Higher N 2 O production in sequencing batch reactors compared to continuous stirred tank reactors: effect of feast-famine cycles

Nitrous oxide (N 2 O) 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 N 2 O 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 NO 2 − -N removal efficiencies over 99.9 %, but the N 2 O-N emissions from the SBR reached ∼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 N 2 O accumulation ratio was ∼0.1 % during the feast period, increased to ∼1.9 % in the first five hours of the famine period, and then gradually reached ∼2.3 % at the end of famine. Batch experiments showed that limiting extracellular electron donor is required for N 2 O accumulation in cells from the SBR-famine period and that cells from the CSTR do not accumulate N 2 O 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 N 2 O production in the SBR.