"Granular activated carbon (GAC) treatment of drinking water was investigated to determine its effects on the microbiological characteristics of potable water distribution systems. Though GAC is increasingly used to remove organic chemicals and reduce trihalomethane precursors from drinking water, concern exists that the practice may lead to increased microbial concentrations in the finished water. Microbial parameters examined were total coliforms (TC), endotoxins, standard plate count (SPC), membrane standard plate count (m-SPC), and acid-fast organisms (AF). An 18-month field investigation was held at two full-scale water treatment plants, and a pilot-scale study was conducted on GAC columns receiving prechlorinated, coagulated, and filtered water. The 18-month field investigation showed that such treatment does increase SPC and m-SPC organisms but has no effect on any of the other microbial parameters investigated. Furthermore, conventional postchlorination following GAC treatment appeared to mitigate any impact on the distributed water quality. Within the water distribution system itself, pH, temperature, and turbidity appeared to be positively correlated with SPC and m-SPC densities, with only sporadic inverse correlations to chlorine concentration. The pilot plant GAC studies using coagulated, postfiltration water were conducted to determine the effect of empty bed contact time (EBCT), velocity, and prechlorination on the microbial dynamics in GAC columns. Increasing EBCT by providing greater bed depth tended to stimulate microbial growth, but a decrease in velocity tended to depress microbial growth. Increasing levels of prechlorination tended to yield greater microbial concentrations on the carbon itself and to effect a slight increase in effluent microbial densities."