Aim of the present study was to assess and evaluate the impact of various kinds of impurities of biodiesel-derived raw glycerol feedstock, upon microbial growth and 1,3-propanediol (1,3-PDO) production by Clostridium butyricum. Preliminary trials in 200-mL anaerobic bottles revealed that the presence of NaCl at a concentration of 4.5% (w/w of glycerol) in growth medium imposed an evident inhibitory effect, in contrast with phosphoric salts. However, the application of NaCl at elevated quantities during batch bioreactor experiments [up to 30% (w/w of glycerol)], did neither affect the microbial growth, nor the 1,3-PDO production. Moreover, when oleic acid was added into the growth medium at 2% (w/w of glycerol), a total preclusion of the strain was observed. In order to further investigate whether the nature of oleic acid itself or the presence of the double bond induced the inhibitory phenomenon, stearic acid was added into the medium at the same concentration (2%, w/w, of glycerol). Indeed, no inhibitory effect was observed in the fermentor, suggesting that the presence of the double bond may play a key role in the growth behavior of the microorganism. Finally, methanol effect was tested in batch and continuous bioreactor operations. Interestingly enough, the alcohol addition did not affect the microbial bioconversion of glycerol into 1,3-PDO, even when imposed at relatively high concentrations (10%, w/w, of glycerol) in batch-bioreactor operations. In continuous experiments, methanol was added when steady state had been achieved, and although in one case high concentration was added into the chemostat (5 g/L), the system re-obtained a steady state without indications of negative effect upon biomass production due to the alcohol.
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