Abstract: The present invention relates to a method and an apparatus for biological purification of wastewater with no excess activated sludge by subjecting a machanically and/or chemically pretreated wastewater to the conditions of total endogenous respiration at aeration with one or more surface turbine type aerators of 5.10.sup.-2 to 12.10.sup.0.2 kW/m.sup.3 of wastewater, at a microorganism to food ratio of 20 to 100. The height of the liquid level in the basin is at least 5 m. There is provided a high-grade effluent at low capital and operational investment.