## 3 Results
### Microbiological Changes During Distribution of Chlorinated Water  
Drinking water distributed from the \"canal water treatment plant\" was monitored for changes in cell numbers and regrowth potential. The DWDS was fed not only by the treatment plant shown in Figure 3A, but also by three other DWTPs that supply drinking water with disinfectant residual to a large European city. Samples were taken in June, September and December 2022. In total, 31 sampling points were included and sorted based on increasing distance from the DWTP (Figure 4). These samples represent water with increasing residence time, with the maximum water age reaching approx. 100 h before leaving the DWDS and entering individual plumbing systems. The first seven samples (1-7) were taken from different pumping stations, and samples 8 and 9 from transport pipes. The average water temperatures at the time of sampling were 20.6 \({}^{\circ}\)C (lowest: 17.9 \({}^{\circ}\)C, highest: 22.6 \({}^{\circ}\)C) in June, 18.2 \({}^{\circ}\)C (lowest: 15.6 \({}^{\circ}\)C, highest: 20.0 \({}^{\circ}\)C) in September and 9.0 \({}^{\circ}\)C (lowest: 6.6 \({}^{\circ}\)C, highest: 10.7 \({}^{\circ}\)C) in December. The \(\text{ICC}_{\text{day 0}}\) values tended to increase with increasing residence time (Figure 4A). The highest intact cell numbers were obtained in June, increasing from approx. 10\({}^{4}\) cells/mL (proximal sampling sites) to 10\({}^{5}\) cells/mL (distal sampling sites). In September and December, the cell numbers also increased throughout the distribution, but on a lower level (trendlines are indicated in Figure 4A). Additionally, the maximal intact cell numbers (\(\text{ICC}_{\text{day 7}}\)) were subject to seasonal variation on the three different sampling dates. In June, the maximal cell numbers were highest and showed less variation throughout the distribution, whereas in September and December, the maximal cell numbers tended to increase from a lower level. In the September and December sampling rounds, samples 4 and 5 (from pumping stations early in the distribution system) did not show much regrowth, likely due to extensive damage inflicted on the cells by chlorine. This phenomenon did not occur in samples later in the distribution system, as the chlorine effect was probably not as pronounced. The corresponding ICC regrowth factors (the ratios of \(\text{ICC}_{\text{day 7}}\)/\(\text{ICC}_{\text{day 0}}\)) along the entire DWDS showed great variation, ranging from factors of 3 to 3241. In other words, the concentrations of intact bacteria could increase by these factors after the quenching of chlorine and seven days of incubation. When looking at the relative ratios of intact and membrane-damaged cells, the proportion of intact cells increased in June with increasing residence time (Figure 4B). In December, high proportions of damaged cells were maintained throughout the distribution, probably due to less pronounced chlorine decay at lower water temperatures. The ratios of intact and damaged cells in September showed an intermediate profile. Figure 4: Changes in intact cell numbers, regrowth potentials, relative proportions of intact cells and proportions of dissolved and biologically fixed nutrients along a network distributing chlorinated  
drinking water for three selected months (June, September and December). Samples were chosen to reflect water with increasing distance from the DWTP and increasing water age. (**A**) Changes in ICC\({}_{\text{day}\,0}\) and ICC\({}_{\text{day}\,7}\) along the DWDS together with exponential trendlines. Green and red shading was used for visual simplification with most data points in the shaded areas. The corresponding regrowth factors are shown underneath. The values are averages from three repeated measurements of the corresponding sample (technical repeats). (**B**) Changes in the relative ratios of membrane-intact and membrane-damaged cells throughout the distribution.