Processes, applications and legislative framework for carbonized anaerobic digestate: Opportunities and bottlenecks. A critical review

• Published in: Energy conversion and management 2022-07, Vol.263, p.115691, Article 115691
• Main Authors: Catenacci, Arianna, Boniardi, Gaia, Mainardis, Matia, Gievers, Fabian, Farru, Gianluigi, Asunis, Fabiano, Malpei, Francesca, Goi, Daniele, Cappai, Giovanna, Canziani, Roberto
• Format: Article
• Language: English
• Subjects: Agricultural wastes; Anaerobic digestion; Anaerobic processes; Animal manures; Biorefineries; Catalysts; Char; Crop residues; Electrochemistry; Energy recovery; Food waste; Hydrothermal carbonization; Legislation; Pollutants; Process parameters; Pyrolysis; Raw materials; Sewage sludge; Sludge digestion; Soil application; Substrates; Sustainability; Toxicity testing
• ISSN: 0196-8904; 1879-2227
• DOI: 10.1016/j.enconman.2022.115691

[Display omitted] •Thermochemical conversion processes for digestate treatment are reviewed.•Emphasis is placed on digestate-based char production and application.•Digestate-based char is mainly investigated for soil application or energy recovery.•Legislative issues still prevent full-scale char soil application and energy recovery.•Next research efforts should face techno-economic and environmental constraints. Char is a valuable product obtained from thermochemical conversion processes of different biomass feedstocks, mainly pyrolysis and hydrothermal carbonization (HTC). In this work, anaerobic digestion (AD) integration with pyrolysis/HTC is critically reviewed, considering anaerobic digestates as feedstocks for char production. This virtuous interconnection can boost sustainable digestate valorization in the circular economy framework. Different substrates for AD are investigated, including sewage sludge, food waste, agricultural residues, and animal manure. The available thermochemical technologies, including pyrolysis, HTC and other processes are considered, analyzing the effects of substrate characteristics and process parameters on char quality. The possible fields of char application are successively presented, including agricultural application, energy recovery, pollutants adsorption, catalysts production, and electrochemical technologies; the advantages and drawbacks of each application are highlighted. Limitations still preventing the full-scale application of digestate-derived char production and utilization include the variability in substrate characteristics and the presence of undesired pollutants (especially in sewage sludge digestate), full-scale development of thermochemical plants, lacking legislative frameworks, uncertain economic sustainability, limited eco-toxicological studies, and stakeholders’ acceptance. Future research needed on the topic is finally depicted, with the aim of widening digestate reuse applications, as thermochemical processes may prevent safety concerns linked to direct agricultural reuse, leading to sustainable biorefinery platforms.