A promising application for microbial pretreatment of lignocellulosic materials is for increasing biogas yield in the anaerobic fermentation process. Anaerobic diges­tion of organic waste and residues not only provides a good solution for the sustainable processing and treat­ment of large amounts of biomaterials, but also leads to value-added renewable energy production. Natural lignocellulosic materials can only be converted to biogas at a very low efficiency due to their resistance to anaerobic digestion. The low biogas conversion rate results from the resistance to enzymatic attack by the biomass due to the tight association of lignin, cellulose, and hemicellulose. Under anaerobic conditions, cellulose and hemicellulose can be degraded during biogas production but not lignin (Fernandes et al., 2009). Pretreatment procedures to increase the accessibility of holocellulose are necessary to increase biogas production. Different pretreatment methods, including physical and chemical pretreatments, effectively enhance anaerobic digestion, but these proce­dures have disadvantages as described beforehand. A microbial pretreatment followed by another step of bio­logical process seems very promising and close to prac­tical application as shown by some following examples.

Pretreatment of wheat straw with Pleurotus sp. "flor­ida" doubles both cellulase digestibility of the treated biomass and the resulting biogas yield, compared with untreated wheat straw (Muller and Trosch, 1986). Pre­treatment of softwood in the presence of wheat bran with the white-rot fungus C. subvermispora, which can effectively degrade the lignin component, enhanced methane fermentation of softwood to 35% of the theoret­ical yield, based on holocellulose content of the biomass. In contrast, pretreatment with Pleurocybella porrigens, which has a lower ability to decompose lignin, led to no significant changes (Amirta et al., 2006).

Application of a lignocellulose degrading composite microbial system with high xylanase activity (XDC-2), instead of a pure culture of microorganisms for biomass pretreatment has also been tested. XDC-2 is composed of 26 different clones from three phyla: Clostridiales, Pro — teobacteria, and Bacteriodetes. However, these degrade mainly carbohydrate but not lignin. After a 5-day pretreatment with XDC-2, corn stalk was efficiently degraded by nearly 45%, and the cellulose and hemicel — lulose contents were decreased by 22.7% and 74.1%, respectively. Biodegradability of the pretreated biomass is improved resulting from changes in chemical struc­ture due to decreased holocellulose content. Compared with untreated corn stalks, total biogas production and methane yield were increased by 68.3% and 87.9%, respectively, and the technical digestion time (T80) was shortened by 35.7% (Yuan et al., 2011).

Effectiveness of biological pretreatments in enhancing corn straw biogas production has also been reported with complex microbial agents including yeast (S. cerevisiae, Coccidioides immitis, and Hansenula anomala), cellulolytic bacteria (Bacillus licheniformis, Pseudomonas sp., Bacillus subtilis, and Pleurotus florida), and the lactic acid bacteria Lactobacillus deiliehii. A 15-day pretreatment of corn straw at ambient temperature led to reduced contents of total lignin, cellulose, and hemicellulose, and increased con­tent of hot-water extractives. Anaerobic digestion of the pretreated material resulted in 33.07% more biogas yield, 75.57% more methane yield, and 34.6% shorter technical digestion time compared with the untreated sample (Zhong et al., 2011).

In conclusion, under proper conditions, microbial/ biological pretreatment can be an effective method for improving biodegradability and enhancing downstream biological conversion efficiency of biomass into bio­energy and other value-added bioproducts.