Anaerobic digestion using cocoa residues as substrate: Systematic review and meta-analysis

Abstract

This systematic review analyses the potential of cocoa residues for energy conversion using anaerobic digestion and evaluates strategies for optimizing methane production yields, retrieving information from papers published on anaerobic digestion of cocoa residues in journals indexed in Scopus, Web of Science, and PubMed databases. The articles were selected according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method. The effects of operational and compositional variables were evaluated by multivariate (PCA) and bivariate (correlation) statistics. A total of twenty-two studies were selected for this systematic review, and five of them were considered for meta-analysis. This search evidenced a lack of studies on the anaerobic digestion of cocoa residues, leading to, for example, high heterogeneity in methane yields between studies (163 to 382 mL CH4 gVS−1). Operational and compositional conditions in the anaerobic digestion were comparable (statistically significant correlations), and anaerobic digestion was considered efficient in the treatment of cocoa residues. In the meta-analysis, optimizing strategies did not significantly affect methane yields. Nonetheless, pretreatment using organic solvents can lead to inhibitory effects on the process. Anaerobic co-digestion can be considered a satisfactory strategy depending on the origin of the co-substrate. In conclusion, this review has worldwide relevance for the waste treatment sector and regarding the search for sustainable energy alternatives. To the best of our knowledge, this study was the first one that used quantitative methods such as multivariate bivariate statistical approaches and meta-analysis to answer questions related to the energy conversion of cocoa residues using anaerobic digestion.