Water purification using sand

Slow sand filters are used to purify drinking water. Each filter consists of a large tank containing a bed of sand through which water passes at typical rates of 0.1–0.3 m h^–1. Water is cleaned by physico–chemical and biological processes occurring at the air–water interface, within the bulk water, over the surface of the sand, and within the bed of sand. The processes found in sand filters replicate many of those found in natural sand banks and sandy beaches.