Optimization and kinetic study of biogas production from rice husk through solid-state alkaline pretreatment method

Biogas production of raw and solid-state alkaline pretreated rice husk (RH) was investigated at mesophilic conditions. Three concentrations of sodium hydroxide was used to prepare the samples namely; 1, 3 and 5% at 100 and 120°C. The 3% pretreated RH at 100°C (3N100) produced the highest cumulative biogas of 4705 ml followed by 5% pretreated RH at 120°C sample with a cumulative volume of 4590 ml. The combined severity factor (CSF) showed that a moderate severity treatment was needed to improve biogas production for solid-state pretreated RH. First order kinetic model and the modified Gompertz model were used to evaluate biogas production while Chen and Hashimoto model was used to determine the critical hydraulic retention time (HRT_critical). The biodegradability constants (k₀) ranged between 0.5163 to 1.1294 with 3N100 and 5N120 having the highest values of 0.8086 and 1.1294 d⁻¹ respectively. Modified Gompertz model showed there was marginal percentage difference between experimental and predicted biogas production except for 1N100 with percentage difference of 11.57%. The HRT_critical for 3N100 was calculated to be 11.60 days. The optimum conditions for solid-state alkaline pretreatment of RH were 3% concentration of NaOH and reaction temperature of 100°C