Activated sludge method

A method of removing nitrogen and phosphorus from waste water by using activated sludge including aerobic bacteria and changing alternately operational modes in a treatment basin, in accordance with a time schedule, which comprises the steps of:

• adding the waste water continuously and gradually into a mixed liquor in a treatment basin which consists of the waste water and the sludge;
• agitating the mixed liquor in said treatment basin during continuous and gradual adding of the waste water until it presents an anoxic condition and a succeeding anaerobic condition so that denitrification is caused in the anoxic condition and phosphorus is then released from the sludge in the anaerobic condition;
• aerating the mixed liquor in the aerobic condition during the continuous and gradual adding of the waste water into the mixed liquor in the aerobic condition so that nitrification is accomplished while phosphorus is ingested by the sludge;
• repeating a cycle consisting of said agitating and aerating steps at least two times so that each cycle is finished within two hours, the ratio of the agitating time to the aerating time being between one to one and five to one;
• settling the agitated and aerated mixed liquor for separating it into a supernatant liquid and the sludge;
• and
• discharging the supernatant liquid as an effluent and reusing the settled sludge for the treatment of waste water.