This study aims at investigating the relationship between the nitric oxide (NO) and nitrous oxide (N2O) production rates with the ammonia oxidation rate (AORsp) in an enriched AOB culture. Different concentrations of ammonia were applied in a sequential batch reactor (SBR) performing partial nitritation in order to determine the effect of AORsp on N2O and NO production rates. Results showed that NO linearly correlates with the AORsp whereas N2O presents an exponential relationship. The effect of changes on the dissolved oxygen (DO) concentration on the overall NO and N2O emissions was assessed by increasing and decreasing the DO maintaining a constant pH at 7. When DO decreased the AORsp was maintained at the level achieved with the starting DO and led to lower NO and N2O emissions than when DO was increased. Finally, the effect of pH on N2O and NO was also tested by maintaining the DO at 1.5–2 mg O2/L while pH was gradually decreased from 8 to 6.5. Results show that NO was chemically produced due to the addition of HCl when decreasing the pH whereas N2O was only produced biologically and was not affected by the addition of HCl

This study was funded by the Spanish Government (MINECO) (CTM 2011-27163 and CTM 2015-66892-R) and the European Commission (FP7-PEOPLE-2011-CIG 303946). ). M. Pijuan and A. Ribera-Guardia acknowledge the Ramon y Cajal research fellowship (RYC-2009-04959) and the FPI PhD grant (BES-2012-052753) respectively provided by the Spanish Government