Upcycling the anaerobic digestion streams in a bioeconomy approach: A review

Gaseous and liquid anaerobic digestion (AD) streams, currently are at best used for electricity and heat production or simply spreading at the fields, respectively. However, electricity and heat are economically produced from other renewables and advanced fertilizers are needed to avoid leaching and boost nutrients capture. Hence, AD seeks new opportunities to support circular bioeconomy. The overall objective of this review is to present state-of-the-art resource recovery routes for upcycling the AD streams to reduce carbon footprint and formulate alternative products to increase sustainability. Technical barriers and integrated systems to upcycle AD streams through biological means are presented. New technologies and methods to capture CH4, CO2 and nutrients from the digested residual resources are presented, as a) methanotrophs cultivation to be used as feed ingredients; b) CO2 conversion and micro-nutrients capturing from microalgae to be valorized for a wide range of applications (e.g. biofuels, food and feed, fertilizers, bioactive compounds); c) CO2 transformation to biodegradable plastics precursors (e.g. Polybutylene succinate, Polyhydroxyalkanoate); d) digestate valorization for biochar production to support efficient agricultural usage. Moreover, the environmental factors and life cycle assessment perspectives of the novel biorefinery routes are revised highlighting the need for regionalized models or assessments that can reveal the most sustainable routes based on local conditions and requirements. Despite AD poses some positive characteristics related to environmental benefit and emissions reduction, the present work reveals that the novel routes can further enhance sustainability metrics supporting circular bioeconomy.

Automated summary

This review aims to explore resource recovery routes for upgrading anaerobic digestion streams through biological means to reduce carbon footprint and enhance sustainability. New technologies and methods for capturing CH4, CO2, and nutrients from digested residual resources are presented for producing a variety of beneficial products to support circular bioeconomy.