Risk Assessment for Safe Drinking Water Supplies - Global Drinking Water Management and Conservation - page 131

Although all viruses should be considered, rotavirus is selected as the reference
virus for risk assessment because of the prevalence of infection in children and the
possibility of severe outcomes due to this virus. It is also assumed that if this
reference virus were controlled, this would ensure control of all other similar
viruses. If a source water has a mean concentration of approximately 1 rotavirus/
100 L, a water treatment plant would need consistently to achieve at least a 4-log
reduction in virus concentration in order to meet the acceptable reference risk level
of 10 − 6 DALY/person per year. Thus, a minimum 4-log reduction and/or inacti-
vation of viruses has become established as a health-based treatment goal. But
many source waters may require more than a 4-log reduction as the treatment goal
to meet the acceptable level of risk as explained next.
It should be noted that a source water concentration of 1 rotavirus/100 L of water
generally represents groundwater sources and relatively pristine surface water
sources. In North America, many surface water sources will have virus concen-
trations in the range of 1

100 viruses per 100 l of water or even more. In that case,
obviously the treatment plant would need to strive for even higher log reductions in
order to achieve the reference risk level of 10 − 6 DALY/person per year. Thus, if the
source water concentration was 10 rotavirus per liter (1,000 per 100 L), then the
treatment plant would need to have 7-log reduction in order to achieve the reference
level of risk.
To summarize, (a) 4-log reduction of viruses is fine for good quality source
water, such as groundwater or pristine water sources, but for most surface waters,
the treatment plants would need to receive additional log reduction credits, possibly
through a multibarrier approach, and (b) susceptibility to infection of subpopula-
tions is still elusive: it looks as though science has not yet rendered an unambiguous
de

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population. But new can-
didate contaminants and their MCLs will be added only if the risk-adjusted bene

nition of susceptibility and what is a “ susceptible ”

ts

exceed the costs to the community. The cost bene t analysis will re ect the

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acceptable risk as DALY/per person per year, speci ed as the community standard

of acceptable level of risk. In other words, regulatory practice implies some mea-
sures of risk aversion. But since 1996, the US EPA goes further; it

risk
assessment when a new contaminant is added with its MCL to the list of regulated
substances. The

“ builds in ”

of a contaminant is a deciding factor in whether the
substance is controlled or not.

“ riskiness ”

6.4 Case Studies of Risk Assessment

We present four case studies, two from developing countries, Bangladesh and
Uganda; two from developed countries, Iceland and Australia. These case studies
show the wide variety of application of risk assessment programs.