An improved process for the removal of organic substances and nitrogen compounds from waste water by means of micro-organism, wherein waste water to be treated is passed continuously through a denitrification zone, effluent from the denitrification zone is passed into a nitrification zone, and at least a portion of the effluent from the nitrification zone is recycled back to the denitrification zone. A portion of the waste water to be treated is fed directly into the nitrification zone as well as to the denitrifcation zone.