## 3 Results
### Scenario 2
\begin{table}
\begin{tabular}{c c} \hline \hline
**Plant Characteristics** & **Scenario 1** \\ \hline Substrate & Manure + cheese whey (25\%) \\ TS content of feed & 10\% \\ TS of the feed after adding HTC process water & 8.9\% \\ Organic loading rate (based on reactor working volume) & 2.2 kg/m\({}^{3}\) d \\ Digester inlet (volumetric flow) & 40.4 m\({}^{3}\)/d \\ Digester thermal demand & 5141 MJ/d \\ Biogas production from manure + cheese whey & 976 m\({}^{3}\) \\ Biogas production after adding HTC process water & 1076 m\({}^{3}\) \\ Energy in biogas & 23,108 MJ/d \\ Mass flow of dewatered digestate (30\% TS content) & 8476 kg/d \\ TS mass flow of digested material submitted to HTC & 2543 kg/d \\ HTC energy demand & 11,490 MJ/d \\ Thermal energy recovery (from flash steam) & 2432 MJ/d \\ Mass flow of hydrochar (at 40\% TS content) & 5105 kg/d \\ Energy demand for hydrochar drying & 8635 MJ/d \\ Energy from hydrochar combustion (85\% energy recovery) & 30,277 MJ/d \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main calculated parameters of the integrated system: anaerobic digestion and HTC Scenario 2. Manure and cheese whey (25% _v/w_) used as substrate. in the digestion process and in composting to attain the conversion of digestate into active compost enriched with hydrochar. Compared with transforming the whole digestate into hydrochar, the thermal demand of the HTC process is 11,490 MJ/d. However, if this option is disregarded in preference of treating just a fraction of digestate, then the energy demand is considerably decreased but the benefits of greater biogas production are retained. The amount of digestate to be treated by HTC to produce hydrochar needed in biological processes was estimated. The amount of digestate to be treated by HTC is then reduced to 15% of the total digestate stream. Thus, the volume of HTC water was also reduced, affecting the total amount of biogas produced. However, given the low methane yield of this stream, the effect is irrelevant. The volume of biogas produced under this assumption is 990 m\({}^{3}\)/d, whereas this value was 1076 m\({}^{3}\)/d when the whole digestate stream was treated by the HTC system. The more significant benefit comes from the much lower demand for energy associated with the HTC, which is 1724 MJ/d. In addition, since hydrochar would be used as a supplement in biological processes, the drying step after dewatering is unnecessary, reducing the thermal energy demand of the whole process. If biogas is used as fuel in a CHP system, electricity and thermal energy are produced simultaneously. For an electrical efficiency of 34% and a thermal efficiency of 51%, the energy recovered as heat (10,984 MJ/d) can clearly compensate for the thermal demand of the process. However, one additional consideration that should be carefully evaluated is that HTC operates under batch conditions. Given the small amount of digestate to be treated, then only one single reactor can be used for the process. The reactor would receive the total mass of steam to be injected in a single cycle. If an injection flow of 10 kg/min is considered, which is equivalent to a steam injection time of 38 min, then the hourly demand of the HTC unit rises to 2700 MJ/h while the engine can produce a flow of 458 MJ/h as maximum. A fraction of biogas must be sent to the boiler to cover the thermal demand of the process. This feature results in a lower production of electricity because of the thermal needs associated with the peak demand imposed by the batch HTC process, decreasing the global efficiency. However, even if this inefficiency is considered, the energy available for the engine will be 19,826 MJ/d (which is 93% of the total amount produced). The thermal demand can be thus met by the engine and a recovery boiler capable of using engine exhaust gases and biogas as fuel. The thermal energy produced from the engine under this configuration will be 10,236 MJ/d with a daily electricity output of 1920 kWh  
The thermal demand of the digester is scarcely affected, amounting to 4390 MJ/d.