Comparison of Different Pretreatments of Rice Straw Substrate to Improve Biogas Production

Abstract

Chemical and biological pretreatments (with NaOH, HCl, CO(NH2)2 and cellulase) were used to pretreat rice straw at ambient temperature (about 20 °C) to improve its biodegradability and increase anaerobic biogas production. The NaOH and CO(NH2)2 pretreatments reduced the percentage contents of hemicellulose and lignin. The HCl pretreatment mainly dissolved the hemicellulose and resulted in decreases of 12.5–7.1% of the hemicellulose. The percentage content of cellulose showed a dramatic decrease, from 38.3 to 10.9%, after the cellulase pretreatment. Compared with untreated rice straw substrate, the total biogas yield ratios were 3.38–5.91, 1.63–2.99, 1.93–5.22 and 3.62–6.45, with a hydraulic retention time of 30 days, under NaOH, HCl, CO(NH2)2 and cellulose pretreatments, respectively. The highest yields of biogas and methane were obtained from 40 U/g total solids (TS) cellulase-pretreated rice straw (20.433 and 9.918 L respectively). Biogas production yields of volatile solids (VS) were 123.7, 273.8, 318.5, 353.5 mL/g for control, 6% CO(NH2)2-, NaOH- and 40 U/g TS cellulase-pretreated rice straw substrate, respectively. Compared to untreated rice straw substreates, cumulative biogas production yields increased 16–103, 25–122% for NaoH- and cellulase-pretreated rice straw substrate, respectively. The results suggested that the highest removal efficiencies of TS and VS were obtained from 6% NaOH-pretreated (53.80 and 36.80%), 6% CO(NH2)2-pretreated (54.90 and 36.10%) and 40 U/g TS cellulase-pretreated (51.30 and 37.30%) rice straw substrate. In short, NaOH, HCl, CO(NH2)2 and cellulase pretreatment was suitable to enhance the biogas production. However, to choose the optimal treatment, the energy requirements relative to the energy gain as extra biogas production have to be taken into account, as well as the costs of chemicals or enzymes.