## 4 Discussion
### Biological Stability of Drinking Water and Its Measurement  
We use the term \"biological stability\" in the sense of small regrowth factors. This contrasts with the definition of \"stability\" on the basis of low colony or cell numbers enforced by residual disinfectants. This type of'stability' is lost when the disinfectant decays during stagnation or heating. The overall results of this study, from raw water to tap, corroborate that drinking water produced from surface waters has an intrinsic regrowth potential. The regrowth potential is largely due to the water treatment process. While all current water treatment steps serve a purpose and ensure excellent drinking water quality from a chemical and microbiological perspective (e.g., the removal of fecal pathogens in a multibarrier process), this research raises the question of whether current treatment should be supplemented with an additional step: biological stabilization. Converting dissolved nutrients into biomass that is hygiically safe might help to increase its resilience to the establishment of microorganisms that can cause esthetic, technical or hygienic problems. This applies especially to DWIs, which are not strictly controlled. Occupying the ecological niches might lower the risk of unwanted bacteria intrinsic to water such as _Legionella_, _Pseudomonas_, _Aeromonas_ or _Mycobacterium_. The correlation between the abundance of dissolved nutrients and the risk of hygienic problems is currently under-studied and needs to be addressed in future research. What seems clear, however, is that the current definition of biological stability needs to be clarified. The fact that drinking water comes with regrowth factors in the range reported here is hardly compatible with biological stability. An excellent basis for the concept of biological stability has been provided by [PERSON] et al. [66; 67] and [PERSON] et al. [23]. Some of the results presented here (e.g., the lack of biological stabilization by soil passage or slow sand filtration) show deviations from those of other studies. These differences might be partly explained by methodological differences. The classical AOC assay is based on the reproductive ability of _Pseudomonas fluorescens_ P-17 and _Spirillum_ NOX added to pasteurized samples, followed by the quantification of colony-forming units after 5-25 days [68; 69; 70]. The ability of these two strains to metabolize the complex pool of carbon compounds sometimes differs from that of more complex microbial communities [29]. Other assays use complex microbial communities (e.g., from a river or finished drinking water) that are seeded into the water samples after removing or killing the indigenous bacterial community by filtration (0.22 \(\upmu\)m) or pasteurization, followed by incubation at different temperatures and time periods and the quantification of bacteria by flow cytometry, turbidity or ATP measurement [21; 29; 50; 71; 72]. The evaluation of treatment performance on the basis of a complex microbial community was seen as advantageous [29]. In contrast, our approach did not include water pasteurization, filtration or seeding (the latter was only necessary in cases of certain ozonated waters) to avoid modification of the bioavailability of organics, and the lysis of biologically fixed nutrients or their removal. We also intentionally did not add a nitrogen or phosphorous source to limit the analysis to organic carbon. Instead, regrowth potentials were determined in unmodified samples using the autochthonous heterotrophic microbial community naturally contained in the corresponding water sample without prior laboratory treatment. The metabolic capacity of the microbial consortium can be seen to be well adapted to the nutrient spectrum contained in the specific sample. This approach allows for the assessment of both regrowth factors and the upper growth limits, each bearing important but distinct information.