The Immobilization Of Syngas Trophic Anaerobic Acetogen

Global warming caused by carbon dioxide emissions and the shortage of fossil fuels have led to an increasing demand for new renewable energy technologies.Anaerobic acetogen can convert syngas,mainly composed of CO,CO2 and H2,into organic acids or alcohols through Wood-Ljungdahl metabolic pathway,with high potential in energy production and sustainable development.The final product of acetogen M.thermoacetica syngas fermentation,can be used as Yarrowia lipolytica substrate to synthesize oil,which greatly improves the application value of syngas fermentation.However,the syngas fermentation is still the rate limiting step,and the yield of acetic acid is low.In addition,it requires a membrane device for intercepting anaerobic acetogen into the second femtation reactor,which will complicate the operation and increase the additional cost.Therefore,cell immobilization was used in this study for increasing the reusability of anaerobic bacteria,maintaining cell density,simplifying the separation process of cells and products,and reducing costs.In this study,two immobilization methods,adsorption method and embedding method,were compared,and calcium alginate embedding method was selected for further optimization.Furthermore,the study attempted to enhance the mechanical strength of gel particles by glutaraldehyde crosslinking,adding molecular sieve powder and optimizing the concentration of phosphate in the medium.The final embedding conditions were as follows:sodium alginate concentration 3%,molecular sieve powder addition amount 3%,CaCl2 concentration 4%,cell embedding amount 0.6 g/L,embedding temperature 60?,and the phosphate concentration in the medium was 0.2 g/L.In the batch culture of immobilized cells,at 0.1 and 0.2 vvm gas flow rates,the space-time yields of acetic acid were 1.1 g L-1 d-1 and 1.2 g L-1 d-1 in the airlift reactor,and that was 1.5 g L-1 d-1 and 2.1 g L-1 d-1 in the bubble column reactor,respectively.In semi-continuous fermentation of immobilized cells,at 0.2 vvm gas flow rate,the space-time yields of acetic acid in airlift and bubbling reactors were 1.6 g L-1 d-1 and 3.2 g L-1 d-1,respectively.The immobilized cells were reused in batch culture,the trace O2 in the environment did not affect the cell activity in the process of replacing the culture medium and cleaning the immobilized particles.During the semi-continuous fermentation of immobilized cells,the production rate of acetic acid did not decrease until 30 days.In addition,the immobilized cells do not need a membrane reactor to intercept the bacteria,and the fermentation broth can flow directly from the reactor for the next reaction.Therefore,the immobilization method realizes the reuse of anaerobic bacteria,enhances the tolerance of anaerobic bacteria to the environmental changes,and simplifies the subsequent cell separation steps.