Nitrous Oxide Production and Hydroxylamine Accumulation in a Partial Nitritation Sequencing Batch Reactor : Comparison of Different Operational Strategies

Sequencing batch reactors (SBRs) performing partial nitritation (PN) for treating high-strength ammonium wastewater are known to exhibit elevated levels of nitrous oxide (NO) emissions. This study investigated NO production and hydroxylamine accumulation in a PN-SBR operated using three distinct strategies. The NO emission factor (EF) and net production rate (N2OR) were determined under stable conditions for (i) single feeding with continuous aeration and one microaerobic stage before settling (strategy I), yielding EF = 4.4% and N2OR = 14 mg N g VSS d; (ii) single feeding with multiple microaerobic stages distributed throughout the cycle (strategy II), yielding EF = 13.5% and N2OR = 85 mg N g VSS d; and (iii) step feeding with one single microaerobic stage before settling (strategy III), yielding EF = 10% and N2OR = 45 mg N g VSS d. The distribution of microaerobic stages throughout the cycle (strategy II) promoted the highest hydroxylamine accumulation (0.18 mg N L) during the aerated stage, whereas strategy I showed the lowest accumulation (0.01 mg N L). A strong positive correlation (R ≥ 0.9) was observed among the specific ammonium oxidation rate (AOR), specific N2OR, and bulk liquid hydroxylamine concentration during the aerated stages.