Disinfection efficacy was evaluated in four pilot-scale systems that were operated at four water utilities across North America. Through this approach, factors which are difficult to simulate in a laboratory environment (such as water properties, process conditions, climatic conditions) were assessed, in order to determine their influence on disinfectant effectiveness. The results from this study show that a critical level of disinfectant residual (Ccrit) was necessary for inhibiting heterotrophic bacterial growth to less than 500 CFUml−1 in the effluent of these pilot-scale systems. When free chlorine was the disinfectant, this critical residual concentration was found to be approximately 0.6 mg L−1. A total chlorine concentration of 1 mg L−1 resulted in low bacterial counts with chloramine disinfectants, though approximately 10–20% of this residual was found to be comprised of slowly degradable organic chloramines which are less effective as disinfectants. A higher ductile iron pipe corrosion rate was observed with a free chlorine disinfectant as compared with chloramines. The results from this study are likely to be of interest to water utilities responsible for the operation and optimization of drinking water distribution systems.

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