Ecological effect evaluation of biogas project integrated with solar-ground source heat pump system and slurry recirculation

Abstract: Biogas engineering is an effective method for agricultural waste treatment among many methods. It can recycle on multi-layers and dispose wastes. It is an important department of sustainable agricultural development mode to achieve production capacity, and protect environment. There were many methods for evaluating the environmental effect and the sustainability of a biogas engineering such like analytic hierarchy process, cost benefit method and life cycle assessment (LCA) method. Emergy analysis method, established by Odum, could bring a new category and grade of material and energy conversion, and the conversion and quantitative analysis using the unified benchmark. The method considered many factors such like the natural resources, labor and the other factors which were usually neglected by the other methods. For sustainability assessment, biogas system provided a better way of analysis. For the aim of evaluating comprehensive ecological effect of biogas plant with the application of new ecological technologies, we chose "D" Biogas plant as the research object. And we set the traditional biogas engineering operation mode "coal-fired heating and keep warming insulation + biogas slurry discharged directly" as pattern I; "biogas slurry recycling technology" as pattern II; "solar energy-ground source heat pump heating technology" as pattern III; and both techniques used in "D" Biogas plant as pattern IV. Based on energy system diagram and the emergy-based indexes, the system were analyzed in comprehensive performance from economic production efficiency, environmental effect as well as sustainability of the ecological economic effects on biogas plant with the solar-ground source heat pump system and slurry recirculation. The results indicated that due to the application of solar-ground source heat pump system, nonrenewable purchased resources reduced by 8.80%, environmental sustainability index (ESI) increased by 18.75%. The results showed that it was an effective alternative way to the use of fossil energy, which showed obvious ecological economic benefit. At the same time, owing to the application of slurry recirculation, groundwater input fell by 65%, and the fees of treating with the disposal of raw digestate would reduce by 1.77E+05 dollar/a. So the environmental loading ratio decreased by 68.52%, environmental sustainability index (ESI) increased from 0.03 to 0.38, the system sustainability increased. Anyhow, the application of the two typical ecological technologies combined with "biogas slurry recycling in water saving and emission reduction technology" and the "solar energy-ground source heat pump heating in thermal insulation technology" in "D" Biogas plant improved greatly in economic efficiency, environmental and ecological benefits.