Abstract: A process for biological purification of waste water by the activated sludge method wherein the waste water is continuously passed through one or more treatment zones and a clarification zone, wherein the waste water treated in the last treatment zone at the outlet from the zone under suitable control of aeration and stirring is continuously divided into a predominantly aqueous fraction and a sludge-containing fraction, and wherein a portion of the sludge fraction is recirculated and mixed with non-treated waste water, of which the former is subjected to clarification, and the latter is recirculated to the same or a preceding treatment zone.

Abstract: A process and an apparatus for biological purification of waste water by the activated sludge method, wherein the waste water is continuously passed through one or more treatment zones and a clarification zone in the same tank under such conditions that an accumulation of sludge occurs in the treatment zone from which the treated waste water is withdrawn, and wherein the direction of flow through the tank is reversed, so that the non-treated waste water, after accumulation of sludge has occured at the outlet end of the tank, in a succeeding period, is introduced at this end, and treated waste water is withdrawn at the opposite end.

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.