Who’ll Stop the Rain and Reuse It
To limit untreated sewage overflows, Pitt framework identifies smarter ways to reuse stormwater
Many older cities across the eastern United States rely heavily on aging sewer systems that use a single pipe network to carry away rainwater and sewage. With such systems, however, heavy rains can cause huge problems. Unable to handle the deluge, the system spills a mix of stormwater and untreated sewage, called combined sewer overflows (CSO), into nearby waterways. In Pittsburgh alone, where roughly three-quarters of the sewered area relies on combined pipes, these overflows send billions of gallons of diluted sewage into the region's rivers each year.
A solution to help limit CSO involves capturing and repurposing stormwater, but cost concerns can deter implementation. Researchers at the University of Pittsburgh Swanson School of Engineering are seeking to change that. Professors Vikas Khanna and Sarah Haig, with PhD student Ahteshamul Haq and Pitt alumnus SriGanesh V. Pennathur (BS EE ’26), have developed a decision-support framework that automatically identifies cost-effective, energy-efficient, and low-emission treatment systems for captured stormwater based on how the water will ultimately be used.
Their research “From runoff to circular resource: an integrated optimization framework for cost, energy, and GHG performance in urban stormwater systems” (DOI: 10.1038/s41545-026-00585-4) was published in the journal npj Clean Water. It provides planners with a valuable tool to help design viable, resilient, low-emission systems for water reuse, reducing the strain on single pipe networks and keeping untreated sewage out of local ecosystems.
"Cities like Pittsburgh face a legacy infrastructure problem, and it gets worse as storms in the region become more frequent and heavier," said Haq, first author of the paper. “Right now, that rain only adds load to the sewer. Captured and treated to the right level, it becomes a usable resource.”
Underpinning the team’s research is a “One Water” approach, which views all sources of water – whether from the tap, a nearby river, stormwater, or even the sewer – as parts of one interconnected urban water cycle. The approach explores the many ways different kinds of water can be used across an area.
“Stormwater doesn’t need to be treated to drinking-water standards to be a valuable resource,” said Haig, associate professor of civil and environmental engineering. “Instead, we can think about treating stormwater to be fit for its intended use. If the water is going to be used for irrigation, industrial processes, infrastructure, or surface-water recharge, we may not need the same level of treatment required for drinking water. Matching treatment to the end use could reduce the cost and energy required while still allowing us to put that water to beneficial uses."
To illuminate the varying costs of treating stormwater for each use, the team developed a mixed-integer nonlinear programming model built on a menu, or “superstructure,” of all plausible water treatment steps and how they can be wired together.
The menu integrates cost, energy, and life-cycle greenhouse gas (GHG) emissions data as it applies to various technologies used to treat water, such as coagulation, sedimentation, constructed wetlands, filtration, membranes, and disinfection. Whatever water use a planner might consider, the model solves separately for the lowest cost, the lowest energy use, and the lowest emissions.
While the team used Pittsburgh as its case study, the framework is built on published U.S. stormwater quality data and the water-quality standards for each end use, so other cities with aging combined sewer systems can apply it. Planners can incorporate local electricity prices, grid carbon intensity, and regulations to determine their unique situation. The model gives them a quantitative way to decide how to best use captured stormwater.
“We found that matching treatment intensity to the intended use can lower costs and avoid unnecessary treatment,” said Khanna, professor and interim chair of the Department of Civil and Environmental Engineering. “The goal is not to treat every gallon to the highest possible standard. It is to provide the appropriate treatment for how that water will be used.”
For drinking-water production, the researchers found that treating typical-strength stormwater could have lower treatment costs and greenhouse-gas emissions than seawater desalination. Even stormwater with relatively high pollutant concentrations remained competitive with other alternative water sources in the study’s treatment-only comparison.
The team’s research reveals how something that increasingly causes untreated sewage to spill into waterways can become an asset to a city. Instead of overwhelming old pipes, it can be used to make new concrete, recharge surface water, and cool industrial equipment.
As Haq said, “By decreasing flood damage and reducing costly pollution from overflows, cities can help offset the cost of building a stormwater reuse system.”
By showing how captured runoff can be treated for uses ranging from irrigation and concrete production to industrial supply and drinking water, the research reframes stormwater as more than an urban burden. With appropriate treatment and infrastructure, it could become part of a more circular and resilient water system.
This research was supported by the Pitt Momentum Funds program at the University of Pittsburgh.