## 3 Results
### Impact of Tap Water Quality on Yeast Biomass Yield and Lipid Content
At a similar time to the culture in the distilled water-based medium, the oxygenation level of the medium dropped sharply, signalling the entry of the cells into the logarithmic phase of growth. After 30 h, a reduced oxygen demand was observed by a decrease in stirrer speed. It was not supplemented additionally, as an accumulation of unused substrate, impaired biomass separation, and problems with excessive foaming of the culture were observed in the first culture. Figure 1: Biomass yield after 96 h yeast culture in medium with waste carbon source, where medium ingredients were dissolved in water: Water C—control redistilled water; Water P, B and U—tap water collected at different water intakes (\(p<0.05\)). Due to the short duration of the 20 h culture (Figure 2c), a batch mode was used without introducing carbon sources. The degree of oxygenation of the medium decreased rapidly, as in the previous variants. Still, it remained at the appropriate assumed level of 30% until the end of the culture, when the stirrer speed reached the maximum set value of 600 rpm. The cells did not enter the stationary growth phase as in the previous two variants. The observed oxygenation changes were due to the yeast cells utilizing the oxygen present in the medium. In more extensive cultures, due to the intensive multiplication of  
Figure 2: Changes in stirrer speed and oxygenation level of the culture medium during yeast culture in the bioreactor carried out in the medium prepared based on distilled water (**a**), in a tap water-based medium (**b**) and during a 20 h yeast culture in a bioreactor conducted in a tap water-based medium (**c**). y yeast cells, it was necessary to add oil during the culture, i.e., an additional portion of the carbon source. For each culture cycle, changes in the biomass content of the culture medium were observed. In a study by [PERSON] et al. (2021) [37], yeast cultures of _Y. lipolytica_ in bioreactors showed a significant decrease in dissolved oxygen during the early logarithmic phase of growth, leading to an increase in stirrer speed until reaching 600 rpm after 21 h, with oxygenation remaining around 5%. Similarly, the experiments showed a sharp decrease in oxygenation with increased agitator speed, but substrate oxygenation remained at around 30% [37]. [PERSON] et al. (2022) [38] observed a similar trend, noting a sharp decrease in substrate oxygenation after 12 h, which varied depending on the carbon sources used (glucose and olive oil), highlighting the significant oxygen requirement of yeast for growth. The cited experiments presented a comparable trend to our study, indicating that, irrespective of the carbon source and medium, _Y. lipolytica_ yeast cells require significant amounts of oxygen for cell growth [38]. In a study by [PERSON] et al. (2018), it was also shown that the level of oxygenation affects lipid synthesis, cell morphology and physical properties, and the maximum lipid content was recorded at 30-35% saturation (44.8%), while higher aeration seemed to reduce the lipid content and promote biodegradation of the accumulated SCO [39]. As demonstrated in Figure 3, the pH of the medium during the cultivation of _Y. lipolytica_ yeast remained consistent across all experiments. Initially, the pH level remained stable for approximately 14 to 16 h, followed by a rapid decline, reaching its minimum around 22 h in cultures lasting 65 h. After this point, the pH level remained at a similar level for the remainder of the culture. In the experiment conducted in a distilled water-based medium, the pH level was higher at the beginning of the experiment; however, after 20 h, it had decreased to a level that was lower than that of the variant with tap water. The initial pH of the medium for each of the three cultures was set at 5, and the final pH in the 65 h cultures in the experiment with medium prepared with distilled water was within 2, while in the tap water it was within 3. Following the completion of the 20 h culture, the culture was concluded when the pH level reached approximately 4.