Efficacy of disinfectant residualon microbial intrusion: A review of experiments

To assess the state of knowledge regarding the effectiveness of distribution system disinfectant residuals during accidental microbial intrusions, the authors reviewed laboratory-, pilot-, and full-scale experimental studies. The persistence of microorganisms in the biofilms of simulated mains at the laboratory scale was much longer than in pilot-scale settings, under the typical disinfectant residual concentrations used in distribution systems. Most of the studies reviewed used culture-based methods for the enumeration of microorganisms, which are known to underestimate the number and diversity of microorganisms. Because the distribution system is the last barrier between the treatment plant and the consumer, intrusion is of concern to utility managers. Conditions experienced during some repairs—zero- or low-flow conditions and/or delay in reinstatement of disinfection—are likely to be critical for the persistence of intruded microorganisms in the biofilm, a fact that underscores the importance of adequate chlorination and flushing procedures. For the research community, this review highlights the need for additional studies using methods (e.g., molecular tools) that allow the detection of viable but nonculturable microorganisms.