A continuous cultivation of Clostridium acetobutylicum ATCC 824 is described using a two-stage design to mimic the two phases of batch culture growth of the organism. A hydrophobic pervaporation unit was coupled to the second fermentor containing the highest solvent titers. This in situ product recovery technology efficiently decreased butanol toxicity in the fermentor while the permeate was enriched to 57-195 g L-1 total solvents depending on the solvent concentrations in the fermentor. By the alleviation of product inhibition, the glucose concentration could be increased from 60 to 126 g L-1 while the productivity increased concomitantly from 0.13 to 0.30 g L-1 h(-1). The continuous fermentation was conducted for 1172 h during which the pervaporation was coupled to the second fermentor for 475 h with an average flux of 367 g m(-2) h(-1). The energy consumption was calculated for a 2 wt.% n-butanol fermentation broth and compared with the conventional process. (C) 2012 Elsevier Ltd. All rights reserved.