Biological degradation of rice straw with thermophilic lignocellulolytic bacterial isolates and biogas production from total broth by rumen microorganisms

Abstract

Rice straw, a common agricultural residue, is abundant in Thailand and can be converted to biogas to reduce dependence on fossil resources for energy. Biological degradation of rice straw followed by anaerobic digestion (AD) for biomethane production (BMP) was investigated. Thermophilic lignocellulolytic bacteria for biological degradation were isolated from soil samples collected from Dong Han and Kok Soong National Forest around Roi-Et Rajabhat University, Roi-Et Province, Thailand. Cellulase and ligninolytic enzymes were assayed using standard microbiological methods. Isolate(s) with high cellulolytic and ligninolytic activities were then used to degrade rice straw in basal medium at 50 ± 2 °C for 5, 10, 15, and 20 days. Total broth was anaerobically digested with rumen microorganisms in 1.2-m3 PP digester at 39 ± 0.5 °C for 30 days. A total of 106 thermophilic bacterial isolates were sampled from soil, out of which 96 (90.6%) and 16 (15.1%) isolates tested positive for cellulolytic and ligninolytic activities, respectively. Unlike laccase and manganese peroxidase (MnP), lignin peroxidase (LiP) activity was not detected. Isolate RUFR60 showed both high cellulolytic and ligninolytic activities. Highest biogas and methane (CH4) yields (318.15 and 177.93 mL/gTS, respectively) were recorded from 20 days degraded rice straw which were not significantly (p > 0.05) different from those (315.10 and 171.72 mL/gTS, respectively) recorded from 15 days degraded rice straw. Optimal degradation period of 15 days with thermophilic lignocellulolytic bacteria was efficient and economical for BMP from rice straw.