Performance evaluation for a stormwater treatment train incorporating sedimentation and geomedia-augmented biofiltration

Project overview

Urban stormwater runoff carries with it pollutants, such as nutrients (including nitrogen and phosphorus), bacteria (such as E. coli), metals, road salt, pesticides, and flame retardants. Discharging untreated runoff into watersheds, lakes, and streams harms aquatic ecosystems and poses a risk to human health. Conventional stormwater biofiltration systems can remove metals, hydrophobic organic contaminants (like polyaromatic hydrocarbons, PAHs), and suspended solids. However, they are not reliably effective at removing E. coli, nutrients, and polar organic contaminants (like pesticides). 

In Minnesota, E.coli levels are of particular concern, and frequently cause beach closures. In the city of Two Harbors, Minnesota, Agate Bay Beach periodically closes due to high levels of E. coli, and in 2016, it became one of eight beaches on Minnesota’s Lake Superior North Shore to be listed on the State’s Impaired Waters List for E. coli. 

Therefore, new stormwater treatment approaches are needed, particularly for removing nutrients and E. coli. One novel approach to removing phosphorus is the use of iron-enhanced sand (IES). However, IES does not effectively remove E. coli. Biochar, a charcoal-like substance produced by heating wood waste under oxygen-limited conditions, has been demonstrated to remove E. coli, as well as nitrogen, metals, and organic contaminants. Combining biochar and iron to enhance stormwater filter systems might allow stormwater filters to remove both phosphorus as well as E. coli and other pollutants. For this reason, the Lake County Soil and Water Conservation District is installing an iron and biochar-augmented filtration system to treat contaminated runoff that is discharged to Agate Bay.

One challenge to this approach is that biochar efficacy varies widely, depending on production characteristics. Additionally, sourcing a locally produced, commercially available biochar can be difficult. Moreover, the presence of natural organic matter can foul the surfaces of biochar particles, potentially leading to reduced treatment efficacy during long term operation of a biochar-enhanced filtration system. To address these challenges, additional research is needed. 

The Natural Resources Research Institute (NRRI) is partnering with the Lake County SWCD to evaluate the long-term contaminant removal performance of biofiltration systems amended with biochar. NRRI’s research project begins with laboratory analyses of biochar efficacy. Then, NRRI and the Lake County SWCD will evaluate the contaminant removal performance of Lake County’s treatment system during its first two years of operation. 

Research questions

• What is the efficacy of a stormwater treatment train consisting of a two-stage ditch sedimentation system followed by a geomedia-augmented biofilter for treatment of urban runoff at the catchment scale?
• What types of novel contaminants (e.g., trace organic contaminants, bacteria, pathogens) are present in urban runoff, and how effectively are they removed by the treatment system?
• How does system performance evolve over the first two years of operation, and what are the associated changes in microbial communities established on the filter media?
• What improvements to receiving water quality are achieved during the first two operational years of the system?