Occurrence and Roles of Comammox Bacteria in Water and Wastewater Treatment Systems: A Critical Review

Abstract Nitrogen removal is a critical process in water treatment plants (WTPs) and wastewater treatment plants (WWTPs). The recent discovery of a novel bacterial process, complete ammonia oxidation (comammox, CMX), has refuted a century-long perception of the two-step conversion of NH3 to NO3–. Compared with canonical nitrifiers, CMX bacteria offer undeniable advantages, such as a high growth yield propensity and adaptability to nutrient- and growth-limiting conditions, which collectively draw attention to validate the aptness of CMX bacteria to wastewater treatment. As there has been no comprehensive review on the relevance of CMX bacteria for sustainable water and wastewater treatment, this review is intended to discuss the roles and applications of CMX in the removal of nitrogen and pollutants from water and wastewater. We took into account insights into the metabolic versatilities of CMX bacteria at the clade and subclade levels. We focused on the distribution of CMX bacteria in engineered systems, niche differentiation, co-occurrence and interactions with canonical nitrifiers for a better understanding of CMX bacteria in terms of their ecophysiology. Conceptualized details on the reactor adaptability and stress response of CMX bacteria are provided. The potential of CMX bacteria to degrade micropollutants either directly or co-metabolically was evaluated, and these insights would be an indispensable advantage in opening the doors for wider applications of CMX bacteria in WWTPs. Finally, we summarized future directions of research that are imperative in improving the understanding of CMX biology.