Integrating comammox into anammox-dominated one-stage mainstream partial nitrification/anammox: nitrifier niche...

Mainstream partial nitrification/anammox (PN/A) provides a low-carbon process pathway for municipal nitrogen removal. However, one-stage PN/A deployment remains constrained by unstable ammonia-oxidizing bacteria (AOB)-mediated nitritation, adaptive resurgence of nitrite-oxidizing bacteria (NOB), and inadequate nitrite availability for anammox under variable influent conditions. This review evaluates complete ammonia-oxidizing (comammox) bacteria as an ecological strategy to enhance mainstream PN/A stability by reconfiguring nitrifier competition and nitrite partitioning. Comammox Nitrospira exhibit high ammonia affinity, persistence under low dissolved oxygen, and preferential retention under long solids retention times. These traits enable sustained ammonia oxidation under conditions that often compromise canonical AOB. Although comammox possess nitrite-oxidizing capacity encoded by nitrite oxidoreductase, reactor observations indicate that ammonia oxidation by comammox exceed their subsequent nitrite oxidation. This kinetic imbalance enables transient nitrite accumulation and facilitates anammox nitrite uptake when biomass spatial organization promotes nitrite transfer to anammox before Nitrospira reoxidation. Conditions that favour comammox also diminish the competitive advantage of many canonical NOB, reframing NOB control from transient chemical inhibition toward ecological constraint through comammox niche occupation. The resulting comammox-anammox interaction is governed by nitrite partitioning among ammonia oxidation, nitrite oxidation and anammox uptake. Endogenous partial denitrification further recycles residual nitrate to nitrite when nitrate accumulation limits nitrogen removal. Current evidence supports comammox integration as a promising, boundary-defined strategy for stabilizing anammox-dominated one-stage mainstream PN/A. Future efforts should focus on validating nitrogen transformation pathways, improving the retention of functional biomass, demonstrating long-term reactor-scale performance, and developing predictive modelling that accounts for uncertainty.