# Enzymatic Activity in the Anaerobic Co-Digestion of Cavitated Coffee Waste and Sewage Sludge
## 4 Discussion
[34], the occurrence of heavy metals in larger quantities in the AnD process depends on the component that is fed to the bioreactors. Furthermore, foods added into bioreactors generally include various trace amounts of metals, so CW could also contain HMs in its composition. [PERSON] et al. [35] draw attention to the fact that the heavy metal content in coffee beans may depend on the location of cultivation, type of fertilization and atmospheric deposition. In this study, the application of CW to the feedstock did not significantly increase the HMs. Only after the AnD process, a significant increase was observed in the content of Fe, Cu, Zn, Mn, Ni, Cd, Cr, Pb and Hg in the digestate. [PERSON] et al. [36] and [PERSON] et al. [37] demonstrated that an increase in HMs during AnD, particularly regarding the Ni concentration, influences the growth of methanogenic bacteria, contributes to the synthesis of cofactor F430 for urease, and impacts the activity of cellulase during various steps of the AnD process. The correlation analyses showed a positive and negative correlation between the Ni concentration in the digestate and \(\beta\)-Glu and phosphomonoesterases activity, respectively. As shown by [PERSON] et al. [38], glucose metabolism correlates with the Cr concentration, while the concentration of Zn plays an important role as a metallic enzyme activator and cofactor of various enzymes and stimulates microbial growth in AnD. According to [PERSON] et al. [39], the increase in trace metals in the feedstock can have a positive effect on the growth of methanogens as well as contribute to the growth of various enzymatic reactions and participate in the formation of methane. Enzymes are involved in the decomposition of organic matter in their catalytic centers, which include transition metals, which participate as cofactors in electron transport [14]. In addition, [PERSON] et al. [38] noted that when a bioreactor's trace element content is insufficient, this may cause improper AnD operation as well as increases in ammonia and volatile fatty acids. Besides heavy metals, light metals, such as Ca, Mg, K and Na, are also present in bioreactors after feedstock addition and can have a significant impact on microbial growth [34]. In the present study, using HDC for CW in the feedstock influenced the increase in the concentration of Na and K as well as decreased the Ca content in the medium. In turn, the analysis of the digestate indicates a significant decrease in the concentration of Mg, P and Ca after using HDC. Probably, the cavitation removes absorbed compounds from the feedstock matrix, which can be utilized more easily by the microorganisms involved in the AcD process. As reported by [PERSON] et al. [34], during the decomposition of organic matter, light metals are released into the solution, which can significantly affect the activity of microorganisms. However, when there are excessive amounts of these metals in solution, they can have an inhibitory effect on microbial growth by destabilizing cell membranes, with a consequent reduction in biogas production, which was not observed in this research. Enzymes are essential for the anaerobic digestion process; they play an important role in key biochemical responses, catalyzing reactions necessary for the proper functioning of microorganisms, the decomposition of organic pollutants and the formation of organic matter [40]. Many scientists have reported that there is still insufficient knowledge regarding the activities of enzymes and microorganisms participating in waste decomposition during the AnD process [38; 41]. This is due to the fact that each AnD process is different because of the components introduced and it is impossible to replicate the same process 100% of the time. For example, a slight fluctuation in macro- or micro-elements can change the activity of the microbial population and thus the enzyme activity, influencing the AnD process and biogas production [13; 38].