NOVELTY - A waste water purification apparatus comprises anaerobic tank (21) which purifies waste water which flows in from waste water inflow tube, aerobic tank (22) connected to anaerobic tank, methane-fermentation apparatus (41) which decomposes sludge in waste water with methane bacterium, waste water delivery valve (40) which makes waste water which flowed in from anaerobic tank flow in into methane-fermentation apparatus, and digestive liquid delivery valve (45) which sends out digestive liquid which had biological oxygen demand (BOD) value reduced by methane bacterium.    USE - A waste water purification apparatus.    ADVANTAGE - The apparatus can maintain sewage disposal rate to appropriate level.    DETAILED DESCRIPTION - A waste water purification apparatus comprises anaerobic tank (21) which purifies waste water which flows in from waste water inflow tube, aerobic tank (22) connected to anaerobic tank, treated-water discharge pipe which discharges treated water purified by aerobic tank, methane-fermentation apparatus (41) which decomposes sludge in waste water with methane bacterium, generates methane and reduces BOD value of waste water, waste water delivery valve (40) which makes waste water which flowed in from anaerobic tank or waste-water inflow tube flow in into methane-fermentation apparatus, digestive liquid delivery valve (45) which sends out digestive liquid which had BOD value reduced by methane bacterium rather than natural water of waste water, first solenoid valve provided in anaerobic tank inflow tube, second solenoid valve provided in aerobic tank inflow tube, and third solenoid valve provided in treated-water side inflow tube. The digestive liquid delivery valve branches to anaerobic tank inflow tube, aerobic tank inflow tube, and treated-water side inflow tube. The anaerobic tank inflow tube is connected to anaerobic tank. The aerobic tank inflow tube is connected to aerobic tank. The treated-water side inflow tube is connected to treated-water discharge pipe. The BOD control part determines outflow destination of digestive liquid by comparison with upper limit and minimum of BOD value of digestive liquid that BOD sensor in methane-fermentation apparatus detects and BOD reference value of aerobic tank. The first solenoid valve is opened and digestive liquid is made to flow into anaerobic tank when BOD value of digestive liquid is larger than upper limit of BOD reference value of aerobic tank. The second solenoid valve is opened and digestive liquid is made to flow into aerobic tank when BOD value of digestive liquid is smaller than upper limit of BOD reference value of aerobic tank and aerobic tank is larger than minimum of BOD reference value. The third solenoid valve is opened and digestive liquid is made to flow into treated-water drain pipe when BOD value of digestive liquid is smaller than minimum of BOD reference value of aerobic tank.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of waste water purification apparatus. (Drawing includes non-English language text).    Anaerobic tank (21)    Aerobic tank (22)    Waste water delivery valve (40)    Methane-fermentation apparatus (41)    Digestive liquid delivery valve (45)