Transcript No Slide Title

Risk assessment and health
based targets for microbial
water quality
Prof. Nicholas J. Ashbolt
School of Civil & Environmental Engineering
University of New South Wales
Sydney, Australia
Cooperative Research Centre for
Water Quality and Treatment
Public health based water
quality guidelines
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Health impacts of waterborne disease
vary in severity and impact
(Marginal) cost-effectiveness of
additional measures decreases steadily
Decide which risk is tolerable…
against current background of costs
and population health
Then define tolerable exposure
WHO harmonised approach
for provision of safe water
Basic control approaches
RISK MANAGEMENT
(HACCP)
Water quality
objectives
HEALTH
TARGETS
Tolerable
risk
Other management
objectives
Define measures and interventions
(requirements, specifications)
based upon objectives
Assess
environmental
exposure
Risk
Assessment
Define key risk points and
audit procedures for overall
system effectiveness
Define analytical verifications
(process, public health)
PUBLIC
HEALTH
OUTCOME
Fewtrell &
Bartram 2001
Pathogens are primarily from faeces,
with various modes of transmission:
Excreta from humans and animals
Land
Runoff
Sewage
Solid Waste
Landfills
Oceans and
Estuaries
Rivers and
Lakes
Groundwater
Irrigation
Crops
Aerosols
Shellfish
Recreation
Adapted from Charles P. Gerba et al. 1975
Water
Supply
Human
Cholera: cases & case fatality rates
Classic O1
El Tor biotype of Vibrio cholerae
Cholera outbreaks appear to originate in travel
and trade from Bangladesh to Pakistan and then
spread throughout south-central Asia.
E. coli outbreak in Walkerton, Canada
Agents associated with waterborne
outbreaks (CDC, USA)
Viral AGI
15%
Giardia
11%
Bacterial
Chemical
10%
1%
AGI
43%
Cryptosporidium
20%
AGI - acute gastrointestinal illness
Epidemic to endemic illnesses
as detected by epidemiologic studies
Number of Cases
Outbreak detected
Threshold for detection for an outbreak
Endemic
rate
Undetected outbreak
Hyperendemic
Time
Frost et al. (1996) J. Am. Wat. Wk. Assoc. 88(9):66-75
Sporadic
How much diarrhoea is
tolerated: developed regions?
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Current rates (by strict definition):
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USA 1.05 episodes per person a year (Mead et al.
1999)
UK 0.19-0.55 episodes per person a year (Wheeler et
al. 1999)
Australia 0.92 episodes of gastroenteritis per person a
year (Hellard et al. 2001)
Gastro in USA, hospitalisation rate of 4.4%,
fatality rate 0.03% for community cases
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Foodborne illness was estimated to cause 36% of
cases, 34% of hospitalisations and 67% of deaths
(Mead et al. 1999)
So how much diarrhoea is
tolerable?
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To translate the risk of developing a
specific illness to disease burden per
case, the metric DALYs is used
Disability Adjusted Life Years (DALYs)
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mortality: years of life lost (YLL)
morbidity: years lived with disability (YLD)
DALY = YLL + YLD
Conceptually simple:
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health loss = N x D x S
 N = number of affected persons
 D = duration of adverse health effect
 S = measure for severity of the effect
Hypothetical example of DALY
Disability weight
1
0.8
0.6
Acute
(infectious) disease
0.4
Premature
death
YLL
0.2
YLD
0
0
20
40
Age
60
Residual
disability
80
Disease burdens for different
water contaminants
Reference Pathogens
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Viruses
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Bacteria
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Rotavirus (Adenovirus)
Campylobacter & STEC (developed)
STEC and Vibrio cholerae
(developing)
Parasitic Protozoa
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Cryptosporidium/Giardia
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Entamoeba histolytica
Helminths
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Ascaris lumbricoides
Using DALYs to derive Water
Quality Targets (WQT)
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Tolerable mortality risk 10-5 per lifetime  1.4 x
10-7 per year (for a lifespan 70 years)
Corresponds to  10-6 DALY per year.person
Disease burden for e.g. rotavirus in
developed countries is 0.014 DALY per case
Tolerable incidence of rotavirus associated
gastroenteritis is 7.14 cases per Mpy
Use consumption data and Dose-Response
model to derive WQT, accounting for sensitive
fraction in the population
Linking tolerable disease burden and
source water quality: Ref. pathogens
Cryptosporidium
Campylobacter
Rotavirusa
Pathogens/L in source
10
100
10
Health outcome target
(per person a year)
10-6 DALYs
10-6 DALYs
10-6 DALYs
Annual risk of
diarrhoeal illnessb
1 per 1600
1 per 4000
1 per 11 000
Drinking-water quality
1 per 1600
litres
1 per 8000
litres
1 per 32 000
litres
4.2 log10
units
5.9 log10
units
5.5 log10 units
Parameter
Performance targetc
a Data from high-income regions. In low-income regions, severity is typically
higher, but drinking-water transmission is unlikely to dominate.
b For the susceptible population (100%, 100%, 6% respectively).
c Performance target is a measure of log reduction of pathogens based on source
water quality (e.g. 4.2 log10 = 99.994% reduction).
Linking tolerable disease burden and
source water quality: Ref. pathogens
Cryptosporidium
Campylobacter
Rotavirusa
Pathogens/L in source
10
100
10
Health outcome target
(per person a year)
10-6 DALYs
10-6 DALYs
10-6 DALYs
Annual risk of
diarrhoeal illnessb
1 per 1600
1 per 4000
1 per 11 000
Drinking-water quality
1 per 1600
litres
1 per 8000
litres
1 per 32 000
litres
4.2 log10
units
5.9 log10
units
5.5 log10 units
Parameter
Performance targetc
a Data from high-income regions. In low-income regions, severity is typically
higher, but drinking-water transmission is unlikely to dominate.
b For the susceptible population (100%, 100%, 6% respectively).
c Performance target is a measure of log reduction of pathogens based on source
water quality (e.g. 4.2 log10 = 99.994% reduction).
Linking tolerable disease burden and
source water quality: Ref. pathogens
Cryptosporidium
Campylobacter
Rotavirusa
Pathogens/L in source
10
100
10
Health outcome target
(per person a year)
10-6 DALYs
10-6 DALYs
10-6 DALYs
Annual risk of
diarrhoeal illnessb
1 per 1600
1 per 4000
1 per 11 000
Drinking-water quality
1 per 1600
litres
1 per 8000
litres
1 per 32 000
litres
4.2 log10
units
5.9 log10
units
5.5 log10 units
Parameter
Performance targetc
a Data from high-income regions. In low-income regions, severity is typically
higher, but drinking-water transmission is unlikely to dominate.
b For the susceptible population (100%, 100%, 6% respectively).
c Performance target is a measure of log reduction of pathogens based on source
water quality (e.g. 4.2 log10 = 99.994% reduction).
Linking tolerable disease burden and
source water quality: Ref. pathogens
Cryptosporidium
Campylobacter
Rotavirusa
Pathogens/L in source
10
100
10
Health outcome target
(per person a year)
10-6 DALYs
10-6 DALYs
10-6 DALYs
Annual risk of
diarrhoeal illnessb
1 per 1600
1 per 4000
1 per 11 000
Drinking-water quality
1 per 1600
litres
1 per 8000
litres
1 per 32 000
litres
4.2 log10
units
5.9 log10
units
5.5 log10 units
Parameter
Performance targetc
a Data from high-income regions. In low-income regions, severity is typically
higher, but drinking-water transmission is unlikely to dominate.
b For the susceptible population (100%, 100%, 6% respectively).
c Performance target is a measure of log reduction of pathogens based on source
water quality (e.g. 4.2 log10 = 99.994% reduction).
Issues in adapting risk-based performance
target setting to local circumstances
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The choice of reference pathogens is mainly
based on availability of data
The pathogens illustrated may not be priority
pathogens in all regions of the world
Wherever possible, country- or site-specific
information should be used in assessments of
this type
If no specific data are available, an
approximate risk estimate can be based on
default values in guidelines
Presented point estimates do not account for
variability and uncertainty
Accounting for poor performance
in the risk assessment
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Nominal performance can be described
stochastically (i.e. as a distribution)
Similarly failure duration and effect can
be described stochastically
Failures leading to
Waterborne illness
in the USA
Treatment
deficiency
35.4%
Distribution
31.8%
Surface
Groundwater
4.5% Unknown
18.2%
9.1%
Example failure in raw water
reservoir during a rain event
Validation of reservoir models
www.cwr.uwa.edu.au/~ttfadmin/model/inflow
Myponga
Burragorang
(26 GL, 30m)
(2000GL, 90m )
measured predicted measured predicted
Inflow °C
10
Insertion
°C
14.7
14.9
12.5-12.7
12.6
Travel
time
Inflow
dilution
30 h
27 h
7d
6.5 d
7.1 x
8x
1.9-2.9 x
2.5 x
10.5
Example of a failing treatment1
1 log nominal
100 pathogens
entering barrier
0.3 log failure
a) Schematic of failing process, say 10% of operational time
b) Performance for pathogen removal, bimodal outcome
1Teunis
et al. (2004) Short-term fluctuations, WHO, Geneva
Daily risks (grey curves), Annual
risks: with and without failure
In summary
Basic control approaches
RISK MANAGEMENT
(HACCP)
Water quality
objectives
HEALTH
TARGETS
Tolerable
risk
Other management
objectives
Define measures and interventions
(requirements, specifications)
based upon objectives
Assess
environmental
exposure
Define key risk points and
audit procedures for overall
system effectiveness
Define analytical verifications
(process, public health)
PUBLIC
HEALTH
OUTCOME
Risk
Assessment
Rationale for multiple barriers:
Probability of reaching at least 2 log10
removal (P<0.01)
Each barrier (p0=0.1, p1=0.03) and probability of failure
Pf= 0.1, 0.3, 0.5, 0.7, and 0.9. Note that at least two but
no more than seven barriers necessary.
Combining WQT ( 5 µDALY), pathogen #
in source waters & system performance
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Estimate the percent removal
necessary for safe water provision
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Need for catchment improvement, and/or
treatment
QMRA also provides data on
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Variability (inherent to the system)
Uncertainty (lack of knowledge of system)
Target ranges at critical control points
Selection of appropriate new systems
Are health targets met
1
Links between WSP & QMRA
Assemble team and other resources
Risk Characterisation
Integration into risk assessment
Describe water supply
Variation and uncertainty
Define intended use
Effect Assessment
Dose-response data
Construct system flow diagram +verify
Host characterisation
Health outcome
2
Exposure Assesment
Identify and prioritize hazards
(background level and incidents)
Description of the system (source to tap)
3
Description of risk scenario’s
Assess occurrence of pathogens in source water
Assess elimination of pathogens by treatment
Identify Control Points
Set Critical Limits
Establish monitoring
Recontamination during storage/distribution
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5
6
Consumption of drinking water
Dose estimation
Hazard identification
Characterisation of pathogens
Health effects
Establish corrective actions
Establish validation and verification
Problem formulation
Define scope and boundaries
Establish record keeping
Assess (un)certainties and constraints
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The principles of HACCP
(hazard analysis critical control point)
Construct & valid process flow diagram
Initial steps
1. Identify hazards and preventative measures
2. Identify Critical Control Points
The seven principles
of HACCP
3. Establish critical limits
4. Identify monitoring procedures
5. Establish corrective action procedures
6. Validate/verify HACCP plan
7. Establish documentation and record keeping
Not drinking water, but upstream
for key index organisms
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In developing regions ~ 2/y gastro rate and
for expatriates from developed regions ~ 3/y
An appropriate health target from waterborne
exposure could be 0.8% gastro, given that is
what is currently tolerated today in developed
regions
With an additional safety factor ~ 0.01% (10-2
per annum illness risk) would be about as
strict as could be defensible.
Particularly given that extra community dollars
spent on improved water treatment may take
dollars away from other (more beneficial)
areas of health protection.
What is tolerable risk for
other industries?
How does it compare to 1 infection/100
people a year?
Annual Probability of Failure
Target risk based on consequences
of failure for industries
10
Merchant
shipping
10-1
10-2
Mobile drill
rigs
10-3
Drowning associated with
large boats
10-4
10-5
* P  10
4
10-6
f
1
Allen’s Equ.
KT
n
10
Commercial
aviation
100
Consequence of Failure (lives lost)
K, A & W are factors regarding the redundancy of the structure
T is the life of the structure; n is the number of people exposed to risk
Pf  10 7 TA
W n
1000
Linking tolerable disease burden and source
water quality  1 µDALY per year
Parameter
Units
Raw water quality
(CR)
Organisms/L
Treatment effect
needed to reach
tolerable risk (PT)
Percent
reduction
Drinking-water
quality (CD)
Organisms/L
Consumption of
unheated drinkingwater (V)
L/d
Exposure by
drinking water (E)
Organisms/d
CD = CR × (1 -PT)
E = CD × V
Cryptosporidium
Campylobacter
Rotavirus
10
100
10
99.994
99.99987
99.99968
6.3 × 10-4
1.3 × 10-4
3.2 × 10-5
1
1
1
6.3 × 10-4
1.3 × 10-4
3.2 × 10-5