Backwashing-induced nitrous oxide emissions in denitrifying biofilters: Dissolved N2O accumulation, physical stripping, and enzymatic imbalance.

Controlling nitrous oxide (N2O) emissions during hydraulic disturbances is important for the low-carbon operation of denitrifying biofilters (DNBFs). This study combined orthogonal batch assays with 40-day continuous-flow monitoring to evaluate gaseous N2O emission factors (EFs), dissolved N2O accumulation, and backwashing-induced release under different operating conditions. Batch assays showed that N2O EFs and phase partitioning were highly sensitive to operational conditions, with dissolved N2O reaching up to 0.032 mg/L under high filtration-rate conditions and N2O EF ranging from 5.79% to 15.13%. During continuous operation, the 24-h backwashing regime exhibited the highest N2O EF and dissolved N2O concentration despite lower nitrogen removal performance, reaching 2.80 ± 0.28% and 0.0412 ± 0.0025 mg/L, respectively. Transient monitoring revealed the rapid release of the accumulated dissolved N2O pool during backwashing, with gaseous N2O rising from 0.0005 mg/L (baseline) to 0.01955 mg/L over 15 min and dissolved N2O declining from approximately 0.0390 to 0.0065 mg/L. Endpoint enzyme assays indicated an apparent imbalance between nitric oxide reductase (Nor) and nitrous oxide reductase (Nos) activities under frequent backwashing. Microbial community analysis showed lower Sphaerotilus and higher Acidovorax and Dechloromonas under 24-h backwashing. These findings indicate that the dissolved N2O accumulation and transient physical stripping jointly influence N2O emissions in DNBFs and should be considered when optimizing backwashing strategies.