Process and apparatus for separating off suspended matter from liquids

An apparatus comprising a gas-introduction reactor and a downflow reactor and a pressure release flotation cell for separating off suspended matter from liquids which are present in the gas-introduction reactor by pressure release flotation wherein the apparatus provides said gas introduction reactor having means for introducing gas into the liquid which contains said suspended matter, said downflow reactor comprises means for conducting the gasified liquid containing suspended matter in the downflow reactor at a flow velocity of 0.01 m/s to 0.3 m/s downwardly against the natural direction of bubble rise, while said downflow reactor further includes means for introducing gas in a lower portion of said downflow reactor which contacts said downwardly flowing gasified liquid containing suspended matter, withdrawing a gas-treated liquid containing suspended matter at the bottom of the downflow reactor and feeding it to said pressure release flotation cell wherein suspended matter attached to microbubbles is separated off by floating away from the liquid, and wherein the downflow reactor has a slenderness ratio of height to diameter (H/D)>1, and is arranged in the gas-introduction reactor, said gas introduction reactor having a liquid depth of 5 to 30 m.