Capturing contaminants of emerging concern with biofiltration

Project overview

This project was a continuation of two previous studies of biofiltration media mixes. You can read about those projects by following these two links:

Contaminants of Emerging Concern (CECs) are newly recognized stormwater pollutants that are not yet regulated, such as pesticides, herbicides, tire-related contaminants, and pharmaceuticals. When these compounds enter our water bodies, they harm fish, mussels, insect larvae, and humans. Therefore, stormwater managers need cost-effective designs for stormwater treatment systems that are capable of removing CECs from stormwater. 

Cities, counties, and watershed districts currently rely on stormwater ponds and biofiltration basins to remove other pollutants, such as sediment and nutrients, from runoff. Biofiltration systems have not been designed to remove CECs, but they have the potential to do so. This research explored how biofiltration systems could be optimized to improve CEC removal from stormwater runoff.

To do that, the research team conducted outdoor experiments to determine the best materials (sand, compost, biochar) and mix ratios for capturing CECs. Using this knowledge, the team aimed to optimize the mixes to produce the most cost-effective design. 

Research questions

  • Which biofiltration media mix ratios of sand, compost, and biochar are optimal for removing CECs from stormwater runoff?
  • Which CECs are present in urban runoff when it enters biofiltration systems, and at what concentrations?
  • Which CECs are captured by one or more existing field-installed biofiltration systems, and at what efficiency?
  • Does CEC capture in mesocosm experiments correlate with CEC capture in field-scale biofiltration systems?

Research findings

  • Among the tested mixes, 1% biochar + 99% sand was most effective at removing CECs from stormwater runoff. Biochar provided greater and longer-lasting removal of several pesticides, tire-derived contaminants, and 1H-benzotriazole than sand or compost + sand. 
  • CECs were detected in urban stormwater runoff, stormwater ponds, and the Mississippi River, with concentrations varying by contaminant, location, and season. Tire-derived chemicals peaked during spring snowmelt, while pesticides peaked in early summer. Pharmaceutical concentrations generally decreased as runoff volume increased. These patterns indicate that CECs enter biofiltration systems from multiple sources, including urban and agricultural runoff, wastewater, and precipitation.
  • Field-installed biofiltration systems reduced the overall amount of CECs reaching receiving waters, although removal varied by contaminant. CECs that readily attach to organic matter and solids tended to be removed more effectively. Overall CEC removal was similar in filters containing iron-enhanced sand alone and those containing iron-enhanced sand with biochar.
  • CEC removal patterns observed in laboratory column experiments were generally consistent with those measured in field-installed biofilters. In both settings, removal varied by contaminant, with some CECs being removed more effectively than others. These results suggest that laboratory experiments can help predict how biofiltration practices will perform under field conditions.

What does this mean for Minnesota?

This project helps us understand how to design biofiltration practices that can capture CECs before they are carried into our water bodies, which can help stormwater managers cost-effectively meet their stormwater treatment goals. Ultimately, this will help to preserve and improve water resources and habitats throughout Minnesota. 

Additional research is underway that will expand our understanding of how to support plant growth on biofiltration media mixes.

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