## 2 Materials and Methods
### AD Experiments and Process Monitoring  
Biochemical methane potential tests were carried out according to VDI 4630 [66]. The volumetric biogas production was measured using glass manometers (1-L gas storage) considering the temperature within the digester and ambient conditions. The biogas composition was analyzed with portable biogas monitors (BIOGAS 5000 and GAS 5000, Geotech, Coventry, UK) from biogas collected and stored in bags (PLASTIGAS, Linde, Pullach, Germany). The specific biogas and CH4 production were related to oDM and calculated for standard conditions (1013 hPa, 0 \({}^{\circ}\)C, dry gas). The configuration of each digester, the installed gas measurement system, and the storage bag as used in this study is depicted in [PERSON] et al. [74]. The experiments were conducted in two batch test series at 25/40 \({}^{\circ}\)C and at 23/35 \({}^{\circ}\)C using 2-L insulated glass vessels with preprogrammed heating, which were automatically stirred for 60 s/h. In each test series, 12 digesters were used (thereof four blank variants). The operating temperature of 35 and 40 \({}^{\circ}\)C was selected to represent typical German mesophilic biogas processes. The lowered operating temperature of 25 \({}^{\circ}\)C in the first test series was chosen in order to achieve an adequate reduction of the heat demand and in order to remain close to the mesophilic temperature level (Table 1). The temperature of 23 \({}^{\circ}\)C was the lowest possible temperature in the laboratory. It was therefore chosen in the second test series. The detailed experimental setup is provided in Table 2.  
In the first test series, the biogas composition was measured on day 5 (only variants with OFMSW as feedstock) and day 56 (all digesters). In the second test series, the biogas measurements of day 24 were used to extrapolate the CH4 yield results. Therefore, the measured biogas yields (mL/g\({}_{\text{oDM}}\)) from day 25 to days 35 and 77 were transferred to CH4 yields (mL/g\({}_{\text{oDM}}\)) based on the CH4 concentration (%) measured on day 24. This \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Series 1 T25**} & \multicolumn{3}{c}{**Series 1 T40**} & \multicolumn{3}{c}{**Series 2 T23**} & \multicolumn{3}{c}{**Series 2 T35**} \\ \cline{2-9}
**Variant** & \begin{tabular}{c} **Inoc.** \\ **(blank)** \\ \end{tabular} & \begin{tabular}{c} **Inoc. and** \\ **OFMSW** \\ \end{tabular} & \begin{tabular}{c} **Inoc. and** \\ **(blank)** \\ \end{tabular} & \begin{tabular}{c} **Inoc. and** \\ **(blank)** \\ \end{tabular} & \begin{tabular}{c} **Inoc. and** \\ **OFMSW** \\ \end{tabular} & \begin{tabular}{c} **Inoc. and** \\ **(blank)** \\ \end{tabular} & \begin{tabular}{c} **Inoc. and** \\ **OFMSW** \\ \end{tabular} & \
\begin{tabular}{c} **Inoc. and** \\ **(blank)** \\ \end{tabular} &
\begin{tabular}{c} **Inoc. and** \\ **OFMSW** \\ \end{tabular} \\ \hline \multirow{2}{*}{Inoculum} & 0.8 L DSS & \multirow{2}{*}{1.2 L water} & 0.8 L DSS & 0.8 L DSS & 1 L DSS & 1 L DSS & 1 L DSS & 1 L DSS & 1 L DSS \\  & 1.2 L water & & 1.2 L water & & 1 L water & 1 L water & 1 L water & 1 L water \\ \multirow{2}{*}{Feedstock} & - & 15 g DM & - & 15 g DM & - & 10 g DM & - & 10 g DM \\  & & OFMSW & - & OFMSW & - & OFMSW & - & OFMSW \\ \multirow{2}{*}{Retention time (d)} & 56 & 56 & 56 & 56 & 77 & 77 & 35 & 35 \\  & 56 & 5; 56 & 5; 56 & 8; 24 & 8; 24 & 8; 24 & 8; 24 \\ \multirow{2}{*}{Day with gas analysis} & \multirow{2}{*}{56} & \multirow{2}{*}{5; 56} & \multirow{2}{*}{5; 56} & \multirow{2}{*}{5; 56} & \multirow{2}{*}{8; 24} & \multirow{2}{*}{8; 24} & \multirow{2}{*}{8; 24} \\  & 2 & 4 & 2 & 4 & 2 & 4 & 2 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental setup of the anaerobic digestion experiments of both test series at 25/40 \({}^{\circ}\)C and 23/35 \({}^{\circ}\)C. Digested sewage sludge (DSS) was used as inoculum (Inoc. ), while dry matter (DM) of the organic fraction of municipal solid waste (OFMSW) served as feedstock.