Valorisation of digestate: Characteristics, products, processes and potential

Anaerobic digestion (AD) plays a critical role in meeting renewable energy needs and managing solid and liquid waste streams, thereby making significant contributions towards sustainability and the circular economy. However, AD generates a large amount of digestate as a by-product. Digestate contains undigested organic fractions, micro and macro-nutrients, inorganic materials, trace metals etc. It is essential to develop appropriate ways for managing and valorising digestate for truly harnessing the potential of AD. This review critically analyses the current state of the art on valorisation and management of digestate. It starts with a review of key digestate characteristics that have a great impact on the valorisation potential and products. Different valorisation pathways based on the analysis of digestate properties are then reviewed. The interaction among characteristics of digestate, valorisation technologies, potential products and economic challenges is critically reviewed and discussed. Furthermore, the separation technologies, advanced valorisation processes and their potential are discussed. Techno-economic aspects are briefly discussed. Some comments on policies and regulations are included. Key conclusions on the state of the art, specific suggestions for further research, and some comments on the outlook are included at the end. The review will be useful to researchers, technologists, and policymakers interested in sustainability and the circular economy.