Impact of particle size of cell carrier on caproate fermentation in a cell immobilized system: Focusing on the improvement of caproate production in batch and continuous operation modes

Wheat straw is a superior matrix for cell immobilization, yet the influence of its particle size on caproate fermentation is unclear. In this study, the influence of particle size of wheat straw on caproate production, biofilm formation and mass transfer was investigated in the cell immobilized system. Results showed that the maximum caproate production achieved 14.4 g/L in batch tests and the optimum particle size of wheat straw was in the range of 5–10 mm. Analysis on the biomass, extracellular polymeric substances (EPS), Fourier transform infrared spectroscopy (FTIR), Thermogravimetry (TG) and zeta potential revealed that the biofilm of Clostridium kluyveri was formed on the cell carrier, which explained the high caproate production at the optimum condition. The highest mass transfer coefficient of 0.738 m/s was observed with the optimum particle size. Bubble formation on the cell carrier promoted the floating of cell carrier, which benefited the mass transfer from fermentation broth to immobilized cells as well as biofilm formation. Taking the optimized wheat straw as cell carrier in the continuous operation mode, the highest mean caproate production reached 19.6 g/L, and the mean caproate production achieved 18.3 g/L when the food waste-based ethanol and acetate were used as substrates. The high caproate production was ascribed to the formation of thick biofilm on the cell carrier which kept its chemical structure stable during the long term fermentation.