## 4 Discussion
### Regrowth in Distribution Systems  
The distribution system studied here distributed water with hypochlorite as a secondary disinfectant. The cell numbers increased with water age. Strong seasonality was observed, with substantially higher ICC levels and higher relative proportions of intact cells in summer than in winter. This seasonal variation is primarily attributed to faster regrowth kinetics and higher chlorine demand in months with higher water temperatures, whereas in the colder months, the chlorine residuals persisted longer, causing a stronger disinfection effect. It is important to note that this biological effect was measurable even when chlorine levels were below the detection threshold using the DPD method. These observations align with earlier research. [PERSON] et al. [51] linked higher heterotrophic plate counts to lower levels of chlorine residuals and extended water retention times in a French distribution system. Similarly, [12] attributed increases in cell numbers (quantified by flow cytometry) in a chloramphenicol Swedish DWDS to distance from the DWTP, longer residence times and increased contact time of bulk water with pipe biofilm. Elevated water temperatures in summer led to higher TCC (1.51-5.24-fold increase) in some locations compared to colder months. And also, applying flow cytometric quantification, [52] reported increasing cell numbers with prolonged water residence times in the chlorinated DWDS of Riga, Latvia. The bacterial growth was ascribed to the loss of disinfectant residuals with prolonged retention times. A later study of the same DWDS described seasonal variations, with the ICC counts in winter being at least one log lower than in summer [53], consistent with the data presented here. These variations were attributed to changes in water temperature, with a direct effect on bacterial growth, but the extent varied depending on the water source and underlying treatment. Bacterial dynamics in the periphery of a large chlorinated DWDS was also corroborated in a recent study reporting shorter bacterial community turnover times and, therefore, reduced biological stability following the depletion of the disinfection residual for the sampling site with the greatest distance from the treatment plant [9]. Also, in non-chlorinated systems, temperature plays a pivotal role; for instance, a major German city experienced a strong increase in the cell count of heterotrophic bacteria at the end of a very hot summer with water temperatures up to 25 \({}^{\circ}\)C [8]. The latter example shows that the absence of secondary chlorination does not ensure biological stability. Higher temperatures have been reported to shorten growth lag phases and enhance growth rates and maximal cell numbers [54]. Different bacterial genera respond differently to temperature elevations [54]. Temperature likely plays a crucial role in increasing the probability of elevated cell numbers in summer, despite higher AOC concentrations typically found in winter in raw surface waters [42; 55]. This contrasts with higher AOC removal efficiencies observed for biological filtration processes like GAC or SSF at higher temperatures [44]. Overall, bacterial growth in the DWDS appears to be more influenced by temperature than by AOC [23]. However, bacterial growth can only occur if a nutrient basis is available. In addition to seasonal variations in actual cell numbers (ICCday0), profound differences in ICCday7 were observed between sampling months, with the highest concentrations occurring in summer. The reasons for these seasonal fluctuations in maximal growth potential remain unclear, but could relate to variations in organic compounds in raw water (canal water) and/or more efficient TOC conversion into AOC due to higher chlorine reactivity in summer. The use of secondary disinfection has been reported to significantly alter not only the community composition, but also the nutrient pool [23].