Fact Sheet
Health-Based Targets for Microbial Safety of
Drinking Water: Introduction
Pathogenic micro-organisms (pathogens) in source waters
present a challenge for those tasked with providing safe
drinking water. Water quality managers and treatment
plant operators work hard to ensure that water supplied
to consumers is pathogen-free. To achieve this, they are
guided by the Australian Drinking Water Guidelines (ADWG),
although advice from the ADWG on this issue could be
improved. One proposal currently being considered is the
introduction of health based targets (HBTs) for microbial
safety of drinking water. This Fact Sheet provides a brief
explanation of HBTs and some rationale for their introduction
into the ADWG.
to ensure that drinking water is free of micro-organisms,
through measuring the absence of E. coli in a 100mL water
sample and no other pathogen detections in tested samples.
This approach is problematic as the absence or presence of
E. coli does not give an accurate measure of the risk of illness
because it is primarily an indicator for bacteria and much of
the current burden of disease in Victoria results from other
microorganisms such as viruses and protozoa. It could be
called the ‘no detection’ regime.
WHAT IS A PATHOGENIC MICRO-ORGANISM?
The process of deriving HBTs for the microbial status of
drinking water is complex. Some of the challenges include:
Pathogenic behaviour means that the organism (microbes
such as bacteria, protozoa and viruses) is capable of
causing disease to a host, such as a human. Legionella,
Cryptosporidium and E. coli are common examples of
pathogens, often mentioned in the media.
WHAT ARE HEALTH-BASED TARGETS?
It is not possible to undertake an activity with zero risk. HBTs
is a term used to define the level of risk that is acceptable
to the community to protect public health. A HBT is a
benchmark that defines the safety of drinking water. This
benchmark can be expressed either as a water quality target
or as a treatment goal.
WHAT IS THE CURRENT ADWG APPROACH TO
CONTROLLING MICROBIAL ACTIVITY?
The ADWG provides a framework for the good management
of drinking water supplies. While the ADWG includes HBTs
for chemical and radiological parameters in the form of water
quality standards, using a prescriptive end point approach, a
different approach is currently adopted for microbial safety.
For microbial activity the framework focuses on E. coli.
The ADWG states that pathogenic micro-organisms should
be absent from drinking water. In terms of water safety,
the current ADWG does not prescribe a numerical value
for microbial safety. The only established definitions are
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WHAT ARE THE CHALLENGES OF DERIVING HBTS FOR
MICROBIAL QUALITY OF DRINKING WATER?
•
Unlike chemicals (which tend to be homogeneously
distributed through water), micro-organisms are
distributed heterogeneously, so that small volume
testing is unlikely to give an accurate estimate of microorganism concentration in raw or treated water;
•
A single exposure to a pathogenic micro-organism can
result in an acute illness, whilst, except in extreme cases,
illness to exposure to chemicals in water requires longterm exposure or exposure to comparatively high doses;
•
At the concentrations that can result in acute illness,
micro-organisms may not impart a taste or odour to the
drinking water;
•
Individuals respond differently to the same exposure
to various micro-organisms, such that it is extremely
difficult to derive universal guideline values for
acceptable concentrations of pathogenic microorganisms in treated drinking water;
•
There are currently no reliable, real-time or on-line
tests for the presence of pathogenic micro-organisms,
and even if there were, the lack of homogeneity in the
concentration of micro-organisms in drinking water may
undermine the integrity of monitoring.
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HBT METRICS AND TREATMENT PLANT OPERATIONS
Two HBT metrics for microbial pathogens in drinking water
that are most often discussed are the ‘infection rate’ metric
and the ‘DALY’ (disability adjusted life year).
The USEPA has adopted the ‘infection rate’ metric at
a level of 1:10,000 people per year. This metric does
not differentiate based on the severity of the infection,
which could be relatively minor (for instance, showing
no symptoms) or severe. In contrast, the World Health
Organisation (WHO) has adopted the DALY, which is a
measure of the health impact of a disease on a population.
The major difference between the ‘infection rate’ metric and
the DALY is that the DALY does consider the severity of the
illness. A separate fact sheet on DALYs has been published by
WaterRA and is available on the website (waterra.com.au).
The important thing to note is that the acceptable rate of
infection or illness is a statistical construct. The ultimate aim
is that by adopting a HBT for the microbial quality of drinking
water, no-one will be exposed to pathogenic micro-organisms
through consumption of drinking water. The acceptable rate
of infection recognises that consumption of drinking water is
not completely risk free, but can be made an extremely low
risk activity by proper management.
Operationally, it is important to understand that HBTs are
a mechanism to work out what treatment is necessary for
a given source water. It is not practical, nor required, to
measure ‘infection rates’, ‘DALYS’ or for that matter any other
HBT metric during normal operations. A HBT for microbial
quality of drinking water will define the level of water
treatment that is required to achieve an acceptable level of
health risk, in turn providing a level of certainty that safe
drinking water has been produced.
WHAT ARE THE IMPLICATIONS OF INTRODUCING HBTS
FOR MICROBIAL CONTROL OF DRINKING WATER?
The introduction of HBTs for microbial control of drinking
water would define the level of water treatment required to
achieve an acceptable level of health risk for a given source
water quality. You can see that this is a slight shift from the
current ADWG ‘no detection’ approach.
It is anticipated that in most cases the level of treatment
that currently takes place will be found to be adequate,
and therefore the introduction of HBTs will not result in
significant capital expenditure for many utilities. In some
cases, more likely to be in the regional areas, the current
level of treatment will be inadequate.
Using a catchment to tap risk management approach will
require risk management planning to characterise the
level of bacterial, viral and protozoan hazards in source
waters. It will also require treatment barriers to reduce
or remove microbial hazards to an acceptable level. For
most catchments this would typically mean maximising
performance of clarifiers, filters and disinfection.
The important outcome with this approach is that water
businesses will need to understand source water risk and
demonstrate that they have reliable barriers to effectively
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manage each group of microbial hazards (bacteria, viruses,
protozoa). Reliable on-line monitoring, to ensure corrective
actions can be implemented prior to supply, is also an
important element of this approach.
A key implication of the introduction of HBTs is that they
would provide a robust and defensible metric which would
allow for the determination of the level of water treatment
required for a defined health benefit. This is a significant
improvement over the current ADWG approach, which
provides no guidance on how to determine adequate water
treatment for microbial control.
A significant benefit to the adoption of HBTs is the increased
emphasis on the effectiveness of the treatment processes
and the ability that this provides to implement corrective
actions in real-time, if a problem is detected, to prevent
affected water from being delivered to customers.
SUMMARY
The Australian water community has expressed some degree
of apprehension regarding the inclusion of HBTs into the
ADWG, and specifically regarding what it actually means to
implement HBTs. Most treatment plant operators would be
pleased to know that they already manage their treatment
plants for a range of HBTs for non-microbial parameters.
The main driver and implication for introducing HBTs will be
to have a defensible basis for defining the treatment level
required for a given source water (normally expressed as a
‘log reduction’) and then specify how the plant should be
operated to achieve this treatment target.
This series of fact sheets was initiated by the Cooperative Research Centre
for Water Quality and Treatment (CRC WQT) in 2003. The fact sheets are
largely based on research carried out in the CRC WQT and its successors,
WQRA (2008-2013) and Water Research Australia (WaterRA, 2013 - ).
Since 2008 the research has been funded entirely by the Members of
WaterRA, who comprise Australian water utilities, universities, engineering
and consulting companies and government agencies. Our Members provide
annual contributions in the form of money, time, expertise and other in-kind
contributions. The research conducted investigates issues of concern to
the water industry and will benefit all Australians through improved water
treatment and quality.
Visit our website: waterra.com.au
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