Effects of Surface Water Treatment for Drinking Water Production, Distribution and Heating on Biological Stability

Drinking water treatment, from a microbiological perspective, efficiently removes hygienically relevant microorganisms, making the water safe for human consumption. The water quality is strictly regulated. An aspect that is often overlooked, however, is biological stability. In this study, we assessed the effect of drinking water treatment of surface waters on biological stability. Biological stability was assessed as the difference between the actual cell concentrations naturally contained in the water at the time point of sampling and the maximal cell concentrations in the same sample obtained after a batch growth assay. Whereas raw waters were biologically stable, treatment resulted in a loss of biological stability and also partly in an increase in the maximal cell numbers. Treatment steps reducing biological stability resulted in the conversion of biologically fixed nutrients to dissolved nutrients. The stabilizing effect of biologically active filters was annihilated by disinfection at the end of treatment. The lack of biological stability was especially evident when distributing chlorinated water, where actual cell numbers and, in part, regrowth potentials tended to increase throughout transport with strong seasonal variations. Comparing cold drinking water at house entrances in different buildings across Germany, regrowth tests resulted in an average regrowth factor of 54 with high spatial and temporal variability. Biological instability was further increased in drinking water installations during water heating, which contributed to an additional shift towards dissolved nutrients, giving room to microbiological changes once the water cools down and stagnates. It remains to be determined whether the biological stabilization of drinking water can increase its microbiological resilience towards the growth of hygienically relevant bacteria.