Due to the toxic nature of butanol, the initial substrate concentration is limited to <80 gL-1 (usually 60 gL-1). A substrate concentration in excess of this results in a high residual substrate, thus resulting in inefficient sugar utilization and increased BOD (biological oxygen demand) load for wastewater treatment. How­ever, recent developments in downstream processing (recovery) of ABE have made it possible to use concentrated sugar solutions for this fermentation. During the fermentation, the toxic products are removed simultaneously, thus relieving inhibition that results in the utilization of more substrate. The details of the recovery techniques are given in the recovery section (3.1). Employing butanol removal techniques, sugar solutions containing 161 gL-1 glucose (C. beijerinckii; Ezeji et al., 2003) and 227 gL-1 lactose (C. acetobutylicum; Qureshi and Maddox, 2005) have been successfully used. Use of concentrated glucose and lactose solutions has resulted in the production of 76 and 137 gL-1 ABE, respectively. In such fermentations, fewer acids are produced, thus improving the ABE yield. In another process, 200 gL-1 lactose was successfully fermented in a batch reactor of C. acetobutylicum when integrated with product recovery by gas stripping (Maddox et al., 1995). This system resulted in the production of 70 gL-1 ABE with a productivity of 0.32 gL-1h-1 as compared to 0.07 gL-1h-1 in the control batch reactor. Studies reported in this section demonstrated that a fermentation medium containing over three times the sugar concentration as compared to a batch reactor can be successfully fermented when integrated with product removal techniques.