Microbial Risk Modeling for Main Breaks (PDF)

A microbial risk model was developed for main breaks to evaluate the impact of flushing and disinfection as needed to mitigate the potential contamination risk. Of protozoan, virus, and bacterial risks examined, estimated risks from viruses were the highest, suggesting a 7-log reduction to achieve a 1/10,000 risk target. To accomplish this level of risk reduction, it was necessary to use a flushing velocity of at least 3 ft/s and disinfection with a free chlorine CT (disinfectant concentration multiplied by time) of at least 100 mg x min/L. The risk model also sheds light on alternative risk-management options, the impact of specific field conditions, and potential for future model improvements.