Nowadays, biological nitrogen removal has been widely used in wastewater treatment, which is the largest environmental biotechnology in the world. The recent discoveries of novel nitrogen-cycling microorganisms (e.g. anammox, ammonia-oxidizing archaea, comammox Nitrospira and nitrite/nitrate-dependent anaerobic methane oxidation microorganisms) have provided new opportunities to achieve sustainable nitrogen removal and recovery from wastewater. This talk will review the biochemistry, microbiology, and ecophysiology of these newly discovered microorganisms, and discuss the current knowledge associated with their corresponding pathways. Moreover, this talk will introduce several examples to promote the versatile applications of novel nitrogen-cycling microorganisms for achieving circular economy in wastewater treatment. It aims to facilitate finding new approaches and solutions to help safeguard the environment for future generations.