The research, conducted by Tiku T. Tanyimboh of the University of the Witwatersrand and North Lanarkshire in the United Kingdom, together with Nokuthula R. Nyawo of the University of the Witwatersrand and the Johannesburg Roads Agency, tackles a deceptively simple question: when a developer must choose between conventional piped drainage, rainwater harvesting systems and detention basins, how should that choice be made? The authors argue that previous studies have typically addressed fragments of the problem. Some evaluated hydrologic performance without cost. Others considered water supply reliability for rainwater tanks in isolation, without examining how those tanks alter the behavior of the entire drainage network. Very few combined probabilistic rainfall modelling, full hydrologic-hydraulic simulation of the complete drainage infrastructure, and whole-life cost analysis within a single decision framework.

To close that gap, the team developed a generic, parsimonious and computationally efficient method for sizing rainwater harvesting tanks based on the modal storm, the representative storm event that captures the dominant rainfall characteristics of a site. This SuDS-oriented tank-sizing approach deliberately sidesteps the elaborate continuous simulations and extensive historical data requirements that have traditionally made probabilistic tank design burdensome. By anchoring the sizing calculation to the modal storm, the method delivers tank capacities consistent with the statistical structure of local rainfall while remaining fast enough to embed in iterative design and optimization workflows. The tank sizing also moves beyond the conventional focus on water supply reliability alone, instead treating the tank as a component of the drainage system whose storage volume attenuates runoff and reshapes the hydrographs that downstream pipes and channels must convey.

With tank sizing resolved, the researchers built a multicriteria decision-making framework around the analytical hierarchy process, a structured technique for weighting and combining multiple, often conflicting, objectives. The framework compares alternative SuDS-based integrated drainage designs across infrastructure performance, whole-life costs and a broader set of sustainability factors. The analytical hierarchy process, originally formulated by Thomas Saaty, asks decision makers to perform pairwise comparisons among criteria and alternatives, translating qualitative judgments into quantitative priorities. Here those priorities were complemented by entropy-based criteria weighting, a technique that derives objective weights from the spread and discriminating power of the performance data itself, reducing the subjectivity that can undermine purely judgment-based weighting schemes.

The test bed for the framework was a large, real-world urban residential development on a greenfield site in Johannesburg, South Africa, a city whose climate and regulatory context make it a compelling laboratory for drainage research. The authors modelled the hydrologic and hydraulic response of the complete drainage infrastructure under each design alternative, routing runoff generated by probabilistically derived design storms through the network to assess whether pipes and channels retained adequate freeboard and conveyance capacity. Greenfield sites are particularly instructive because they allow designers to compare conventional and sustainable approaches from first principles, without the legacy constraints that complicate retrofit studies.

One of the study’s most striking results concerns hidden redundancy in existing design standards. The analysis revealed excess flow capacity in the drainage network, which translated directly into high marginal construction costs. The authors attribute this oversizing to implicit constraint duplication within the drainage design standards, meaning that multiple clauses in the governing guidelines effectively impose the same hydraulic requirement, each adding its own margin of safety. When these overlapping constraints stack, the resulting infrastructure is significantly larger, and therefore significantly more expensive, than a technically defensible design requires. In an era when municipal budgets are stretched thin, the finding suggests that simply harmonizing design standards could free substantial resources without sacrificing flood protection.

The cost analysis carried a second sobering message. Operation and maintenance expenditure accounted for approximately 35 to 47 percent of the net present value of the drainage schemes examined. Whole-life costing, which discounts construction, operation, maintenance, rehabilitation and end-of-life costs over the asset’s service horizon, is frequently championed in principle but neglected in practice, with decisions dominated by lowest capital cost. The study’s numbers make clear that ignoring the operational tail of a drainage project’s life distorts the comparison between options. Detention basins, for example, may appear inexpensive at construction yet accrue maintenance liabilities associated with sediment removal, vegetation management and outlet structure upkeep, while rainwater harvesting systems require pump maintenance, tank cleaning and eventual component replacement.

Rainwater harvesting carries an additional dividend that conventional drainage cannot match: it converts a nuisance into a resource. In a water-stressed country such as South Africa, where national planning documents have repeatedly warned of supply deficits, harvested rainwater can displace potable demand for non-potable uses such as toilet flushing, irrigation and laundry. The study’s framework captures this dual function, crediting harvesting designs both for runoff attenuation and for water conservation, while detention basins earn their place through flood peak reduction and conventional systems through their simplicity and familiarity. By scoring every alternative on the same weighted criteria, the framework exposes trade-offs that single-metric evaluations conceal, such as how much additional capital a rainwater harvesting scheme must absorb to deliver a given improvement in network hydraulic performance.

The broader significance of the work lies in its methodological generality. The authors describe their approach as generic, meaning it can be transplanted to other catchments, climates and regulatory regimes by substituting local rainfall statistics, unit costs and design standards. The parsimonious tank-sizing method lowers the computational barrier that has kept smaller municipalities and design consultancies out of probabilistic SuDS analysis, and the multicriteria structure accommodates additional criteria, from water quality improvement to amenity and biodiversity value, as local priorities demand. As climate change allowances force designers to plan for intensified rainfall, frameworks of this kind provide a defensible, transparent and reproducible basis for spending scarce infrastructure funds where they deliver the most value.

For engineers, planners and municipal officials, the practical takeaways are concrete. Audit drainage design standards for duplicated constraints before committing to oversized pipes. Bring operation and maintenance costs to the table at project inception, where they can shape the choice of technology rather than merely haunt the asset register. Treat rainwater tanks not just as water supply devices but as distributed storage that reshapes catchment response. And above all, evaluate drainage alternatives as integrated systems, using structured multicriteria methods that make every assumption explicit. The study suggests that the path to affordable, resilient urban drainage runs not through any single technology, but through smarter, more honest comparisons among them.

Subject of Research: Multicriteria evaluation of sustainable urban drainage design options combining hydrologic-hydraulic performance, rainwater harvesting tank sizing and whole-life cost analysis.

Article Title: Multicriteria Evaluation of Sustainable Drainage Options: Comparison of Conventional Drainage, Rainwater Harvesting and Detention Basins

Article References: Tanyimboh, T. T., & Nyawo, N. R. (2026). Multicriteria Evaluation of Sustainable Drainage Options: Comparison of Conventional Drainage, Rainwater Harvesting and Detention Basins. Water Resources Management, 40(12), Article 527. https://doi.org/10.1007/s11269-026-04878-8

Image Credits: AI Generated

DOI: 10.1007/s11269-026-04878-8

Keywords: sustainable drainage systems, stormwater management, rainwater harvesting, detention basins, multicriteria decision making, analytical hierarchy process, whole-life costing, urban flooding, runoff attenuation, water resources management, Multicriteria, Evaluation

The research, published in the journal Microplastics and Nanoplastics, focused on two coastal sampling sites in Greater Jakarta, a metropolitan area of more than thirty million people where land subsidence, saltwater intrusion and failing piped infrastructure have pushed many communities toward alternative water sources. Rainwater harvesting systems, which collect and store rooftop runoff for domestic use, have become an attractive coping strategy in this setting. But until now, little was known about whether the water these systems capture carries microscopic plastic particles and the chemical additives that leach from plastic products.

The team, led by Annisa Fitri Mustafa of the School of Environmental Sciences at Universitas Indonesia, together with colleagues including corresponding author Hayati Sari Hasibuan, set out to establish a baseline. Rather than counting plastic particles visually, as many earlier studies have done, they employed two of the most sensitive analytical techniques available for environmental contamination. Microplastic polymers were identified using pyrolysis gas chromatography-tandem mass spectrometry, known as Py-GC/MS/MS, a method that thermally breaks plastic molecules into characteristic fragments that can be identified with high confidence. Plastic additives were measured with ultra-high performance liquid chromatography-tandem mass spectrometry, or UHPLC-MS/MS, which can detect trace organic chemicals at extremely low concentrations.

The results paint a nuanced picture. In samples drawn from the rainwater harvesting systems, the researchers detected three common plastic polymers: polyvinyl chloride, or PVC, polyethylene terephthalate, or PET, and polymethyl methacrylate, or PMMA. PVC is ubiquitous in pipes, gutters and building materials, while PET is the polymer of beverage bottles, and PMMA, better known as acrylic, appears in construction and consumer products. Critically, the polymer composition and concentrations differed between the two sampling locations, suggesting that contamination is not uniform but shaped by local conditions.

That site-specific variation matters, the authors argue, because it hints at multiple contamination pathways. Rainwater passing over roofs, gutters and storage tanks can pick up fragments shed by the harvesting infrastructure itself, particularly where PVC piping and plastic tanks are involved. At the same time, atmospheric deposition may deliver plastic particles and dissolved additives directly into open storage. Jakarta’s intense urban activity, from traffic and waste burning to construction, generates a constant load of airborne particles, and plastic debris is increasingly recognized as a component of that urban aerosol. The study suggests that the observed contamination may reflect interactions among local environmental conditions, atmospheric deposition and the materials of the harvesting systems themselves.

The chemical side of the analysis proved equally revealing. After careful blank correction, the researchers detected plastic additives in both rainwater and rainwater harvesting system samples. These additives, which include compounds such as bisphenol A and phthalate plasticizers like DEHP, DBP and DEP, are incorporated into plastics to improve flexibility, durability or heat resistance. They are not chemically bound to the polymer matrix, meaning they can migrate out of plastic products and into surrounding water over time. Several of these compounds are recognized endocrine disruptors, capable of interfering with hormone systems even at low doses, which is why their presence in harvested drinking water sources warrants attention.

Intriguingly, no target microplastic polymers were detected above the method detection or quantification limits in the analyzed rainwater sample, even though additives were found in it. The authors are careful about how this discrepancy is interpreted. One possibility is that dissolved or particulate chemical additives travel through the atmosphere differently from intact polymer particles, or that the limited sampling did not capture polymer-contaminated rainfall events. The study was explicitly designed as a preliminary baseline investigation with a limited sampling design, and the researchers acknowledge potential blank-related uncertainty for some additives and the absence of direct source attribution. They stress that the findings should be read as preliminary baseline evidence rather than definitive measurements of exposure.

That cautious framing is a hallmark of the study’s quality assurance approach. The analytical work relied on method detection limits, quantitative detection limits, initial and continuing calibration verification, and multiple reaction monitoring in the tandem mass spectrometers, all standard tools of good laboratory practice. Pyrolysis-gas chromatography with tandem mass spectrometry is increasingly favored in microplastics research because it avoids some of the visual misidentification problems that plague microscopy-based counts, and pairing it with liquid chromatography-tandem mass spectrometry allows both the particles and their chemistry to be assessed in parallel.

For Jakarta, the findings arrive at a politically and environmentally sensitive moment. Large parts of the Greater Jakarta coastal plain are sinking by several centimeters each year, and Indonesia is proceeding with plans to relocate its capital while investing heavily in water infrastructure. In neighborhoods where piped water is unreliable or saline, rooftop harvesting is often promoted as a resilient, decentralized solution. The new results do not argue against rainwater harvesting, but they do suggest that system design, materials selection and maintenance could influence the quality of stored water. First-flush diversion, filtration, and the use of less additive-rich storage materials are among the practical questions that follow from this work.

The authors call for future studies with expanded sampling, improved quality control and direct characterization of rainwater harvesting system materials, which would allow contamination to be traced to specific components of the collection infrastructure. They also point toward the need to understand how atmospheric deposition varies across the urban landscape, and whether seasonal monsoon dynamics in the Indonesian archipelago modulate the delivery of plastic particles and additives to rooftop catchments. As microplastics research matures worldwide, studies like this one underscore that the plastic pollution problem is not confined to rivers and oceans. It now extends into the sky above cities and into the rain that falls from it, quietly entering the alternative water systems on which millions of coastal residents may depend.

Subject of Research: Microplastic polymers and plastic additives in rainwater and rainwater harvesting systems in coastal Jakarta, Indonesia

Article Title: Microplastics and plastic additives in rainwater and rainwater harvesting systems in the Greater Jakarta Coastal Region, Indonesia

Article References: Mustafa, A. F., Ika, A. R., Chen, J.-W., Hartono, D. M., Hasibuan, H. S., & Chang-Chien, G.-P. (2026). Microplastics and plastic additives in rainwater and rainwater harvesting systems in the Greater Jakarta Coastal Region, Indonesia. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00228-y

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00228-y

Keywords: microplastics, plastic additives, rainwater harvesting, Jakarta, Indonesia, PVC, PET, PMMA, phthalates, bisphenol A, coastal water quality, Py-GC/MS/MS