raw water 2 Feed water 2 Permeate 2 Concentrate 2 Ion mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l Ca Mg Na K NH Ba Sr CO HCO SO Cl F NO B SiO CO TDS ph Raw water Feed water Concentrate CaSO4 / Ksp * 100: 0% 0% 0% SrSO4 / Ksp * 100: 0% 0% 0% BaSO4 / Ksp * 100: 0% 0% 0% SiO2 saturation: 0% 0% 0% Langelier Saturation Index Stiff & Davis Saturation Index Ionic strength Osmotic pressure 0.7 bar 0.7 bar 4.5 bar 207

71 6.0.Design for RO high pressure pump and ERD The energy usage in RO system is an aggregate value of pumping energy for delivery of raw water, head loses in pretreatment, energy to drive the high pressure pump, energy of treatment of product water, energy of pumping product water to the distribution system and electric energy required for the operation of auxiliary equipment (Figure 6.1). Sizing of high pressure pumps and energy recovery devices is based design parameters of the RO membrane units and configuration of the desalination plant. The selection of type of pumping equipment, including selection of material of construction is based on the type of application, specific site conditions and project specifications. The general approach is to minimize number of pumping stapes in the plant through configuration of proper hydraulic profile. The hydraulic profile of the RO plant will vary with application and site conditions. In most cases the pumping requirement will consist pumping to deliver raw water to the plant site with sufficient head to pass the pretreatment step. 6.1.Raw water supply and transfer pumps In brackish systems treating well water this pressure bust will be provided by well pumps that would generate sufficient head for the raw water to pass the cartridge filters with effluent pressure within the suction head required by the high pressure pump. In RO systems in which the raw water is stored initially in the raw water reservoir, there will be a transfer pump to generate head for passing raw water through the pretreatment system. Similar consideration of pumping system configuration will apply to brackish system operating on surface water. 208

72 E tot = E rw + E prt + E hp + E prod + E serv + E aux Feed supply Prod. pumping 6% 8% 21% 30% 35% 15% 22% 52% 80% HP Pump NF BRO SWRO E ~ 0.9 kwhr/m3 (3.4 kwhr/kgallon) E ~ 1.2 kwhr/m3 (4.5 kwhr/kgallon) E ~ 3.5 kwhr/m3 (13.2 kwhr/kgallon) Figure 6.1. Energy usage in RO desalination systems In this case the pretreatment will be more extensive and could include media filtration followed by cartridge filters. In case of pressurized media filtration only one pumping step after raw water storage reservoir will be required to pass raw water through the pretreatment. In case of gravity filters, the approach is to use gravity filters and the filtrate effluent clear well as the buffer reservoir. This way the number of pumping steps in the pretreatment can be limited to pumping from the filtrate clear well through cartridge filters to the suction of high pressure pumps. The pretreatment in seawater RO systems, treating seawater from beach wells, is configured in a similar way as it is in the brackish plants. Accordingly the pumping requirements in the pretreatment section of the plant are similar as it is in the brackish plants. However, majority of seawater RO plants treat seawater delivered from the open sea intakes. The 209

73 hydraulic profile and pumping requirement will depend on type of pretreatment utilized. The possible alternatives could include: - Gravity media filtration followed by cartridge filters - Submersed (vacuum driven) membrane filtration - Pressurized medial filtration followed by cartridge filters - Pressurized membrane filtration In the above first two pretreatment alternatives the gravity media filtration and submersed membrane filtration units provide the required buffer storage capacity and the pressure bust is provided by the transfer pump after the filtrate storage clear well. In case of pressurized media filtration or pressurized membrane filtration pretreatment system, the usual configuration is to have a raw water storage reservoir prior to pretreatment system. Transfer pumps at the outlet of storage reservoir provide pressure bust required to pass the pretreatment system. Additional pumping units will be require after the pretreatment to pump feed water to the suction of high pressure pumps. Selection of type of transfer pumps will depend on site conditions and preference of the system designer. The current tendency is to utilize split case one two impeller horizontal pumps over the vertical can turbine pumps, whenever possible. The horizontal pumps, having smaller volume, are built utilizing smaller weight of metal alloy, therefore, are usually less expensive than vertical can pumps. Vertical pumps can be installed on the top of clear well, where horizontal pumps require side access, which is not always available at sites with limited area. The selection of materials of construction of transfer pumps will depend on the salinity of raw water. In brackish applications stainless steel grade, equivalent to 316 L will be sufficient. In seawater applications the pumps should be made of high alloy steels, equivalent to the super duplex alloy. If variability of feed pressure in RO unit is expected, due to fluctuation of feed water temperature and/or salinity, the system designer should have decision which pumping stage in the system will provide the variable head. Accordingly, the pump designated to provide the variable head will be driven by electric motor equipped with a variable frequency drive (VFD). As the VFD introduces additional energy transfer inefficiency in the range of 2%, the general approach is to install a variable speed drive on the lower energy demand motors in the system to reduce efficiency losses. Except for small systems the transfer pumps are configured as a group of parallel unit pumping to a common manifold rather than operate as dedicated pump to individual membrane trains. In this arrangement the transfer pumps could be bigger and it is usually sufficient to have one spare transfer pump for 4 5 transfer pumps in operation. 6.2.High pressure pumps 210

74 The decision on selection and configuration of high pressure pump will be driven by similar considerations as discussed in the above section regarding the transfer pumps. As the high pressure pumps are main contributor to the plant total energy use, it is important to select pumps of high hydraulic efficiency. In brackish and nanofiltrtaion applications the common configuration is to have high pressure pump dedicated to individual trains. In seawater applications high pressure pumps can be configured as train dedicated pumps or group in a pressure center configuration, as shown in Figure 6.2. In pressure center configuration the high pressure pumps form a group of pumps pumping to the common high pressure feed manifold. The number of high pressure pumps is significantly smaller than number of RO trains. The pumps are of high capacity and usually of high efficiency. Reduction of number of high pressure pumps and high efficiency results in reduced capital cost and reduced energy consumption. Figure 6.2. Pressure centers configuration of a large capacity SWRO plant. Due to high efficiency of the pumps and high efficiency of modern energy recovery devices, there is little energetic penalty for reducing recovery rate and the recovery rate can vary in relatively wide range with very small effect on the energy usage of the system. Accordingly when 1 2 trains are taken out of operation (if for example a cleaning of membrane elements is required), the system will continue to operate with the same number of pumps and energy recovery devices but at lower recovery rate. The reduced recovery rate will result in lower average osmotic pressure of the feed concentrate stream. Therefore at a constant feed 211

75 pressure the remaining trains in operation will produce additional quantity of permeate, as compared to the regular operating conditions. 6.3.Optimized control methods for high pressure pump discharge head and capacity Application of energy recovery devices (ERD) in RO systems Energy recovery devices (ERD s) are applied in RO systems for the purpose to reduce energy consumption. Addition of ERD increases system cost. Therefore, energy savings should be sufficient to provided net economic benefits as calculate through life cycle cost. Operation of ERD in RO membrane unit results in energy saving according to efficiency of ERD, available concentrate flow and concentrate pressure. Moving from seawater application to brackish application, both the flow rate of concentrate and concentrate pressure are being reduced. In parallel to reduced availability of discharged energy the economic incentive of utilizing ERD decrease as well. Example of potential energetic benefits in RO applications is illustrated through results listed in Table 6.1. Table 6.1. Calculation of energy total energy usage in RO systems of permeate capacity of 40,000 m3/day Application type NF RO low salinity RO high salinity Seawater RO System recovery rate, % 85% 80% 65% 50% Raw water pressure, bar Feed pressure, bar Concentrate pressure, bar Permeate pumping pressure, bar Energy, raw water, kwhr Energy, feed pump, kwhr Energy recovered from the concentrate, kwhr (%) -9.9 (0.9) (5.2) (14.5) (35.3) Energy product, kwhr Energy total, kwhr Specific energy, kwhr/m3 (kwhr/kgallon) 0.66 (2.5) 1.02 (3.9) 1.61 (6.1) 3.89 (14.7) It is evident from the results listed in Table 6.1 that application of ERD has noticeable effect on 212

76 overall power consumption only in high pressure, low recovery RO applications Selection of ERD All modern ERD s have a proven record of providing reliable operation in field conditions. Therefore, the selection of type ERD for given application is mainly based on economic benefits. The important parameter in evaluation is the local electricity rate. At location with low electricity rate utilization of low cost and lower efficiency energy recovery equipment will more beneficial. At locations of high electricity rates, energy recovery equipment with premium efficiency will be more beneficial. Additional considerations are maintenance cost, footprint, simplicity of operation and hydraulic considerations Pelton wheel The Pelton Wheel ERD is shown on Figure

77 Figure 6.3. Pelton Wheel. The device consist of drum with buckets mounted on a shaft. The concentrate exits the concentrate nozzles under pressure and impinges the buckets. The kinetic energy of the concentrate stream creates rotation of the drum and the shaft that is connected through the electric motor to the high pressure pump (Figure 6.4) provides rotation energy to the electric motor, reducing electric load required for operation of the high pressure feed pump. The Pelton Wheel chamber, where the rotor is located, operates under atmospheric pressure. Therefore, the Pelton Wheel equipment has to be installed at elevation that will provide sufficient hydrostatic head for concentrate to flow under gravity at sufficient velocity to the concentrate outfall. It is important that the outlet channel, located below the Pelton Wheel device, will be designed properly to prevent foaming of the concentrate as it flows out of the Pelton Wheel rotor. At some installations an excessive foaming has been experienced as shown on Figure Pump discharge pipe High pressure pump Electric motor Pelton wheel Feed water pipe Concentrate pipe Pumping system at the Larnaca plant Figure 6.4. Pelton Wheel electric motor high pressure pump unit. The Pelton Wheel can receive concentrate flow from number of RO trains, however, the usual 214

78 operation mode is according to the configuration shown in Figure 6.4, an integrated unit connected to a single train. In the past Pelton Wheel ERD has been used both in brackish and seawater applications. Today their application is limited to SWRO almost exclusively. Hydraulic efficiency of Pelton Wheel is the range of 84% and up to 88% in very large units. 215

79 Figure 6.5. Concentrate foaming at the Pelton Wheel outlet Energy requirement (E) of a RO membrane unit equipped with Pelton Wheel is calculated according to the following equation. The units are kwhr/m3 of permeate. E = * (Pf/( p * m * R) * f - Pc * (1 R) * t/ m * c ) (6.1) Where: Pf is feed pressure DPc is the differential pressure of the concentrate available for the ERD p, m and t are efficiencies of high pressure pump, electric motor and ERD respectively f and c are densities of the feed and concentrate streams Turbocharger Hydraulic Turbocharger is configured as two impellers with the blades in opposite directions connected through the common shaft (Figure 6.6). Figure 6.6. Configuration of Hydraulic Turbocharger The turbine impeller receives kinetic energy of the concentrate that induces rotation of the turbine impeller that provides torque to the pump impeller which creates differential pressure boost of the feed stream. 216

80 Brackish RO train equipped with Hydraulic turbocharger in interstage position is shown on Figure 6.7. The pictures also shows other components of turbocharger assembly that includes two motorized valves: the bypass valve and backpressure control valve. Hydraulic Turbocharger operating in seawater unit is shown in Figure 6.8. In this case the turbocharger unit is placed after the high pressure pump providing pressure boost to feed stream. Relation for calculation of pressure boost available from Hydraulic Turbochrger is listed on Figure 6.9. The pressure boost is function of turbocharger efficiency, available concentrate pressure and ration of flow of concentrate to the feed stream. The turbocharger consist of two turbine impellers, so the combined efficiency is multiplier of efficiency of individual impellers. Assuming maximum efficiency of turbine device of 0.9, the Interstage Concentrate Figure 6.7. Brackish RO train with Hydraulic Turbocharger in the interstage position 217

81 Figure 6.8. Hydraulic Turbocharger positioned after high pressure pump in seawater RO unit combined efficiency of the ERD will be 0.81 (or 81%). Such high efficiency is seldom achievable. In large seawater units efficiency could reach 78% - 80%. In smaller brackish units efficiency of 65% - 75% is more common. P = T ef R cf (P c P e ) P pressure boost T ef turbocharger efficiency R cf ratio of concentrate to feed flow (or interstage flow) P c concentrate pressure at the RO unit exit P e concentrate pressure at the turbocharger exit 218

82 Figure 6.9. Examples of configurations of seawater (left) and brackish (right) RO units with Hydraulic Turbocharger P = T ef R cf (P c P e ) Example - seawater RO P = 0.78 * (50/100) * ( ) = 26 bar (377 psi) Example - brackish RO P = 0.78 (16/40) ( ) = 3.9 bar (56 psi) Figure Example of calculations of pressure boost provided by Hydraulic Turbocharger in seawater and brackish RO membrane unit The latest development is Hydraulic Turbocharger unit equipped with electric motor (figure 6.11). Such unit provides additional flexibility of increasing pressure boost beyond what is available through recovery of energy of the concentrate stream. 219

83 Figure Hydraulic Turbocharger equipped with electric motor Pressure exchangers (isobaric devices) The isobaric energy recovery devices are positive displacement energy recovery pumping units or very high hydraulic efficiency, frequently reaching 94 % - 96% even in small size units. The schematic configuration of RO membrane unit with isobaric device is shown on Figure In this configuration the freed water stream is split into two streams. One stream (F1) is directed to the high pressure pump (P1). The high pressure pump increases pressure of the F1 stream to the required feed pressure. The flow rate of the F1 stream is approximately equal to the flow rate of the permeate stream from the RO membrane unit. The second stream (F2) is directed to the isobaric ERD. Flow rate of stream F2 is approximately equal to flow rate of the concentrate stream from the RO membrane unit. In the isobaric ERD, the F2 stream exchanges energy with the concentrate stream. At the exit from the isobaric ERD the pressure of the F2 stream is somewhat lower than the required feed pressure. The additional pressure boost is provided by the circulation pump P2. Prior to the entrance to the membrane unit, both streams are combined together. 220

84 High pressure pump 60 bar, 870 psi 100 m3/hr, 440 gpm P - 50 m3/hr, 220 gpm P1 F1 50 m3/hr, 220 gpm F2 60 bar, 870 psi 58 bar, 840 psi, 50 m3/hr, 220 gpm 1 bar, 15 psi P2 F1 = Permeate F2 = Concentrate F1 F2 Circulation pump 55 bar, 797 psi Power Recovery device 2bar, 30 psi 100 m3/hr, 440 gpm F2-50 m3/hr, 220 gpm Energy consumption of RO process: 2.14 kwhr/m3 (8.01 kwhr/kgallon) 46% reduction Figure Schematic configuration of RO membrane unit with isobaric energy recovery device. Two commercial types of isobaric ERD dominate at present the seawater RO applications market. They are DWEER and PX. Both are positive displacement devices but of different configuration. The configuration of DWEER is shown on Figure It consists of two parallel cylinders with floating pistons. The cylinders are connected together at one end through the link valve (concentrate end) and on the other end through manifold with check valves (seawater feed end). The two cylinders operate in opposite cycles of filling in by RO concentrate and seawater. In the energy exchange cycle, high pressure concentrate enters on cylinder through the link valve and replaces under high pressure seawater that filled the cylinder before at low pressure. When the floating piton reaches the end of the cylinder, the link valve, positioned at the other end, changes its position and allows the concentrate to be replaced by low pressure seawater. The second parallel cylinder operates in the same way but its cycle sequence timing is shifted to perform opposite filling and discharge to the operation of the first cylinder. 221

85 Figure Configuration of DWEER energy recovery device The isobaric devices make by DWEER could have flow capacity of up to 300 m3/hr. Number of units can be connected together of large flow capacity is require. An assembly of large size DWEER units at the 300,000 m3/day SWRO Ashkelon, Israel, desalination plant is shown on Figure The configuration of PX isobaric ERD is shown on Figure The PX configuration consists of ceramic rotor in a vessel made of composite material. The ceramic rotor has number of radial spaced parallel channels passing through the rotor. The PX vessel has four connections, two on each end of the vessel. In a similar way as in operation of DWEER, high pressure concentrate fills one of the rotor channels, pressurized seawater that filled the channel before at low pressure. The rotor is in continuous rotation and the channel filled with concentrate moves to position that connects (through the rotor channel) the inlet of low pressure seawater and discharge of concentrate. At this position low pressure seawater fills the channel and concentrate volume that has filled the channel previously is discharged from the device. In a continuous process rotor channels are filled with low pressure seawater, pressurized with high pressure concentrate and as the rotor 222

86 rotates, the concentrate is replaced with low pressure seawater again. Figure DWEER isobaric EDR assembly operating in 330,000 m3/day SWRO plant, Ashkelon, Israel. 223

87 Figure Configuration of PX energy recovery device (ERI) The flow capacity of PX is lower than DWEER, in the range of 50 m3/hr. In a similar way as DWEER units, the PX units can be arranged in parallel assemblies as shown on Figure

88 Figure Large assembly of PX ERD s. In the DWEER ERD the concentrate and seawater feed are separated by floating piston. In the PX ERD there is direct contact between concentrate and seawater. For this reason mixing between concentrate and seawater is somewhat higher in PX device. On the average in DWEER the mixing of two streams is usually less than 3%. In the PX device the mixing can reach up to 6% of the seawater feed flow rate. Because, in each case only about 50% of the feed flow is treated with ERD, the effect of salinity increase of the feed is only half on the volumetric mixing in each case. Introduction of isobaric ERD s brought a significant decrease of energy requirement in SWRO applications. Current Isobaric ERD s are not very effective in brackish application due to significantly different ratios of concentrate to feed flows rates in BWRO. Incentives for application of ERD in brackish RO units are much lower than in SWRO (see Table 6.1). In brackish applications Hydraulic Turbocharger is at present a more cost effective solution than isobaric ERD s. Among new ERD devices one more interesting is ISave introduce recently by Danfoss. ISave is 225

89 an integrated unit consisting of pump, energy recovery unit and a motor, shown on Figure Figure ISave ERD introduced by Danfoss. The flow capacity of ISave units is limited to about 40 m3/hr. However, the advantage of ISave is elimination of the circulation pump, required in system utilizing DWEER or PX Cost and economic benefits of ERD Configuration of pumping unit depends on application, method of delivery of raw water to the plant, selection of pumping equipment and plant size. The membrane trains pumping units usually consist of transfer pumps, cartridge filters (depending on filtration pretreatment type) high pressure pumps and energy recovery devices. Listing of components of pumping system for a 100,000 m3/day SWRO unit, treating high salinity gulf seawater at recovery rate of 45%, is provided in Table 6.2. Table 6.2. Comparison of operating parameters of pumping unit in SWRO 100,000 m3/day plant. Feed flow m3/day 231,200 Concentrate flow m3/day 129,200 Permeate flow M3/day 102,000 Energy recovery device type Isobaric Pelton Turbocharger 226