U of M researchers are tackling two significant environmental challenges with a single, innovative solution: using shredded, recycled tires to filter urban stormwater runoff.
Every year, more than 270 million tires are discarded, creating vast stockpiles that can attract mosquitoes and rodents and pose a combustion risk. At the same time, urban stormwater—laden with pollutants such as antifreeze, oil, and sediment—contributes to phosphate contamination in local waterways. New research led by John Gulliver, professor emeritus with the Department of Civil, Environmental and Geo- Engineering and a CTS scholar, seeks to address both issues simultaneously.
“Currently, there aren’t many options for removing phosphates from stormwater runoff in Minnesota,” Gulliver says. “Existing treatment methods are expensive and limited to surface filtration. Our hope is that recycled tires could offer a low-cost, practical way to treat stormwater underground, in places such as beneath parking lots where surface treatments aren’t feasible.”
The new study, sponsored by the Minnesota Local Road Research Board (LRRB), builds on a previous project that demonstrated recycled tires’ potential for phosphate retention in stormwater treatment systems. However, that initial research also revealed that tires may leach heavy metals into the environment. A key finding was that naturally occurring biofilms—slimy collections of microbial organisms—could absorb these leached metals, mitigating the environmental concerns.
The current project aimed to determine if these beneficial biofilms would develop in the real-world conditions of an underground stormwater treatment system—and if the shredded tires could be safely used for stormwater filtration and storage. The team conducted a series of three tests to find out.
For the first test, shredded tire pieces were placed in one-gallon buckets and subjected to alternating wet and dry periods using Mississippi River water, simulating natural rainy and dry conditions. Some buckets were kept open and others lidded to mimic varying humidity conditions in an underground chamber. After each “flush” of river water, samples were analyzed for biofilm growth, phosphate levels, and heavy metals. The results were encouraging: biofilms quickly developed on the tire material and effectively absorbed leached metals, ensuring water quality remained within environmental standards.
The second test focused on per- and polyfluoroalkyl substances (PFAS), commonly known as “forever chemicals” because they take an extremely long time to break down. Researchers investigated whether the tire aggregate could absorb PFAS from stormwater without releasing more into it. Through testing in which tire pieces were separated into their components in a solution, the team found that while the tires didn’t significantly absorb PFAS, they also didn’t release these chemicals at concerning levels.
In the final test, wooden cedar chunks were added to the shredded tire material to see whether they could further reduce metal leaching. Ultimately, the robust biofilm growth proved sufficient to mitigate the release of heavy metals from the tires, rendering the addition of wood chunks unnecessary.
Overall, the study’s findings demonstrate the strong potential for safely incorporating shredded, recycled tires into underground stormwater systems.
“Using tire-derived aggregate for underground stormwater treatment offers significant advantages,” Gulliver explains. “These include cost savings through more efficient stormwater management and better use of discarded tires, land savings by reducing the need for new tire landfills, and environmental benefits created by preventing harmful phosphates from entering our rivers, ponds, and lakes.”
—Megan Tsai, contributing writer
