Editorial: Many roads to anammox

Anaerobic ammonium oxidation (Anammox) has been proposed as one of the most promising alternatives to achieve an energy-efficient urban wastewater treatment plant (WWTP). Over the past few decades, anammox-based processes have been successfully implemented in sidestream treatment. However, the evolution of anammox bacteria, cell biology, and new anammox microorganisms is still not clear. The engineering applications of anammox at low temperatures and achieving a promising way to supply nitrite for anammox in mainstream treatments are still limited. Additionally, the function and contribution of anammox bacteria to the nitrogen cycles of different ecosystems need to be determined. In order to promote the application of anammox and to further explore the connection between anammox and other microorganisms, our Research Topic contains articles on the studies of anammox and nitrogen removal. This Research Topic includes five articles that are innovative and interesting and report achievements in the process of anammox and efficient biological nitrogen removal strategies from the perspectives of metabolomics, metagenomics, and microbial ecology. Anammox is an important process in the nitrogen cycle, widely occurring in terrestrial, freshwater, and marine ecosystems. Partial nitritation, as a promising way to support nitrite for anammox, has received a lot of attention in this Research Topic. In order to explore the diversity and function of bacteria involved in nitrogen removal, Xu et al.analyzed the microbiota compositions of different kinds of sludge at a full-scale partial nitritation-anammox (PN/A) plant. This investigation uncovered the partition mechanism of anammox and nitrifiers in a full-scale PN/A plant. It is the first assessment of the microbiota compositions of three different types of sludge in a full-scale sidestream PN/A plant, providing a contribution for guiding the replication and development of fullscale PN/A systems. In this PN/A plant, biofilm sludge exhibited a higher anammox functional activity, and suspended sludge presented a higher partial nitritation activity. The highest numbers of anammox bacteria were detected in biofilm sludge, and ammonia-oxidizing bacteria (AOB) were mainly present in suspended sludge. OPEN ACCESS