Methane and nitrous oxide emission from different treatment units of municipal wastewater treatment plants in Southwest Germany

We measured the atmospheric emission rates of methane (CH(4)) and nitrous oxide (N(2)O) in two wastewater treatment plants in Southwest Germany, which apply different treatment technologies. Dissolved gas concentrations and fluxes were measured during all processing steps as well as in the discharge receiving streams. N(2)O isotopocule analysis revealed that NH(2)OH oxidation during nitrification contributed 86–96% of the N(2)O production in the nitrification tank, whereas microbial denitrification was the main production pathway in the denitrification tank in a conventional activated sludge (CAS) system. During wastewater treatment using a modified Ludzack-Ettinger system (MLE) with energy recovery, N(2)O was predominantly produced by the NO(2)(-) reduction by nitrifier-denitrification process. For both systems, N(2)O emissions were low, with emission factors of 0.008% and 0.001% for the MLE and the CAS system, respectively. In the effluent-receiving streams, bacterial denitrification and nitrification contributed nearly equally to N(2)O production. The CH(4) emission from the MLE system was estimated as 118.1 g-C d(-1), which corresponds to an emission factor of 0.004%, and was three times lower than the emission from the CAS system with 0.01%.