## 4 Discussion
### Effect of Water Treatment  
Different treatment steps have different effects on biological stability. Ozonation was one of the treatment steps in this study that led to substantially increased regrowth potentials. It is well known that oxidative water treatment results in highly unstable water [31, 32, 33]. Part of the reason is the increased availability of assimilable nutrients due to a modified substrate composition [23, 34]. The majority (60-80%) of the AOC newly formed by ozonation was shown to comprise organic acids [35]. Also, phytoplankton contained in surface water can contribute substantially to the organic carbon load when subjected to ozonation [36]. The increase in AOC is thereby dependent on ozone dose and contact time [37]. Another reason for the instability is the lack of microbial competition [23]. Oxidation opens up ecological niches previously occupied by the bacteria that have been killed or damaged by the treatment, resulting in a \"microbiological\" reset. Among oxidative treatments, ozonation is seen as having the strongest impact due to its high reactivity [34]. Ozonation is typically followed by biological filtration to remove excess AOC [3]. Biological filters are sites of intense metabolic transformations and induce changes in the microbiome [38,39]. Biologically active GAC filters play a dominant role. In a study assessing the effect of GAC filters in a pilot plant treating surface water, different GAC filters receiving water treated with different ozone doses were reported to remove up to 70% of the AOC newly produced by ozone [40]. However, in none of the filters the overall AOC was reduced to concentrations given prior to ozonation. Without prior ozonation, the same authors reported that one GAC filter even produced AOC. This effect, whereby GAC filtration could raise the AOC of treated surface water, has also been described by [PERSON] et al. [41]. In other words, GAC filters in these examples increased the regrowth potential of the water. A Japanese study reported little or no AOC removal in an ozonation-biological activated carbon process [42]. In our study, a GAC-mediated reduction in maximal intact cell numbers was seen for the canal water (Figure 3), whereas for the river water, both the multilayer and the GAC filter led to an increase in maximal cell numbers (Figure 1). These examples show that the effects of GAC filters vary, and filtration does not necessarily always lead to an overall reduction in assimilable nutrients. The reason probably lies in the fact that GAC filters are primarily operated to adsorb trace substances (pesticides, pharmaceuticals, etc.). To maintain a high adsorption capacity, charcoal is frequently exchanged, while a higher filter age in combination with optimized backwashing would be required for the microbiology to establish and to reach stability and maturity while, at the same time, removing excess nutrients. A surprising finding was the increase in regrowth potential by the slow sand filter in canal water treatment (Figure 3). The increase in maximal cell numbers (ICC\({}_{\text{day}}\)) was accompanied by a substantial decrease in ICC\({}_{\text{day}}\), resulting in a high regrowth factor. Slow sand filtration provides treatment through the physical filtration of particles, biological turnover of biomass and organics in the upper layer and intermittent removal of the latter. AOC removal efficiencies between 5 and 40% have been reported [43]. SSF has also been shown to reduce the biomass production potential (BPP) based on measurements of cumulative ATP concentrations and, therefore, metabolically active biomass in a batch growth test [44]. In the current case, the reason for the lack of biological stabilization by the slow sand filter was seen in the mode of operation: frequent mechanical disturbance of the upper filter layer and insufficient removal of the upper layer (\"Schmutzdecke\").