Managing stormwater runoff has a long and ancient history, with storm drains found in archeological digs dating as far back as the Bronze Age. They were – and still are – a way to quickly move water away from streets and other infrastructure to lakes or streams.
But that stormwater carries pollutants – heavy metals from vehicle traffic, along with phosphorus and nitrogen from lawns – especially in that first one inch flush of rainfall.
That’s why the Department of Transportation has implemented low impact development designs that mimic nature's way of filtering water through soil to groundwater. Alongside roadways are engineered bioswales that use a sand/compost mix to filter the runoff. Sand, of course, is a mined product and the compost must be purchased.
Could other more readily available and more cost effective resources be as effective as filtering media? An initial NRRI study completed in 2022 showed that peat, salvaged from a road construction project, was an effective, free and local filtering material. Could other industrial byproducts, collected from greater Minnesota, also work?
Waste Not
That initial study was expanded to Phase Two, which resulted in a Minnesota Department of Transportation report: Re-Use of Minnesota Waste Material in Sustainably Designed Soils, published in September. David Saftner, University of Minnesota Duluth Civil Engineering professor, was principal investigator on the Phase Two project with Mei Cai and Matt Aro as NRRI co-leads. This effort went statewide to find waste materials from industry partners who were happy to find uses for their byproducts.
The challenge was to find materials that are beneficial soil amendments, continually produced and headed to a landfill or otherwise wasted. Shipment and storage of materials was also considered.
“It was not too difficult to find waste materials, and mostly it was through professional connections,” said Mei Cai, NRRI lead researcher on the project. “And each material source had different requirements for how their byproduct is used, so we had to be very transparent.”
After exploring the possibilities in 15 waste materials, nine made the final cut. A paper mill provided ash, degritter and lime mud. A contractor company provided recycled concrete aggregate. A sugar manufacturing plant provided beet waste and VersaLime (made from calcium carbonate). A company that makes products with peat provided a peat/biochar mix. A sawmill provided sawdust. And the U.S. Army Corps of Engineers provided dredged sediment from the Mississippi River.
Results & Impact
Three mixtures of these organic (sawdust, peat/biochar, degritter) and inorganic materials (concrete aggregate, dredge sediment) were developed and run through a variety of tests both in the lab and on-site roadside:
- Recycled concrete aggregate (80%) + Ash sawdust (20%)
- Recycled concrete aggregate (80%) + Peat/Biochar (20%)
- Dredge Sediment (80%) + Degritter (20%)
The researchers found that these combinations effectively, and similarly, captured stormwater runoff, removed pollutants and promoted the growth of native plants.
Full Circle
A life cycle assessment was also conducted by NRRI Scientist Matt Aro. The environmental impact of the three top-performing mixes were very similar. But he found that the most important element in applying these materials is to keep them within 100 miles of the road site where they’ll be used.
“Transportation was the biggest driver in each of the impact categories,” said Aro. “We thought that might be true, but now we have the data to verify that.”
Recycled cement and ash sawdust soil mix had the lowest environmental impacts, followed closely by a mix of recycled cement with peat/biochar. Further research, before this is widely adopted, would be an economic assessment of using these waste materials versus compost and sand.
Based on the research findings, the project team created a comprehensive and easy-to-follow design guide for selecting and applying waste materials as soil amendments in stormwater treatment.
“This is a win-win for MnDOT and for the industries with waste materials from their processes,” added Cai. “The design guide allows road construction crews to implement these resources to save money and use waste resources, keeping them out of overburdened landfills.”
The project was funded by MnDOT’s Local Road Research Board.
PHOTO TOP: NRRI crew is joined by NRRI Executive Director Rolf Weberg to dig test plots for a variety of soil mixtures made with waste resources as part of Phase One of this research effort. (Archive Photo)