Filtration systems treat water by passing it through beds of granular materials (e.g. sand) that remove and retain contaminants. Conventional, direct, slow sand, and diatomaceous earth filtration systems all do a good job of removing most protozoa, bacteria, and viruses (if coagulation is used). Bag and cartridge filters generally do not remove any viruses and few bacteria.
Conventional filtration is a multistage operation. First, a chemical coagulant such as iron or aluminum salts is added to the source water. The mixture is then stirred to induce tiny suspended particles to aggregate to form larger and more easily removable clots, or “flocs.” These coagulated masses, or “flocs,” are then allowed to settle out of the water, taking many contaminants with them. Once these processes are complete, water is passed through filters so that remaining particles attach themselves to filter material.
Direct filtration is similar to conventional filtration, except that after the coagulant is added and the mixture stirred, there is no separate settling phase. Rather, the suspended particles are destabilized by the coagulant and thus attach more readily to the filter material when the water is then filtered.
Slow sand filtration systems have no coagulation and, usually, no sedimentation step. Water is induced to pass slowly downward through a bed of sand some two to four feet (0.6 to 1.2 meters) deep. A biologically active layer forms along the upper surface of the sand bed, trapping small particles and degrading some organic contaminants.
Biosand filtration is a point-of-use analog to slow sand filtration, but its effectiveness is much less well established than the latter.
Diatomaceous earth filtration uses the fossil shells of tiny marine organisms as the filter through which raw source water is fed. The earth physically filters particle contaminants from the water.
Bag and cartridge filters are simple and easy-to-operate systems that use a woven bag or a cartridge with a wound filament filter or pleated filter to physically strain microbes and sediment from source water.
Ceramic filters are mostly being used for point of use applications. In developing countries, they are manufactured locally-sometimes as a self-financed micro-enterprise.
Most filtration systems use “backwashing” to clean the system. This produces wastewater that has to be managed properly.