Biosand filter

A biosand filter (BSF) is a point-of-use water treatment system adapted from traditional slow sand filters. Biosand filters remove pathogens and suspended solids from water using biological and physical processes that take place in a sand column covered with a biofilm. BSFs have been shown to remove heavy metals, turbidity, bacteria, viruses and protozoa. BSFs also reduce discoloration, odor and unpleasant taste. Studies have shown a correlation between use of BSFs and a decrease in occurrence of diarrhea. Because of their effectiveness, ease of use, and lack of recurring costs, biosand filters are often considered appropriate technology in developing countries. It is estimated that over 200,000 BSFs are in use worldwide.

The household biosand filter was proposed by Dr. David Manz in the late 1980s at the University of Calgary, Canada. The system was developed from the slow sand filter, a technology that has been used for drinking water purification since the 1800s. Initial lab and field tests were conducted in 1991; the system was patented in 1993 and was implemented in the field in Nicaragua. The Canadian non-profit company Center for Affordable Water and Sanitation Technology (CAWST) was co-founded in 2001 by David Manz and Camille Dow Baker to promote education and training in water purification and sanitation including using this technology, and to continue developing it. A privately owned company, Hydraid Biosand Water Filter produces and distributes plans for filters.

Biosand filters are typically constructed from concrete or plastic. At the top of the filter, a tightly fitted lid prevents contamination and unwanted pests from entering the filter. Below this, the diffuser plate prevents disturbance of the biofilm when water is poured into the filter. Water then travels through the sand column, which removes pathogens and suspended solids. Below the sand column, a layer of gravel prevents sand from entering the drainage layer and clogging the outlet tube. Below the separating layer is the drainage layer consisting of coarser gravel that prevents clogging near the base of the outlet tube.

Pathogens and suspended solids are removed by biological and physical processes that take place in the biolayer and the sand layer. These processes include:

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