## 3 Results
### Effect of Surface Water Treatment on Cell Numbers and Bacterial Regrowth Potentials
#### 3.1.3 Canal Water Treatment  
Next, we looked into microbiological changes within a water works processing canal water as raw water. The canal water was diverted from a major canal and subsequently passed through a reservoir before entering the treatment works. An overview of the subsequent treatment is shown in Figure 3A. The cell numbers from the water intake at the canal through the water reservoir to the raw water inlet at the treatment works remained similar (Figure 3B). Water at the inlet contained 4.2 \(\times\) 10\({}^{6}\) total cells/mL and 2.7 \(\times\) 10\({}^{6}\) intact cells/mL. Seven days of incubation at 22 \({}^{\circ}\)C resulted in little change in the ICC levels, with low regrowth factors, suggesting biological stability. The first two treatment steps (coagulation/flocculation/clarification and rapid sand filtration) resulted in a decrease in day 0 cell numbers but did not considerably affect the regrowth potential. The residence in the open buffer reservoir, on the other hand, led to an interim increase in cell numbers. As the ICC\({}_{\text{day 7}}\) value was slightly below the ICC\({}_{\text{day 0}}\) value (resulting in negative regrowth), the water remained biologically stable. Surprisingly, the roof-covered slow sand filter had a strong impact on cell numbers, which dropped to 1.4 \(\times\) 10\({}^{5}\) cells/mL (TCC) and 1.2 \(\times\) 10\({}^{5}\) cells/mL (ICC) (Figure 2B). The ICC\({}_{\text{day 7}}\), at the same time, increased strongly to 7.0 \(\times\) 10\({}^{6}\) cells/mL, suggesting a substantial loss of biological stability with a regrowth factor of 56.4. GAC passage, in contrast, increased day 0 cell numbers and lowered the maximal cell number, but did not lead to complete biological stabilization (regrowth factor: 9). Whereas UV disinfection had no impact on cell numbers, the addition of hypochlorite (added to the drinking water tank) resulted in a strong decrease to 8.2 \(\times\) 10\({}^{4}\) cells/mL (TCC) and 2.3 \(\times\) 10\({}^{4}\) cells/mL (ICC), as measured in the outflow of the pumping station. Assuming plug flow conditions and flow rates between 2580 and 4050 m\({}^{3}\)/h, the residence time between hypochlorite addition and the pumping station was in the range between 345 and 541 min. The residual chlorine concentrations measured at the pumping stations were 0.25 mg/L (total chlorine) and 0.18 mg/L (free chlorine). The fact that chlorination also affected TCC values and not only ICC suggests that the disinfection was sufficiently strong to cause cellular disintegration and not only damage. The strong disinfection was probably also responsible for the increase in maximal cell numbers to 8.7 \(\times\) 10\({}^{6}\) intact cells/mL, representing an increase by a factor of 3.3 compared to the raw water intake. The regrowth factor reached a maximum of 378 at this point. Figure 3: Changes in the cell numbers and regrowth potentials for the canal water treatment. (**A**) A treatment train overview including coagulation/flocculation/clarification, rapid sand filtration (RSF), storage in an open buffer basin, slow sand filtration (SSF), GAC filtration and disinfection with UV, and hypochlorite addition to the drinking water tank. The water entered the DWDS via a pumping station. The DWDS was supplied with water from two additional DWTPs. (**B**) Changes in TCC\({}_{\text{day}}\) 0, ICC\({}_{\text{day}}\) 0, ICC\({}_{\text{day}}\) 7 and colony counts in cfu/mL throughout the treatments. The dotted line serves as a reference for the ICC\({}_{\text{day}}\) 7 value of the raw water intake. 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. Changes in cell numbers did not show much correlation with colony counts at 22 \({}^{\circ}\)C, which showed a decrease throughout treatment, with the exception of the open water basin.