Membrane cartridge filters have wide application in water filtration and drinking water treatment. Under the LT2ESWTR, cartridge filters would be eligible to receive Cryptosporidium removal credit as a membrane filtration process. In this case, the cartridge filter element would constitute a membrane module for the purposes of the rule. The ability of these modules to be subjected to direct integrity testing in the field during the course of normal operation, a feature that has not been widely utilized in association with cartridge filters in municipal water treatment applications, is a critical aspect of these systems that distinguishes what is considered to be MCF under the LT2ESWTR.

Membrane cartridge filters are manufactured by placing flat sheet membrane media between a feed and filtrate support layer and pleating the assembly to increase the membrane surface area within the cartridge. The pleat pack assembly is then placed around a center core with a corresponding outer cage and subsequently sealed, via adhesive or thermal means, into its cartridge configuration. End adapters, typically designed with a double o-ring sealing mechanism, are attached to the filter to provide a positive seal with the filter housing.

The principles of MCF system operation are similar to those for MF/UF systems in that MCF removes particles via a sieving mechanism. While cartridge filtration has not traditionally been considered a membrane treatment process, a cartridge filtration device that utilizes membrane filtration media capable of removing particles 1.0 mm and larger and which can be subjected to direct integrity testing would satisfy the definition of membrane filtration under the LT2ESWTR (40 CFR 141.719(b)(1)). However, many cartridge filters have pore sizes that are larger than 1.0 mm, and the utilization of a direct integrity test with these filters is relatively new in drinking water treatment applications. Thus, it is important to note that only cartridge filters that meet both criteria of the definition are considered membrane filtration under the rule.