a method for biological purification of wastewater in which nitrogen phosphorous compounds are biologically degraded in a biotank . the biotank is operated with suspended activated sludge and with fixed film biomass for producing purified water that is directly removable from the tank . oxygen input to the biotank is controlled and regulated for operating the tank under different oxygen environmental conditions . an aerobic zone is formed near the water surface of the tank , and an anoxic zone is formed near the bottom of the tank . at least one transition zone connects the aerobic zone and the anoxic zone . the method produces a total concentration exceeding 3 kilograms of dry solids per cubic meter of suspended activated sludge and fixed film biomass in the tank volume that is used .