New opportunities for agricultural digestate valorization: current situation and perspectives
In the agricultural sector of many European countries, biogas production through anaerobic digestion (AD) is becoming a very fast-growing market. AD is a simple and robust process that biologically converts an organic matrix into biogas and digestate, the latter corresponding to the anaerobically non-degraded fraction. So far, digestate has been mostly used at farm-scales for improving soils. However, its ever-increasing production induces problems related to transport costs, greenhouse-gas emissions during storage and high nitrogen content that constrains its use to land application only. Consequently, research on alternative valorisation routes to reduce its environmental impact and to improve the economical profitability of AD plants should draw increasing interest in the future. This review therefore focuses on the different alternatives of digestate valorisation, apart from land applications: (I) the use of the digestate liquor for replacing freshwater and nutrients in algae cultivation; (II) the use of solid digestate for energy production through biological (i.e. AD, bioethanol) or thermal processes (i.e. combustion, hydrothermal carbonization and pyrolysis); (III) the conversion of solid digestate into added-value products (char or activated carbons) through a pyrolysis process.