In view of achieving the effective biological pretreatment, the process can be com­bined with physical and chemical treatment methods as the main drawback of biological pretreatment is loss of polysaccharide (cellulose/hemicellulose) and the longer pretreatment duration than chemical and physical pretreatment. Combina­tion of biological pretreatment with chemical/physical pretreatment can enhance the fermentable sugar conversion from biomass and can improve the performance of pretreatment as compared to sole pretreatment. It is obvious that chemical/physical pretreatment prior to biological pretreatment allows the substrate more assessable for microbes to degrade lignin. Therefore, optimization is required in order to min­imize the overall cost of the pretreatment, time, and energy and maximize the fermentable sugar yield after the enzymatic treatment. This combination can be carried out by two ways (i) chemical/physical treatment prior to biological pretreat­ment, (ii) chemical/physical treatment after biological pretreatment. The combined biological pretreatment with chemical/physical treatment and pretreatment process conditions are summarized in Tables 1.2 and 1.3, respectively.

Taniguchi et al. [142] treated rice straw with steam explosion prior to biological pretreatment using P. ostreatu and found that the pretreatment duration could be reduced from 60 days to 36 days required for obtaining 33 % net glucose yield. Yu et al. [129] reported that the treatment time could be reduced from 60 days to 18 days with considerable sugar yield, when rice straw was pretreated with H2O2 (2 %, 48h).Itoh et al. [143] reported that ethanol yield could be increased by 1.16 times when biological pretreatment was carried out prior to organosolv treatment by using C. subvermispora and saved 15 % electrical energy. Indeed, biological treatment can also be used in lignin-based oil production. For example, Fomitopsis sp. IMER2 was used in removal of amorphous region of cellulose from corn stover and resulted a significant increase in the oil yield from 32.7 % to 50.8 % in pyrolysis process. Therefore, it can be concluded that biopretreatment favors thermal decomposition of corn stover [134].