Contribution of nitrous oxide emissions from wastewater treatment to carbon accounting

Abstract

This paper describes research that investigated the contribution of nitrous oxide (N2O) emissions from wastewater treatment to the greenhouse gas emissions of a wastewater treatment plant (WWTP). The research provided several months of robust data from a large-scale WWTP serving a population equivalent of 284,000. N2O emissions were monitored online at the ventilation system of a covered activated sludge (AS) plant, therefore capturing the complete off-gas stream. This methodology eliminated errors incurred through sampling of small percentages of emission areas and allowed representative continuous measurements. Nitrogen load and dissolved oxygen (DO) were also monitored. To address seasonal variation, data were recorded in two extensive phases. In addition, three separate 24-hour surveys were conducted. Emissions of CO2, CH4 and N2O associated with treatment were calculated using the UK Water Industry Research carbon accounting workbook. This study measured N2O emissions from the AS process (nitrification and denitrification) equivalent to 17.5% of the annual GHG emissions (tonnes CO2e) from processes at the WWTP. The emissions were within the range of published N2O emissions. The diurnal profiles confirmed literature findings of a trend of increased N2O emissions when the DO decreased. The DO in the high rate zone of the aeration lanes should be kept above 1 mg l−1 to avoid favourable conditions for N2O emissions during nitrification.