## 3 Results
### Scenario 1
Several authors have reported digestion enhancement when char is added as carbon conductive material to the anaerobic process [74; 75; 76], indicating that the presence of this external material serves as electrons' transport leading to an innovative pathway for improving performance [77]. [PERSON] et al. [78] reported that the addition of hydrochar favored methane production rate thanks to the abundance of surface oxygen-containing functional groups. Therefore, adding char back to the digestion process would provide an additional benefit for energy recovery regarding the greater biogas production expected. A dose of 10 g/L was evaluated by [PERSON] et al. [79], reporting an increase in biogas yield up to 22%. This same dose was tested by [PERSON] et al. [80] when digesting citrus waste. [PERSON] et al. [81] reported doses between 2-10 g/L with an average enhancement in methane yields of 47% under mesophilic conditions. Higher doses were tested by [PERSON] et al. [75]--50 g/L in a single application to the reactor--and by [PERSON] et al. [82]--(30 g/L)--under biochemical methane potential (BMP) tests reporting no detriments in the fermentation. Other doses reported per mass of substrate are those tested by [PERSON] et al. [83], with values between 0.5-2.5 g char/g substrate. A low amount would be assumed to avoid penalizing energy recovery from hydrochar combustion in the hypothetical case here studied. The addition of char supplement to the digester will consider a dose of 5 g/L of hydrochar with a 15% increase in methane yield. Under this assumption, the biogas production is increased to 958 m\({}^{3}\)/d, but the solid balance is hardly affected. This is explained because the system experiences a low increase in digestate concentration associated with char addition, but the enhancement in volatile solid removal offsets this effect. Thus the global energy balance is further improved when a small fraction of char is supplemented to the digester. The amount of energy now available from the combustion of char would be 26,400 MJ/d, and the energy demand for hydrochar drying is scarcely increased. The energy balance leads to positive results regarding the valorization of the material by transforming it into biogas and heat derived from hydrochar combustion. Similar results have been also reported by [PERSON] et al. [84] indicating that a cascade approach integrating digestion and HTC is the best option for dealing with organic wastes, but a responsible energy source needs to be supplemented to cover the high energy demand of the HTC process. [PERSON] et al. [85] reported the performance of HTC and digestion as a combined approach for treating organic wastes. These authors also brought out the same conclusion regarding the better energy efficiency of this alternative when the moisture content in the waste stream is high. However, this analysis is obviating the high installation costs of such thermal hydrotreatment requiring specialist vessels to support the high pressure associated with harsh conditions and install a high-pressure boiler to provide the necessary steam. Pyrolysis of the digestate would be also an alternative treatment for obtaining energy. However, in this latter case, the drying of the dewatered slurry would need an excessive amount of heat. The pyrochar produced would be much lower because this process has a lower char yield against that of hydrothermal carbonization [86]. Therefore, oil and gas products from pyrolysis would also be used for covering the thermal demand for drying and to keep running the pyrolysis unit [53].