## 1 Introduction  
According to the European Economic and Social Committee, the success of the circular economy is based on three pillars. The first pillar is about energy consumption (i.e., the efficiency in energy sources). The second pillar is about reducing the cost of product processing (i.e., sustainable technological processes for waste-out systems designing), and the third one concerns employment. Biogas production through anaerobic digestion (AD) has proven to provide direct benefits to the second pillar and indirect benefits to the rest of the pillars, achieving the transition into the circular economy, a sustainable approach for the biorefinery concept [1]. Currently, agriculture is one of the industrial sectors that generates the largest amount of wastes. Therefore, 1.3 billion tonnes of food are yearly discarded during production handling,storage processing, and distribution, with an estimated carbon footprint of 3.3 billion tonnes of CO\({}_{2}\) equivalent of greenhouse gases (GHG) released into the atmosphere per year [2]. Agroindustrial wastes such as sugarcane bagasse, wheat straw, wheat bran, and many others are cheapest and abundantly available. Their valorization by AD has long been investigated and implemented for bioenergy production [3,4]. By-products from sugar beet plant after the sugar extraction process may be bio-converted into bioenergy through AD [5]. Similarly, a large amount of manure produced in livestock facilities is a potential candidate for clean energy production such as biogas [6]. Moreover, livestock production in the European Union is high, being Spain and Germany the largest producers of pigs, while France held the largest number of bovines [7]. It has been reported that AD of animal manure can save up to 20% of the dairy processing facility's energy demand [8]. Despite their suitability for the anaerobic treatment, the use of manure as feedstock in AD could inhibit to methanogenic archaea due to the ammonia released during the process [9,10]. Manure has been traditionally used as a fertilizer for agricultural soils due to its high content in nitrogen and phosphorus. Nevertheless, this uncontrolled practice may have environmental problems, such as groundwater contamination due to infiltration, or greenhouse gas emissions [11]. Furthermore, lignocellulosic agroindustrial wastes and by-products lack of some macro and micronutrients [12,13]. The AD of both kinds of wastes separately may be improved and optimized by co-digestion as this biomass has different/complementary characteristics [14,15]. Sugar beet by-products (SBB) are rich in carbon while manure is a potential substrate to compensate nitrogen deficiency in the anaerobic process, together with minerals and essential nutrients. Moreover, manure provides a high buffer capacity, which could neutralize volatile fatty acids (VFAs) released by agroindustrial wastes in the AD process [16,17]. Besides, the manure type plays an important role in the AD process. Hence, some manure resulted in more effective in the co-digestion process with agroindustrial wastes than others [18]. This effect returns to several factors such as the composition of each manure according to its origin (animal digestive tract), the food given to animals in the course of animal husbandry, and the synergy between the manure and agroindustrial waste used in co-digestion [19, 20, 21]. Despite the numerous benefits of co-digestion, the mixture of substrates with different characteristics could create either synergistic interactions of microorganisms or antagonist interactions, which in this case, can worsen the anaerobic process [22]. To date, several previous studies have investigated anaerobic co-digestion of agroindustrial wastes with livestock manure to enhance biogas production and in most of these studies; synergistic effect between the two kinds of substrates has been suggested to be the reason of the process improvement.