Resolving N2O Dynamics in Low-DO BNR: Pilot-Scale Insights from Cyclic Aeration Operation

Abstract Biological nutrient removal (BNR) processes are being increasingly deployed to meet stringent nitrogen limits, yet energy-efficient operation can create conditions (low dissolved oxygen and nitrite accumulation) that may elevate direct nitrous oxide (N2O) emissions. N2O dynamics were investigated in a pilot-scale sequencing batch reactor operated at the Hayward Water Pollution Control Facility (California) and fed with low-strength trickling filter effluent. The SBR was seeded with full-scale activated sludge and operated with cyclic aeration to promote partial nitritation–denitritation. The reactor was run through five operational phases that varied aerobic SRT (aeSRT) (4–8.2 d) and supplemental carbon dosing strategy (MicroC and synthetic fermentate). Online sensors provided high-resolution measurements of dissolved oxygen (DO), pH, oxidation–reduction potential (ORP), ammonium/nitrate (NH4+/NO3–), and dissolved N2O, while grab sampling resolved NH4+, NO2–, and NO3– profiles across the cycle. N2O emission factors were inferred from dissolved N2O measurements rather than directly measured from off-gas emissions. Bench-scale batch assays evaluated nitrification activity under controlled DO. Across all phases, stable nitrification was maintained under low-DO cyclic operation, while denitrification completeness depended strongly on carbon availability. At a cycle-average COD:N 3.6, denitrification was frequently incomplete, resulting in nitrite carryover and reduced total inorganic nitrogen (TIN) removal. Increasing COD:N to 7.9 enabled near-complete NOx removal and substantially improved TIN removal. Dissolved N2O exhibited a consistent cycle pattern: net accumulation during anoxic stages and net depletion during aerated stages, consistent with anoxic production/consumption coupled to loss during aeration. Carbon source, availability, and anoxic reaction time strongly influenced anoxic N2O behavior: MicroC dosing promoted N2O accumulation during anoxic intervals, whereas synthetic fermentate drove rapid N2O reduction and minimized net N2O accumulation, particularly at higher COD:NOx. While increased N2O accumulation did occur with MicroC, higher dosing rates and/or longer anoxic hydraulic retention times helped mitigate N2O risk. These results highlight carbon form and NOx-responsive dosing as key levers to balance shortcut nitrogen removal performance with low N2O emissions under cyclic aeration.