# Enzymatic Activity in the Anaerobic Co-Digestion of Cavitated Coffee Waste and Sewage Sludge
## 3 Results
### Macro- and Micro-Element Content of Feedback and Digestate  
In the conducted study, higher macro- and micro-element concentrations were observed in the digestate, as compared to those in the feedstock. A significant increase in the  
Figure 3: The changes in biogas/methane yield and methane content depending on the investigated variants. The results are presented as mean \(\pm\) SD. R0—with SS, R1—non-cavitated and with CW, R2—CW cavitated for 20 min, R3—CW cavitated for 30 min. Figure 2: The organic compound removal in the investigated variants. The results are presented as mean \(\pm\) SD. COD—chemical oxygen demand, TS—total solids, VS—volatile solids, R0—with SS, R1—non-cavitated and with CW, R2—CW cavitated for 20 min, R3—CW cavitated for 30 min. metal concentration in the digestate was due to the reduced TS during the fermentation process (Figures 4 and 5). The macro-element concentrations of the feedstock and digestate are shown in Figure 4. The magnesium content, depending on the experimental variant, in the feedstock was from approx. 6832 mg/kg TS (F + R1) to 7442 mg/kg TS (F + R3) while in the digestate, it was from approx. 13,707 mg/kg TS (D + R2) to 18,567 mg/kg TS (D + R1). The use of the 20 min and 30 min HDC indicated a decrease in the Mg content of the digestate compared to D + R1. The largest phosphorus content in the feedstock was observed for variant F + R0 while, for D + R1, this was in the digestate. An increase in the P concentration of about approx. 6% was observed for both feedstock and digestate after the 30 min HDC compared to the 20 min pre-treatment duration. The potassium content of the feedstock was highest after the use of HCD F + R3 (11 894 mg/kg TS), by approx. 8%, compared to F + R0. A similar dependence was observed in the digestate, where the highest K content was for D + R3, approx. 26%, as compared to D + R0. Figure 4: Effects of CW and HDC on changes in macro-element concentrations in feedstock and digestate. The results are presented as mean \(\pm\) SD. Different letters above the error bars indicate statistically significant differences evaluated by the Tukey test at \(p<0.05\). The content of calcium in the feedstock was at a similar level of approx. 25,000 mg/kg TS for all variants. In contrast, the significantly highest Ca content for the variant in the digestate was observed in D + R1. The concentration of Ca in the digestate after the use of HCD at 20 min and 30 min was at a similar level of approx. 84,707 mg/kg. In the samples with CW after HCD, a significant increase in sodium in the feedstock by approx. 60% was observed compared to F + R0. In this study, the content of sodium in the digestate ranged from 7620 mg in D + R0 to 8760 mg/kg TS in D + R1 (Figure 4). Figure 5: Effects of CW and HDC on changes in micro-element concentrations in feedstock and digestate. The results are presented as mean \(\pm\) SD. Different letters above the error bars indicate statistically significant differences evaluated by the Tukey test at \(p\) < 0.05. As shown in Figure 5, the concentration of trace elements in the feedstock and digestate depended on the experimental variants used. The Fe and Cu concentrations in the feedstock, regardless of the experimental variant, were at similar levels of approx. 7549 mg/kg TS and 250 mg/kg TS, respectively, while in the digestate, the highest value of Fe and Cu in D + R1 were observed. The use of HCD influenced a decrease in the Fe and Cu concentration in the digest by approx. 21% and 13%, respectively (after 20 min of HCD), while after 30 min of HCD, it decreased by approx. 15% and 8%, respectively, compared to D + R0. The Zn content of the feedstock ranged from approx. 582 mg/kg TS (F + R1) to 660 mg/kg TS (F + R3). In the study, one observed that the use of HCD influenced an increase in the Zn in the feedstock by approx. 6% (20 min HCD) and 11% (30 min HCD). In turn, a decrease in the Zn content was also observed in the digestate after HCD.