Biofilms recall versatile capability of anammox integrating with heterotrophs and advances in application for wastewater treatment: A critical review

Biofilms are the most favorable form of microorganism growth in nature. With the urgent demand for energyefficient wastewater treatment today, biofilms have become a crucial tool for the implementation of new treatment technologies. Anammox-based processes are gaining global attention due to their low cost and high nitrogen removal efficiency. However, the application of anammox in mainstream wastewater treatment is still confined to laboratory settings and faces two major bottlenecks: 1) the relatively slow growth rate of anammox bacteria and 2) unstable nitrite generation. Fortunately, these issues are expected to be relieved through the integration of biofilm technologies that couple autotrophic and heterotrophic pathways. Moreover, both laboratory and full-scale evidence from recent studies highlight the great potential and versatile capability of anammox-based biofilms integrated with heterotrophic microorganisms. A solid understanding of biofilm formation, control, and optimization has thus become increasingly important for further application. This paper provides a comprehensive review of existing methods and microbial mechanisms for enhancing treatment in anammox-based biofilm systems. It critically analyses the potential roles of biofilm formation in anammox process, spatial development of microbial ecology, bioreactor control, associated challenges and perspectives in practical applications. This review is anticipated to contribute significantly to advancing the understanding of biofilm reactor design and operation in the application of new biotechnology.