Full-scale studies of factors related to coliform regrowth in drinking water

Abstract

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؇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 g/liter had 82% more coliform-positive samples and 19 times higher coliform levels than freechlorinated systems with average AOC levels less than 99 g/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.

Escherichia coli O157 can grow in natural freshwater at low carbon concentrations

“…A minimum concentration of 100 mg AOC app l -1 was needed by Vibrio cholerae O1 to grow in natural freshwater . Correspondingly, a significant negative correlation between the occurrence of coliforms in drinking water systems and AOC concentrations lower than 100 mg l -1 was reported (LeChevallier et al, 1996). It seems that also for E. coli O157 a certain threshold value of AOCapp for growth exists.…”

Effect of storage tank material and maintenance on household water quality

“…Free chlorine concentrations decreased from 0.47 mg/L in water leaving the treated surface water supply point to concentrations at or near the detection limit (0.02 mg/L) at the house. It has been reported that significant growth of total coliforms occurs in water when the free chlorine concentrations are less than 0.2 mg/L (LeChevallier et al, 1996). Measured free chlorine levels in the distribution system and at the household level (i.e., cisterns and storage tanks) are one tenth of that value.…”

“…Various field studies have shown that significant bacterial growth can occur in water of 15°C or higher (Donlan et al, 1994;Smith et al, 1989;Donlan & Pipes, 1988;Fransolet et al, 1985). LeChevallier et al (1996) found that coliform bacteria occurred more frequently and in higher concentrations at water temperatures > 15°C. That study also observed an 18-fold increase of coliforms in chlorinated systems when temperatures were increased from 5°C to > 20°C.…”

Phylogenetic diversity of drinking water bacteria in a distribution system simulator

“…USEPA surface water treatment rules state that heterotrophic plate counts (HPC) should be no more than 500 CFU/ ml (U.S. Environmental Protection Agency 1989). Prokaryotic and eukaryotic microorganisms that are present in water leaving the treatment plant, though probably inactivated by the disinfection process, may recover and grow in water distribution system (WDS) biofilms (LeChevallier et al 1996). Sections of biofilms in distribution systems can periodically break off and enter the bulk water (Daly et al 1998;Stoodley et al 2002).…”