###### Abstract
It is estimated that in the EU, this is the largest industrial sector, with a turnover of EUR 1.1 trillion [10]. Moreover, this sector is responsible for the generation of significant amounts of residues, the effective management of which is still a challenge. Among this group, the residues from orange juice and beer production constitute a significant share. The first is represented mostly by orange peel (OP) and pulp (PL), whereas the second is represented by brewery spent grain (BSG). The first two have been used as fertilizer, animal feed, charcoal production, as well as for the extraction of essential oils and pectin [11]. In turn, BSG has been applied as livestock feed, or as raw material for the production of enzymes, biopolymers, biocides and biofuel [12]. Taking into consideration the significant energy consumption of the food industry, the application of such wastes in AD processes becomes a favorable choice. However, the anaerobic transformation of these wastes is still a technological problem [13; 14]. They may contain substances that are recognized as AD inhibitors, e.g., terpen, phenols and ammonium [3]. Additionally, some wastes are lignocellulosic in nature, which indicates poor degradability within anaerobic transformation [14]. On the other hand, both substrates are rich in many vitamins, such as C, A and B, minerals, amino acids and supply a high organic load that may stimulate methanogenic activity. Moreover, their application may improve digestate quality [15; 16; 17]. The main aims of this study were the following: (1) to evaluate the effect of the addition of selected industrial food wastes on the fate of micro- and macro-elements within anaerobic transformation; (2) to examine the effect of co-substrate application on biogas/methane production, volatile solids removal (VRS) as well as process stability; (3) to define the relationship with HMs content and biogas production; (4) to determine the possibility of the digestate for agricultural use. In the present study, the main solid by-products of orange juice and beer processing were chosen as the food industrial wastes. These residues, represented by OP, PL and BSG, were selected taking into consideration their significant amounts, and the ongoing problem with their effective management. Moreover, all of these substances indicated a potential for biogas production [13]. The co-substrates were added to municipal sewage sludge (SS), the main by-product of wastewater processing. The experiment was conducted in batch reactors under mesophilic conditions. The multi-component mixtures were examined. It should be noted that in terms of these substrates, similar investigations have not been conducted previously. The basic novelty of this study is that it presents a holistic approach, in which many factors have been taken into account, i.e., biogas production, process efficiency as well as environmental aspects of the proposed technology. Importantly, the obtained results may be implemented on a technical scale, thus improving the energy balance in many wastewater treatment plants (WWTPs). Moreover, for food producers, the application of this technology may allow for effective waste management that is consistent with the assumptions of the circular economy.