Modelling of nitrous oxide emissions from an A2/O process treating municipal wastewater

Nitrous oxide (N2O) is a greenhouse gas significantly contributing to the greenhouse effect and potentially generated during the biological nutrient removal in wastewater treatment plants (WWTPs). The 3 possible microbial pathways for the N2O production are the incomplete hydroxylamine oxidation, the nitrifier denitrification and the heterotrophic denitrification. The first two, both followed by Ammonia Oxidising Bacteria (AOB), are considered as the most important N2O production mechanisms. In this work, a modified version of the IWA ASM2d model expanded to describe nitrification and 4-step denitrification was combined with a 2-pathway model describing N2O production by AOB. The ultimate goal was the development of a mathematical tool predicting the N2O emissions out of municipal wastewater treatment data. Although low dissolved oxygen (DO) is applied to decrease the energy requirements in WWTPs, our model estimates that low DO (<1.8 mgL 1 ) combined with a high influent ammonium (ΝΗ4 + ) concentration (>20.0 mgL -1 ) are conditions working to the advantage of partial nitrification and triggering N2O production through the nitrifier denitrification pathway.