Decision Tool Charts a Circular Wastewater Future for a Zimbabwean Mining Town

In the drought-prone mining town of Zvishavane, in Zimbabwe’s Midlands Province, the local wastewater treatment plant has long been a one-way street: sewage flows in, partially treated effluent flows out into the Runde River catchment, and everything of value in between is lost. A new study by Roberta Mavugara, Mark Matsa and Rameck Defe of Midlands State University, published in Discover Green Chemistry, argues that this linear model is not an inevitability but a choice, and it offers one of the most detailed roadmaps yet for how a small, cash-strapped city in the Global South could turn its sewage into fertilizer, electricity and clean water.

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