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.