## 1 Introduction  
The organic fraction of municipal solid waste (OFMSW) is commonly treated in Europe through anaerobic digestion (AD) and/or composting as best available techniques--BAT [1]. According to ECN Status Report 2019 [2] on European Bio-Waste Management, out of a total of 47.5 million tons of treated bio-waste, 64% is anaerobically digested, 26% is composted and around 10% is subject to an integrated anaerobic/aerobic treatment. The case of Italy is emblematic, since the produced digestate cannot be directly applied on farmland, so post-composting is almost always applied after AD [3]. In Italy, the stream of OFMSW sent to integrated plants increased by +81.8% in 2019 with respect to 2015, and there is still room for an increase in the future [4]. In actual integrated plants where OFMSW is digested and then composted, this second phase can be a bottleneck for the management of the whole system, thus requiring new strategies to improve the plant productivity. The C2 Land project (supported by EIT Climate-KIC, Call 2019) moves towards this direction: it aims to design an industrial scaling model producing a valuable co-compost from green wastes with OFMSW digestate and its hydrochar produced through Hydrothermal Carbonization (HTC). With HTC, any kind of biomass can be treated in the temperature range 180-250 \({}^{\circ}\)C in liquid water at saturation pressure, producing hydrochar (HC, solid phase), liquor (liquid phase) and a gaseous phase (consisting mainly of carbon dioxide) [5]. The main operative parameter governing the HTC process is the temperature, and directly connected to temperature is pressure since it should maintain water in the liquid phase; other important factors are residence time, solid load (or dry biomass to water ratio), pH, but also feedstock composition, HTC heating rate and the possible use of catalysts [6; 7; 8]. One of the C2 Land's scenarios proposes that a fraction of the OFMSW digestate is diverted to an HTC reactor to be carbonized. Then the liquor is recirculated back to the anaerobic digester, while the HC produced is sent to the composting plant together with the remaining OFMSW digestate. The phytotoxicity characteristics exhibited by the HC [9; 10; 11] should be eliminated by the aerobic stabilization, as shown in the study by [PERSON] et al. [12]. The final product, named \"hydrochar co-compost\", may have the same characteristics of a high-quality compost as the actual standard compost. Moreover, the C2 Land project will analyze the effect of the hydrochar co-compost on greenhouse gas (GHG) emissions from soil application, to measure any positive effect on this parameter, as measured in the study by [PERSON] et al. [13]. Besides, the biogas enhancement could improve the economic and social benefits for all the plants which perform biomethane upgrading for injection into natural gas network, as stated in the study by [PERSON] et al. [14]. The aim of this paper is to present the design and assembly of the bench-scale batch bioreactors built to investigate the co-composting process of OFMSW digestate and its HC mixed in different proportions. In addition, some preliminary results of the process performed and data of the \"hydrochar co-compost\" product are shown, as regards to total solids, volatile solids, and stabilization index through dynamic respirometric index (DRI). This parameter was measured by the adiabatic solid respirometer AIR-nl [15; 16] on samples before and after composting. DRI is recognized as a good indicator of stability of biomass and compost and could be used in industrial-scale composting plants for the determination of the respiration/metabolic activity of the feedstock online [17]. Many previous studies reported the design of lab- or pilot-scale bioreactors used for composting biomass, whose stability was evaluated by measuring the respirometric index.