Effects of early biofilm formation on water quality during commissioning of new polyethylene pipes

Mature biofilms are considered beneficial to non-chlorinated Danish drinking water distribution systems, as they increase the microbiological stability of the water. During commissioning of new pipes in the distribution network, biofilms start developing on the new pipe wall, and are influenced by numerous factors, such as the water quality, pipe material, flow velocity and existence of biofilm upstream. In the early stage of biofilm formation, before a mature biofilm is reached, the development of a biofilm may impact the water quality negatively. However, this short-term effect of biofilm development remains poorly understood. In this study, measurements of microbiological water quality parameters (HPC, ATP, qPCR, DAPI, amplicon sequencing and online microbiological sensor-mobilBACMON), upstream and downstream of a newly commissioned PE pipe, showed a clear effect of early biofilm formation on water quality. In general, high heterotrophic plate counts (highest HPC 22 °C = 870 CFU mL−1, day 12), but a low microbiological diversity (lowest Shannon index 2.3, day 8) was observed around 10 days after pipe commissioning. By day 15, the diversity increased to upstream levels (Shannon index approx. 4.5). HPC decreased to upstream levels after 15–20 days. This study highlights the importance of obtaining and maintaining a stable biofilm composed of natural bacteria originating from groundwater in pipes of non-chlorinated systems to improve the water quality for consumers. Therefore, optimizing the pipe commissioning procedures can benefit the utility and the consumers. Finally, this study shows the strength of a “multiple-line-of-evidence-approach” for both detailed microbiological assessment and routine monitoring of the biological stability of drinking water in distribution systems.