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 (where applicable) varies depending on the type of membrane filtration system and the feed water quality. For example, although hollow-fiber microfiltration (MF) and ultrafiltration (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 / micron rating of the selected prefiltration process may range from as small 100 mm to as large as 3,000 mm or higher, 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 nanofiltration (NF) and reverse osmosis (RO) utilize non-porous semipermeable membranes that cannot be backwashed and are almost exclusively designed in a spiral-wound configuration for municipal water treatment applications, 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 a turbidity less than approximately 1 NTU or a silt density index (SDI) less than approximately 5, cartridge filters with ratings ranging from about 5 to 20 mm are commonly used NF/RO prefiltration. However, if the feed water turbidity or SDI exceeds these values, a more rigorous method of particulate removal, such as conventional treatment (including media filtration) or MF/UF membranes, is recommended as pretreatment for NF/RO.

In conventional applications of cartridge filtration technology, high quality source waters are treated, and thus additional prefiltration is typically not required. However, membrane cartridge filtration (MCF) systems may warrant some level of prefiltration, possibly using more conventional cartridge or bag filters, to protect and extend the life of the more effective and more expensive MCF, particularly if the MCF system is applied to surface waters specifically for the purposes of LT2ESWTR compliance. Since MCF is a new class of technology defined under the LT2ESWTR, pretreatment practices for MCF will likely evolve as this technology is applied and may be similar to those for MF/UF.