Hydrothermal Treatment of Digestates: Challenges and Perspectives

Anaerobic digestion (AD) effectively treats organic waste and biomass. This process recovers energy as biogas, while a by-product known as digestate requires proper treatment. The goal of enhancing biogas production as a way of increasing the share of renewable fuels in the transport and industrial sectors comes with the drawback of managing digestates. When the use of digestate as an organic amendment is not feasible, thermal processes are the alternatives currently available to obtain valuable energetic products. Hydrothermal treatment is a promising option among various thermal processes because it is expected to have a lower thermal demand when combined with anaerobic digestion (AD). This contrasts with scenarios where gasification or pyrolysis is used as the accompanying technology. However, several issues regarding the treatment of by-products (process-water and hydrochar contaminants) need to be resolved, along with high operating and maintenance costs, before these technologies can be truly adopted by digestion plants. The present document reviews the current state of the art in hydrothermal carbonization and liquefaction as technologies for treating digestate, focusing on the key aspects that require further research and development.