Resilient simultaneous nitrification and denitrification in extreme industrial environments: Mechanisms, adaptive strategies, and future perspectives

Simultaneous nitrification and denitrification (SND) represent a resource-efficient paradigm for biological nitrogen removal, yet its application to extreme industrial effluents is constrained by the inherent fragility of its spatial gradients and coupled kinetics. This critical review discusses the distinct physiological and physicochemical impediments imposed by high salinity, elevated temperature, and pH fluctuations. Specifically, it elucidates how these stressors induce ionic displacement within the extracellular polymeric substance (EPS) matrix, create a kinetic-solubility paradox by decoupling oxygen supply from metabolic demand, and disrupt transmembrane bioenergetics via substrate speciation shifts. Moving beyond descriptive analysis, the review synthesizes targeted adaptive strategies to restore SND fidelity, focusing on (1) the reconstruction of stratified microenvironments using biofilm reactors to shield sensitive guilds; (2) the kinetic matching of functional consortia to synchronize nitrification and denitrification rates; and (3) the optimization of electron fluxes to mitigate the carbon penalty associated with stress responses. Finally, the review proposes a roadmap transitioning from empirical parameter optimization to holistic systems engineering, advocating for the convergence of predictive synthetic ecology and digital-twin-enabled control. This framework aims to transform SND into a robust, low-emission standard for managing the complex challenges of extreme wastewater treatment.