The BioSand Filter (BSF) is a household-scale, intermittently-operated slow sand filter invented in its current by Dr. David Manz while he was at the University of Calgary in early 1990s. Slow sand filtration is typically used on a massive scale with filter beds as large as football fields to service large urban centers. They are frequently used as one step in a multi-barrier treatment scheme (e.g. filtration followed by disinfection). Historically slow sand filters were operated continuously which helped keep the biological layer moist and flourishing. The processes that occur in the sand column of the BSF are no different than those described in the previous section. What is unique about the BSF is its design that is able to sustain a biological layer between uses. Two elements of the design contribute to this preservation of the biological layer. First, the BSF is designed to hold 5 cm (2 inches) of water above the top surface of the sand column. Second, a diffuser level prevents entering water from disturbing the top layer of sand. The aim behind these design elements is to provide the greatest protection to the biological layer and the most beneficial environment for its growth. The biological layer needs a continuous stable water environment, and standing water which includes basic nutrients, such as organic matter and oxygen.
How does it work?
This filter is made of concrete on the outside with layers of sand used for water purification on the inside.
The water is filtered by both the sand and by a biological layer that develops at the surface of the water. This layer is known as the biofilm.
It is made of many organisms like algae, plankton, protozoa, and bacteria. Together, these organisms actively trap and break-down organic matter, including other disease-causing organisms, making the water safe to drink.
The BioSand Filter can purify between 20 and 60 liters per hour and can last for decades with simple maintenance.
How are communities chosen to be given Water Filters?
Clear Cambodia takes a number of factors into consideration when choosing which communities should benefit from the Water Filter Program. Firstly, the community must engage with the program, actively participating and collaborating with the Clear Cambodia team. The community must be accessible by road, have a poor water source and need filters in order to access clean water, and sometimes the government of a particular province will suggest specific communities that are needy and in more desperate need of water. Clear Cambodia will go and verify the information and then decide about whether to work in these communities. Often they are the places where filters are offered.
Within the community, Clear Cambodia offers BSFs to families who have poor water sources and who indicate that they need and want a filter.
Slow sand filters have been shown to remove almost all the disease-causing organisms found in water. The Bio-Sand Filter has proven as effective as traditional slow sand filters, in both laboratory and field tests. The filter has been tested by various government, research, and health institutions, as well as by non-governmental agencies. Overall, these studies have shown that the Bio-Sand Filter removes:
• More than 90% of fecal coliform
• 100% of protozoa and helminths 50-90% of organic and inorganic toxicants
• Up to 67% of iron and manganese
• Most suspended sediments
How in the community gets a BSF?
Families/communities who: Have poor water sources indicate that they need and want a filter. It is hard to figure out who are rich and who are poor in a community- sometimes people have a large house but there is nothing inside; not even food. Clear Cambodia will check for signs of wealth aside from just the house.