Higher N2O production in sequencing batch reactors compared to continuous stirred tank reactors: effect of feast-famine cycles

Nitrous oxide (N2O) is a potent greenhouse gas that can be emitted during the biological treatment of wastewater. In this study, a comparison of the long-term performance characteristics and N2O production of sequencing batch reactors (SBR) and continuous stirred tank reactors (CSTR) during nitrite-based denitrification was undertaken. It was found that both reactors had NO2-N removal efficiencies over 99.9 %, but the N2O-N emissions from the SBR reached similar to 2.3 % of the removal nitrite-N load, while in the CSTR it never exceeded 0.1 %. High frequency sampling during one operation cycle of the SBR demonstrated that the N2O accumulation ratio was similar to 0.1 % during the feast period, increased to similar to 1.9 % in the first five hours of the famine period, and then gradually reached similar to 2.3 % at the end of famine. Batch experiments showed that limiting extracellular electron donor is required for N2O accumulation in cells from the SBR-famine period and that cells from the CSTR do not accumulate N2O when either nitrite or carbon is limiting. Another notable difference in the two reactor communities was the high level of accumulation of intracellular granules, most likely polyhydroxybutyrate (PHB), in cells during the feast period in the SBR. Metagenome assembly and binning found that one genome (PRO1), which is a Thauera, accounted for over half the metagenomic reads in both reactors. Neither shifts in gene regulation nor community composition explained the observed differences in reactor performance suggesting some post-transcriptional regulation obligatorily linked to antecedent conditions underly increased N2O production in the SBR. (C) Higher Education Press 2023