Hybrid Nitrous Oxide Production from a Partial Nitrifying Bioreactor: Hydroxylamine Interactions with Nitrite

Area of Science:

• Environmental Microbiology
• Biogeochemical Cycles
• Wastewater Treatment

Background:

• Nitrous oxide (N₂O) is a potent greenhouse gas with significant emissions from wastewater treatment processes.
• Partial nitrification (PN) bioreactors are employed for nitrogen removal, but N₂O production mechanisms require elucidation.
• Ammonia-oxidizing bacteria (AOB) play a key role in nitrification and are implicated in N₂O generation.

Purpose of the Study:

• To investigate the specific mechanisms and pathways of nitrous oxide (N₂O) production in a partial nitrification (PN) bioreactor.
• To quantify the contribution of different pathways, including N-nitrosation, to N₂O generation by ammonia-oxidizing bacteria (AOB).
• To assess the role of abiotic reactions in N₂O formation within the PN system.

Main Methods:

• Enrichment of AOB from a sequencing batch reactor (SBR) and subsequent N₂O pathway tests.
• Isotopolog analysis of N₂O using stable isotope spiking (¹⁵N-labeled nitrite and hydroxylamine).
• Quantification of N₂O isotopologs (⁴⁴N₂O, ⁴⁵N₂O, ⁴⁶N₂O) and measurement of AOB functional gene mRNA levels (haoA, nirK, norB).

Main Results:

• ¹⁵NH₂OH spiking significantly boosted N₂O production and increased mRNA levels of AOB functional genes.
• Predominant production of ⁴⁵N₂O (46%) indicated N-nitrosation hybrid reaction as a major pathway, coupling ¹⁵NH₂OH with ¹⁴NO₂^-.
• Significant abiotic N₂O production via the hybrid pathway was observed, accounting for approximately 51% of total N₂O under specific conditions (¹⁵NH₄⁺ and high NO₂^-).

Conclusions:

• The N-nitrosation hybrid reaction is a dominant pathway for N₂O production in PN bioreactors, involving both biotic and abiotic processes.
• Understanding these multiple N₂O production pathways is essential for developing strategies to mitigate greenhouse gas emissions from wastewater treatment.
• The study highlights the complexity of N₂O formation and the need for targeted interventions to improve the environmental footprint of PN systems.