## 3 Results and Discussion
### Removal Efficiency of Micropollutants from Water
#### 3.5.2 Nutrient Removal  
The removal of excess nutrients, particularly nitrogen compounds (nitrate, nitrite, and ammonium) and phosphate ions, from wastewater is important to prevent eutrophication. Eutrophication is the excessive growth of algae and other aquatic plants that is fueled by these nutrients. This growth can lead to oxygen depletion, causing significant harm to  
Figure 4: COD removal efficiency by _R. mucilaginous_ CMGB-G1 (P1) and _R. mucilaginous_ CMGB 188 (P2).  
Figure 5: Yeasts biomass amount at the end of the experiment.  
aquatic life [71]. The removal of these nutrients from wastewater can be achieved through various methods, including active metal reduction, electrochemical catalytic reduction, chemical precipitation, adsorption, and biological treatment [31; 72; 73; 74; 75].  
In Figure 6, the results obtained for the determination of the reduction efficiency of nitrate, nitrite, ammonium, phosphate, and sulfate ions by the two yeast strains are presented based on the type of sample tested. The key factors that influenced the removal capacity of these ions were the yeast strain used and the type of heavy metal.  
Figure 6: Yeast nutrient removal efficiency at the end of the experiment of: (**a**) nitrate ions; (**b**) nitrite ions; (**c**) ammonium ions: (**d**) phosphate ions; (**e**) sulphate ions.  
For nitrate ions (Figure 6a), a reduction ranging from 80 to 95% was observed. Also, no clear trend was observed in the behavior of the two yeast strains in the presence of nitrate ions, each having a higher or lower removal efficiency depending on the type of wastewater tested. When the biosurfactant was absent, _R. mucilaginousa_ CMGB-G1 (P1) exhibited the highest reduction efficiencies (94-95%) for nitrate ions in the presence of Pb and Cd ions, compared to 87% in synthetic wastewater (_Sw_). _R. mucilaginousa_ CMGB188 (P2) also displayed a reduction efficiency ranging from 85% (_Pb_) to 94% (_Sw_). In the presence of the biosurfactant, notably lower efficiencies were observed, generally falling between 80 and 89%, with a peak of 94% for _R. mucilaginousa_ CMGB188 (P2) in the absence of metal ions. Regarding nitrite ions (Figure 6b), the greatest reduction of 89% was observed for the same yeast strain, but in the presence of synthetic wastewater contaminated with Cd\({}^{2+}\) ions. The strain of _R. mucilaginousa_ CMGB188 (P2) exhibited reduction efficiencies of up to 85% for synthetic wastewater and water containing _Bio Y_, as well as for water contaminated with Pb\({}^{2+}\) and Cd\({}^{2+}\). Similar behaviors were observed in the reduction of phosphate ions (Figure 6d) (between 43 and 66%, generally higher values for _R. mucilaginousa_ CMGB188 (P2) and sulfate ions (between 59 and 86% but for _R. mucilaginousa CMGB-G1_ (P1).  
A particular case was observed in the reduction of ammonium ions (Figure 6c), where the majority of tested samples exhibited an increase in their ion concentrations throughout the experiment. Reductions of up to 51% were observed in synthetic wastewater with Cd\({}^{2+}\) (_R. mucilaginousa_ CMGB-G1 (P1) and synthetic wastewater _Bio Y_ (_R. mucilaginousa_ CMGB188 (P2). Smaller reductions were noted for _R. mucilaginousa_ CMGB-G1 (P1) in the presence of Pb\({}^{2+}\) (29%) and in the presence of Pb\({}^{2+}\) and _Bio Y_ (5%). _R. mucilaginousa_ CMGB188 (P2) exhibited similar reductions, but in water contaminated with Cd\({}^{2+}\) and Pb\({}^{2+}\). All the other samples showed increases in ammonium ion concentrations ranging from +35% (_R. mucilaginousa_ CMGB-G1 (P1) in _Sw_) up to + 145 and + 168% for the rest of the samples. A possible explanation for the accumulation of ammonium ions could be that during the development of yeast strains, nitrogen is metabolized and transformed into ammonium ions, which correlates with an increase in conductivity (see Table 2) [60].