Effects of influent COD/N ratios on nitrous oxide emission in a sequencing biofilm batch reactor for simultaneous nitrogen and phosphorus removal

The characteristics of N(2)O 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 N(2)O 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 N(2)O concentration indicated that the nitrite inhibition threshold for N(2)O reduction increased with the increased C/N ratios, which means the higher influent C/N ratios could lower the inhibition of nitrite on N(2)O reduction. Finally, more PHB and residual organic substances were provided to denitrification at a high C/N ratio, resulting in less total N(2)O emission was achieved at a high C/N ratio in the A/O/A SBBR.