## 3 Results and Discussion
### The Effectiveness of Removing Ammonium Nitrogen from Wastewater  
The results of the research confirmed that ammonium nitrogen can be removed from wastewater using the microalgae _C. vulgaris_ and _S. armatus_, clinoptilolite, and their mixture. An analysis of the data shows that clinoptilolite plays a decisive role in the removal of ammonium nitrogen. In wastewater to which clinoptilolite was added in a mixture with microalgae, gradual decreases in N-NH4\({}^{+}\) concentration were observed throughout the time (Figure 5C). [PERSON] et al. (2022) [29] improved the biological treatment of nitrogenous wastewater using a hybrid embedding of materials synthesized and applied in microbial immobilization to form novel biocomposites. Polyvinyl alcohol/sodium alginate (PVA/SA) beads were prepared. By means of the addition of layered double hydroxides (LDHs) to the hydrogel beads, the authors provided effective protection for the proliferation of immobilized microorganisms. The efficient removal of nitrogen was achieved by the immobilized microorganisms thanks to the processes of the simultaneous nitrification/denitrification and assimilation in the bioreaction system [9]. The adsorption of ammonium nitrogen onto  
Figure 4: Microscopic images of sludges after the sorption processes (400\(\times\)). alginate is caused mainly by electrostatic attraction between functional groups and the nutrient ions in the aqueous solution [9]. Figure 5: Removal efficiency of ammonium nitrogen: **(A)—**_C. vulgaris_, **(B)—**_S. armatus_, **(C)—**microalgae and zeolite. Under dynamic conditions, the mixing process took place evenly throughout the column volume, which was conducive to the removal of ammonium nitrogen from the wastewater. Numerous authors confirm the beneficial effect of mixing on the increase in the efficiency of N-NH\({}_{4}\)+ removal. [PERSON] and [PERSON] (2012) noted on the last day of research the total removal of ammonium nitrogen from wastewater by _C. vulgaris_, with an initial concentration of 33.6 mg N-NH\({}_{4}\)+/L in the case of mixing samples. The authors showed that in unmixed samples, the ammonium nitrogen was not removed [17]. Similar results were obtained by [PERSON] et al. (2008), who showed that the mixing time and the initial concentration are the most important factors affecting the removal of ammonium nitrogen by clinoptilolite. The above-mentioned authors showed not only the beneficial effect of mixing on the ammonia removal efficiency, but also that the best ammonia removal efficiency is achieved after only 3 h [30]. This is confirmed by the result recorded by the author of this paper that clinoptilolite was the fastest in sorbing N-NH\({}_{4}\)+ compared to the other sorbents used in this research. The effectiveness of removing ammonium nitrogen from wastewater with high concentrations of ammonium nitrogen by employing the sorption materials used is shown in Figure 5.  
These results are consistent with the results obtained by [PERSON] and [PERSON] (1996) [31]. The researchers studied the effect of nitrogen on the growth of microalgae of the _C. vulgaris_ species and found that the removal of ammonium nitrogen depends, among other factors, on its initial concentration in wastewater, algal biomass concentration, strain, and chlorophyll a content [31]. However, the efficiency of ammonium nitrogen removal obtained by the above-mentioned authors did not exceed 54%. In dynamic conditions, the addition of successive portions of wastewater every 10 days increased temporarily the concentration of this pollutant in wastewater, which influenced the efficiency of N-NH\({}_{4}\)+ removal from wastewater using microalgae of the species _Chlorella vulgaris_, _Scenedesmus armatus_, and clinoptilolite. After replacing the second portion of wastewater (from day 20), the efficiency of N-NH\({}_{4}\)+ removal from wastewater by all applied sorbents increased. The best results were achieved after using a mixture of both microalgal strains with clinoptilolite (75-86%) or clinoptilolite alone (86-92%) (Figure 5C).