Bioretention, a nature-based treatment practice, has significant potential for reducing the threat of microbial pollutants from urban stormwater runoff to receiving water bodies. The overall goal of this research was to evaluate the removal efficiency for bacteria from urban stormwater runoff in bioretention systems and the potential of an engineered media (iron oxide-coated sand (IOCS)) for enhancing bacterial removal. This investigation was accomplished through laboratory column studies coupled with field tests.

Column studies on the transport and destruction of Escherichia coli O157:H7 strain B6914 (a surrogate of pathogenic E. coli) in conventional bioretention media (CBM) and IOCS demonstrated that the bacteria were well removed in CBM (a mean 70% efficiency), but IOCS significantly enhanced the capture of strain B6914 (a mean 99.4% efficiency) due to the greater positive charge and surface roughness. However, the decay of trapped strain B6914 cells was much faster in CBM compared to the IOCS. More than 99.98% of B6914 cells attached to CBM died off within one week, while approximately 48% of trapped cells still survived in the IOCS after one week. Predation by indigenous protozoa in the CBM appears to play a dominant role in the faster decline of the number of trapped B6914 cells in CBM. Additionally, long-term (18 months) column experiments indicated that during the periodic application of simulated rainfall, the removal efficiency for strain B6914 improved over time, achieving 97% or higher efficiency after six months.

Consistent with the laboratory studies, two years of field studies showed that bioretention systems reduced the concentration of indicator bacteria in the outflow during most storm events and increased the probability of meeting specific water quality criteria. The concentration of indicator bacteria in the input flow generally increased with higher daily temperature. No clear trend for the bacterial removal efficiency with respect to temperature was found in laboratory and field studies. However, the bacterial decay coefficients in CBM increased exponentially with elevated temperature.

Based on these results, it is concluded that CBM not only achieves good removal for bacteria, but also has the potential to render the process sustainable.