Page 370 - Water and wastewater engineering
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REVERSE OSMOSIS AND NANOFILTRATION 9-9

                      Pressure vessel  Anti-telescoping  Concentrate (brine)
                                       support          seal
                                                                Snap ring
           Permeate
                        Membrane     Membrane      Membrane     End cap
            water        element      element       element
          Concentrate    module       module        module        Source water inlet
           outlet
                                   Brine seals  O-ring
                                             connector
            FIGURE 9-6
           Cross section of pressure vessel with three spiral-wound RO elements.
           ( Source:  U.S. AID, 1980.)
          extracted. A two-stage arrangement is shown in  Figure 9-7 a and schematically in  Figure 9-7 b. A
          three-stage arrangement is shown schematically in  Figure 9-7 c.
              A schematic of a typical RO or NF facility is shown in  Figure 9-8 .

                 Pretreatment
           The first pretreatment is to prevent scaling by silica (SiO  2  ) and sparingly soluble salts such as
          calcium carbonate and calcium sulfate. Scale control consists of pH adjustment and/or addition of
          an antiscalant. Typically, the antiscalants are proprietary polymeric compounds.
                Unlike ion exchange columns, RO/NF systems are not backwashed. Therefore, the second
          pretreatment process is filtration to remove particulate matter that will clog the feed channels or
          accumulate on the membrane surface. For surface water sources granular filtration or membrane
          filtration may be required. The minimum filtration requirement regardless of the water source is
          a cartridge filter rated in the 1 to 25    m range with a typical rating of 5    m. The maximum feed
          water turbidity recommended by manufacturers is 1 NTU with a preferred turbidity of less than
          0.2 NTU (Bergman, 2005).
               Disinfection may also be required to prevent biological fouling. Even though groundwater
          is expected to have a very low microbial population, when the membrane is out of service, the
          population on the membrane can quickly multiply. Chlorine solutions may be used for CA mem-
          branes, but other techniques such as ultraviolet irradiation, or chlorination followed by dechlori-
          nation are used for PA membranes.

            Post-treatment
           Because the RO/NF membranes do not remove gases, these are stripped after the RO/NF unit.
          The primary gases of concern are hydrogen sulfide and carbon dioxide. The removal of H  2  S is to
          prevent odor complaints. CO  2   is removed because it forms carbonic acid.
              The permeate has a low pH as a result of removal of alkalinity (buffering capacity) from
          the water and the addition of acid to prevent scaling. This water is corrosive to the distribution
          system. In addition to stripping CO  2  , addition of a base and corrosion inhibitor is generally re-
          quired. Split treatment and blending may aid in corrosion control. Maximum Contaminant Limits
          (MCLs) for constituents such as arsenic must be considered if split treatment is part of the corro-
          sion control strategy.

            Concentrate Stream
           The concentrate stream is under high pressure as it leaves the RO/NF unit. Energy recovery sys-
          tems are often used in reducing the pressure.
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