A bio-sand filter (BSF) is a point-of-use water treatment system adapted from traditional slow sand filters. Bio-sand 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, bio-sand filters are often considered appropriate technology in developing countries. It is estimated that over 200,000 BSFs are in use worldwide.
2 Bio-sand filter components
3 Filtration processes
3.1 During the run
3.2 Pause period (idle time)
5 Removal of contaminants
5.2 Heavy metals
6 Health benefits
The household bio-sand 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 Bio-sand Water Filter produces and distributes plans for filters.
Bio-sand filter components
Basic Diagram of a Concrete Bio-Sand Filter
Bio-sand 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:
Mechanical trapping: Suspended solids and pathogens are trapped in the spaces between the sand grains.
Predation: Pathogens are consumed by microorganisms in the biolayer.
Absorption: Pathogens are absorbed into each other and to suspended solids in the water and sand grains.
Natural death: Pathogens finish their life cycles or die because there is not enough food or oxygen.
During the run
The high-water level (hydraulic head) in the inlet reservoir zone pushes the water through the diffuser and filter, then decreases as water flows evenly through the sand. The flow rate slows because there is less pressure to force the water through the filter. The inlet water contains dissolved oxygen, nutrients and contaminants. It provides the oxygen required by the microorganisms in the biofilm. Large suspended particles and pathogens are trapped in the top of the sand and partially plug the pore spaces between the sand grains. This causes the flow rate to decrease.
Pause period (idle time)
Idle time typically comprises greater than 80% of the daily cycle; during this time, microbial attenuation processes are likely to be significant. Most removal occurs where water is in contact with the biofilm. The processes that occur in the biofilm have not been identified. When the standing water layer reaches the level of outlet tube, the flow stops. Ideally, this should be high enough to keep the biofilm in the sand layer wet and allow oxygen to diffuse through the standing water to the biolayer. The pause period allows microorganisms in the biolayer to consume the pathogens and nutrients in the water. The rate of flow through the filter is restored as they are consumed. If the pause period is too long, the biolayer will consume all of the pathogens and nutrients, and will die, reducing the efficiency of the filter when it is used again. The pause period should be between 1 and 48 hours. Pathogens in the non-biological zone die from a lack of nutrients and oxygen.
Over time, particles accumulate between the filter's sand grains. As more water is poured, a biofilm forms along the top of the diffuser plate. Both of these occurrences cause a decrease in flow rate (clogging and bio-clogging). Although slower flow rates generally improve water filtration due to idle time [APS1], it may become too slow for the users’ convenience. If flow rates fall below 0.1 litre/minute, it is recommended by CAWST to perform maintenance.] The "swirl and dump", or wet harrowing cleaning technique, is used to restore flow rate. About 1 US gallon (3.8 l) is poured into the filter before cleaning (assuming the filter is empty). The upper layer of sand is then swirled in a circular motion. Dirty water from the swirling is dumped out and the sand is smoothed out at the top. This process is repeated until flow rate is restored. Cleaning the diffuser plate, outlet tube, lid, and outside surfaces of the filters regularly is also recommended. Long-term sustainability and efficacy of bio-sand filters depends on education and support from knowledgeable support personnel.
Removal of contaminants
Results for turbidity reductions vary depending on the turbidity of the influent water. Turbid water contains sand, silt and clay. Feed turbidity in one study ranged from 1.86 to 3.9 NTU. In a study water was obtained from sample taps of water treatment plants from three local reservoirs. It poured through a slow sand filter and results showed that turbidity decreased to a mean of 1.45 NTU. In another study using surface water a 93% reduction in turbidity was observed. As the biofilm above the sand ripens, turbidity removal increases. Although bio-sand filters remove much turbidity, slow sand filters, which have a slower filtration rate, remove more.
There is limited research on removal of heavy metals by bio-sand filters. In a study conducted in South Africa, the filter removed about 64% of iron and 5% of magnesium.
The Bio-Sand filter is the same process the Creator designed into the Earth's hydologic cycle used to purify water. The Earth filters most of it and the Sun takes care of the rest through evaporation, condensation and precipitation. This process can be duplicated using the on-board Water distillers, both electric and non-electric units.