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 digestate cannot be used as an organic amendment, thermal processes provide alternatives for producing valuable energy products. Hydrothermal treatment is particularly promising due to its lower thermal requirements when paired with anaerobic digestion (AD), unlike gasification or pyrolysis. However, challenges such as managing by-products like process water and hydrochar contaminants, along with high operating and maintenance costs, need to be addressed before these technologies can be widely adopted in 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. This review examines the existing gaps in the treatment and management of process water, as well as the techno-economic barriers faced when deploying hydrochar-related technologies and integrating them with digestion plants.