Change from chlorine residual distribution to no chlorine residual distribution in groundwater systems

In this study, microbiological data in distribution systems were compared when changing from chlorine residual-distribution to no chlorine residual-d- istribution. In a groundwater system the bacteriological quality of the water with chlorination showed always heterotrophic plate counts below 5 CFU/mL and the regrowth potential of the water was extremely low. The distribution situation in contrast was a dead end line, with high retention times. The chlorination to about 0.1 mg/L free chlorine led to taste and odour complaints therefore a switch to no residual-distrib- ution was done. Shortly after this change in the samples with higher retention times, the heterotrophic plate counts increased to 60 to 80 CFU/mL, decreasing after 4 to 6 weeks again to values below 5 CFU/mL as before.