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MEMBRANE PROCESSES 13.13

lst-Pass RO 2nd-Pass RO
(High-Pressure Seawater (Low-Pressure Brackish
Membranes) i st-Pass Water Membranes)
Permeate
/ 2nd-Pass Concentrate
/' / t (Recycle)
Pretreated~~~
Seawater x ~ i ~ - -
~ i ~ " 2nd-Pass
Break ~ | Permeate
Tank v ....... .~
Concentrate Bypass
(to Energy Recovery Device) i
i
Clearwell
FIGURE 13.11 Example two-pass RO seawater desalination system.

Sometimes small flat-sheet membranes with varying pore sizes are used with bench
test equipment to characterize feedwater in terms of solute fractions in various molecular
weight size ranges. This is sometimes called molecular weight fingerprinting and can be
used for membrane selection.
Feedwater Pretreatment
Most membrane treatment systems require some pretreatment for source water. The type
of pretreatment system required depends on feedwater quality, membrane type, and de-
sign criteria for the membrane unit. Pretreatment may be used to
• Condition the feedwater to allow membrane treatment to be effective, for example, us-
ing coagulants to create particles large enough to be removed by microfiltration
• Modify the feedwater to prevent membrane plugging, fouling, and scaling; maximize
the time between cleanings; and prolong membrane life

Design of specific components needed to pretreat membrane feedwater is discussed in
other chapters of this book. In general, surface waters require more extensive pretreat-
ment than groundwaters because of the presence of significant levels of suspended solids
and biological matter.
For example, Figure 13.12 shows typical groundwater and surface water pretreatment
for RO and NF processes. In some situations, pretreatment can be relatively simple, such
as adding chemicals, such as acid or scale inhibitor, and cartridge filtration. Other waters
may require pretreatment as extensive as a full conventional water treatment plant with
coagulation, flocculation, sedimentation, and filtration.
Suspended Solids Control. Essentially all membrane systems require control of sus-
pended solids in the feedwater to protect the membranes from excessive solids loading
and/or to improve overall system performance.
Pretreatment for UF and MF systems typically includes strainers or bag filters rated
in the 100- to 500-/zm size range. Membrane manufacturers usually specify the maximum
feedwater suspended solids concentration or turbidity allowed for their particular prod-
ucts. If the raw water solids (and organic) content is high, pretreatment coagulation and
clarification are sometimes used. The cost of this more extensive pretreatment is at least
partially offset by cost savings in the MF/UF system cost, because it can be designed at
increased flux rates as a result of the improved feedwater quality.