Fil′ter-bed.
A settling pond whose bottom is a filter. It may consist of a reservoir 5 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 is the operation. See filter.