Low pressure transients can be mechanisms for intrusion of
microbial contaminants into the distribution system. Prior research
has demonstrated episodes of negative pressure (-16 psi) in drinking
water systems following planned or unplanned pump shutdowns.
This poster paper models the risk of infection from
these negative pressure events and evaluates mitigation options. A
quantitative microbial risk assessment (QMRA) model developed
estimates of norovirus and rotavirus concentrations in untreated
sewage based on a Monte Carlo analysis of censored published values.
The volume of sewage that could intrude into the system 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. Sensitivity analysis showed that the coincidence of
consumption (the probability of consuming the slug of contaminated
water) has the strongest influence on risk and was related to the
duration of the negative pressure event. Estimates of the disinfectant
demand due to the intruded diluted sewage, and virus inactivation,
showed that maintenance of free chlorine residuals 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.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 1 MB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 1 |
| Published : | 11/01/2009 |
