## 4 Discussion
### Regrowth Potential in Drinking Water Installations and Effect of Water Heating  
Even if regrowth potentials are not fully met in the distribution system, this is likely the case in the DWIs, where water can stagnate in pipes with high surface area-to-volume ratios for extended periods and can reach substantially higher temperatures compared to the distribution network [18; 19]. In buildings receiving drinking water with residual disinfectants, disinfect decay is nearly unavoidable [57; 58]. Plumbing system characteristics and stagnation under these conditions can lead to substantial increases in cell numbers and alterations in the drinking water microbiome [19; 59; 60]. Stagnation times peak during apartments vacancy or holidays. Water sampled here was collected at the house entrance of multifamily buildings and, thus, largely excluded these factors. Also, the effect of plastic materials within the plumbing system, which can further increase regrowth potentials [61], is not reflected in the data from the house entrance, which, therefore, does not represent the \"worst case\" scenario. Nevertheless, cold water samples at the house entrance exhibited maximal intact cell numbers, exceeding the ICCday0 levels by factors ranging from 6 to 579. The impact of regrowth potentials on biofilm formation on pipe surfaces and overall water hygiene remains unknown but warrants investigation. Within the DWI, a portion of the water undergoes the production of PWH. Prolonged heating of water to lethal temperatures (common in storage boilers) results in biologically unstable water, evidenced by strong regrowth after cooling. The D values (meaning the time required to achieve a one-log reduction in colony-forming units) for waterborne indicator bacteria at 65 \({}^{\circ}\)C were reported to be only a few seconds [62]. The release of intracellular contents from thermally lysed or damaged microorganisms can promote necrotic growth [63]. Also, the thermal degradation of TOC into more bioavailable forms in hot water boilers has recently been discussed as an additional source of AOC [22]. Complex organic molecules may be thermally degraded, becoming available for microbial assimilation. Both nutrient release and new AOC formation contribute to strong (re)growth, with the effects more pronounced at higher temperatures and at extended heat exposure times. Lethal treatments effectively lead to a \"microbiological reset\", as vacated biological niches can be subsequently colonized by new organisms. It has been reported before that PWH contains a different bacterial community from that in PWC prior to heating [22; 64]. Heating water to high temperatures thus adds to treatment steps like chlorination or ozonation (and other oxidative treatment steps) leading to a loss of biological stability in the sense of increasing regrowth factors. The temperature to which PWC is heated has a direct impact on the subsequent bacterial regrowth if not consumed immediately. The higher the temperature, the greater the reduction in ICC levels and the higher the regrowth factors. In typical scenarios, regrowth occurs when hot water reaches peripheral water pipes in the installation system. Once hot water consumption ceases, the remaining water cools down and stagnates for extended periods of time. The water is, at the same time, seeded with bacteria present at the pipe surfaces [64]. The dissolved nutrients contained in the bulk water can subsequently be metabolized, generating new biomass. The increased AOC produced by heating has been reported to promote biofilm formation and to elevate the risk of Legionella manifestation [65]. In cases of appliances with plastic components, the nutrients contained in the bulk water add to those leaching from materials (e.g., EPDM in pipe seals or shower hoses). Regrowth in initially heated drinking water with disinfectant is facilitated by faster disinfectant decay in hot water compared to cold water [57; 58].