Balancing Microbial and Chemical Risks

• Waterborne cholera, diarrhea, and other diseases – Both developing and industrial countries

• Water disinfection vs. waterborne pathogens

• INFECTIOUS WATERBORNE DISEASE RISKS– (Endemic) Waterborne outbreaks: inadequate or no di

sinfection• Cryptoporidiosis: Crytosporidium (protozoa)• Bacteria: Shigella and enterohemorrhagic E. coli• Other microbial contaminants: Cyclospora, microsporidia, Leg

ionella, Aeromonas, Mycobacterium, and adenoviruses• Some gastrointestinal illness: waterborne – contamination of t

he distribution system partly responsible for endemic gastroenteritis

• MANAGING MICROBIAL AND DBP RISKS– Disinfection DBPs (chemical risk)– No disinfection waterborne disease– How to reduce to the formation of DBPs

• Moving the point of disinfection• Using an alternative disinfectant• Removing DBP precursors from the water

– Costs of Controlling Microbial and DBP Risks• Costs saved by the prevention of infectious disease may be

as much as eight to twelve times greater than the cost of the water system

• Coagulation and clarification effective removal of both pathogens and precursor materials

• GAC (granular activated carbon) too expensive

• CONSIDERATION OF COMPETING RISKS– Risk trade-off between exposure to DBPs and

exposure to waterborne pathogens– Identification of:

• The DBPs that pose the greatest health risks• How a proposed treatment strategy or technology will affect

the removal (and risk) of these DBPs and microbial contaminants

• The costs associated with these water technologies

– Developing countries: life expectancy is too low (diarrhea) how then can we observe cancer patients?

• BALANCING RISKS – THE PROBLEM STATEMENT– What specific water treatment option(s)? optimum red

uction in adverse health consequencies from both DBPs and waterborne microbial contaminants

– Notable imbalance between the real and perceived riks associated with disinfection of drinking water

• Ex) Diarrhea and gastroenteritis perceived to be less important

– How to assess risk from complex mixtures of contaminants (bacteria, viruses, protozoa, inorganic chemicals, DBPs, and other organic chemicals)?: various water quality conditions

• PREVENTION-EFFECTIVE ANALYSIS– Cost-effectiveness: treatment of drinking water vs. a

public health intervention (microbial risk : chemical risk)– Duration and severity of illness and the public’s

perception of the degree of risk

• PROPOSED APPROACH TO ASSESS RISKS FROM D/DBPs and MICROBES– Primary effects: cancer, adverse pregnancy outcomes, g

astroenteritis– Secondary effects: long-term sequelae of acute disease,

side effects of treatment of acute disease– Both occurrence and severity of disease should be taken

into account– The mixture of DBPs and pathogens to which humans ar

e exposed can vary with time and location within a water distribution system dose-response data for a mixture of DBPs?

• PROPOSED APPROACH TO ASSESS RISKS FROM D/DBPs and MICROBES– Water Treatment Scenarios to Define Exposure

• Type and characteristics of source water• Type and amount of disinfectant used and contact time with

precursor material• Range of water pH and temperature• Organic material and bromide ion concentration in source water• Water treatment to remove precursor material or DBPs

– Risk Estimates• Highly uncertain at present More data needed