## 3 Results  
### Scenario 1  
Assumptions made for Scenario 1 considered the feeding material to be diluted at a 10% TS to avoid problems regarding floating straw and forming a scum layer on the top of the reactor. The biogas production expected for this system is 752 m\({}^{3}\)/d, if only manure is fed into the reactor. The addition of process water from the HTC system would lead to 852 m\({}^{3}\)/d, representing a 13% increase in gas production, which is almost incidental considering the requirements of installing such a high-pressure system. This low increment is due to the low methane yield of the HTC process water. Table 2 presents the values of the main parameters studied for the different scenarios and Figure 2 provides an outline of the process with main streams represented. The energy content in biogas was estimated considering a methane composition of 60% and the LHV of this gas. The digester thermal demand was calculated by considering the incoming substrate and disregarding the sensible enthalpy of the HTC process water. \begin{table}
\begin{tabular}{c c} \hline Plant Characteristics & Scenario 1 \\ \hline Substrate & Manure \\ TS content of feed & 10\% \\ TS of the feed after adding HTC process water & 8.5\% \\ Organic loading rate (based on reactor working volume) & 1.8 kg/m\({}^{3}\) d \\ Digester inlet (volumetric flow) & 38.2 m\({}^{3}\)/d \\ Digester thermal demand & 4120 MJ/d \\ Biogas production from manure & 752 m\({}^{3}\) \\ Biogas production from manure and HTC process water & 852 m\({}^{3}\) \\ Energy in biogas & 18,300 MJ/d \\ Mass flow of dewatered digestate (30\% TS content) & 7970 kg/d \\ TS mass flow of digested material submitted to HTC & 2392 kg/d \\ HTC energy demand & 10,755 MJ/d \\ Thermal energy recovery (from flash steam) & 3245 MJ/d \\ Mass flow of hydrochar (at 40\% TS content) & 4717 kg/d \\ Energy demand for hydrochar drying & 7521 MJ/d \\ Energy from hydrochar combustion (85\% energy recovery) & 27,778 MJ/d \\ \hline \end{tabular}
\end{table} 
Table 2: Main calculated parameters of the integrated system: anaerobic digestion and HTC. The digested material is dehydrated at 30% TS content, assuming that inorganic solids remain constant during the biological process. Therefore, the mass of solids treated in the HTC is 2392 kg/d. This material is loaded into the reactor under batch operating conditions. Thus loading and unloading operations need to be coupled with the continuous production of sludge. For this reason, two HTC units were assumed with a reaction working time of 1 h and a steam injection time of 45 min. The discharge of the HTC system should allow the production of flash steam for energy recovery, whereas the charge and unloading of the reactor tank were assumed to take place in 3 h for each operation, needing two reactors for completing the cycle to treat the whole digested stream derived from the fermentation. If the total dehydrated sludge produced in a single day is to be treated in these thermal units, the boiler needs to provide the whole steam demand associated with the HTC treatment system for a 90 min period distributed in two cycles. The amount of steam added to the HTC process, considering that adiabatic conditions are reached for saturated water at 200 \({}^{\circ}\)C (15 bar) would be 1200 kg/batch with a steam flow of 1602 kg/h. The steam flow is provided by a boiler for a daily period of 90 min. The energy needed for producing the steam flow is then 5380 MJ/h, as estimated for 85.7% boiler efficiency. The energy demand associated with heating was 1932 MJ for the two working reactors with 5.5 m\({}^{3}\) of volume. This value is to be added to the energy demand of the boiler, finally accounting for the HTC system already needing 57% of the energy produced from biogas. Thus the recovery of thermal energy is imperative to increase process feasibility.