Anaerobic digestion and the effect of hydrothermal pretreatment on the biogas yield of cocoa pods residues

Abstract The current research investigated the possibility of valorizing cocoa pods residues through anaerobic digestion and the possibility of increasing the biogas yield by hydrothermal pretreatment. Using a central composite surface response methodology, the effect of temperature and reaction time on the hydrothermal pretreatment process was studied. Temperature and reaction time was varied between 150–220 °C and 5–15 min respectively. The result show that untreated cocoa pods residues has a biogas potential of 357 l(N)/kgVS and a methane content of 55%. The effect of hydrothermal pretreatment on the biogas yield was diverse. Severities below 3.0 resulted in increased biogas yield. However, higher severity resulted in lower biogas yield. The optimum biogas yield (526.38 l(N)/kgVS) was obtained at 150 °C and 15 min, which represents a severity of 2.65.