Risk assessment and health based targets for microbial water quality

Prof. Nicholas J. AshboltSchool of Civil & Environmental Engineering University of New South WalesSydney, Australia

Cooperative Research Centre for Water Quality and Treatment

Public health based water quality guidelines

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

HEALTHTARGETS

Basic control approaches

Water qualityobjectives

Other managementobjectives

Define key risk points andaudit procedures for overall

system effectiveness

Define analytical verifications(process, public health)

Define measures and interventions(requirements, specifications)

based upon objectivesRisk

Assessment

Assess environmental

exposure

PUBLICHEALTH

OUTCOME

RIS

K M

AN

AG

EM

EN

T(H

AC

CP

)

Tolerable risk

Fewtrell &Bartram 2001

Pathogens are primarily from faeces, with various modes of transmission:

Excreta from humans and animalsExcreta from humans and animals

HumanHuman

ShellfishShellfish CropsCrops AerosolsAerosols

Oceans andEstuaries

Oceans andEstuaries

Rivers andLakes

Rivers andLakes IrrigationIrrigation

Solid WasteLandfills

Solid WasteLandfillsSewageSewageLand

Runoff

LandRunoff

RecreationRecreation WaterSupply

WaterSupply

GroundwaterGroundwater

Adapted from Charles P. Gerba et al. 1975

Cholera: cases & case fatality rates

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.

Classic O1

E. coli outbreak in Walkerton, Canada

Agents associated with waterborne outbreaks (CDC, USA)

AGI43%

Viral AGI15%

Bacterial10%

Giardia11%

Chemical1%

Cryptosporidium20%

AGI - acute gastrointestinal illness

Epidemic to endemic illnessesas detected by epidemiologic studiesN

um

ber

of

Cases

Time

Threshold for detection for an outbreak

Endemic rate

Outbreak detected

Undetected outbreak

Hyperendemic

Sporadic

Frost et al. (1996) J. Am. Wat. Wk. Assoc. 88(9):66-75

How much diarrhoea is tolerated: developed regions?

Current rates (by strict definition): 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 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? To translate the risk of developing a specific

illness to disease burden per case, the metric DALYs is used

Disability Adjusted Life Years (DALYs) mortality: years of life lost (YLL) morbidity: years lived with disability (YLD) DALY = YLL + YLD

Conceptually simple: 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

YLD

Hypothetical example of DALY

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80

Age

Dis

abil

ity

wei

gh

t

Residual disability

Premature death

Acute (infectious) disease

YLL

Disease burdens for different water contaminants

Reference Pathogens

Viruses Rotavirus (Adenovirus)

Bacteria Campylobacter & STEC (developed) STEC and Vibrio cholerae

(developing) Parasitic Protozoa

Cryptosporidium/Giardia Entamoeba histolytica

Helminths Ascaris lumbricoides

Using DALYs to derive Water Quality Targets (WQT)

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

ParameterCryptospor-

idiumCampylo-

bacterRotavirusa

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

Performance targetc 4.2 log10 units

5.9 log10 units

5.5 log10 units

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

ParameterCryptospor-

idiumCampylo-

bacterRotavirusa

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

Performance targetc 4.2 log10 units

5.9 log10 units

5.5 log10 units

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

ParameterCryptospor-

idiumCampylo-

bacterRotavirusa

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

Performance targetc 4.2 log10 units

5.9 log10 units

5.5 log10 units

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

ParameterCryptospor-

idiumCampylo-

bacterRotavirusa

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

Performance targetc 4.2 log10 units

5.9 log10 units

5.5 log10 units

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 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

Nominal performance can be described stochastically (i.e. as a distribution)

Similarly failure duration and effect can be described stochastically

Distribution31.8%

Groundwater18.2%Unknown

9.1%

Surface4.5%

Treatmentdeficiency 35.4%Failures leading to

Waterborne illness in the USA

Example failure in raw water reservoir during a rain event

Validation of reservoir models www.cwr.uwa.edu.au/~ttfadmin/model/inflow

Myponga(26 GL, 30m)

Burragorang(2000GL, 90m )

measured predicted measured predicted

Inflow °C 10 10.5

Insertion °C

14.7 14.9 12.5-12.7 12.6

Travel time

30 h 27 h 7 d 6.5 d

Inflow dilution

7.1 x 8 x 1.9-2.9 x 2.5 x

Example of a failing treatment1

1Teunis et al. (2004) Short-term fluctuations, WHO, Geneva

a) Schematic of failing process, say 10% of operational timeb) Performance for pathogen removal, bimodal outcome

100 pathogensentering barrier

1 log nominal

0.3 log failure

Daily risks (grey curves), Annual risks: with and without failure

In summaryHEALTH

TARGETSBasic control approaches

Water qualityobjectives

Other managementobjectives

Define key risk points andaudit procedures for overall

system effectiveness

Define analytical verifications(process, public health)

Define measures and interventions(requirements, specifications)

based upon objectivesRisk

Assessment

Assess environmental

exposure

PUBLICHEALTH

OUTCOME

RIS

K M

AN

AG

EM

EN

T(H

AC

CP

)

Tolerable risk

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 failurePf= 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

Estimate the percent removal necessary for safe water provision Need for catchment improvement, and/or treatment

QMRA also provides data on 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

Assemble team and other resources

Identify Control Points

Identify and prioritize hazards(background level and incidents)

Describe water supply

Define intended use

Construct system flow diagram +verify

Set Critical Limits

Establish monitoring

Establish corrective actions

Establish validation and verification

Establish record keeping

Problem formulationDefine scope and boundaries

Assess (un)certainties and constraints

Hazard identificationCharacterisation of pathogens

Health effects

Exposure AssesmentDescription of the system (source to tap)

Description of risk scenario’s

Assess occurrence of pathogens in source water

Assess elimination of pathogens by treatment

Recontamination during storage/distribution

Consumption of drinking water

Dose estimation

Risk CharacterisationIntegration into risk assessment

Variation and uncertainty

Effect AssessmentDose-response data

Host characterisation

Health outcome

1

2

3

4

7

56

Links between WSP & QMRA

The principles of HACCP(hazard analysis critical control point)

Construct & valid process flow diagram

1. Identify hazards and preventative measures

2. Identify Critical Control Points

3. Establish critical limits

4. Identify monitoring procedures

5. Establish corrective action procedures

6. Validate/verify HACCP plan

7. Establish documentation and record keeping

The seven principlesof HACCP

Initial steps

Not drinking water, but upstreamfor key index organisms

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?

Target risk based on consequences of failure for industries

K, A & W are factors regarding the redundancy of the structureT is the life of the structure; n is the number of people exposed to risk

Consequence of Failure (lives lost)1 10 100 1000

Ann

ual P

roba

bilit

y of

Fai

lure 10

10-1

10-2

10-3

10-4

10-5

10-6

Merchantshipping

Mobile drillrigs

Drowning - associated withlarge boats

Dams

Commercialaviation

Construction Ind. Res. & Info. Assoc.

EquationAllen’s Equ.

nWTAPf

710n

KTPf410

*

Linking tolerable disease burden and source water quality 1 µDALY per year

Parameter Units Cryptos-poridium

Campy-lobacter

Rota-virus

Raw water quality (CR)

Organisms/L 10 100 10

Treatment effect needed to reach tolerable risk (PT)

Percent reduction

99.994 99.99987 99.99968

Drinking-water quality (CD)

Organisms/L 6.3 × 10-4 1.3 × 10-4 3.2 × 10-5

Consumption of unheated drinking-water (V)

L/d 1 1 1

Exposure by drinking water (E)

Organisms/d 6.3 × 10-4 1.3 × 10-4 3.2 × 10-5

CD = CR × (1 -PT) E = CD × V