## 4 Conclusions  
The energy assessment of high temperature and pressure systems was evaluated considering different scenarios where anaerobic digestion of the material was integrated to recover energy as biogas. The hydrothermal treatment of digestate was the best performing option when contemplating the thermal demand associated with this process. The treatment of the whole digestate under the HTC process resulted in high thermal demand. Yet, even in this case, the energy requirement could be fully provided by the use of the hydrochar as fuel in the same process, thus providing thermal energy for steam production and heat for hydrochar drying. In addition, the mass and energy calculations here performed demonstrated that the HTC process can be operated without the need for supplementary fossil fuels, solely using the energy contained in the incoming waste. The most favorable case was co-digestion which significantly increased the biogas productivity of the digester, even with the use of a low organic content stream as with cheese whey herein considered. The partial treatment of digestate by HTC allows hydrochar to be used as a supplement in the digestion process and as an additive if composting of digestate is selected as the final stabilization stage. This configuration increases biogas production and secures full coverage of the thermal demand, given that the HTC unit now has a significantly lower volume. Additionally, the use of hydrochar in the biological process eliminates the drying step, thus reducing the thermal demand of the whole treatment sequence. When conventional thermal hydrolysis was evaluated as a pre-treatment stage before the digestion process, then the size and number of reactor units dedicated to this aim present a high thermal demand. Even though heat recovery was considered and the digester heat demand was eliminated in this configuration, the energy surplus obtained from this strategy does not compensate for the comparatively high demand of the thermal process. \begin{table}
\begin{tabular}{c c c} \hline
**Scenario Main Characteristics** & **Strength** & **Weakness** \\ \hline \multirow{3}{*}{Scenario 1: treatment of manure (AD 1 + HTC 2)} & Great reduction of waste final disposal & High energy demand for running the HTC unit \\  & Char obtained as product and used as fuel to supply & (10,755 MJ/d) \\  & energy demand & Combustion of hydrochar requires drying, increasing \\  & Treatment of HTC process water in the digestion unit & heat demand \\ \hline \multirow{3}{*}{Scenario 2: treatment of manure and co-substrate (cheese whey), (AD + HTC, compost production)} & Production of active compost supplemented with hydrochar & Producing active compost requires additional land \\  & Enhanced biogas production & A market should be available for \\  & Reduced thermal demand of the global process. & commercializing compost \\  & HTC unit treats a fraction of digestate to cover the hydrochar & Economic feasibility of the plant should be \\  & amount needed in biological conversion & considered in future work due to the low market \\  & & & \\ \hline \multirow{3}{*}{Scenario 3: Thermal hydrolysis of manure and co-substrate (cheese whey), (AD)} & Enhanced biogas production & \multirow{3}{*}{Excessive energy demand for running the thermal} \\  & Reduction of digestate disposal due to the higher conversion & hydrolysis unit (27,580 MJ/d) \\ \cline{1-1}  & of manure VS & \\ \hline \end{tabular}  
* AD: anaerobic digestion. 2 HTC: hydrothermal carbonization.  
\end{table} 
Table 5: Strengths and weaknesses of each scenario evaluated considering the use of digestion for producing biogas and the introduction of thermal treatment for digestate vaporization (Scenario 1 and 2) or as a pre-treatment (Scenario 3). **Author Contributions:** Conceptualization, R.G. and J.G.-C.; methodology, D.C.-P.; validation, A.A.-P., R.G. and R.S. ; formal analysis, X.G. ; investigation, A.A.-P. and M.E. ; resources, X.G. ; data curation, M.E. and A.A.-P.; writing--original draft preparation, X.G. ; writing--review and editing, R.G.