Influence of environmental conditions on the amount of N2O released from activated sludge in a domestic waste water treatment plant

Waste water purification is characterized by intensive mineralization and nitrification processes. Because of the high O2 demand, temporarily anaerobic conditions may be produced, and denitrification by nitrifying organisms as well as heterotropic denitrification may contribute to N2O release. In situ measurements (1993–1994) suggest that N2O is released from activated sludge in a domestic waste water treatment plant at an average rate of 1040 μg m−2h−1 with a range between zero and 6198 μg m−2h−1. The production of N2O seems to be related to the concentration of NO 2 − and NO 3 − as well as to the pH. In the waste water about 75–200 μg N2O l−1 is dissolved. This N2O is released after discharge into the receiving waters. The N2O is produced essentially by nitrification rather than by heterotropic denitrification. On a long-term scale the increasing use of mechanical-biological waste water purification plants world-wide may add increasingly to the anthropogenic production of N2O, although the present amount of N2O produced is negligible compared to its global terrestrial production.