Collier's New Encyclopedia (1921)/Filter bed
FILTER BED, a settling pond whose
bottom is a filter. It may consist of a
reservoir five feet deep, with a paved
bottom covered with open-jointed tubular
drains leading into a central conduit.
The drains are covered with a layer of
gravel, and a top layer of sand. The
water is delivered upon the surface
uniformly, and the rate of subsidence is
about six inches an hour. The more

rapid the rate (other things being equal)
the less effective the operation.