SYSTEM FOR REMOVING NITROGEN AND PHOSPHORUS IN DRAIN AND REMOVING METHOD

PROBLEM TO BE SOLVED: To provide a treating system capable of efficiently removing nitrogen and phosphorus in drain. SOLUTION: A drain is treated by providing an oxygen-free biological treating zone 4a at a head and an anaerobic biological treating zone 5 successively, and moreover, alternately arranging plural aerobic biological treating zones 6a and 6b and at least one oxygen-free biological treating zone 4b. A dissolved oxygen and combined oxygen in the drain and sludge are consumed and reduced in the oxygen-free biological treating zone at the head, and a stable and efficient phosphorus releasing is realized in the anaerobic biological treating zone in absolutely oxygen-free state, and the phosphorus in the drain is favorably removed by phosphorus-over-intake at the following aerobic biological treating zone and oxygen-free biological treating zone, and the nitrogen is removed by repetition of nitrification and denitrification.