Abstract
The characteristics of N2O emissions from an anaerobic/aerobic/anoxic (A/O/A) sequencing biofilm batch reactor (SBBR) were investigated under different influent COD/nitrogen (C/N) ratios (from 1–4). Results indicated that the C/N ratios affected the quantity of polyhydroxybutyrate (PHB) and residual organic substances after the anaerobic period, resulting in the largest N2O emission during aerobic period occurred at a C/N of 2. Moreover, during the anoxic PHB-driven denitrification period, the rapid decline in the dissolved N2O concentration indicated that the nitrite inhibition threshold for N2O reduction increased with the increased C/N ratios, which means the higher influent C/N ratios could lower the inhibition of nitrite on N2O reduction. Finally, more PHB and residual organic substances were provided to denitrification at a high C/N ratio, resulting in less total N2O emission was achieved at a high C/N ratio in the A/O/A SBBR.
Highlights
The characteristics of N2O emissions from an anaerobic/aerobic/anoxic (A/O/A) sequencing biofilm batch reactor (SBBR) were investigated under different influent chemical oxygen demand (COD)/nitrogen (C/N) ratios
During the anoxic PHB-driven denitrification period, the rapid decline in the dissolved N2O concentration indicated that the nitrite inhibition threshold for N2O reduction increased with the increased C/N ratios, which means the higher influent C/N ratios could lower the inhibition of nitrite on N2O reduction
More PHB and residual organic substances were provided to denitrification at a high C/N ratio, resulting in less total N2O emission was achieved at a high C/N ratio in the A/O/A SBBR
Summary
The concentration of the mixed liquor suspended solids (MLSS) was maintained at 3000–3350 mg/L (A certain volume of activated sludge scraped from the biofilm was measured and calculated as the MLSS in SBBR). A constant airflow for aeration was introduced through a fine air diffuser at the bottom of the reactor, and the aeration rate was maintained at 40 L/h (DO concentration was 1.45–2.20 mg/L). The endpoint of the aerobic period was decided by the real-time control method as introduced by Peng et al.[14], and the entire cycle was maintained for 12 h. N2O generation (rg; mg N L−1sec−1; Eq (1)), accumulation (ra; mg N L−1sec−1; Eq (2)) and emission (re; mg N L−1sec−1; Eq (3)) rates were calculated using balances based on on-line N2O liquid concentration measurements. When N2O diffused from water to air, the N2O emission rate can be calculated by Equation (3).
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