Organic Waste Valorisation in The Circular Bioeconomy: A Critical Review of Composting, Anaerobic Digestion, and Technological Integration

Abstract

The amount of organic waste generated in the world has reached critical limits, and in the year 2016, over 2 billion tonnes of municipal solid waste were produced, with 44-60% of this waste being organic. The under-managed waste stream worsened the issues of GHG emissions, soil degradation and resource depletion, all working against the circular bioeconomy paradigm. The two main biological valorization routes to recover organic waste as valuable products are aerobic composting and anaerobic digestion (AD), in which it is transformed into stable compost and biogas, respectively. Until now, their potential is limited by technological, economic and policy challenges, mostly in low- and middle-income countries. This review, however, covered the mechanistic, operational and environmental aspects of aerobic composting and AD and investigates these aspects in relation to their performance indicators, co-digestion possibility, compost quality parameters, energy yields and GHG balance based on evidence retrieved from more than 60 peer-reviewed studies. Additionally, the review delved deeper into integration frameworks, such as combined composting-AD systems and synergies between local and technological aspects, as enablers of a closed-loop circular bioeconomy. Results showed that co-digestion of food wastes with lignocellulosic substrates increased methane production by 20-60%, while the thermophilic composting process considerably reduced the pathogen level and the bioavailability of heavy metals. Finally, the paper highlighted operational and policy bottlenecks and the research significance of scaling waste valorisation in a globally applicable framework of the circular bioeconomy.