Sand filter

A sand filter or "bacteria bed" is a layer of sand that is used to clean the water of bacteria and other microorganisms.^[1] It is a type of filter.^[2]

A layer of sand or gravel is used when sewage goes through sewage treatment.^[1] Before sewage comes to the bacteria bed, it should already have gone through part of the sewage treatment.^[3] At the bacteria bed, sewage gets exposed to (or has contact with) air, while microorganisms make some change to that sewage.^[1] The oxygen in the water kills the microbes, which are anaerobic.