## 3 Results  
### Impact of Tap Water Quality on Yeast Biomass Yield and Lipid Content  
The analysis compared the physicochemical parameters of redistilled water (Water C, control) and tap water samples from three districts in Warsaw: Praga-Pohudnie (Water P), Bielany (Water B), and Ursynow (Water U) (Table 1). Significant differences were observed in conductivity, hardness, and the concentrations of key ions. Water C exhibited the lowest conductivity (3.0 \(\upmu\)S/cm) and negligible concentrations of dissolved substances, confirming its purity. In contrast, tap water samples displayed significantly higher conductivity, with the highest value recorded in Water P (1021 \(\upmu\)S/cm). This suggests greater mineral content. The hardness of the tap water varied considerably, with the highest value in Water U (400 mg/L) and the lowest in Water B (230 mg/L). This variation is linked to the geological composition of the water sources, with Water U likely containing a higher proportion of groundwater, rich in calcium and magnesium. Chloride concentrations followed a similar trend, with Water P showing the highest value (186 mg/L), indicating the influence of water disinfection and distribution system characteristics. Iron and manganese levels remained low in most samples, except for Water U, which exhibited increased concentrations (18 \(\upmu\)g/L Fe, 10 \(\upmu\)g/L Mn). This suggests possible leaching from older pipes or a higher contribution of deep groundwater. Sulfates were also highest in Water U (162 mg/L), further supporting the hypothesis of groundwater influence. Nitrate levels were relatively low, with the highest concentrations detected in Water P (5.41 mg/L) and Water B (5.31 mg/L), reflecting possible surface water contamination. In contrast, Water U contained significantly lower nitrate concentrations (0.71 mg/L). The observed variations are attributed to differences in water sources, treatment processes, and local distribution system conditions [32; 33; 34]. Tap water from the left bank of the Vistula (Water P) showed higher mineralization, likely due to its origin from river infiltration. In contrast, water from the right bank (Water B and Water U) exhibited distinct compositions, with Water U's characteristics resembling water from bad pipe conditions. These results highlight the impact of geological factors, treatment methods, and infrastructure on the final composition of tap water used to prepare culture media for yeast. To confirm the safety of the water used in this study, the values obtained were compared with the parameters specified in the Regulation of the Minister of Health of 7 December 2017 on the quality of water intended for human consumption [Dz.U. 2017 poz. 2294]. All the values obtained in the laboratory tests were within the permissible limits specified in this regulation, which confirms that the water used in the study was safe [35]. The results presented in Figure 1 show the biomass yield of yeast grown in a shaken flask culture in a nitrogen-limited medium with a waste carbon source prepared with tap water from different water supply sources (Water P, B, and U) and control redistilled water (C). Statistical analysis did not reveal any significant differences among the various water sources, suggesting that the different water supplies did not significantly impact yeast biomass growth in flask-shaken cultures. It was also confirmed that the _Y. lipolytica_ yeast could be cultured in untreated tap water, which is beneficial from the point of view of commercial large-scale production. In a study by [PERSON] et al. (2019) [36] on the effect of seawater on biomass and microbial oil production, it was observed that in a medium containing seawater, the _Y. lipolytica_ strain produced significantly more biomass than in a freshwater medium (36.4 and 48 g/L, respectively), which could be related to the fact that higher osmotic stress redirected the carbon flux from glycerol to biomass instead of to lipids.