## 3 Results and Discussion
### The Variation of Micro- and Macro-Elements within AD
#### 3.2.1 Macro-Elements Content
The highest K content was observed in the three-component AcoD in the presence of PL and BSG (R7 and R8). The application of all co-substrates did not lead to the limit value of 400 mg/L being exceeded, thus allowing for effective biogas production. The highest values of Mg were reported in the presence of PL for both the two- and three-component AcoD. The related enhancements exceeded 50%. Despite this fact, the Mg content was established at an optimal level below 720 mg/L [36,37]. Similarly to the other macro-elements, the Ca concentration was the highest in the ternary mixtures in the presence of PL and BSG. This metal plays an important role in the formation of microbial aggregates [38,39]. Importantly, in all of the AcoD series, the Ca contents were established at levels considered as stimulatory for AD efficiency (100-1035 mg/L) [3,37,40]. The S content was also enhanced for each case as compared to the control, reaching a comparable level with the highest content in the three-component AD. The observed values were below 800 mg/L, i.e., a concentration known to inhibit the AD process [40]. This macro-element has a strong influence on metabolic activity; the sulfate-reducing bacteria compete with methanogens for intermediate product energy sources [41]. In the case of Na, there was no significant influence of PL application observed on its content (R2 and R3), while using OP (R4, R5) as well as OP and BSG increments above 35% occurred (comparing the values expressed in g/kg). As in the case of other macro-elements, its concentration was established at an optimal ratio below 350 mg/L [42]. While analyzing all of the results, a clear tendency was noticed in R6, i.e., in the presence of OP and BSG. In this case, as compared to the other AcoD series, the minor increases in the nutrient content were accompanied by the lowest methane production (Figure 2). This fact may be related with specific BSG absorption properties. Previous studies indicated that the activated carbon produced from this lignocellulosic waste is characterized by better adsorption properties than many commercial products. Moreover, it has a potential to absorb phenolic compounds, as well as some metal anions, e.g., Ni, Fe, Cr, Cd and Pb, from wastewater [43]. However, limonee still appeared to inhibit the process. A surprisingly different effect was obtained using PL and BSG (R7 and R8). This time, the improved feedstock composition resulted in enhanced biogas production, as well as stable process performance. This fact, in particular, indicated different properties of these orange wastes (PL and OP), and emphasized a synergistic effect occurring in the presence of PL and BSG. Moreover, it should be mentioned that in the two-component mixtures,there were no negative effects on biogas production as well as no increased accumulation of metals within the AD process (Figure 3). The digestate quality is a key factor regarding its further application, particularly in soil fertilization and remediation. Digested biomass contains macro-elements that are easily available for plants, while the bioavailability of P in digested sludge varies between 40 to 80% [44]. Digested SS may also supply some micronutrients, e.g., amino acids or vitamins that are otherwise not added to the soil. Furthermore, its application may improve the structure of the soil. Such a solution is beneficial regarding environmental and economic aspects; it also implements assumptions of the new Circular Economy Action Plan for a cleaner and a more competitive Europe [45]. However, digestate application is strictly regulated because of contamination risks for HMs, pharmaceuticals, as well as pathogens [28; 46; 47; 48]. As it was shown in the study, increases in almost all of the analyzed macro-elements occurred within AcoD, as compared to the feedstock composition (Table 3). The introduction of co-substrates resulted in increased contents of macro-elements in the digestate.