Center for Affordable Water and Sanitation Technology
A biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community drinking water treatment for 200 years.
The biosand filter is smaller (about 1 m tall, 0.3 m wide on each side) and adapted so that it does not flow continuously, making it suitable for use in people’s homes. The filter container can be made of concrete or plastic. It is filled with layers of specially selected and prepared sand and gravel. The sand removes pathogens and suspended solids from contaminated drinking water. A biological community of bacteria and other micro-organisms grows in the top 2 cm of sand. This is called the biolayer. The micro-organisms in the biolayer eat many of the pathogens in the water, improving the water treatment.
Contaminated water is poured into the top of the biosand filter at least once per day (but not continuously). The water poured into the top of the filter slowly drips through the holes in the diffuser, and flows down through the sand and gravel. Treated water flows out of the outlet tube. No power is required - the filter works by gravity. It should take about 1 hour to get 12-18 litres of filtered drinking water.
Pathogens and suspended solids are removed through biological and physical processes that take place in the sand. These processes include: mechanical trapping, predation, adsorption, and natural death.
The biosand filter has been studied in the field and in labs. It has been shown to remove the following from contaminated water:
Designed for developing country contexts. CAWST headquarters in Calgary, Alberta, Canada.
Goal 6: Ensure availability and sustainable management of water and sanitation for all.