## 3 Results and Discussions
### Productivity and Energetic Assessment of the Integrated System of HTL and ACD Subsystems  
The digestate residue was characterized and the measured values are presented in Table 5. Table 5 shows that the measured average percentage contents of elemental carbon (C), hydrogen (H), nitrogen (N), oxygen (O) and ash content on a dry basis (percentage of kg-element/kg of dry digestate) present in the digestate from the ACD process were 28.64 wt.%, 4.45 wt.%, 2.8 wt.%, 24.6 wt.% and 39.53 wt.%, respectively, with sulphur (S) content being negligible. Additionally, Table 5 shows that the average percentage contents of carbohydrate, crude protein and lipid present in the digestate, on a dry basis (percentage of kg-carbohydrate or protein or lipid/kg of dry digestate) were 41.4 wt.%, 17.5 wt.% and 1.6 wt.%, respectively. The mean specific gravity and mean pH value of the digestate were also determined to be 1.02 and 8.23, respectively. An assessment of elemental carbon mass flow in the substrate mixture of Mix-4, into the ACD process, compared to the sum of the mass of the elemental carbon in the biogas stream and the mass of the elemental carbon in dried digestate has been undertaken. The mass of elemental carbon fed to the anaerobic system in Mix 4 is shown to be comparable to the total mass of elemental carbon of the product streams of biogas and digestate with a relative error of 0.08. The methods employed in the mass balance calculation are presented in detail in Section S2 of the supplementary document. This establishes the accuracy of the methods employed above. Possible reasons for the difference between the mass of elemental carbon fed to the anaerobic system and that in the combined biogas stream and dried digestate stream are also discussed in Section S2 of the supplementary document. The high percentage of carbohydrate content, 41.4 wt.%, of the digestate is not unexpected, as carbohydrate typically constitutes the major organic component present in the biogas digestate. Previous work showed that the carbohydrate content of AD digestate residue was as high as 54 wt.% [72]. The measured carbohydrate content suggests the presence of hemi-cellulosic and cellulosic carbohydrate forms in the AD digestate residue, since these carbohydrate forms are characterized by their poor solubilization rates and their degradation difficulties [73]. The average crude protein content of 17.5 wt.% and the average lipid content of 1.6 wt.% were comparable to the average crude protein content and average lipid content of anaerobic digestate from a waste water treatment plant, as reported in a previous study, as approximately 15 wt.% and 1 wt.%, respectively [72]. The high average ash content (39.53%) of the digestate may be explained to be due to the high ash content of the original substrates anaerobically digested. Differences in the organic composition of the digestate obtained in this study and other digestates reported in the literature may be due to disparate compositions of the original AD substrates. Employing Equations (7)-(10) above, the yields of the biocrude, gas product, biochar and soluble solids present in the post-hydrothermal liquefaction (post-HTL) water were estimated from the \begin{table}
\begin{tabular}{c c} \hline
**Biogas Digestate** & **Value** \\ \hline Lipid content 1 (wt.\%) & 1.60 (0.2) \\ Protein content 1 (wt.\%) & 17.50 (0.09) \\ Ash content 1 (wt.\%) & 39.53 (0.39) \\ Carbohydrate content 1 (wt.\%) & 41.40 (0.44) \\ Carbon content 1 (wt.\%) & 28.64 (0.10) \\ Nitrogen content 1 (wt.\%) & 2.80 (0.02) \\ Sulphur content 1 (wt.\%) & Trace \\ Hydrogen content 1 (wt.\%) & 4.45 (0.07) \\ Oxygen content 1 (wt.\%) & 24.60 (0.40) \\ pH value of wet digestate & 8.32 (0.32) \\ Bulk specific gravity at 20 \({}^{\circ}\)C & 1.02 (0.02) \\ \hline \end{tabular} Standard errors are shown in parentheses based on duplicate measurements; 1 measured on a dry digestate basis.  
\end{table} 
Table 5: Characteristics of digestate residue. measured physicochemical properties of the digestate.