## 3 Results and Discussion  
### Digestion Efficiency  
As shown in Table 2, the application of co-substrates resulted in improvement in VSR (volatile solids removal), as compared to that for the control reactor. The highest enhancement of approx. 25% was observed at the lowest doses of both orange wastes. The optimum pH range for AD microorganisms should vary between 6.5 and 8.0. In turn, a VFA/ALK ratio of 0.23-0.3 constitutes a recommended range for stable AD performance [2; 19; 20; 21]. In all of the experimental series, the stability parameters were at a favorable level for the AD process (Table 2). However, in the presence of the highest doses of both orange wastes, the adverse effect on the VFA/ALK ratio was noted. This observation was related with the application of substrates that indicated low ALK and acidic pH (Table 1). It should be noted that in the presence of BSG in three-component digestion, a different trend was achieved. In this case, the introduction of co-substrate with significant ALK provided adequate buffering capacity in the reactors. Such beneficial effects of BSG have been reported previously in the case of co-digestion of acid cheese whey and SS [22]. The essential efficiency parameters were the biogas and methane production, as well as the methane content in biogas. As shown in Figure 2, in almost all of the co-digestion series, an improvement in the first and a deterioration in the second were observed, as compared to SS mono-digestion (R1). The significant enhancement in both values related only to the three-component AcoD (R8) supplied by 5 g of PL and 1.5 g BSG. In this case, the average values were 522 and 395 mLg/VS for biogas and methane production, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reactor** & **Volatile Solid Removal (VRS)** & **VFA/ALK** & **pH** \\  & **\%** & **-** & **-** \\ \hline R1 & 55.6 \(\pm\) 4.7 & 0.052 \(\pm\) 0.003 & 7.44 \(\pm\) 0.01 \\ R2 & 69.5 \(\pm\) 4.1 & 0.058 \(\pm\) 0.002 & 7.46 \(\pm\) 0.01 \\ R3 & 59.8 \(\pm\) 4.0 & 0.048 \(\pm\) 0.003 & 7.42 \(\pm\) 0.02 \\ R4 & 69.8 \(\pm\) 4.6 & 0.056 \(\pm\) 0.001 & 7.49 \(\pm\) 0.03 \\ R5 & 60.9 \(\pm\) 3.2 & 0.048 \(\pm\) 0.001 & 7.46 \(\pm\) 0.01 \\ R6 & 63.3 \(\pm\) 1.8 & 0.058 \(\pm\) 0.002 & 7.43 \(\pm\) 0.01 \\ R7 & 57.3 \(\pm\) 1.1 & 0.066 \(\pm\) 0.001 & 7.45 \(\pm\) 0.02 \\ R8 & 59.7 \(\pm\) 1.3 & 0.062 \(\pm\) 0.001 & 7.44 \(\pm\) 0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of volatile solid removal (VRS) and stability parameters in the corresponding series (average values and standard deviations are given). Beneficial results were also obtained in two-component AcoD in the presence of both OP doses (R4, R5), for which the biogas production was increased by over 21% as compared to the control [23]. Importantly, in this case, the decreased methane content in biogas did not decrease the methane, yielding a value below that for SS mono-digestion. Such a tendency is a common effect in the AcoD of substrates with a significant content of carbohydrates [1]. The decreased methane concentration may be attributed to the limonene presence in OP, which may adversely affect methanogens [24]. It should be pointed out that a decreasing tendency was observed only in the three-component mixtures in the presence of OP (R6). This effect occurred despite a relatively high VRS and favorable VFA/ALK ratio may be related with the presence of both AD inhibitors, such as limonene and phenol compounds, which was confirmed in the authors' study [23]. Limonene is classified as an essential oil that exhibits toxic effects on AD microorganisms [25]. In turn, acetate-utilizing methanogenesis is particularly sensitive to the inhibitory effects of phenolic compounds [26]. Microbial activity may also be disturbed by high organic loads within the AD of various food wastes [27].