# Evaluation of Eco-Efficiency of Two Alternative Agricultural Biogas Plants
## 4 Determinants of Environmental Efficiency of Agricultural Biogas Plants--Discussion  
In the literature of the subject, LCA studies for biogas plants operating based on maize silage are mainly presented. The use of energy crops, including their cultivation, is treated as the \"hotspot\" of the entire life cycle of biogas installations [19]. The results of environmental burdens of the electricity production process in the co-generation system from biogas where maize silage is the only feedstock into the fermentation process were compared to the environmental impact of power plants operating based on non-renewable fuel [19]. The studies presented in this paper indicate that the waste-to-energy process is more effective in circumstances when maize silage does not comprise the predominant part of the feedstock, but rather a less significant fraction, besides waste from the agri-food industry. This is a very important remark from the circular economy point of view, because the main assumption of this idea is to minimise the consumption of primary raw materials and provide an effective (from an environmental and economic point of view) waste management. Therefore, the presented research results can be used to formulate recommendations and solutions to guarantee high energy efficiency, which is an element of the circular economy. In the presented study, it was also indicated that the method of providing feedstock for the methane fermentation process is a very important aspect of the environmental efficiency of a biogas plant. Literature sources report that the provision of the feedstock, including its long-distance transport, results in the highest environmental burden of agricultural biogas plants [20]. According to [PERSON] et al. [21], the distance over which a feedstock is transported influences the potential of global warming, particularly in the case where biomass with a low potential for biogas production is the substrate. On the other hand, according to [PERSON] et al. [22], transport constitutes a considerable environmental burden also for other types of raw materials with higher dry mass parameters. Furthermore, authors of earlier studies [10] have recognised that agricultural biogas plants have a favourable impact on the environment only in cases when a biogas plant is located at a maximum distance of 20 km from the cropland of maize cultivated for energy purposes. All of these issues found confirmation in the studies performed. The conducted LCA analysis indicates that the highest negative environmental impact in the case of biogas Plant A is associated with the feedstock provision stage. In the case of biogas Plant A, the wheeled transport of substrates to fermentation tanks is the major burden for the environment at this stage. The environmental aspects related to transport comprise interventions which mainly contribute to respiratory disorders triggered by effects of inorganic compounds (in the _human health_ category), global warming (in the _climate change_ category), and renewable energy use (in the _resources_ category). The high values of eco-indicators obtained in the three specified impact categories suggest that the combustion of fossil fuel during wheeled transport and the use of agricultural machines is the factor contributing to the highest environmental burdens of agricultural biogas plant operation. In the case of Biogas Plant B, a significant limitation of wheeled transport and its replacement by the gravitation pipeline during dosing of the majority of substrates has an impact on obtaining much lower values of the eco-indicator in those categories. The sale of heat energy to external customers is also an important factor influencing the eco-efficiency of a biogas plant. Biogas Plant B, which has the possibility to sell heat energy, demonstrates a lower environmental impact than Biogas Plant A, which does not have this possibility at the moment.