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Ordering Information:
ORDER NUMBER: 90761
DATE AVAILABLE: Summer 1999
Printed Report |
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PRINCIPAL INVESTIGATORS:
S.A. Sattar, C. Chauret, V.S. Springthorpe, D. Battigelli, Morteza Abbaszadegan,
and Mark LeChevallier
OBJECTIVES:
The overall objective of this study was to understand the environmental factors
that influence the survival or inactivation of protozoan parasite cysts and
oocysts, and their susceptibility to subsequent disinfection. This refers to
the physical, chemical and biological factors prevailing in natural or managed
water environments, and to the stressors that may be encountered or applied
during in-plant treatment.
BACKGROUND:
Cysts/oocysts are resistant forms of protozoa shed from infected hosts, and
subsequently discharged into receiving waters through various point (e.g., sewage
effluent) and nonpoint (e.g., land run-off) sources. The resistance of these
forms to many stressors, including the disinfectants used in water treatment,
makes them a potential hazard for water consumers and a challenge for the water
industry. Understanding the factors that affect inactivation of such pathogens
in watersheds may encourage better management practices.
HIGHLIGHTS:
The physical factors that were most influential in parasite decline were water
temperature, and sunlight or blacklight. Within the confines of the study, the
chemical composition of water did not play a large role in oocyst decline. The
greatest decline observed was attributable to biological components, between
0.2 and 5 μm in diameter, present in the natural
waters tested. However, results varied between watersheds and seasons. Generally,
Giardia muris appeared more sensitive to stressors than did Cryptosporidium.
Aged or stressed oocysts were no less resistant to disinfection than those freshly
shed.
APPROACH:
Parasite survival experiments were performed in standard hard water as well
as in natural water samples from different watersheds in Canada and the U.S.
Relevant controls were included in all experiments. Most experiments were conducted
in vitro in small polypropylene tubes, but some experiments were conducted in
situ using flow-through tanks and dialysis cassettes. Physicochemical environmental
stressors were applied separately and in various combinations. Early indications
that biological components in water can affect parasite inactivation were followed
by more detailed investigations of this phenomenon. Parasite survival was assessed
as a function of time by excystation assays and counts of total cyst or oocyst
numbers. Aged or stressed oocysts were used, in comparison with fresh ones,
for disinfection experiments.
RESULTS/FINDINGS:
Physicochemical Stressors
In general, Giardia muris cysts were very sensitive to stressors at ambient
temperatures. Therefore, most experiments used only Cryptosporidium.
Increased temperature had the most influence on parasite decline in standard
hard water. Sunlight or long wavelength UV (blacklight) was also detrimental.
Water hardness, pH, and redox had little effect in the ranges tested. Sand shearing
and freezing/thawing were more damaging. The anion and metal content of the
water played no role in oocyst decline in vitro. However, the influence of chemicals
in situ in the aquatic environment may be more prominent and would require further
investigation.
Biological Factors
While biological activity emerged as perhaps the most prominent stressor in
some water sources, the effects were not observed across the board. This may
be attributable to different microbial populations between water sources and
seasons. Concentration of the indigenous biota, between 0.2 and 5 μm
in diameter, can serve to pinpoint the importance of microbial antagonistic
activity in any watershed. Tests with individual bacterial isolates or groups
of isolates failed to demonstrate significant oocyst inactivation. This suggests
that the organism(s) responsible may not be cultivable or may act in complex
consortia.
Disinfection
Aged or stressed oocysts were no less resistant to disinfection than were those
freshly shed.
IMPACT:
This study is a building block upon which models, essential to justify
good watershed management, could be designed to predict the inactivation of
Cryptosporidium oocysts and Giardia cysts. Findings of this study
show that Cryptosporidium oocysts survive well and thus travel far from
the pollution source. This emphasizes the importance of applying best management
practices throughout the watershed.
Aged/stressed oocysts were as resistant to disinfection as freshly excreted ones. Thus, it is important for utilities to implement effective removal and disinfection practices. Accordingly, the data generated in previous studies on disinfection kinetics (CT values, etc.) of fresh oocysts must be regarded as accurate for evaluating CT values of environmental isolates.
PARTICIPATING UTILITIES:
- American Water Works Services Company, Inc., East St. Louis, Ill.
- Regional Municipality of Ottawa-Carleton, Ottawa, Ont., Canada
- City of Cornwall, Cornwall, Ont., Canada
- Regional Municipality of Waterloo, Kitchener-Waterloo, Ont., Canada