Controlling ammonia in aquatic environments: a critical review linking monitoring, prevention, and recovery.

Ammonia is a key inorganic nitrogen species connecting aquatic chemistry, ecosystem toxicity, and atmospheric particulate formation. In water, environmental risk is not governed solely by total ammonia nitrogen (TAN), but by the speciation equilibrium between NH 3 and NH 4 + , controlled by pH, temperature, and hydrological variability. Increasing nitrogen loads from wastewater effluents, anaerobic digestion residues, and agricultural activities generate short-term concentration peaks that are difficult to capture by conventional monitoring and complicate regulatory management. At the same time, tightening discharge limits and circular-economy policies require strategies that both mitigate environmental impacts and enable nutrient recovery. However, ammonia is still typically addressed separately in environmental chemistry, agricultural management, and wastewater engineering, preventing coordinated mitigation. Here, we review environmental ammonia in aquatic systems using an integrated chemical-environmental-technological perspective. The major points are the following: (i) chemical speciation governs toxicity, volatilization, and environmental transport more strongly than total concentration alone; (ii) analytical monitoring approaches, from colorimetric and ion-selective electrode measurements to sensors and passive sampling techniques, are essential for interpreting environmental exposure and guiding regulation; (iii) upstream measures, including controlled-release fertilizers and urease inhibitors, reduce emission peaks at the source; (iv) physicochemical and biological treatment processes such as air stripping, membrane contactors, adsorption, and nitrification enable ammonia removal and recovery from wastewater and digestate; and (v) recovered ammonia can serve as a secondary nitrogen resource within circular nutrient management. Integrating monitoring, prevention, treatment, and reuse provides a framework for data-driven ammonia control across environmental and engineered water systems.