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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. This biological layer matures over one to three weeks, depending on volume of water put through the filter and the amount of nutrients and micro-organisms in the water.
A perforated diffuser plate or basin is used to protect the biolayer from disturbance when water is poured into the filter.
Pathogens and suspended material are removed from the water through a combination of biological and physical processes. These occur both in both the biolayer and within the sand bed. These processes include: mechanical trapping, adsorption/attraction, predation and natural death: 1.Mechanical trapping: Sediments, cysts and worms are removed from the water by becoming trapped in the spaces between the sand grains. Over time the spaces become smaller, enabling the filter to trap smaller particles sooner in the sand bed. The filter can remove some inorganic compounds and metals if they are attached to other materials or each other. 2.Adsorption or attachment: Viruses are adsorbed or become attached to the sand grains. Once attached, they are metabolized by the cells or are inactivated by antiviral chemicals produced by the organisms in the filter. Certain organic compounds are also adsorbed in the sand and thus removed from the water. 3.Predation: The microorganisms within the "schmutzdecke" or biological layer consume bacteria and other pathogens found in the water, thereby providing highly effective water treatment. 4.Natural death: Food scarcity, less than optimal temperatures and a relatively short life span will cause pathogens to die off and become nutrients for other micro-organisms.
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====Operation====
Contaminated water is poured into the top of the filter on an intermittent basis. The water slowly passes through the diffuser, which dissipates the initial force of the water, and percolates down through the biolayer, sand and gravel. Treated water naturally flows from collects in a pipe at the base of the filter and is propelled through plastic piping encased in the concrete exterior and out of the outletfilter for the user to collect in a safe water container.
The biolayer is the key pathogen removing component of the filter. Without it, the filter is significantly less effective. It may take up to 30 days to establish the biolayer depending on inlet water quality and frequency of use. The water from the filter can be used during the first few weeks while the biolayer is being established, but disinfection is recommended during this time, as during regular on-going use.
The biolayer requires oxygen to survive. When water is flowing through the filter, dissolved oxygen in the water is supplied to the biolayer. During pause times, when the water is not flowing, the oxygen is obtained by diffusion from the air. Correct installation and operation of the biosand filter has a water level of approximately 5 cm above the sand during the pause period. It is this design feature that distinguishes the Biosand filter from other slow sand filters, allowing for small scale construction and intermittent use. The layer of water is shallow enough for oxygen to diffuse through, providing the biological layer with enough oxygen to develop. A water depth of greater than 5 cm results in lower oxygen diffusion to the biolayer. A water depth less than 5 cm may evaporate quickly in hot climates and cause the biolayer to dry out.
A pause period is needed between uses to allow time for the microorganisms in the biolayer to consume pathogens in the water. The recommended pause period is 6 to 12 hours with a minimum of 1 hour and maximum of 48 hours.
====Treatment Efficiency====
Slow sand filters have been proven to almost entirely remove the disease-causing organisms found in water. The Biosand filter adaptation has proven as effective as traditional slow sand filters, in both laboratory and field tests.
In conjunction with the introduction of the technology to communities, the filter has been tested by various government, research, and health institutions, as well as by non-governmental agencies. CAWST carried out water analyses of 107 long term Biosand filter users in Haiti in 2005 and found average removal effectiveness of 98.5% (measuring E. coli, the indicator bacteria recommended by the WHO).
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