# Evaluation of Eco-Efficiency of Two Alternative Agricultural Biogas Plants
## 3 Results
### Life Cycle Assessment
The greatest positive effects were observed in the case of the subcategories, _aquatic eutrophication_ (category _ecosystem quality_), followed by _global warming_ (category _climate change_), _non-carcinogens_ (category _human health_), and _non-renewable energy_ (category _resources_). The results of the studies explicitly indicate that the stage of providing the feedstock is mainly responsible for environmental burdens of Biogas Plant A, comprising mainly transport of raw materials, particularly in liquid form with a low dry mass and low biogas efficiency. In the case of Biogas Plant B, where liquid raw materials are transported by a gravitation pipeline, the negative impact is mitigated (Table 3). On the other hand, in both analysed biogas plants, the stage of energy production and application of the digestate to cropland shows a favourable environmental impact. The favourable environmental impact of the digestate application to cropland is associated with a possibility to limit the application of mineral fertilisers to cropland and related emission, mainly at the stage of production of these fertilisers, in exchange for the application the digestate with natural fertiliser properties. The negative values of the eco-indicator for the energy production stage, indicating favourable environmental effects, result from the substitutive character between electricity and heat \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Stages of Operation**} & **Maze** & **Rye** & **Providing** & **Energy** & **Digestate** & **Digestate** \\  & **Cultivation** & **Cultivation** & **Feedstock** & **Production** & **Storage** & **Application** \\ \cline{2-7}  & & \multicolumn{5}{c}{**[mPt/1 MWh]**} \\ \hline A & 404.12 & - & 1905.79 & \(-\)276.21 & 0.01 & \(-\)18.66 \\ B & 2.67 & 8.82 & 2.58 & \(-\)311.15 & 59.65 & \(-\)16.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Negative and positive environmental impact of the analysed biogas plants expressed as a result of cumulative eco-indicator, broken down into stages of a biogas plant operation. Figure 2: Environmental effects of the biogas plan operation expressed as impact categories. generated in the biogas plant and the energy produced in the conventional power plant or co-generation plant. Biogas Plant A demonstrates a smaller favourable impact (by 34.94 mPt/1 MWh). Biogas Plant A uses only 25% of the generated heat for its own needs (ambient air emission makes up the remaining part). Biogas Plant B has a much higher level of heat use (75%). In the case of Biogas Plant B, part of the generated heat is managed for its own needs for the purpose of the heating of tanks and social and office rooms of the biogas plant, whereas the remaining part is sold in the form of steam to a neighbouring distillery. The type of feedstock used--more precisely, the indicator of biogas productivity for the specific substrate--also affects the differences in the eco-indicator results. This indicator is estimated based on the content of dry mass and organic dry mass in the specific quantity of the substrate. Thus, the more liquid the substrate, the lower the productivity. This means that in order to obtain the same quantity of biogas a much higher volume of slurry (biogas productivity amounts to approximately 36 m\({}^{3}\)/Mg of a substrate) than maize silage (230 m\({}^{2}\)/Mg of a substrate) has to be provided. Therefore, in the case of Biogas Plant A, the limitation of slurry used in favour of maize silage would result in considerable limitation of wheeled transport and a reduction of the related adverse environmental impact. However, the fact should be highlighted that for the proper operation of the biogas plant in the mesophilic fermentation technology the dry mass content in the fermentation chamber must amount to approximately 12%. Therefore, it is impossible to replace all liquid substrates with substrates with high biogas productivity.