## 2 Materials and Methods  
### Selection of Full-Scale BGPs in Germany and China  
Three Chinese (CN) BGPs were selected in Beijing to conduct a one-year monitoring, within 100 km of the China Agricultural University (Beijing), where the laboratory for the chemical analysis is located. The key selection criteria included (1) willingness of the BGP operators to cooperate; (2) sampling possibilities; and (3) availability of operation data records. Three German (DE) full-scale BGPs under the current BMP III were selected to make general comparisons with CN BGPs in all aspects. They are all located in Baden-Wurttemberg. DE1 was built in 2011, commenced operation in 2012, and holds a FiT contract under the EEG 2009 for 20 years. It consists of a 1500 m\({}^{3}\) heated continuous stirred-tank reactor (CSTR) with a mesophilic average working temperature of 42.3 \(\pm\) 1.9 \({}^{\circ}\)C. Cow manure from the packed bedding stalls (CM) and liquid manure from cow (LM) and grass silage (GS) are fed directly to the digester, while the digestate flows into a gas-tight storage tank of 1500 m\({}^{3}\), for subsequent application as fertilizer on-site. The biogas drives a 250 kW combined heat and power (CHP) generator (Mototryp Scania-Schnell, ES 2507; 2G Bio-Energietechnik AG, Heek, Germany). Power is sold to the power grid and heat is partially used for warming up the digesters, and the rest is sold to local residential houses in the surrounding area. Under its EEG-2009 contract, the annual revenue is derived from the basic power tariff, EEG bonus for energy crops, CHP unit and manure, and heating supply. One-year monitoring was conducted in September 2017 to August 2018. DE2 was built in 2009 and commenced operation in 2011. It holds an FiT contract under EEG 2009 for 20 years. It consists of one 1880 m\({}^{3}\) heated CSTR (average thermophilic working temperature at 49.3 \(\pm\) 1.0 \({}^{\circ}\)C) and one 1880 m\({}^{3}\) complete mixed secondary digester (heated). CM, LM, maize silage (MS), and cereal leftovers (CL) are fed directly to the digester, while the digestate flows into three gas-tight storage tanks of 13,539 m\({}^{3}\) in total, for subsequent fertilizer application on-site. The biogas drives four CHP generators, including three 400 kW generators and one 350 kW generator (E2842 LE322, E3268 LE232, Elektro Hagl KG, Geisenfeld, Germany). Power is sold to the power grid and heat is partially used for heating digesters, and the rest is sold to local residential houses. Underits EEG-2009 contract, the annual revenue is derived from the basic power tariff, EEG bonus for energy crops, CHP unit, manure and power flexibility scheme, and heating supply. One-year monitoring was conducted from September 2016 to August 2017. DE3 was built in 2013 and commenced operation in the same year. It holds an FiT contract under the EEG 2012 for 20 years. It consists of an 847 m\({}^{3}\) CSTR digester (heated) (mesophilic average working temperature at 41.9 \(\pm\) 0.2 \({}^{\circ}\)C). Chicken solid manure (CSM), CM, LM, MS, and GS are fed directly to the digester, while the digestate flows into a storage tank of 1742 m\({}^{3}\), for subsequent application on-site. The biogas drives one 75 kW CHP generator. Power is sold to the power grid, and heat is completely used for heating the digesters. Given the capacity of this plant (75 kWh CHP unit), the annual revenue only comes from the basic power tariff. One-year monitoring was conducted from September 2017 to August 2018. In DE BGPs, all initial investments were made by the farmers (in this case, also the BGP operators). Loans obtained from commercial bank(s) to cover the payment had differing loan terms and conditions based on the assessment by each of the banks. During the operation, income was generated from the sale of electricity and heat to repay the principal loan and interest to the bank(s). CN1 was built in 2007 and commenced operation in the same year.