Design a Slow Sand Filter for a population of 40,000 with an average rate of water supply of 150 lpcd.
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The filtration rate of a slow sand filter is usually one-twentieth (or less) of the rate of a rapid sand filter or pressure filter.
These filters require large areas of land and a correspondingly large quantity of filter media (sand) and base material (gravel).
Cleaning of the filter is done by surface scrapping which may involve a lot of labor. A slow sand filter is suitable when the availability of land, labor, filter media are at a low cost.
A distinguishing property of slow sand filters is the availability of a thin layer, called the schmutzdecke, which results on the surface of the sand bed and contains a large variety of biologically active microorganisms. It is a very simple and effective technique for purifying surface water.
It will remove practically all of the turbidity from the water as well as most of the pathogens without the addition of chemicals. If turbidity of raw water is high then plain sedimentation would be required to reduce turbidity to some extent so that the fillers arc not unduly loaded.