Sand filters (multi-layered)
Sand filtration is used for the removal of suspended solids, as well as floating and settable particles. The waste water flows vertically through a bed of fine sand and / or gravel. Present particles are removed by adsorption or physical encapsulation. If the pressure drop across the filter becomes too large, it must be backwashed.
Before rinsing, the dirt is released from the sand by starting to inject air from bottom to top.
Filtered water then flows through the filter bed in countercurrent. The contaminated material is released and flows with the flushing water. The filtration process can resume.
An important advantage of a sand filter is the simple installation with which in many cases a considerable return can be obtained.