Nitrous oxide emission and nutrient removal in aerobic granular sludge sequencing batch reactors

Application of aerobic granular sludge into wastewater treatment is promising due to its excellent settling ability and high microbial concentrations. However, its spatial structure could induce incomplete denitrification, leading to generation of nitrous oxide (N2O) – a potent greenhouse gas. Under the temperature of 14 ± 4 °C, three identical laboratory-scale aerobic granular sludge sequencing batch reactors (SBRs) were established to treat synthetic wastewater simulating a mixture of liquid pig manure digestate and municipal wastewater at three aeration rates (0.2, 0.6 and 1.0 L air/min) and three COD:N ratios (1:0.22, 1:0.15 and 1:0.11). The studies show the proportions of N2O emission to the influent nitrogen loading rate at the aeration rates of 0.2, 0.6 and 1.0 L air/min were 8.2%, 6.1% and 3.8% at a COD:N ratio of 1:0.22; 7.0%, 5.1% and 3.5% at a COD:N ratio of 1:0.15; and 4.4%, 2.9% and 2.2% at a COD:N ratio of 1:0.11, respectively. With NO2− as the only nitrogen source in the liquid phase, the specific N2O generation rates via denitrification were 1.7, 1.6 and 1.3 μg N2O/(g SS· min) at the aeration rates of 0.2, 0.6 and 1.0 L air/min, respectively, which were 40.9%, 44.8%, 39.9% higher than those with NO3− as the only nitrogen source, respectively. N2O generation by aerobic granular sludge due to NH4+–N nitrification was not sensitive to the aeration rate, and the average specific N2O generation rate was 0.8 ± 0.02 μg N2O/(g SS· min). [Display omitted] ► It is the first time to study N2O emission by aerobic granules using microsensors. ► N2O emission to the influent nitrogen loading rate was up to 2.2–8.2%. ► N2O emission was affected by the aeration rate and the COD:N ratio. ► N2O generation with NO2− as the nitrogen substrate was higher than with NO3−.