###### Abstract  
Drinking water treatment, from a microbiological perspective, efficiently removes hygiically 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 hygiically relevant bacteria. D 2025 11 May  
a disinfection residual. Within drinking water distribution systems (DWDSs), water is typically assumed to be stable if only minor changes in colony numbers are determined by cultural methods based on standard nutrient agars. Concentrations tend to be substantially below 100 colony-forming units (cfu)/mL on nutrient-rich agars, although heterotrophic plant counts using R2A medium with a lower nutrient concentration can exceed these numbers by a factor of 100-200 [8]. For fecal indicators like _E. coli_, coliforms or enterococci, strict monitoring guarantees their absence, and in rare cases of coliform occurrence, countermeasures are taken quickly. The current diagnostic system has served well for many decades and assures microbiology safety within DWDSs. Nevertheless, when applying cultivation-independent methods, research has shown that water microbiology can undergo profound changes in DWDSs [9; 10; 11]. When examining changes in total cell numbers in a chlorinated DWDS fed from a drinking water treatment plant (DWTP) treating surface water, increases in cell numbers were reported with increasing distance from the DWTP, seasonal temperature fluctuations with higher counts in summer (1.51-5.24-fold increase), longer residence times and increased contact of bulk water with pipe biofilm [12]. A combination of flow cytometry and sequencing also detected substantial changes in flow cytometric fingerprints and shifts in the composition of bacterial communities in drinking water distributed from a surface water works that added chlorine dioxide at the end of treatment [13]. These changes were not detected using colony counts. Changes have also been observed in DWDSs without disinfect residual, but with UV disinfection prior to distribution. The drinking water microbiome re-established quickly after UV disinfection [14]. It was found that water at the point of entry of a building in close proximity to the water works shared a much more similar sequencing profile with the water prior to UV than after UV disinfection. Changes in microbiological water quality can be even more pronounced in drinking water installations (DWIs) than in the DWDSs.