Enhancement of methane production from sunflower oil cakes by dilute acid pretreatment

The conversion of sunflower oil cake (SOC) into methane by mesophilic anaerobic digestion was the object of this study. The effect of a combined dilute acid-thermal pretreatment (acid concentration and temperature) on solubilisation and methane potential was investigated using a central composite design (CCD). For temperatures up to 170 °C, solubilisation of each parameter (total organic carbon, sugars and proteins) increased with the severity of the pretreatment (high temperature and high acid concentration). Methane production was higher for pre-treated samples than for the untreated samples (195 mL CH4/g VS). The highest yield (302 ± 10 mL CH4/g VS) was obtained after acid pretreatment at 170 °C. At this temperature, acid concentrations lower than 1% had no significant impact on methane production in comparison to thermal treatment alone. The volume of methane produced by the soluble fraction reached more than 60% of total methane production. An increase in methane production was correlated to the concentration of organic carbon in the liquid phase of samples pretreated at 130–170 °C with acid. At temperatures higher than 170 °C, some recalcitrant compounds were formed in the liquid phase.