## 2 Materials and Methods
### AD Experiments and Process Monitoring
procedure had to be conducted in accordance with SARS-CoV-2 laboratory regulations (limited laboratory access). The inoculum was added without further degassing (already treated anaerobically at the treatment plant). However, blind variants were carried out in duplicate, determining the residual biogas potential of the DSS. According to VDI 4630 [66], DSS contains a broad variety of microorganisms. Therefore, all digesters started with the same initial conditions at 37 \({}^{\circ}\)C, reaching the designated operating temperature several hours later (no long-time acclimation period for microorganisms was executed). In variants with added tap water, the water was preheated to 37 \({}^{\circ}\)C. The experiments were stopped as soon as the biogas formation rate per digester remained close to 1 mL/h. Only the digesters at 23 \({}^{\circ}\)C (unheated) were operated with an increased retention time (approximately doubled compared to the digesters at 35 \({}^{\circ}\)C). For both test series, the same OFMSW sample was used. The specific biogas yield of the mixture of OFMSW and DSS in each digester (Table 2) was calculated as  
\[SBG_{OFMSW\ and\ DSS}=\frac{BG_{OFMSW\ and\ DSS}}{m_{oDM,OFMSW}+m_{oDM,DSS}}, \tag{2}\]  
where \(SBG_{OFMSW\ and\ DSS}\) (mL/g\({}_{\text{oDM}}\)) is the specific biogas yield from the mixture of OFMSW and DSS, \(BG_{OFMSW\ and\ DSS}\) (mL) is the total gas yield from the mixture of OFMSW and DSS (used as inoculum), and \(m_{oDM,OFMSW}\) and \(m_{oDM,DSS}\) (g\({}_{\text{oDM}}\)) are the organic mass of OFMSW and DSS, respectively. The specific biogas yield of OFMSW alone was calculated as  
\[SBG_{OFMSW}=\frac{BG_{OFMSW\ and\ DSS}-BG_{DSS}}{m_{oDM,OFMSW}}, \tag{3}\]  
where \(SBG_{OFMSW}\) (mL/g\({}_{\text{oDM}}\)) is the specific biogas yield from OFMSW alone, and \(BG_{DSS}\) (mL) is the biogas yield from DSS, i.e., from the corresponding blanks. The specific CH\({}_{4}\) yield of the mixture of OFMSW and DSS was calculated as  
\[SMY_{OFMSW\ and\ DSS}=\sigma_{OFMSW\ and\ DSS}\cdot SBG_{OFMSW\ and\ DSS}, \tag{4}\]  
where \(SMY_{OFMSW\ and\ DSS}\) (mL/g\({}_{\text{oDM}}\)) is the specific CH\({}_{4}\) yield from the mixture of OFMSW and DSS, and \(\sigma_{OFMSW\ and\ DSS}\) (-) is the measured volume concentration of CH\({}_{4}\) in the biogas from the mixture. The specific CH\({}_{4}\) yield of the blanks was calculated as  
\[SMY_{DSS}=\sigma_{DSS}\cdot SBG_{DSS}, \tag{5}\]  
where \(SMY_{DSS}\) (mL/g\({}_{\text{oDM}}\)) is the specific CH\({}_{4}\) yield from DSS (blanks), \(SBG_{DSS}\) (mL/g\({}_{\text{oDM}}\)) is the specific biogas of the DSS, and \(\sigma_{\text{DSS}}\) (-) is the measured volume concentration of CH\({}_{4}\) in the biogas from the blanks. Finally, the specific CH\({}_{4}\) yield of OFMSW alone was calculated as  
\[SMY_{OFMSW}=\frac{\sigma_{OFMSW\ and\ DSS}\cdot BG_{OFMSW\ and\ DSS}-\sigma_{DSS} \cdot BG_{DSS}}{m_{oDM,OFMSW}}, \tag{6}\]  
where \(SMY_{OFMSW}\) (mL/g\({}_{\text{oDM}}\)) is the specific CH\({}_{4}\) yield from OFMSW alone. In experiments with two subsequent gas analyses (Table 2), measured CH\({}_{4}\) concentrations have been applied for the corresponding time interval. For all experiments, \(SMY_{DSS}\), \(SMY_{OFMSW}\), and \(SMY_{OFMSW\ and\ DSS}\) results were plotted based on mean values (n = 2 for DSS variants, n = 4 for variants containing OFMSW and DSS) on day 35 in order to evaluate the goodness of fit. The coefficient of determination (R\({}^{2}\)) was determined with the help of a linear regression line.