In order to achieve a sustainable water-quality management, nitrous oxide emissions from wastewater treatment plants need to be reduced due to their relevance for climate protection. Since nitrous oxide can not only occur as a product of nitrification, but also as an product of denitrification in the course of biological wastewater treatment, the course of denitrification was investigated more detailed in the context of this diploma thesis. The research took place on a laboratory-scaled pilot plant, in the laboratory of the Institute for Water Quality, Resource Management and Waste Management of the Vienna University of Technology. The research was divided into three phases. In the first research phase were the general optimization and regulation of the operation performed (aeration intensity, duration of the aerated and non-aerated interval, etc.). In the second phase, special tests were carried out on individual days by metering the additional substrate (ethanol or methanol) intermittently during the denitrification. The intention was to increase the availability of organic carbon (COD) during denitrification and at the same time to investigate its influence on nitrous oxide accumulation in dissolved form. In the third phase the additional substrate was added directly in the inflow, to enable the increased availability of the organic carbon during the whole treatment. Throughout the operation of the pilot plant, N2O emissions were measured on-line in order to capture the variations over a longer period of time, depending on load and operating conditions. N2O emissions were continuously measured in both the liquid phase and the gas phase. On-line measurement of the N2O concentration in the air space was carried out with the N2O-IR spectrometer. An N2O microsensor (Unisense) was used for the continuous determination of dissolved nitrous oxide in the activated sludge. The individual experiments have contributed to a better understanding of the factors influencing nitrous oxide accumulation in the course of denitrification and have shown how nitrous oxide accumulation can be prevented, above all, by increasing the amount of the available COD in wastewater. Increasing COD availability or decreasing the N/COD ratio significantly reduces the accumulation of N2O during denitrification by enabling a complete process of denitrification. Both types of COD sources, that have been used (ethanol and methanol), have ensured success, whereat the activated sludge required an additional adaption period in order to use methanol as a carbon source.