Characteristics of Nitrous Oxide (N2O) Emissions from Intermittently-Aerated Sequencing Batch Reactors Treating the Separated Liquid Fraction of Anaerobically Digested Pig Manure

In this study, the N2O emission from an intermittently aerated sequencing batch reactor (IASBR-1) treating the separated liquid fraction of anaerobically digested pig manure (SLAP) was investigated. The wastewater had chemical oxygen demand (COD) concentrations of 11,540 ± 860 mg l−1, 5-day biochemical oxygen demand (BOD5) concentrations of 2,900 ± 200 mg l−1and total nitrogen concentrations of 4,041 ± 59 mg l−1, with low COD:N ratios (2.9, on average) and BOD5:N ratios (0.72, on average). Synthetic wastewater, simulating the SLAP with similar COD and nitrogen concentrations but with higher BOD5 concentrations of up to 11,500 ± 100 mg l−1, was treated in another identical reactor (IASBR-2) to compare the effects of carbon source on nutrient removals and N2O emissions. In steady-state, soluble N2O accumulated in the non-aeration periods, with the highest N2O concentrations measured at the end of the non-aeration periods. There was a significant reduction in N2O concentrations during the aeration periods with reductions occurring immediately on commencement of aeration. The mean N2O emissions in an operational cycle were 253.6 and 205.3 mg for IASBR-1 and IASBR-2, respectively. During the non-aeration periods, only 8.3% and 8.4% of total N2O emissions occurred in IASBR-1 and IASBR-2, respectively; while during the aeration periods, 91.7% and 91.6% of N2O emissions took place in IASBR-1 and IASBR-2, respectively. The mean specific N2O generation rates were 0.010 and 0.005 mg (g VSS·min)−1 in the aeration periods, 0.024 and 0.021 mg (g VSS·min)−1 in the non-aeration periods for IASBR-1 and IABSR-2, respectively. Mean nitrogen removal via N2O emissions was 15.6% and 10.1% for IASBR-1 and IASBR-2, respectively. The IASBR-1 with low influent BOD5 concentrations emitted and generated more N2O.