Civil Engineering Reference
In-Depth Information
prechlorination of the feed stream can create unwanted DBPs. Alternative disinfectants,
such as hydrogen peroxide, chloramines, and ultraviolet radiation,
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applied to the
feedwater can be used. Chemical cleaners are also commonly used to control microbial
growth, though reliance on frequent membrane cleaning is not desirable due to costs
associated with the cleaning operation and plant downtime. Nearly all membrane clean-
ers include a detergent to remove biofilms from the membrane surface. Other generic
components of membrane cleaning solutions for control of microbial growth include
enzymes, biocides, chelating agents, and chaotropic agents.
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Prefiltration or other contaminant removal processes can reduce microbial growth
by reducing the loading on the membranes. These pretreatments are further described
under suspended solids control and organics control.
Organics Control
While some RO and NF systems are designed to remove organics,
in many cases organic compounds can be problematic because they cause fouling and
other damage to membranes. Natural organic material can bind reversibly and irre-
versibly to the membrane, reducing the membrane permeability to water. The impact
of the organics can vary greatly from one application to another. Typically, pilot testing
is used to determine the effect of natural organic matter on membrane performance.
Oils and other hydrocarbons, as well as organic solvents, can also bind to or otherwise
damage membranes.
Pretreatment options for removing organics include conventional coagulation, floc-
culation, sedimentation, and granular-bed filtration; in-line coagulation with MF or
UF; or treatment with activated carbon or iron oxide particles followed by MF or UF.
Some research has shown positive results for these processes reducing fouling by
organics;
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however, experience with controlling organic fouling, particularly with sur-
face waters, is relatively limited.
Fouling by organics can also be controlled to some extent with membrane cleaning.
Cleaning agents used to remove organic compounds include detergents, oxidizing
agents, and bases. Some organic materials, however, bind irreversibly to membranes
and cannot practically be removed.
pH Control
Depending on the raw water and other pretreatments, pH control up-
stream of the membranes may be necessary. The manufacturer should recommend the
acceptable pH range for a given membrane. Cellulose acetate membranes are usually
operated in a pH range of 4.0 to 6.5. Outside of this range, the material can begin to
undergo hydrolysis. Polyamide membranes can typically tolerate pH up to 10.
pH can also affect the properties of solutes and membranes. Natural organic matter
will take on a different configuration depending on pH, thereby influencing its re-
moval.
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The charge of ions is important to ion passage through membranes and can
be influenced by pH.
RO and NF Process Configurations
Plant Process Configuration.
Figure 15-15 shows a basic process schematic for RO
and NF plants; typically, additional treatment steps are required to achieve treatment
objectives. Prefiltration may be accomplished with a fine mesh screen, cartridge filter,
microfilter, or ultrafilter. More extensive pretreatment may include in-line coagulation
or conventional treatment. Most RO and NF membranes remove a large percentage of
