## 1 Introduction
### AD of OFMSW  
OFMSW can be digested in dry AD processes at DM contents >15-20% FM or wet AD processes at DM contents <15-20% FM. [PERSON] et al. [56] found that the same CH\({}_{4}\) yields can be achieved in practice with wet and dry AD processes, which was also confirmed by [PERSON] et al. [57] and [PERSON] and [PERSON] [58]. OFMSW, in its natural state, is indicated with a DM content >20% and complex composition, including substances such as lignin that hardly contribute to biogas yields. Therefore, it is often digested in dry processes such as plug-flow or batch digesters. If OFMSW is used in wet processes (e.g., via stirred tank reactors), it has to be diluted to a pumpable condition through its mixture with other substrates or water [1; 22].  
On a global level, biowaste (e.g., food waste as a component of OFMSW) is mostly digested at mesophilic temperatures in wet AD plants [15]. Due to the simpler construction of an unheated psychrophilic biogas plant, this method is currently an important form of supplying CH\({}_{4}\) in developing countries and emerging economies. Micro digesters (household size) can be found millions of times in China (42 million, but also 7000 AD plants with an electrical capacity of approximately 0.1-1 MW), India (5 million), and Africa, along with other Asian countries (0.7 million). The generated biogas is mostly used for cooking stoves [15; 19; 59]. In other regions of the world, micro digesters are barely used, and large(r) scale biogas plants are common, e.g., 2200 biogas plants operate in the USA with an installed total capacity of 977 MW, leading to an average plant size of approximately 0.45 MW. Including landfill and sewage AD plants, approximately 17,800 AD plants with an installed electrical capacity of 10.5 GW operate in the EU out of which approximately 700 are utilizing OFMSW and industrial biowaste. An increase in the number of biogas plants for OFMSW and industrial biowaste is expected as a separate collection of OFMSW is envisaged EU-wide [15; 19].  
By 2018, approximately 9500 biogas plants with an installed electrical capacity of 5 GW were operated in Germany, mainly based on energy crops [60; 61]. Most agricultural biogas plants are equipped with stirred tank reactors at mesophilic temperatures with defined feeding intervals. The generated biogas is mostly used for the combined generation of electricity and heat. In the currently operating approximately 215 biogas plants, biogas is upgraded to biomethane and fed into the gas grid. Psychrophilic biogas plants are uncommon in Germany [22; 58; 62]. In 2015, 1392 biogas plants in Germany were registered to use biowaste as feedstock, but only 337 actually used it as the main substrate or in co-digestion. Then, again, 75 biogas plants are specialized on OFMSW (share of OFMSW compared to the total feedstock >90%) operating as dry AD system at a thermophilic and mesophilic temperature in equal share [58; 60].  
According to the literature [63; 64; 65], the heat demand of AD systems depends on substrate characteristics, operating temperature, and geographic region, and on AD parameters such as digester type or plant size. The benchmarking of German biogas plants [65] has proven that heat demands for digester heating can be between 0 and 100% of the generated heat. Especially small biogas plants with an electrical capacity below 150 kW and plants utilizing substrates with high water contents show higher heat demands. Typically, the digester heating demand is 30 \(\pm\) 20% of the generated heat. A lower digester heating demand would allow heat export, and hence the profitability of biogas plants could be increased.