Evaluation of nitrous oxide gaseous emissions from a partial nitritation reactor operating under different conditions

In this study, continuous measurements of nitrous oxide gaseous emissions from a lab-scale partial nitritation (PN) chemostat treating ammonium-rich (1500 mgN/L) synthetic wastewater were performed, in order to achieve a deeper knowledge about its potential environmental impact. The reactor was operated at constant hydraulic retention time (HRT, 1 d) and nitrogen loading rate (NLR, 1500 mgN/L·d), with different influent alkalinity/nitrogen molar ratios (alk/N, 1 and 1.3) and dissolved oxygen concentrations (DO, 5.0, 3.0, 2.0, 1.5 and 1.0 mgO2/L). As DO ranged between 5.0 and 1.5 mgO2/L, the partial conversion of ammonium to nitrite was successfully achieved, with a negligible production of nitrate and nitrous oxide. When alk/N was increased from 1 to 1.3 (DO was not limiting, i.e. 2 mgO2/L), the increase in ammonium-nitrogen removal rate (ANRR, from 717±17 to 945±21 mgNH4-N/L·d) and the simultaneous decrease in specific N2O-N production (from 0.33±0.01 to 0.23±0.01% of ANRR) were observed. When DO was set to 1.0 mgO2/L, partial nitritation was irreversibly compromised, and a corresponding increase in specific N2O-N production was observed (from 0.22±0.01 to 0.61±0.03% of ANRR). The results indicate a clear correlation between N2O production and autotrophic ammonium to nitrite oxidation rate.