## 3 Results
### Scenario 3
If heat recovery from flash steam is considered, then a portion of this energy can be recovered, reducing the energy deficiency. If this is the case, for the given operating conditions and using the flash steam calculator [58], the energy contained in flash steam will be 8042 MJ. Under similar assumptions for estimating the energy recovery for the previous scenarios, an 85% recovery capacity will lead to 6840 MJ available, which is still far from the deficit associated with the thermal treatment. Additional energy is recovered from cooling the pretreated slurry to the digester temperature. Considering that the incoming stream to the digester should be at 37 \({}^{\circ}\)C and also taking into account that process water is added to adjust solid content to 10%, then an additional amount of energy would be available through the heat exchanger dedicated to this aim. This recovery is considered to contain low-quality energy if water is used as a transfer fluid. Thus the energy obtained from cooling the slurry to a temperature of 55 \({}^{\circ}\)C will be available for other uses, and this quantity is 4885 MJ/d. The slurry stream is mixed with cheese whey (25% (\(v/w\)) addition--keeping the co-digestion assumption also for this case) and process water (attaining a volumetric flow of 32.5 m\({}^{3}\)/d). If the mixture of these streams takes place by directly combining the hot slurry stream with the other influents to adjust solid content, then a final temperature of the mixture of 46 \({}^{\circ}\)C is reached. This value comes from assuming a cheese whey temperature of 15 \({}^{\circ}\)C and a process water temperature of 35 \({}^{\circ}\)C. The final temperature reached can cover digester thermal demand, including losses associated with digester walls. Thus the heat demand of the digester is eliminated. However, this energy recovery is insufficient to cover the demand of the whole system. For the present scenario, a feed solid content of 15% TS was set. However, this solid concentration adversely affects the capacity of this fluid to be pumped. Low-solid content sludge behaves as a pseudoplastic fluid, but at high-solids content (with TS values in the range of 7-15%), thixotropic behavior is exhibited [108]. Sludge viscosity increases significantly with an increase in total solid content, but on the contrary, decreases with the increase in shear rate. Thus, future aspects being considered are the actual feasibility of applying thermal treatments as either a pre-treatment or post-treatment stage, when pumping capacity and slurry handling becomes a challenging task with an increase in the solid content. Scenario 2 evaluating thermal post-treatment of digestate considered that a slurry stream dewatered at 30% TS content is submitted to HTC treatment. However, loading and unloading operations for this system will set severe constraints regarding the handling of this stream with such a high TS value. Comparing these two scenarios at similar TS  
Figure 5: Representation of the main treatment stages in Scenario 3 (thermal hydrolysis and anaerobic digestion of hydrolyzed manure). Cheese whey is used also as co-substrate in this scenario. values, (it should be borne in mind that thermal hydrolysis units can handle sludge streams between 12.5 and 16% for CambiTM Process and 20% for the ExelisTM process [109]), then the amount of energy needed for running the HTC unit will rise to 25,365 MJ which makes the process unfeasible. If this same configuration is evaluated when only a fraction of the digestate is submitted to hydrotreatment, 3800 MJ will be needed. Further research is needed to find optimal configurations and evaluate the plant's electrical demand once the solid content of the slurry is established. Another relevant factor is the possible toxicity of hydrochar material when used in biological processes, just as it has already been reported for biochar [110].