Low nitrous oxide production through nitrifier-denitrification in intermittent-feed high-rate nitritation reactors

Area of Science:

• Environmental microbiology
• Wastewater treatment
• Biogeochemical cycles

Background:

• Nitrous oxide (N₂O) is a potent greenhouse gas.
• Autotrophic nitrogen conversion processes, particularly nitritation, are significant sources of N₂O.
• Understanding N₂O production pathways and mitigation strategies is crucial for wastewater treatment.

Purpose of the Study:

• To quantify N₂O production rates and pathways in lab-scale nitritation systems.
• To identify operational conditions that minimize N₂O emissions.
• To elucidate the dominant N₂O production mechanisms under high nitritation efficiency.

Main Methods:

• Operation of two lab-scale sequencing batch reactors with intermittent feeding.
• Maintaining low dissolved oxygen (DO) set-points and temperatures of 20-26 °C.
• In situ application of ¹⁵N labeled substrates to trace N₂O pathways.

Main Results:

• High nitritation efficiency (>93% ammonium conversion to nitrite) was maintained for ~300 days.
• Net N₂O production was low (~2% of oxidized ammonium) under optimal conditions.
• N₂O production rates transiently increased with pH fluctuations after feeding.
• Nitrifier denitrification was identified as the dominant N₂O production pathway.

Conclusions:

• Intermittent feeding and low DO set-points effectively control nitritation and minimize N₂O emissions.
• Nitrifier denitrification is a key pathway for N₂O production in these systems.
• Optimized operational strategies can significantly reduce greenhouse gas emissions from nitrogen removal processes.