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SLOW SAND AND DIATOMACEOUS EARTH FILTRATION 9.29

mated or operated manually. Where automation is used, the precoat cycle, body feed, and
washing cycles may be initiated and performed automatically. The washing cycle may be
automatically initiated when maximum head loss is reached, or the staff operator may be
alerted by an alarm so that the cycle may be manually initiated.
A flow-measuring device on the outlet of pressure filters controls system flow with a
throttling valve on the product water line. Most installations use a supply pump that op-
erates at a fixed head, and the flow is controlled by the throttling valve. In some instal-
lations, it may be advantageous to use a variable-speed supply pump instead of the con-
trol valve to maintain required flow. For vacuum filters, a level control may be added to
maintain a relatively fixed water level in the filters while the flow device controls outlet flow.

BIBLIOGRAPHY

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Calgon Carbon Corporation. GAC Sandwich Filter. Pittsburgh, Pa.: Water Services, February 1994.
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