## 3 Results and Discussion
### Efficiency Potentials through Lowered Process Temperatures  
By reducing operating temperatures, the efforts and costs for digester heating and technical equipment can be reduced or (depending on the ambient temperature) even completely eliminated. In general, the heat demand of biogas plants arises due to heat losses of the digester and because of the necessity to heat the incoming substrate (Appendix A). The main proportion of the heat demand is related to substrate heating. Therefore, it is advisable to preheat the amount of OFMSW before starting the AD process. In practice, this can be conducted by self-heating processes with a short phase of aerobic treatment, which is stopped as soon as the OFMSW reaches the designated process temperature for AD. However, this process leads to lower energy potentials of the OFMSW. Saved heating costs could possibly compensate for lower gas yields, especially if, as can be seen, for example, with DSS, the CH\({}_{4}\) concentration increases due to the lower operating temperatures. The reduction of treatment costs for biogas purification is a possible secondary effect of lower operating temperatures. However, larger digester volumes with increased HRT, and sepa  
Figure 4: Specific methane yield (SMY) on day 35 of the experiment based on organic dry matter (oDM) for the organic fraction of municipal solid waste (OFMSW) and digested sewage sludge (DSS) alone and in the mixture. Gas volume at 1013 hPa, 0 \({}^{\circ}\)C; mean values \(\pm\) standard deviation including trend lines with n = 2 for DSS, n = 4 for OFMSW, n = 4 for OFMSW and DSS. rate sanitation systems (biowaste) seem to become necessary. The balance between energy savings due to lower digester temperatures, changes in biogas yields, and infrastructural costs has to be investigated in further research based on detailed simulation to determine optimum conditions due to various interdependencies. In this study, an operating temperature of 25 \({}^{\circ}\)C for the AD of OFMSW together with DSS delivered similar results, compared to an AD temperature of 40 \({}^{\circ}\)C, and could be beneficial. Although the experiments for AD of OFMSW were conducted in quadruplicate with relatively low SD, the results have to be verified in further experiments at 25 \({}^{\circ}\)C, especially due to the nonlinear correlation (Figure 4) and the findings of other authors [25; 31; 52] that suggest a linear correlation between SMY and process temperatures. Moreover, the exclusion of the 25 \({}^{\circ}\)C variants delivered well-fitting linear trends. The estimation of an OFMSW-based AD plant with an annual throughput of 10,000 tons of OFMSW (Appendix A) can provide a basic understanding of the influence of the process temperature. When assuming the same energy yield provision at 25 \({}^{\circ}\)C, compared to the energy yield at 40 \({}^{\circ}\)C, the annual heat demand of the AD plant decreases by approximately 10%. However, the hypothesis of heightened efficiencies in the AD of OFMSW together with DSS through lowered operating temperatures cannot be answered conclusively. If an operating temperature of 25 \({}^{\circ}\)C proves in further research that it can deliver similar energy yields, an efficiency increase could be realistic. Otherwise, efficiency potentials through lowered operating temperatures have to be determined based on detailed models with a cost-oriented optimization approach. Notwithstanding the above, the co-digestion of OFMSW together with DSS could also lead to further synergies. First, the SBG\({}_{\text{DSS}}\) and SMY\({}_{\text{DSS}}\) could be increased through a continued treatment in AD. Second, the DSS could serve as a suitable inoculum delivering a broad variety of microorganisms for the AD of OFMSW with its changing characteristics. Furthermore, the co-digestion of OFMSW, which has a high DM content, together with DSS, which has a low DM content, could be a beneficial option to achieve suitable DM contents for wet digestion systems such as stirred-tank reactors without adding water.