The crucial role of activated sludge models (ASMs) on wastewater treatment processes: Developments, applications, and future perspectives

Activated sludge models (ASMs), the most widely used mathematical models for biological wastewater treatment, offer a simplified matrix-based representation of pollutant biochemical degradation. As understanding of wastewater treatment mechanisms has advanced, the simplifying assumptions of general ASMs have proven unreasonable under certain conditions, prompting their improvement. Existing reviews often focus on the specific application of ASMs, with limited comprehensive analyses of their multi-dimensional extensions and cross-model integrations. This review provides the first systematic overview of the latest developments in ASMs, focusing on model mechanism extension and multi-scale model integration. In terms of mechanism extension, the incorporation of new theories and secondary reaction has enhanced the accuracy of models in simulating membrane bioreactor systems, phosphorus removal, and industrial wastewater treatment. It has also quantified the generation and dissipation pathways of N 2 O and provided a basis for sludge reduction and sedimentation control. Regarding model integration, this review focuses on the coupling interfaces between ASMs and other models, such as anaerobic reaction models, convection-diffusion theory, hydrodynamic models, and machine learning. These coupled models enable full-scale simulation from micro-level biochemical reactions to macro-level environmental dynamics. Finally, the review emphasizes that future ASMs developments should focus on improving mechanisms and addressing emerging contaminants. It highlights that integrating artificial intelligence can serve as a key tool to balance model accuracy and parameter identifiability. The present review aims to establish a systematic research framework for ASMs, analyze the limitations of existing models, and ultimately provide insights for enhancing the precision and application of ASMs in wastewater treatment.