Abstract: A filtration apparatus whose interior is divided into a filtrate zone and a filtering zone. The filtering zone is adapted to receive a stream of raw-water and a lower portion of its volume is filled with filtering grains, and the filtrate zone is adapted to receive a filtrate obtained from passage of said stream of raw-water via the filtering grains and a perforated member. The apparatus further includes a pressure reducing device in fluid communication with the lower portion of the filtering zone and with an upper portion thereof. The pressure reducing device is adapted to receive a stream of water and responsively to continuously remove filtering grains from said filtering zone and separate filtration residues therefrom by the reduction of pressure conditions thereinside, and direct a stream comprising the stream of water and the separated filtering grains and filtration residues to the upper portion of said filtering zone.