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.