Assessment of Full-Scale N₂O Emission Characteristics and Testing of Control Concepts in an Activated Sludge Wastewater Treatment Plant with Alternating Aerobic and Anoxic Phases

This work aims to obtain full-scale N₂O emission characteristics translatable into viable N₂O control strategies and conduct full-scale testing of the proposed N₂O control concepts. Data of a long-term monitoring campaign was first used to quantify full-scale N₂O emission and probe into the seasonal pattern. Then trends between N₂O production/emission and process variables/conditions during typical operating cycles were revealed to explore the dynamic N₂O emission behavior. A multivariate statistical analysis was performed to find the dependency of N₂O emission on relevant process variables. The results show for the first time that relatively low/high N₂O emission took place in seasons with a decreasing/increasing trend of water temperature, respectively. Aerobic phase contributed to N₂O production/emission probably mainly through the hydroxylamine pathway. Comparatively, heterotrophic bacteria had a dual role in the anoxic phase and could be responsible for both net N₂O production and consumption. Incomplete denitrification might contribute mainly to the N₂O production/emission in the anoxic phase and the accumulation of N₂O to be significantly emitted in the following cycle due to the competition between different denitrification steps for electron donors. Therefore, properly extending the length of anoxic phase could serve as a potential control means to regulate N₂O accumulation in the anoxic phase. The full-scale testing not only verified the efficacy of reduced dissolved oxygen set-point in reducing N₂O emission by 60%, but also confirmed the proposed concepts of control over the aerobic and anoxic phases collectively.