## 1 Introduction  
Recently, the growing energy demand as well as the depletion of traditional fuels has resulted in the constant search for new solutions for sustainable energy production. Therefore, the application of food waste in the anaerobic digestion (AD) process has gained increasing attention. This is the dominant technology that allows various organic by-products to be converted into biogas as an energy carrier [1]. Importantly, the food sector is recognized as one of the most energy- and water-/raw material-intensive industries [2]. Simultaneously, it generates a significant amount of valuable by-products. The use of these wastes in AD is dictated by many factors, including a significant content of organic matter and the presence of many micro- and macro-elements that might stimulate AD microbes [3]. Another important factor in favor of the application of such wastes is the reduction of its management costs. This technology aligns with a circular economy model by converting food waste into valuable resources, e.g., biogas and nutrient-rich digestate, thus limiting the negative impact of this industry on the environment [4]. Despite significant developments in this technology, there are still many gaps in the use of specific wastes in AD. This group includes orange juice processing by-products, represented mainly by orange peel, accounting for about 40-50% of the total fruit weight [5]. It mainly contains essential oils, flavonoids, pectin, and fibers. It might be a source of essential oils or pectin [6; 7]. Therefore, it has many applications in cosmetics, food, and pharmaceutical industries, as well as in biomaterial production [8]. However, those applications entail additional financial outlays. Additionally, due to its high liability and significant moisture content, as well as its minor calorific value, conventional methods of orange waste disposal, such as animal feed, landfilling, incineration, composting, and thermal treatment, are inadequate in terms of investment, environmental effect, and sustainability [2]. Simultaneously, according to the U.S. Department of Agriculture Foreign Agricultural Service (USDA FAS), in the coming years, a continuous increase in orange production is expected. In 2023-2024, approx. 48.8 million tons is forecast, which will result in huge amounts of waste that will need to be effectively managed. However, the previous studies indicated that mono-digestion is still challenging mainly due to the presence of essential oils, e.g., d-limonene, a natural compound present in orange peels that can inhibit microbial activity during its AD [9]. Other factors that make it difficult to use these wastes in mono-digestion include phenols, a significant content of organic substances, and the presence of barely biodegradable lignins. Several strategies have been proposed to overcome the mentioned difficulties. The first includes the use of a pre-treatment strategy to reduce the d-limonene content, such as steam distillation [10], ensiling [11], or leaching of limonene using hexane as solvent [12]. Another possibility is the reduction of toxic compounds by using carbonaceous sorbents, e.g., biochar, that adsorb the toxic agents onto their surface [13; 14]. However, those methods require additional financial outlays. In recent years, the co-digestion strategy has gained increasing attention mainly because of the relatively low cost and ease of implementation, possibility of using a wide range of various organic wastes, and environmental friendliness [15]. However, the crucial benefits of implementing this strategy are enhanced methane production as well as providing valuable nutrient-rich digestate [16]. Nevertheless, the main challenge for successful application is the selection of a suitable additive with complementary composition to the main component, as well as the adoption of adequate operational parameters [17].