Spatially resolved nitrous oxide emissions in wastewater treatment processes

Area of Science:

• Environmental Science
• Environmental Engineering
• Atmospheric Chemistry

Background:

• Nitrous oxide (N2O) is a potent greenhouse gas emitted by wastewater treatment plants (WWTPs).
• The specific origins and dominant sources of N2O emissions within WWTPs, particularly from primary treatment, are not fully understood.
• Methodological limitations have historically led to the underestimation of N2O emissions from primary treatment stages.

Purpose of the Study:

• To investigate and quantify N2O emissions from various treatment stages in Chinese WWTPs.
• To identify the dominant contributors to N2O emissions (primary vs. biological treatment) under different plant operating conditions.
• To elucidate the mechanisms and influencing factors (e.g., influent load, diurnal/seasonal variations) of N2O formation and emission in WWTPs.

Main Methods:

• Spatially resolved N2O emission measurements using zonal odor control systems at seven WWTPs.
• Intensive diurnal and seasonal monitoring at two representative WWTPs (biological-dominant and primary-dominant).
• On-site batch experiments to assess N2O generation from primary treatment processes and microbial contributions.

Main Results:

• Average N2O emission factors varied from 0.014% to 0.122%, with either primary or biological treatment being the main source depending on plant conditions.
• In biological-dominant plants, influent ammonium-nitrogen (NH4+-N) loads strongly correlated with N2O emission rates.
• In primary-dominant plants, sewer-transported N2O contributed significantly (104-168%) to total emissions, and primary treatment microbial production accounted for 43.6% of its gaseous emissions, with higher emissions observed in winter.

Conclusions:

• Primary treatment processes represent a substantial and previously underestimated source of N2O emissions from WWTPs.
• Understanding N2O formation mechanisms and emission hotspots requires considering both primary and biological treatment stages, as well as influent characteristics and environmental factors.
• Accurate N2O emission quantification and source identification are crucial for developing effective mitigation strategies in WWTPs.