Optimization for improved biomethane yield from cashew nut hulls through response surface methodology

The increase of cashew nut-processing plants in Burkina Faso caused high amount of cashew nut waste. The waste is either discharged into the environment or used as fuel, causing significant harm to both the environment (such as global warming) and human health (such as respiratory diseases). To address these issues, this project proposes an alternative biotechnological method for recovering the waste and producing biogas. The aim is to contribute to the energy autonomy of processing units. The objective of this study was to optimize biomethane yield from cashew nut hulls using response surface methodology. A central composite design (CCD) methodology with three independent variables was applied to investigate the effects of inoculum concentration, substrate concentration, and incubation temperature on biogas, biomethane, and methanogenic bacteria numbers. The experimental design involved using 28 glass bottles (300 ml) as batch reactors with a working volume of 2/3. The optimal biogas, CH₄, and methanogenic bacteria numbers were estimated using the numerical and graphical optimization tool of Design Expert Software®. Surface response design results revealed that experimental results were best fitted into a quadratic polynomial model with regression coefficient values of more than 0.80 for all responses. Optimized preparation conditions for biomethane production were 50% inoculum concentration, 2% for substrate concentration, and 45 °C for temperature. Optimal conditions made it possible to note biomethane production of 157.7 mL/g VS with a load reaching 7 × 10³ CFU/mL for methanogenic bacteria. This study has defined the optimum parameters for producing biomethane from cashew nut shells. The units will be able to use these results to add value to cashew nut shell waste and achieve energy autonomy.