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PCT No. PCT/EP97/00411 Sec. 371 Date Sep. 11, 1998 Sec. 102(e) Date Sep. 11, 1998 PCT Filed Jan. 30, 1997 PCT Pub. No. WO97/28094 PCT Pub. Date Aug. 7, 1997A 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.