211680 Estimating the risk of virus intrusion into drinking water systems

Wednesday, November 11, 2009: 10:30 AM

Mark W. LeChevallier, PhD , Innovation & Environmental Stewardship, American Water, Voorhees, NJ
     
Approach: A quantitative microbial risk assessment model developed estimates of norovirus and rotavirus concentrations in untreated sewage based Monte Carlo analyses of censored published values. The volume of intruded sewage was based on a logarithmic distribution of the duration of negative pressure transients and an estimate of the distribution leakage. The dilution of the intrusion volume was calculated based on the distribution of flow in the system and resulted in an estimation of the final virus concentration. Dose response data for norovirus and rotavirus along with known estimates of water consumption were used to estimate the risk of infection based on the distribution of customers (demand) within the system.

Conclusions: Sensitivity analysis showed that the coincidence of consumption had the strongest influence on risk and was related to the duration of the negative pressure event. Estimates of free chlorine inactivation nearly eliminated the risk, but viruses persisted for more than 24 hours in chloraminated systems. A single event in an undisinfected system could pose significant risk (>1/10,000 annual risk of infection), but data on electrical power interruptions showed that pump shutdowns could happen as frequently as once per month. Overall the QMRA model was useful for organizing known data, estimating data gaps, and testing hypotheses for future research.

Learning Objectives:
1. Demonstrate that low pressure transients can result in intrusion of microbes into the distribution system. 2. Illustrate how modeling of risk can be used to evaluate mitigation options. 3. Provide information and guidance that will help public health officials better understand drinking water risks and investigate waterborne outbreaks.

Presenting author's disclosure statement:

Qualified on the content I am responsible for because: I am the director of research for the largest water utility in the US and have >30 years of experience in drinking water quality research. I hold numerous leadership and advisory positions.
Any relevant financial relationships? No

I agree to comply with the American Public Health Association Conflict of Interest and Commercial Support Guidelines, and to disclose to the participants any off-label or experimental uses of a commercial product or service discussed in my presentation.

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