8. Sustainability and Indicator System

To evaluate the effects of biogas on agro-ecosystem from a systematic perspective, we discussed the current situation of household biogas and identified its main factors that may have impacts on agro-ecosystem. An indicator framework covering social, environmental, and economic aspects was set up. A case study of Gongcheng was then conducted to evaluate the combined impact of biogas project through the proposed indicator framework. In addition, to integrate the socioeconomic-environmental relationships, we innovatively introduced extended exergy analysis to the sustainability evaluation of biogas project. Besides, new extended exergy-based sustainability evaluation indicators that unified greenhouse economic performance, gas emissions, and resource depletion were first proposed to identify biomass conversion pathway planning and as goals for possible system sustainability optimization. The sustainability of a “Three-in-One” biogas production system in southern China was then evaluated based on the proposed framework. Results showed that there was a significant positive effect resulted from the application of biogas, and the integrated benefits have been improved by 60.36%, implying that biogas as a substitute energy source can promote the sustainable level of rural areas. According to the extended exergy analysis, the biogas project has a higher renewability (0.925) and economic return on investment ratio (6.82) and a lower GHG emission intensity (0.012) compared with other renewable energy conversion systems.