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 ( $E{F}_{N_{2}O}$ ) of 5.67% ± 1.68%. The contribution of N₂O emitted through the gas phase was 97.1%, with a flow-based emission factor ( $FBE{F}_{N_{2}O}$ ) 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 ( $FBR{F}_{N_{2}O}$ ) of 0.11 × 10⁻³ g N₂O-N/L. Although CO₂ flow-based emission factor ( $FBE{F}_{C{O}_2}$ ) 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 ( $GW{I}_{N_{2}O}$ of 83.6% and $GW{I}_{C{O}_2}$ of 16.4%).