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This study demonstrates that microoxic conditions (0.02–0.2 mg-O₂/L dissolved oxygen) significantly enhance ammonium oxidation rates at bioanodes in bioelectrochemical systems, achieving 228 ± 0.4 g-N m⁻³ d⁻¹ compared to 120 ± 21 g-N m⁻³ d⁻¹ under anoxic conditions. Hydroxylamine (NH₂OH) was identified as a rate-limiting intermediate in both pathways. The dominant end-product was dinitrogen gas with 75% conversion efficiency under microoxic conditions and 100% under anoxic conditions, with minimal N₂O production (2.1%), demonstrating selective nitrogen removal without greenhouse gas emissions.

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Abstract

Bioelectrochemical systems (BESs) are considered to be energy-efficient to convert ammonium, which is present in wastewater. The application of BESs as a technology to treat wastewater on an industrial scale is hindered by the slow removal rate and lack of understanding of the underlying ammonium conversion pathways. This study shows ammonium oxidation rates up to 228 ± 0.4 g-N m–3 d–1 under microoxic conditions (dissolved oxygen at 0.02–0.2 mg-O2/L), which is a significant improvement compared to anoxic conditions (120 ± 21 g-N m–3 d–1). We found that this enhancement was related to the formation of hydroxylamine (NH2OH), which is rate limiting in ammonium oxidation by ammonia-oxidizing microorganisms. NH2OH was intermediate in both the absence and presence of oxygen. The dominant end-product of ammonium oxidation was dinitrogen gas, with about 75% conversion efficiency in the presence of a microoxic level of dissolved oxygen and 100% conversion efficiency in the absence of oxygen. This work elucidates the dominant pathways under microoxic and anoxic conditions which is a step toward the application of BESs for ammonium removal in wastewater treatment.

Keywords

Anoxic watersHydroxylamineAmmoniumOxidizing agentAmmoniaWastewater

Identifiers

DOI
10.1021/acs.est.3c02227
Journal
Environmental Science & Technology
Year
2023