Intensification of biogas production from rice straw using anaerobic digestion based on pre-treatment with ultrasound

The current work illustrates a novel method of pre-treating rice straw using ultrasound (US) as well as using ultrasound coupled with anaerobic digestion (AD) to intensify biogas production. The primary objectives were to evaluate the effectiveness of ultrasound in increasing the utilization of rice straw and to optimize the conditions for maximum biogas yield. Important parameters such as ultrasonic power (0.2-1 W/mL), duty cycle (20%-80%), and substrate loading (2%-10% w/v) were varied to understand their effects during pre-treatment. The results showed that the maximum increase in soluble chemical oxygen demand (sCOD), with a final value of 13,500 mg/L (an increase of 64.63%), was achieved under optimum conditions of ultrasonic power of 0.4 W/mL, a duty cycle of 50%, and a substrate loading of 6% w/v. Additionally, the study evaluated the effect of low-intensity US exposure during AD with pre-treated rice straw at varying irradiation times (10-30 min) and duty cycles (20%-60%). The optimal conditions of ultrasonic time of 20 min and a duty cycle of 50% resulted in nearly four times higher biogas generation compared to untreated samples. The current research successfully demonstrates the efficient use of US in the feedstock pre-treatment and also in AD process, leading to significant intensification in biogas production within a shorter time frame.