## 1 Introduction
Other benefits should also be added regarding the safety of the product and improvements in the rheological properties of digestate (lower viscosity and improved dewaterability) [19; 25]. Other studies focus on energy evaluations dealing with global process performance, indicating that the energy demand of the additional equipment associated with the thermal hydrolysis unit may be higher than the energy derived from digestion enhancement [26; 27]. This disadvantage may be offset by a much lower amount of sludge needing final disposal [28], along with a lower volume of reactor required to treat the pre-treated sludge. Therefore, it is of great importance to analyze the performance of the complete process since additional benefits, including synergies associated with the improvement of the global thermal balance, may be attained [29]. Additionally, the application of thermal treatments to other processes also dealing with the treatment of wastes should be explored as a way for increasing global efficiency using, in this case, the wide experience attained in sludge pre-treatment. Reports regarding the performance of thermal hydrolysis of agricultural, animal wastes or organic wastes usually deal with small-scale experimental work [6; 30; 31; 32]. Results reported by [PERSON] et al. [33] indicated an increase in biogas production which can be double the value obtained from fresh lignocellulosic substrate. Conditions tested in their study evaluated the temperature in a range of 100-180 \({}^{\circ}\)C at a duration of 10-120 min. However, other authors have reported a decrease in digestion performance with the severity of the thermal pre-treatment, indicating the formation of recalcitrant compounds such as furfural and 5-hydroxy methyl furfurals [34; 35] and the accumulation of recalcitrant structures with an increasing pre-treatment severity [10]. The HTC process retains similarities with thermal hydrolysis regarding the severe treatment conditions of both techniques. In the latter, the temperature is controlled to avoid the accumulation of excessive recalcitrant compounds because the subsequent treatment stage will be a biological process. Thus HTC can be interpreted as a further severe treatment where conditions are set to maintain the transformation of the feedstock into complex compounds with high carbon content. [PERSON] et al. [36] found that the duration of this process did not have a significant effect on the fixed carbon content of the remaining material, but on the contrary, the increase in temperature had an inverse effect on solid yield and, to a lesser extent, in the energy content. The solid product derived from HTC, also known as hydrochar, has been demonstrated to attain benefits when added as a supplement in anaerobic digestion and composting, similar to those reported by biochar obtained from pyrolysis processes [37; 38; 39; 40; 41; 42]. Other reports have also demonstrated that char may act as an active compound, improving nutrient management in crops [43; 44]. Given these features, the coupling of HTC processes and anaerobic digestion leads to additional improvements associated with the enhancement in organic degradation and reactor productivity. Additionally, the process water derived from the HTC may also be used as a substrate in the digestion stage, thus eliminating the problems derived with the final disposal of this high organic content fraction. There are several reports in the literature reporting the biogas potential of this HTC liquid stream [39; 45; 46]. The present manuscript aims to assess the energy demand of the HTC process integrated into a conventional digestion plant treating cattle manure. Results obtained are compared with those derived from the hypothetical pre-treatment of the same material under thermal hydrolysis. Co-digestion was also considered to increase the amount of energy available in the treatment plant. Cheese whey was selected as the co-substrate due to its high carbohydrate content and being readily degradable.