## 3 Results and Discussion
### Removal Efficiency of Micropollutants from Water  
3.5.1. Chemical Oxygen Demand (COD)  
COD is a measure of the amount of oxygen required to break down organic matter in water and is an important parameter in water quality analysis because it can be a measure of the wastewater treatment efficiency [35].  
The results obtained in this study (Figure 4) show a significant reduction in COD by up to 86% compared to the reference (508 \(\pm\) 9.6 mg O\({}_{2}\)/L) value. It is important to emphasize that the synthetic wastewater samples containing a biosurfactant showed COD values comparable to or even higher than the synthetic wastewater without Pb\({}^{2+}\) and Cd\({}^{2+}\) ions. In the presence of metal ions, without biosurfactants, COD reduction ranged from 42% for Cd\({}^{2+}\) to 57% for Pb\({}^{2+}\). It is interesting to note that _R. mucilaginous_CMGB 188 (P2) showed, on average, a higher COD reduction capacity than _R. mucilaginous_CMGB-G1 (P1), considering all types of samples tested. Moreover, the results obtained in this study are comparable with a previous study [31] conducted by our group, where the efficiency of removing COD from synthetic wastewater was tested using five different yeast strains: _Kluyveromyces marxianus_ CMGBP16, _Saccharomyces cerevisiae_ S228C, _Saccharomyces cerevisiae_ CM6B70, _Saccharomyces cerevisiae_ CMGB234, and _Pichia anomala_ CMGB88. The results showed a COD removal efficiency of up to 70%. In a different investigation, a reduction in COD ranging from 50% (_Candida krusei_) to 71.6% (_Candida albicans_) was observed when using a 1% (\(v/v\)) inoculum rate, but after 25 days of the experiment [70].  
The capacity for COD removal appears to be affected by the quantity of yeast biomass obtained at the end of the 120 h experiment (Figure 5). This is contingent upon the experimental conditions, including the presence or absence of heavy metal ions and the biosurfactant in the water sample. By analyzing Figure 5, it is evident that changing the biomass amount leads to a similar behavior as in the COD analysis. Consequently, the maximum biomass (66-76 mg/L) was observed in the case of synthetic wastewater with biosurfactants. It is noteworthy to mention that slightly elevated values of the biomass amount were obtained in the case of _R. mucilaginous_ CMGB-G1 (P1).  
In wastewater samples without biosurfactants, the amount of biomass decreased from ca. 70 mg/L (synthetic wastewater (_Sw_)) to 52 mg/L (_R. mucilaginous_ CMGB188 (P2)) and 58 mg/L (_R. mucilaginous_ CMGB-G1(P1)) in the presence of Pb\({}^{2+}\), respectively, and to 42 mg/L (_R. mucilaginous_ CMGB-G1 (P1)) and 44 mg/L (_R. mucilaginous_ CMGB188(P2)) in the presence of Cd\({}^{2+}\), respectively. In the combined presence of metal ions and biosurfactants in wastewater, the highest amount of biomass of about 62 mg/L was obtained in the presence of Pb\({}^{2+}\) for the _R. mucilaginous_ CMGB-G1 (P1) strain, followed by 58 mg/L for the _R. mucilaginous_ CMGB188 (P2) strain. The lowest values in the presence of biosurfactants were obtained for Cd\({}^{2+}\) (between 47 and 52 mg/L), and the highest values were obtained for the _R. mucilaginous_ CMGB188 (P2) strain.