The scale of the material is far from trivial. An adult horse of roughly 500 kilograms produces an estimated 20 to 31 kilograms of manure and bedding per day, amounting to some 9 to 11 tonnes annually. In regions with dense horse populations, those figures rival municipal bio-waste streams. The County of Wesel in Germany’s Lower Rhine basin, for example, counts more than 8,500 horses alongside 458,000 inhabitants; in 2024 the area collected 32,700 tonnes of bio-waste and 23,300 tonnes of green waste, while horse manure is estimated at around 75,000 tonnes. Yet because many stables sit far from farmland that could recycle those nutrients, manure increasingly generates disposal costs and storage constraints rather than agronomic value.

Chemically, horse manure is a distinctive feedstock. Pure manure contains about 1.0 to 1.7 percent nitrogen on a dry basis, much of it water-soluble and plant-available, along with 0.5 to 1.3 percent phosphorus as P2O5, roughly 1.1 percent potassium as K2O, and 84 to 95 percent organic matter. Its carbon-to-nitrogen ratio typically ranges from 20:1 to 32:1, but bedding materials such as straw or wood shavings can push mixtures above 50:1. That matters agronomically: high C:N materials trigger net nitrogen immobilisation in soil, reducing short-term plant-available nitrogen and potentially depressing yields. If manure is marketed as a fertiliser substitute, customers may perceive weak or even negative fertiliser effects, undermining willingness to pay. The bedding is therefore not a passive bulking agent but a design variable that shapes composting behaviour, nutrient dynamics, and ultimately market positioning.

Controlled composting resolves many of these problems. Thermophilic phases thermally degrade pathogens, parasite eggs, and weed seeds, while microbial activity stabilises labile nitrogen compounds into more predictable, plant-available forms. Finished horse manure composts reported in the literature contain total nitrogen of 1.7 to 2.3 percent, phosphorus up to 1.3 percent, potassium around 1.2 percent, and a near-neutral pH of 6.4 to 6.7, with the C:N ratio falling from an initial 27.3:1 to roughly 15.9:1 and moisture dropping to between 8 and 15 percent. Practical benchmarks, such as the German Organic Waste Ordinance, converge on sustaining temperatures above 55 degrees Celsius for more than two weeks. Studies of small-scale composting of manure with wood shavings show that weekly turning improves hygienisation uniformity, since outer pile layers can otherwise remain insufficiently sanitised. That quality assurance is what opens quality-sensitive horticulture, landscaping, and growing-media markets to manure-derived products.

Emissions, however, constitute the central technical trade-off. Composting generally suppresses methane relative to passive storage because aerobic conditions inhibit methanogenesis, but poorly managed piles can still release considerable methane. Meanwhile, ammonia volatilisation and nitrous oxide emissions can rise, eroding the product’s nitrogen content and contributing to eutrophication and climate forcing. Turning aerates the pile and cuts methane, yet can simultaneously increase ammonia losses by exposing ammonium-rich zones during thermophilic, alkaline phases. Research on dairy manure even shows that pile mixing can raise total measured greenhouse gas emissions while lowering methane alone. The authors conclude that good composting is not a compliance exercise but a core production competence: process control determines hygiene, nitrogen retention, odour, and customer-perceived value. Facilities with optimal aeration and exhaust gas treatment are preferable, which favours centralised processing where horse density or cooperative logistics allow efficient feedstock aggregation.

The systematic EU mapping reveals a structured, application-driven compost market in which soil improvement dominates, followed by fertilising functions and a smaller but meaningful segment for growing-media components. Solid composts prevail, while compost-biochar blends and vermicomposts are increasingly documented. Application fields extend beyond agriculture into horticulture, landscaping, urban greening, engineered soils, remediation, and green roof substrates. That breadth is strategically significant for horse manure, because peri-urban stables are often spatially closer to urban green infrastructure markets than to bulk agricultural outlets, reducing transport costs and enabling circularity branding that raises willingness to pay. The trade-off is that these markets demand higher product consistency and safety, reinforcing the case for controlled composting and rigorous quality management rather than passive pile storage.

Organisational form emerges as a decisive determinant of economic viability. Centralised private plants exploit economies of scale and professional quality assurance but require collection logistics and sufficient horse density. Municipal and public-private systems monetise composting partly through avoided disposal costs and internal use of compost in public green spaces, aligning well with the peri-urban geography of horse keeping. Decentralised models suit small holdings with limited investment capacity, where the business case rests on avoided container rental, haulage, and disposal contracts rather than product sales, though governance mechanisms are needed if products leave the site. Notably, the literature review found no documented cases of co-composting horse manure with municipal bio-waste, despite the apparent synergy: blending manure with kitchen and garden waste would raise nitrogen and potassium in the finished compost and could improve the economics of both streams.

Product differentiation offers the most promising frontier. Vermicomposting can unlock premium horticultural markets through higher microbial activity and nutrient availability, but it demands prior hygienisation and careful process management, raising complexity and risk. Compost-biochar blends present a more scalable strategy: biochar incorporated during composting improves nutrient retention and microbial habitat, and its carbon can persist in soils for decades to centuries, qualifying as a plausible carbon dioxide removal pathway. Voluntary carbon markets have begun recognising biochar-based removals, suggesting that carbon monetisation is more credible in compost-biochar models than in composting alone, provided monitoring, reporting, and verification frameworks are in place. Formulation design also follows application logic: fertiliser-oriented composts target C:N ratios of 8:1 to 12:1, soil improvers sit between 12:1 and 18:1, and carbon-storage or remediation blends exceed 18:1, often above 30:1 with woody biomass or biochar.

Regulation threads through every business model. Under the EU Fertilising Products Regulation (Regulation (EU) 2019/1009) and national frameworks such as Germany’s Fertilizer Act, Fertilizer Ordinance, and Bio-waste Ordinance, requirements for storage capacity, spreading periods, and waste classification shape what is legally and commercially feasible. Directive 2008/98/EC mandates separate biowaste collection, and the EU Soil Strategy for 2030 raises demand for organic matter inputs in degradation-prone regions such as Southern Europe. The authors’ central message is that viable models must treat regulatory conformity and quality assurance as core capabilities enabling market access, not external constraints. Horse-specific data on emissions, pharmaceuticals, and antibiotic resistance genes remain sparse compared with cattle and pig systems, and systematic evaluation of decentralised peri-urban systems is scarce. Future work combining horse-specific process monitoring with economic modelling across organisational structures would strengthen the evidence base for policy and investment. The larger conclusion is striking: Europe’s horse manure problem is really a design problem, and its solution lies in engineering purpose-built compost products, organisational models, and carbon strategies around the material’s distinctive chemistry.

Subject of Research: Business models for utilising horse manure through compost-based value chains in the European Union

Article Title: Business Models for Horse Manure Utilisation in the European Union: Compost-Based Products, Market Pathways, and Carbon Integration

Article References: Pleissner, D., Podßun, P., Hölscher, P., & Friege, H. (2026). Business Models for Horse Manure Utilisation in the European Union: Compost-Based Products, Market Pathways, and Carbon Integration. Waste and Biomass Valorization. https://doi.org/10.1007/s12649-026-03776-9

Image Credits: AI Generated

DOI: 10.1007/s12649-026-03776-9

Keywords: horse manure, composting, circular economy, soil improver, biochar, carbon credits, peri-urban agriculture, EU Fertilising Products Regulation, waste valorization, growing media, greenhouse gas emissions, business models

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