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The Biosand Water Filter
HOW IT WORKS
Four actions take place to purify the water that comes through Biosand Water Filters. It starts when untreated water is poured into the top of the filter:
1. The layer of water below the diffuser plate that sits atop the sand creates a biologically active zone. Here, resident bacteria will consume any available source of food, and cannibalize other bacteria. Most of the larger bacteria – cholera, fecal coliform, typhoid (which is a form of salmonella), and bacterial dysentery (caused by shigella) – are killed here. The dead bacteria not only provide food to keep this biozone active, but their debris also forms a layer on the top of the sand (known as the schmutzdecke), providing additional filtering.
1. The layer of water below the diffuser plate that sits atop the sand creates a biologically active zone. Here, resident bacteria will consume any available source of food, and cannibalize other bacteria. Most of the larger bacteria – cholera, fecal coliform, typhoid (which is a form of salmonella), and bacterial dysentery (caused by shigella) – are killed here. The dead bacteria not only provide food to keep this biozone active, but their debris also forms a layer on the top of the sand (known as the schmutzdecke), providing additional filtering.
2. The sand used in the filter (quarry sand or high riverbank sand) has coarse, sharp edges. These surfaces carry a slight electric charge that attracts certain bacteria and viruses, which adhere to the surface of the sand and die. This is called adsorption.
3. Since the filter is wet and full of water, there is little or no oxygen in the sand column below the top few inches. This creates a deadly environment for most the bacteria and pathogens that contaminate drinking water. This death from lack of oxygen is called anaerobic die-off.
4. Most bacteria and pathogens that contaminate drinking water are relatively large. As water moves down through the filter, the sand provides a final treatment process – physical filtration – that strains out parasites, worms, remaining bacteria, and fine-grain sediments.
By virtue of these four different treatments, biosand filters have been shown to eliminate 95-99% of bacteria and viruses, and virtually all parasites and worms. The filter is also effective at removing iron and manganese, and can be adapted to remove arsenic as well.
To be 100% effective, a complete treatment system would finish with a disinfection step, such as the addition of bleach after filtering or use of a solar/UV disinfection system. However, because of the dramatic reduction of contaminants that result from the filtering process, use of biosand filters (even without disinfection) will result in significant reductions in disease, provided there is clean water storage and good household sanitation practices. Between continual use of the Biosand Water Filter and good handwashing practices, likely up to 80% of waterborne diseases could be eliminated.
3. Since the filter is wet and full of water, there is little or no oxygen in the sand column below the top few inches. This creates a deadly environment for most the bacteria and pathogens that contaminate drinking water. This death from lack of oxygen is called anaerobic die-
4. Most bacteria and pathogens that contaminate drinking water are relatively large. As water moves down through the filter, the sand provides a final treatment process – physical filtration – that strains out parasites, worms, remaining bacteria, and fine-
By virtue of these four different treatments, biosand filters have been shown to eliminate 95-
To be 100% effective, a complete treatment system would finish with a disinfection step, such as the addition of bleach after filtering or use of a solar/UV disinfection system. However, because of the dramatic reduction of contaminants that result from the filtering process, use of biosand filters (even without disinfection) will result in significant reductions in disease, provided there is clean water storage and good household sanitation practices. Between continual use of the Biosand Water Filter and good handwashing practices, likely up to 80% of waterborne diseases could be eliminated.
For more information about the science behind Biosand Water Filters, visit the Center for Affordable Water and Sanitation Technology (CAWST) online at www.cawst.org.