Towards a full circular economy in biogas plants: Sustainable management of digestate for growing biomass feedstocks and use as biofertilizer

Abstract

The digestate is a prospective biofertilizer and potential source of income for many biogas plants worldwide. However, its actual impact on the soil properties and biomass yield is still unexploited. The different digestates from eight agricultural biogas plants were researched in terms of their chemical composition and the fertilizing potential. The results obtained from digestate chemical analysis indicate that the digestate biomass had large amount of nitrogen (up to 73 g kg−1 fresh mass) and potassium (up to 25 g kg−1 fresh mass). The value of the digestate was estimated in the range of 2.88–7.89 EUR Mg−1 for liquid digestate and 7.62–13.61 EUR Mg−1 for solid digestate based on the commercial fertilizer market price of nitrogen, potassium phosphorus, organic carbon, Cu, Zn, Fe and Mg. The digestate produced at the 1 MW biogas plant is worth EUR 941–2095 per day in addition to energy sales income. The application of digestate on low-fertility land in areas close to the biogas plant allows the production of up to three-fold more biomass suitable for biogas production. The digestate’s application on semi-natural grass biomass production in the low-fertility soils near the biogas plants could be an alternative strategy for the biogas plant feedstock portfolio diversification.