Heterotrophic nitrification and aerobic denitrification process: Promising but a long way to go in the wastewater treatment

The traditional biological nitrogen removal (BNR) follows the conventional scheme of sequential nitrification and denitrification. In recent years, novel processes such as anaerobic ammonia oxidation (anammox), complete ox-idation of ammonia to nitrate in one organism (comammox), heterotrophic nitrification and aerobic denitrifica-tion (HN-AD), and dissimilatory nitrate reduction to ammonium (DNRA) are gaining tremendous attention after the discovery of metabolically versatile bacteria. Among them, HN-AD offers several advantages because individ-ual bacteria could achieve one-stage nitrogen removal under aerobic conditions in the presence of organic car -bon. In this review, besides classical BNR processes, we summarized the existing literature on HN-AD bacteria which have been isolated from diverse habitats. A particular focus was given on the diversity and physiology of HN-AD bacteria, influences of physiological and biochemical factors on their growth, nitrogen removal perfor-mances, as well as limitations and strategies in unraveling HN-AD metabolic pathways. We also presented case studies of HN-AD application in wastewater treatment facilities, pointed out forthcoming challenges of HN-AD in these systems, and presented modulation strategies for HN-AD application in engineering. This review may help improve the existing design of wastewater treatment plants by harnessing HN-AD bacteria for effective ni-trogen removal. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Traditional biological nitrogen removal (BNR) involves sequential nitrification and denitrification, but in recent years, novel processes like anaerobic ammonia oxidation and comammox have gained attention. Among them, heterotrophic nitrification and aerobic denitrification (HN-AD) is particularly promising for one-stage nitrogen removal in the presence of organic carbon under aerobic conditions. This review focuses on the diversity, physiology, performance, challenges, and strategies of HN-AD bacteria.