10 is above the submerged drum reactors 12 an overflow pipe elbow 20 connecting the vessels 11 and 15 serves for maintaining the envisaged water level 18 in the vessel 11 for this purpose a horizontal section ending in the vessel 11 of the overflow pipe elbow 20 is arranged at a height above the water level 18 a vertical pipe section leading into the vessel 15 of the overflow pipe elbow 20 ends below the water level 21 in the vessel 15 of the second biological stage 13 the water to be treated namely the ground water provided with reducing agents passes through a feed pipe elbow 22 into the vessel 11 of the first biological stage 10 in this illustrative embodiment the horizontal pipe section of the feed pipe elbow 22 enters an upper region of the vessel 11 and its long vertical pipe section protrudes deep into the water which is to be denitrified in the first biological stage 10 in the present illustrative embodiment the feed pipe elbow 22 enters the water from above at about one third of the height of the water level 18 the overflow pipe elbow 20 and the feed pipe elbow 22 are allocated to approximately diagonally opposite corner regions of the vessel 11 of the first anoxic biological stage 10 the upper side 23 of the vessel 15 which in this case has only one submerged drum reactor 14 of the second.