Filter apparatus using floating filter medium

A filter apparatus using floating filter medium comprising a filter tank separated by a water-penetrable partition wall into an upper section forming a filtrate reservoir and a lower section forming a filter chamber, floating filter medium in the filter chamber, means for feeding water to be filtered into the bottom of the filter chamber, the water being caused to flow upwardly passing through the floating filter medium and through the partition wall, the resulting filtrate being removed from the filtrate reservoir, the filter apparatus being further characterized by the improvement comprising an agitating blade provided at a central location in said filter chamber as viewed from above for causing upward flow of water streams, a pit provided beneath said filter chamber, a back-flow washing water discharge pipe provided in the form of a siphon-shaped pipe with a discharge opening thereof located adjacent the bottom of the pit, said discharge pipe being provided with a siphon breaker having an air feed port positioned at a level adjacent the bottom of the filter chamber, said means for feeding water including a supply pipe having a feed opening positioned at a suitable location at or adjacent the bottom of the filter chamber and outside the pit for feeding the water to be filtered into said filter chamber.