Bacterial survival and regrowth in drinking water systems

Weekly variations in free and total chlorine residuals, and their impact on microbial concentrations were monitored for the IIT Kharagpur (IITKgp) water supply to determine possible correlations between heterotrophic bacterial density, total and faecal coliform density, and total and free chlorine residuals. All correlations between HPC, coliforms and free and total chlorine residuals were found to be insignificant, suggesting that coliform survival and growth are not inhibited by low levels of residual chlorine ranging from 0.01 to 0.41 mg Cl2/L and total chlorine ranging from 0.02 to 1.23 mg Cl2/L. Growth studies were carried out to measure bacterial growth rates in chlorinated drinking water using turbidity. An effort was also made to establish correlations between Cell Density (cfu/L) and Turbidity (NTU), which would allow turbidity to be used as a tool for measurement of bacterial growth in real time. Bacteria were grown in nutrient broth solutions diluted 10-fold with chlorinated tap water to simulate low substrate chlorinated drinking water environments and the average growth rate was 0.196 +/- 0.049 1/h (n = 5, CoV = 25%). No bacterial inocula were added to these samples and chlorine residuals in these samples were 0.14 +/- 0.06 mg Cl2/L and 0.06 +/- 0.05 mg Cl2/L for total chlorine and free chlorine, respectively.