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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