Sand filters are used for water purification. There are three main types

Rapid (gravity) sand filters

Upflow sand filters

Slow sand filters

All three methods are used extensively in the water industry throughout the world.

The first two require the use of flocculation chemicals to work effectively whilst slow sand filters can produce very high quality water free from pathogens, taste and o dour without the need for chemical aids.

Sand filters become clogged with floc after a period in use and they are then back washed or pressure washed to remove the floc. This backwash water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed off as a waste-water stream. In some countries the sludge may be used as a soil conditioner. Inadequate filter maintenance has been the cause of occasional drinking water contamination.

Sand filters are occasionally used in the treatment of sewage as a final polishing stage (see Sewage treatment). In these filters the sand traps residual suspended material and bacteria and provides a physical matrix for bacterial decomposition of nitrogenous material, including ammonia and nitrates, into nitrogen gas.