It is the process of separating colloidal and suspended impurities from water by passing it through a porous bed made of fine sand and other proper sized granular materials. Filtration is carried out in a sand filter. It consists of a top thick layer of fine sand, placed over coarse sand layer followed by gravel. Water comes from the top.
Small micro-organisms present in sand voids decompose organic matter and remove some of the biological impurities.