## 2 Methods
### Sampling Sites and Sample Collection  
Water samples were collected in AOC-free vials (50 mL or 100 mL). The sampling sites were heat-disinfected with a flame following standard procedures. Depending on the sampling site, the first 200 mL to 1 l of water was discarded prior to sampling. The sample bottles for collecting water with chemical disinfect contained thiosulfate (final concentration 18 mg/L) for quenching. The samples were subsequently stored at 4 \({}^{\circ}\)C (\(\pm\)2 \({}^{\circ}\)C) and transported cold to the laboratory. The samples were either analyzed directly after receipt or at the latest within 24 h after sampling in cases of longer transport time. The data obtained from this first analysis are referred to as day 0, representing the microbiological status at the time point of sampling. The water works treating river water was sampled on 25 July 2018. Treatment included slow sand filtration (SSF), ground passage (GP) and ozonation followed by passage through multilayer filters and, subsequently, granular activated carbon (GAC) filters (Figure 1A). The filtrate was subject to UV disinfection prior to collecting the finished water in a clean water tank from which it entered the DWDS. No sampling site was available after slow sand filtration. The water works treating reservoir water was sampled on 3 September 2024. Treatment consisted of flocculation followed by floc filtration in multilayer filters (MLF) containing anthracite-N and quartz sand for the removal of turbidity, TOC-reduction and manganese removal (Figure 2A). A second filtration over calcium carbonate served to increase the hardness. The filtrate of the second filter was subjected to disinfection with UV-light followed by chlorine dioxide prior to the collection of the finished water in a drinking water tank before distribution. The water works treating canal water was sampled on 22 June 2022. Treatment included pH adjustment with H\({}_{2}\)SO\({}_{4}\), coagulation using polyaluminium chloride with subsequent flocculation, and clarification followed by rapid sand filtration (RSF) in a dual-layer filter (Figure 3A). The filtrate entered an open buffer basin, which fed a slow sand filter (SSF). The SSF was covered by a roof. The filtrate was further treated by GAC filtration, followed by UV disinfection and pH adjustment with NaOH prior to the addition of sodium hypochlorite. The water entered the DWDS via a pumping station. All water works were also sampled on other days and in different seasons. Although the absolute values could differ, the same trends in cell counts and regrowth potentials could be confirmed. Water samples from multifamily buildings were taken between March 2021 and January 2024 at the entry point of the corresponding building (point of entry, POE), typically before or directly after the water meter. The buildings were located in different geographical cities across Germany: Kiel, Hamburg, Neuruppin, Berlin, Hameln, Kassel and Mulheim an der Ruhr. These cities receive drinking water from different water works utilizing different raw waters (groundwater, enriched groundwater and river water). The corresponding water works were not related to the ones in the examples shown in Figures 1-3. Samples were typically taken in 100 mL AOC-free Schott bottles. Hot water samples from a single-family home plumbing system in Mulheim an der Ruhr were taken from a tap receiving water from a 5 L under-sink water heater (STIEBEL ELTRON GmbH & Co. KG, Holzminden, Germany). The heater was turned on only for the duration of the experiment. The first sample consisted of cold water collected after discarding approx. 50 L of water. Subsequently, the heater was set to different target temperatures with increasing heat: 40, 55, 65 and 80 \({}^{\circ}\)C. The actual temperature reached could differ slightly from the set value.