## 3 Results
### Effect of Surface Water Treatment on Cell Numbers and Bacterial Regrowth Potentials  
#### 3.1.1 River Water Treatment  
A DWTP treating river water serves as the first example. Treatment comprised slow sand filtration (SSF), ground passage (GP) and ozonation followed by water passage through multilayer filters (MLFs) with anthracite and quartz sand and GAC filters (Figure 1A). The filtered water was subsequently subjected to UV disinfection prior to storage in a drinking water tank from where it was fed into the distribution system. Three sampling points within the distribution system were included in this study. The cell numbers in the river water on the day of sampling (day 0) were 4.0 \(\times\) 10\({}^{6}\) total cells per ml (TCC) and 2.6 \(\times\) 10\({}^{6}\) intact cells per ml (ICC) (Figure 1B). Similarly to other surface waters, the ICCday7 value of the river water was lower than the ICCday0 value due to the die-off of phototrophic bacteria and the establishment of a different microbial community during sample incubation. This corresponds to an ICC\({}_{\text{day\,7}}\)/ICC\({}_{\text{day\,0}}\) regrowth factor of 0.4. The combination of slow sand filtration and ground passage led to a decrease of more than one log-scale in day 0 cell numbers, whereas the maximal cell numbers after regrowth (ICC\({}_{\text{day\,7}}\)) remained comparable to that of the river water. The regrowth factor increased to 4. Ozonation, as expected, resulted in a strong decrease in ICC. The reduction in TCC was due to the destruction of bacteria through ozone, which exceeds the mere loss of membrane integrity. The maximal cell numbers (ICC\({}_{\text{day\,7}}\)) increased as a result of the ozonation converting TOC partially into AOC. The regrowth factor reached a maximum of 353. The subsequent increase in cell numbers in MLF and GAC filtrates reflected the beginning of biological stabilization. Surprisingly, the ICC\({}_{\text{day\,7}}\) values also continued to increase, indicating that both filters introduced nutrients instead of removing them. However, overall, the regrowth factors gradually decreased. The next treatment step consisted of UV disinfection prior to the collection of the finished drinking water in a storage tank from where it entered the distribution network. The day 0 cell numbers of the drinking water remained constant after UV disinfection, storage and distribution. The effect of UV on cell viability could not be captured, as photochemical disinfection does not impact cell integrity. The ICC\({}_{\text{day\,7}}\) value showed a slight decrease in the finished water (\(1.4\times 10^{6}\) cells/mL), possibly caused by the turnover of dissolved nutrients during water storage in the clean water tank. Compared with the raw water (ICC\({}_{\text{day\,7}}\): \(1.0\times 10^{6}\)), however, the treatment resulted in an overall increase in maximal cell numbers by 37.5%. Water was subsequently distributed and did not contain a disinfect residual. The samples in the distribution showed stable ICC\({}_{\text{day\,0}}\) values but differences in maximal cell numbers at the various sampling points. These variations were also seen in later sampling events. The water at different sampling points in the DWDS showed regrowth factors between 5 and 29 compared to 9 for the finished water. The changes in the relative proportions of HNA bacteria reflect the changes in the bacterial community composition (Figure 1B). The combined effect of slow sand filtration and ground passage resulted in a sharp drop in HNA bacteria, making this microbiological parameter more similar to groundwater, where LNA bacteria typically dominate. The high proportion of HNA bacteria in the ozonated water should not be overinterpreted, as the ratio is based on very few intact cells. Passage over the MLF and GAC filters was accompanied by significant changes in the HNA proportion, indicating that the water was enriched by bacteria specifically populating these filters.