Innovative pretreatment strategies for biogas production

•Substrates for biogas might be indigestible, hard to digest, toxic materials.•Indigestible materials can be gasified to syngas and then fermented to biogas.•Recalcitrant biomass can go through a variety of pretreatments.•Toxic substrate can be digested with or without a pretreatment.•Right choice of digesting reactor and criteria can resolve some of these challenges. Biogas or biomethane is traditionally produced via anaerobic digestion, or recently by thermochemical or a combination of thermochemical and biological processes via syngas (CO and H2) fermentation. However, many of the feedstocks have recalcitrant structure and are difficult to digest (e.g., lignocelluloses or keratins), or they have toxic compounds (such as fruit flavors or high ammonia content), or not digestible at all (e.g., plastics). To overcome these challenges, innovative strategies for enhanced and economically favorable biogas production were proposed in this review. The strategies considered are commonly known physical pretreatment, rapid decompression, autohydrolysis, acid- or alkali pretreatments, solvents (e.g. for lignin or cellulose) pretreatments or leaching, supercritical, oxidative or biological pretreatments, as well as combined gasification and fermentation, integrated biogas production and pretreatment, innovative biogas digester design, co-digestion, and bio-augmentation.