Filtration

After coagulation and settling remove most suspended matter, filtration polishes the water by passing it through a bed of granular media, usually sand, sometimes with anthracite on top.

Slow sand filters run at low rates. A biological layer forms on the surface and removes particles and many pathogens. They need large areas and are cleaned by scraping the top layer. Rapid sand filters run many times faster, rely on prior coagulation, and are cleaned by backwashing, reversing the flow, often with air scour, to lift and clean the bed.

Filter design picks a loading rate, the number of units (so that one can be out for washing), media size and depth, and the underdrain and wash-water systems. Head loss rises as the bed clogs; when it reaches a limit or turbidity breaks through, the filter is washed.

Filtration is followed by disinfection, usually chlorination, to kill remaining pathogens and keep a residual in the network.