The research, led by Viney P. Aneja and Srijan Sengupta of North Carolina State University, together with Swarnali Sanyal of the University of Illinois Urbana-Champaign and William H. Schlesinger of the Cary Institute of Ecosystem Studies, examined ammonia emissions from swine operations that had retrofitted their waste management systems with anaerobic digestion. The team combined direct field measurements with outputs from a semiempirical mass-transfer model, a computational tool that estimates how quickly gaseous ammonia escapes from liquid manure surfaces based on chemical and physical conditions. Their central finding is striking: ammonia flux from so-called biogas secondary lagoons — open lagoons that receive digestate, the liquid residue left after anaerobic digestion — was statistically significantly higher than ammonia flux from conventional open-air lagoons used in traditional manure management, with the difference holding even after the analysis controlled for temperature, the single most important driver of volatilization.

To understand why this happens, it helps to follow the chemistry of nitrogen through a digester system. In a conventional lagoon, raw manure sits in open basins where organic nitrogen compounds are gradually mineralized to ammonium, and some of that ammonium escapes to the air as ammonia gas. In an anaerobic digester, by contrast, microbes decompose the organic matter in an oxygen-free environment, breaking down proteins and urea far more completely and rapidly. This process converts a large share of the organic nitrogen into total ammoniacal nitrogen, the pool of dissolved ammonia and ammonium ions from which gaseous ammonia can volatilize. At the same time, digestion consumes volatile acids and raises the pH of the digestate. Because the equilibrium between ammonium ions and free ammonia gas shifts strongly toward the gas phase as pH rises, digestate emerging from a digester is essentially primed for ammonia loss the moment it is exposed to air.

That exposure comes quickly. After digestion, the liquid effluent is typically transferred to a secondary storage lagoon before being pumped onto cropland as fertilizer. The study’s model simulations confirm that both during secondary storage and during subsequent land application under typical management practices, the elevated total ammoniacal nitrogen concentration and higher pH of digestate dramatically enhance the potential for volatilization. In effect, the digester concentrates the nitrogen problem: it converts slow, diffuse organic nitrogen into a chemically reactive, easily airborne form, then hands that material to an open lagoon where wind and warm temperatures can strip it into the atmosphere. The regression analysis, which statistically isolated the effect of the management system from confounding variables, found the difference between biogas secondary lagoons and conventional lagoons to be highly significant, with a p-value below 0.001 — a level of statistical confidence rarely achieved in environmental field studies.

The consequences extend well beyond the fence lines of the farms themselves. Ammonia is classified as a reactive nitrogen compound, and once released, it does not stay ammonia for long. In the atmosphere it reacts with acidic species such as nitric and sulfuric acids to form fine particulate matter, microscopic particles known as PM2.5 that penetrate deep into human lungs and are linked to asthma, cardiovascular disease, and premature death. Deposited back onto land and water, ammonia and its reaction products acidify soils, over-fertilize sensitive natural ecosystems in a process called eutrophication, and contribute to biodiversity loss in nitrogen-limited habitats. Some of the deposited nitrogen ultimately converts to nitrate, which can leach into groundwater and contaminate drinking water supplies — a cascade of impacts the study’s authors explicitly flag as a risk of degraded air quality and water quality relative to manure managed in systems open to the atmosphere.

The timing of this research matters. Across the United States, the livestock industry is actively retrofitting existing animal waste management systems at CAFOs to collect biogas, driven by a combination of renewable energy incentives, carbon credit markets, and corporate climate pledges. In states such as North Carolina and Iowa, major utility partnerships have funneled hundreds of millions of dollars into covering swine lagoons with anaerobic digesters. The shift from using animal waste solely as crop fertilizer toward producing biofuels is routinely advocated as an environmentally friendly strategy because of its potential to reduce greenhouse gas emissions, particularly methane, which traps far more heat per molecule than carbon dioxide over a twenty-year time horizon. The new findings do not dispute the climate accounting for methane. Instead, they reveal a blind spot in how the environmental performance of digesters is evaluated — one focused almost exclusively on carbon while ignoring the reactive nitrogen consequences of the same technology.

The study’s synopsis is blunt: anaerobic digesters used for biogas production from animal manure processing carry negative consequences for society and the environment and may not represent the cleaner energy future they are advertised to be. The authors caution that retrofitting digesters without integrated nitrogen management may simply shift environmental impacts from methane to reactive nitrogen, exchanging a visible, well-regulated climate problem for a diffuse air and water pollution problem that is harder to measure and easier to overlook. This kind of pollution shifting is a familiar trap in environmental engineering; solutions that solve one problem in isolation frequently create or amplify others when the full system is examined. The digester-ammonia connection is a textbook example, hidden in plain sight because the emissions occur at secondary lagoons and applied fields rather than at the digester itself.

Importantly, the researchers do not argue that biogas should be abandoned. Their conclusion is conditional but constructive: the environmental risk can be substantially reduced if digester systems are paired with mitigation technologies for animal waste management. Such technologies exist. Solid-liquid separation and acidification of digestate can lower pH and suppress the ammonia equilibrium shift. Covering secondary lagoons, capturing or scrubbing emitted ammonia, and applying digestate with injection or rapid incorporation into soils rather than surface spraying can all cut volatilization losses substantially. The key insight is that these controls must be designed into the retrofit from the beginning, not bolted on after air quality problems emerge. A digester project that captures methane but leaves hot, high-pH digestate evaporating in an open lagoon has, by the study’s measurements, made local air quality worse, not better.

For policymakers, the study lands at a sensitive moment. Regulatory frameworks in the United States reward digesters primarily for their methane capture, and renewable natural gas credits are priced on carbon metrics alone. If ammonia penalties were factored into the environmental accounting — as they increasingly are in European assessments of biogas systems — the calculus of which waste management strategies deserve public support could shift. The authors’ field-based evidence, strengthened by independent model confirmation, gives regulators a quantitative basis for requiring nitrogen management plans as a condition of digester incentives. It also gives communities near CAFOs, many of which are already overburdened by odor and particulate pollution, a new line of evidence in ongoing debates about the true cost of industrial livestock production.

Ultimately, the research is a reminder that environmental virtue in one dimension does not guarantee virtue in all. Capturing methane from manure is genuinely valuable for the climate, and the study does not challenge that. But the biogas story told to the public has been incomplete. The full lifecycle of digested manure — from the microbial transformations inside the digester to the moment the resulting liquid drifts off a secondary lagoon on a summer afternoon — determines the real environmental footprint of this technology. As the United States accelerates its retrofit of livestock operations in the name of renewable energy, the message from Biogeochemistry is clear: measure the nitrogen, manage the digestate, and do not assume that a covered lagoon means a cleaner farm. Without that vigilance, the cleaner energy future may arrive with an invisible plume of ammonia attached.

Subject of Research: Ammonia emissions from anaerobic digesters used for biogas production at swine concentrated animal feeding operations.

Article Title: Biogeochemical reactions in anaerobic digesters for biogas production yield enhanced ammonia emissions

Article References: Biogeochemical reactions in anaerobic digesters for biogas production yield enhanced ammonia emissions. (n.d.). https://doi.org/10.1007/s10533-026-01372-6

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01372-6

Keywords: anaerobic digestion, biogas, ammonia emissions, reactive nitrogen, CAFOs, swine manure, digestate, methane capture, air quality, water quality, volatilization, renewable energy

The motivation is grounded in stark numbers. Global meat production has grown more than fourfold since 1961, rising from 71 million tonnes to 337 million tonnes in 2020, and cattle production has doubled over the same period. Livestock are indispensable to human nutrition, but they are also a major climate burden. Ruminants produce between 250 and 500 litres of methane per animal per day through enteric fermentation, the microbial digestion process in the rumen. Of the estimated 86 teragrams of methane released annually by domesticated livestock, dairy cattle alone account for approximately 18.9 teragrams. Lactating cows, which eat more than dry cows or heifers, emit roughly twice as much methane as their non-lactating counterparts. Projections suggest that methane emissions from dairy farming could rise by 30 percent by 2050 if current practices continue.

In arid regions such as Qatar, the problem is compounded by heat. Cattle are sensitive to the temperature-humidity index, or THI, a combined measure of air temperature and relative humidity that indicates heat stress. When the THI exceeds the animals’ thermoneutral zone, cows respond with sweating, altered respiration, and elevated skin temperature, and milk production suffers. Conventional open sheds or naturally ventilated barns with water spraying and fogging struggle to maintain acceptable THI under Qatar’s extreme ambient temperatures and intense solar irradiance, and these open systems allow methane to escape uncontrolled into the atmosphere. The new design closes that loop, both thermally and chemically.

The proposed barn houses 100 mature lactating cows weighing 500 kilograms or more in a tie-stall configuration, following established reference designs for manure collection. The architectural model, built in Autodesk Revit, incorporates insulated walls and roof elements that cut the overall heat-transfer coefficients dramatically — from 2.242 to 0.139 W/m²/K for the walls and from 3.440 to 0.105 W/m²/K for the roof. Insulation proved to be far more than a comfort measure: sensitivity analysis showed it reduces monthly cooling loads by at least 15 percent, a substantial saving given that cooling is the single largest energy consumer in the design. The building envelope is modelled against Doha’s weather data using ASHRAE Fundamentals methods, accounting for conduction through the envelope, solar heat gain through windows, metabolic heat from the animals themselves, and ventilation loads.

At the heart of the climate-control strategy is a vapor-compression HVAC system consisting of an air-handling unit and a chiller, sized with Carrier’s Hourly Analysis Program and ducted according to the equal-friction method with a friction loss of 1 pascal per metre. The system maintains a barn setpoint of 18°C — comfortably within the thermal comfort zone for dairy cows — and regulates humidity between 50 and 60 percent through integrated humidifier and dehumidifier components. Air is distributed through 24 supply diffusers and 12 exhaust diffusers, each 450 millimetres square, mounted in a 5-metre-high ceiling. The target air velocity at cow level is between 1 and 2 metres per second, fast enough to remove heat, moisture, and harmful gases without causing drafts that stress the animals. Crucially, the ventilation system is closed and mechanical, which means the exhaust air — and the methane it carries — can be routed somewhere useful rather than vented to the sky.

To verify that the air actually moves the way the designers intended, the team ran computational fluid dynamics simulations in ANSYS Fluent 2022 using the standard k–ε turbulence model, solving the continuity, momentum, energy, and species-transport equations for the airflow around the animals. The CFD results predict temperatures of approximately 20°C around the animals and air velocities consistently within the 1–2 m/s target band, with generally uniform circulation across the animal zone. The species-transport formulation also allowed the researchers to estimate methane concentration in the barn air, which depends on cow weight, ventilation rate, and air density. For cows above 500 kilograms, an emission factor of 3.5 to 4.5 applies; at the design conditions of 18°C and 46 litres per second of ventilation per cow, the modelled methane concentration sits near the lower end of a 0–3 percent parametric range used to characterise the downstream power cycle.

That downstream component is a Brayton cycle, the same thermodynamic arrangement used in gas-turbine power plants, consisting of a compressor, combustion chamber, and turbine. In a conventional Brayton cycle, ambient air enters the compressor, is compressed from 101 to 1000 kilopascals, and is heated by burning fuel. Here, the innovation is twofold. First, the compressor intake is not ambient air but the methane-containing exhaust stream drawn from the barn, which carries more chemical energy than air alone. At 1500 K and 1000 kPa, methane has a specific enthalpy of 4943 kJ/kg compared with 1637 kJ/kg for air, so even dilute methane enriches the working fluid. Second, the combustion fuel is not natural gas but cow manure, which has a heating value of 11,729 kJ/kg. Combustion gases leave the chamber at approximately 1200 K and expand through the turbine to generate electricity. Mass and energy balances for each component were solved using the first law of thermodynamics, with a fuel-to-air ratio of 1:10.

The performance numbers are nuanced and honest. Across the analysed methane concentrations of 0 to 3 percent, power output and cycle efficiency rise only slightly with methane enrichment: at 1 percent methane, the model predicts 17.68 kW of power at a cycle efficiency of 21.34 percent, while at 3 percent these figures reach 17.77 kW and 21.6 percent. The researchers are explicit that the electrical output is governed primarily by the manure fuel; the dilute methane in the recovered ventilation air contributes only marginally to power. Its principal role is greenhouse-gas mitigation through thermal oxidation — controlled combustion in the high-temperature chamber converts methane to carbon dioxide and water. Because carbon dioxide has a far lower global warming potential than methane (25 versus a much higher value for methane over 100 years), this conversion yields a substantial net climate benefit.

The emissions accounting quantifies that benefit precisely. Using a 100-year global warming potential of 25 for methane and the stoichiometric combustion reaction CH₄ + 2O₂ → CO₂ + 2H₂O, the researchers calculate that one gram of methane produces 2.75 grams of carbon dioxide. For the 100-cow barn, the system is modelled to capture and process approximately 18 tonnes of methane annually, corresponding to a 400.5-tonne CO₂-equivalent reduction in methane-attributable emissions — an 89 percent reduction in the greenhouse-gas burden directly attributable to methane at the barn boundary. The authors caution that this figure excludes indirect emissions, such as grid electricity used for cooling, which would be addressed in a full life-cycle assessment.

The researchers are equally candid about the study’s boundaries. This is a conceptual design and feasibility study, not an experimentally validated or economically optimised system. The CFD and thermodynamic results are numerical predictions that would benefit from experimental validation or comparison with field data. Methane capture efficiency, leakage, maintenance requirements, safety controls, techno-economic assessment, and full life-cycle analysis were all outside the present scope. Performance is also sensitive to operating conditions: methane concentration in the exhaust air rises with cattle weight and falls as ventilation rate increases, creating a design tension between air quality, cooling demand, and methane enrichment that future work must resolve. The authors recommend testing the concept across different geographies, cattle types, and ventilation strategies before advancing it toward practical implementation.

Even with those caveats, the significance of the design lies in its integration. Previous efforts have attacked the problem piecemeal — dietary manipulation and breeding to reduce enteric methane, anaerobic digestion to convert manure to biogas, or barn designs focused solely on animal welfare. Earlier polygeneration studies by some of the same authors demonstrated that methane and manure from dairy farms could yield 17 MW of electricity and 1350 cubic metres of freshwater per day, or drive systems with overall energy efficiencies of up to 81.6 percent. The new work is the first, according to the team’s comparison of the literature, to fold barn-level THI design, methane mitigation, and power generation into a single architectural and thermodynamic scheme — so that the building that houses the cows is also the machine that cools them, scrubs their methane, and powers the farm.