NOVELTY - City sewage, which is phosphorus and ammonia containing water, organic industrial wastewater, and polyphosphate accumulating bacteria with the capability to ingest phosphorus are sequentially processed in an anaerobic tank (2) in a reaction tank (1), a low dissolved oxygen (DO) tank (4) located in the front half in an aerobic tank (3), a high DO tank (5) similarly located in the second half in the aerobic tank, and a sedimentation basin (6).    USE - Removal of phosphorus and nitrogen by anaerobic, low dissolved-oxygen, and high dissolved-oxygen activated sludge method.    ADVANTAGE - The method shortens denitrification and phosphorus-removal time, and enables efficient removal of nitrogen and phosphorus.    DETAILED DESCRIPTION - City sewage, which is phosphorus and ammonia containing water, organic industrial wastewater, and polyphosphate accumulating bacteria with the capability to ingest phosphorus are flown into an anaerobic tank in a reaction tank and stirred, and activated-sludge liquid mixture containing nitrifying bacteria which oxidize ammonium ion to nitrous acid and nitrate ion, and the denitrifying bacteria which reduces nitrified nitrous acid and nitrate ion to nitrogen gas, are mixed. While the polyphosphate accumulating bacteria which live in an activated-sludge liquid mixture, under the anaerobic condition in which dissolved oxygen (DO) does not exist, release a phosphate ion in a liquid mixture, nitrous acid and nitrate ion contained in the recycle sludge returned to the anaerobic tank from a sedimentation basin are denitrified to nitrogen gas by denitrifying bacteria within the anaerobic tank, and nitrogen is removed. When the activated-sludge liquid mixture, which flows out of the anaerobic tank flows into a low DO tank located in the front half in an aerobic tank into which air is supplied as bubbles, the polyphosphate accumulating bacteria, which live in an activated-sludge liquid mixture in the low DO state, absorb the phosphate ion in a liquid mixture. Nitrifying bacteria nitrify ammonium ion to nitrous acid and nitrate ion, at the same time it produces a polyphosphate granule in a cell. When denitrifying bacteria denitrifies nitrified nitrous acid and nitrate ion to nitrogen gas, phosphorus and nitrogen are simultaneously removed out of liquid mixture. When the liquid mixture, which flows out of the low DO tank flows into a high DO tank similarly located in the second half in the aerobic tank, at the same time the polyphosphate accumulating bacteria, which live in an activated-sludge liquid mixture in the high DO state, absorb a phosphate ion further. Nitrifying bacteria nitrify ammonium ion which remains in an activated-sludge liquid mixture without the ability to nitrify to nitrous acid and a nitrate ion in the low DO tank. The above processes are controlled such that the range of oxidation-reduction potential (ORP) value (where, ORP value is measured with an ORP meter (7) installed in the anaerobic tank) which causes a phosphate ion to be released efficiently from an activated-sludge liquid mixture is maintained. The ORP value measured with an ORP meter (8) installed in the low DO tank, and DO value measured with DO meter (9), or ammonium-ion value measured with an ammonium-ion meter, and DO value measured with DO meter (10) installed in the high DO tank are controlled to be maintained at preset range respectively by manual control or automatic operation. The activated-sludge liquid mixture formed by passing through the above processes is sent to a sedimentation basin. A portion of the activated-sludge liquid mixture sent to the sedimentation basin is returned to the anaerobic tank. The returned activated-sludge liquid mixture, the city sewage which is new phosphorus- and ammonia-containing water, and organic industrial wastewater are passed through the above processes.    DESCRIPTION OF DRAWING(S) - The drawing shows a process flow diagram of the phosphorus and nitrogen removal method. (Drawing includes non-English language text)    Reaction tank (1)    Anaerobic tank (2)    Aerobic tank (3)    Low DO tank (4)    High DO tank (5)    Sedimentation basin (6)    ORP meters (7,8)    DO meters (9,10)