Slow sand filtration

Slow Sand Filtration
Passage of raw water through a bed of sand at low velocity, resulting in substantial removal of chemical and biological contaminants.
Sludge ...

slow sand filtration A process involving passage of raw water through a bed of sand at low velocity (generally less than 0.4 m/hr) resulting in substantial particulate removal by physical and biological mechanisms.

See also: Conservation, Aggregate, Solid waste, Safe, Wildlife