Methane Production from Previously Fermented Cacao Waste Pod Husks

Abstract The wastes from cacao processing represent approximately 80% of the fruit weight and are rich in readily biodegradable carbohydrates and cellulosic and lignocellulosic compounds. There is little information about the processing of these wastes, and few authors report the potential of these wastes to produce energy through anaerobic digestion. As an alternative to disposing of these wastes in the field, associated with negative consequences, such as soil pollution, this research deals with the fermentation of the cacao pod husks under different organic loading rates and/or solids retention times to enhance hydrolysis through the production of acids and alcohols. Ethanol, lactic, acetic, propionic, and butyric acids were identified in fermented cacao pod husks. The digestates from the fermentation were tested for methane production. From the main results, the following conclusions can be drawn. The fermentation efficiency depends on the solids retention time, and the hydrolysis rate is inversely proportional to the organic load and directly proportional to solids retention times. Metabolic displacements show sudden changes in the composition of the volatile acid due to solids retention time. Methane production did not tend to be associated with the organic load or solids retention times. The specific methane production increased with decreasing substrate concentration, indicating substrate inhibition. Graphical Abstract