A comprehensive review on recent biological innovations to improve biogas production, Part 2: Mainstream and downstream strategies

This review is aimed at detailing and discussing biological innovations and optimizations including upstream, mainstream, and downstream approaches in biogas production elaborated in the recently published patents and research papers. In the first part of this review, upstream strategies including pretreatments by fungal, microbial consortium, and enzymatic as well as some other biological methods including microaeration, composting, ensiling, and genetic and metabolic engineering were comprehensively presented and duly discussed. Despite the fact that these approaches hold great promise for improving the quantity and quality of the evolved biogas, the need for more processing time and extra equipment are the main drawbacks of such strategies. In the second part of this review, mainstream and downstream strategies are reviewed, and their merits and limitations are outlined. Bioaugmentation, anaerobic co-digestion, and integrated biogas production are evaluated as mainstream strategies. Finally, downstream biological approaches used for removal of CO2, H2S, and other impurities are reviewed and discussed. Among the mainstream biological techniques, bioaugmentation is highly advised to accelerate start-up periods at commercial scale while anaerobic co-digestion is recommended to not only enhance biogas yield throughout the operation but also to take advantage of different waste streams. Such mainstream strategies could contribute to improving the economic facets of biogas plants. Despite their currently underdeveloped status, downstream techniques for biological removal of biogas impurities are expected to replace their physiochemical counterparts at industrial scale in future owing to increasingly stringent environmental regulations.