## 3 Results and Discussions
### Productivity and Energetic Assessment of the Integrated System of HTL and ACD Subsystems
The estimated yields of the HTL product fractions are presented in Figure 9.  
Figure 9 shows that a low biocrude yield of 9.55 wt.% (percentage of kg-biocrude/kg-dry digestate) was estimated from the HTL of the digestate. This biocrude is an energy dense product that can serve as a liquid fuel and also as a valuable source of platform biochemicals [74]. The low biocrude yield was not unexpected, since the lipid content of the HTL feedstock is a crucial factor influencing HTL biocrude yield [75]. The mean lipid content of the HTL feedstock in this study was only 1.6 wt.%. The HTL insoluble biochar yield was estimated to be 32 wt.% (percentage of kg-biochar/kg-dry digestate), which is within the range of 1.3-35.0 wt.% (percentage of kg-biochar/kg-dry digestate) measured for HTL processing of different biomass feedstocks [45]. The soluble solids present in the post-HTL water constitutes another HTL product stream and was estimated to be relatively high at 38.19 wt.% (percentage of kg-soluble solids/kg-dry digestate) compared to typical values ranging from 4.6-31.2 wt.% (percentage of kg-soluble solids/kg-dry digestate) of soluble solid yields obtained from biomass reported in the literature [45]. Due to the typically low concentrations of the soluble solids in the post-HTL water, it may not be a viable approach to undertake further energy intensive product recovery steps [76]. The gas yield of 20.27 wt.% (percentage of kg-gas/kg-dry digestate) was also estimated. This gas product is typically composed of CO\({}_{2}\) gas [77; 78]. The low concentrations of soluble solids in the post-HTL water and the typically high concentration of CO\({}_{2}\) in the gas product imply that the post-HTL water and the gas product may not present immediate secondary application opportunities. However, based on the combined yields of biochar and biocrude of over 40 wt.% (percentage of kg-useful product/kg-dry digestate), further experimental investigation of the HTL processing of digestate may be justified as a basis of future work. This is because biochar and biocrude may constitute valuable products that may provide additional energetic and economic benefits to the ACD system. This implies that in addition to the utilization of the biocrude product as a liquid fuel or source of biochemicals, the HTL biochar product may be utilized in soil remediation, carbon sequestration and as a cheap adsorbent for the removal of impurities such as phenol, heavy metals and dye from waste water [79]. Applying Equations (12) and (13), the \(AOS_{c}\) was determined to be \(-0.3255\) and the HHV of the energy dense biocrude product was subsequently estimated to be 34 MJ/kg. The calculated HHV of the biocrude product is within the range expected for the typical biocrude product from the HTL processing of biomass, which was reported to be in the range from 30 MJ/kg to 38 MJ/kg [80]. Applying Equations (14)-(20) for different digestate moisture contents (fractional) ranging from 0.999 wt. to 0.5 wt., the NER was determined and is presented in Figure 10.  
Figure 9: Theoretically determined yield of the major HTL products from the digestate feedstock: db indicates dry basis. Figure 10 shows that the NER negatively correlates with increments in the moisture content of the digestate, with the fractional moisture content of the digestate necessary for environmental sustainability being less than 0.733 wt. The result suggests that it may be necessary to incorporate a compromise between the need to employ a large mass of the buffer solution and the need to achieve a favorable environmental performance with NER > 1. Additionally, Equation (10) suggests that an improved waste heat recovery from the integrated system of HTL and the ACD system will also serve to improve overall environmental performance.