Cartridge Filter (CF)

Cartridge filters were once considered only as a point-of-use water treatment method for removal of larger particles. However, breakthroughs in filter design, such as the controlled use of blown microfiber filters (as opposed to wrapped fabric or yarn-wound filters), have tremendously broadened cartridge filter utilization. Cartridge filters fall into two categories: depth filters or surface filters

Depth Cartridge Filters

In a depth cartridge filter the water flows through the thick wall of the filter where the particles are trapped throughout the complex openings in the medium. The filter may be constructed of cotton, cellulose and synthetic yarns, chopped fibers bound by adhesives, or “blown” microfibers of polymers such as polypropylene.

The most important factor in determining the effectiveness of depth filters is the design of the porosity throughout the thick wall. The best depth filters for many applications have lower density on the outside and progressively higher density toward the inside wall. The effect of this “graded density” (Figure 9) is to trap coarser particles toward the outside of the wall and the finer particles toward the inner wall. Graded-density filters have a higher dirt-holding capacity and longer effective filter life than depth filters with constant density construction.

Depth cartridge filters are usually disposable and cost-effective, and are available in the particle-removal size range of 0.5 to 100 microns. Generally, they are not an absolute method of filtration since a small amount of particles within the micron range may pass into the filtrate. However, there are an increasing number of depth filters in the marketplace that feature near-absolute retention ratings.

Surface Filtration – Pleated Cartridge Filters

Pleated cartridge filters typically act as surface filters. Flat sheet media, either membranes or nonwoven fabric materials, trap particles on the surface. The media are pleated to increase usable surface area. Pleated filters are usually not cost-effective for water filtration, where particles greater than one micron quickly plug them. However, pleated membrane filters serve well as submicron particle or bacteria filters in the 0.1- to 1.0-micron range and are often used to polish water after depth filters and other treatment steps in critical applications. Pleated filters are usually disposable by incineration, since they are constructed with polymeric materials, including the membrane. Newer cartridges also perform in the ultrafiltration range: 0.005- to 0.15-micron.

Ultrafiltration (UF) Cartridge Filters

UF membrane cartridges perform much finer filtration than depth filters but are more expensive and require replacement as the filter becomes “blinded,” i.e., covered with an impervious thin coating of solids. Typically the smaller the pore the more quickly this blinding occurs. To avoid blinding of the pores, point-of-use ultrafiltration cartridges are built in a spiral-wound configuration to allow crossflow mode operation to help keep the surface clean by rinsing away the solids. Point-of-use ultrafiltration cartridges are used to remove colloids, pyrogens and other macromolecular compounds from ultrapure water.