## 4 Discussion  
The data reported in Figure 6 show that the bioreactors were able to reproduce the thermal profile of the actual industrial composting plant (10 days thermophilic, five of which at 60 \(\pm\) 5 \({}^{\circ}\)C, and 17 days mesophilic at 37 \(\pm\) 3 \({}^{\circ}\)C, Figure 6), with oxygen  
Figure 8: Online measurements of Dynamic Respirometric Index (DRI) (expressed in mg O\({}_{2}\) kgvs\({}^{-1}\) h\({}^{-1}\)) during composting. Figure 7: Percentage consumption of: (**a**) total solids (TS) and (**b**) volatile solids (VS) on dry basis of the biomass due to composting process. ** No statistically significant difference. content set always >14%. Working with a water bath, the air was always in water vapor saturation condition. As regards Table 3, since this composting process did not have a final drying phase but was instead always performed with air in saturated vapor condition, the MC at the end was still high. This helps the stabilization process, because moisture content is a limiting factor for microbial activity if below 60% [29]. Actually, [PERSON] et al. [29] found that a moisture content in the range of 60-70% provides maximum activity, while if in the range 30-60%, the lower the moisture content, the lower the microbial activities. Many studies reported 55-65% as the best range of moisture content for composting, explaining that the problem associated with moisture content higher than 65% is the possibility to decrease oxygen diffusion through biomass particles, leading to the activation of anaerobic processes [30; 31; 32; 33]. In this study, two operative parameters were adopted in order to overcome this risk: the air flow rate was kept continuously high (about 0.6 L min\({}^{-1}\) kg\({}^{-1}\)) to help oxygen diffusion, while the mechanical turning/mixing of the pile in the sector at the transition from thermophilic to mesophilic conditions allowed a reallocation of particles and air pathways, enabling the access of microorganisms to further sources of organic matter (as noted in the study by [PERSON] et al. [34]). The results shown in Table 4 suggest that the TS and VS in HC are more recalcitrant than in digestate, but further analyses are going to be performed in C2 Land project to confirm this finding (chemical, agronomic and soil tests). The main feature to be confirmed is the elimination of the phytotoxicity of HC after composting. In this way, the project will pave the way for industrial-scale aerobic stabilization of hydrochar co-compost. In particular, the advantage of insertion of HTC before composting from a plant management point of view is double. On the one hand, if OFMSW mass flow is kept constant, HTC can reduce the material flows entering the composting facility, which gives rise to lower volumes needed for composting areas. On the other hand, for integrated anaerobic/aerobic plants with residual capacity in biodigesters, besides the benefit deriving from biogas enhancement due to HTC liquor recirculation [35; 36; 37], the upstream flow of OFMSW could be increased, maintaining the unchanged capacity of the composting plant; the higher the quantity of digestate diverted to the HTC unit, the higher the amount of digestate that can be handled by the combined HTC-composting path. Consequently, the fraction of hydrochar substituting digestate in composting mix increases as well: this is the reason why we decided to test various mixes which differed in the amount of HC substituting the digestate. With regards to respirometric activity during the process, it is shown in Figure 8 that all the mixes, after composting, present DRI values much lower than 500 mg O\({}_{2}\) kg\({}_{\text{ys}}\)\({}^{-1}\) h\({}^{-1}\), recommended limit for high-quality compost [38], meaning that the process was effective in stabilizing the feedstock. The MIX 75% has a very low consumption in solids (Figure 7 and Table 4), even if the DRI indicates an initial metabolic activity comparable to the other mixes, even slightly higher.