Reuse and Valorization of Solid Digestate Ashes from Biogas Production

Digestate is produced in large quantities by the anaerobic digestion process, which is recognized to be a promising technology for producing bioenergy from biological waste. Digestate is a highly humid by-product containing organic and inorganic substances, including nutrients that make it suitable for soil applications. However, it can be considered a high-risk environmental contaminant if it is not correctly treated. For these reasons, thermochemical treatment is one of the alternatives for valorizing the digestate, leading to a high ash quantity. This review aims to investigate the formation of ash derived from thermochemical valorization treatments of digestate. Furthermore, considering the compositions of the elements present in these ashes, an additional objective is to identify possible prospects for the reuse of these ashes following a circular economy approach.