## 1 Introduction
Nevertheless, typical anaerobic digestion systems are not sufficiently efficient for today's demand [13]. The alternative may be a combination of modern reactors with enhanced biomass retention capacity and optimized digestion conditions (pH, temperature, HRT, among others) to obtain higher methane yields and productivity [8; 13; 14]. In the selection of a suitable bioreactor, the biomass retention capacity is an important consideration, because anaerobes grow slowly during the metabolic generation of butanol, ethanol, hydrogen, and methane [15]. This is particularly important in a bioreactor configuration that decouples HRT from solids retention times (SRT). These reactors, that are usually named high-rate reactors, were initially developed in the late 1970s with the introduction of the up-flow anaerobic sludge blanket (UASB) reactor [16]. The decoupling of HRT and SRT enables the maintenance of a significantly higher SRT/HRT ratio than in a continuous stirred tank reactor (CSTR) and prevents the washout of slow-growing anaerobes. Therefore, high-rate anaerobic systems are maintained at a sufficiently high biomass level inside the bioreactor [17]. In addition, environmental conditions are well preserved under optimal bioreactor performance parameters. The organic loading rates in these systems typically vary from 5 to 30 kg\({}_{\text{COD}}\)/(m\({}^{3}\cdot\)d), although higher rates have been reported [15; 18]. The two main types of high-rate systems include suspended and attached growth. The expanded granular sludge bed (EGSB) reactor is a suspended high-rate system, which has been used in industrial wastewater treatment. The implementation of ECSB reactors for biogas production has grown very fast in the last two decades [19]. Interestingly, ECSB reactors appeared as an improvement on UASB reactors, which allow high height-diameter relations for achieving high superficial velocities of \(>\)4 m/h ECSB per 1.5 m/h UASB [20; 21]. ECSB reactor technology was developed to optimize internal mixing and solve problems which are typically found in the practical operation of UASB reactors, such as the occurrence of dead zones, preferential flows, and short circuits, among others. Consequently, ECSB reactors provide better substrate--biomass contacts within the treatment system by expanding and intensifying the sludge bed and hydraulic mixing, respectively [1; 20]. This study aimed to assess the performance of the AcoD of three manure-based agro-industrial mixtures in three different ECSB reactors employed in a continuous operation mode. The AcoD of manure-based mixtures has acquired more relevance due to changes in the German Renewable Energy Sources Act (EEG), as a result of which only small liquid manure plants and waste digestion plants continue to benefit from the original remuneration system outlined in 2012 [11]. Furthermore, the German government encourages the use of natural fertilizers to reduce greenhouse gas emissions in the recycling of nitrogen [22]. The substrates were collected in the federal state of Nordrhein-Westfalen, Germany. In particular, the optimal compositions of two of the considered agro-industrial mixtures were determined using an approach initially designed by our group. The optimal composition of the third mixture was determined in [PERSON] et al. [23] using the approach of [PERSON] et al. [24]. The observed performance characteristics of the laboratory-scale reactors will form the basis for operation optimization and scale-up to pilot plant scale.