Raw water flows down

wards
through the filter bed and as the suspended matter- which has usually
been treated by addition of a coagulant like alum-

is retained on the
sand surface and between the sand grains immediately below the surface.
There is steady rise in the loss of

head as the filter process continues
and the flow reduces once the Pressure sand filter drop across the
filter is excessive.
The Pressure sand filter is now taken out of
service and cleaning of the filter is effected by flow reversal. To
assist in

cleaning the bed, the backwash operation is often preceded by
air agitation through the under drain system. The process of air
scouring agitates the sand with a scrubbing action, which loosens the
intercepted particles. The filter is now ready to be put back

into
service. Picture Low Flow PSF (FRP Vessel) and High flow custom Vessel
(MS Vessel) Sand filter - Sand filters are used for

water purification.

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