Production of chemicals in thermophilic mixed culture fermentation: mechanism and strategy

Thermophilic mixed culture fermentation (TMCF) has advantages such as high substrate degradation rate (e.g. 0.5 d(-1) of 60 degrees C vs 0.2 d(-1) of 35 degrees C), hydrogen and methane yields (e.g. 3-4 mol-H-2/mol-glucose at 70 degrees C vs 1-2 mol-H-2/mol-glucose at 35 degrees C) and low gas solubility (e.g. H-2 decrease 10% and CH4 decrease 30%) for the treatment of hot wastewater. However, the low biomass concentration, complexity of metabolites and high-running cost still limit its application to some extent. Here, the typical metabolic reactions in this process are briefly summarized. Then, the recent advances in alkali TMCF, anaerobic membrane bioreactor, individual metabolite production, thermophilic microbial fuel cells and thermophilic syngas fermentation are reviewed. The following separation processes of biogas upgrading, gas stripping and electrodialysis are also reviewed to separate and purify the metabolites in TMCF. Lastly, to meet the demands of better overall performance and lower-running cost, the prospects in the fields of direct interspecies electron transfer, medium chain fatty acids production and energy cost reduction are highlighted, but new strategies in TMCF still need to be developed in the future.