Microorganisms Die-Off Rates in Urban Stormwater Runoff

Stormwater best management practices (BMPs) are often considered effective tools to mitigate the effects of stormwater pollutants before they are discharged to receiving waters. However, BMP performance for microorganism removal is not well documented. Microorganism die-off in stormwater runoff is a complex process involving various types of environmental factors such as temperature, sunlight, dissolved oxygen, salinity, etc. This bench-scale study was designed to determine the effects of individual factors that influence the microorganism die-off in BMPs. In this study, organism-specific die-off constants in stormwater were developed assuming temperature and sunlight as the major influential parameters. The temperature study indicated that the organisms persisted at higher levels at lower temperatures. The die-off rate constants increased with increasing temperatures. These observed temperature effects on die-off were well documented in other literature as well. Out of all the organisms studied, total coliforms had a much slower die-off rate. Fecal coliforms, fecal streptococci, E. coli, and enterococci have similar die-off rate constants. The temperature coefficient values obtained in this study are similar to the ones reported in the literature. Except for total and fecal coliforms, the effect of sunlight on the die-off constant is significant. The initial concentrations of organisms in the stormwater have an effect on the die-off rates.