Sand filter

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

1. rapid (gravity) sand filters
2. upflow sand filters (e.g. DynaSand)
3. slow sand filters including bio-sand filters

The first two are are used extensively in the water industry and the third is used in water supply as a cheap and effective way of removing pathogens and solids from raw drinking water.

Passing flocculated water through a rapid gravity sand filter strains out the floc and the particles trapped within it. The medium of the filter is sand of varying grades. As water flows through the sand, impurities such as solids precipitate and in some cases even bacterial particles are filtered out, reducing turbidity. Where taste and odour may be a problem (organoleptic impacts), the sand filter may include a layer of activated carbon to remove the taste and odour. Sand filters may become clogged with floc after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into special settling tanks so that the floc can precipitate out and is then disposed of as waste material. In some countries this may be used as a soil conditioner. Inadequate filter maintenance has been the cause of occasional drinking water contamination. A good source of information is the American Water Works Association (AWWA).

A different type of sand filter is sometimes used for sewage treatment as well. The sand traps suspended material and bacteria living in the sand decompose ammonia and nitrates into nitrogen gas.