Assessment of Nitrous Oxide and Carbon Dioxide Emissions and the Carbon Footprint in an Aerobic Granular Sludge Reactor Treating Domestic Wastewater

Abstract

The assessment of nitrous oxide (N2O) and carbon dioxide (CO2) emissions from wastewater treatment processes using aerobic granular biomass is still scarce and most of them deal with synthetic wastewater in well-controlled conditions. Also, the greenhouse gases footprint from aerobic granular process has been poorly described despite a strong impact on climate change. This work quantified and analyzed the N2O and CO2 emissions, as well as the treatment efficiency of a pilot-scale sequencing batch reactor with aerobic granular sludge (AGS) fed with real domestic wastewater. The operational conditions applied to the reactor allowed the development of AGS (1.7 ± 0.40 gVSS/L), with good settleability (SVI30 < 50 mL/g) and removal efficiencies of 84% for BOD5 and 65% for NH4+-N. The relative amount of nitrogen denitrified to N2O was 22.9%, which corresponded to an emission factor of 5.67% ± 1.68%. The contribution of N2O emitted through the gas phase was 97.1%, with a flow-based emission factor of 3.65 × 10−3 g N2O-N/L, while the contribution of N2O released through the liquid phase was 2.9%, with a flow-based release factor of 0.11 × 10−3 g N2O-N/L. Although CO2 flow-based emission factor was higher (0.297 g CO2/L), in terms of carbon footprint, the global warming impact of N2O was five times higher than the impact of CO2 emissions ( of 83.6% and of 16.4%).