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WATER TREATMENT: SAND FILTRATION
Sand filtration can be either rapid or slow. The difference between the two is not a simple matter of the speed of filtration, but in the underlying concept of the treatment process. Slow sand filtration is essentially a biological process whereas rapid sand filtration is a physical treatment process.
Slow sand filters have an advantage over rapid sand filters in that they produce microbiologically "clean" water which should not require disinfection to inactivate any bacteria, although the addition of a disinfectant to provide a residual for the distribution system is still advisable. However, because of their slow flow rate, slow sand filters require large tracts of land if they are to supply large populations and can be relatively labour intensive to operate and maintain. As the reestablishment of the schumtzdecke takes several days, the plant has to have sufficient capacity to supply the water demand when one or more filters are out of action.
Rapid sand filtration is now commonly used worldwide and is far more popular than slow sand filtration. The principal factor in this decision has been the smaller land requirement for rapid sand filters and lower labour costs. However, rapid sand filters do not produce water of the same quality as slow sand filters and a far greater reliance is placed on disinfection to inactivate bacteria. It is also worth noting that rapid sand filters are not effective in removing viruses.
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