Influence of disinfection processes on the microbial quality of potable groundwater in a laboratory-scale system model

Groundwater was used to evaluate the influence of the disinfection processe s on the microbial quality of potable water distribution systems using labo ratory-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, chloram ine, ozone and UV irradiation was found to be equally effective in the elim ination of coliform bacteria during the first hours (20 min-2 h) after disi nfection. Complete elimination of coliforms in hydrogen peroxide treated wa ter occurred after 48 h. More than 4 log cfu. ml(-1) (average killing rate) heterotrophic bacteria were killed by all the disinfectants with the excep tion of hydrogen peroxide (average killing rates: 3-2 log cfu. ml(-1)). How ever, ozone was highly effective within the first 2 h as shown by the avera ge 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 c oncentrations of disinfectant residuals. Bacterial regrowth, however, could be detected earlier with chlorine (after 20 min-average regrowth rate 0.06 4 h(-1), average generation time 10.95 h), ozone (after 2 h-average regrowt h rate 0.202 h(-1), average-generation time 5.04 h) and UV treated water (a fter 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 (1 4 days) residuals were found to inhibit bacterial regrowth in potable water .