Slow sand filtration water treatment plants
Practical Action
SLOW SAND FILTERS
The main characteristic of slow filtration is that it eliminates bacteria and viruses in the
water. It is a very effective mechanism because it simulates the natural purification process
that occurs when rain water goes through the crust of the earth down to subterranean rivers.
A thin, jelly-like film forms on the surface of the sand, digesting and eliminating impurities in
the water. Once the water has gone through this layer it seeps into the sand bed, where the
main part of the purifying treatment takes place, and where pure water is obtained.
During the filtration process, impurities come into contact with - and are trapped by - the
grains of sand on the surface. The impurities then go through a chemical and biological
degradation process to become simpler, more inoffensive forms that dissolve or remain on the
sand bed as inert matter until the filter is cleaned.
Other advantages of slow filtration: 1) no chemical substances need be applied, 2) the
system can be managed by a fairly unskilled operator on a regular basis.
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