## 3 Results and Discussion
### Influence of Process Temperature on AD of OFMSW
In total, the peak values for the biogas production rates of the variants at 23 and 25 \({}^{\circ}\)C were lower, but from approximately day 5 to day 10, the biogas production rates were higher or at a similar level, compared to the variants at 35 and 40 \({}^{\circ}\)C. This could be attributed to a short adaption phase of the microorganisms as the diversity of microorganisms decreases with lowered temperatures [53; 54]. For all variants, the biogas production occurred primarily in the first two weeks allowing conclusions for HRT in practice. The anaerobic treatment of DSS could be increased by one week in order to reduce the residual biogas potential of the DSS. For OFMSW, an HRT of 30 to 40 days seems appropriate in wet digestion with low shares of OFMSW in the mixture. In this study, an inoculum/substrate-ratio of approximately 0.3 in terms of oDM was applied. The HRT depends on various factors such as the organic loading rate or the digestion principle. Therefore, a recommendation of HRT of 30 to 40 days for OFMSW cannot be generalized. \begin{table}
\begin{tabular}{c c c c} \hline
**Variant** & **SBG (mL/g\({}_{\text{oDM}}\))** & **SMY (mL/g\({}_{\text{oDM}}\))** & **\(\sigma\) (\%)** \\ \hline DSS T40 (56 d) & 105.45 \(\pm\) 2.16 & 74.75 \(\pm\) 2.92 & 70.89 \(\pm\) 4.22 \\ DSS T35 (35 d) & 80.75 \(\pm\) 23.92 & 57.88 \(\pm\) 17.09 & 71.69 \(\pm\) 0.07 \\ DSS T25 (56 d) & 78.03 \(\pm\) 10.10 & 62.06 \(\pm\) 10.38 & 79.53 \(\pm\) 3.05 \\ DSS T23 (77 d) & 57.67 \(\pm\) 14.86 & 46.71 \(\pm\) 12.50 & 80.99 \(\pm\) 0.86 \\ \hline OFMSW T40 (56 d) & 552.80 \(\pm\) 17.22 & 325.17 \(\pm\) 6.13 & 58.82 \(\pm\) 0.84 \\ OFMSW T35 (35 d) & 424.49 \(\pm\) 31.59 & 268.35 \(\pm\) 26.74 & 63.22 \(\pm\) 0.67 \\ OFMSW T25 (56 d) & 623.91 \(\pm\) 56.22 & 364.19 \(\pm\) 24.71 & 58.37 \(\pm\) 1.41 \\ OFMSW T23 (77 d) & 270.19 \(\pm\) 33.47 & 171.89 \(\pm\) 21.19 & 63.62 \(\pm\) 2.05 \\ \hline \end{tabular}
\end{table} 
Table 4: Specific biogas yields (SBG), methane yields (SMY), and CH\({}_{4}\) concentrations (\(\sigma\)) of the digested sewage sludge (DSS) and the organic fraction of municipal solid waste (OFMSW) at different temperatures and retention times; mean values \(\pm\) standard deviation. The comparison of process temperature-dependent SMY\({}_{\text{DSS}}\) and SMY\({}_{\text{OFMSW}}\) and SMY\({}_{\text{DSS}}\) on day 35 of the experiments is presented in Figure 4 in order to determine correlations between process temperatures and CH\({}_{4}\) yields for OFMSW and DSS. In the literature, different authors [25; 31; 39; 40; 52] stated a linear correlation between CH\({}_{4}\) yield and process temperature in AD for other substrates. As can be seen in Figure 4, the SMY\({}_{\text{DSS}}\) seemed to be linear dependent on the process temperature. The SMY\({}_{\text{OFMSW}}\) and the SMY\({}_{\text{OFMSW}}\) and DSS delivered different results. While the exclusion of the results for AD of OFMSW at 25 \({}^{\circ}\)C led to a relatively well-fitting linear trend, the joint reflection did not deliver a linear trend. However, the co-digestion of OFMSW with its various ingredients [1], together with DSS as inoculum delivering a variety of microorganisms [66], is not yet tested extensively. When assuming a linear correlation, lower CH\({}_{4}\) yields through lower process temperatures could have been overcompensated by other process parameters in the AD of OFMSW and DSS at 25 \({}^{\circ}\)C. Nevertheless, it cannot be excluded that the process temperature at approximately 25 \({}^{\circ}\)C provides favorable overall conditions. In additional research, this should be validated by detailed process monitoring and through repetition of the experiment.