Advances and challenges of anammox-based PN/A and PD/A coupled processes in treating diverse wastewater qualities: A review.

The anammox process is critical for sustainable nitrogen removal, yet widespread use faces operational, environmental and microbial challenges. This review evaluates recent advances in anammox-based coupled processes, particularly PN/A and PD/A, highlighting their adaptation to varied wastewater types. PN/A has been extensively validated at full scale for high-ammonia wastewaters, including sludge digestion liquor, landfill leachate, and industrial effluents, and is now being extended to mainstream municipal applications. However, persistent barriers such as NOB suppression, sensitivity to low temperature, and heterotrophic competition under high C/N conditions continue to limit its performance. In contrast, successful PD/A deployment in mainstream wastewater depends on innovative solutions to temperature-related constraints, including biofilm buffering, metabolic adaptation, and kinetic optimization. The performance of both PN/A and PD/A systems is closely tied to wastewater composition (e.g. such as salinity, organic load, and the presence of toxic compounds) and its influence on microbial kinetics. Emerging innovations, including EPS-enriched biofilms, granular sludge, quorum sensing microbial regulation, and AI-driven controls have enhanced system resilience. Furthermore, integrated approaches enabling simultaneous nitrogen and phosphorus removal and novel reactor configurations are expanding the practical applicability of anammox processes, supporting resource recovery goals. This review synthesizes mechanistic insights, highlights full-scale implementation cases, and outlines emerging frontiers such as nanotechnology-enhanced biofilms and digital twin modelling for process optimization. By bridging microbial ecology with advanced process engineering, this work provides strategic direction for scaling up anammox-based systems in pursuit of energy-neutral and sustainable wastewater treatment under tightening environmental regulations.