Retention and survival of E. coli in stormwater biofilters: Role of vegetation, rhizosphere microorganisms and antimicrobial filter media

- Retained E. coli concentration decreased with increasing filter media depth.
- Retained E. coli concentrations in rhizosphere and bulk soils were similar.
- E. coli die-off rate increased in the presence of rhizosphere microbes.
- Root exudates from biofilter plants has a net negative effect on E. coli survival.

The public health risks associated with pathogens in urban stormwater have been well established, making it necessary to ensure adequate treatment of the stormwater before it is discharged into recreational water bodies or is harvested for reuse. Biofilters, also known as stormwater bioretention systems or raingardens, have shown promising, yet variable, results in reducing indicator bacteria in stormwater. Different biofilter design elements, such as filter media composition and vegetation type, have been found to cause this variable removal performance. Although plants play a key role in the treatment of pollutants, relatively little work has been conducted to understand the importance of interactions between vegetation and the biofilter microbial community on fecal microbial removal. A laboratory-scale biofilter experiment was conducted using Escherichia coli as the test fecal microorganism. Biofilter columns with differing soil media and vegetation types were dosed over a two month span, during which inflow and outflow samples were collected to evaluate system performance. The columns were then decommissioned to collect rhizosphere and bulk soil samples. Root exudates were extracted and used in an E. coli survival study to evaluate their contribution to system performance. The study demonstrated that the antagonistic effects of root exudates/rhizosphere microbes and Cu2+ exchanged zeolite antimicrobial filter media adversely impact the survival of E. coli retained within stormwater biofilters. Furthermore, leaf and flower/seed extracts of L. continentale showed some potential antibacterial activity against E. coli. This work supports the concept that natural processes in biological systems can deliver effective results in the removal of fecal microorganisms, and should be promoted to the extent possible in stormwater green infrastructure.