## 2 Materials and Methods
### Selection of Full-Scale BGPs in Germany and China
It consists of two 700 m\({}^{3}\) upflow solid reactors (USR) with an average working temperature of 31.3 \(\pm\) 5.1 \({}^{\circ}\)C, a low mesophilic range. Biogas was supplied to seven villages with a total of 1700 households for cooking. CSM was mixed with digestate and/or water for ease of pumping and fed to the digesters (heated by electricity or solar energy), while the digestate flowed into a storage tank of 1500 m\({}^{3}\) for subsequent application on farmland or circulation for feeding. After removing H\({}_{2}\)O and H\({}_{2}\)S, biogas was compressed through two 22 kW biogas compressors, stored in four 8 bar storage tanks (40 m\({}^{3}\) each), and was then supplied to households. One-year monitoring was conducted from October 2017 to September 2018. CN2 was built in 2008 and commenced operation in the same year. It was equally financed by the government and the village committee. It consists of four 450 m\({}^{3}\) CSTR digesters (heated by air source heating pumps or electricity or biogas burning) with a mesophilic average working temperature of 38.0 \(\pm\) 2.2 \({}^{\circ}\)C. After mixing with freshwater in the mixing tank (about 60 m\({}^{3}\)/d), the substrates, including CSM and CM, were fed directly to the digesters, while the digestate flowed into a storage tank of 1742 m\({}^{3}\), for sequent application on the village-owned farmland and orchards. The biogas was compressed by two 11 kW compressors (42F, AL-160M/4 TF, Nord Drivesystems Nord Co., Ltd., Suzhou, China) and stored in six 8 bar storage tanks with a total capacity of 280 m\({}^{3}\). Biogas was supplied to approximately 2000 households in one village for cooking. Monitoring was conducted from October 2017 to July 2018 as this BGP was closed down afterward due to the natural gas supply to local households. CN3 was built in 2007 and commenced operation in 2008. It was equally financed by the government and the village committee. It consists of two 160 m\({}^{3}\) USR digesters (heated) with an average working temperature of 32.0 \(\pm\) 2.2 \({}^{\circ}\)C in the low mesophilic range. Pig manure (PM) was fed directly to the digester, while the digestate flowed into the underground storage tank with 450 m\({}^{3}\) (not fully covered, no biogas collection), for sequent application in the vineyard and other agricultural fields (free of charge and delivered to the field for farmers). The biogas was compressed by an 11 kW compressor and stored in one 8 bar storage tank of 30 m\({}^{3}\). Biogas was supplied to 184 households in one village for cooking. One-year monitoring was conducted from October 2017 to September 2018. In all CN BGPs, the village committee is responsible for the daily operation, as well as managing the income and the assumption of operation costs (including labor and all maintenance cost). Investment from the central government is a type of grant, requiring no repayment. Income is sourced from biogas sold to the local households. Table 1 presents the details of these BGPs. Figure 1 presents the general BGP schemes of DE BGPs and CN BGPs. Figure 1: Diagram of the generally selected German (**A**) and Chinese (**B**) biogas plants.