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The application of Appropriate Technology

The biosand filter is a modified form of the traditional slow sand filter in such a way that the filters can be built on a smaller scale and can be operated intermittently. These modifications make the biosand filter suitable for household or small group use. The biosand filter can be produced locally anywhere in the world using materials that are readily available.

It was considered impractical to operate a slow sand filter intermittently, due to the need for a continuous supply of food and oxygen. However, Dr. Manz of the University of Calgary re-designed the traditional sand filter, making it suitable for intermittent use at a household level. This adaptation, brilliant in simplicity, consists of raising the under drain pipe back up to between 1 and 8 cm above the sand level, ensuring a foolproof method for maintaining the water level just above the sand. Manz proved that, even when water is not continually added to the filter, oxygen can still permeate into the water to reach the organisms living in the sand by diffusion accross this shallow layer of standing water.

Intermittently-operated slow sand filters can be small units that easily supply enough clean water for a family. Therefore, they are particularly suited for use in low-income countries, where the majority of people rely on untreated, contaminated surface water. Find out in detail how intermittently-operated slow sand filters work.

A couple of great manuals from Biosandfilter.org on making moulds and casting biosand filters for domestic use:

Another document about casting the filters from concrete, this one is from CAWST (Centre for Affordable Water and Sanitation Technology):