Methanogenic Potential of Cocoa Processing Waste for the Production of Renewable Energy and Mitigation of the Environmental Impact

The cultivation of cocoa (Theobroma cacao Linnaeus) plays a significant socioeconomic role in the south of the world, particularly in Asia, Africa, Latin America and the Caribbean area, although being mainly consumed in developed countries. The cocoa industry generates a large amount of waste during the harvesting process, including cocoa pod husk (CPH) and cocoa mucilage exudate (CME). These by-products are generally disposed into healthy plantations without any treatment, causing crop losses. Improper disposal also generates environmental problems due to the production of greenhouse gases, unpleasant odors, spread of diseases, soil acidification, bacterial proliferation and insect reproduction. Anaerobic digestion enables the degradation of the biodegradable organic fraction present in biowastes, transforming it into biogas and a valuable fertilizer, i.e. digestate. Ripe cocoa fruits of the forastero variety from the Equatorial Guinea were pre-treated to obtain cocoa pod husk and were subsequently processed in an anaerobic digestion process to evaluate their energy potential. The tests were performed in batch conditions at 38°C, according to the Biochemical Methane Potential (BMP) standards. The inoculum used in the process was represented by digestate from a 1MWe dairy biogas plant. Two different ratios of VSinoculum/VSsubstrate were adopted, 2:1 (I/S2) and 1:1 (I/S1). Methane yield of the two theses resulted similar, indicating that the process was capable of reaching the maximum BMP with both inoculum/substrate ratios. The methane yield referred to the VS content resulted of 110.1 NL/kgVS for I/S2 and 120.3 NL/kgVS for I/S1.