QUANTIFICATION OF EMITTED AND RETAINED N2O IN A MUNICIPAL WASTE-WATERTREATMENT-PLANT WITH ACTIVATED-SLUDGE AND NITRIFYING-DENITRIFYING UNITS

About 0.4-0.5 Tg nitrogen (1 Tg = 10(9) kg) are annually channeled thr ough German waste water treatment plants. To quantify the gaseous N2O losses during the conversion of the above nitrogen inputs in situ meas urements were carried out in the activated sludge, the nitrification-d enitrification unit and the outflow of the waste water treatment plant of Giessen, Germany, during 1993-1996. Self-constructed floating PVC chambers were used to collect N2O. The gas was sucked and trapped on m olecular sieve traps and quantified in the laboratory by gaschromatogr aphy (ECD). Until 1994 the municipal waste water of Giessen was purifi ed only by an activated sludge unit. From 1994-1996 the waste water of meanwhile 150.000 inhabitants was splitted and purified partly by the activated sludge and partly by newly installed nitrification-denitrif ication units. Since 1996 the waste water is purified completely by ni trification-denitrification. In addition to N2O emissions, NH4+-N, NO3 --N, NO2--N, BOD5, water temperature, pO(2) pH and the N2O dissolved i n the waste water were determined. The dissolved N2O was quantified ga schromatographically after heating the samples for 80 min at 80 degree s C. Generally speaking all compartments of the waste water treatment plant, Giessen, emitted annually about 188 kg N2O-N which corresponds roughly to about 0.02 % of the N input. During the winter period 1995- 1996 an additional amount of 1005 kg N2O-N was discharged as dissolved N2O into the receiving river Lahn. These considerable amounts of N2O leaving a waste water treatment plant in dissolved form should not be overlooked in nitrogen balances. Second, the well and homogeneously ae rated nitrification tank released about 45 times more N2O than the den itrification or 94 times more than the activated sludge unit. This sur prising result should be considered if waste water treatment plants ar e extended more and more by nitrification-denitrifcation units. Finall y, the contributions of waste water treatment plants to the overall Ge rman N2O budget seems to be relatively small so far. Because this very effective and relatively stable greenhouse gas N2O may increase in mo dern waste water treatment plants, the release of this trace gas shoul d not be underestimated as potential global hazard on a longterm scale .