Heterotrophic denitrification plays an important role in N₂O production from nitritation reactors treating anaerobic sludge digestion liquor

Nitrous oxide (N₂O) emissions from nitritation reactors receiving real anaerobic sludge digestion liquor have been reported to be substantially higher than those from reactors receiving synthetic digestion liquor. This study aims to identify the causes for the difference, and to develop strategies to reduce N₂O emissions from reactors treating real digestion liquor. Two sequencing batch reactors (SBRs) performing nitritation, fed with real (SBR-R) and synthetic (SBR-S) digestion liquors, respectively, were employed. The N₂O emission factors for SBR-R and SBR-S were determined to be 3.12% and 0.80% of the NH4+-N oxidized, respectively. Heterotrophic denitrification supported by the organic carbon present in the real digestion liquor was found to be the key contributor to the higher N₂O emission from SBR-R. Heterotrophic nitrite reduction likely stopped at N₂O (rather than N₂), with a hypothesised cause being free nitrous acid inhibition. This implies that all nitrite reduced by heterotrophic bacteria was converted to and emitted as N₂O. Increasing dissolved oxygen (DO) concentration from 0.5 to 1.0mg/L, or above, decreased aerobic N₂O production from 2.0% to 0.5% in SBR-R, whereas aerobic N₂O production in SBR-S remained almost unchanged (at approximately 0.5%). We hypothesised that DO at 1mg/L or above suppressed heterotrophic nitrite reduction thus reduced aerobic heterotrophic N₂O production. We recommend that DO in a nitritation system receiving anaerobic sludge digestion liquor should be maintained at approximately 1mg/L to minimise N₂O emission. © 2014 Elsevier Ltd.