Document Type

Article

Publication Date

2026

DOI

10.1002/wer.70593

Publication Title

Water Environment Research

Volume

98

Issue

10

Pages

e70593 (1-18)

Abstract

Low–dissolved oxygen (DO) biological nutrient removal is an attractive option for energy and cost reduction, but it is not common practice as critical outcomes remain uncertain. Open questions include whether ammonia-oxidizer oxygen kinetics are fixed or can adapt to operating conditions; whether low DO increases simultaneous nitrification and denitrification (SND); whether good settling can be maintained; and whether N₂O emissions rise or fall at low DO. In this study, a continuously fed pilot anaerobic/oxic (A/O) process was run with 6.5- to 8.5-day solids retention time (SRT), stable temperature, and constant influent flow, and the operating DO was decreased from 2.0 to 0.2 mg O2/L over multiple months while maintaining complete nitrification. In situ ammonia uptake rate, ammonia oxidizer oxygen affinity (KO) (via two independent batch methods), quantitative PCR (qPCR)-based community composition, floc size, settling, and SND were monitored throughout the transition to low DO. N₂O was measured before and after the low DO transition. Ammonia oxidizer K₀ decreased significantly with reduced operating DO, driven primarily by a shift toward comammox Nitrospira (CMX), particularly Clade A, which comprised > 80% of the nitrifier community at the lowest operating DO. SND, measured as total inorganic nitrogen (TIN) removal across the aerobic zone, did not increase at lower DO within this well-mixed A/O configuration. Settling performance remained acceptable across setpoints. The low-DO sampling campaign had lower N2O emissions than the high-DO campaign, coincident with CMX enrichment. Total supplied airflow was reduced by 44% at 0.2 relative to 2.0 mg O₂/L, indicating reduced aeration demand.

Rights

© 2026 The Authors

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Data Availability

Article states: "The data that support the findings of this study are available from thecorresponding author upon reasonable request."

ORCID

0009-0008-3286-7174 (Bradshaw)

Original Publication Citation

McCullough, K., McIntosh, L., Gagnon, A., Dawson, W., Bradshaw, H., Klaus, S., Hiripitiyage, D., Sturm, B., Rosso, D., Bott, C. B., & Vanrolleghem, P. A. (2026). Long-term low-DO operation changes the ammonia oxidizer community and oxygen kinetics. Water Environment Research, 98(10), Article e70593. https://doi.org/10.1002/wer.70593

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