Abstract: A submerged filter includes filtering matter in the form of a granular material contained in a vat having a floor for supporting the granular material. The liquid to be treated flows from the bottom to top and is brought into contact with a gas serving for the treatment of the liquid and circulating in the same direction. The filtering material is washed with water and air. The liquid to be treated, the treating gas, and the wash fluids are introduced into the filtering mass through the floor by means of nozzles which extend through the floor. Some of the nozzles essentially serve for introducing the treating gas, while others serve for introducing the liquid to be treated and the fluids for washing the filtering material.