Nitrous oxide emissions from aerobic granular sludge

Abstract

The emissions of climate-relevant nitrous oxides from wastewater treatment with aerobic granular sludge (AGS) are of special interest due to considerable structural as well as microbiological differences compared with flocculent sludge. Due to the compact and large structures, AGS is characterised by the formation of zones with different dissolved oxygen (DO) and substrate gradients, which allows simultaneous nitrification and denitrification (SND). N2O emissions from AGS were investigated using laboratory-scale SBR fed with municipal wastewater. Special attention was paid to the effects of different organic loading rates (OLR) and aeration strategies. Emission factors (EF) were in a range of 0.54% to 4.8% (gN2O/gNH4-Nox.) under constant aerobic conditions during the aerated phase and different OLR. Higher OLR and SND were found to increase the N2O emissions. A comparative measurement of two similarly operated SBR with AGS showed that the reactor operated under constant aerobic conditions (DO of 2 mg L−1) emitted more N2O than the SBR with an alternating aeration strategy. Total nitrogen (TN) removal was significantly higher with the alternating aeration since non-aerated periods lead to increased anoxic zones inside the granules. The constant aerobic operation was found to promote the accumulation of NO2-N, which could explain the differences in the N2O levels.