Abstract: An apparatus for separating suspended solids from a liquid includes a tank, a floating bed of granular filter material held at the upper portion of the tank at the time of filtration, a feed pipe for feeding the liquid at a position between a filtering section and a sedimenting section, a hollow member or members which are arranged within the tank at a position below the position of the filter bed but above the position of the feed pipe, an air supply pipe which blows air up through the hollow member or members, thereby creating a turbulence required for effective backwashing, an arrangement for bringing the filter bed down to a position where the lowermost surface of the filter bed is held below a position substantially flush with the top end of the hollow member or members, a means for discharging a filtrate from the tank which is arranged in the top portion thereof, and a means for discharging a deposit of the suspended solids from the tank, which is equipped at the bottom portion thereof.