When SSF fermentation is conducted via yeast, it is suggested to recirculate the process stream. The purpose is to maintain a high concentration of ethanol and dissolved solids and therefore to reduce the energy requirements in distillation and evaporation units ( expensive steps) and reduce fresh water usage. Based on the study by Alkasrawi et al (2002) for SSF process,when 60% of the fresh water was replaced by stillage, ther was no changed for ethanol yield and productivity but ethanol production cost reduced by 17%. When the liquid after SSF was partially recycled, 40% fresh water was replaced;final ethanol yield was not affected with initial productivity decreased; ethanol cost reduced 12%.
However, the disadvantage is that ccumulation of compounds released, or formed in the hydrolysis and fermentation steps may be toxic to enzymes or yeast.
So, we at least have effective enzymes and robust microorganisms for fermentation for this initial consideration of stream liquor circulation, right?
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