Advancing twin transition in wastewater management for nitrogen cycle

Ammonia-nitrogen is a major anthropogenic pollutant in agricultural, municipal, and industrial wastewater worldwide. However, it can be transformed into a valuable resource through the twin transition that integrates green technological innovations with digital intelligence, thereby enabling circular and climate-resilient wastewater management. Conventional biological, physico-chemical, and membrane-based processes have significantly advanced nitrogen removal, yet they remain constrained by high energy consumption, operational instability, and limited resource recovery potential. This study introduces a twin transition perspective for ammonia-nitrogen management, synthesizing recent advances across four key transformation pathways: hybrid system integration, resource recovery technologies, life cycle assessment, and artificial intelligence-driven optimization. Building on this synthesis, a novel sustainability-digital coupling strength framework is proposed to classify nitrogen management systems into three evolutionary stages: passive digitalization, reactive automation, and a fully realized twin transition characterized by predictive, multi-objective optimization. Furthermore, a three-phase deployment roadmap is presented to facilitate practical implementation, prioritizing digital readiness and process optimization in the short term, scaling hybrid resource-recovery systems in the medium term, and ultimately achieving autonomous, energy-neutral, and circular nitrogen management in the long term. Overall, this study reframes ammonia nitrogen from a pollutant to a strategic resource and provides a comprehensive pathway toward intelligent, low-carbon, and sustainable wastewater management systems.