Transcript Epidemiology and Toxicology of Disinfection By

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
disinfection
• Cryptoporidiosis: Crytosporidium (protozoa)
• Bacteria: Shigella and enterohemorrhagic E. coli
• Other microbial contaminants: Cyclospora, microsporidia,
Legionella, Aeromonas, Mycobacterium, and adenoviruses
• Some gastrointestinal illness: waterborne – contamination of
the 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
reduction 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,
gastroenteritis
– 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
are 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