Project overview
Biofiltration is one of the most robust stormwater treatment practices available to stormwater practitioners. However, some recommended biofiltration media mixes have been shown to export phosphate, potentially contributing to water quality impairments. Stormwater practitioners need to know which biofiltration media components can be trusted to capture and retain phosphorus.
In Phase I of this research, we measured plant growth rate, phosphate release, and filtration rate of common biofiltration media mixes, containing a variety of locally available materials. Questions remained as to the long-term performance of these mixes. One of these questions was “how do these mixes perform when road salt is introduced into the system?” In addition, because mixes with high organic compost content are known to release phosphate, there is interest in media mixes with low organic content. However, their ability to support plant growth had been untested.
To address these lingering questions, Phase II of this research aimed to:
Identify which locally available biofiltration media mixes can
effectively support plant growth and microbial function,
attain high filtration rates, and yet,
not release phosphate.
Document simple tests or metrics, and/or design specifications, that practitioners can use to reliably design a biofiltration practice that effectively filters stormwater.
Identify local sources of biofiltration media materials.
Ensure that the results of this investigation are made available to stormwater practitioners, by incorporating the knowledge gained into guidance materials and training opportunities.
Research findings
Organic materials vary substantially based on supplier, source material, and other factors not tested during this research. Due to this variability, the results from this study cannot be used to predict the performance of compost, peat, or other organic material.
Simple tests and metrics do not appear to reliably predict the potential for phosphate release from organic materials.
A lab-scale indoor discovery experiment found no correlation between the ratio of compost (0% to 20%) in a media mix and the germination of ten different vegetation species commonly used in Minnesota rain gardens.
For the media and materials tested in outdoor mesocosm experiments:
Food-based and leaf-based composts supported healthy plant vegetation growth, but released phosphate and nitrate
Replacing food- and leaf-based compost with sphagnum or reed sedge peat accomplished phosphate capture, but produced poor vegetation growth
Leaf-based compost media mixes amended with biochar or spent lime reduced phosphate export, but not below inflow concentration
Leaf-based compost media mixes amended with iron accomplished phosphate capture, but produced poor vegetation growth
Adding simulated road salt to stormwater inflow increased phosphate export from leaf-based compost
What does this mean for Minnesota?
These results will be incorporated into professional training programs, continuing education programs, and stormwater certification programs. By filling existing knowledge gaps related to the effectiveness of various biofiltration media mix components, this study helps stormwater practitioners design biofiltration practices that accomplish effective stormwater filtration, preventing the pollution of Minnesota’s surface waters.
This project laid the foundation for additional research. You can read about those projects by visiting Capturing contaminants of emerging concern with biofiltration and Bioretention media for supporting plant growth and limiting nutrient export.
Reports and Presentations
- Final Report 2022
- Project update 2021