Abstract: A method of biological wastewater treatment, having an anaerobic zone, an anoxic zone and an aerobic zone, where raw wastewater is brought to the anaerobic zone, then, the wastewater is directed to the anoxic zone and eventually to the aerobic zone, afterwards, a part of the content of the aerobic zone is returned to the anoxic zone, and the rest of the wastewater from the aerobic zone is directed together with the sludge, directly or through additional zones, to a secondary settling tank where the treated wastewater is separated from the sludge, and the treated wastewater and excess sludge located on the bottom of the secondary settling tank are discharged outside the secondary settling tank, and the rest of the sludge accumulated in the secondary settling tank is passed to the anaerobic zone directly or via the anoxic zone in the process of external recirculation.