NOVELTY - A waste water is purified using biological reactor having an anaerobic tank and an aerobic tank. Wastewater is introduced into anaerobic tank, reacted with anaerobic microbe in an anaerobic tank, and pollutant material is decomposed. Anaerobic processing water is provided, and anaerobic processing water is introduced into aerobic tank from the anaerobic tank. When the detected value of nitrogen oxide exceeds a threshold value, the amount of aeration of anaerobic processing water by aeration mechanism is increased, to obtain membrane separation activated-sludge.    USE - Processing of membrane separation activated-sludge (claimed).    ADVANTAGE - The membrane separation activated-sludge is efficiently processed with reduced energy consumption, and suppressed generation of fouling and nitrogen oxide.    DETAILED DESCRIPTION - A waste water is purified using biological reactor having an anaerobic tank and an aerobic tank. Wastewater is introduced into anaerobic tank, reacted with anaerobic microbe in an anaerobic tank, and pollutant material is decomposed. Anaerobic processing water is provided, and anaerobic processing water is introduced into aerobic tank from the anaerobic tank. The anaerobic processing water is sent to the membrane separation unit provided in the aerobic tank, and anaerobic processing water is aerated to an aeration mechanism in the presence of activated sludge containing an aerobic microbe. The anaerobic processing water is reacted with an aerobic microbe in aeration atmosphere, and pollutant material is decomposed in the anaerobic processing water. The anaerobic processing water is separated into solid and liquid by membrane separation unit, and treated water which permeated filtration membrane is discharged from the aerobic tank. Nitrogen oxide produced within aerobic tank is detected. When the detected value of nitrogen oxide exceeds a threshold value, the amount of aeration of anaerobic processing water by aeration mechanism is increased, to obtain membrane separation activated-sludge. An INDEPENDENT CLAIM is included for membrane separation activated sludge process equipment.