## 3 Results and Discussion
### BGPs Outputs and Performance  
#### 3.2.1 Outputs and Performance of DE BGPs  
As presented in Table 3, DE1 had a daily OLR of 2.31 kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d, a HRT in the heated system of 81 days, and a HRT in the entire gas-tight system of 162 days. It produced a total of 835,765 m\({}^{3}\)/a biogas, which generated 1,673,661 kWh of electricity and 1,628,128 kWh of heat. This led to a CH\({}_{4}\) productivity of 0.82 m\({}^{3}\)CH\({}_{4}\)/m\({}^{3}\)/d and a CH\({}_{4}\) yield of 354 m\({}^{3}\)CH\({}_{4}\)/t\({}_{\text{ODM}}\).  
DE2 had a daily OLR of 4.05 kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d, a HRT in the heated system of 58 days, and a HRT in the entire gas-tight system of 267 days. It produced a total of 3,817,055 m\({}^{3}\)/a biogas, which generated 8,648,634 kWh of electricity and 3,368,255 kWh of heat. This led to a productivity of 1.41 m\({}^{3}\)CH\({}_{4}\)/m\({}^{3}\)/d and a CH\({}_{4}\) yield of 349 m\({}^{3}\)CH\({}_{4}\)/t\({}_{\text{ODM}}\).  
DE3 had a daily OLR of 2.10 kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d, a HRT in the heated system of 72 days, and a HRT in the entire gas-tight system of 234 days. It produced a total of 3,817,055 m\({}^{3}\)/a biogas, which generated 293,622 kWh of electricity. Heat was not recorded, as it was only for internal consumption. It had a CH\({}_{4}\) productivity of 0.49 m\({}^{3}\)CH\({}_{4}\)/m\({}^{3}\)/d and a CH\({}_{4}\) yield of 234 m\({}^{3}\)CH\({}_{4}\)/t\({}_{\text{ODM}}\).  
For these three DE BGPs, the HRT in the gas-tight system is longer than the period required according to the EEG 2017, which states that the HRT in the entire gas-tight system that is connected to a gas consumption device should last for at least 150 days. This is the case for installations commissioned after 31 December 2016 and digestate storage facilities after 31 December 2011 [4], even though this requirement is not mandatory for these three BGPs. The OLR was between 2.10 and 4.05 kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d, which is in the middle range of results from other research conducted in more than 140 German BGPs, resulting in a daily OLR between 0.4 and 7.0 kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d [10; 11; 23; 24]. Regarding the CH\({}_{4}\) yield, DE1 and DE2 had a similar value. However, as manure was the dominant substrate in DE3, it had a much lower productivity (0.49 m\({}^{3}\)CH\({}_{4}\)/m\({}^{3}\)/d). This result represents the lower end among other similar plants that were monitored under the BMP I, of which eight BGPs applied manure as main substrates (83-88% of the total FM), with an OLR range of 0.8-5.1 kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d and a productivity of 0.41-0.89 m\({}^{3}\)CH\({}_{4}\)/m\({}^{3}\)/d [10]. Such  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Items** & **DE1** & **DE2** & **DE3** & **CN1** & **CN2** & **CN3** \\ \hline Biogas production (m\({}^{3}\)/d) & 2290 & 10,458 & 804 & 431 & 581 & 91 \\ Productivity (m\({}^{3}\)CH\({}_{4}\)/m\({}^{3}\)/d) & 0.82 & 1.41 & 0.49 & 0.17 & 0.18 & 0.17 \\ Yield (m\({}^{3}\)CH\({}_{4}\)/t\({}_{\text{ODM}}\)) & 354 & 349 & 234 & 222 & 199 & 106 \\ Electricity production (kWh/d) & 4585 & 23,695 & 1757 & - & - & - \\ Heat production (kWh/d) & 4461 & 9228 & Not recorded & - & - & - \\ OLR (kg\({}_{\text{ODM}}\)/m\({}^{3}\)/d) & 2.31 & 4.05 & 2.10 & 0.78 & 0.91 & 1.55 \\ HRT (day)-heated system & 81 & 58 & 72 & 36 & 30 & 16 \\ HRT (day)-gas-tight system & 162 & 267 & 234 & 74 & 47 & 16 \\ \hline \hline \end{tabular} HRT = hydraulic retention time, ODM = organic dry matter, OLR = organic loading rate.  
\end{table}
Table 3: Outputs and performance of selected German (DE) and Chinese (CN) biogas plants.  
differences could be caused mainly by: (1) The characteristics of the substrates; and (2) working temperatures.