Effect of free chlorine application on microbial quality of drinking water in chloraminated distribution systems

Abstract

The potential for bacterial regrowth and the formation of disinfection by‐products in distribution systems are issues of concern for many drinking water utilities. The principal objective of this study was to examine bacterial growth/regrowth in two chloraminated distribution systems before, during, and after a free chlorine flush. Raw water for both systems was of similar quality, but treatment trains differed, primarily in their use of ozone. To avoid culture‐based technique biases, cell counts were measured directly by nucleic acid staining and subsequent analysis by flow cytometry. During the free chlorine flush period, lower cell counts were obtained at all sampled locations in one distribution system, but only at long detention times in the second distribution system. Assimilable organic carbon levels were generally greater in the second distribution system, where ozonation was used in the treatment train. At an intermittent‐use facility, nitrification was mitigated after hydrant flushing led to the appearance of a free chlorine residual. However, without additional hydrant flushing, the free chlorine residual decayed and led to elevated levels of chloroform. In comparing tap water from both distribution systems with several commercially available bottled waters, bacteria levels were found to be higher in more than half of the bottled waters tested.