## 3 Results and Discussion
### Other Indicators of Wastewater Treatment  
Microalgae had an impact on pH value fluctuations due to metabolic processes. Over time, in all samples, the pH values of the wastewater decreased from the initial values to the final values of pH = 6.6-7.7. These results are in agreement with some previous studies. It was reported that low pH (5.0) as well as high pH (9.0) strengthened the ammonia toxicity of _C. vulgaris_[21]. The researchers found that the maximum biomass concentration and the highest cell viability for cell growth was pH = 7.0. [PERSON] et al. (2019) also indicated that during their studies, the ammonium removal efficiency reached peak value at pH = 8.0. When the pH of wastewater was lower (<6.5) or higher (9.5), the ammonium removal efficiency decreased [19]. On three occasions, the replacement of a portion of wastewater with raw wastewater had an impact on the fluctuations of the pH value of the wastewater, and temporary increases and decreases in pH value were observed. In the samples of treated wastewater, the concentration of oxygen dissolved during the tests was monitored. The observed dissolved oxygen concentrations were a reflection of the ongoing photosynthesis process. A decrease in the content of dissolved oxygen in the wastewater was noted, from 9.3 mg/L to 11.8 mg/L to 4.8-5.8 mg/L. This was because the adaptation processes of the microalgae to the new environment (wastewater) occurred. The increase in the concentration of DO observed in the following days indicated the beginning of the active phase of assimilation of ammonium nitrogen. From the 30 th day, after replacing the last portion of wastewater, the concentration of dissolved oxygen began to decrease in all samples and reached its lowest values in the range of 3.4-6.2 mg/L whichwas unfavorable for microalgae culture and N-NH\({}_{4}\)+ binding. In the control sample (K), the trends in changes in the concentration of dissolved oxygen in the wastewater to the 28 th day were also observed. The concentration of dissolved oxygen varied from 8.8 mg/L on the first day to 6.3 mg/L on the 28 th day. Then, a decrease in dissolved oxygen concentration to 1.5 mg/L was observed, which was determined after 40 days. In the remaining samples (with the addition of sorbents), the concentration of dissolved oxygen in the treated wastewater was higher than in the control sample, and at the end, it was 3.4-6.4 mg/L. In wastewater samples treated with sorbents, the concentration of volatile fatty acids was determined (Figure 2). The initial concentration of volatile fatty acids in the wastewater was 158 mg CH\({}_{3}\)COOH/L. After the tests, the concentration of volatile fatty acids in the treated wastewater was lower. In the samples in which microalgae were used in the form of a suspension, the concentrations of volatile fatty acids were in the range of 69 to 79 mg CH\({}_{3}\)COOH/L for samples CvB and 51 mg CH\({}_{3}\)COOH/L for samples ScB. In the wastewater treated with both strains (CvA and ScA) of immobilized microalgae, the VFAs were higher at 96 to 103 mg CH\({}_{3}\)COOH/L. These results indicated that microalgae can use an external organic carbon source (e.g., the volatile fatty acids present in wastewater), which enables their growth in a wastewater with high turbidity. The carbon contained in volatile fatty acids is more easily absorbed when the microalgae are present in suspended form. This can be explained by the better access to the components dissolved in the wastewater. These results are in agreement with the achievements of other researchers that indicate that volatile fatty acids (VFAs), thanks their simple chemical structure, are a source of easily assimilable carbon for microorganisms. Therefore, they play a key role in biological wastewater treatment processes [24]. Figure 2: Volatile fatty acids after the experiment. The research showed that the use of clinoptilolite also reduced the concentration of volatile fatty acids in wastewater.