## 3 Results
### Scenario 1
If the production of flash steam from the HTC reactor is considered and conditions of saturated water are assumed, then the amount of flash steam to be produced from the two cycles would be 993.4 kg/d at 1.3 bar. If 85% of the energy can be recovered, then 2268 MJ/d are now available to cover some of the thermal demand of the plant. An additional amount of energy can also be recovered from the cooling of the HTC process water. For a flow of 5200 L/d of HTC process water, an additional 977 MJ/d can be obtained as thermal energy. However, this latter energy is of lower quality since it is derived from cooling a liquid stream from 90 to 40 \({}^{\circ}\)C. This heat can be recovered by mixing the hot process water stream with the fresh manure slurry to increase thus its temperature. Valorizing hydrochar by combustion is a process requiring the drying of this material and, therefore, increasing the energy demand of the global process. The energy needed for drying was calculated assuming a water content of 60% in the dewatered hydrochar. Thus the thermal energy needed for attaining a 10% moisture content was 797.2 kWh per ton of water evaporated, translating into 7521 MJ/d of energy to dry this material. The valorization of hydrochar by combustion is necessary to cover the high energy demand of the process. However, alternative uses of hydrochar have been proposed. The  
Figure 2: Representation of the main treatment stages in Scenario 1 (anaerobic digestion and subsequent HTC treatment of the whole digestate stream). use of char as an active supplement in organic fertilizers, the production of activated carbon, or its use as an adsorbent to remove bacterial pathogens are some examples [69; 70; 71]. However, the high energy demand of the global process necessitates a reliance on fossil fuels which translates into higher operational costs and negative effects associated with CO\({}_{2}\) fossil fuel emissions. Based on assumptions regarding the LHV of hydrochar, the energy in this material would be 27,974 MJ/d. This amount is enough to cover the thermal demand of the HTC process and drying. Therefore, there is no need for diverting biogas to produce heat, as this fuel can be fully valorized for electricity production, which yields 1728 kWhe per day. If the combined digestion and HTC system is compared with the single production of biogas from manure, then only 33% of energy is recovered. This calculation was made for biogas being the only fuel susceptible of valorization. The value was calculated as the ratio between the energy contained in biogas and that of fresh manure as well as considering data reported by [PERSON] et al. [72], with an HHV of 19.1 MJ/kg (free ash manure) reported along with the ultimate analysis for this material. Evidently, the direct combustion of either manure or digestate can be performed, but in the first case, no gaseous fuel is obtained capable of valorization in a CHP unit. If digestate is directly used as fuel, then assuming values reported by [PERSON] et al. [48] for digestate composition, the LHV of digested manure would be 11.4 MJ/kg, which translates into 27,340 MJ/d energy. The energy demand for drying would be 15,251 MJ/d after dewatering this material to a solid content of 30% (as in the previous case). This quantity can be easily provided by the combustion of the digestate if 85% of heat recovery is obtained from combustion. However, this strategy does not account for benefits regarding the production of hydrochar and positive effects associated with its application as it would be the use of char as a supplement in the same digestion process for producing active fertilizers. [PERSON] et al. [73] studied the combustion characteristics of digested material and the pyrochar and hydrochar obtained from the thermal processing of digestate, and reported better combustion characteristics for digestate and pyrochar, whereas hydrochar presented a poor combustion scenario.