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 firstly 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.