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Membrane Systems Planning and Design 351
also be required. Because the design of spiral-wound membranes generally does not permit
reverse flow, NF and RO membrane systems are not backwashed. For these systems,
membrane fouling is controlled primarily with chemical cleaning, as well as through flux
control and crossflow velocity. The inability of spiral-wound membranes to be backwashed is
one reason that NF and RO membranes are seldom applied to directly treat water with high
turbidity and/or suspended solids.
3.3. Chemical Cleaning
Chemical cleaning is another means of controlling membrane fouling, particularly those
foulants such as inorganic scaling and some forms of organic and biofouling that are not
removed via the backwash process. As with backwashing, chemical cleaning is conducted for
each membrane unit separately and is typically staggered to minimize the number of units
undergoing cleaning at any time. While chemical cleaning is conducted on both MF/UF and
NF/RO systems, because non-porous, semi-permeable membranes cannot be backwashed,
chemical cleaning represents the primary means of removing foulants in NF/RO systems.
Although cleaning intervals may vary widely on a system-by-system basis, the gradual
accumulation of foulants makes eventual chemical cleaning virtually inevitable. Membrane
cartridge filters are an exception; however, in that, cartridge filters are usually designed to be
disposable and thus are typically not subject to chemical cleaning.
As with backwashing, the goal of chemical cleaning is to restore the TMP of the system to
its baseline (i.e., clean) level. Any foulant that is removed by either the backwash or chemical
cleaning process is known as reversible fouling. Over time, membrane processes will also
typically experience some degree of irreversible fouling, which cannot be removed through
either chemical cleaning or backwashing. Irreversible fouling occurs in all membrane
systems, albeit over a wide range of rates, and eventually necessitates membrane replacement.
There are a variety of different chemicals that may be used for membrane cleaning, and
each is generally targeted to remove a specific form of fouling. For example, citric acid is
commonly used to dissolve inorganic scaling, and other acids may be used for this purpose as
well. Strong bases such as caustic are typically employed to dissolve organic material.
Detergents and surfactants may also be used to remove organic and particulate foulants,
particularly those that are difficult to dissolve. Chemical cleaning may also utilize concentrated
disinfectants such as a strong chlorine solution to control biofouling. Due to the variety
of foulants that are present in many source waters, it is often necessary to use a combination
of different chemicals in series to address multiple types of fouling. The various types of
chemical cleaning agents used are summarized in Table 8.4.
Numerous proprietary cleaning chemicals are also available, and these specialty cleaning
agents may be useful in cases in which more conventional chemicals are ineffective. For
example, under some circumstances, enzymatic cleaners have been found to be effective at
dissolving organic contaminants. Chemical cleaning options are more limited for membranes
that cannot tolerate oxidants and/or extreme pH levels. A chemical cleaning regimen may be
specified by the manufacturer or identified based on site-specific pilot testing and source