to free nitrogen which leaves in its gaseous form the bacteria use the energy thus produced partly for basic metabolism and growth and partly for storaging phosphate in the form of polyphosphate the partially treated water from the filter 2 flows through the conduit 7 to the filter 3 wherein aerobic conditions are maintained this means that a biological conversion of ammonia nitrogen into nitrate is effected therein simultaneously with any organic matter present being decomposed hereby nitrate containing waste water is produced of which a part is recycled to the filter 2 through the recycling conduit 9 after a suitable period the plant is changed over by supplying the untreated waste water to the filter 2 as shown in fig2 and the water leaving the filter 2 is conducted to the filter 1 through the conduit 6 from where following treatment it is conducted to the filter 3 through the conduit 11 in phase 2 anaerobic conditions are maintained in the filter 2 and anoxic conditions in the filter 1 whereas aerobic conditions are maintained in the filter 3 like in phase 1 after a suitable period of time the plant is again changed over and a new phase 1 is initiated in the described embodiment of the process according to the invention an accumulation of phosphorus accumulating bacteria is effected in filters 1 and 2 and part of the bacteria are removed from the filters at intervals by backwashing when the.