Bioenergy recovery from cellulosic biomass through anaerobic digestion and the functional microbes' investigation

Abstract

Food waste is the largest category (36%) of municipal solid waste being disposed of at landfills in Hong Kong. Co-digestion of food waste with the sewage sludge has been proposed as an alternative way to deal with this abundant carbohydrates-rich solid waste. Such co-digestion approach is supposed to not only lighten the burden of the landfills but also produce biogas that could be further utilized to generate electricity, lowering the carbon footprints of the sewage treatment works. Before the pilot scale application, lab-scale experiments are necessary to evaluate the operational feasibility and the stability.

The abundant cellulosic substances like the agricultural residuals or the solid waste (e.g. beer lees) are those candidate sustainable sources for biogas generation. Comparing with the most common organic substrates (e.g. starch, proteins, lipids), biogas generation from the cellulosic substance requires the unique activity of the cellulolytic anaerobes in preliminarily cracking down its recalcitrant structure. Fatty acids metabolizers and the methanogens are the other two dominant microbial groups that deserve thorough understanding as they play equally significant roles in all the methanogenic reactors.

Groups of those functionally interacting microorganisms essentially drive the whole bioconversion process in the anaerobic reactors. Knowledge accumulated in the anaerobic technology might provide some insights on the potential fine-tuned control of the whole system, while a thorough understanding is impossible without the perception on its microbial foundations. Well-meeting with this need is the recently introduced multi-omics tools that have provided an excellent opportunity to accelerate the scientific endeavors in illuminating the functioning anaerobic community.

In this study, the application of the anaerobic technology starts with the lab-scale experiments to evaluate the operational parameters for efficient and steady codigestion of food waste with sewage sludge at mesophilic temperature (~35 ℃). The optimal co-digestion performance was observed at an HRT of 25 days and the food waste to feeding sewage sludge ratio of 2:8. In the medium-size semi-continuous reactor operated under such conditions, the volatile solid reduction (VSR) ratio of 56% and the methane yield of 598 ml CH4/g-VSR were detected. 16S rRNA gene-based phylogenetic analysis was applied to characterize the taxonomy affiliation of the dominant microbial populations in the co-digestion reactors.

Enrichment of the cellulose converting consortia was conducted with substrates of a-cellulose and beer lees, respectively. Gene-centric metagenomics analysis indicated that, comparing with a previously enriched thermophilic consortium, bio-augmentation on the methanogenesis step might weight more than that on the hydrolysis step to further improve the cellulose degrading efficiency (~70%) at mesophilic temperature. In the current era of high-throughput sequencing, huge amounts of sequence/genome information are being generated, while the bottleneck shifted a little bit to the proper and prompt interpretation of these sequences/genomes. A pipeline was established in this study to reliably assign the genomes to six categories regarding to their cellulolytic capability; and the subsequent genome-centric metagenomics facilitated an illumination of the beer lees fermenting consortia at the level of individual populations. Each genome was interpreted on the aspects of its cellulolytic competency, the fatty acids and methane metabolism.