White rot fungi pretreatment to advance volatile fatty acid production from solid-state fermentation of solid digestate: Efficiency and mechanisms

Anaerobic digestion has been widely applied throughout the world for lignocellulosic biomass treatment and energy recovery. However, the solid digestate from anaerobic digestion still contains a rather large fraction of poorly anaerobic degradable lignocellulosic fibers due to inhibition of lignin, which deeply limits the bioenergy production from lignocelullosic biomass. Therefore, a novel fungal pretreatment method using P. sajor-caju and T. versicolor was investigated to advance the solid-state fermentation of solid digestate and improve the production of fermentative volatile fatty acids (VFAs). The results showed that a maximum VFA yield of 240 mg COD/g VS was obtained from solid digestate pretreated by P. sajor-caju in 6 weeks, which was 1.17-fold and 1.24-fold higher than that of the autoclaved group and raw substrate, respectively. The mechanisms indicated that these fungal strains could grow on the solid digestate and secrete ligninolytic enzymes such as laccase and manganese peroxidase to degrade lignin in different extents. Besides, fungal pretreatment substantially changed the solid digestate characteristics such as cellulose/lignin ratio and the presence of specific functional groups. Moreover, fungal pretreatment using P. sajor-caju effectively damaged the structure and increased surface area and pore size of the solid digestate, which is beneficial to further VFA production.