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[[Image:CAWST_Biosand.png|thumb|right|150px| Cross-section through a concrete biosand filter]]The biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community water treatment for hundreds of years. The BSF is smaller and adapted for intermittent use, making it suitable for households.
Water treatment is carried out by the sand inside the filter. The filter container can be made of concrete, plastic or any other water-proof, rust-proof and non-toxic material, though concrete has several advantages.
The concrete BSF typically uses a box about 0.9 m tall by 0.3 m square, or about 0.3 m in diameter. The filter box is cast from a steel mold or made with pre-fabricated pipe. The container is filled with layers of sieved and washed sand and gravel (also referred to as filter media). There is a standing water height of 5 cm above the sand layer. The different layers trap and eliminate sediments, pathogens and other impurities from the water. Similar to in slow sand filters, a biological layer of microorganisms (also known as the biolayer or schmutzedecke) develops at the sand surface, which contributes to the water treatment.
A perforated diffuser plate or basin is used to protect the biolayer from disturbance when water is poured into the filter.
| process=Filtration
| watcrit=Turbidity < 50 NTU
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- Cement can be easily acquired in most developing countries. <br>
- People are familiar with the construction techniques using cement. <br>
- Household labour or volunteer labour can be utilized in the manufacturing process <br>
- The concrete container is heavy and durable. It does not need to be replaced as often as a plastic container <br>
- The spout piping is located inside the filter. Consequently, it is less prone to damage than a plastic filter, which has piping on its exterior <br>
- Plastic fabrication usually requires the importation of raw material or the finished product. Depending on your views of international trade, this can be an advantage or disadvantage. <br>
- The life expectancy of a plastic model is substantially less than a concrete model. The ultimate disposal of plastic is a concern for the environment <br>
- Encouraging and empowering local people to completely manage their own project increases local sustainability and motivates others to take action for safer water. This process can only be facilitated if the product supply is managed locally. In most cases, if a plastic model is used, there will always be a dependence on outside manufacturers and distributors to supply filters at a higher cost. <br>
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====Manufacturing====
Local production of filters is most commonbecause materials are readily available all over the world. Molds can be borrowed, rented, bought or constructed locally. Filters can be constructed at a central production facility, or in the community. Filter sand and gravel can be prepared (sieved and washed) on-site or nearby.
Materials required are