Mw. Lechevallier et al., FULL-SCALE STUDIES OF FACTORS RELATED TO COLIFORM REGROWTH IN DRINKING-WATER, Applied and environmental microbiology, 62(7), 1996, pp. 2201-2211
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 assimilab
le organic carbon (AOC), coliforms, disinfectant residuals, and operat
ional parameters, Coliform bacteria were detected in 27.8% of the 2-we
ek sampling periods and were associated with the following factors: fi
ltration, temperature, disinfectant type and disinfectant level, AOC l
evel, corrosion control, and operational characteristics, Four systems
in the study that used unfiltered surface water accounted for 26.6% o
f the total number of bacterial Samples collected but 64.3% (1,013 of
1,576) of the positive coliform samples, The occurrence of coliform ba
cteria was significantly higher when water temperatures were > 15 degr
ees C. For filtered systems that used free chlorine, 0.97% of 33,196 s
amples contained coliform bacteria, while 0.51% of 35,159 samples from
chloraminated systems contained coliform bacteria, The average densit
y of coliform bacteria was 35 times higher in free-chlorinated systems
than in chloraminated water (0.60 CFU/100 ml for free-chlorinated wat
er compared with 0.017 CFU/100 ml for chloraminated water), Systems th
at maintained dead-end free chlorine levels of <0.2 mg/liter or monoch
loramine levels of <0.5 mg/liter had substantially more coliform occur
rences than systems that maintained higher disinfectant residuals, Fre
e-chlorinated systems with AOC levels greater than 100 mu g/liter had
82% more coliform-positive samples and 19 times higher coliform levels
than free-chlorinated systems with average AOC levels less than 99 mu
g/liter. Systems that maintained a phosphate-based corrosion inhibito
r 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 col
iform bacteria within a distribution system is dependent upon a comple
x interaction of chemical, physical, operational, and engineering para
meters, No one factor could account for all of the coliform occurrence
s, and one must consider all of the parameters described above in devi
sing a solution to the regrowth problem.