Pathogen Removal
As with all slow sand filters, the removal of pathogens occurs in the BioSand filter due to a combination of biological and mechanical processes. Organic material is trapped at - or very close to - the surface of the sand, forming a biological layer or 'schmutzdecke'. Over a period of one to three weeks, micro-organisms colonize this part of the filter, where organic food and oxygen derived from the water abounds. These micro-organisms consume bacteria and other pathogens found in the water, thereby providing highly effective water treatment. In addition to this process (known as predation), pathogens are removed as food scarcity and less than optimal temperatures causes their death and subsequent collection at the surface of the sand.
Viruses are adsorbed (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.
Cysts and worms are removed from the water by becoming trapped in spaces which lie between the sand grains.
Between uses,a layer of water (5 cm deep) is maintained above the sand at all times. It is this design feature that distinguishes the Biosand filter from other slow sand filters and which allows for both small scale construction and for intermittent use.
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