"A review on nitrous oxide (N2O) emissions during biological nutrient removal from municipal wastewater and sludge reject water" by Theoni Maria Massara, Simos Malamis, Albert Guisasola, Juan Antonio Baeza, Constantinos Noutsopoulos and Evina Katsou
Nitrous oxide (N2O) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as the nitritation/denitritation and the completely autotrophic N-removal. The microbial pathways leading to the N2O emission include the hydroxylamine oxidation, the nitrifier denitrification by ammonia oxidizing bacteria (AOB) and the heterotrophic denitrification. In this work, a critical review of the existing literature on the N2O emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the N2O emissions are identified: low dissolved oxygen, low chemical oxygen demand to nitrogen ratio, elevated pH combined with low temperature, the accumulation of nitrite and the slow growth of denitrifying bacteria. However, there is no common pattern in reporting the N2O generation amongst the cited studies, fact that complicates its evaluation. When simulating N2O emissions, all the microbial pathways along with the potential contribution of abiotic N2O production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of these models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous N2O dynamics. Thus, the choice of the N-removal process, the optimal selection of parameters and the establishment of validated dynamic models combining multiple N2O pathways are essential for studying the emissions mitigation.