FULL-SCALE STUDIES OF FACTORS RELATED TO COLIFORM REGROWTH IN DRINKING-WATER

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.