Evaluation of circular development mode for dairy cow industrial parks based on emergy value analysis

The circular agriculture mode with the biogas project as a link is one of the critical modes to reduce waste pollution and improve resource utilization in industrial parks. This research was to construct an emergy evaluation index system suitable for the circular agriculture mode of the industrial park. A typical representative industrial park in Hebei Province was chosen as a case study. The results of four common biogas projects were compared, including anaerobic fermentation using continuous stirred tank reactor (CSTR), wet two-phase anaerobic digestion, dry sequential batch anaerobic digestion and dry steady anaerobic digestion. To compared with each other, the effect of the circular agriculture mode with the biogas project as a link was evaluated. In the industrial park, a total of 5 500 dairy cows were bred annually and the park had a breeding area of 6.05×104 m2. A biogas project that could treat 1 000 m3 of dairy cow manure per day was built. The CSTR anaerobic fermentation biogas project was used to produce the biogas. The biogas slurry was used as a liquid fertilizer for the planting industry, forming a circular agricultural system for multi-level utilization of agricultural waste. The results showed that, compared with the other three biogas engineering modes, the CSTR anaerobic fermentation biogas project was more suitable for the industrial park. The total emergy input of this mode was 1.69×1018 sej. Among them, the emergy value of renewable environmental resources was 3.87×1016 sej and the emergy value of non-renewable environmental resources was 5.24×1016 sej. The emergy input of environmental resources, accounting for 5.38% of the total emergy input. The renewable organic emergy value was 3.70×1017 sej and the non-renewable supplement emergy value was 1.23×1018 sej. The emergy input from external resources dominates, accounting for 94.62% of the total emergy input. The input of raw materials and emergy was the central part of the external resource input. The emergy yield was 6.55×1017 sej, of which the energy value of maize, milk, biogas, organic fertilizer and other products was 4.18×1017 sej, accounting for 63.77% of the output emergy value. And the emergy value of reused resources such as biogas residue and biogas slurry was 2.37×1017 sej, accounting for 36.23% of the output emergy value. Based on the analysis of emergy input and output, the total index values in the industrial park were further calculated and the results showed that the emergy input rate of the circular agriculture mode with the CSTR anaerobic fermentation biogas project was 17.57. It was lower than the other three modes and the goal of reducing system resources was achieved. The emergy yield ratio of the CSTR anaerobic fermentation biogas project was 0.41. The proportion of wastes such as biogas residue and biogas slurry in the system was higher than those of the other three modes, decreasing the overall input cost of the system and improving the production efficiency of the system. The environmental loading ratio of the CSTR anaerobic fermentation biogas project was 0.33. The emergy sustainable index of the CSTR anaerobic fermentation biogas project was 1.23, indicating that the system has more vitality and development potential. On the whole, the overall benefit of this model was better than the other three models, and it was the most suitable biogas project mode for the industrial park. The database and the constructed evaluation index system of emergy value is also relevant to the evaluation of other industrial parks. The results can be used to evaluate the circulation model on the same scale and to analyze important parameters such as the ratio of cultivation to breeding scale and the treatment capacity of biogas projects in the system.