Study On Residual Broth Produced During Continuous Fermentation With A PDMS Membrane Bioreactor System

Ethanol could be produced with raw material which contains starch, such as corn, wheat and tapioca and molasses by saccharification, fermentation and distillation. The distillate product would be further dehydrated to become the fuel ethanol. As a substitute of fossil fuels, fuel ethanol is a perfect clean energy. It was thought as a solar energy which transformed by bioprocess. It can make sure the safety of energy sources of a country and improve air environment to some extent.The traditional batch fermentation for ethanol production exist some disadvantages, such as high energy-consumption, high water-consumption and high waste water release. With continuously elevating energy-cost and water-cost, particularly environmental problems, we must develop an innovative fermentation technology for producing fuel ethanol.The membrane bioreactor system for continuous fermentation was constructed based on the composite silicone rubber membrane (PDMS) with high selectivity to ethanol, which prepared in our laboratory. In this system, ethanol product was separated in situ by Pervaporation, and then inhibition of the ethanol to the yeast was reduced. The system could be steadily running for a long time. During the ferment period, ethanol and yeast concentration in the broth and the ethanol production rate were keeping steady. The ethanol production rate was 4.5 times of the conventional batch fermentation process. But the time of the continuous fermentation was limited by aging of the yeast. We could achieve a continuous ferment experiment of 500 hours.There were cumulative inorganic salts in ferment broth, which were fed into the broth as nutriments. In addition, there were nonvolatile co-products in the broth, which could not be removed by Pervaporation. The accumulated inorganic salts, nonvolatile coproducts and dead cell were effects on the yeast cell growth. After the fermentation was ended, the residual broth must be treated with. The residual broth volume was only the one-sixth of conventional batch fermentation processes, and had much different properties. The total solid of the residual broth was 76.71g/L, corresponding to a TS concentration of 7.52%. The ethanol concentration in the residual broth was about 2% (wt), and the COD was 1.2×10~5mg/L. There were also other valuable co-products such as protein, cellulose, saccharides, glycerol, esters, fusel oil, and organic acid and so on, besides the yeast. To utilize the residual broth and achieve a zero-discharge of the waste water has become an important technical problem in the continuous fermentation system.In this paper, the process technology for dealing with the residual broth was investigated in experiment, and the DDGS technology was imaged for the future industrial practice. The recovery byproduct from the residual was mostly used for animal feed, while the remaining was returned to the ferment system as the culture medium. We thought this technology to have considerable economic interest. This technology would need only some usual apparatus and machines, such as decanter centrifuges, multiple reduced pressure evaporators, spray dryers. These machines were proven safe and reliable in industrial practices. We could accordingly believe the imaged technology logical.