The researchers tackled a problem that has frustrated engineers and municipal planners for years: how do you choose the right combination of technologies when every option carries trade-offs across cost, health, environment and technical feasibility? Traditional tools fall short. Life cycle assessment quantifies environmental burdens but cannot weigh them against financial metrics or stakeholder priorities. Techno-economic assessment reduces everything to net present value while ignoring social acceptability and ecosystem effects. Both are data-hungry and time-consuming, which makes them awkward fits for municipalities that lack both. The team instead turned to the New Energy and Resource from Urban Sanitation Decision Support Tool, or NEREUS DST, a framework designed to evaluate entire treatment trains, not just individual processes, across water, energy and nutrient recovery simultaneously.

What makes NEREUS distinctive is its mathematical engine: a weighted multi-objective integer nonlinear programming model that treats technology selection as an optimization problem. Users feed in local influent characteristics, including chemical oxygen demand, total suspended solids, total nitrogen, total phosphorus and heavy metals, along with the resources they want to recover and weights reflecting local priorities. A knowledge library supplies country-specific discharge regulations and unit process data, allowing the tool to screen out trains that would violate Zimbabwean environmental standards. The output is not a single technology but an integrated sequence of processes, each chosen for how well it connects with the others.

To ground the model in reality, the researchers spent twenty weeks sampling influent at the Mabula wastewater treatment plant, the sole treatment facility serving Zvishavane’s roughly 55,000 residents. The results were striking. Biochemical oxygen demand averaged 314.85 milligrams per liter, a high-strength reading the authors attribute to chronic water rationing: when households use less water, the same organic load arrives in a smaller, more concentrated stream. Chemical oxygen demand averaged 597 milligrams per liter, and the BOD-to-COD ratio of 0.6 indicated high biodegradability. Total nitrogen and total phosphorus came in at 54 and 19 milligrams per liter respectively. Counterintuitively, the very scarcity that stresses the town’s water supply makes its sewage a richer resource stream, concentrating the nutrients and organic matter that recovery technologies feed on.

Equally important was the human input. The team consulted ten experts, including the town engineer, using purposive sampling to establish the criteria weights that drive the optimization. Environmental sustainability emerged as the top priority at 33 percent, followed by technical criteria at 28 percent, economic at 22 percent and social at 17 percent. Within the environmental category, health impacts ranked highest, a reflection of post-pandemic heightened concern for public safety. On the economic side, capital cost dominated, and on the technical side, stakeholders demanded technology readiness level six or above, meaning systems already demonstrated in comparable settings. That preference for proven technology, the authors note, creates a paradox: the most innovative solutions are precisely the ones local operators are least comfortable adopting.

Running the model produced a three-part treatment train. For nutrients, the tool recommended retrofitting the plant with struvite precipitation using ferric chloride, a mature process that recovers roughly 40 percent of phosphorus and 47 percent of nitrogen as a slow-release, multi-nutrient fertilizer. For energy, it proposed coupling anaerobic digestion with pyrolysis of the digestate, recovering about 25 percent of the energy embedded in the sludge as biogas, biochar, syngas and bio-oil. For water, it selected electrodialysis combined with double membrane filtration and chlorine dioxide disinfection, achieving up to 93 percent water recovery suitable for industrial and irrigation reuse. The full retrofit carried an estimated capital cost of US$655,910, with annual operating costs near US$249,949, offset by projected income generation of roughly US$575,645 per year, implying a payback period of about two years.

That payback figure, however, comes with caveats the authors are careful to spell out. The two-year return assumes stable markets for struvite fertilizer and willing industrial off-takers for reclaimed water, neither of which is guaranteed in a context where farmer acceptance, pricing against conventional fertilizers and distribution networks all remain uncertain. The coupled digestion-pyrolysis system demands specialized chemical and thermal process engineering skills that are scarce within the town council, and intermittent power supply could destabilize sensitive membrane and pyrolysis processes. There is also a subtler critique embedded in the analysis: the tool’s knowledge library may favor engineered solutions over lower-tech alternatives such as constructed wetlands, which could better match local operational capacity even if they recover fewer resources.

Institutional barriers loom as large as technical ones. Wastewater management, agricultural extension and energy regulation in Zimbabwe sit under different ministries, and the country lacks quality standards for recovered products like struvite or clear regulations for specific water reuse applications, leaving investors and operators in a regulatory gray zone. Public acceptance presents its own hurdle, since products derived from human waste carry a well-documented stigma that only sustained communication and strict quality control can overcome. The authors argue that these barriers do not invalidate the tool’s recommendations but rather define the journey required to reach them, and they propose a phased, modular implementation: begin with struvite recovery and biogas optimization using mature technologies, then add pyrolysis later once capacity and financing allow.

On the financing question, the study advocates a hybrid model combining municipal capital, concessional loans from development banks such as the African Development Bank, and private investment from energy and water services companies. It also calls for partnerships with local universities, including Midlands State University itself, for staff training and process monitoring, and for a national Resource Recovery and Reuse Policy with product quality guidelines and fiscal incentives for circular utilities. Before any of this happens, the authors stress, experimental validation is essential: the model’s predicted yields and energy balances have not yet been tested at the plant, and pilot-scale trials must precede full retrofitting.

The broader significance of the work lies less in Zvishavane’s specific numbers than in its demonstration of how decision support tools can be adapted to contexts they were never designed for. NEREUS had not previously been tested in an environment of intermittent water supply and severe fiscal constraint, and the study shows both its power and its limits. The tool can identify a technically optimal destination, but reaching it requires attention to governance, financing, community trust and institutional coordination that no algorithm can supply. For hundreds of small cities across sub-Saharan Africa facing the same pressures of urbanization, water scarcity and aging treatment infrastructure, the message is that circular wastewater systems are technically achievable and potentially self-financing, but only if planners plan for the socio-technical transition, not just the machinery.

Subject of Research: Multi-criteria decision analysis of resource recovery pathways from municipal wastewater in Zvishavane, Zimbabwe

Article Title: A multi-criteria decision analysis for sustainable resource recovery from municipal wastewater in Zvishavane, Zimbabwe using the NEREUS DST

Article References: Mavugara, R., Matsa, M., & Defe, R. (2026). A multi-criteria decision analysis for sustainable resource recovery from municipal wastewater in Zvishavane, Zimbabwe using the NEREUS DST. Discover Green Chemistry, 1(1), Article 13. https://doi.org/10.1007/s44509-026-00017-z

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00017-z

Keywords: wastewater, resource recovery, NEREUS DST, multi-criteria decision analysis, struvite precipitation, anaerobic digestion, pyrolysis, water reuse, circular economy, Zimbabwe, nutrient recovery, decision support tool

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