## 5 Pilot-scale and Farm-scale Plants for AD of RS  
Energy recovery in the form of biogas/methane from agricultural residues is highly interesting for the agricultural sector, as it is an option to produce local energy, replace fossil fuels, and thus, become self-sufficient and sustainable in terms of energy supply [122]. Introducing AD plants into the farm system, treating the animal manures along with the straws for the recovery of energy has a great potential to reduce the demand for fossil energy and reduction of greenhouse gas emissions [123]. Numerous bench-scale experiments have been carried out on RS digestion to define the optimal temperature [25; 26; 42], substrates for co-digestion [39; 65; 66], mixing intensities and duration [87; 92], OLR [28; 40; 41], sources of inoculum [38], S/I ratio [69], recycling LD to the digester [36; 98], addition of TEs and their bioavailability [110]. Bench-scale studies are important to understand the principles but it is challenging to design and operate a farm-scale plant based on these microcosms [124]. Besides, the monitoring of a farm-scale plant is necessary to obtain detailed information and a better understanding of technology [125]. It also helps to discover practical operational aspects such as the length of start-up phase and energy and co-substrates input and to check if the system is sustainable on a long-term basis [36; 125]. These aspects are often missed in lab tests. [PERSON] et al. [124] conducted a batch pilot-scale study using untreated RS and PWW to define TS, straw-to-wastewater ratio, digestion temperature, and digestion time. RS was co-digested in two pilot-scale batch digesters with different amounts of PWW (Digester A: 50 kg dry straw, 150 L PWW, 20% TS; Digester B: 50 kg dry straw, 60 L PWW, 20% TS) for 189 days. The digestion temperature inside the digester A was between 30 and 40 \({}^{\circ}\)C. The digester B was initiated with ambient conditions (20-25 \({}^{\circ}\)C) and constant heating was applied later on to maintain mesophilic temperatures. The CH\({}_{4}\) yield from digester A was 231 L/kg VS and from the digester, B was 12 L/kg VS. The lower temperature and lesser amount of PWW were the limiting factors for lower CH\({}_{4}\) yield from digester B. The amount of PWW was not sufficient to establish a stable microbial community. The results of digester A were compared with the results of another study [57] of the same magnitude where the pretreated RS was digested. The pretreated (cutting into 7-8 cm pieces and treated with white-rot fungi) RS requires only 89 days for digestion while the untreated RS needs 189 days. Moreover, the pretreated RS can be adequately digested at ambient temperature (19-30 \({}^{\circ}\)C), while the untreated RS requires the mesophilic conditions in the presence of approximately 67% more PWW [57]. Other than RS, [PERSON] et al. [126] studied the dry AD of roadside grass (RSG) with manure in a pilot-scale plant at 37 \({}^{\circ}\)C with three digesters. The overall average CH\({}_{4}\) yield (226 L/kg VS) from three digesters obtained in this study [126] was similar to the yield obtained by [PERSON] et al. [124] during the co-digestion of RS with PWW. A pilot-scale plant was successfully operated with a stable AD process treating RS and manure in the BIORIST project [24]. No further studies were found to compare the performance of AD of RS with other substrates at the pilot scale. Data from a farm-scale system treating RS is limited to one study [36]. During the initial loading event, 727 t of RS and 285 t of PWW was loaded in a batch digester. Approximately 1300 t of water was added to the digester overtime during the first year. The co-digestion of RS with PWW was completed after 422 days, and the cumulative energy production was 295 MWh. The results showed a start-up period of 200 days and the authors attributed this long phase to high straw to wastewater ratio, low ambient temperature (<15 \({}^{\circ}\)C), and low recycling rate of LD (< 0.04 m\({}^{3}\)/m\({}^{3}\)straw d\({}^{-1}\)).