Valorization of Macaúba husks from biodiesel production using subcritical water hydrolysis pretreatment followed by anaerobic digestion

Abstract Macauba husks are inedible lignocellulosic wastes obtained after extracting oil from the fruit for biodiesel production. The objective of this study was to valorize macauba husks through subcritical water hydrolysis (SWH) pretreatment followed by anaerobic digestion (PT+AD) in comparison to a control reactor (CR). The semi-continuous parameters were: 10 g of dried husk; flow rate of 10 mL/min at 200 °C and 14 MPa and for a total reaction time of 40 min. For AD, mesophilic conditions (35 °C) were kept for both experimental trials. For the PT+AD experiment, digestion was performed for 38 days and for 39 days for the CR experiment. The chemical oxygen demand (COD) removal for the PT+AD reactor was 48% and 43% for CR, indicating the pretreatment improved digestibility of the organic feed. Similarly, the CH4 yield per unit of COD for 590 mL CH4/g COD for the PT+AD experiment compared with 57 mL CH4/g COD for the CR experiment. The corresponding biogas yield was 161% greater than in the PT+AD experiment compared with the CR. Energy analysis revealed a corresponding benefit from SWH for production of heat and power, and several recommendations were made for decreasing the heat required by SWH to make the process self-sufficient. Macauba cultivation has promise as an energy crop that can be grown locally in Brazil and similar tropical locations for co-production of biodiesel from the extracted oil and biogas from the husks, thereby avoiding greenhouse gas emissions by replacement of traditional fossil sources.