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 review, authored by Katarzyna Chojnacka, classifies biorefinery residues into three functional nutrient fractions, each demanding a different combination of chemical transformation and separation. Ammonium-rich aqueous streams, such as the liquid fraction of anaerobic digestate, concentrate nitrogen and potassium in dissolved, mobile forms that plants can access immediately but that also carry elevated risks of volatilisation, leaching, salinity, and phytotoxicity. Carbonaceous solid matrices, including biochars and hydrochars, embed nutrients within organic and mineral frameworks whose fertiliser value is expressed through release kinetics and soil-microbe-mediated transformations. Inorganic mineral concentrates, chiefly biomass ashes, deliver nutrients as salts, oxides, carbonates, and phosphates, but face the strictest contaminant qualification requirements. The central insight is that bulk elemental analysis is fundamentally insufficient: the same total phosphorus content can reside in immediately soluble ammonium phosphate, in slowly dissolving calcium phosphate minerals, or in recalcitrant iron-bound forms with negligible short-term plant availability.

This speciation problem is where heterogeneous catalysis earns its central role. The review draws a deliberately sharp line between true catalytic reactions, in which a solid catalyst accelerates a chemical transformation and is regenerated within a catalytic cycle, and complementary non-catalytic operations such as adsorption, ion exchange, reactive crystallisation, and membrane separation. Acid- and base-catalysed conversion, heterogeneous redox chemistry, and electrocatalytic nitrogen transformation are treated as genuinely catalytic, while struvite precipitation, for example, is classified as reactive crystallisation because the magnesium source is incorporated stoichiometrically into the mineral product. This mechanistic discipline matters because sorbents, stoichiometric reagents, and separation media are frequently mislabelled as catalysts, obscuring what each process can and cannot achieve and preventing valid comparisons of performance, regeneration, and technological readiness.

The evidence for catalytic control of nutrient speciation is strongest in thermochemical processing. Catalytic pyrolysis over H-ZSM-5 zeolites mediates how fuel nitrogen partitions between gas-phase ammonia and hydrogen cyanide and nitrogen retained in liquid and solid products: in model amino-acid studies, the zeolite increased the gas-phase nitrogen fraction from 38.54 percent to 53.95 percent while reducing nitrogen in tar and char by 69.22 percent and 58.07 percent respectively. Hydrothermal carbonisation of wet biomass offers a second lever, with temperature governing nitrogen solubilisation into process water and phosphorus partitioning into less soluble apatite-type phases at higher severity. Perhaps most strikingly, co-pyrolysis of piggery biogas residue with ten percent Fenton sludge as an iron and magnesium source at 600 degrees Celsius promoted conversion towards high-activity apatite inorganic phosphorus and increased phosphorus bioavailability by 41 to 48 percent relative to the undoped control, demonstrating how waste-derived mineral additives can drive phase transformation during conversion itself.

For aqueous streams, the review catalogues a spectrum of surface-engineered functional materials. Metal oxides such as ferrihydrite capture phosphate through inner-sphere surface complexation, transitioning towards amorphous iron-phosphate precipitation at higher surface loading. Zeolites provide practical ammonium exchange, with alkaline-activated and lanthanum-impregnated variants achieving simultaneous ammonium and phosphate removal at reported capacities of 23.9 and 21.2 milligrams of ammonium-nitrogen per gram respectively. Layered double hydroxides recover phosphate by anion exchange and, for calcined materials, through reconstruction via the memory effect; MgFe-Zr-coated magnetic particles sustained 75 to 97 percent phosphate adsorption after one hour across fifteen laboratory adsorption cycles with fourteen intervening desorption cycles. Metal-modified biochars emerge as the only material class with documented functions in both aqueous and carbonaceous-solid fractions, acting as phosphate sorbents in solution and as nutrient carriers and contaminant immobilisers in soil.

The review is equally candid about the gap between laboratory promise and field reality. A three-tier testing hierarchy is proposed as standard practice: single-component model solutions to establish mechanism, spiked real matrices to quantify the performance discount from competitive co-ions and dissolved organic matter, and unmodified real residues under repeated operational cycling for process design. Tier-one experiments dominate the current evidence base, and the authors argue that prioritising tiers two and three would represent the single most important methodological change for improving the transition from laboratory demonstration to process deployment. Catalyst stability under real residue conditions, which are aqueous, moderately acidic to alkaline, and rich in dissolved organics, sulphur, and chloride species, is identified as the primary practical barrier, with fouling, leaching, structural transformation, and poisoning often acting simultaneously.

Nitrogen recovery illustrates both the maturity spectrum and the catalytic frontier. Stripping of ammonium from digestate liquid fraction followed by acid scrubbing to ammonium sulphate remains the most commercially deployed route, while membrane-based concentration offers a complementary path that recovers water alongside nutrients. The most catalytically explicit approach is electrocatalytic oxidation of ammonia to nitrate on NiOOH electrode surfaces bearing hydroxyl and oxygen vacancies, a mechanistically characterised pathway that converts ammoniacal nitrogen into nitrate rather than merely capturing it. However, the review notes that this approach rests on a single model-solution study, that validation in real digestate matrices has not been reported, and that any recovered nitrate product would face the EU limit of 100 milligrams of nickel per kilogram of dry matter, making direct measurement of nickel transfer from the electrode mandatory before CE marking is possible.

On the product side, the review evaluates five fertiliser categories against release mechanism, agronomic performance, and soil-microbe interactions. Mineral precipitates such as struvite-type phosphates and ammonium salts from stripping-scrubbing carry the strongest field evidence, with struvite performing comparably to soluble phosphate sources on acid soils but releasing phosphorus more slowly on near-neutral to alkaline soils. Organo-mineral composites and biochar-based slow-release fertilisers show the clearest benefit from chemically or thermochemically engineered precursors but require more field validation, with binder type, pyrolysis temperature, and nutrient formulation jointly determining mechanical strength and release dynamics. Coating-controlled products offer the most precise timing control but add manufacturing cost and raise microplastic concerns that are driving development of biodegradable shells. Across all architectures, soil pH, texture, moisture, and biological activity remain decisive external controllers of actual nutrient delivery, and release claims require soil-context qualifiers.

Environmental, techno-economic, and regulatory dimensions complete the picture. Life cycle assessment studies identify energy-intensive drying and pH-control chemicals, rather than the recovery steps themselves, as the dominant environmental hotspots, and methodological inconsistency in functional units and allocation methods is large enough to reverse the environmental ranking of competing technologies. Under the EU Fertilising Products Regulation, struvite qualifies under Component Material Category 12, biomass ash under CMC 13, and compliant biochar under CMC 14, with Article 19 providing a harmonised end-of-waste route that converts compliant residues into CE-markable products. Yet regulatory gaps persist for antibiotics, antimicrobial resistance genes, pharmaceuticals, and microplastics, none of which currently carry EU-wide limits in CE-marked products. The review concludes that closing nutrient loops at a scale that genuinely reduces primary mineral fertiliser demand will require harmonised assessment methods, reliable contaminant monitoring as a mandatory quality gate, and, above all, multi-season field trials that isolate the contribution of catalytic processing to real agronomic performance, because a catalytic innovation without a demonstrated connection to crop response remains incomplete.

Beyond the technical chemistry, the review’s framing carries implications for how biorefinery operators might sequence investments. Because the three residue fractions respond to different processing logic, a single facility handling digestate, process water, and ash simultaneously would need parallel recovery trains rather than one universal technology, reinforcing the argument for adaptive process design that can accommodate feedstock-driven variability. The observation that feedstock origin is a primary determinant of both nutrient forms and trace element burdens suggests that regional sourcing strategies, not just reactor engineering, will shape product consistency.

The methodological critique embedded in the review also deserves attention from the wider research community. By distinguishing model-substrate evidence from real-matrix evidence and giving interpretive priority to the latter, the authors provide a template for evaluating claims across the nutrient recovery literature. Many reported capacities and efficiencies, including the zeolite exchange and layered double hydroxide adsorption figures, derive from simplified solutions whose competitive chemistry differs substantially from digestate process water. Readers should therefore treat such numbers as upper bounds indicative of mechanism rather than as deployment forecasts.

Finally, the policy analysis highlights a quiet but consequential alignment: the classification of phosphate rock as a critical raw material creates regulatory momentum that could accelerate permitting and financing for recovery facilities, while the end-of-waste pathway under the Fertilising Products Regulation offers a defined route from residue to CE-marked product. The binding constraint, on this reading, is less the availability of individual technologies than the absence of harmonised contaminant standards and multi-season agronomic validation, gaps that the review identifies as the field’s most urgent research priorities.

Subject of Research: Nutrient recovery and fertiliser production from biorefinery residues using heterogeneous catalysis and complementary processes

Article Title: Heterogeneous catalysis and complementary processes enable nutrient recovery and fertiliser production from biorefinery residues

Article References: Chojnacka, K. (2026). Heterogeneous catalysis and complementary processes enable nutrient recovery and fertiliser production from biorefinery residues. Discover Green Chemistry, 1(1), Article 28. https://doi.org/10.1007/s44509-026-00028-w

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00028-w

Keywords: heterogeneous catalysis, biorefinery residues, nutrient recovery, circular fertilisers, struvite precipitation, biochar, phosphorus recovery, nitrogen recovery, slow-release fertilisers, circular bioeconomy, digestate, EU Fertilising Products Regulation