BioSand Filters remove 95.0 to 99.0% of organic contaminants, including bacteria, viruses, protozoa, worms, and particles. Safe water produced by the filters is free of discoloration, odor, and unpleasant taste , and can be used for drinking, food preparation, personal hygiene, and sanitation. Most common home-based models can produce between 20 and 60 litres of water per hour.
The BioSand Water Filter (BSF) was developed by Dr. David Manz, while a professor and researcher at the University of Calgary, in Calgary, Alberta. BSF development began in 1990 and has continued, involving numerous individuals and organizations that actively develop and deploy the technology. While several commercial and community-scale implementations exist, the largest use of BSF technology has been in the humanitarian arena. The relief organization Samaritan's Purse has installed 70,000 filters world-wide , and has recently embarked on an initiative, called Turn on the Tap to install at least 65,000 more filters by 2010.
Most BioSand Filters are constructed from concrete, though new plastic models are being tested. Gravel and sand are layered inside the filter with a PVC collection pipe situated at the base of the filter. Contaminated water from rain, surface, or ground sources is poured through the top of the filter and passes through a plate that diffuses the stream and blocks large contaminants (e.g. stones, large twigs, leaves).
The top few centimeters of the sand trap the bulk of micro-organisms, which accumulate and develop into a highly active food chain, called the Biological Layer or Schmutzdeke. The biological layer, which must remain partially wet, traps and feeds on the micro-organisms and contaminants in the water. Further filtration occurs in the lower layers of sand and gravel, which removes contaminants that cause odour, cloudiness, and taste.
A 2007 study conducted by the School of Public Health at the University of North Carolina at Chapel Hill indicates that BioSand Water Filters can reduce the incidence of diarrheal illness by up to 40 percent.
Over time, the top layers of sand may become clogged with material, causing flow rates to drop. A simple stirring or skimming of the top layer of sand is usually sufficient to restore optimal flow. Frequency of needed maintenance is dependent on the quality of the source water. Although longitudinal studies have not been completed, Samaritan's Purse reports that filters have remained in effective operation for over ten years.