## 3 Results
### Effect of Surface Water Treatment on Cell Numbers and Bacterial Regrowth Potentials
#### 3.1.2 Reservoir Water Treatment  
A water treatment plant treating reservoir water serves as the second example. Treatment consisted of flocculation followed by floc filtration in multilayer filters (MLFs) containing anthracite-N and quartz sand for the removal of turbidity, TOC-reduction and manganese removal. A second filtration over calcium carbonate served to increase the hardness. The filtrate of the second filtration step was subjected to disinfection with UV-light and chlorine dioxide prior to collection of the finished water in a drinking water tank before distribution (Figure 2A). We measured the microbiological parameters in raw water, the effluent of the second filter, the disinfected water and the finished water at the outlet of the drinking water tank.  
Figure 2: Changes in the cell numbers and regrowth potentials for the reservoir water treatment. (**A**) A treatment train overview including two filter steps and disinfection with UV and chlorine dioxide. (**B**) Changes in \(\text{TCC}_{\text{day 0}}\), \(\text{ICC}_{\text{day 0}}\), \(\text{ICC}_{\text{day 7}}\) and the percentage of HNA bacteria throughout the treatment. The dotted line serves as a reference for the \(\text{ICC}_{\text{day 7}}\) value of the raw reservoir water. The values are averages from three repeated measurements of the corresponding sample (technical repeats). (**C**) Changes in the relative proportions of biologically fixed nutrients and dissolved nutrients as a percentage of the total assimilable nutrient pool.  
The cell numbers in raw water at the time of sampling were 2.0 \(\times\) 10\({}^{6}\) (TCC\({}_{\text{day 0}}\)) and 1.4 \(\times\) 10\({}^{6}\) cells/mL (ICC\({}_{\text{day 0}}\)) (Figure 2B). Approximately 72% of cells were intact, which is within the typical range for surface water. The effluent of the second filtration step contained lower cell numbers (TCC: 4.8 \(\times\) 10\({}^{5}\), ICC: 2.6 \(\times\) 10\({}^{5}\)), of which 55% were intact. The maximal cell number (ICC\({}_{\text{day 7}}\)) dropped in the two filtration steps. Directly after the addition of ClO\({}_{2}\), the concentration of intact cells dropped to 1.4 \(\times\) 10\({}^{5}\) cells/mL, corresponding to 34% intact cells. The ICC values in water from the clean water tank dropped further to 3.7 \(\times\) 10\({}^{4}\) cells/mL (8% intact cells) due to the longer exposure time to the disinfectant. Treatment resulted in a reduction in the maximal cell number, with ICC\({}_{\text{day 7}}\) of the finished water being 2.9 \(\times\) 10\({}^{5}\) cells/per ml. Compared with the raw water (ICC\({}_{\text{day 7}}\): 1.3 \(\times\) 10\({}^{6}\)), treatment thus decreased the maximal cell number by 88%.  
When looking at the nutrient ratios, the nutrients were biologically fixed in the form of biomass in the raw water (Figure 2C). This proportion dropped to 51% after the second filtration, and the remaining nutrients were dissolved. Disinfection and concomitant killing of cells eventually resulted in a drop in biologically fixed nutrients to less than 13% in the finished water, and 87% of nutrients were dissolved. Overall, treatment thus led to an inversion of the two nutrient reservoirs.