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In batch reactors from which product is not removed simultaneously, butanol concentrations in excess of 20 g/L are rarely achieved due to product (butanol) inhibition, and this limits the use of substrate sugar solutions to no greater than 55–60 g/L. Use of such dilute sugar solution results in the requirement for large process volumes. In this case, product is recovered by distillation. This results in the removal of 980 g of water from 1 L of fermentation broth as the top product of the distillation column, and to recover only 20 g butanol as the bottom product, thus making it an energy intensive process. Recycle of water is essential to reduce the process volume. In addition, recycle of reaction intermediates such as acetic and butyric acids cannot normally be performed in batch fermentation. In order to utilize reaction intermediates and recycle water, toxic butanol should be simultaneously removed from the fermentation broth using one of the in-situ product removal techniques such as gas stripping, pervaporation, adsorption, or liquid–liquid extraction. In such integrated systems, sugar amounts in excess of 1400 g/L reactor volume have been possible as compared to 55–60 g/L in non-integrated batch reactors, and this has resulted in the production of 461 g/L total acetone–butanol–ethanol (ABE), of which butanol was the main product. This also resulted in the improvement of productivity by a factor of at least 3. In non-integrated batch systems productivity of the order of 0.30 g/L · h are achieved while in the above-mentioned system it is of the order of 1 g/L · h. Such integrated systems have been successful for substrates of glucose, whey permeate, and cellulosic biomass. For cellulosic biomass systems (wheat straw and sweet sorghum bagasse), simultaneous saccharification, fermentation, and product recovery (SSFR) have been successful. Hence, integrated butanol-producing systems have benefits of water recycle, use of reaction intermediates, improvement of productivity, and use of concentrated sugar solutions.