## 3 Results and Discussion
### Influence of Process Temperature on AD of OFMSW  
For both test series, the influence of process temperatures on AD of OFMSW together with DSS is depicted in Figures 1-3. Figure 1 presents the results for SBGOFMSW and DSS and SMYOFMSW and DSS in the different digester configurations. The ratio of oDM from OFMSW to oDM from DSS according to the specifications of VDI 4630 [66] was 0.30 in test series 1 and 0.32 in test series 2. From a digester perspective, the influence of the OFMSW on biogas and CH\({}_{4}\) yields per digester (SBGOFMSW and DSS and SMYOFMSW and DSS) was relatively low due to a larger share of inoculum (DSS), compared to OFMSW. Figure 2 presents the results for specific biogas yields (SBG) and methane yields (SMY) for OFMSW and DSS alone at the different process temperatures. In both test series, higher temperatures were beneficial for SBG\({}_{\text{DSS}}\) and SMY\({}_{\text{DSS}}\). Based on the results for SBG\({}_{\text{DSS}}\) and SMY\({}_{\text{DSS}}\) it \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Material**} & **DM** & **oDM** & **C** & **H** & **N** & **O** \\  & **(\% FM)** & **(\% DM)** & **(\% DM)** & **(\% DM)** & **(\% DM)** & **(\% DM)** \\ \hline \multirow{2}{*}{DSS, series 1} & 3.33 & 58.55 & \multirow{2}{*}{n.a. 1} & \multirow{2}{*}{n.a. 1} & \multirow{2}{*}{n.a. 1} & \multirow{2}{*}{n.a. 1} & \multirow{2}{*}{n.a. 1} \\  & \(\pm\)1.84 & \(\pm\)0.40 & & & & \\ \multirow{2}{*}{DSS, series 2} & 4.07 & 60.21 & 30.38 & 4.50 & 4.11 & 21.22 \\  & \(\pm\)0.01 & \(\pm\)0.17 & \(\pm\)0.11 & \(\pm\)0.06 & \(\pm\)0.04 & \\ \multirow{2}{*}{OFMSW, series 1 and 2} & 33.28 & 77.88 & 39.49 & 5.29 & 2.13 & 30.97 \\  & \(\pm\)5.43 & \(\pm\)1.37 & \(\pm\)2.55 & \(\pm\)0.35 & \(\pm\)0.32 & \\ \hline \end{tabular} \({}^{1}\) not analyzed.  
\end{table} 
Table 3: Dry matter content (DM), organic dry matter content (oDM), chemical elements of digested sewage sludge (DSS) used as inoculum, and organic fraction of municipal solid waste (OFMSW) used as substrate; mean values \(\pm\) standard deviation. can be assumed that the residual CH\({}_{4}\) potential of DSS in wastewater treatment plants could be better exploited through mesophilic temperatures in the storage tank serving as a second digester or by increasing the HRT for sewage sludge in the main AD unit. In test series 1, the variants using OFMSW as feedstock achieved similar SBGOFMSW and DSS and SMYOFMSW and DSS per digester (Figure 1). By subtracting the yields of the blank variants (DSS) for the calculation of SBGOFMSW and SMYOFMSW (Figure 2) at 25 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, the lower SBG\({}_{\text{DSS}}\) and SMY\({}_{\text{DSS}}\) at 25 \({}^{\circ}\)C led to slightly higher yields for OFMSW at 25 \({}^{\circ}\)C. In general, the two test series showed strong differences regarding the final SBG and SMY. While the SBGOFMSW at 25 \({}^{\circ}\)C (after 56 d) was 623 mL/g\({}_{\text{GDM}}\), the SBGOFMSW at 23 \({}^{\circ}\)C (after 77 d) dropped by 57.7%. The results for the SBGOFMSW and SMYOFMSW digested at 25 and 23 \({}^{\circ}\)C showed that an extended HRT in test series 2 (21 d) was not sufficient to deliver similar results. However, the results (25 \({}^{\circ}\)C beneficial, 23 \({}^{\circ}\)C disadvantageous) should be questioned because all yields were lower in test series 2, even for variants that were operated at 35 \({}^{\circ}\)C. Nevertheless, it can be deduced that the microorganisms do not perform efficiently without heating or strongly increased HRT, which was confirmed by [PERSON] et al. [51]. As mentioned, the SMYOFMSW at 35 \({}^{\circ}\)C also decreased by approximately 17.5%, compared to OFMSW digested at 40 \({}^{\circ}\)C, probably due to the lower activity of microorganisms [55]. Although both test series were executed with the same procedure, the differences between the OFMSW variants at 23, 25, 35, and 40 \({}^{\circ}\)C could also be influenced by variations in the quality of the DSS and the potential inhomogeneity of OFMSW samples.