Abstract: A method and an apparatus for biological purification of waste water by the activated sludge method, whereby the waste water is continuously passed through several treatment zones in which the waste water is successively subjected to nitrification and denitrification. The treatment zones are arranged circularly around a supply zone for waste water, whereby the supply zone is alternatingly connected with one of the treatment zones, and optionally a clarification zone for dividing the treated waste water into a water fraction and a sludge fraction.

Abstract: The present invention entails a nitrification-denitrification wastewater treatment process where nitrification and denitrification is carried out in three separate treatment basins. First, wastewater influent is directed from a supply line into a first basin where the wastewater is subject to denitrification. Wastewater held in a second basin is subjected to nitrification while the influent wastewater is directed into the first basin. Effluent from the first basin that is being filled is directed directly to a third basin which performs both denitrification and nitrification at various times during the total process. Purified wastewater from the third basin is directed to a clarifier which separates activated sludge from the purified wastewater and the activated sludge is returned and mixed with the influent wastewater. After a selected time period the influent wastewater being directed to the first treatment basin is switched to where the wastewater influent is now directed into the second treatment basin.