Appl. Environ. Microbiol., 07 1996, 2201-2211, Vol 62, No. 7
MW LeChevallier, NJ Welch and DB Smith
An 18-month survey of 31 water systems in North America was conducted to
determine the factors that contribute to the occurrence of coliform
bacteria in drinking water. The survey included analysis of assimilable
organic carbon (AOC), coliforms, disinfectant residuals, and operational
parameters. Coliform bacteria were detected in 27.8% of the 2-week sampling
periods and were associated with the following factors: filtration,
temperature, disinfectant type and disinfectant level, AOC level, corrosion
control, and operational characteristics. Four systems in the study that
used unfiltered surface water accounted for 26.6% of the total number of
bacterial samples collected but 64.3% (1,013 of 1,576) of the positive
coliform samples. The occurrence of coliform bacteria was significantly
higher when water temperatures were > 15 degrees C. For filtered systems
that used free chlorine, 0.97% of 33,196 samples contained coliform
bacteria, while 0.51% of 35,159 samples from chloraminated systems
contained coliform bacteria. The average density of coliform bacteria was
35 times higher in free- chlorinated systems than in chloraminated water
(0.60 CFU/100 ml for free-chlorinated water compared with 0.017 CFU/100 ml
for chloraminated water). Systems that maintained dead-end free chlorine
levels of < 0.2 mg/liter or monochloramine levels of < 0.5 mg/liter
had substantially more coliform occurrences than systems that maintained
higher disinfectant residuals. Free-chlorinated systems with AOC levels
greater than 100 micrograms/liter had 82% more coliform-positive samples
and 19 times higher coliform levels than free-chlorinated systems with
average AOC levels less than 99 micrograms/liter. Systems that maintained a
phosphate-based corrosion inhibitor and limited the amount of unlined cast
iron pipe had fewer coliform bacteria. Several operational characteristics
of the treatment process or the distribution system were also associated
with increased rates of coliform occurrence. The study concludes that the
occurrence of coliform bacteria within a distribution system is dependent
upon a complex interaction of chemical, physical, operational, and
engineering parameters. No one factor could account for all of the coliform
occurrences, and one must consider all of the parameters described above in
devising a solution to the regrowth problem.
Copyright © 1996, American Society for Microbiology
Full-scale studies of factors related to coliform regrowth in drinking water
American Water Works Service Company, Inc., Voorhees, New Jersey 08043, USA.
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