## 3 Results and Discussion
### Influence of Process Temperature on AD of OFMSW
All final SBG and SMY for DSS and OFMSW alone and corresponding CH\({}_{4}\) concentrations are presented in Table 4. The CH\({}_{4}\) concentrations of OFMSW did not vary as strongly as the CH\({}_{4}\) concentrations of the DSS at different temperatures. Between the SBG\({}_{\text{DSS}}\) and SMY\({}_{\text{DSS}}\) at 25 and 40 \({}^{\circ}\)C and between the DSS at 23 and 35 \({}^{\circ}\)C, a difference of almost 10%-points with higher CH\({}_{4}\) concentrations at lower temperatures was measured. Similar increases in CH\({}_{4}\) concentrations through reduced process temperatures were discovered in the literature [32]. It can be concluded that the inoculum is the major and liquid component in the digester, and therefore, the biogas composition can be influenced toward higher CH\({}_{4}\) concentrations through lowered temperatures. Figure 1: Specific biogas yield (SBG) and specific methane yield (SMY) for the mixture of the organic fraction of municipal solid waste (OFMSW) and digested sewage sludge (DSS); mean values \(\pm\) standard deviation (n = 2 for DSS, \(n\) = 4 for OFMSW and DSS) based on organic dry matter (oDM), gas volume at 1013 hPa, 0 \({}^{\circ}\)C.  
circumstance that the DSS already was AD treated, and the remaining oDM suggests a low digestibility. However, both series showed that there is still energy potential within the DSS that could be exploited better. In Germany, the average SBG production from sewage sludge amounts to 520 mL/g\({}_{\text{oDM}}\)[22]. The determined residual SBG potentials equaled approximately 10-20% of this average, showing that efficiency enhancements are possible. In addition, the results for SMY\({}_{\text{DSS}}\) at 23 \({}^{\circ}\)C coincide with those of [PERSON] et al. [39], in which the transition to the psychrophilic temperature range is accompanied by a doubled HRT to reach approximately similar CH\({}_{4}\) yields. The mean values for SBG\({}_{\text{OFMSW}}\) and SMY\({}_{\text{OFMSW}}\) (Table 4) were measured between 270 and 624 mL\({}_{\text{biogas}}\)/g\({}_{\text{oDM}}\) and was between 172 and 364 mL\({}_{\text{CH4}}\)/g\({}_{\text{oDM}}\), corresponding to approximately 27-63% (biogas) and 34-73% (CH\({}_{4}\)) of the stoichiometric yields. The variant at 25 \({}^{\circ}\)C delivered the best results but was indicated with the largest SD. As mentioned, this result can also be attributed to the calculation method presented in Equation (2) to Equation (2). When neglecting the two best performing digesters at 25 \({}^{\circ}\)C (SBG\({}_{\text{OFMSW}}\) of 700 and 650 mL/g\({}_{\text{oDM}}\)), the average SBG\({}_{\text{OFMSW}}\) drops to 570 mL/g\({}_{\text{oDM}}\) equaling the SBG\({}_{\text{OFMSW}}\) at 40 \({}^{\circ}\)C. In total, an operating temperature of 25 \({}^{\circ}\)C could be beneficial regarding the potential ratio between saved energy costs (heating) and CH\({}_{4}\) yields at a probably similar level. A study [51], in which an adaptation from mesophilic to psychrophilic AD temperatures took several months, could explain the results for the OFMSW variant at 23 \({}^{\circ}\)C. For the evaluation of SBG\({}_{\text{OFMSW}}\) and SMY\({}_{\text{OFMSW}}\) at 35 \({}^{\circ}\)C (Table 4), the reduced HRT (21 d) also has to be considered. Depending on the reactor design, typical OFMSW yields (SBG) in Germany vary between 80 and 120 mL/g\({}_{\text{FM}}\)[58]. Normalized on oDM by using the DM and oDM contents of OFMSW (Table 3), SBG yields vary between 309 and 463 mL/g\({}_{\text{oDM}}\). The determined gas potentials are similar considering the influence of laboratory conditions. In addition, Figure 3 presents the biogas production rates for the digester configurations for both test series. The variants at higher temperatures showed higher production rates and thus higher methanogenic activity at the beginning of the experiment. This is also found in other studies [25; 31; 52].