Heterotrophic denitrification plays an important role in N 2 O production from nitritation reactors treating anaerobic sludge digestion liquor Qilin Wang a , Guangming Jiang a , Liu Ye a,b , Maite Pijuan a,c , Zhiguo Yuan a,* a Advanced Water Management Centre (AWMC), The University of Queensland, QLD 4072, Australia b School of Chemical Engineering, The University of Queensland, QLD 4072, Australia c Catalan Institute for Water Research (ICRA), Technological Park of the University of Girona, 17003, Spain article info Article history: Received 13 April 2014 Received in revised form 28 May 2014 Accepted 2 June 2014 Available online 11 June 2014 Keywords: Nitrous oxide Heterotrophic denitrification Nitritation Anaerobic digestion liquor Free nitrous acid Dissolved oxygen abstract Nitrous oxide (N 2 O) 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 differ- ence, and to develop strategies to reduce N 2 O 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 N 2 O emission factors for SBR-R and SBR-S were determined to be 3.12% and 0.80% of the NH þ 4 -N oxidized, respectively. Heterotrophic denitrification supported by the organic carbon pre- sent in the real digestion liquor was found to be the key contributor to the higher N 2 O emission from SBR-R. Heterotrophic nitrite reduction likely stopped at N 2 O (rather than N 2 ), with a hypothesised cause being free nitrous acid inhibition. This implies that all nitrite reduced by heterotrophic bacteria was converted to and emitted as N 2 O. Increasing dis- solved oxygen (DO) concentration from 0.5 to 1.0 mg/L, or above, decreased aerobic N 2 O production from 2.0% to 0.5% in SBR-R, whereas aerobic N 2 O production in SBR-S remained almost unchanged (at approximately 0.5%). We hypothesised that DO at 1 mg/L or above suppressed heterotrophic nitrite reduction thus reduced aerobic heterotrophic N 2 O pro- duction. We recommend that DO in a nitritation system receiving anaerobic sludge digestion liquor should be maintained at approximately 1 mg/L to minimise N 2 O emission. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Nitrous oxide (N 2 O) is not only a potent greenhouse gas, with a global warming potential of approximately 265 times stronger than carbon dioxide (CO 2 )(IPCC, 2013), but also leads to the destruction of the stratospheric ozone layer (Ravishankara et al., 2009). Wastewater treatment systems have been identi- fied as a source of N 2 O. N 2 O is produced during both nitrifica- tion and denitrification processes (Desloover et al., 2012; Law * Corresponding author. Tel.: þ61 7 3365 4374; fax: þ61 7 3365 4726. E-mail addresses: zhiguo@awmc.uq.edu.au, wangqilin666@hotmail.com (Z. Yuan). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 62 (2014) 202 e210 http://dx.doi.org/10.1016/j.watres.2014.06.003 0043-1354/© 2014 Elsevier Ltd. All rights reserved.