The Mechanism of Aggravating N ₂ O Emission in a Partial Nitritation Reactor with Increasing bCOD/NH ₄ ⁺ –N

Partial nitritation (PN) is an important source of nitrous oxide (N2O) emissions, and a low biodegradable chemical oxygen demand to ammonium (bCOD/NH4+–N) ratio tends to aggravate N2O emissions. However, the specific release characteristics and potential mechanisms with increasing bCOD/NH4+–N within a low ratio range have not been elucidated. In this study, a continuous-flow PN reactor was operated using food waste digestate characterized by fluctuating low bCOD/NH4+–N ratios. When the ratio rose from 0.12 to 1.36, the gaseous N2O emission factor increased from 3.2% to 25.8%. Three mechanisms were inferred to be associated with this increase: (1) To maintain the nitrite-to-ammonium ratio at higher bCOD/NH4+–N, the ammonia oxidation efficiency (AOE) was elevated from 57.7% to 75.1%, which increased the electron flux used for nitrite reduction; (2) to sustain the nitrite-to-ammonium ratio, the aeration coefficient was increased, which elevated the N2O mass transfer coefficient (KLa) from 32 to 705 d–1 and consequently enhanced N2O emissions; (3) the increasing bCOD/NH4+–N induced a nonuniform upregulation of functional genes, the (nirK+nirS)/nosZ rose from 2.62 to 3.14, enhancing the N2O-producing capacity. Overall, AOE-driven electron flux enrichment, aeration coefficient-driven KLa enhancement, and uneven functional gene promotion may have jointly contributed to the increase in N2O emissions with increasing bCOD/NH4+–N.