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

In this study, a laboratory-scale partial nitritation (PN) reactor was fed with a synthetic medium simulating the ammonium-rich wastewater produced by the anaerobic digestion of food waste. The reactor was operated at constant hydraulic retention time (1 d) and nitrogen loading rate (1.5 g N/L d), with different influent alkalinity to ammonium-nitrogen molar ratios (Alk/N, 1 and 1.3) and dissolved oxygen (DO) concentrations (5.0, 3.0, 2.0, 1.5, and 1.0 mg O-2/L). For each operating condition tested, nitrous oxide (N2O) gaseous emissions from the PN reactor were measured via infrared gas-filter correlation, in order to achieve a deeper understanding of process potential environmental impact. As DO concentration ranged between 5.0 and 1.5 mg O-2/L, the partial conversion of ammonium to nitrite was successfully achieved, with negligible nitrate production and nitrous oxide emission. When Alk/N was increased from 1 to 1.3 (DO was not limiting), the increase in ammonium-nitrogen oxidation rate (AOR, from 717 +/- 17 to 945 +/- 21 mg NH4-N/L d) and the simultaneous decrease in N2O-N emission factor (from 0.33% +/- 0.01% to 0.23% +/- 0.01% of AOR) were observed. When DO was set to 1.0 mg O-2/L, PN was irreversibly compromised, and a corresponding increase in N2O-N emission factor was observed (from 0.22% +/- 0.01% to 0.61% +/- 0.03% of AOR). Nitrifier denitrification was suggested as the main pathway contributing to N2O emission. Minimization of anoxic conditions may contribute in reducing greenhouse gas emissions even at low DO concentrations, as long as they are not process-limiting.