The aim of this study was to compare the level of removal and inactivation of indigenous bacteria during drinking water production as evaluated by culture techniques and epifluorescent microscopic counts of metabolically active bacteria (in situ respiring bacteria i.e. able to metabolise CTC: cyano 2,3-ditolyl tetrazolium chloride). Two sets of experiments were designed: a) bacterial counts through a full scale drinking water treatment plant (multibarrier treatment including coagulation-flocculation-settling, sand filtration, ozonation, biological GAC filtration, post-chlorination) and distribution system; b) benchscale disinfection studies in order to re-evaluate the C.t values necessary to inactivate laboratory grown E. coli or indigenous bacteria from water by ozone and chlorine. Main conclusions of this study are: a) significant amounts of in situ respiring bacteria (undetected by the classical culture techniques) are detected in finished water; b) the efficiency of ozone and chlorine recorded by microscopic counts of active bacteria is much less than supposed by classical enumerations of culturable bacteria; c) previous results reported in the literature may have largely overestimated the bactericidal efficiency of disinfectants used for producing drinking water.

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