Recycled tires could potentially be used as a low-cost building material that retains phosphate in stormwater treatment systems, but they unfortunately can leach heavy metals into the environment. U of M researchers have been looking into the use of biofilms as a means of mitigating this problem.
“There is always a need to develop low-cost, minimal operations and maintenance treatment methodologies for urban runoff,” says John Gulliver, professor emeritus in the U of M’s Department of Civil, Environmental, and Geo- Engineering and member of the research team.
Previous research in 2023 from the St. Anthony Falls Laboratory noted that tire-derived aggregate—which consists of recycled, shredded tires—works as a cheap alternative to gravel or sand in underground stormwater treatment systems.
“[Tire-derived aggregate] is a useful resource generated from waste tires, avoiding disposal in landfills or combustion in energy generation facilities,” Gulliver, a CTS Research Scholar, says. As a bonus, the study observed that tire aggregate absorbs and removes dissolved phosphate—a major pollutant in stormwater runoff. The problem is that tire aggregate also tends to leach heavy metals such as iron, zinc, and copper.
In a 2024 follow-up study, U of M researchers tested whether biofilms—slimy collections of microbial organisms that attach to surfaces—could help solve this problem.
The researchers grew their own biofilms by placing sterilized tire aggregate in buckets and filling them with synthetically prepared stormwater. The biofilm-coated tire material was then run through flushes of water treated with various pollutants and nutrients to simulate storm events.
The results of the test were positive. The samples with biofilms leached lower levels of iron and zinc than samples without (copper didn’t leach much in either scenario). This could be attributed to the microbes absorbing the metals into their cell matrixes (“biosorption”) or the microbes adsorbing the metals onto their surface. They also found that biofilms did not interfere with the tire aggregate’s ability to absorb phosphate.
The researchers also used the study to get an idea of the lifespan of a tire-based stormwater system. In the tests, the materials could achieve greater than 95 percent phosphate reduction for up to 72 flushes—which represents about nine years’ worth of storms in the upper Midwest.
The study also demonstrated that the nutrients in stormwater runoff will feed and maintain the biofilm, and that “wire-exposed” tire-derived aggregate—a manufacturing byproduct that contains exposed iron wires mixed in with the tire material—provides a surface that seems to promote biofilm growth and retain phosphate even better than traditional tire aggregate.
The project was funded by the Minnesota Local Road Research Board.
—Sophia Koch, contributing writer
