Index >> Drinking Water Microbiology >> Rapid Sand Filters

The removal of impurities from raw water in rapid filtration is affected by a combination of different processes like straining, sedimentation, adsorption and bacterial and biochemical processes.
In rapid sand filters straining of impurities is not an effective process because of the large pore size of the filter bed. In addition, sedimentation also will not be an effective process as filtration rate is high. Thus, relatively less impurities are retained by the filterbed through straining and sedimentation.
Adsorption of impurities having an electric charge, onto the filter bed grains with an opposite charge is the only effective process taking place in rapid sand filtration. The filter bed material itself has a natural static charge.

This can be corrected by a phenomenon known as back washing. Here water is forcibly introduced into the filters in the upward direction from the bottom at a rate of about ten times the normal filtration rate.

This process expands the sand bed and causes the sand grains to be agitated so that the clogged material is washed off from them. Sometimes backwashing may not be sufficient to keep the filter clean. In that case along with water air also can be used for backwashing