## 3 Results and Discussion
### Bacterial Inactivation  
#### 3.3.1 Testing Natural River Water for Bacterial Contamination  
Figure 7 shows the physicochemical and microbiological characteristics of water in the Bystrystia river. The pH values tend to increase slightly downstream (Figure 7a) and range from 8.20 to 8.65, which is generally acceptable for drinking water [27]. Increased water pH values may have natural or anthropogenic causes, i.e., leaching of alkali salts from the riverbed and surface runoff from agricultural areas. In turn, changes in the TDS parameter are likely caused by anthropogenic factors (Figure 7a). The highest TDS values occur at points 5-7, i.e., within and below the urban area, indicating human influence. A slight increase in TDS was also recorded near the smaller town of Yezupil (point 9 in Figures 2a and 7a). The  
Figure 6: The kinetics of H\({}_{2}\)O\({}_{2}\) decomposition in the flow reactor at different initial concentrations of H\({}_{2}\)O\({}_{2}\). **(a,b)** The changes in H\({}_{2}\)O\({}_{2}\) concentration over time. **(c)** The changes in H\({}_{2}\)O\({}_{2}\) concentration plotted on a logarithmic scale. WHO does not set any guidelines for TDS in drinking water [28]. Measured TDS values range from 112 to 166 ppm (Figure 7a) and are lower than the level of 600 ppm considered good for water taste. The color of the river water tends to increase, especially at points 5-9 (Figure 7b). The observed increase in water color may be caused by the leaching of humic acids and iron ions from the soil. An increase in iron concentrations was also observed (Figure 7b), which confirms the above explanation. The iron content increases to almost 0.3 mg/L, which the WHO considers a critical level [28]. The recorded iron concentrations are not dangerous, but indicate the need for further monitoring. Figure 7c shows the water hardness values corresponding to the total content of calcium and magnesium ions. At points 1-6, the river water has a hardness of approximately 2.4 mM. Further along the river, the hardness decreases to 1.9-2 mM (Figure 7c). This effect is caused by the inflow of another river (Bystrtytsia Solotvynska) between sampling points 6 and 7 (Figure 2a). Comparison of Figure 7a,c suggests that the TDS changes are not due to calcium and magnesium ions. The changes in TDS are probably caused by sodium ions of anthropogenic origin. Figure 7d shows the number of bacteria at the different sampling points indicated in Figure 2a. The number of _E. coli_ bacteria was determined using two different tests: the classic Endo agar test and the Compact Dry test (Section 2.5). The two tests were found to have different sensitivities, with the Endo agar test detecting approximately twice as many bacteria as the Compact Dry test (Figure 7d). On the other hand, the Compact Dry test has the advantage of providing information on both the _E. coli_ count and the total coliform count (Figure 2b). Typically, microbiological contamination of rivers varies significantly depending on the sampling location and time. For example, the _E. coli_ count in a small river in Germany ranges from 1 \(\times\) 10\({}^{3}\) to 4.7 \(\times\) 10\({}^{4}\) CFU/L [2]. High population density and a hot climate  
Figure 7: Characteristics of natural water taken from the Bystrtytsia river: (**a**) pH value and total dissolved salt, (**b**) water color and iron concentration, (**c**) total hardness, and (**d**) the number of bacteria. can cause very high bacterial loads. An example is the main river of Ecuador, where the number of _E. coli_ bacteria ranges from 5 \(\times\) 10\({}^{4}\) to 2.5 \(\times\) 10\({}^{5}\) CFU/L [1]. In the Bystrytsia Nadvirnianska river, the bacterial count at sampling points 1-3 (above the city) ranges from 240 to 500 CFU/L (Figures 2a and 7d). The reason for such low bacterial contamination is the fact that the river flows through rural areas. However, the river in urban areas has a much higher bacterial load.