Abstract: A method of biological purification of sewage in a purification plant in order to decompose organic material and in order to remove inorganic compounds, mainly nitrogen compounds, wherein the plant comprises three tanks only, of which at least two sequentially serve as a feeding tank for the sewage and at least one as a clarification tank, and wherein the sewage at any time is subjected to an aerobic treatment in the plant by means of aeration and simultaneous circulation of the sewage, and wherein a tank, which during a period serves as feeding tank, at least during a part of such period is driven in such a manner that sewage is only circulated, substantially, under anaerobic conditions, the tank being indirectly connected, during at least said part of said period, to a tank serving as clarification tank, viz.