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Bacteriological analysis were performed on collected water samples from a conventional water treatment pilot plant in Cincinnati, Ohio in which granular activated carbon (GAC) has been used as the final process to assess the impact of GAC on the bacteriological quality and incidence of antibiotic resistant bacteria in water produced. Heterotrophic bacterial counts (HPC) at 20 °C was stabilized at 102 to 194 cfu mL−1 and did not markedly differ at different water treatment processes. On the other hand, slight reduction in HPC was observed for the effluent produced from sand filter and GAC contactors. Effluents produced from both the sand filter and GAC contactors showed 2 log reduction in coliforms count. Fecal coliform showed the same rate of reduction as a result of sand filtration, while it reached undetectable numbers in the effluent of GAC contactors. Subculturing the isolated strains in tryptic soy broth revealed that 61.3, 61.5, 12.6 and 8.5% of HPC at 28 °C, total coliforms and fecal coliform, respectively were non-culturable. In this case, R2A or R3A broth was used as subculturing media. The incidence of coliform resistant strains among isolates varied significantly according to the source of water samples. Multiple antibiotic resistance (MAR) was not always high in the same samples in which the overall resistance was high. The species composition varied considerably in different water samples. Selection for bacteria exhibiting resistance to antibiotic or antibiotics was observed under some experimental conditions using different doses of chlorine. The antibiotic resistance character was mostly transferable. As a conclusion, the use of GAC has no observable adverse effect on the bacteriological quality of the water produced from the pilot plant under investigation.

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