Nitrogen in water is removed through biological, physical-chemical, or hybrid methods, and the right choice depends on the form of nitrogen present, the concentration, and whether you’re treating municipal wastewater, drinking water, agricultural runoff, or industrial effluent. Most large-scale nitrogen removal still relies on bacteria that convert ammonia to nitrate and then to harmless nitrogen gas, but a growing set of alternatives handles situations where biology alone falls short. The field has changed substantially in the past two decades, with newer processes cutting energy use and even recovering nitrogen as a useful product rather than simply disposing of it.
Why Nitrogen Needs to Come Out in the First Place
Nitrogen enters water primarily as ammonia, nitrate, or nitrite. Each form causes different problems. Ammonia is directly toxic to fish at low concentrations and consumes dissolved oxygen when bacteria convert it in rivers and lakes. Nitrate in drinking water poses health risks, particularly for infants; the regulatory limit for nitrate in public drinking water supplies was originally set to prevent a condition called methemoglobinemia (sometimes called “blue baby syndrome”), though researchers now recognize that other health effects may also matter.1PubMed Central. Drinking Water Nitrate and Human Health: An Updated Review When excess nitrogen of any form reaches rivers, estuaries, and coastal waters, it feeds algal blooms that suffocate aquatic life and create dead zones. The goal of nitrogen removal is to convert these reactive forms into nitrogen gas, which simply escapes to the atmosphere as a harmless component of the air we breathe, or in some newer approaches, to capture the nitrogen for reuse.
Traditional Biological Removal Through Nitrification and Denitrification
The workhorse of nitrogen removal at wastewater treatment plants worldwide is a two-step biological process. In the first step, nitrification, one group of bacteria converts ammonia to nitrite and then to nitrate. This requires oxygen, so treatment plants aerate the water during this stage. In the second step, denitrification, a different group of bacteria converts nitrate into nitrogen gas. These bacteria need an oxygen-free environment and a carbon source to fuel their metabolism.2PubMed Central. Biological nitrogen removal from low carbon wastewater The carbon source is typically organic matter already present in the wastewater, though operators sometimes add an external source like methanol or acetate when the wastewater doesn’t contain enough on its own.
Treatment plants achieve these conditions by moving water between aerated and non-aerated zones, or by cycling aeration on and off in the same tank. The process works reliably and handles the nitrogen loads typical of domestic sewage, but it has real drawbacks. Aeration is energy-intensive, often accounting for the largest share of a plant’s electricity bill. The denitrification step needs that carbon source, which adds cost if the wastewater is carbon-poor. And the process produces sludge that must be managed.
A full-scale example of how plants optimize this approach comes from a facility that repurposed existing equipment for a sidestream treatment system, achieving about 81% ammonia reduction and 53% total nitrogen reduction by using primary effluent as the carbon source instead of purchasing chemicals.3PubMed. Full-scale N removal from centrate using a sidestream process with a mainstream carbon source That kind of creative plumbing, routing carbon-rich water from one part of the plant to feed bacteria in another, is a common way operators squeeze more performance out of existing infrastructure.
Anammox and Other Advanced Biological Shortcuts
One of the biggest shifts in biological nitrogen removal over the past couple of decades has been the discovery and engineering of anaerobic ammonium oxidation, known as anammox. In this process, specialized bacteria from the Planctomycete group convert ammonium and nitrite directly into nitrogen gas without needing oxygen or an external carbon source.4PubMed Central. Anaerobic ammonium oxidation for treatment of ammonium-rich wastewaters At least two genera within the Planctomycetales have been identified as capable of driving this reaction in wastewater systems.5PubMed. Microbiology and application of the anaerobic ammonium oxidation (‘anammox’) process
The appeal is clear: anammox uses roughly 60% less energy than conventional nitrification-denitrification because you skip most of the aeration, and you don’t need to buy a carbon source. The catch is that anammox bacteria grow extremely slowly, which makes starting up a reactor a patience-testing exercise that can take months. The bacteria are also sensitive to temperature swings and certain inhibitors. Still, anammox-based systems are now operating at full scale in plants around the world, particularly for treating high-ammonia sidestreams like the water that comes off dewatered sludge.
Another advanced approach is simultaneous nitrification and denitrification (SND), which collapses both steps into a single reactor. Aerobic granular sludge makes this possible: the dense granules create oxygen-rich conditions on their outer surfaces and oxygen-depleted zones in their interiors, so nitrifying and denitrifying bacteria coexist in the same pellet. Modeling studies have attributed roughly 75% of denitrification during aerobic periods to this simultaneous process within granular sludge.6PubMed. Model-based evaluation of simultaneous nitrification and denitrification in aerobic granular sludge systems Under optimized conditions, ammonia removal can reach 100% when the ratio of organic matter to ammonia in the incoming water is tuned correctly.7PubMed. Simultaneous nitrification and denitrification in an aerobic reactor with granular sludge originating from an upflow anaerobic sludge bed reactor Granular sludge systems have also been configured to handle phosphorus removal alongside nitrogen, making them attractive for municipal wastewater treatment where discharge limits cover both nutrients.8PubMed. Enhanced simultaneous nitrification, denitrification and phosphorus removal through mixed carbon source by aerobic granular sludge
Air Stripping for Ammonia
Air stripping is a physical-chemical method that works well when ammonia concentrations are high and biological treatment is impractical or too slow. The principle is simple: ammonia in water exists in a balance between ammonium ions (which stay dissolved) and ammonia gas (which can escape). By raising the pH and blowing air through the water, you shift that balance toward the gas form and physically push ammonia out of the liquid.9South African Journal of Chemical Engineering. Ammonia removal form municipal wastewater by air stripping process: An experimental study
pH has the strongest influence on how well air stripping works, though its effect levels off above about pH 10; pushing higher than that doesn’t meaningfully improve removal.10Process Safety and Environmental Protection. Effect of pH, temperature and air flow rate on the continuous ammonia stripping of the anaerobic digestion effluent Higher temperatures and higher air flow rates also help. The method is especially popular for landfill leachate, which can contain very high ammonia levels. A pilot-scale study treating landfill leachate in a closed air stripping tower removed an average of 98% of ammonia nitrogen, and the stripped ammonia was captured using phosphoric acid to produce ammonium phosphate, a material with potential use as fertilizer.11PubMed. Ammonia recovery from air stripping process applied to landfill leachate treatment That recovery step is worth noting because it turns a waste stream into something useful rather than just transferring the problem from water to air.
Air stripping does have practical limitations. It requires chemicals to raise pH, which generates costs and creates a secondary waste stream. In cold climates, ammonia gas is much less volatile, so towers need to be larger or heated. And if you don’t capture the ammonia on the back end, you’ve merely moved a water pollution problem into an air pollution problem. Open-air stripping towers are increasingly falling out of favor for this reason, replaced by closed systems with acid scrubbers.
Ion Exchange and Membrane Filtration
Ion exchange uses materials that swap harmless ions for nitrogen-containing ones. Zeolites, naturally occurring or synthetic mineral ion exchangers, are particularly well known for their ability to remove ammonium from water because they preferentially grab ammonium ions over competing ions like calcium or magnesium.12Elsevier / Water Research. Ammonium removal using ion exchange and biological regeneration In a typical setup, water flows down through a packed column of zeolite. When the zeolite is saturated, it’s regenerated with a salt solution, and the cycle repeats. This is a proven, low-tech approach that works well for polishing water to meet strict discharge limits or for small community drinking water systems.
Membrane filtration takes a different approach. Reverse osmosis (RO) pushes water through membranes tight enough to reject dissolved ions, including nitrate. One challenge is that standard RO removes everything, stripping out beneficial minerals alongside nitrogen. Researchers have explored hybrid systems using nanofiltration as a first stage to selectively remove non-target ions, followed by RO to remove nitrate. One such scheme achieved water recoveries above 90% while producing water with balanced mineral content suitable for drinking.13Elsevier / Chemical Engineering Journal. Selective nitrate removal from groundwater using a hybrid nanofiltration–reverse osmosis filtration scheme Membrane systems are energy-intensive and generate a concentrated waste stream (the reject brine) that needs disposal, so they’re typically reserved for drinking water treatment or situations where biological processes can’t do the job.
Breakpoint Chlorination
If you’ve ever shocked a swimming pool to get rid of that sharp chloramine smell, you’ve used a version of breakpoint chlorination. When enough chlorine is added to ammonia-containing water, it first forms chloramines, then breaks them down into nitrogen gas and hydrochloric acid. Computational chemistry studies have clarified the mechanism: ammonia reacts with hypochlorous acid to form monochloramine, then dichloramine, which preferentially decomposes to nitrogen gas rather than forming the third possible product, trichloramine.14JOURNAL OF CHEMICAL ENGINEERING OF JAPAN. A Computational Mechanistic Study of Breakpoint Chlorination for the Removal of Ammonia Nitrogen from Water
Breakpoint chlorination is fast and doesn’t require specialized equipment, but it consumes large amounts of chlorine, produces byproducts that can be problematic in their own right (disinfection byproducts like trihalomethanes), and only works economically when ammonia concentrations are relatively low. It’s most commonly used as a finishing step in drinking water treatment or for emergency situations rather than as a primary nitrogen removal strategy.
Constructed Wetlands and Passive Treatment
For agricultural runoff and stormwater, engineered treatment plants are often impractical. Constructed wetlands offer a nature-based alternative. These shallow, planted basins slow water flow and create conditions where nitrogen removal happens through a combination of plant uptake, microbial denitrification in the sediment, and ammonia volatilization. Research on wetlands treating agricultural runoff found that absolute nitrogen removal was best predicted by the nitrogen load coming in and the hydraulic shape of the wetland (how water flows through it), while relative removal depended more on how much emergent vegetation covered the water surface.15PubMed. Wetland nitrogen removal from agricultural runoff in a changing climate
Biochar, a charcoal-like material produced from organic waste, is another passive sorbent gaining attention. Unmodified biochar has modest nitrogen-removal capacity, but versions modified with metals show significantly higher uptake of both ammonium and nitrate due to changes in surface charge and increased metal oxides on the biochar surface.16PubMed. Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water Biochar can be used as a soil amendment after it’s loaded with nitrogen, effectively recycling the nutrient back to land. The technology is still more research than routine, but it fills a niche for decentralized, low-energy treatment.
Woodchip Bioreactors for Farm Tile Drainage
One of the simplest and most cost-effective tools for removing nitrate from agricultural drainage is the woodchip bioreactor. These are trenches or beds filled with wood chips that intercept tile drain water before it reaches surface waterways. Bacteria living on the wood chips use the carbon in the wood as an energy source to convert nitrate to nitrogen gas under the oxygen-poor conditions inside the bed.17Water Supply. Sizing an open-channel woodchip bioreactor to treat nitrate from agricultural tile drainage and achieve water quality targets
Early field trials demonstrated the concept convincingly: two 200-liter fixed-bed bioreactors containing coarse sand, wood chips, and leaf compost treated farm tile drainage with nitrate-nitrogen concentrations of 3 to 6 mg/L down to below 0.02 mg/L, and operated maintenance-free for a year.18Journal of Contaminant Hydrology. Removal of agricultural nitrate from tile-drainage effluent water using in-line bioreactors The technology has scaled up considerably since then and is now increasingly deployed across agricultural regions to control nonpoint source nitrate pollution.19PubMed. Climate change effects on denitrification performance of woodchip bioreactors treating agricultural tile drainage Climate change does introduce uncertainty here: warmer temperatures speed up microbial activity (potentially improving removal), but changing rainfall patterns can alter flow rates and nitrogen loads in ways that are harder to predict and design for.
Microalgae for Nitrogen Recovery
Rather than converting nitrogen to gas and losing it, some approaches capture it in a form that can be reused. Microalgae absorb ammonium and nitrate as nutrients for growth, pulling nitrogen out of the water and storing it in their biomass. The harvested algae can then be used as animal feed, fertilizer, or even converted to biofuel. Research on microalgae treating swine wastewater, one of the most nitrogen-rich waste streams in agriculture, has shown the approach can achieve environmentally sustainable and economically viable treatment, though most studies so far have focused on improving removal and accumulating biomass while information on the underlying mechanisms is still developing.20PubMed. Microalgae-driven swine wastewater biotreatment: Nutrient recovery, key microbial community and current challenges
Pairing microalgae with other technologies can push performance further. One study combined electrochemical pretreatment with a microalgae species to treat swine wastewater, reaching over 89% total nitrogen removal and over 96% ammonia removal. The harvested algal biomass was rich in saturated fatty acids desirable for biodiesel production, adding an energy-recovery dimension to the treatment.21PubMed. Integrating electrochemical pretreatment with microalgae treatment for nitrogen and phosphorus removal and resource recovery from swine wastewater The appeal of microalgae lies in this dual benefit: you clean the water and produce something valuable, rather than just disposing of waste. The practical barriers are land and light requirements, harvesting costs, and the difficulty of maintaining stable algal cultures when the incoming wastewater composition varies.
Electrochemical and Catalytic Methods
Electrochemical oxidation uses electricity to destroy ammonia at an electrode surface or to generate oxidants (like hydroxyl radicals and active chlorine) that react with ammonia in solution. A study treating actual dyeing wastewater demonstrated complete ammonia removal in 60 minutes of electrolysis under optimized conditions, with about 88% of the nitrogen converted selectively to harmless nitrogen gas rather than other byproducts.22PubMed Central. Process Optimization of Electrochemical Oxidation of Ammonia to Nitrogen for Actual Dyeing Wastewater Treatment This kind of speed makes electrochemical methods attractive for industrial applications where treatment time is limited and concentrations are high.
Bio-electrochemical systems merge microbial activity with electrochemistry. A microbial electrolysis cell (MEC) was shown to achieve about 58% total nitrogen removal by coupling anodic ammonium oxidation with hydrogenotrophic denitrification in a single chamber, and its nitrate removal reached over 97%.23PubMed. Bio-electrochemical nitrogen removal in wastewater: Coupling anodic ammonium oxidation with hydrogenotrophic denitrification in a microbial electrolysis cell These systems are still largely in the lab, but they represent an intriguing direction: using a small electrical input to boost what biology can do on its own.
On the catalytic side, researchers have developed supported palladium catalysts that reduce nitrate and nitrite with hydrogen gas. The selectivity challenge is real: you want to produce nitrogen gas, not ammonia, which would defeat the purpose in drinking water treatment. Careful catalyst design using palladium combined with a second metal like copper achieved nitrate removal to below drinking water standards while keeping ammonia formation under 0.5 mg/L after removing 100 mg/L of nitrate.24Elsevier / Catalysis Today. Catalytic reactions in the liquid phase Development of catalysts for a selective nitrate and nitrite removal from drinking water Catalytic reduction could eventually complement or replace ion exchange in small drinking water systems, though scaling it up remains a challenge.
The Nitrous Oxide Problem
Any discussion of nitrogen removal methods that ignores greenhouse gas emissions is incomplete. Nitrous oxide (Nâ‚‚O) is a potent greenhouse gas, roughly 270 times more powerful than carbon dioxide over a 100-year horizon, and biological nitrogen removal processes can release it as an intermediate. This is particularly concerning because wastewater treatment plants are already significant energy consumers, and if the nitrogen they remove is partly escaping as Nâ‚‚O, the climate impact of the treatment itself could undercut its environmental benefits.
Research at a full-scale combined nitrogen and phosphorus removal plant found that operational adjustments alone could reduce the facility’s carbon footprint by an estimated 10% without major upgrades or increased costs.25PubMed. Strategies for mitigating nitrous oxide production and decreasing the carbon footprint of a full-scale combined nitrogen and phosphorus removal activated sludge system The idea of harvesting that Nâ‚‚O as an energy source has been floated, since it can support combustion. But the recoverable energy turns out to be negligible compared to a plant’s total energy consumption, and the complexity and cost of capturing it make the approach impractical, along with the environmental risk of residual dissolved Nâ‚‚O escaping to the atmosphere.26PubMed. Engineering feasibility, economic viability and environmental sustainability of energy recovery from nitrous oxide in biological wastewater treatment plant
One promising avenue for reducing Nâ‚‚O emissions is sulfur-driven autotrophic denitrification. This process uses sulfur-based compounds rather than organic carbon as the electron donor for converting nitrate to nitrogen gas, and it offers lower energy consumption, less sludge production, and reduced greenhouse gas emissions compared to conventional denitrification.27PubMed. Sulfur-based electron donor driven autotrophic denitrification for nitrate removal: Mechanisms, performance, and nitrous oxide emission The trade-off is that sulfur denitrification produces sulfate, which can be problematic in some discharge scenarios, and it doesn’t handle ammonia, so it only addresses the nitrate portion of the nitrogen load.
When the Water Fights Back
Not all nitrogen-laden water cooperates with treatment. Industrial effluents and landfill leachates often contain high salinity, extreme pH, toxic organics, or heavy metals that inhibit the microorganisms responsible for biological nitrogen removal. Elevated salt concentrations in particular can deteriorate microbial activity and increase Nâ‚‚O emissions, though systems can gradually acclimate to moderate salinity levels over time.28PubMed. Effect of salinity on N2O production during shortcut biological nitrogen removal from landfill leachate
For these challenging streams, the toolkit usually combines multiple methods in sequence. A common approach for landfill leachate, for instance, might use air stripping to knock down the bulk ammonia load, followed by biological treatment for the remaining nitrogen, and a membrane or chemical polishing step to meet final discharge limits. Food-processing wastewater with extremely high organic loads might need anaerobic digestion first to reduce the carbon content, then conventional nitrification-denitrification for the nitrogen. The underlying principle is that no single method handles every water matrix perfectly, and treatment trains, sequences of complementary technologies, are the norm for difficult waste streams.
Low-carbon wastewater presents the opposite challenge. When there isn’t enough organic matter to fuel denitrification, operators face a choice between adding an expensive external carbon source, switching to anammox-based processes that don’t need carbon, or exploring autotrophic denitrification alternatives that use inorganic electron donors instead.2PubMed Central. Biological nitrogen removal from low carbon wastewater Each option carries its own cost and operational complexity, and the best fit depends on the plant’s scale, existing infrastructure, and discharge requirements. The reality of nitrogen removal is that the chemistry is well understood, but the engineering always comes down to matching the method to the water you actually have.