Submitted to: Journal of Environmental Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/21/2011
Publication Date: 8/1/2011
Citation: Zhang, L., Seagren, E.A., Davies, A.P., Karns, J.S. 2011. Long-term Sustainability of Escherichia coli Removal in Conventional Bioretention Media. Journal of Environmental Engineering. 137(8):651-759. Interpretive Summary: The movement of bacteria from animal feces, particularly those that may be pathogenic to humans, into waters used for irrigation, recreation, or that are sources of drinking water is a problem in both urban and agricultural settings. This study showed that the movement of E. coli bacteria in runoff water can be retarded by binding to a simple substance made from soil, sand, and mulch (conventional bioretention medium, CBM) and that the bacteria bound to this matrix are killed by other organisms. In urban environments constructed pits containing CBM can be used to slow the flow of water running off from parking lots and other impervious surfaces and remove bacterial contaminants. In agricultural settings such constructed bio-retention could be used to reduce the numbers of bacteria from manure fertilizers reaching water sources.
Technical Abstract: Bioretention has significant potential for reducing bacterial levels in urban stormwater discharge. The long-term performance of bacteria removal was evaluated using column studies over an 18 month period, during which synthetic urban stormwater runoff was loaded to conventional bioretention media (CBM) columns once every two weeks. CBM initially achieved a mean of 72% removal efficiency for Escherichia coli O157:H7 strain B6914. The removal efficiency improved over time, achieving 97% or higher efficiency after six months. The trapped B6914 cells died off rapidly between runoff application events. Mechanistic studies indicated that decreased porosity and increased hydrodynamic dispersion observed in mature CBM are favorable for improving the physical straining of cells and for bacterial adhesion. The temporal change in surface charge on CBM may not be a key factor in the improved bacterial removal. Indigenous protozoa in the CBM grew logistically, and may play an important role in the enhancement of bacterial capture and the rapid decline in the numbers of trapped bacteria via predation. Overall, the long-term bacterial removal process in CBM can be efficient and sustainable.