Sand dunes are not only made of sand
but also of water. You don't have to dig very far to see the moisture
in the sand. One of the unique features of sand is that it helps filter
water that passes through it. Go out to the sand and have the students
dig into the sand and describe what they find. Talk about why they think
it is cool and wet. Ask the students where they think the water came
from and where they think it goes. Doing Castles in the Sand prior to
this activity helps students understand how sand and water work together
through a process called capillary action. 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.
There are three main types; All three methods are used extensively in
the water industry throughout the world.

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