The average N2O emission rate
The average N2O emission rate for a two-week monitoring period was calculated by summing up the N2O emissions from the individual cells. The emission from each cell was found by multiplying the off-gas N2O concentration from the cell by the measured airflow to that cell. For practical reasons it is not possible to measure the off-gas concentration in every cell simultaneously. For this study the N2O emissions from the anoxic zones are assumed to be small and have been ignored (see ref. 1 for more information).
The average N2O emission rate for MLE4 for the two-week period was calculated to 4.15 kg N2O-N/d. Based on the average influent total nitrogen load to the plant during this period with 25% going to MLE4, an emission factor (EFN2O) of 0.005 kg N2O-N/kgNinfluent (0.5% N2O-N/TNinfluent ) can be deducted.
Long-Term Monitoring Recommended
The survey was conducted over a short period and the recommendation is that longer-term monitoring will provide further confidence in the EFN2O figure. Monitoring for a period of at least a year would be required to understand the seasonal variations.
1Unisense Environment (2024, February). ”N2O Mass Transfer Coefficient Calculation from Aeration Field Size and Air Flow”. unisense-environment.com/manuals/
2Prediction of Wastewater Treatment Greenhouse Gas Emissions Using a Real-Time Model, David Hume (Mott MacDonald), Kenny Williamson (Watercare Services Ltd.), Kevan Brian (Watercare Services Ltd.), Nick Dempsey (Mott MacDonald) Water New Zealand Conference and Expo 2022.