Total ammonia nitrogen, or TAN, is the usual regulatory measure, but it does not by itself describe ammonia’s toxicity. In water, NH₃ and NH₄⁺ form an equilibrium controlled by pH, temperature and hydrological conditions. NH₃ is the more toxic form and is also more readily volatilized into the atmosphere. The review cites a toxicity comparison in which NH₃ is 190 times more toxic than NH₄⁺. Consequently, two water bodies with identical TAN concentrations can present very different risks: warmer or less acidic conditions shift more ammonia toward the molecular form, while the same total concentration can be diluted or concentrated as flows change. Chemical speciation must therefore be considered alongside concentration when environmental exposure is assessed.

Timing is another central problem. Periodic sampling can miss the short concentration peaks created by storms, seasonal agricultural activity, low streamflow or disturbances at wastewater plants. Lower winter temperatures can also slow nitrifying microbes in treatment systems, allowing ammonia to accumulate. Long-term monitoring data cited by the review suggest that exceedances of recommended U.S. freshwater criteria are relatively rare, but sparse sampling may underestimate brief peaks. The authors distinguish diffuse agricultural sources, which strongly influence regional averages, from localized events that can dominate ecological exposure. They recommend monitoring strategies matched to the objective: laboratory methods for accuracy, online sensors for process control, and passive samplers for time-integrated measurements where episodic events and spatial heterogeneity matter.

Regulatory systems reflect this complexity differently. European Union directives combine wastewater nutrient limits, agricultural nitrogen controls, industrial permitting and broader water-quality goals. China uses function-based surface-water categories whose ammonia-nitrogen limits rise from 0.15 to 2.0 mg per litre as permitted uses become less protective. Australia and New Zealand instead emphasize ecological risk and site-specific conditions, with default ammonium trigger values ranging from about 1 to 100 micrograms per litre and protection-based criteria for total ammonia. Latin America, the review notes, lacks a harmonized regional framework for reactive nitrogen, leaving governance largely divided among national sectors. These contrasts demonstrate that a numerical ammonia limit cannot be interpreted independently of jurisdiction, ecosystem type, exposure duration and the chemical form measured.

Prevention at the source can reduce the concentration peaks that dominate risk. Polymer- or sulfur-coated urea and other controlled-release fertilizers delay nitrogen availability, while urease inhibitors slow the conversion of urea to ammonia. The review summarizes field reductions of roughly 8–37% in NH₃ volatilization for controlled and coated fertilizers, about 54% for urease inhibitors and about 68% for controlled-release fertilizers, although results depend on products and field conditions. Nitrification inhibitors can also alter nitrogen persistence and mobility, but their effectiveness and trade-offs vary. These measures do not eliminate emissions: they spread release over longer periods, lower maxima and sometimes prolong low-level emissions, changing both the monitoring burden and the influent conditions that downstream treatment systems must handle.

For wastewater and digestate, treatment choices must balance removal against energy use, stability and the possibility of recovering nitrogen. Air stripping transfers NH₃ from liquid into a gas stream, while hollow-fiber membrane contactors combine desorption with absorption into an acidic solution. In one packed-column experiment, ammonia removal reached 98% at pH 11.94, 40.5 °C and an 80:1 liquid-to-gas ratio. Membrane-contacting studies likewise found that raising pH and temperature improved mass transfer, while aeration increased transfer from biogas digestate. Such separations are attractive for concentrated streams because the captured ammonia can be converted into fertilizer salts, but the reviewed results come from particular reactor configurations, matrices and operating ranges rather than universal performance estimates.

Biological pathways offer alternatives with different resource demands. Partial nitritation–anammox oxidizes about half the ammonium to nitrite, after which anammox bacteria combine nitrite with the remaining ammonium to form nitrogen gas. Reported nitrogen-removal efficiencies generally range from 70% to more than 90%, with optimized systems approaching 98–99% ammonia removal. The process can reduce aeration, external carbon demand and sludge production, but stable operation requires careful control of dissolved oxygen, nitrite and temperature; nitrite above roughly 400 mg per litre has been reported as toxic to anammox organisms. Simultaneous nitrification–denitrification uses oxygen gradients within flocs, biofilms or granules, with total nitrogen removal commonly above 80% and optimized biofilm systems near 98%. Neither result removes the need for stable reactor design and precise aeration control.

Emerging pathways include ferric iron-mediated anaerobic ammonium oxidation, known as Feammox, in which Fe(III) replaces oxygen as the terminal electron acceptor. Feammox could offer low-energy nitrogen removal without external organic carbon, but low conversion rates, incomplete knowledge of the microorganisms and difficulty regenerating bioavailable Fe(III) limit present application. The review also identifies a wider sustainability problem: nitrification and denitrification can produce nitrous oxide, a powerful greenhouse gas, and real systems may simultaneously support processes that regenerate ammonia. These limitations explain why the authors reject a search for one universally superior process. Membrane contactors, air stripping, adsorption, precipitation and biological treatment each occupy a niche determined by waste composition, concentration, desired product, climate, energy price, infrastructure and regulation.

Recovered ammonia can become a secondary nitrogen resource rather than waste. Absorption into sulfuric or phosphoric acid yields ammonium salts used as fertilizers; sulfuric acid generally showed better long-term capacity, while phosphoric acid offered rapid initial removal but declined as ammonium phosphate precipitated and transfer became limiting. Struvite, a magnesium ammonium phosphate fertilizer, can precipitate near pH 9 with suitable magnesium and phosphorus ratios, and reviewed studies report more than 90% ammonia removal in tested systems. Recovered ammonia may also support carbon-dioxide capture, linking wastewater treatment with carbon management, although ammonia volatility, solid precipitation, regeneration energy, corrosiveness and leakage remain constraints. The review argues that recovery must be judged by product quality, transport, market demand and life-cycle performance as well as removal efficiency. It identifies digital monitoring, hybrid treatment trains and circular nutrient use as promising directions, while emphasizing that broader deployment still requires full-scale validation, standardized data systems, techno-economic assessment and life-cycle assessment.

Subject of Research: Integrated monitoring, prevention, treatment and recovery of ammonia in aquatic environments

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

Article References: Lindovský, J., & Komendová, R. (2026). Controlling ammonia in aquatic environments: a critical review linking monitoring, prevention, and recovery. Environmental Monitoring and Assessment, 198(11), Article 1238. https://doi.org/10.1007/s10661-026-16001-w

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16001-w

Keywords: ammonia pollution, total ammonia nitrogen, ammonia speciation, aquatic toxicity, environmental monitoring, wastewater treatment, nutrient recovery, agricultural emissions, circular nitrogen management, Controlling, ammonia, aquatic