What’s the best way to achieve successful mainstream partial nitritation-anammox application?

Partial nitritation-anammox (PN-A) process, which significantly reduces energy and chemical input compared to the conventional biological nitrogen removal process, is a game-changing nitrogen-removal alternative. PN-A treating municipal wastewater (mainstream PN-A) may shed light on achieving a more sustainable energy-neutral municipal wastewater treatment, but practical applications are still not available. This review is therefore critically conducted to identify the challenges impeding the application of mainstream PN-A. At temperatures ≥ 25 °C, PN-A process treating mainstream wastewater can achieve comparable nitrogen removal performance to conventional processes, but the effluent quality may need to be improved to meet stringent discharge standards; at temperatures < 20 °C, long-term process instability, low nitrogen removal efficiency, and poor effluent quality are the main operating difficulties. The reasons are attributed to challenges in maintaining sufficient anammox activity and effective NOB suppression. A framework of operable solutions to these difficulties and challenges is suggested, among which the efficient anammox biomass retention should be prioritized. The effectiveness and practical availability of these solutions are also assessed. Ultimately, depending on technological readiness, a roadmap for the PN-A application is proposed to make the best of the benefits of PN-A and promote the implementation of energy-neutral/positive municipal wastewater treatment.