Matagenomic-metatranscriptomic analysis of the acidogenic reactor in a two-phase anaerobic digestion process (PI: Professor Jonathan Wong)
On global basis, about 1.3 billion tonnes of MSW (municipal solid waste) is produced every year. Of these, food wastes represent 30-50% (~40% in Hong Kong) in MSW and their disposal by landfilling occupies precious lands and cause emission of methane resulting in global warming. Anaerobic digestion (AD) is a viable treatment option as it produces energy and simultaneously divert the waste from landfilling. Traditionally AD was performed using single-phase reactors, with an imbalance between acid production and consumption resulting in digester running under sub-optimal feeding rate and inefficient methane generation. Thus two-phase AD reactor design, which physically separates the acid production (acidogenesis/acetogenesis) and acid consumption (methanogenesis) phase, has been demonstrated with the advantages of optimizing the reactors individually, increasing process efficiency and digester stability. However, the methane yields are only marginally higher than the single-phase reactors. Analysis of carbon mass balance indicated that ~20% of C in the substrate is lost as CO2 emission in the acidogenic reactor. From the literature, it is clear that the off-gas include both CO2 and H2 in significant quantities. We hypothesize that the off-gas from the acidogenic reactor can be diverted to the methanogenic reactor, where homoacetogens or hydrogenotrophic methanogens could use the CO2 and H2 to produce acetate or methane, respectively, thus the overall yield of methane would be increased. The key factors affecting the metabolic pathways are low pH, acid accumulation and headspace H2 concentration in the acidogenic reactors. These interactions have not been adequately addressed and the few available literatures often report the shift in dominant microbial species without linking them to the shift in metabolic pathways. A combined metagenomic-metatranscriptomic approach could reveal the complete array of microbial species and functional attributes that can be used to decipher the factors affecting the generation of different acidogenic products such as fatty acids and solvents, and the overall shift in metabolic pathways under different operating conditions and reactor-environments. Understanding these microbe-metabolite interactions which influence the metabolic pathways is the key to further increase the methane yield. Thus this proposal aims at revealing (i) the microbial interactions in the acidogenic reactor of a two-phase AD setup using advanced metagenomic-metatranscriptomic analysis and (ii) the factor affecting the acidogenic leachate quality, i.e. the feed for the second-stage methanogenic reactor, on the methane production as well as potential ways to reduce the CO2 loss. The knowledge developed through the metagenomic analysis can be used to adjust the operating conditions and reactor environment to target specific metabolite production; thus have the potential to develop an effective AD process that have the scientific relevance in different areas of biofuel production.