## 5 Conclusions  
In the framework of the C2 Land project, four bioreactors were designed and assembled to reproduce the composting process of an actual integrated anaerobic/aerobic industrial plant treating OFMSW. The biocells were built with three sectors for triplicate test, matching with the internal support allowing the internal up-flow air to pass through their holed bottom side. The fresh air circulation mode was controlled by automatic valves and probes measuring internal air and biomass temperature and oxygen content (to keep it always higher than 14%), connected to an Arduino based PLC. The OFMSW digestate from an industrial plant was treated in an HTC lab reactor at 200 \({}^{\circ}\)C for 3 h, the wet hydrochar obtained after filtration was used as feedstock for co-composting experiments in the bioreactors. The mixes to be co-composted consisted of half of waste green as bulking agent and half of OFMSW digestate. This latter part was substituted with an increasing percentage of hydrochar (0, 25, 50, 75%). The co-composting process was performed with a continuous air flow passing through a static feedstock, with 10 days at thermophilic conditions and 17 days of mesophilic stabilization. The total solids TS before composting were respectively equal to 38, 40, 41, 39% and reached the values of 29, 33, 34, 36% after composting, testifying the high moisture content of the produced hydrochar co-composts. This condition could have possibly led to anaerobic reactions during composting, but it was counterbalanced by the adoption of a high air flow rate and one feedstock mixing performed at the end of the thermophilic phase and before starting the mesophilic stabilization. Volatile solids (VS), as expected, were consumed during composting; the variation in VS linearly decreased at increasing hydrochar percentage, with values equal to 26, 18, 13% respectively for MIX 00, 25, 50%, while the last feedstock tested (MIX 75%) did not show statistically different VS values before and after composting. The values of the maximum 24h-mean of the Dynamic Respirometric Index (DRI\({}_{\text{24,max}}\) in mg O\({}_{\text{2}}\) kgvs\({}^{-1}\) h\({}^{-1}\)) measured with the standardized AIR-nl solid respirometer before and after composting were respectively 1340 vs. 340 for MIX 00%, 826 vs. 124 for MIX 25%, 1287 vs. 128 for MIX 50%, 1201 vs. 186 for MIX 75% (values in mg O\({}_{\text{2}}\) kgvs\({}^{-1}\) h\({}^{-1}\)). The Dynamic Respirometric Indexes measured from the oxygen consumption rates during the process were in accordance with the aforementioned standardized DRI values. This result suggests that the aerobic stabilization of the mixture hydrochar-OFMSW digestatewas effective during composting. This indicates also that the bioreactors designed to compost solid wet biomasses in a wet environment with temperature and oxygen control worked well for the purpose. The experimental campaign reported in this study proposes an interesting solution to composting plant managers for process optimization, using a zirconia probe for oxygen content measurement online. The production of hydrochar co-compost and the industrial model proposed within this project can be beneficial to plants suffering from excessive quantities of waste to be treated, but also to plants with residual capacity in digesters for productivity enhancement. To complete the objectives of the C2 Land project, further analyses are going to be performed on the hydrochar co-composts (elemental analysis, germination tests, plant growth bioassays, HPLC on extracts, GHG emissions from soil tests) to better understand the quality of these products in respect to regulatory standards and to the actual \"normal\" compost. Conceptualization, methodology, D.S., G.A. and L.F.; software, D.S. ; validation, D.S., G.A. and L.F.; data curation, D.S. ; writing--original draft preparation, D.S. ; writing--review and editing, G.A. and L.F.; visualization, D.S. ; supervision, G.A.