Guidelines for Canadian Drinking Water Quality Guideline Technical Document

Guidelines for Canadian Drinking Water Quality Guideline Technical Document
Guidelines for
Canadian Drinking
Water Quality
Guideline Technical Document
Escherichia coli
Health Canada is the federal department responsible for helping the people of Canada
maintain and improve their health. We assess the safety of drugs and many consumer
products, help improve the safety of food, and provide information to Canadians to help
them make healthy decisions. We provide health services to First Nations people and to
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needs of Canadians.
Published by authority of the Minister of Health.
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document –
Escherichia coli
is available on Internet at the following address:
www.healthcanada.gc.ca
Également disponible en français sous le titre :
Recommandations pour la qualité de l’eau potable au Canada : Document technique –
Escherichia coli
This publication can be made available on request in a variety of alternative formats.
© Her Majesty the Queen in Right of Canada,
represented by the Minister of Health, 2013
This publication may be reproduced without permission provided the source is fully
acknowledged.
Pub. Number: 130021
Cat.: H144-7/2013E-PDF
ISBN: 978-1-100-21737-6
Guidelines for
Canadian Drinking
Water Quality
Guideline Technical Document
Escherichia coli
Prepared by the
Federal-Provincial-Territorial Committee on
Drinking Water
of the
Federal-Provincial-Territorial Committee on
Health and the Environment
Health Canada
Ottawa, Ontario
March, 2012
This document may be cited as follows:
Health Canada (2012). Guidelines for Canadian Drinking Water Quality: Guideline Technical
Document — Escherichia coli. Water, Air and Climate Change Bureau, Healthy Environments
and Consumer Safety Branch, Health Canada, Ottawa, Ontario. (Catalogue No H144-7/2013EPDF).
The document was prepared by the Federal-Provincial-Territorial Committee on Drinking Water
of the Federal-Provincial-Territorial Committee on Health and the Environment.
Any questions or comments on this document may be directed to:
Water, Air and Climate Change Bureau
Healthy Environments and Consumer Safety Branch
Health Canada
269 Laurier Avenue West, Address Locator 4903D
Ottawa, Ontario
Canada K1A 0K9
Tel.: 613-948-2566
Fax: 613-952-2574
E-mail: [email protected]
Other Guideline Technical Documents for the Guidelines for Canadian Drinking Water Quality
can be found on the following web page: www.healthcanada.gc.ca/waterquality
ii
Table of Contents
Part I. Overview and Application................................................................................................ 1
1.0
Guideline ............................................................................................................................. 1
2.0
Executive summary............................................................................................................. 1
2.1
Significance of E. coli in drinking water systems and their sources ...................... 1
2.2
Sampling and testing for E. coli .............................................................................. 2
2.3
Treatment technology ............................................................................................. 2
3.0
Application of the guideline................................................................................................ 2
3.1
Municipal-scale drinking water supply systems ..................................................... 3
3.1.1 Monitoring E. coli in water leaving the treatment plant ............................. 3
3.1.2 Monitoring E. coli within water distribution and storage systems ............. 3
3.1.3 Notification................................................................................................. 3
3.1.4 Corrective actions ....................................................................................... 3
3.2
Residential-scale and private drinking water systems ............................................ 4
3.2.1 Testing requirements ................................................................................... 4
3.2.2 Notification ................................................................................................. 5
3.2.3 Corrective actions for disinfected supplies ................................................. 5
3.2.4 Corrective actions for non-disinfected wells .............................................. 6
Part II. Science and Technical Considerations .......................................................................... 7
4.0
Significance of E. coli in drinking water ............................................................................ 7
4.1
Description .............................................................................................................. 7
4.2
Sources .................................................................................................................... 7
4.3
Survival ................................................................................................................... 7
4.4
Role of E. coli as an indicator of microbiological safety........................................ 8
4.4.1 Role in source water monitoring ................................................................ 9
4.4.2 Role in treatment and distribution system monitoring ............................... 9
4.4.3 Considerations for residential-scale systems ............................................ 10
5.0
Analytical methods ........................................................................................................... 10
5.1
Presence–absence procedure ................................................................................. 11
5.2
Membrane filter procedure ................................................................................... 12
5.3
Multiple tube fermentation procedure .................................................................. 13
6.0
Sampling for E. coli .......................................................................................................... 13
6.1
Sample collection .................................................................................................. 13
6.2
Sampling frequency considerations ...................................................................... 14
6.3
Location of sampling points.................................................................................. 16
7.0
Treatment technology ....................................................................................................... 16
7.1
Municipal scale ..................................................................................................... 16
7.1.1 Level of treatment necessary .................................................................... 17
7.1.2 Physical removal ....................................................................................... 17
7.1.3 Disinfection ............................................................................................... 17
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
iii
7.2
7.1.3.1 Chemical disinfection ................................................................. 17
7.1.3.2 UV light disinfection .................................................................. 19
Residential scale.................................................................................................... 19
8.0
Risk assessment ................................................................................................................ 20
8.1
International considerations .................................................................................. 21
9.0
Rationale ........................................................................................................................... 21
10.0
References ......................................................................................................................... 22
Appendix A: Decision Tree for Routine Microbiological Testing of Municipal Scale Systems . 30
Appendix B: Decision Tree for Routine Microbiological Testing of Residential Scale Systems 31
Appendix C: List of Acronyms ..................................................................................................... 32
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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March 2012
Escherichia coli
Part I. Overview and Application
1.0
Guideline
The maximum acceptable concentration (MAC) of Escherichia coli is none detectable per
100 mL.
2.0
Executive summary
Escherichia coli is a member of the coliform group of bacteria that is naturally found in
the intestines of humans and warm-blooded animals. As it is not usually found naturally in other
environments such as on plants or in soils or water, the presence of E. coli in a water sample is a
good indicator of recent faecal contamination. The ability to detect faecal contamination in
drinking water is a necessity, as pathogenic microorganisms from human and animal faeces in
drinking water pose the greatest danger to public health.
Health Canada recently completed its review on the usefulness of E. coli as an indicator
of faecal contamination in drinking water systems. This guideline technical document reviews
and assesses available literature on the uses of E. coli as an indicator of pathogen presence in
source waters, the potential for E. coli to be present in the absence of recent faecal
contamination, and improvements in methods for the detection of E. coli. From this review, the
guideline for E. coli in drinking water is established as a maximum acceptable concentration of
none detectable in 100 mL of water.
2.1
Significance of E. coli in drinking water systems and their sources
E. coli monitoring should be used, in conjunction with other indicators, as part of a multibarrier approach to producing drinking water of an acceptable quality. The number, frequency,
and location of samples for E. coli testing will vary according to the type and size of the system
and jurisdictional requirements.
Surface water and groundwater under the direct influence of surface water (GUDI) are
commonly impacted by faecal contamination from either human or animal sources and, as a
result, usually contain E. coli. If E. coli monitoring results are available for these sources, they
can be used as part of the source water assessment to identify changes in its microbiological
quality. The presence of E. coli in any groundwater sources indicates that the groundwater is
contaminated by faecal material and is microbiologically unsafe for drinking without further
treatment.
Monitoring for E. coli in treated water at the treatment plant and in the distribution
system is carried out to provide information on the adequacy of drinking water treatment and on
the microbial condition of the distribution system. The presence of E. coli in water leaving a
treatment plant signifies that treatment has been inadequate, while the presence of E. coli in the
distribution system, when water tested immediately post-treatment is free of E. coli, suggests that
post-treatment contamination with faecal material has occurred. The presence of E. coli at any
point in the treated water indicates there is a potential health risk from consuming the water.
Additional actions to be taken in these cases include notifying the responsible authorities, issuing
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Escherichia coli (March 2012)
a boil water advisory, investigating the cause of the contamination, and implementing corrective
actions.
Although the presence of E. coli is a good indicator of recent faecal contamination, such
contamination is often intermittent and may not be revealed by the examination of a single
sample. Therefore, if a sanitary inspection shows that an untreated supply is subject to faecal
contamination, or that treated water is subject to faecal contamination during storage or
distribution or is inadequately treated, the water should be considered unsafe, irrespective of the
results of E. coli analysis.
2.2
Sampling and testing for E. coli
As a minimum, water leaving a municipal-scale treatment plant should be sampled and
tested at least weekly for E. coli as part of the verification process in a source-to-tap multi-barrier
approach. In many systems, the water leaving the treatment plant will be tested well in excess of
the minimum requirements. In a distribution system, the number of samples for this
bacteriological testing should be increased in accordance with the size of the population served,
and the samples should be taken at regular intervals throughout the month.
Sampling frequencies in residential-scale and small private systems may vary from
jurisdiction to jurisdiction but should include times when the risk of contamination is greatest,
for example, after spring thaw, heavy rains, or dry periods. New or rehabilitated wells should
also be sampled initially to confirm acceptable bacteriological quality.
Proper procedures for collecting samples must be observed to ensure that the samples are
representative of the water being examined. A minimum volume of 100 mL of water should be
collected for testing, and testing should be started as soon as possible after collection.
2.3
Treatment technology
Generally, minimum treatment of supplies derived from surface water or GUDI sources
should include filtration (or technologies providing an equivalent log reduction credit) and
disinfection. Groundwaters less vulnerable to faecal contamination should receive adequate
treatment for the removal/ inactivation of enteric viruses, unless exempted by the responsible
authority based on site-specific considerations, such as historical and on-going monitoring data.
In systems with a distribution system, a disinfectant residual should be maintained at all times.
3.0
Application of the guideline
Note: Specific guidance related to the implementation of drinking water guidelines
should be obtained from the appropriate drinking water authority in the affected jurisdiction.
E. coli is currently the best available indicator of recent faecal contamination in drinking
water systems. Consequently, detection of E. coli in any drinking water system is unacceptable.
For decision-making, the focus is the positive detection of E. coli, regardless of quantity.
However, although quantitative results are not precise, they can be used to provide an indication
of the magnitude of a problem and thus inform the public health response. Faecal contamination
is often intermittent and may not be revealed by the examination of a single sample. Therefore, if
a sanitary inspection shows that an untreated supply is subject to faecal contamination, or that
treated water is subject to faecal contamination during storage or distribution or is inadequately
treated, the water should be considered unsafe, irrespective of the results of E. coli examination.
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Escherichia coli (March 2012)
3.1
Municipal-scale drinking water supply systems
3.1.1 Monitoring E. coli in water leaving the treatment plant
E. coli should be monitored at least weekly in water leaving a treatment plant. If E. coli is
detected, this indicates a serious breach in treatment and is therefore unacceptable. E. coli tests
should be used in conjunction with other indicators, such as residual disinfectant and turbidity
monitoring as part of a multi-barrier approach to producing drinking water of acceptable quality.
While the required frequency for all testing at the treatment plant is prescribed by the responsible
authority, best practice commonly involves a testing frequency beyond these minimum
recommendations based upon the size of system, the number of consumers served, the history of
the system, and other site-specific considerations.
3.1.2 Monitoring E. coli within water distribution and storage systems
In municipal scale distribution and storage systems, the number of samples collected for
E. coli testing should reflect the size of the population being served, with a minimum of four
samples per month. The sampling points and testing frequencies for E. coli in treated water
within distribution and storage systems will be prescribed by the responsible authority.
3.1.3 Notification
If E.coli is detected in a municipal-scale drinking water system, the system
owner/operator and the laboratory processing the samples should immediately notify the
responsible authorities. The system owner/operator should resample and test the positive site(s)
and adjacent sites. If resampling and testing confirm the presence of E. coli in drinking water, the
system owner/operator should immediately: (1) issue a boil water advisory1 in consultation with
the responsible authorities, (2) carry out the corrective actions described below, and (3)
cooperate with the local responsible authority in any surveillance for possible waterborne disease
outbreaks (see Appendix A for decision tree). In addition, where E. coli contamination is
detected in the first sampling—for example, positive sample results from a single site, or from
more than one location in the distribution system—the owner or the responsible authority may
decide to notify consumers immediately to boil their drinking water or use a safe alternative
source and initiate corrective actions without waiting for confirmation.
3.1.4 Corrective actions
If the presence of E. coli in drinking water is confirmed, the owner of the waterworks
system should carry out appropriate corrective actions, which could include the following
measures:
Verify the integrity and the optimal operation of the treatment process.
Verify the integrity of the distribution system.
Verify that the required disinfectant residual is present throughout the distribution
system.
1
For the purpose of this document, the use of the term “boil water advisory” is taken to mean advice given to the
public by the responsible authority to boil their water, regardless of whether this advice is precautionary or in
response to an outbreak. Depending on the jurisdiction, the use of this term may vary. As well, the term “boil water
order” may be used in place of, or in conjunction with, a “boil water advisory.”
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Escherichia coli (March 2012)
Increase disinfectant dosage, flush water mains, clean treated-water storage tanks
(municipal reservoirs and domestic cisterns), and check for the presence of crossconnections and pressure losses. Water should be dechlorinated before being discharged
into fish-bearing waters. The responsible authority should be consulted regarding the
methods available, as well as the correct procedure, for carrying out dechlorination.
Sample and test the positive site(s) and locations adjacent to the positive site(s). Tests
performed should include those for E. coli, total coliforms, disinfectant residual, and
turbidity. At a minimum, one sample upstream and one downstream from the original
sample site(s) plus the finished water from the treatment plant as it enters the distribution
system should be tested. Other samples should be collected and tested after a sampling
plan appropriate for the distribution system has been implemented.
Conduct an investigation to identify the problem and prevent its recurrence; this would
include measuring raw water quality (e.g., bacteriology, turbidity, colour, assimilable
organic carbon [AOC], conductivity) and variability.
Continue selected sampling and testing (e.g., bacteriology, disinfectant residual,
turbidity) of all identified sites during the investigative phase to confirm the extent of the
problem and to verify the success of the corrective actions.
If a boil water advisory is issued, it should be rescinded only after a minimum of two
consecutive sets of samples, collected 24 hours apart, show negative results demonstrating full
system-wide integrity (including acceptable microbiological quality, turbidity, and/or
disinfectant residuals). Additional negative results may be required by the local responsible
authority. Further information on boil water advisories can be found in Guidance for Issuing and
Rescinding Boil Water Advisories. Only a history of data together with the verification of the
suitability of the system design and its operation and maintenance can be used to confirm the
long-term integrity of a supply.
Minimum treatment of supplies derived from surface water sources or groundwater under
the direct influence of surface waters should include adequate filtration (or technologies
providing an equivalent log removal/inactivation) and disinfection. For groundwater sources less
vulnerable to faecal contamination, adequate treatment is recommended to ensure removal/
inactivation of enteric viruses, unless these sources are exempted by the responsible authority
based on site-specific considerations such as historical and on-going monitoring data. In systems
with a distribution system, a disinfectant residual should be maintained at all times. The
appropriate type and level of treatment should take into account the potential fluctuations in
water quality, including short-term water quality degradation, and variability in treatment
performance.
3.2
Residential-scale and private drinking water systems
3.2.1 Testing requirements
Testing frequencies for residential-scale2 systems will be determined by the responsible
authority and should include times when the risk of contamination is greatest, for example, in
early spring after the thaw, after an extended dry spell, or following heavy rains. Owners of
2
For the purposes of this document, a residential-scale water supply system is defined as a system with a minimal or
no distribution system that provides water to the public from a facility not connected to a municipal supply.
Examples of such facilities include private drinking water supplies, schools, personal care homes, day care centres,
hospitals, community wells, hotels, and restaurants. The definition of a residential-scale supply may vary between
jurisdictions.
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
private supplies should be encouraged to have their water tested during these same periods. New
or rehabilitated wells should also be tested before use to confirm microbiological safety.
3.2.2 Notification
The presence of E. coli in a residential-scale or private drinking water system
demonstrates that the source or the system has been impacted by recent faecal contamination; as
a result, the water is unsafe to drink. The drinking water should be immediately retested to
confirm the presence of E. coli. The responsible authority should advise the owner to boil the
drinking water or to use a safe alternative source in the interim. If resampling confirms that the
source is contaminated with E. coli, the corrective actions described in the next section should be
taken immediately. As a precautionary measure, some jurisdictions may recommend immediate
corrective actions without waiting for confirmatory results (see Appendix B).
3.2.3 Corrective actions for disinfected supplies
The first step, if it has not already been taken, is to conduct a sanitary survey to evaluate
the physical condition of the drinking water system as applicable, including water intake, well,
well head, pump, treatment system (including chemical feed equipment, if present), plumbing,
and surrounding area.
Any identified faults should be corrected before proceeding. If all the physical conditions
are acceptable, some or all of the following corrective actions may be necessary:
In a chlorinated system, verify that a disinfectant residual is present throughout the
system.
Increase the disinfectant dosage, flush the system thoroughly, and clean treated water
storage tanks and domestic cisterns. Water should be dechlorinated before being
discharged to fish-bearing waters. The responsible authority should be consulted
regarding the methods available and the correct procedure for carrying out
dechlorination.
For systems where the disinfection technology does not leave a disinfectant residual, such
as UV or ozone, it may be necessary to shock chlorinate the well and plumbing system;
further information on shock chlorination is available in the factsheet What’s in Your
Well? A Guide to Well Water Treatment and Maintenance (www.hc-sc.gc.ca/ewhsemt/pubs/water-eau/well-puits-eng.php).
Ensure that the disinfection system is working properly and maintained according to
manufacturer’s instructions.
After the necessary corrective actions have been taken, samples should be collected and
tested for E. coli to confirm that the problem has been corrected. If the problem cannot be
corrected, additional treatment or a new source of drinking water should be considered. In the
interim, any initial precautionary measures should continue; for example, drinking water should
continue to be boiled or an alternative safe source of water should continue to be used.
Minimum treatment of supplies derived from surface water sources or groundwater under
the direct influence of surface waters should include adequate filtration (or technologies
providing an equivalent log removal/inactivation) and disinfection. For groundwater sources less
vulnerable to faecal contamination, adequate treatment is recommended to ensure
removal/inactivation of enteric viruses, unless these sources are exempted by the responsible
authority based on site-specific considerations such as historical and on-going monitoring data.
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Escherichia coli (March 2012)
3.2.4 Corrective actions for non-disinfected wells
The first step, if it has not already been taken, is to conduct a sanitary survey to evaluate
the physical condition of the well, well head, pump, plumbing, and surrounding area.
Any identified faults should be corrected before proceeding. If all the physical conditions
are acceptable, then the following corrective actions should be carried out:
Shock-chlorinate the well and plumbing system. Further information on this topic is
available in the factsheet What’s in Your Well? A Guide to Well Water Treatment and
Maintenance (www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/well-puits-eng.php).
Flush the system thoroughly and retest to confirm the absence of E. coli. Confirmatory
tests should be done no sooner than either 48 hours after tests indicate the absence of a
chlorine residual or 5 days after the well has been treated. Local conditions may
determine acceptable practice. Water should be dechlorinated before being discharged to
fish-bearing waters. The responsible authority should be consulted regarding the methods
available and the correct procedure for carrying out dechlorination.
If the water remains contaminated after shock-chlorination, further investigation into the
source of the contamination should be carried out. If the source cannot be found or corrected,
either an appropriate disinfection device or well reconstruction or replacement should be
considered. Drinking water should be boiled or an alternative safe source of water should
continue to be used in the interim.
A boil water advisory should be rescinded only after a minimum of two consecutive sets
of samples, collected 24 hours apart, show negative results. Further information on boil water
advisories can be found in Guidance for Issuing and Rescinding Boil Water Advisories.
Additional tests should be taken after 3 to 4 months to ensure that the contamination has not
recurred. Only a history of data can be used to confirm the long-term integrity of a supply when
applied jointly with sanitary surveys. Further information on routine monitoring can be found in
section 6.0.
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Part II. Science and Technical Considerations
4.0
Significance of E. coli in drinking water
4.1
Description
Escherichia coli is a member of the coliform group of bacteria, part of the family
Enterobacteriaceae, and described as a facultative anaerobic, Gram-negative, non-spore-forming,
rod-shaped bacterium. The vast majority of waterborne E. coli isolates have been found to be
capable of producing the enzyme β-glucuronidase (Martins et al., 1993; Fricker et al., 2008b,
2010), and it is this characteristic that currently facilitates their detection and identification.
Further information on the coliform group of organisms can be found in the guideline technical
document on total coliforms (Health Canada, 2012a).
4.2
Sources
As a member of the Enterobacteriaceae family, E. coli is naturally found in the intestines
of humans and warm-blooded animals. Unlike other bacteria in this family, E. coli does not
usually occur naturally on plants or in soil and water, although there is evidence that some strains
may be able to survive and grow in soils (Winfield and Groisman, 2003; Byappanahalli et al.,
2006; Ishii et al., 2006). Within human and animal faeces, E. coli is present at a concentration of
approximately 109 cells per gram (Edberg et al., 2000) and comprises about 1% of the total
biomass in the large intestine (Leclerc et al., 2001). Although E. coli are part of the natural faecal
flora, some strains of this bacterium can cause gastrointestinal illness along with other, more
serious health problems. Faecal concentrations of the typical non-pathogenic E. coli, used to
indicate recent faecal contamination, will always be greater than those of the pathogenic strains,
even during outbreaks. Further information on illness-associated E. coli strains can be found in
Guidance on Waterborne Bacterial Pathogens (Health Canada, 2013a).
4.3
Survival
The survival time of E. coli in the environment is dependent on many factors including
temperature, exposure to sunlight, presence and types of other microflora, and the type of water
involved (e.g., groundwater, surface water, or treated distribution water) (Foppen and Schijven,
2006). In general terms, E. coli survives for about 4–12 weeks in water containing a moderate
microflora at a temperature of 15–18°C (Kudryavtseva, 1972; Filip et al., 1987; Edberg et al.,
2000). A concept that is receiving increasing attention within the scientific community is that if
favorable conditions (e.g., nutrients, temperature, reduced environmental stresses) are present in
the environment, E. coli may be capable of prolonged survival and growth, even in the absence
of recent faecal contamination. Several studies have been published demonstrating evidence of
the survival and growth of E. coli populations in nutrient-rich environments such as soils, beach
sand, cyanobacterial bloom material and mats of the algal species Cladophora, in both tropical
and temperate environments (Carrillo et al., 1985; Hardina and Fujioka, 1991; Power et al.,
2005; Byappanahalli et al., 2006; Ishii et al., 2006; Heuvel et al., 2010). Compared to soils or
masses of vegetative material, most source water environments provide conditions that are less
protective and less nutrient-rich. As a result, bacterial regrowth is not expected to be a concern in
a water setting. E. coli is generally the most sensitive of the coliform bacteria to environmental
stressors and does not survive as long in the environment as do protozoans and some viruses
(Edberg et al., 2000). E. coli does have similar survival rates to many faecal bacterial pathogens
(Jimenez et al., 1989; Artz and Killham, 2002; Karim et al., 2004; Cook and Bolster, 2007).
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Escherichia coli (March 2012)
The ability of E. coli to survive and grow in distribution system biofilms has been the
subject of several recent studies. In several research settings that used model and pilot-scale
systems, E. coli were able to survive in the biofilm under various conditions, although in most
cases colonization was transient (Fass et al., 1996; Williams and Braun-Howland, 2003; Lehtola
et al., 2007). In full-scale drinking water distribution systems, when E. coli has been detected in
the biofilm, it generally makes up a small portion of the total coliforms isolated (Lechevallier et
al., 1987; Blanch et al., 2007; Juhna et al., 2007). Although in some instances E. coli can be
detected in biofilms in distribution systems, they do not seem to be a significant component of
the biofilm matrix; consequently, the detection of E. coli in a water distribution system is a good
indication of recent faecal contamination.
4.4
Role of E. coli as an indicator of microbiological safety
Although modern microbiological techniques have made the detection of pathogenic
bacteria, viruses, and protozoa possible, it is currently not practical to attempt to routinely isolate
them from drinking water (Allen et al., 2000; Payment and Pintar, 2006). Reasons for this
include the large number of different pathogens that exist, their uneven distribution in water, and
the time and expense associated with routine monitoring of all pathogens. It is better to use a
variety of indicators that are less difficult, less expensive, and less time consuming to monitor,
which will encourage a higher number of samples to be tested, giving a better overall picture of
the water quality and, therefore, better protection of public health. Of the contaminants that may
be regularly found in surface and groundwater sources, pathogenic microorganisms from human
and animal faeces pose the greatest danger to public health. For this reason, the ability to detect
faecal contamination in drinking water is a necessity for ensuring public safety. As early as the
19th century, E. coli was recognized as a good indicator of faecal contamination, but there were
no easy, simple, and low-cost methods to easily speciate E. coli until the late 1980s (Edberg et
al., 2000). Of the coliform group of organisms, E. coli are considered a more specific indicator of
faecal contamination and can be rapidly and easily enumerated in water. In addition to being
faecal specific, E. coli do not usually multiply in the environment, are excreted in the faeces in
high numbers (approximately 109 cells per gram) making detection possible even when greatly
diluted, and have a life span on the same order of magnitude as those of other enteric bacterial
pathogens. These features make E. coli the best available indicator of faecal contamination. A
subset of the total coliform group, known as the thermotolerant coliforms coliforms that have
the ability to ferment lactose at 44–45 C previously referred to as faecal coliforms has been
used as a surrogate for E. coli in water quality testing. Thermotolerant coliforms were considered
more faecal specific than total coliforms, and, given that E. coli testing was difficult,
thermotolerant coliform detection was used routinely. Advances in E. coli detection methods
have made the need for thermotolerant coliform testing in drinking water quality management
redundant.
Although E. coli is the best available indicator of recent faecal contamination, there are
waterborne illnesses that result from pathogens not transmitted by the faecal–oral route and,
therefore, the detection of faecal indicators does not provide any information on their potential
presence. No indicators are currently known for such pathogens. Further information can be
found in Guidance on Waterborne Bacterial Pathogens (Health Canada, 2013a). The best means
of safeguarding against the presence of waterborne pathogens in drinking water, including nonfaecal pathogens, is the application of the multi-barrier approach that includes adequate
treatment, a well-maintained distribution system, and source protection. This approach can
reduce both faecal and non-faecal pathogens to non-detectable levels or to levels that have not
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
been associated with human illness. E. coli sampling should be used as part of this approach to
verify the microbiological water quality and safety. Bacteriological analysis needs to be used in
conjunction with numerous other indicators and process controls to reliably produce drinking
water of an acceptable quality.
4.4.1 Role in source water monitoring
E. coli is well recognized as an indicator of recent faecal contamination; consequently
numerous studies have attempted to link the presence of this indicator with the presence of
specific faecal pathogens in both surface and groundwater sources. For bacterial faecal
pathogens, such as Salmonella, pathogenic E. coli, and Campylobacter, studies have shown that,
in general, E. coli can be used to indicate the increased potential for all these pathogens to be
present in both surface and groundwaters (Mitchell and Starzyk, 1975; Schaffter and Parriaux,
2002; Jokinen et al., 2010), although this relationship is not always found (Dorner et al., 2007).
On the other hand, enteric protozoan pathogens can have little (Medema et al, 1997; Atherholt et
al., 1998; Payment et al., 2000) or no (Rose at al., 1988, 1991; Chauret et al., 1995; Stevens et
al., 2001; Hörman et al., 2004; Dorner et al., 2007; Sunderland et al., 2007) correlation to faecal
indicators in surface water sources. In the cases where a correlation has been reported, it is with
Giardia and at high indicator levels (Wallis et al., 1998). Studies investigating the presence of
protozoa in groundwater sources are lacking.
A relationship between the presence of E. coli and of enteric viruses in surface water
sources has been reported when the water source is known to be contaminated by human faecal
pollution (Payment and Franco, 1993; Payment et al., 2000; Ashbolt et al., 2001; Hörman et al.,
2004). In a study of groundwater quality in three provinces in Canada, Locas et al. (2007, 2008)
reported that most groundwater sites that did not contain any faecal or bacterial indicators were
also free of any enteric viruses. However, the detection of enteric viruses in the absence of
indicator bacteria has been reported in these and other groundwater studies (Abbaszadegan et al.,
1998, 1999; Borchardt et al., 2003, 2004, Locas et al., 2007, 2008). Although the presence of E.
coli is not necessarily associated with the presence of specific pathogens, studies have shown a
link between E. coli presence in groundwater and the development of gastrointestinal illness
(Craun et al., 1997; Raina et al., 1999).
4.4.2 Role in treatment and distribution system monitoring
Monitoring for E. coli at the treatment plant and in the distribution and storage system
provides information on the adequacy of drinking water treatment and on the microbial condition
of the distribution system. E. coli are more susceptible to many of the disinfectants commonly
used in the drinking water industry than are protozoans and some viruses (Edberg et al., 2000).
Nevertheless, if a multi-barrier, source-to-tap approach is in place and each barrier in the
drinking water system has been controlled to ensure that it is operating adequately based on the
quality of the source water, then E. coli can be used as part of the verification process to show
that the water has been adequately treated and is therefore of an acceptable microbiological
quality. The presence of any E. coli in water leaving a treatment plant or in any treated water
immediately post-treatment signifies inadequate treatment and is unacceptable.
The presence of E. coli in the distribution and storage system, when water tested
immediately post-treatment is free of E. coli, suggests that post-treatment contamination has
occurred. Post-treatment contamination, for example, through cross-connections, back
siphonage, low pressure events, contamination of storage reservoirs, and contamination of mains
from repairs, have been identified as causes of distribution system contamination linked to illness
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Escherichia coli (March 2012)
(Craun, 2001; Hunter, 2005). The presence of any E. coli in the distribution and/or storage
system is unacceptable.
4.4.3 Considerations for residential-scale systems
E. coli is an indicator of recent faecal contamination. Therefore, their presence in
disinfected residential-scale systems provides evidence of inadequate disinfection. The presence
of E. coli in non-disinfected wells indicates that the well is being affected by faecal
contamination. Therefore, the water should be boiled or an alternative safe source of water
should be used, and an investigation to determine the source of the contamination needs to be
carried out. Further information is outlined in section 3.2. If the contamination source cannot be
found or corrected, either an appropriate disinfection device or well reconstruction or
replacement should be considered.
5.0
Analytical methods
Currently, three methods are routinely used to detect Escherichia coli organisms in water:
the presence–absence (P-A) method, which is a qualitative test, as well as two quantitative
methods, membrane filter (MF) and multiple tube fermentation (MTF). A detailed description of
each method is given in Standard Methods for the Examination of Water and Wastewater
(APHA et al., 2005). All three detection methods use cultivation to detect or confirm the
presence of E. coli. Newer molecular methods are available for the detection of pathogens or
indicators. Although much progress has been made in this area, molecular methods still have
numerous challenges and are not used for routine monitoring. The literature and/or the
responsible authority should be consulted for further information regarding these methods.
Cultivation media can be broadly categorized into two types: (1) enzyme-based media
that contain fluorogenic or chromogenic substrates for the specific detection and confirmation of
the bacterium in a single step (Feng and Hartman, 1982; Ley et al., 1988); and (2) presumptive
coliform detection media that require a second step to confirm the presence of E. coli.
Methods that detect and confirm the presence of E. coli in a single step are based on the
presence of the enzyme β-glucuronidase. This isa unique constitutive enzyme that is found in the
vast majority of E. coli isolates, as well as someShigella spp., and Salmonella spp., but is rarely
present in other coliforms (Manafi et al., 1991, Fricker et al., 2008b, 2010). The most publicized
methods use the β-glucuronidase activity of E. coli to hydrolyse 4-methylumbelliferyl-β-Dglucuronide to form 4-methylumbelliferone, which fluoresces under longwave ultraviolet (UV)
light (Feng and Hartman, 1982). A distinct advantage of enzyme-based methods is that no
confirmation step is required. Both presence-absence and quantitative results are possible,
depending on the enzyme-based method being used. Some enzyme-based methods also inhibit
non-coliform bacterial growth, and thus non-coliform bacteria cannot interfere with the recovery
of coliforms. This design is based on the principle that only the target microbe, in this case E.
coli, can utilize vital nutrients from the media (Rompré et al., 2002). For these reasons, the use of
enzyme-based methods is recommended. Various enzyme-based methods have been approved by
the U.S. Environmental Protection Agency (U.S. EPA) as acceptable means for the detection of
E. coli in drinking water (U.S. EPA, 1992; Allen et al., 2010). Enzyme-based methods have also
been developed that are capable of detecting total coliforms and simultaneously differentiating E.
coli (Edberg et al., 1988).
Presumptive coliform media, such as lauryl tryptose broth, m-Endo media or EC media,
can make a presumptive determination but cannot distinguish E. coli colonies from other types of
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Escherichia coli (March 2012)
coliforms. Therefore, a confirmation step is required. Several options are available for
confirmation of E. coli. For example, the classical “IMViC” test uses biochemical reactions to
differentiate the members of the coliform group. Various media and reagents, which are
available commercially prepared, are needed to complete the test. Also, E. coli confirmation can
be done by subjecting the coliform-positive sample to media that tests for the enzyme βglucuronidase (APHA et al., 2005). The main disadvantages to using presumptive coliform
media are the necessity of a confirmation step, which requires additional time (24 hours) to
complete the analyses, and the possible interference with recovery by non-coliform bacteria.
Although multiple types of tests can be used, variability exists among these tests in their
sensitivity for the detection and quantification of E. coli. The method and media composition
used, as well as the presence of non-coliform bacteria in the sample, have varying effects on the
results (Olstadt, 2007; Fricker et al., 2008a, 2008b). It is also important to use validated or
standardized methods to make correct and timely public health decisions.
All analyses for E. coli should be carried out as directed by the responsible authority. In
many cases, the responsible authority will recommend or require the use of accredited
laboratories. In some cases, it may be necessary to use other means to analyze samples in a
timely manner, such as non-accredited laboratories or on-site testing using commercial test kits
by trained operators. To ensure reliable results, a quality assurance (QA) program, which
incorporates quality control (QC) practices, should be in place. In addition to the QA/QC
program, any test kits used should meet minimum requirements for accuracy, detection
(sensitivity), and reproducibility, and be used according the manufacturer’s instructions.
5.1
Presence–absence procedure
The P-A test was developed as a more sensitive, economical, and efficient means of
analyzing drinking water samples (Clark and Vlassoff, 1973). This procedure is currently the
preferred method in many jurisdictions for verifying the bacteriological safety of public drinking
water supplies (i.e., the absence of E. coli). Essentially the P-A test is a modification of the MTF
procedure (see section 5.3) in which only one analysis bottle per sample is used. This method can
be used with either enzyme-based media or presumptive coliform media (e.g., using lauryl
tryptose broth), with follow-up E. coli confirmation. Commercial test kits using enzyme-based
media have been developed for P-A testing. Studies performed on the effectiveness of the
commercial tests compared with classical MTF and MF approaches showed that the commercial
kits were usually as sensitive as the MTF approach for the detection of E. coli, and sometimes
more sensitive for the detection of total coliforms (Rompré et al., 2002). Also, data illustrate that
some media based on enzyme-based methods can detect injured coliforms within 24 hours
(Edberg and Edberg, 1988).
In comparative tests using lactose-based media, the P-A method was shown to be at least
as sensitive as the MF and MTF techniques for the recovery of both total coliforms and E. coli
(Clark, 1980; Jacobs et al., 1986; Pipes et al., 1986; Clark and El-Shaarawi, 1993), and it
required a similar amount of time to obtain results. Technically, P-A testing (using lactose-based
and enzyme-based technologies) is simpler than the MF and MTF procedures and has a shorter
processing time (less than 1 minute per sample).
P-A testing does not provide any information on the actual concentration of organisms in
the sample. The quantitation of organisms is sometimes used to assess the extent of the
contamination, and as such is considered a benefit of the more quantitative methods such as the
MF and MTF methods. For decision-making, the focus is the positive detection of E. coli,
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Escherichia coli (March 2012)
regardless of quantity; as the guideline for E. coli in drinking water is none per 100 mL,
qualitative results are sufficient for protecting public health.
5.2
Membrane filter procedure
The MF procedure was introduced to bacteriological water analysis in 1951, after its
capability to produce results equivalent to those obtained by the MTF procedure was
demonstrated (Clark et al., 1951; Goetz and Tsuneishi, 1951). Both enzyme-based media and
presumptive coliform media can be used. With this technique, the water sample is passed
through a filter that retains bacteria. The filter is then placed on a standard presumptive coliform
medium or on a medium containing substrates designed for the detection of the enzyme βglucuronidase (Dufour et al., 1981; Ciebin et al., 1995) and incubated. The advantages of the
technique were quickly recognized because it made the examination of larger volumes of water
practical. Sensitivity and reliability were increased, whereas time, labour, equipment, space, and
material requirements were significantly reduced. The MF technique remains the method of
choice in some jurisdictions for the routine enumeration of coliforms in drinking water; however,
this method may underestimate the number of viable coliform bacteria in a sample. Standard
Methods for the Examination of Water and Wastewater provides 95% confidence limits for MF
results (APHA et al., 2005). When the MF method is used with media capable of detecting the
enzyme β-glucuronidase, it is an efficient means of enumerating E. coli in water; however, it
does not satisfactorily solve the problems linked to the presence of non-culturable bacteria
(discussed below) (Rompré et al., 2002). Commercial agar is available for routine enumeration.
The major concern for this and other methods that use stressful selective media (i.e.,
media that contain inhibitory chemicals for non-target organisms) is an inability to enumerate
bacteria that have been subjected to sublethal injury (e.g., caused by chlorination) in a treatment
plant or distribution system. The resultant false-negative findings could lead to the acceptance of
water of potentially hazardous quality. Although stressed organisms may not grow on selective
media, they can recover through a resuscitation process. Detection of stressed coliforms, in
general, has been improved using enhanced recovery media such as m-T7 (LeChevallier et al.,
1983) or through the addition of substances, such as catalase and/or sodium pyruvate, to m-Endo
or m-FC media (Calabrese and Bissonnette, 1990). Since these media are not specific for E. coli,
additional confirmation steps are still needed. To overcome the need for a confirmation step,
substrates such as 4-methylumbelliferyl-β-D-glucuronide can be added to selective and nonselective coliform media. As described above, the β-glucoronidase enzyme produced by E. coli
will cleave the substrate in the media, resulting in a fluorescent product that can be visualized
under UV light.
High turbidity can also interfere with the MF method. The retention of particulate matter
by the filter can interfere with colony development and the production of surface sheens and/or
fluorescence by presumptive coliforms or E. coli. Similarly, concentrations of heterotrophic
bacteria in excess of 500 colony-forming units (CFU) per millilitre can interfere with coliform
recovery when using presumptive coliform media (Geldreich et al., 1972; Clark, 1980;
Burlingame et al., 1984), even with the addition of substrates for detecting the β-glucuronidase
enzyme. Most water supplies that maintain a total chlorine residual of 0.2 mg/L have a
heterotrophic plate count (HPC) below 500 CFU/mL (LeChevallier, 1990). Further information
on HPC along with their significance in drinking water, can be found in the guidance document
on heterotrophic plate count (Health Canada, 2012b). Background colony counts can also be
used for determining whether there is interference with coliform recovery when using
presumptive coliform media. Although this method is quantitative, clumping of E. coli or
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Escherichia coli (March 2012)
masking because of growth of other microorganisms, can also lead to underestimations in
concentrations. Since E. coli should not be present in treated drinking water, these
underestimations are not as much of a concern as are false-negative findings.
5.3
Multiple tube fermentation procedure
In the MTF procedure, 10-fold dilutions of the water to be tested are added to tubes
containing the appropriate medium (5 or 10 tubes per dilution) and incubated. Both enzymebased media and presumptive coliform media can be used. For drinking water, dilution should be
unnecessary because of the expected low counts. Commercial kits using enzymatic methods have
been developed for enumeration by the multiple tube technique (Rompré et al., 2002). Results
are reported as a most probable number (MPN). The MPN is only a statistical estimate of the
number of bacteria that, more than any other number, would probably give the observed result; it
is not an actual count of the bacteria present. Standard Methods for the Examination of Water
and Wastewater provides 95% confidence limits for MPN results (APHA et al., 2005). Studies
performed on the effectiveness of the commercial tests compared with classical MTF and MF
approaches showed that the commercial kits were usually as sensitive as the MTF approach for
the detection of E. coli and sometimes more sensitive for the detection of total coliforms
(Rompré et al., 2002).
Similar to the situation with the MF procedure, high densities of non-coliform bacteria
and the inhibitory nature of some presumptive coliform MTF media may have an adverse
influence on E. coli detection. For example, many heterotrophic bacteria can inhibit the detection
of E. coli (Waksman, 1941; Hutchison et al., 1943; Means and Olson, 1981). In addition, the
recovery of coliforms from gas-negative MTF tubes has demonstrated the presence of inhibitory
compounds in the MTF media (Evans et al., 1981; McFeters et al., 1982). In response to these
findings, APHA et al. (2005) recommends treating all tubes with turbidity, regardless of gas
production, as presumptive coliform-positive tubes. Clumping of coliforms can lead to an
underestimation of their concentrations in both MF and MTF methods.
The MTF procedure has a longer turnaround time for results and the MF procedure has
largely replaced it for routine examinations of drinking water when using presumptive coliform
media. However, the MTF technique is used more extensively with enzyme-based tests when
conditions render the MF technique unusable, for example, with turbid, coloured, or grossly
contaminated water.
6.0
6.1
Sampling for E. coli
Sample collection
Proper procedures for collecting samples must be observed to ensure that the samples are
representative of the water being examined. Detailed instructions on the collection of samples for
bacteriological analysis are given in APHA et al. (2005). To avoid unpredictable changes in the
bacterial flora of the sample, examination should be started as soon as possible after collection.
The sample should be transported to the laboratory in a cooler with ice or cooling packs (at 5 ±
3°C), to minimize changes in populations and concentrations (Dutka and El-Shaarawi, 1980;
McDaniels et al., 1985; ISO, 2006). As well, samples should be protected from direct contact
with the ice or cooling packs to prevent freezing during transport. Ideally, the interval between
collection of the sample and the beginning of its examination should not exceed 24 hours
(Bartram and Rees, 2000), and analysis within 8 hours is recognized as the preferred time
interval (Bartram and Rees, 2000; APHA et al., 2005). In remote areas, up to 48 hours may be an
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Escherichia coli (March 2012)
acceptable time interval; however, the implications of the extended holding time should be
discussed with the responsible authorities. When delays are anticipated, a delayed incubation
procedure, described in APHA et al. (2005), should be employed or consideration given to onsite testing. Alternatively, if normal transportation time exceeds 24 or 48 hours (depending on
circumstances noted above), the sample should be processed and arrangements made to have
another sample collected as soon as the first sample is received. Thus, if the first sample contains
E. coli, a repeat sample will already have been received or will be in transit. Samples should be
labelled with the time, date, location, type of sample (e.g., raw water, distribution system),
sampler’s name, and identification number (if used), along with the disinfectant residual
measurements and any special conditions. In most cases, much of this information, along with
the identification number linked to the sample bottle, is recorded on accompanying submission
forms and, in cases where samples are collected for legal purposes, chain-of-custody paperwork.
When examination will be delayed, it is particularly important to record the duration and
temperature of storage, as this information should be taken into consideration when interpreting
the results.
A minimum volume of 100 mL of water should be examined to obtain a reliable estimate
of the number of organisms (using MTF or MF) or to obtain an accurate P-A result at the
expected low levels in treated drinking water. For the MTF method, a test series consisting of
one 50-mL volume and five 10-mL volumes is suggested in the World Health Organization’s
International Standards for Drinking-Water for water expected to be of good quality (WHO,
1971). Examination of larger volumes, such as in groundwaters with very low levels of
contamination, can increase both the test sensitivity and the test reliability. Smaller volumes,
dilutions, or other MTF combinations may be more appropriate for waters of poor quality.
6.2
Sampling frequency considerations
The World Health Organization lists the following factors that should be taken into
account when determining sampling frequency for municipal scale systems (WHO, 1971, 1976,
2004):
past frequency of unsatisfactory samples;
source water quality;
the number of raw water sources;
the adequacy of treatment and capacity of the treatment plant;
the size and complexity of the distribution system; and
the practice of disinfection.
These variables preclude application of a universal sampling frequency formula. Instead,
the sampling frequency and location of sampling points should be decided upon by the
responsible authority after due consideration of local conditions—for example, variations in raw
water quality and a history of treated water quality. The sampling frequency should meet all
jurisdictional requirements.
As a minimum, water leaving a treatment plant should be tested daily for disinfectant
residual and turbidity and tested at least weekly for E. coli as part of the verification process in a
source-to-tap multi-barrier approach. A guide for the recommended sampling frequency is
provided in Table 1. In many systems, the water leaving the treatment plant will be tested for
these indicators well in excess of the minimum requirements. For supplies where weekly E. coli
testing is impractical (e.g., in small supplies), E. coli sampling may be reduced and other means
of verifying the microbiological quality may be used, such as residual disinfectant
determinations and good process control. Small supplies should also periodically carry out
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Escherichia coli (March 2012)
sanitary surveys as an additional action to verify the safety of the system. The daily sampling
recommendations for disinfectant residual and turbidity testing may not apply to supplies served
by groundwater sources of excellent quality in which disinfection is practised to increase the
safety margin.
In a distribution system, the number of samples for bacteriological testing should be
increased in accordance with the size of the population served. The general practice of basing
sampling requirements on the population served recognizes that smaller water supply systems
may have limited resources available for monitoring. However, because small water supplies
have more facility deficiencies and are responsible for more disease outbreaks than are large
ones (Schuster et al., 2005), emphasis should also be placed on identified problems based on
source-to-tap assessments, including sanitary surveys.
Table 1: Recommended sampling frequency.
Population served
Up to 5000
5000–90 000
90 000+
Minimum number of samples per month*
4
1 per 1000 persons
90 + (1 per 10 000 persons)
*
The samples should be taken at regular intervals throughout the month. For example, if four samples are required
per month, samples should be taken on a weekly basis.
Disinfectant residual tests should be conducted when bacteriological samples are taken.
Further information on monitoring for turbidity can be found in the guideline technical document
for turbidity (Health Canada, 2013b). The majority of samples should be taken in potential
problem areas. Routine verification of the concentration of the disinfectant residual, and the
bacteriological quality of the water ensures that immediate remedial action can be taken if water
of doubtful quality enters a distribution system. It must be emphasized that the frequencies
suggested in Table 1 are only general guides. For small systems, additional guidance may need
to be considered by the responsible authority. In supplies with a history of high-quality water, it
may be possible to reduce the number of samples taken for bacteriological analysis.
Alternatively, supplies with variable water quality may be required to sample on a more frequent
basis. Sampling frequencies in residential-scale and private systems may vary with jurisdiction
but should include times when the risk of contamination is greatest, for example, during spring
thaw, heavy rains, or dry periods. New or rehabilitated wells should also be sampled initially to
confirm acceptable bacteriological quality.
Even at the recommended sampling frequencies for E. coli, there are limitations that need
to be considered when interpreting the sampling results. Simulation studies have shown that it is
very difficult to detect a contamination event in a distribution system unless the contamination
occurs in a water main, in a reservoir, at the treatment plant, or for a long duration at a high
concentration (Speight et al., 2004; van Lieverloo, 2007). Therefore, even if the analytical result
indicates the absence of E. coli, intrusion may be occurring in the distribution system. Some
improvement in detection capabilities were found when sampling programs were designed with
the lowest standard deviation in time between sampling events (van Lieverloo, 2007), such as
samples collected every 5 days regardless of weekends and holidays. This highlights the
importance of implementing a source-to-tap multi-barrier approach, as opposed to relying on
monitoring a single parameter for verifying the microbiological quality of the drinking water.
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6.3
Location of sampling points
In municipal-scale systems, the location of sampling points must be chosen by the
responsible authority. The sampling locations selected may vary depending on the monitoring
objectives. For example, fixed sampling points may be used to help establish a history of water
quality within the distribution system, whereas sampling at different locations throughout the
distribution system may provide more coverage of the system. A combination of both types of
monitoring is common (Narasimhan et al., 2004). Some work has been published on how to
select statistically based random sampling sites (Speight et al., 2004).
Samples should be taken at the point where the water enters a system and from
representative points throughout a distribution system. If the water supply is obtained from more
than one source, the location of sampling points in the system should ensure that water from each
source is periodically sampled. Distribution system drawings can provide an understanding of
water flows and directions and can aid in the selection of appropriate sampling locations. The
majority of samples should be taken in potential problem areas, such as low-pressure zones,
reservoirs, dead ends, areas at the periphery of the system farthest from the treatment plant, and
areas with a poor previous record.
In residential-scale systems, samples are generally collected from the locations
recommended by the responsible authority. More extensive sampling may be necessary,
depending on the system and results from previous samples.
7.0
Treatment technology
The application of a multi-barrier approach, including watershed or well-head protection,
optimized treatment barriers, and a well-maintained distribution system, is the best approach to
reduce the presence and associated health risks of waterborne pathogens to an acceptable level.
E. coli monitoring is part of the multi-barrier approach.
An array of options is available for treating source waters to provide high-quality
drinking water in municipal and residential-scale systems. The quality of the source water will
dictate the degree of treatment necessary. Generally, minimum treatment of supplies derived
from surface water sources or groundwater under the direct influence of surface waters should
include adequate filtration (or technologies providing an equivalent log reduction credit) and
disinfection. All groundwaters should receive adequate treatment for the removal/inactivation of
enteric viruses unless exempted by the responsible authority based on site-specific considerations
such as historical and on-going monitoring data. In systems with a distribution system, a
disinfectant residual should be maintained at all times.
7.1
Municipal scale
In general, all drinking water supplies should be disinfected, and a disinfectant residual
should be maintained throughout the distribution system at all times. In addition, surface water
sources and groundwater under the direct influence of surface water should include physical
removal methods, such as chemically assisted filtration (coagulation, flocculation, clarification,
and filtration) or technologies that provide an equivalent log removal/inactivation of
microorganisms. It is essential that the removal and disinfection targets are achieved before
drinking water reaches the first consumer in the distribution system. Adequate process control
measures and operator training are also required to ensure the effective operation of treatment
barriers at all times (U.S. EPA, 1991; Health and Welfare Canada, 1993; AWWA, 1999).
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7.1.1 Level of treatment necessary
Most source waters are subject to faecal contamination, as such, treatment technologies
should be in place to achieve a minimum 4-log (99.99%) removal and/or inactivation of enteric
viruses and a minimum 3-log (99.9%) removal and/or inactivation of enteric protozoa in
accordance with the guideline technical documents on enteric viruses and protozoa (Health
Canada, 2011, 2012c). Depending on the source water quality, a higher log reduction may be
necessary to produce safe drinking water. Groundwater classified as less vulnerable to faecal
contamination, using procedures determined by the responsible authority, should not have
protozoa present. Therefore the minimum treatment requirements for protozoa would not apply.
However, even these groundwater sources will have a degree of vulnerability and should be
periodically reassessed. In general, protozoa and enteric viruses are more difficult to inactivate
and/or remove than bacterial pathogens. Therefore, water that is treated to meet the guidelines
for enteric viruses and enteric protozoa should have an acceptable bacteriological quality,
including meeting the MAC for E. coli of none detectable in 100 mL of water leaving the
treatment plant.
7.1.2 Physical removal
The physical removal of coliform bacteria can be achieved using various types of
filtration. A recent review of pilot- and full-scale study data concluded that coagulation,
flocculation, and sedimentation processes were associated with 1.7 log bacteria (E. coli,
coliforms, faecal streptococci) removal credit (range, 0.5 to 3.9 log) (Hijnen et al., 2004). In
studies that included pre- and post-disinfection along with coagulation, flocculation,
sedimentation, and filtration, total coliform concentrations were reduced to non-detectable levels
in the finished water (5 to 6 log reduction) (Payment et al., 1985; El-Taweel et al., 2001). A
review of slow sand filtration studies reported a 2.4 log removal credit for bacteria (range, 1.3 to
3.2 log) (Hijnen et al., 2004). Membrane filtration technologies are also capable of removing 4.0
log to greater than 6.0 log of E. coli (NSF, 2002). More detailed information on filtration
techniques can be found in the guideline technical document on turbidity (Health Canada,
2013b).
7.1.3 Disinfection
The commonly used drinking water disinfectants are chlorine, chloramine, UV light,
ozone, and chlorine dioxide. Disinfection is typically applied after treatment processes that
remove particles and organic matter. This strategy helps to ensure efficient inactivation of
pathogens and minimizes the formation of disinfection byproducts (DBPs). It is important to note
that when describing microbial disinfection of drinking water, the term “inactivation” is used to
indicate that the pathogen is no longer able to multiply within its host and is therefore noninfectious, although it may still be present.
7.1.3.1 Chemical disinfection
Currently, chlorine is the most widely used disinfectant in the drinking water industry. It
is a strong oxidant capable of inactivating bacteria and viruses present in bulk water, although, as
with most chlorine-based disinfectants, it is not as effective for the control of protozoans.
Chlorine is also less effective for inactivating organisms present in biofilms. Compared with
chlorine, chloramine is a weaker oxidant. This property is advantageous in that the disinfectant
resides longer in a distribution system. It is therefore easier to maintain a disinfectant residual,
and the disinfectant is better able to penetrate into the biofilm found in the pipes and reservoirs,
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Escherichia coli (March 2012)
leading to superior coliform control (LeChevallier et al., 1990; LeChevallier, 2003). However,
chloramine is less efficient at controlling a sudden pulse of contamination (Snead et al., 1980)
and it can lead to nitrification. Chlorine dioxide is as effective as, and in some instances more
effective than, chlorine. However, this compound is difficult to work with and therefore is not
widely used. Ozone is more efficient for the inactivation of bacteria, viruses, and protozoa
compared with chlorine-based disinfectants, although ozone treatment can result in an increase in
biodegradable organics that can promote bacterial regrowth in the distribution system. Ozone is
highly effective at the point of treatment, but an additional disinfectant (usually chlorine or
chloramine) needs to be added to supply a residual. Maintaining a disinfectant residual will limit
the growth of organisms within the distribution system and, depending on the residual
concentration, contact time, and the pathogens present, a disinfectant residual also may afford
some protection against contamination from intrusion (Besner et al., 2008). The disappearance of
the residual may also provide an immediate indication of the entry of oxidizable matter into the
system or a malfunction of the treatment process.
The efficacy of chemical disinfectants can be predicted based on knowledge of the
residual concentration of disinfectant, temperature, pH, and contact time (AWWA, 1999b). This
relationship is commonly referred to as the CT concept, where CT is the product of “C” (the
residual concentration of disinfectant, measured in mg/L) and “T” (the disinfectant contact time,
measured in minutes). To account for disinfectant decay, the residual concentration is usually
determined at the exit of the contact chamber rather than using the applied dose or initial
concentration. Also, the contact time, T, is often calculated using a T10 value, such that 90% of
the water meets or exceeds the required contact time. The T10 values can be estimated based on
the geometry and flow conditions of the disinfection chamber or basin. Hydraulic tracer tests,
however, are the most accurate method to determine the contact time under actual plant flow
conditions.
CT values for 99% inactivation of E. coli using chlorine, chlorine dioxide, chloramine,
and ozone are provided in Table 2. For comparison, CT values for Giardia lamblia and for
viruses have also been included. In a well-operated treatment system, the CT provided will result
in a much greater inactivation than 99%. From Table 2, it is apparent that, compared with most
protozoans and viruses, coliform bacteria are easier to inactivate using the common chemical
disinfectants. Also, chloramines have a much higher CT value than any of the other disinfectants
listed. This means that to achieve the same level of inactivation with chloramine, a higher
disinfectant concentration or a longer contact time, or a combination of both, is necessary. This is
consistent with the properties of chloramine as a disinfectant, as previously described.
Table 2: CT values for 99% inactivation at 5°C
Free chlorine
6–7
E. colia
(mg·min/L)
0.034–0.05
Chloramines
8–9
95–180
Disinfectant agent
a
pH
Chlorine dioxide
6–7
Ozone
6–7
b
Giardia lambliab
(mg·min/L)
65–93
Virusesb
(mg·min/L)
4.0c
1470
857
0.4–0.75
17
c
5.6c
0.02
1.3
0.6
c
From Hoff (1986); From U.S. EPA (1999); Value for pH 6.0–9.0
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
7.1.3.2 UV light disinfection
Ultraviolet (UV) light disinfection is highly effective for inactivating many types of
pathogens. Further information on inactivation of specific protozoan and viral pathogens can be
found in the guideline technical documents on protozoa and enteric viruses (Health Canada,
2011, 2012c). Similar to ozone, UV light is highly effective at the point of treatment, but an
additional disinfectant (usually chlorine or chloramine) needs to be added to supply a residual.
When using UV light for the inactivation of E. coli (and other bacteria), the bacteria can undergo
photo repair and, to a lesser extent, dark repair (Harris et al., 1987; Schoenen and Kolch, 1992;
Zimmer and Slawson, 2002). However, the amount of repair is not considered significant in
drinking water treatment and distribution.
Log inactivations using UV light disinfection are listed in Table 3. E. coli, because of its
importance as a public health indicator, has been used as a representative bacterial species. For
comparison, UV light doses for representative protozoa and viruses have also been included.
Review of the data on inactivation using UV light (Table 3) shows that, of the representative
organisms, bacteria (in this instance, E. coli) and protozoa require comparable doses of UV light
to achieve the same level of inactivation, whereas certain viruses are much more resistant.
Table 3: UV light dose (mJ/cm2) required for inactivation
Log
inactivation
E. colia,d Cryptosporidiuma
Adenovirusa,b,d
1
1.5–5
2.5
42–58
2
3
4
2.8–9
4.1–14
5.0–18
5.8
12
22
83–111
129–167
167–186
Rotavirusa,c,d Giardiab
7.1–10
2.1
15–20
23–29
36–40
5.2
11
22
a
Based on U.S. EPA (2003).
Adenoviruses are highly UV resistant in comparison with other enteric viruses. Further information is available in Health
Canada’s guideline technical document on enteric viruses (Health Canada, 2011).
c
LeChevallier and Au (2004)
d
Hijnen et al. (2006)
b
7.2
Residential scale
Residential-scale treatment is also applicable to small drinking water systems. This could
include both privately owned systems and systems with minimal or no distribution system that
provide water to the public from a facility not connected to a municipal supply (previously
referred to as semi-public systems).
Various options are available for treating source waters to provide high-quality pathogenfree drinking water. These include filtration and disinfection with chlorine-based compounds or
alternative technologies, such as UV light. These technologies are similar to the municipal
treatment barriers, but on a smaller scale. In addition, there are other treatment processes, such
as distillation, that can be practically applied only to small or individual water supplies. Most
of these technologies have been incorporated into point-of-entry devices, which treat all water
entering the system, or point-of-use devices, which treat water at only a single location—for
example, at the kitchen tap. It is important to note that if point-of-use devices are used instead of
a point-of-entry system, all points of water used for drinking, food and beverage preparation,
hygiene or washing dishes should be equipped with a point-of-use treatment device, to minimize
the potential public health risks when use of microbiologically-contaminated drinking water.
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
The use of UV light has increased owing to its availability, relative ease of operation, and
its ability to inactivate a range of pathogenic organisms. However, scaling or fouling of the UV
lamp surface is a common problem when applying UV light to raw water with moderate or high
levels of hardness, such as groundwater. UV light systems are often preceded by a pretreatment
filter to reduce scaling or fouling. A pretreatment filter may also be needed to achieve the water
quality that is required for the UV system to operate properly. In addition, regular cleaning and
replacement of the lamp, according to manufacturer’s instructions, are critical to ensure the
proper functioning of the unit. Alternatively, special UV lamp-cleaning mechanisms or water
softeners can be used to overcome this scaling problem.
Health Canada does not recommend specific brands of drinking water treatment devices,
but it strongly recommends that consumers look for a mark or label indicating that the device has
been certified by an accredited certification body as meeting the appropriate NSF International
(NSF)/American National Standards Institute (ANSI) standard. These standards have been
designed to safeguard drinking water by helping to ensure the safety of material and performance
of products that come into contact with drinking water.
NSF/ANSI Standard 55 (Ultraviolet Disinfection Systems) provides performance criteria
for two categories of certified systems, Class A and Class B. UV systems certified to NSF/ANSI
Standard 55 Class A are designed to deliver a UV dose that is at least equivalent to 40 mJ/cm2 in
order to inactivate microorganisms, including bacteria, viruses, Cryptosporidium oocysts, and
Giardia cysts, from contaminated water. However, they are not designed to treat wastewater
or water contaminated with raw sewage and should be installed in visually clear water. Systems
certified to NSF Standard 55 Class B systems are intended for a drinking water supply that is
already disinfected, tested, and deemed acceptable for human consumption. NSF Standard 62 for
Drinking Water Distillation Systems also includes reduction claims for bacteria. To meet this
standard, a distillation system must provide a minimum 6 log reduction of bacteria and bacterial
spores. Distillation systems should only be installed at the point of use as the water they have
treated may be corrosive to internal plumbing components.
Certification organizations provide assurance that a product or service conforms to
applicable standards. In Canada, the following organizations have been accredited by the
Standards Council of Canada (www.scc.ca) to certify drinking water devices and materials as
meeting the appropriate NSF/ANSI standards:
Canadian Standards Association International (www.csa-international.org)
NSF International (www.nsf.org)
Underwriters Laboratories Inc. (www.ul.com)
Quality Auditing Institute Ltd. (www.qai.org)
Water Quality Association (www.wqa.org)
International Association of Plumbing and Mechanical Officials (www.iapmo.org).
8.0
Risk assessment
The adoption of a risk-based approach, such as a multi-barrier approach, is essential to
the effective management of drinking water systems (CCME, 2004). This approach should
include assessment of the entire drinking water system, from the watershed or aquifer and intake
through the treatment and distribution chain to the consumer, to assess potential effects on
drinking water quality and public health.
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
A health-based risk assessment for E. coli is not appropriate since E. coli is used only as
an indicator organism. Risk assessments have been done for specific microbiological organisms
that have health implications, such as enteric viruses and the enteric protozoa Cryptosporidium
and Giardia (Health Canada, 2011, 2012c).
Current drinking water guidelines encourage the adoption of a multi-barrier approach to
produce clean, safe, and reliable drinking water. E. coli is a bacteriological indicator that should
be routinely used as part of this approach. E. coli is well recognized as an indicator of recent
faecal contamination. In surface and GUDI water sources, E. coli monitoring results can be used
as part of the source water assessment to look at the microbiological quality of the water and to
indicate changes in this quality. In protected groundwater systems, the presence of E. coli signals
that the groundwater is being impacted by faecal contamination and is microbiologically unsafe
for drinking without further treatment. Monitoring for E. coli, when used in conjunction with a
source-to-tap multi-barrier approach, is used as part of the verification that the drinking water
system is producing water that is microbiologically acceptable.
8.1
International considerations
Other countries use E. coli for similar purposes. The Drinking Water Inspectorate of
England and Wales has included in its regulations a mandatory value of zero E. coli per 100 mL
in water leaving treatment works, in service reservoirs, and at the consumer’s tap (DWI, 2000).
These regulations are based on the European Union’s Council Directive on the quality of water
intended for human consumption, which specifies a maximum contaminant level of zero for E.
coli in 100mL of water (Council of the European Union, 1998). The Australian Drinking Water
Guidelines (NHMRC, 2011) include E. coli as an indicator for faecal contamination and set a
guideline of no E. coli in any sample of drinking water (minimum of 100 mL). The proposed
revision to the Total Coliform Rule in the United States includes a maximum contaminant level
(MCL) and a maximum contaminant level goal (MCLG) of zero for E. coli because it is a more
specific indicator of faecal contamination (than total coliforms) (U.S. EPA, 2010). The presence
of E. coli in a repeat sample or the presence of E. coli in a routine sample followed by a positive
test for either total coliforms or E. coli in the repeat sample exceeds the MCL. A system can also
be non-compliant with the MCL for E. coli if they fail to perform all the necessary testing.
9.0
Rationale
Of the contaminants that may be regularly found in drinking water, those present in
human and animal faeces pose the greatest danger to public health. Faecal pathogens of concern
for transmission through water include bacteria (e.g., Campylobacter, pathogenic E. coli),
viruses (e.g., rotaviruses, enteroviruses), and protozoa (e.g., Cryptosporidium, Giardia).
Routinely detecting pathogenic bacteria, viruses, and protozoa is currently not practical. Reasons
for this include the large number of different pathogens that exist, their uneven distribution in
water, and the time and expense associated with routine monitoring of all pathogens. It is better
to use an indicator that is less difficult, less expensive, and less time consuming to monitor. This
encourages a higher number of samples to be tested, giving a better overall picture of the water
quality and therefore better protection of public health.
Although E. coli are not necessarily related to the presence of specific faecal pathogens,
the presence of E. coli is a definite indicator of the presence of recent human or animal faeces. In
addition to being faecal specific, E. coli do not usually multiply in the environment, are excreted
in the faeces in high numbers, making detection possible even when greatly diluted, and have a
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
life span on the same order of magnitude as those of other enteric bacterial pathogens. This
makes E. coli the best available indicator of faecal contamination and the potential presence of
faecal pathogens. Any level of E. coli in drinking water would be related to faecal contamination
and is deemed unacceptable. Consequently, the guideline for E. coli in drinking water systems is
proposed as a maximum acceptable concentration of none detectable per 100 mL.
Sampling and analysis for E. coli is an easy, relatively quick, inexpensive way of
monitoring the quality of drinking water. The absence of E. coli, when used in conjunction with
a source-to-tap multi-barrier approach, is used as part of the verification that the drinking water
system is producing water that is microbiologically acceptable.
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Escherichia coli (March 2012)
Appendix A: Decision Tree for Routine Microbiological Testing of Municipal
Scale Systems
Routine sampling for E.coli (Ec) and total coliforms
(TC) in public systems
Ec positive*
TC positive**, Ec negative
Single site
contaminated
1st Resample/
retest positive
site and
adjacent sites
Multiple sites
contaminated
Boil Water Advisory****
should be issued
(in consultation with
responsible authority)
Ec positive
Ec positive
1st Reanalyze Coliform
positive sample;
Resample/retest positive
site and adjacent sites
TC positive
Ec negative
Ec negative***
2nd Resample/
retest positive
site and
adjacent sites
TC negative
Ec negative
Ec positive
Investigate the
source of
contamination and
implement corrective
actions
Investigate the
source of
contamination and
implement corrective
actions
Ec negative***
Water considered
safe
Ec positive
1st Resample/
retest positive
site and
adjacent sites
Ec negative
Ec positive
2nd Resample/
retest positive
site and
adjacent sites
2nd Resample/
retest positive
site and
adjacent sites
TC positive
TC negative
No further
corrective actions
required
Ec negative
Water considered
safe
Water considered
safe
Boil Water Advisory****
should be rescinded
(in consultation with
responsible authority)
Continue routine sampling for Ec and TC
*A boil water advisory may be issued on a single site contamination if deemed necessary by the responsible authority
**A boil water advisory may be issued based on a positive total coliform, in the absence of E.coli, if deemed necessary by the responsible authority.
***If a total coliform positive sample is detected during resampling for E.coli, the decision route for detection of a total coliform positive sample, in
the absence of E.coli, should be followed (right-hand side of the decision tree).
****Depending on the jurisdiction, "boil water order" may be used in place of, or in conjunction with, "boil water advisory."
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
Appendix B: Decision Tree for Routine Microbiological Testing of Residential
Scale Systems
Routine sampling for E.coli (Ec) and total coliforms
(TC) in residential scale systems*
TC positive***
Ec negative
Ec positive
Boil Water Advisory**
should be issued
(in consultation with
responsible authority)
Ec positive
1st Resample/
retest positive
site and
adjacent sites
TC positive
Ec negative
Investigate the
source of
contamination and
implement corrective
actions
TC positive
or
Ec positive
TC positive
or
Ec positive
Response will vary and may
depend on:
Extent of contamination
History of the system
Jurisdictional
requirements
Outbreak in progress
Investigate the
source of
contamination and
implement corrective
actions
1st Resample/
retest positive
site and
adjacent sites
TC and Ec
negative
TC positive
or
Ec positive
nd
2 Resample/
retest positive
site and
adjacent sites
1st Resample/
retest positive
site and
adjacent sites
TC positive or
Ec positive
TC and Ec
negative
Water considered
safe
TC negative
Ec negative
Boil Water Advisory***
should be rescinded
(in consultation with
responsible authority)
Continue routine sampling for Ec and TC
TC positive and
Ec negative
TC and Ec
negative
2nd Resample***/
retest positive
site and adjacent
sites
TC and Ec
negative
No further
corrective actions
required
Water considered
safe
* Private systems (eg. an individual well serving a rural home) are responsible for the microbiological quality of the water serving the
system. Nevertheless, health authorities should be willing to provide advice on remedial actions, when necessary.
** Depending on the jurisdiction, "boil water order" may be used in place of, or in conjunction with, "boil water advisory."
*** A boil water advisory may be issued based on a single positive TC result, if deemed necessary by the responsible authority.
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
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Escherichia coli (March 2012)
Appendix C: List of Acronyms
AOC
ANSI
CFU
EPA
GUDI
HPC
MAC
MCL
MCLG
MF
MPN
MTF
NSF
P-A
QA
QC
UV
assimilable organic carbon
American National Standards Institute
colony-forming unit
Environmental Protection Agency (U.S.)
groundwater under the direct influence of surface water
heterotrophic plate count
maximum acceptable concentration
maximum contaminant level (U.S.)
maximum contaminant level goal (U.S.)
membrane filtration
most probable number
multiple tube fermentation
NSF International
presence–absence
quality assurance
quality control
ultraviolet
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document
32
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