## 3 Results
### Scenario 2  
The addition of a co-substrate would increase the amount of biogas available in the system and, therefore, benefit the process's feasibility by increasing revenues derived from biogas valorization. If a variable amount of co-substrate is added from 25 to 50%, then the amount of biogas produced can be estimated based on biochemical methane potential (BMP) data reported by [PERSON] et al. [50], considering that the total solid content of the feed is kept constant at 10% by the addition of process water from digestate dewatering operations. The addition of the co-substrate in a 25% mass percentage causes a moderate increase in biogas production (11%) due to the low solid content of the cheese whey stream here considered. Doubling this quantity to 50% causes just a 34% increase if compared with the case of no co-substrate addition (see Figure 3). Thus the advantage of adding a high methane yield co-substrates is lost by the low organic load of the stream here considered (TS content values based on data reported by [PERSON] et al. [49]). The high content in volatile solids of the co-substrate and its high biodegradability scarcely affect the system's solid balance (Figure 4). Thus, adding this carbon-rich source was estimated to increase the mass of digested solids in the range of 4 to 11%. The assumption considered additive effects in methane production disregarding synergistic effects, which would lead to better results due to the higher gas obtained and lower volume of digestate needing final disposal. Different authors have w  
Figure 3: Daily biogas production obtained from the addition of different co-substrate proportions to the mixture (the mixture is composed of cheese whey and cattle manure). The value of 0 in the graph stands for the baseline case where no co-substrate addition is implemented. Substrate addition was made as a percentage representing volumetric addition of cheese whey to the global mass of the feeding mixture. effects when evaluating mixtures of manures and carbon-rich content substrates [87; 88; 89; 90]. Therefore, the remaining amount of solids in the digested slurry should not be significantly affected. Keeping the conservative assumption of no synergies, then it is evident that adding a low organic content co-substrate for attaining just a 34% gas improvement may not be worthwhile, given the additional investments necessary for transporting and storing the co-substrate. Other authors have reported the use of more concentrated whey streams, as it is the case of [PERSON] et al. [91] with a TS content of 134.87 g/L or [PERSON] et al. [92] reporting a solid content of cheese whey of 84.3 g TS/L. The availability of such type of material will highly enhance biogas production. Another important fact is that the valorization process assumes that the only valuable product (hydrochar) is used for heat production. This strategy is justified by the need to supply the thermal demand to the whole process. However, if heat is obtained in excess, this energy has a low market price. Other valorization options are more attractive, including producing high-quality compost enriched with hydrochar or applying other thermal processes capable of producing higher valuable biofuels, such as gasification or pyrolysis. Digestate can also be valorized for attaining the recovery of nutrients [93], as it would be the use of digestate liquid fraction as media for ethanol production [94] or for producing biofertilizers based on the use of plant growth promoting rhizobacterium (PGPR) [95]. Digestate can also be transformed into methanol and hydrogen via gasification and posterior syngas conversion by water gas shift reaction [96]. Other processes consider the treatment of the recalcitrant carbon fraction of digestate, or of the remaining straw, to be further transformed into ethanol via fermentation [97; 98]. There is a wide range of  
Figure 4: Representation of the main treatment stages in Scenario 2 (anaerobic digestion and subsequent HTC treatment).