Advanced Water Treatment (DESALINATION) معالجة مياه متقدمة EENV 5330 PART 4. Page 1

Transcription

1 Advanced Water Treatment (DESALINATION) معالجة مياه متقدمة EENV 5330 PART 4 Page 1

2 Reverse Osmosis (RO) Introduction. RO membrane structures & materials (general overview) RO membranes modules. RO system general description. Membrane desalination plant components. RO membrane separation. Page 2

3 3.1 Introduction If water of high salinity is separated from water of low salinity via a semipermeable membrane, a natural process of transfer of water will occur from the low-salinity side to the high-salinity side of the membrane until the salinity on both sides reaches the same concentration. This natural process of water transfer through a membrane driven by the salinity gradient occurs in every living cell; it is known as osmosis. Page 3

4 3.1 Introduction In order to remove fresh (low-salinity) water from a high-salinity source water using membrane separation, the natural osmosis-driven movement of water must be reversed, the freshwater has to be transferred from the highsalinity side of the membrane to the low-salinity side. For this reversal of the natural direction of freshwater flow to occur, the high-salinity source water must be pressurized at a level higher than the naturally occurring osmotic pressure. (Fig. 3.1) Page 4

5 Figure 3.1 : Osmosis and reverse osmosis. Page 5

6 3.1 Introduction This process of forced movement of water through a membrane in the opposite direction to the osmotic force driven by the salinity gradient is known as Reverse Osmosis(RO). Page 6

7 3.1 Introduction The applied feed water pressure counters the osmotic pressure and overcomes the pressure losses that occur when the water travels through the membrane, thereby keeping the freshwater on the low-salinity (permeate) side of the membrane until this water exits the membrane vessel. The salts contained on the source water (influent) side of the membrane are retained and concentrated; they are ultimately evacuated from the membrane vessel for disposal. Page 7

8 3.1 Introduction As a result, the RO process results in two streams one of freshwater of low salinity (permeate) and one of feed source water of elevated salinity (concentrate, brine or retentate), as shown in Fig RO membranes reject all suspended solids, but they are not an absolute barrier to dissolved solids. Some passage of dissolved solids will accompany the passage of freshwater through the membrane. The rates of water and salt passage are the two key performance characteristics of RO membranes. Page 8

9 Concentrate Permeate (fresh water) Saline source water Figure3.2: Reverse osmosis process. Semi-permeable membrane Page 9

10 3.2 Reverse Osmosis Membrane Structures, and Materials Reverse osmosis membranes differ by the material of the membrane polymer and by structure and configuration. Based on their structure, membranes can be divided into two groups: conventional thin-film composite. thin-film nanocomposite. Page 10

11 3.2 Reverse Osmosis Membrane Structures, and Materials Based on the thin-film material, conventional membranes at present are classified into two main groups: polyamide cellulose acetate. Depending on the configuration of the membranes within the actual membrane elements (modules), RO membranes are divided into three main groups: spiral-wound hollow-fiber flat-sheet (plate-and-frame). Page 11

12 3.2.1 Conventional Thin-Film Composite Membrane Structure The reverse osmosis membranes most widely used for desalination at present are composed of a semipermeable thin film (0.2 μm), made of either aromatic polyamide(pa) or cellulose acetate(ca). The semipermeable membrane is supported by a 25 to 50 μm microporous layer that in turn is cast on a layer of reinforcing fabric (Fig. 3.3). The 0.2-μm ultrathin polymeric film is the feature that gives the RO membrane its salt rejection abilities. Page 12

13 Figure3.3 Structure of a typical RO membrane. Page 13

14 3.2.2 Thin-Film Nanocomposite Membrane Structure Nanocomposite membranes either: 1. incorporate inorganic nanoparticles within the traditional membrane polymeric film structure (Fig. 3.4) 2. Or they are made of highly structured porous film consisting of a densely packed array of nanotubes (Fig. 3.5) Nanocomposite membranes reportedly have a higher specific permeability (i.e. ability to transport more water through the same surface area at the same applied pressure) than conventional RO membranes at almost similar salt rejection. Nanocomposite membranes have comparable or lower fouling rates in comparison to conventional thin-film composite RO membranes operating at the same conditions, and they can be designed for enhanced Page 14

15 Figure3.4 Polyamide RO membrane with nanoparticles. Page 15

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17 3.2.3 Cellulose Acetate Membranes The thin semipermeable film of the first RO membranes developed in the late 1950s at the University of California, Los Angeles was made of cellulose acetate (CA) polymer. While CA membranes have a three-layer structure similar to that of PA membranes, The main structural difference is that the top two layers (the ultrathin film and the microporous polymeric support) are made of different forms of the same CA polymer. Page 17

18 3.2.3 Cellulose Acetate Membranes CA membranes have a number of limitations, including the ability to perform only within a narrow ph range of 4 to 6 and at temperatures below 35 C (95 F). In order to maintain the RO concentrate ph below 6, the ph of the feed water to the CA membranes has to be reduced to between 5 and 5.5, which results in significant use of acid for normal plant operation and requires RO permeate adjustment by addition of a base (typically sodium hydroxide). Page 18

19 3.2.3 Cellulose Acetate Membranes CA membranes experience accelerated deterioration in the presence of microorganisms capable of producing cellulose enzymes and bioassimilating the membrane material. However, they can tolerate exposure to free chlorine concentration of up to 1.0 mg/l, which helps to decrease the rate of membrane integrity loss due to destruction by microbial activity. Since CA membranes have a higher density than PA membranes, they create a higher headloss when the water flows through the membranes; therefore they have to be operated at higher feed pressures, which results in elevated energy expenditures. Page 19

20 3.2.3 Cellulose Acetate Membranes Despite their disadvantages, and mainly because of their high tolerance to oxidants (chlorine, peroxide, etc.) as compared to PA membranes, CA membranes are used in municipal applications for saline waters with very high fouling potential (mainly in the Middle East and Japan) and for ultrapure water production in pharmaceutical and semiconductor industries. One important benefit of CA membranes is that the surface has very little charge and is considered practically uncharged, as compared to PA membranes, which have negative charge and can be more easily fouled with cationic polymers if such polymers are used for source water pretreatment. In addition, CA membranes have a smoother surface than PA membranes, which also renders them less susceptible to fouling. Page 20

21 3.2.4 Aromatic Polyamide Membranes Aromatic polyamide (PA) membranes are the most widely used type of RO membranes at present. They have found numerous applications in both potable and industrial water production. PA membranes operate at lower pressures and have higher productivity (specific flux) and lower salt passage than CA membranes, which are the main reasons they have found a wider application at present. Page 21

22 3.2.4 Aromatic Polyamide Membranes Another key advantage of PA membranes is that they can operate effectively in a much wider ph range (2 to 12), which allows easier maintenance and cleaning. In addition PA membranes are not biodegradable and usually have a longer useful life 5 to 7 years versus 3 to 5 years. Page 22

23 3.2.4 Aromatic Polyamide Membranes It should be noted that PA membranes are highly susceptible to degradation by oxidation of chlorine and other strong oxidants. For example, exposure to chlorine longer than 1000 mg/l-hour can cause permanent damage of the thin-film structure and can significantly and irreversibly reduce membrane performance in terms of salt rejection. Oxidants are widely used for biofouling control with RO and nanofiltration membranes; therefore, the feed water to PA membranes has to be dechlorinated prior to separation Page 23

24 3.2.4 Aromatic Polyamide Membranes PA membranes have negative charge and can be more easily fouled with cationic polymers if such polymers are used for source water pretreatment. (compare this to CA membranes which are practically uncharged). While CA membranes have a neutral charge, PA membranes have a negative charge when the ph is greater than 5, which amplifies co-ion repulsion and results in higher overall salt rejection. However, it should be noted that when the ph is lower than 4, the charge of a PA membrane changes to positive and rejection is reduced significantly, to lower than that of a CA membrane. Page 24

25 CA or PA? Mainly because of their higher membrane rejection and lower operating pressures, PA membranes are the choice for most RO membrane installations today. Exceptions are applications in the Middle East, where the source water is rich in organics and thus CA membranes offer benefits in terms of limited membrane biofouling and reduced cleaning and pretreatment needs. Because of the relatively lower unit power costs in the Middle East, cellulose acetate membranes provide an acceptable tradeoff between lower fouling rates and chemical cleaning costs on one hand and higher operating pressures and power demand on the other. Page 25

26 CA or PA? However, as newer generations of lower-fouling PA membranes are being introduced on the market, the use of CA elements is likely to diminish in the future. Page 26

27 Table3.1 Comparison of Polyamide and Cellulose Acetate Membranes Page 27

28 3.3 Spiral-Wound, Hollow-Fiber, and Flat-Sheet RO Membrane Elements The two most widely used configurations of membrane elements at present are: Spiral-wound. Hollow-fiber. Until the mid-1990s, hollow-fiber elements were the most prevalent technology used for desalination, But at present the marketplace is dominated by spiralwound RO membrane elements. Page 28

29 3.3 Spiral-Wound, Hollow-Fiber, and Flat-Sheet RO Membrane Elements Other configurations of membrane elements, such as tubular and plate-and-frame, have found application mainly in the food and dairy industries. They are practically never used in conventional municipal brackish or seawater desalination plants, because of their higher costs and equipment space requirements. Page 29

30 3.3.1 Spiral-Wound RO Membrane Elements Spiral-wound membrane elements (modules) are made of individual flat membrane sheets that have the threelayer structure described in the previous section (i.e., ultrathin CA or PA film; microporous polymeric support; and reinforcing fabric (see Fig. 3.3). A typical 8-in.-diameter spiral-wound RO membrane element has 40 to 42 flat membrane sheets. The flat sheets are assembled into 20 to 21 membrane envelopes (leafs), each of which consists of two sheets separated by a thin plastic net (referred to as a permeate spacer) to form a channel that allows evacuation of the permeate separated from the saline source water by the flat sheets (permeate carrier). Page 30

31 3.3.1 Spiral-Wound RO Membrane Elements Three of the four sides of the two-membrane flat-sheet envelope are sealed with glue and the fourth side is left open (Fig. 3.6). The membrane leafs are separated by a feed spacer approximately 0.7 or 0.9 mm (28 or 34 mils) thick, which forms feed channels and facilitates the mixing and conveyance of the feed-concentrate stream along the length of the membrane element (Fig. 3.7). Page 31

32 3.3.1 Spiral-Wound RO Membrane Elements Membranes with the wider 34-mil spacers have been introduced relatively recently and are more suitable for highly fouling waters. In order to accommodate the wider spacers, fewer membrane leafs are installed within the same RO membrane module, which results in a tradeoff between reduced membrane fouling and lower membrane element productivity. Page 32

33 Figure3.6 Flat-sheet membrane envelope. (Source: Hydranautics.) Page 33

34 Figure3.7 Spiral-wound membrane element. Page 34

35 3.3.1 Spiral-Wound RO Membrane Elements Pressurized saline feed water is applied on the outside surface of the envelope. permeate is collected in the space inside the envelope between the two sheets and directed toward the fourth, open edge of the envelope, which is connected to a central permeate collector tube. This collector tube receives desalinated water (permeate) from all flat-sheet leaves (envelopes) contained in the membrane element and evacuates it out of the element. Page 35

36 3.3.1 Spiral-Wound RO Membrane Elements The assembly of flat-sheet membrane leafs and separating spacers is wrapped (rolled) around the perforated permeate collector tube. The membrane leafs are kept in the spiral-wound assembly with a tape wrapped around them and contained by an outer fiberglass shell. The two ends of each RO element are finished with plastic caps referred to as end caps, or seal carriers. The plastic caps are perforated in a pattern that allows even distribution of the saline feed flow among all membrane leafs in the element (Fig. 3.8). Page 36

37 3.3.1 Spiral-Wound RO Membrane Elements The reason the plastic caps are often also referred to as seal carriers is that one of their functions is to carry a rubber brine seal that closes the space between the membrane and the pressure vessel in which the membrane is installed. This seal prevents the feed water from bypassing the RO element (Fig. 3.9). Page 37

38 Figure3.8 Cross-section of an RO membrane element installed in a pressure vessel. Page 38

39 Figure3.9 Membrane elements installed in a pressure vessel. Page 39

40 3.3.1 Spiral-Wound RO Membrane Elements The source water flow is introduced from one end of the element and travels in a straight tangential path on the surface of the membrane envelopes and along the length of the membrane element (see Figs. 3.6 and 3.7). A portion of the feed flow permeates through the membrane and is collected on the other side of the membrane as freshwater. The separated salts remain on the feed side of the membrane and are mixed with the remaining feed water. As a result, the salinity of the feed water increases as this water travels from one end of the membrane element to the other. The rejected mix of feed water and salts exits at the back end of the membrane element as concentrate (brine). Page 40

41 3.3.1 Spiral-Wound RO Membrane Elements As shown in Figs. 3.8 and 3.9, the permeate collector tubes of the individual RO membrane elements installed in the pressure vessel are connected to each other and to the permeate line evacuating the fresh water from the pressure vessel via interconnectors (adaptors) with integral O-rings that seal the connection points and prevent concentrate from entering the permeate collector tubes. The interconnectors with O-rings provide flexible connections between the elements, which allow for their limited movement within the vessel, for some level of flexibility in loading membranes and also facilitate handling transient pressure surges created in the vessels as a result of abrupt shutdown and start-up of the RO system. Page 41

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43 3.3.1 Spiral-Wound RO Membrane Elements Commercially available RO membrane elements are standardized in terms of diameter and length and usually are classified by diameter. Spiral-wound RO membranes are available in 2.5-in., 4-in., 6-in., 8-in., 16-in., 18-in., and 19-in. sizes. A typical 8-in. RO membrane element is shown in Fig At present, the most widely used and commercially available RO elements have a diameter of 20 cm (8 in.), length of 100 cm (40 in.) and brine spacer thickness of 28 mils (0.7 mm). Page 43

44 3.3.1 Spiral-Wound RO Membrane Elements Standard 8-in. elements in a typical configuration of seven elements per vessel can produce the following amounts of freshwater (permeate): For seawater: between 13 and 25 m 3 /day For brackish water: between 26 and 38 m 3 /day Larger 16-in., 18-in., and 19-in. RO brackish and seawater membrane elements are also commercially available. However, these large elements have received limited full-scale application. While 8-in. elements and smaller can be handled manually by a single person (Fig. 3.13), larger RO elements can only be loaded and unloaded by special equipment because of their significant weight. Page 44

45 Figure3.12 Typical 8-in. membrane element. Page 45

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47 3.3.2 Hollow-Fiber RO Membrane Elements In hollow-fiber membrane elements, the 0.1- to 1.0-μm semipermeable film is applied as a coating to the surface of hollow fibers of diameter comparable to that of human hair (42 μm internal diameter, 85 μm external diameter). The hollow fibers are assembled in bundles and folded in a half to a length of approximately 48 in. (1200 mm). The hollow-fiber bundle is 4 to 8 in. in diameter and is located inside a cylindrical housing that is 6 to 12 in. across and 54 in. (1370 mm) long. Page 47

48 3.3.2 Hollow-Fiber RO Membrane Elements Both ends of the bundle are epoxy-sealed to encapsulate the water introduced in the tube in a way that allows all of the concentrate generated in the tube to exit from only one location the back end of the membrane (Fig. 3.14). Page 48

49 Figure3.14 Hollow-fiber RO vessel with two membrane elements. (Source : Toyobo.) Page 49

50 3.3.2 Hollow-Fiber RO Membrane Elements As compared to spiral-wound membrane configuration, hollow-fiber membrane configuration allows approximately 4 times more membrane surface per cubic foot of membrane volume. This higher surface area results in a proportionally lower permeate flux for the same volume of processed water, which in turns reduces concentration polarization and associated scaling potential when the source seawater is of high mineral content. Page 50

51 3.3.2 Hollow-Fiber RO Membrane Elements As a result, a typical hollow-fiber vessel contains only two membrane elements but produces approximately the same volume of water as a conventional RO vessel that contains seven or eight elements. These features make hollow-fiber membrane elements very suitable for high-salinity waters with elevated scaling potential, such as those of the Persian Gulf, the Gulf of Oman (Indian Ocean), and the Red Sea. Therefore, this type of membrane element configuration has found a wider application in the Middle East than in other parts of the world. Page 51

52 3.3.2 Hollow-Fiber RO Membrane Elements Because of the lower permeate flux and higher membrane surface area, the feed water flow regime in a hollow-fiber membrane element is laminar (as compared to nearly turbulent flow that occurs in the spiral-wound elements). This low-energy laminar flow results in little to no scrubbing effect of the feed flow on the surface of the membranes. This low velocity along the membrane surface allows solids and biofilm to attach to and accumulate more easily on the membranes, which in turn makes hollow-fiber membranes more susceptible to particulate fouling and biofouling and more difficult to clean. As a result, this type of element requires more enhanced source water pretreatment to remove particulate foulants from the water and it operates better on waters of low turbidity and SDI, such as those obtained from well intakes. Page 52

53 3.3.2 Hollow-Fiber RO Membrane Elements For comparison, the turbulent flow on the surface of a spiral-wound membrane element makes that membrane configuration more resistant to particulate fouling and biofouling, but because of the higher permeate flux and concentration polarization, it is more prone to mineral scaling. Page 53

54 3.3.3 Flat-Sheet RO Membrane Elements Flat-sheet membrane elements are used in plate-and-frame RO systems (Fig. 3.15). Because of its low membrane packing density which is approximately half that of a spiral-wound system this type of RO system is significantly larger and more costly than a conventional spiral-wound RO system. Therefore, plate-and-frame systems have not found application for municipal water RO desalination. However, under the plate-and-frame configuration, the flat membrane sheets can easily be removed from the module and can individually be hand-cleaned. This allows for better cleaning and facilitates the use of this type of system for high-solids applications such as food processing. Page 54

55 Figure3.15 Plate-and-frame RO unit. Page 55

56 3.4 Reverse Osmosis System General Description This section describes basic configuration and performance parameters of RO systems using spiral-wound membrane elements. Page 56

57 3.4.1 Configuration RO membranes in full-scale installations are assembled in membrane elements (modules) installed in vessels in series of six to eight elements per vessel, the feed water is introduced to the front membrane elements and applied tangentially on the surface of the membranes in a cross-flow direction at pressure adequate to overcome the osmotic pressure of the saline water and the energy losses associated with the separation process. Page 57

58 3.4.1 Configuration A general schematic of an RO system is shown in Fig Key parameters associated with the performance of reverse osmosis systems are discussed in the following subsections. Figure3.16 General schematic of an RO system. Page 58

59 3.4.2 Reverse Osmosis Process Parameters Osmotic Pressure The osmotic pressure P o of a given saline water is calculated by measuring the molar concentrations of the individual dissolved salts in the solution and applying the following equation: Page 59

60 3.4.2 Reverse Osmosis Process Parameters Where: Page 60

61 Example: Calculate the osmotic pressure of Pacific Ocean seawater with a TDS concentration of 35,000 mg/l (see Table 2.1). Table 3.1 shows the estimate of the molar concentration of all salts in the source Pacific Ocean seawater ( m ). i Based on Eq. 3.1: The osmotic pressure of the Pacific Ocean seawater at 25ºC is calculated as: Page 61

62 3.4.2 Reverse Osmosis Process Parameters The relative osmotic pressure per 1000 mg/l of TDS of Pacific Ocean water is 26.8/(35,000/1000) =0.77 bar (11 lb/in 2 ). This ratio is often used as a rule-of-thumb relationship between source water salinity and osmotic pressure, i.e., every 1000 mg/l of salinity results in an osmotic pressure of 0.77 bar (11 lb/in 2 ). Page 62

63 Table3.1 Molar Concentrations of Pacific Ocean Water Salts Osmotic pressure is a parameter that should be calculated individually for the specific source water quality and that rules of thumb for this parameter may often over- or underestimate its actual value Page 63

64 Permeate Recovery The percentage of the feed source water flow Q f that is converted into freshwater flow Q p is defined as the permeate recovery rate P r (Fig. 3.17):.. (3.2) For SWRO: P r is 40 to 65% For BWRO: P r is 65 to 85% Page 64

65 The TDS of the concentrate TDSc can be calculated based on the RO system permeate recovery rate Pr. The actual TDS concentration of the permeate TDS p, and the feed water TDS (TDS f ) using the following formula:. (3.3) Page 65

66 Figure3.17 Recovery of a typical SWRO system. Page 66

67 For the example in Fig. 3.17, the TDS of the concentrate assuming a recovery rate of 50% and a permeate salinity of 200 mg/l is calculated as follows: Page 67

68 The more water passes through the membranes, the more salts pass as well. therefore, the overall permeate water quality decreases with an increase in system recovery. The passing of salts through the membrane can be controlled to some extent by the membrane structure. i.e., membranes with tighter structure will pass fewer salt ions when the RO system is operated at higher recovery. Page 68

69 Membrane Salt Passage Salt passage S p of a membrane is defined as: the ratio between the concentration of salt in the permeate TDS p and in the saline feed water TDS f (see Fig. 3.17); it is indicative of the amount of salts that remain in the RO permeate after desalination... (3.4) Page 69

70 Membrane Salt Rejection Salt rejection S r is a relative measure of how much of the salt that was initially in the source water is retained and rejected by the RO membrane. Page 70

71 For the seawater desalination example depicted in Fig The salt passage of the SWRO membrane for the high end of performance (TDSp = 200 mg/l) is S p = (200/35,000) 100% = 0.57%. The total salt rejection of this membrane is S r =100% 0.57% = 99.43%. Page 71

72 Not all ions are rejected equally by an RO membrane. Usually, the larger the ions and the higher their electrical charge, the better rejected they are. It is also important to note that RO membranes do not reject gases. Also, low-charge monovalent ions such as boron will be rejected at a lower rate than higher-charge monovalent ions such as chloride and sodium. This is a very important feature of RO membranes, because often for practical purposes RO membrane structure can be modified to selectively reject specific ions better. Page 72

73 For example, the RO membrane material can be modified to have a looser structure and remove mainly bivalent ions when the key goal of water treatment is water softening (i.e., removal of calcium and magnesium). Such membranes are often referred to as nanofiltration membranes. Nanofiltration membranes usually reject less than 30% of TDS (as compared to RO membranes, which reject over 90%of TDS), but they reject over 99%of calcium and magnesium and they do that at higher productivity and lower feed pressure. Page 73

74 Net Driving Pressure (Transmembrane Pressure) Net driving pressure(ndp), also known as transmembrane pressure, is: the actual pressure that drives the transport of freshwater from the feed side to the freshwater side of the membrane. The average NDP of a membrane system is defined as : the difference between the applied feed pressure F p of the saline water to the membrane and all other forces that counter the movement of permeate through the membrane, including the average osmotic pressure O p which occurs on the permeate side of the RO membrane, the permeate pressure P p existing the RO pressure vessel, and the pressure drop P d across the feed/concentrate side of the RO membrane. Page 74

75 Net Driving Pressure (Transmembrane Pressure) The NDP can be calculated as follows: The applied feed pressure F p is controlled by the RO system operator and delivered through high-pressure feed pumps. The average osmotic pressure O p of the membrane is determined by the salinity and the temperature of the source water and the concentrate. Page 75

76 Net Driving Pressure (Transmembrane Pressure) The permeate pressure P p (also known as product water back pressure) is a variable that is controlled by the RO plant operator and is mainly dependent on the energy needed to convey permeate to the downstream treatment and/or delivery facilities. Typically, permeate pressure is set at 1 to 2 bar (15 to 30 lb/in2). The osmotic pressure of permeate is usually very small, because the salinity of this stream is low. The pressure drop P d across the feed/concentrate side of the RO membrane depends mainly on the membrane fouling and the RO membrane and system configuration. Page 76

77 Example: in Fig. 3.17, assuming an SWRO system recovery of 50%, saline water feed pressure of F p =56 bar (812 lb/in 2 ), permeate pressure P p =1.4 bar (20.3 lb/in 2 ), and pressure drop across the RO system P d =3.2 bar (46.4 lb/in 2 ), the NDP at which the system operates is determined as follows: 1.Calculate the average salinity on the feed/concentrate side of the RO membrane. Page 77

78 Example: 2. Calculate the average osmotic pressure of the saline water on the feed/concentrate side. 3. Calculate the NDP. Page 78

79 Membrane Permeate Flux Membrane permeate flux ( J) membrane flux : The permeate flow a membrane produces per unit membrane area. It is calculated by dividing the flow rate Qp of permeate produced by a RO membrane element [usually expressed in gallons per day(gpd) or liters per hour (lph)] by the total membrane area S of the element (in square feet or square meters). The flux unit is therefore gal/(ft2 day), also referred to as gfd, or L/(m2 h), also known as lmh. Page 79

80 Membrane Permeate Flux The average permeate flux of the system is calculated by dividing the total flow of permeate produced by all membranes by the total surface area of the membranes. For design purposes, flux is selected as a function of the source water quality and the type of RO membrane used for desalination. The higher the quality of the source water applied to the membranes, the higher the acceptable design flux. Page 80

81 Specific Membrane Permeability (Specific Flux) Specific membrane permeability(smp), also known as specific membrane flux, is a parameter that characterizes the resistance of the membrane to water flow. It is calculated as the membrane permeate flux (J) divided by the net driving pressure (NDP):. (3.8) The standard specific permeability of a given membrane is typically determined for a feed temperature of 25 C and expressed in lmh/bar or gfd/(lb/in2). Page 81

82 Specific Membrane Permeability (Specific Flux) For example, most commercially available seawater desalination RO membranes at present have an SMP of 1.0 to 1.4 lmh/bar [0.04 to 0.06 gfd/(lb/in2)]. For comparison, brackish water RO membranes have a significantly higher specific permeability of 4.9 to 8.3 lmh/bar [0.2 to 0.35 gfd/(lb/in2)]. Page 82

83 Specific Membrane Permeability (Specific Flux) Nanofiltration membranes, which have a looser membrane structure, have an even higher specific permeability than brackish and seawater RO membranes: 7.4 to 15.8 lmh/bar [0.3 to 0.6 gfd/(lb/in2)]. Usually, membranes of lower specific permeability also have higher salt rejection, so there is a tradeoff between lower production and higher water quality. Page 83

84 Actual RO Membrane Specification Page 84

85 3.7 Key Membrane Desalination Plant Components General Overview Seawater contains solids in two forms: suspended. dissolved. Suspended solids occur in the form of insoluble particles (particulates, debris, marine organisms, silt, colloids, etc.). Dissolved solids are present in soluble form (ions of minerals such as chloride, sodium, calcium, magnesium, etc). Page 85

86 3.7 Key Membrane Desalination Plant Components RO desalination plants incorporate two key treatment steps designed to sequentially remove suspended and dissolved solids from the source water. The purpose of the first step source water pretreatment is to remove the suspended solids and prevent some of the naturally occurring soluble solids from turning into solid form and precipitating on the RO membranes during the salt separation process. Page 86

87 3.7 Key Membrane Desalination Plant Components The second step the RO system separates the dissolved solids from the pretreated source water, thereby producing fresh low-salinity water suitable for human consumption, agricultural uses, and for industrial and other applications. Once the desalination process is complete, the freshwater produced by the RO system is further treated for corrosion and health protection and disinfected prior to distribution for final use. Page 87

88 3.7 Key Membrane Desalination Plant Components Figure 3.24 presents a general schematic of a seawater desalination plant. The plant shown in Fig collects water using open ocean intake, which is conditioned by coagulation and flocculation and filtered by granular media pretreatment filters to remove most particulate and colloidal solids, and some organic and microbiological foulants. The filtered water is conveyed via transfer pumps through micronsize filters (referenced on the figure as cartridge filters) into the suction headers of highpressure pumps. Page 88

89 3.7 Key Membrane Desalination Plant Components These pumps deliver the filtered water into the RO membrane vessels at a net driving pressure adequate to produce the target desalinated water flow and quality. The reverse osmosis vessels are assembled in individual sets of independently operating units referred to as RO trains or racks. Page 89

90 Figure3.24 Schematic of a typical seawater desalination plant. Page 90

91 Plant Intake Plant intake is designed to collect source water at a quality and quantity adequate to produce the target volume and quality of desalinated water. Pretreatment The fine microstructure of thin-film composite membranes presently used for desalination by reverse osmosis does not permit passage of particulates contained in the source water or formed during the desalination process. Page 91

92 Reverse Osmosis Separation System The key components of the RO separation system include : filter effluent transfer pumps. high-pressure pumps. reverse osmosis trains. energy recovery equipment. and the membrane cleaning system. Page 92

93 Post-Treatment Post-treatment facilities include equipment for remineralization and disinfection of RO permeate. Some brackish water plants have additional posttreatment facilities for removal of odorous gases naturally contained in the source water, such as hydrogen sulfide. Desalination plants typically generate source water pretreatment and concentrate waste streams, which have to be handled in an environmentally safe and cost-effective manner. Page 93

94 Assignment 3 An SWRO system used to desalinate a seawater of TDS =40,000 mg/l, recovery of 45%, a permeate water rate is 5000 m3/day, saline water feed pressure of Fp =50 bar, permeate pressure Pp =1.5 bar, and pressure drop across the RO system Pd =4.2 bar, if the RO system will produce a permeated water of TDS = 250 mg/l, calculate: 1. The salt rejection percentage, 2. The required feed water rate, 3. The NDP at which the system operates? 4. Specific Membrane Permeability for 1 m2 membrane? Page 94

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