The impact of a seasonal change in loading rate on the nitrous oxide emissions at the WWTP of a tourist region

Nitrous oxide (N2O) is a powerful greenhouse gas (GHG) whose production and emission must be minimised from wastewater treatment plants (WWTPs) to avoid undesirable impacts to climate change and the ozone layer. WWTPs operated in tourist regions undergo large seasonal changes to the influent loading rates of organic matter, nitrogen and phosphorus, which operators must respond to by changing their operational conditions. This study examines the impact of a change in low to high season on the N2O emissions of an activated sludge WWTP in a well-known tourist region in the Algarve, Portugal. While literature studies have suggested that increases in the nitrogen and organic loading rates can promote increased N2O emissions, we have found higher N2O emissions in the low season (7.4% kgN2O-N·kgNH4-N−1), where these loading rates were lower. It was found that the impact of accompanying operational changes to the WWTP outweighed any change caused by the increased loading rate, where the aeration rate showed a significant correlation with N2O emission dynamics. The location of the N2O fluxes observed as well as the dissolved vs gaseous N2O levels suggested that the hydroxylamine oxidation pathway was likely to be of higher relevance towards N2O production as compared to nitrifier denitrification. This study contributes towards the understanding of operational factors impacting N2O emissions at full-scale WWTPs and potential mitigation strategies.