## 1 Introduction
### Operating Temperatures in AD  
AD processes can operate at different temperature levels but definitions of temperature ranges and process optima vary in the literature (Table 1). As can be seen in Table 1, a clear classification is difficult and the gaps between the temperature levels imply smooth transitions. However, according to [PERSON] [20] and [PERSON] et al. [21], process temperatures in AD can be subdivided into four different ranges, namely, psychrophilic (12-15 \({}^{\circ}\)C), psychrotolerant (20-30 \({}^{\circ}\)C), mesophilic (30-40 \({}^{\circ}\)C) and thermophilic (55-75 \({}^{\circ}\)C). In general, microorganisms with a metabolism based on AD are known to exist in a temperature range of \(-\)5-121 \({}^{\circ}\)C. Microorganisms between 70 and 85 \({}^{\circ}\)C are called extremely thermophilic, while the temperature range from 85-121 \({}^{\circ}\)C is described as hyper-thermophilic [22]. Although the higher activity of the methanogenic microorganisms at thermophilic temperature leads to a higher degradation rate and higher biogas yields, the process stability is more sensitive, and maintaining process temperatures requires more energy, which reduces the total energy yield [23; 24; 25; 26; 27]. An additional advantage of thermophilic process temperatures is a lower level of sludge generation and the possibility to fulfill hygienic requirements (e.g., regulated by the German Biowaste Ordinance) for substrates such as OFMSW that need hygienic treatment [22; 28]. Therefore, usually, no separate sanitation systems are required at thermophilic AD plants. [PERSON] et al. [45] discovered that biogas yields via thermophilic AD (55 \({}^{\circ}\)C) of OFMSW together with sewage sludge can be 45-50% higher compared to mesophilic AD (37 \({}^{\circ}\)C). [PERSON] et al. [46] already investigated the influence of mesophilic (35 \({}^{\circ}\)C) and thermophilic (55 \({}^{\circ}\)C) AD of OFMSW on the biogas quantity and quality with a hydraulic retention time (HRT) of 25 d. In this case, the mesophilic AD was less efficient than the thermophilic AD. [PERSON] et al. [37] confirmed these results. Mesophilic AD is indicated with higher process stability than thermophilic AD through greater diversity of microorganisms, and with lower resistance to foaming and lower organic loading rates. At mesophilic temperature, the amount of CO\({}_{2}\) in the biogas is reduced as a higher percentage remains dissolved in the liquid phase, but the conversion rate of cellulose and hemicellulose is lower than in thermophilic AD. Lower process temperatures lead to lower energy demands of AD plants and offer the possibility to use low-temperature waste heat for process heating [22; 23; 27; 47]. [PERSON] et al. [48] proved that psychrophilic AD is indicated with high process stability, which was also confirmed by other studies [49; 50]. At the psychrophilic level, the additional energy input to maintain AD temperature is reduced or nullified depending on the ambient temperatures [32]. The comparatively slow CH\({}_{4}\) generation in AD at lower temperatures can be compensated by larger digester volumes and an increased HRT, reaching similar final CH\({}_{4}\) yields (tested with coffee husks, pulp, and mucilage) to those of mesophilic AD [40; 51]. [PERSON] et al. [32] also concluded that the total CH\({}_{4}\) yields of mesophilic AD (37 \({}^{\circ}\)C) of brewery wastewater are similar to the yields in psychrophilic AD (15 \({}^{\circ}\)C). With lower temperatures, the CH\({}_{4}\) concentrations in the biogas increase. [PERSON] et al. [39] proved that the psychrophilic AD (15 \({}^{\circ}\)C) of sewage sludge delivered approximately half of the CH\({}_{4}\) yield, compared to the AD at 30 \({}^{\circ}\)C at the same time step. The finding of [PERSON] et al. [31], i.e., AD processes at psychrophilic temperature level require an HRT roughly twice as long to achieve the same CH\({}_{4}\) yield, underlines the findings of [PERSON] et al. [39] and [PERSON] et al. [40].