Nitrifier activity as a cross-facility screening indicator of N2O emission potential in full-scale wastewater treatment facilities

초록

Nitrous oxide (N2O) dominates the process carbon footprint of biological nitrogen removal, but direct monitoring remains impractical. N2O emissions were measured at seven municipal wastewater treatment plants and one landfill leachate facility over 53 campaigns using a common flux-chamber protocol, together with water quality and the respirometric activities of ammonia-and nitrite-oxidizing bacteria (sOUR-AOB, sOUR-NOB). Quantified removal-based emission factors ranged from 0.0002% to 31.5% of removed nitrogen and showed little relationship with process configuration. The municipal median on an influent-nitrogen basis was approximately 160-fold lower than the IPCC 2019 default. Machine-learning models reached R2 up to 0.33 under random cross-validation but failed across all 15 algorithm-feature-set combinations under leave-one-facility-out validation. Facility identity alone reached R2 = 0.45, indicating facility-specific rather than transferable relationships. In contrast, an activity index based on sOUR-AOB, the AOB/NOB activity ratio, and inorganic carbon removal identified elevated-emission states (AUC 0.78), and the three-indicator block remained significant after facility adjustment (p = 0.006). Thresholds of sOUR-AOB >= 2.0 mg O2/g MLVSS center dot h and an AOB/NOB ratio >= 3.0 separated tiers with a 72-fold difference in median emission factor. Measuring the top 25% of campaigns captured 42% of emissions, supporting respirometric screening to prioritize N2O measurement and mitigation.

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