Enhancement of paddy straw digestibility and biogas production by sodium hydroxide-microwave pretreatment

Biogas has become a promising energy substitute to fossil fuels. Lignocellulosics, being cheap and renewable resource, could be very well used as a feedstock for biogas generation. Paddy straw is an abundant lignocellulose which is rich in organic matter like cellulose, hemi-cellulose and lignin. It is generally disposed off by burning causing environmental pollution whilst it can be used for biofuel generation. The digestibility of paddy straw is low due to high lignin and silica content. The present study was carried out to enhance paddy straw digestibility and biogas production through sodium hydroxide (NaOH) pretreatment. The paddy straw was pretreated with NaOH by soaking (24 h) in different concentrations of NaOH (2, 4, 6, 8 and 10%) and supplementing with microwave irradiations (30 min, 720 W, 180 °C). 4% NaOH-30 min microwave was found to be the best pretreatment which resulted in 65.0% decrease in lignin content and 88.7% reduction in silica content. This increased digestibility due to reduced lignin and silica content resulted in 54.7% increase in biogas production. Scanning Electron Microscopy of pretreated paddy straw revealed breakdown of lignocellulose structure resulted from the tearing of different layers of cell wall of paddy straw.