## 3 Results
### Repeated Fed-Batch Culture Cultivation Techniques of Microalgae  
Figure 4 presents changes in oxygen concentration during the cultivation of _C. vulgaris_ and _S. armatus_ in wastewater. The observed concentrations of dissolved oxygen reflected the photosynthesis process. During the first 18 days, there was a decrease in the dissolved oxygen content in wastewater, from the initial value ranging from 8.8 mg/L to 11.9 mg/L to the final values of 4.9-6.3 mg/L. This is related to the process of microalgae adaptation to the new wastewater environment. The increase in the dissolved oxygen concentration observed from day 24, indicated the beginning of the logarithmic growth phase. Figure 4: Oxygen concentration values observed during the repeated fed-batch culture cultivation of microalgae. The lines indicate the days when the culture was fed with a new dose of wastewater. From the thirtieth day after replacing the last portion of wastewater the dissolved oxygen concentration began to decrease in all samples, reaching the lowest values in the range of 3.3-6.3 mg/L observed in the last days. This was unfavourable for microalgae breeding. In the control sample (C), changes in the concentration of oxygen dissolved in wastewater from the beginning to the 28 th day were similar to those observed in other samples. The concentration changed from 8.8 mg/L on the first day to 3.0 mg/L on the last day. Figure 5 presents changes in chlorophyll-a concentration during cultivation of _C. vulgaris_ and _S. armatus_ in wastewater. The initial concentration of chlorophyll-a in wastewater at the first day was ca. 5.0 mg/L (Figure 5). In the following days, a decrease was observed in the concentration of chlorophyll-a that lasted, depending on the sample, to the eighth or twelfth day. This could be due to the adaptation of microalgae to new environmental conditions. The fluctuations of chlorophyll-a concentration were observed in all samples (every ten days). In samples with microalgae suspended in solution, regardless of the dose of microalgae, chlorophyll-a production was higher than in wastewater with the addition of microalgae immobilized on alginate beads. This can be explained by the fact that the immobilized microalgae had a limited access to light, which is a key factor in the production of chlorophyll-a. After replacing the wastewater, the highest concentration of chlorophyll-a reached 4.8 mg/L on the 24 th day in the sample with _C. vulgaris_ suspended in the solution. In the sample with _S. armatus_ microalgae suspended in the solution, and the highest concentra  
Figure 5: Chlorophyll-a concentration observed during the repeated fed-batch culture cultivation of microalgae. The lines indicate the days when the culture was fed with a new dose of wastewater. tion of chlorophyll-a was 4.9 mg/L, also on the 24 th day. In the wastewater contained immobilized microalgae, the highest concentration of chlorophyll-a was at the level of 2.6 mg/L after 28 days in samples with _C. vulgaris_ and 1.3 mg/L in samples with _S. armatus_. In the last days of the experiment, chlorophyll-a concentration in the samples with _C. vulgaris_ immobilized ranged from 1.2 to 2.5 mg/L, whereas in the samples with _S. armatus_ it ranged from 0.7 to 1.0 mg/L. In the samples of _C. vulgaris_ microalgae in a suspended form, the concentration of chlorophyll-a was from 2.8 to 4.4 mg/L, whereas for _S. armatus_ this was in the range from 3.5 to 4.2 mg/L. The growth phases of microalgae from _C. vulgaris_ and _S. armatus_ showed similar trends throughout the range studied. Figure 6 presents changes in pH values with time during cultivation of _C. vulgaris_ and _S. armatus_. The analysis of the data presented in Figure 6 indicated that over time the pH value of wastewater decreased in all samples, from the initial values in the range of pH = 7.8-8.4 to the final values of pH ranging from 6.7 to 7.6.