Solvent pretreatment and dosing of trace metals in the anaerobic digestion of rice straw

This research investigated the effects of three different chemical pretreatments on the methane yields of rice straw, which is one of the most abundant agricultural, lignocellulosic 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% (w/w) ethanol was used as organic solvent and the pretreatment was carried out at 150 and 180°C for 30 min. Finally, rice straw was pretreated with a 1.6% (w/w) NaOH solution at 30°C for 24 h in a third experimental setup. This study also focused in improving the rice straw biodegradability through the addition of trace elements. Cobalt, nickel and selenium were individually added to the rice straw at different concentrations. The NMMO, ethanol-organosolv and NaOH pretreatments were particularly effective in enhancing the biodegradability of the rice straw and increasing the biogas yields up to 82, 42 and 21%, 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.