The application of a clean technology for alcohol fermentation was developed by recycling spent broth from the production of biobutanol to reduce energy, save water, protect the environment and reduce wastewater treatment cost. Spent broth was recycled by ultrafiltration and permeate was directly reused as raw water in the production of biobutanol. Ultrafiltration was used to remove most high molecular weight fermentation by-products and proteins, and partially removed some low molecular weight organics. Polysulfone hollow fiber membrane (Romicon HF0.8-0.75 PM500, Koch Membrane Systems, Inc., Wilmington, MA) did not show any significant fouling during ultrafiltration of fermentation broth. It was observed that the increase in temperature and pressure increased rate of permeation (Figure 1). Performance of fermentation showed that the biobutanol titer was reduced from ~21 g/L to ~14 g/L and ~12 g/L in the first and second recycle, respectively (Figure 2). After two consecutive recycle of the broth, glucose consumption continuously decreased which resulted in lower productivity of butanol. The fermentation time, on the other hand, was not prolonged but productivity was reduced from 0.22g butanol/L-hr to approximately 0.17 productivity in the first recycle and 0.14 in the second recycle.The reduction in yield, however, was observed to be less significant from 23% to 21%. The decrease in productivity can be attributed to the accumulation of low molecular weight organics, and salts in the recycled permeate which inhibit fermentation. Based on the results ultrafiltration can be considered as a cost effective method of recycling fermentation broth up to second recycle in this study.
Figure 1. Effect of operating temperature and pressure on ultrafiltration flux
Figure 2. Fermentation kinetics (a) first recycle, (b) second recycle
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