Abstract: A method of biologically cleaning polluted water uses active sludge and at least three zones, where at least one zone operates as a biological treatment zone and at least two zones variably operate as a biological treatment zone and as a clarifying zone. By this method, the polluted water is fed to a zone operating as a permanent biological treatment zone, where a back-mixing is performed during a period between this zone and a zone variably operating as a biological treatment zone and which has operated as a clarifying zone during a previous period. As a result, the drawbacks are overcome which the known T- and V-channel methods are encumbered with.

Abstract: A method and plant for the purification of polluted water by the activated sludge method is provided in that a mixture of polluted water and recycled sludge is subjected to biological treatments in at least two treatment zones, and maintaining for a period such conditions in one of the treatment zones that a sludge precipitation occurs therein, supplying in a subsequent period the mixture of polluted water and recycled sludge directly or indirectly to the zone which, during the preceding period, served as a sludge precipitation zone prior to further treatment thereof in one or more treatment zones, and passing the treated water to a permanent clarification zone and separating it into a water fraction and a sludge fraction, at least a part of the sludge fraction being recycled and mixed with polluted water.