Nitrous oxide and methane emissions from a plug-flow full-scale bioreactor and assessment of its carbon footprint

Abstract

Fugitive greenhouse gas (GHG) emissions in the form of nitrous oxide (N2O) and methane (CH4) have been reported from many different wastewater treatment plants. However, the majority of the current literature only reports emissions during short periods of time and only focuses on one of the two GHGs. In this study, N2O and CH4 emissions from the aerated parts of a plug-flow full-scale bioreactor treating municipal wastewater were studied over five months from November through March. A multiple gas hood collection system was used to simultaneously monitor the first three aerated compartments of the plug-flow bioreactor. Results show temporal variations in N2O emissions with N2O detected during November, no emissions during December and January, and a recovery of emissions from February onwards. In addition, different spatial emissions were found across the three aerated zones, with the highest N2O emissions detected in the second aerated zone. A daily N2O emission pattern was characterised by an N2O peak correlated with the ammonium that arrived in the monitored zone. However, CH4 emissions occurred during the whole monitored period and showed a spatial variability inside the plug-flow bioreactor, presenting the highest emissions in the first aerated zone and then decreasing in the two subsequent zones. In addition, the dynamic carbon footprint (C-footprint) of the bioreactor is presented in which the contribution of the direct and indirect emissions (related to electricity consumption) is assessed. Results show that CH4 emissions account for the majority of the direct emissions. Moreover, CH4 and N2O emissions represent approximately 60% of the total emissions (direct and indirect) originating from the bioreactors.