## 3 Results and Discussion  
### Process Performance  
The performance of the AD process was evaluated based on the efficiency of removing organic compounds expressed as VSs, TS, COD, and sCOD (Figure 1). Regarding the first two parameters, the introduction of co-substrates resulted in improvements in their removal. As compared to MSS mono-digestion, a major growth of approx. 15% was observed in multicomponent co-digestion of IPW and OW (S4). A similar trend occurred in the case of COD; therein, its removal in S4 was enhanced by 26%, as compared to the control series (S1). These facts indicate the effective decomposition of organic compounds in multi-substrate AD. Previous studies indicated that the application of fat-rich wastes to MSS resulted in higher microbial activity that might have led to an enhanced degradation rate [31; 32; 33]. In the research conducted by [PERSON] in MSS and fat-oil-grease co-digestion, a degradation rate improved by 17% was achieved [33]. It should be pointed out that this tendency occurred despite providing a significant load of organic substances in the feedstocks. A different tendency was recorded only for sCOD (Figure 1d)--in this case, the deterioration of its removal. However, the highest decrease was obtained for two-component AD in the presence of IPW. The stability of anaerobic digestion might be influenced by several factors, and maintaining it requires a balance of these factors to ensure optimal microbial activity and prevent system failure. In this study, process stability was evaluated on the basis of pH, ALK, and VFAs. Importantly, in all experimental series the mentioned parameters reached values indicating a stable process performance (Table 2). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Unit**} & \multicolumn{2}{c}{**S1**} & \multicolumn{2}{c}{**S2**} & \multicolumn{2}{c}{**S3**} & \multicolumn{2}{c}{**S4**} \\ \cline{3-8}  & & **F** & **D** & **F** & **D** & **F** & **D** & **F** & **D** \\ \hline TA & mg/L & \(635\pm 19.1\) & \(4896\pm 120\) & \(680\pm 18.9\) & \(4590\pm 135\) & \(694\pm 15.7\) & \(4916.5\pm 135\) & \(732\pm 12.3\) & \(4915.5\pm 140\) \\ VFA & mg/L & \(715\pm 35.2\) & \(224.5\pm 9.8\) & \(725\pm 29\) & \(314\pm 7.9\) & \(694\pm 21\) & \(275\pm 17\) & \(742\pm 15.1\) & \(237\pm 14.1\) \\ pH & - & \(5.62\pm 0.1\) & \(7.35\pm 0.2\) & \(5.5\pm 0.1\) & \(7.25\pm 0.3\) & \(5.75\pm 0.2\) & \(7.38\pm 0.4\) & \(5.75\pm 0.1\) & \(7.4\pm 0.07\) \\ TP & mg/L & \(474\pm 9.5\) & \(648.5\pm 12.3\) & \(550\pm 10.2\) & \(690\pm 145.6\) & \(650\pm 18.1\) & \(760\pm 16.2\) & \(655\pm 21.4\) & \(628\pm 11.3\) \\ TN & mg/L & \(1701\pm 85.1\) & \(2065\pm 80\) & \(1954\pm 54.1\) & \(2300\pm 47\) & \(2033\pm 39.5\) & \(2290\pm 37.1\) & \(2087\pm 39.1\) & \(2064\pm 38.2\) \\ N-NH\({}_{4}\)\({}^{*}\) & mg/L & \(80.2\pm 3.2\) & \(248.25\pm 10.2\) & \(100\pm 9.7\) & \(312\pm 9.9\) & \(109\pm 7.8\) & \(173.3\pm 7.9\) & \(897\pm 5.9\) & \(242\pm 4.7\) \\ P-PO\({}_{4}\)\({}^{3-}\) & mg/L & \(176\pm 13.3\) & \(111.7\pm 8.9\) & \(258\pm 5.7\) & \(114\pm 4.4\) & \(264\pm 5.5\) & \(116\pm 5.7\) & \(266\pm 4.8\) & \(116\pm 7.8\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The pH value and alkalinity (TA), volatile fatty acid (VFA), total nitrogen (TN) and phosphorus (TP), and ammonia nitrogen (N-NH\({}_{4}\)\({}^{*}\)) and phosphate (P-PO\({}_{4}\)\({}^{3-}\)) concentrations in feedstock (F) and digestate (D) in corresponding experimental series (average values \(\pm\) standard deviation are presented). Figure 1: Content/concentration of VSs (**a**), TS (**b**), COD (**c**), and sCOD (**d**) in feedstock (F) and digestate (D) in corresponding experimental series and also the effectiveness of their removal (R); MSS—municipal sewage sludge; OW—orange waste; IPW—ice-cream processing waste. One of the most important factors affecting the AD process is the concentration of VFAs in the reactor. This group of compounds includes acetic, propionic, butyric, valerian, and caproic acids, among others [34].