Submitted to: Water Environment Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/4/2010
Publication Date: 8/16/2010
Citation: Zhang, L., Seagren, E.A., Davies, A.P., Karns, J.S. 2010. The capture and destruction of E. coli from simulated urban runoff using conventional bioretention media and iron oxide-coated sand. Water Environment Research. 82:701-713. Interpretive Summary: Water running off dairy farms or other animal production farms, or from fields fertilized with manure from these operations, is always contaminated with bacteria, some forms of which may be pathogenic. This study examined the potential for engineered bio-retention systems to remove E. coli 0157:H7 from runoff water. These bio-retention systems were originally designed to trap chemicals in run off from urban environments. The system usually consists of a pit filled with compost or some other high-organic matter substrate and planted with trees or bushes. Water runs into the pit and the organic matter removes chemicals before the water is allowed to exit the system. This work was a laboratory study done to compare the ability of a traditional compost-based substrate to remove a surrogate strain of pathogenic E coli 0157:H7 to that of iron-oxide coated sand, a substrate that is expected to have higher bacterial removal capacity. Although iron-oxide coated sand was found to have a greater pathogen removal capacity over the short-term, the bacteria survived much longer on that substrate, indicating that it might represent a reservoir for future pathogen release. In contrast, the compost-based substrate did not remove bacteria as quickly but the bacteria died-off more rapidly. The removal of the bacteria in the compost-boned substrate was shown to be due to protozoan predators which are apparently inhibited on iron-oxide coated sand. This study will aid engineers and water management experts in designing systems for prevention of contamination of ground and surface waters with bacteria from runoff.
Technical Abstract: Given the magnitude of the threat to the quality of receiving water bodies posed by microbial pollutants in urban stormwater runoff, and the untested potential for their removal in bioretention systems, studies were performed to evaluate the removal efficiency of bacteria from simulated urban stormwater runoff using conventional bioretention media (CBM) and iron-oxide coated sand (IOCS). Column studies of the transport and destruction of Escherichia coli O157:H7 strain B6914 (a surrogate of pathogenic E. coli) in CBM and IOCS demonstrated that the bacteria were well removed in CBM (82% efficiency), but IOCS significantly enhanced the capture (99% efficiency) due to the greater positive surface 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. A major factor contributing to these different results was probably the lack of microbial predation and competition in the IOCS, as studied. Indeed, predation and competition from the native microorganisms in CBM were verified to play a dominant role in the rapid destruction of trapped strain B6914 through comparing the bacterial survival in parallel non-sterile media, gamma-irradiated sterile media, and gamma- irradiated sterile media coated with cultured native microorganisms. In particular, protozoan grazing appeared to play an important role, with the die-off of trapped B6914 increasing with increasing concentrations of protozoa in the media.