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Membrane Systems Planning and Design 347
3. OPERATIONAL UNIT PROCESSES
All membrane filtration systems have associated operational unit processes that are
essential for maintaining and optimizing system performance and therefore critical to the
successful implementation of the technology. These operational processes include:
1. Pretreatment
2. Backwashing
3. Chemical cleaning
4. Integrity testing
5. Post-treatment
3.1. Pretreatment
Pretreatment is typically applied to the feed water prior to entering the membrane system
in order to minimize membrane fouling, but in some cases may be used to address other water
quality concerns or treatment objectives. Pretreatment is most often utilized to remove
foulants, optimize recovery and system productivity, and extend membrane life. Pretreatment
may also be used to prevent physical damage to the membranes. Different types of pretreatment
can be used in conjunction with any given membrane filtration system, as determined by
site-specific conditions and treatment objectives. Pilot testing can be used to compare various
pretreatment options, optimize pretreatment, and/or demonstrate pretreatment performance.
Several different types of commonly used pretreatment for membrane filtration systems are
discussed in the following subsections.
3.1.1. Prefiltration
Prefiltration, including screening or coarse filtration, is a common means of pretreatment for
membrane filtration systems that is designed to remove large particles and debris. Prefiltration
can either be applied to the membrane filtration system as a whole or to each membrane unit
separately. The particular pore size associated with the prefiltration process varies depending
on the type of membrane filtration system and the feed water quality. For example, although
hollow-fiber MF and UF systems are designed specifically to remove suspended solids, large
particulate matter can damage or plug the membranes fibers. For these types of systems the
pore size of the selected prefiltration process may range from as small as 100 mm to as large as
3,000 mm orhigher(11, 12), depending on the influent water quality and manufacturer
specifications. Generally, hollow-fiber MF/UF systems that are operated in an inside-out
mode are more susceptible to fiber plugging and thus may require finer prefiltration.
Because NF and RO utilize non-porous semipermeable membranes that cannot be backwashed
and are almost exclusively designed in a spiral-wound configuration for municipal
water treatment, these systems must utilize much finer prefiltration in order to minimize
exposure of the membranes to particulate matter of any size. Spiral-wound modules are
highly susceptible to particulate fouling, which can reduce system productivity, create
operational problems, reduce membrane life, or in some cases damage or destroy the
membranes. If the feed water has turbidity less than approximately 1 NTU or a silt density
index (SDI) less than approximately 5, cartridge filters with a pore size range from about