## 4 Discussion
LX1 depends on the form of nitrogen and decreases according to the formula of NH\({}_{4}\)+-N\(>\) urea\(>\) NO\({}_{3}\)-N. Researchers observed that when nitrate (V) or urea was in the environment, there was a steady increase in microalgae biomass whereas nitrogen was removed in 90% and phosphorus in 100%. When the source of nitrogen was ammonium ions, the growth of microalgae biomass was lower due to release of H\({}^{+}\) ions, which acidified the environment and then only 31.3% of nitrogen and 76.4% of phosphorus were removed [25]. [PERSON] et al. [26] observed that when ammonia is the only source of nitrogen in the environment it causes a rapid decrease in pH, which in turn adversely affects the growth of microalgae. The authors observed that the concentration of ammonia of 330 mg/L caused inhibition of approx. 50% of _C. vulgaris_ cells. The growth of _C. vulgaris_ was inhibited at pH = 8-9, or at a concentration of ammonia of 700 mg/L. These studies proved that the concentration of ammonia and the pH of the environment are the most important factors affecting the growth of microalgae [26]. Numerous authors have shown that different species of microalgae react differently to the nitrogen content in the environment. [PERSON] et al. [27] investigated the growth of _S. dimorplus_ in the presence of nitrogen compounds, where they obtained biomass concentration values ranging from 0.1116 g/L to 0.920 g/L, revealing a stimulating influence of nitrogen compounds on the growth of microalgae biomass. [PERSON] et al. [26] found that when ammonia concentration was 84 mg/L and the pH value was 4, the growth of _C.protothecods_ was limited and the population did not show a logarithmic growth phase [26]. Furthermore, [PERSON] and [PERSON] [21] showed that at low nitrogen concentrations, chlorophyll-asynthesis by _C. vulgaris_ was limited, indicating that the factor limiting the synthesis was low nitrogen concentration. In studies conducted under dynamic conditions, the initial cell number was 3.6 \(\times\) 10\({}^{6}\) cells/L. The use of the carrier for immobilizing the cells was because immobilized microalgae cells were less exposed to damage due to turbulence caused by the mixing process than cells suspended in wastewater and the easier separation processes. The obtained microscopic images confirmed a higher density of microalgae immobilized on alginate beads compared to cells in the form of a suspension. Calculated growth rates of algal biomass (\(\upmu\)) indicated that the inverse relationship and growth rate in the form of suspended microalgae was higher than the growth of microalgae immobilized on alginate beads. The growth rate of microalgae in the samples at a dose of 100 g/L of microalgae was: 2.3 \(\pm\) 0.01 d\({}^{-1}\) (10 CvA sample); 3.8 \(\pm\) 0.03 d\({}^{-1}\) (10 CvB); 2.2 \(\pm\) 0.02 d\({}^{-1}\) (sample 105 cA) and 4.0 \(\pm\) 0.06 d\({}^{-1}\) (sample 105 cB). In the samples with dose of 50 g/L of microalgae the growth rate (\(\upmu\)) was in the order of: 1.2 \(\pm\) 0.01 d\({}^{-1}\) (sample 5 CvA); 2.2 \(\pm\) 0.02 d\({}^{-1}\) (sample 5 CvB); 1.3 \(\pm\) 0.02 d\({}^{-1}\) (sample 5 SCA) and 2.4 \(\pm\) 0.05 d\({}^{-1}\) (sample 5 ScB). This can be explained by the fact that when microorganisms are immobilized on alginate beads, then there is a limitation of diffusion associated with the rate of penetration of the substrate into the flocks or the carrier. This is described by the Monod constant \(K_{m}\) characterizing the affinity of the substrate for microorganisms, with its value depending on the growth of microorganisms and the susceptibility of compounds to decomposition. The constant \(K_{m}\) is higher when microorganisms grow in aggregates, and lower when they occur in a dispersed form in solution [28]. [PERSON] and [PERSON] [7] conducted research on the cultivation of _C. vulgaris_ in municipal wastewater.