## 1 Introduction  
Anaerobic digestion (AD) is an efficient and suitable method for the sustainable management of bio-wastes as well as the production of biofuel [1]. It is a biological degradation process whereby biomass is converted into a mixture of gases called biogas, which consist mainly of methane and carbon dioxide, by the action of a microorganism consortium in the absence of oxygen. It is typically divided into four main stages, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis [2]. However, AD is a very complex and sensitive process, which involves diverse microorganism groups that require different environmental and operational conditions [1]. For example, biomass substrate digestibility and biogas production are significantly affected by the substrate composition and chemistry, such as the carbon-to-nitrogen ratio (C/N), mineral and volatile fatty acid composition, and pH [3]. These are also affected by the operational conditions, including the hydraulic retention time (HRT), substrate loading rate, reactor temperature, and so on. High-efficiency energy production by AD has been commonly achieved through the use of a high organic substrate loading rate. However, the high loading rate affects the stability and efficiency of methane production due to the imbalance between acidification and methanation, which typically result in a significant accumulation of volatile fatty acids and a sharp decrease in pH, leading to system failure [4]. The most popular approaches to improving AD for biogas production include anaerobic co-digestion (AcoD), coupling with dark fermentation, microbial community bioaugmentation, reactor engineering, substrate pretreatments, and the use of enzymes as biocatalysts [1]. AcoD in particular involves the simultaneous digestion of two or more substrates. It has been shown to be a highly viable option for improving biogas production by alleviating the disadvantages of mono-digestion [4]. It also increases the economic feasibility of the process in existing AD plants by increasing methane yields [4]. The advantages of AcoD include the ample supply of macro and micronutrients, a balanced C/N, the dilution of reaction inhibitors, a superior buffering capacity, and the enhancement of biogas production [5]. Despite the advantages of AcoD, it also presents some drawbacks due to the inappropriate selection of co-substrates, co-substrate composition, and operating conditions. Consequently, a poorly researched co-digestion process may result in an instable process, bringing with it a significant reduction in methane production. It is therefore necessary to have a profound comprehension of the co-digestion mixture(s) employed at a lab and pilot plant in order to support full-scale design and operation decisions [3; 6]. With 9632 operating and 9692 forecasted biogas plants in 2020 and 2021, respectively, Germany is the largest producer of biogas in Europe [7]. Its main feedstock for biogas production was initially energy crops. However, due to the current policy framework, Germany has shifted toward the use of alternative substrates such as crop residues, livestock waste, and catch crops [8; 9]. Hence, agro-industrial wastes have gained importance due to their potential as raw materials for obtaining energy [10]. Their use could eventually reduce environmental liabilities and add value to already developed production chains. In 2018, 95% of Germany's mass-specific substrates came from animal excrements (48%) and renewable resources (47%) such as maize or grass silage [11]. AD has been carried out either as a single or co-digestion system. Production plants have been equipped with a gas-tight storage tank and a minimum of two digesters that are connected in series [12]. However, the optimization of HRTs is still required to reach high degradation values [11; 12]. A large share of the studies on anaerobic co-digestion are concerned with the enhancement of biogas production while increasing methane content and shortening the retention time.