Ecological evaluation of biogas from catch crops with Sustainable Process Index (SPI)

Background: Ever increasing global population requires to find additional options or increase the efficiency of food and feed supply to fulfil its dietary needs. In agricultural sector, competing situations with energy supply occur and ask for more sustainable solutions in an ethically correct manner. Methods: The Sustainable Process Index (SPI) provides a powerful method for an ecological evaluation of various processes. The comparison of partial ecological pressures allows to identify main spots of ecological pressure and provides a base for an integrated discussion about ecological improvement. Results: The results show scenarios about different options to change typical agricultural business as usual (BAU) successions. Mulching and fermentation of catch crops show high grades of reduction potential of the ecological footprint evaluated with the SPI method. A comparison to natural gas equivalent shows the direct potential to improve agricultural farming towards higher sustainability. The highest reduction of the ecological footprint can be between 56% in case of summer catch crops with wheat as a main crop and 59% in case of winter catch crops with maize as a main crop in comparison to the BAU scenario without catch crops. Conclusions: Besides energy generation, the use of catch crops instead of main crops in biogas plants has several additional ecological benefits. Leaving main crops untouched for food and feed purposes, the additional seeding of catch crops after the harvest of main crops reduces the risk of erosion and nitrate leaching as well reduce the application of mineral fertiliser. Additionally, soil humus content improves due to the application of fermentation residues to the fields.