AWWA JAW49169

Microbiological data on drinking water in a distribution system were compared before and after the treatment plant discontinued use of a chlorine residual. Bacteriological assessments of chlorinated groundwater consistently showed heterotrophic plate counts (HPCs) <5 colony forming units (cfu)/mL, a count indicating that bacterial regrowth was very unlikely. Because chlorination that produced about 0.1 mg/L free chlorine residual led to taste and odor complaints, chlorination was discontinued, and water that contained no residual was distributed. Shortly after this change, samples with higher retention times produced HPCs as high as 60-80 cfu/mL. After four to six weeks these counts decreased to values again <5 cfu/mL. This short-term increase in HPCs, which has also been observed in other cases, is assumed to be due to a change in the biofilm. Includes 7 references, table, figures.