Enhancement strategies for higher biomethane production yields from the anaerobic digestion of rice straw

This research investigated the effects of two different chemical pretreatments on the methane yields of the lignocellulosic material rice straw, which is one of the most abundant agricultural wastes worldwide. Initially, the raw agricultural residue was pretreated with the organic solvent N-methylmorpholine-N-oxide (NMMO) at 120°C for 3 h. Consequently, batch bio-methane production (BMP) tests were performed under mesophilic conditions for 40 days. In a similar experimental run, the effect of organosolv pretreatment on the rice straw was investigated. 50% ethanol was used as organic solvent and the pretreatment was carried out at 150 and 180°C for 30 min. Furthermore, this study focused in improving the rice straw biodegradability through the addition of trace elements. Iron, cobalt, nickel and selenium were added to the rice straw at different concentrations and in different combinations. Both NMMO and organosolv pretreatments were particularly effective in enhancing the biodegradability of the rice straw and, consequently, increasing the biogas yields up to 82 and 42%, respectively, compared to those of the untreated material. On the other hand, the addition of trace elements had only a minor effect (i.e. up to 8%) on the enhancement of the biogas yield from the AD of rice straw.