Fungal pretreatment methods for organic wastes: advances and challenges in biomass valorization

Food wastes, municipal solid wastes, sewage sludge, plant materials, animal biomasses, aquatic and terrestrial wastes, agricultural and forestry wastes, industrial and domestic wastes and many other lignocellulosic biomasses are grouped under the category of organic wastes or bio-wastes. Various techniques, mainly mechanical (high-pressure homogenization and ultra-sonication), thermal (temperature-based), microwave-assisted, chemical, and biological pretreatments, have been found to be effective in organic waste valorization. Fungal pretreatment of organic wastes is a promising biological technology because of its excellent efficiency in the decomposition of various types of organic wastes, such as food wastes, ligno-cellulosic biomasses, hemicellulose, agricultural wastes, hardwoods, softwoods, switchgrass, spent coffee grounds, park wastes, cattle dung, and solid digestate, which are specifically reviewed. Fungal pretreatment of organic waste materials can generate advantageous products such as biogas, alternative energy sources, monomeric or oligomeric sugar products, and different types of acids. However, the major challenge associated with fungal pretreatment technology is the requirement of a longer time to achieve a greater degree of biomass valorization, which increases the cost and vulnerability to contamination. However, the use of fungal pretreatment with other pretreatment techniques may shorten the time and enhance the functionality of the method with a higher rate of biomass valorization. Heat generation in the fungal pretreatment process and need for feedstock sterilization before fungal pretreatment are some other challenges that need to be properly addressed for its efficient application on an industrial scale. In this review, the use of different fungal pretreatment methods for the valorization of different types of biomasses and production of valuable products is evaluated and discussed. We performed a comprehensive assessment of the fungal pre-treatment of various types of organic wastes together with a concise but effective discussion on organic solid wastes and different pretreatment techniques involved in bio-waste digestion processes. Furthermore, techno-economic analysis, challenges and future perspectives are discussed.