Groundwater was used to evaluate the influence of the disinfection processes on the microbial quality of potable water distribution systems using laboratory-scale units. Coliform bacteria, heterotrophic plate count and total bacteria were used for the evaluation of the bactericidal effectiveness of each disinfectant. The microbial disinfection efficacy of chlorine, chloramine, ozone and UV irradiation was found to be equally effective in the elimination of coliform bacteria during the first hours (20 min-2 h) after disinfection. Complete elimination of coliforms in hydrogen peroxide treated water occurred after 48 h. More than 4 log cfu . ml−1 (average killing rate) heterotrophic bacteria were killed by all the disinfectants with the exception of hydrogen peroxide (average killing rates: 3-2 log cfu . ml−1). However, ozone was highly effective within the first 2 h as shown by the average killing rate of 4 log cfu . ml−1 heterotrophic bacteria in both source waters. The phenomenon of bacterial regrowth was linked to the absence of concentrations of disinfectant residuals. Bacterial regrowth, however, could be detected earlier with chlorine (after 20 min-average regrowth rate 0.064 h−1, average generation time 10.95 h), ozone (after 2 h-average regrowth rate 0.202 h−1, average-generation time 5.04 h) and UV treated water (after 2 h-average regrowth rate 0.263 h−1, average generation time 2.70 h) than chloramine (between 24 h and 48 h-average regrowth rate 0.057 h−1, average generation time 13.87 h) and hydrogen peroxide treated water (after 48 h-average regrowth rate 0.063 h−1, average generation time 12.74 h). The greater persistence of monochloramine (7 days) and hydrogen peroxide (14 days) residuals were found to inhibit bacterial regrowth in potable water.

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