## 3 Results and Discussion
### The Variation of Micro- and Macro-Elements within AD  
#### 3.2.1 Macro-Elements Content  
Balanced feedstock composition is a key factor for effective biogas production. Within the implementation of the AcoD strategy, one of the major decisions involves selecting an adequate co-substrate with a complementary composition to the SS [1,2]. Therefore, conducting analyses of nutrients, metalloids as well as HMs presented in applied materials is particularly important [28]. The characteristics of sole SS may differ remarkably, depending on the wastewater characteristics, as well as the treatment technology adopted at WWTPs. Properly selected co-substrates may significantly change the feedstock composition, in particular, supplying deficient micro- and macro-elements to the main component [2,29]. Among the macro-elements of great importance are considered nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg) and sodium (Na). These components are essential for the microbial growth of microorganisms within AD [30]. The application of the co-substrates resulted in increased contents of macro-elements in the feedstock (Table 3), as compared to the control (R1), in most cases visibly enriching its composition. The exception was the K content in R5, as well as the S content in R2 and R3, both being lower than that for the control. The highest enhancements were observed in the three-component series in the presence of both PL doses and BSG, i.e., for R7 and R8, respectively. This effect was related with the introduction of an additional component in BSG, indicating a high content of TS, VS as well as N, Na, P and S (Table 1). Additionally, the applied PL, as compared to OP, indicated a more valuable composition in terms of TN, P and K contents. Moreover, this orange waste was characterized by higher concentrations of both ALK and COD (Table 1). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Reactor** & **Unit** & **N** & **Na** & **Mg** & **P** & **K** & **Ca** & **S** \\ \hline \hline \multicolumn{8}{c}{Feedstock} \\ \hline R1 & mg/L & \(1074\pm 12.7\) & \(119.6\pm 5.1\) & \(146.5\pm 12.4\) & \(106.1\pm 8.8\) & \(182.9\pm 3  
All of the analyzed macro-elements play an important role within AD. However, a decisive influence on the AD effectiveness corresponds to N, being a crucial nutrient in protein biosynthesis. Its low concentration may lead to a reduction in the metabolic capacity [31]. On the other hand, a high N content may result in ammonia inhibition [3]. It is commonly known that the preferred C/N ratio in feedstock should be between 20-30:1 [31,32]. In the present research, values above 20 were reached only in the mono- and two-component digestions (R1-R5). In turn, the highest methane production was achieved in the three-component series (R8), despite the unfavorable C/N ratio. Often, the application of co-substrates, and hence supplying the digesters with various micro- and macro-elements, may lead to enhanced process performance (despite disadvantage conditions, e.g., low pH, adverse VFA/ALK or C/N ratios). In an anaerobic co-digestion strategy, this influence is known as synergistic effects [2]. Moreover, previous studies also indicated that the toxicity of one compound may be eliminated by the presence of some trace elements in an additional co-substrate. Such a phenomenon was observed in the case of light metals, in particular Na, K and Mg [3,33]. In the present study, all of these were significantly enhanced in the R8 feedstock, as compared to the control (R1), which seems to confirm the above-mentioned regularity. Phosphorus is also needed for appropriate cell growth. Additionally, it plays an important role in the maintenance of an optimal pH during AD [34]. Although significant improvements of over 150% were reached in almost all of the AcoD series, the content of P was still at a relatively low level [3]. In turn, K indicated a stimulatory effect on the AD microorganisms, especially at low concentrations [3,35].