Slow sand filters are perhaps the most efficient means of producing clean drinking water.
Slow sand filters rely on biological processes for their action rather than physical filtration or disinfection. They require no electricity, no chemicals and no filter changes.
How is this possible? The secret is that slow sand filters work through the formation of a gelatinous layer called the hypogeal layer or “Schmutzedecke” on top of a layer of fine sand. This layer consists of bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. As a Schmutzedecke ages more algae tend to develop and larger aquatic organisms may be present including some Endoprocta, Snails and Annelid worms.
The Schmutzedecke is the layer that provides the purification for water treatment — the underlying sand provides the support medium for this biological layer. As water passes through the Schmutzedecke, particles of foreign matter are trapped in the mucilaginous matrix and dissolved organic material is adsorbed and metabolized by the bacteria fungi and protozoa.
The water draining from a well managed slow sand filter can be of exceptionally good quality with no detectable bacterial content.
Blackburn and Associates produce a slow sand filters for residential use.