accommodate variations in the waste water concentration if complete nitrification is not desired or at least substantial nitrification is desired it is possible depending on the characteristics of the waste water to set the cod volume loading of biological reactor 1 at a higher level no upper limit for the nitrogen volume loading in biological reactor 2 has so far been determined volume loadings of up to about 0 4 kg of total nitrogen m 3 d were completely nitrified the recycle of the activated sludge water mixture from biological reactor 2 to biological reactor 1 may vary within a wide range it is expediently calculated and so selected that an operating cost optimum which is dependent on the pumping energy to be delivered on the one hand and on the costs of the neutralizing agent and the costs of the reducing agent in biological reactor 3 on the other hand is achieved if necessary the quantity of nitrate which exceeds the level of denitrification which can be obtained in biological reactor 1 can additionally be reduced in a third stage biological reactor 3 by adding a biologically oxidizable substance for example methanol based on the specified redox potential so that in the event the specified value of the redox potential is exceeded or not achieved the amount of the said substance supplied.