SLOW SAND FILTERS SHOULD ONLY BE CLEANED AT LONG INTERVALS, EVEN RATE OF FILTRATION MAY FALL, HOWEVER SURFACE SHOULD LIGHTLY RAKED OR SCRATCHED, TO LEAVE IT LOOSE, AND RESTORED TO IT'S INITIAL POSITION AT CLEAN LEVEL.

Filtration of water consists of the system passing water through a thick layer of sand which acts as a strainer. Suspended and colloidal matter in the water and also a large number of bacteria are caught in the interstices of the sand during it's passage. There are principally three types of filters ;(a) slow sand filters ;(b) Rapid gravity filters;and (c) Pressure filters. As for as the Rural water supply is concerned generally the thickness of the filter is usually between 0.5 and 1.0 m thick but should be thicker if the sand in the bed is coarse, thinner if the sand is fine. The filtering bed area is calculated from the consumption and the rates of filtration, and if the particle radius is 0.3 to 0.5 mm the bed should be approximately 0.8to 1 m deep, the filtration rate being 0.1 to 0.3 m/h ie 0.1 to 0.3 cubic metre of water through 1 meter square of the filter area per hour. As per guidelines of the World Health Organization Sand -Filter is desirable to continuous flow through the filter at all times. The rate of filtration may fall off in time but the filter should only be cleaned at long intervals, possibly several weeks or even months, since it's efficiency depends on the biological growth on the surface of the sand. Trouble with green growths can be eliminated by covering the filter to keep it perfectly dark since the green algae depend upon light for growth. When it becomes necessary to clean the filter, a very thin layer about 0.5 cm can be scraped off and discarded, following which the surface should be lightly raked or scratched to leave it loose. After several such cleanings, the sand should be restored to it's initial level with clean sand after scraping the surface down to a clean level.