Bioconversion of Kraft Paper Mill Sludges to Ethan

Authors: Li Kang Wei Wang Yoon Y Lee

Publish Date: 2010/01/23

Volume: 161, Issue: 1-8, Pages: 53-66

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Abstract

Paper mill sludge is a solid waste material composed of pulp residues and ash generated from pulping and paper making processes The carbohydrate portion of the sludge has chemical and physical characteristics similar to pulp Because of its high carbohydrate content and welldispersed structure the sludges can be biologically converted to valueadded products without pretreatment In this study two different types of paper mill sludges primary sludge and recycle sludge were evaluated as a feedstock for bioconversion to ethanol The sludges were first subjected to enzymatic conversion to sugars by commercial cellulase enzymes The enzymatic conversion was inefficient because of interference by ash in the sludges with the enzymatic reaction The main cause was that the pH level is dictated by CaCO3 in ash which is two units higher than the pH optimum of cellulase To alleviate this problem simultaneous saccharification and cofermentation SSCF using cellulase Spezyme CP and recombinant Escherichia coli ATCC55124 and simultaneous saccharification and fermentation SSF using cellulase and Saccharomyces cerevisiae ATCC200062 were applied to the sludges without any pretreatment Ethanol yields of 75–81 of the theoretical maximum were obtained from the SSCF on the basis of total carbohydrates The yield from the SSF was also found to be in the range of 74–80 on the basis of glucan The SSCF and SSF proceeded under stable condition with the pH staying near 50 close to the optimum for cellulase Decrease of pH occurred due to carbonic acid and other organic acids formed during fermentation The ash was partially neutralized by the acids produced from the SSCF and SSF and acted as a buffer to stabilize the pH during fermentation When the SSF and SSCF were operated in fedbatch mode the ethanol concentration in the broth increased from 255 and 326 g/L single feed to 45 and 42 g/L respectively The ethanol concentration was limited by the tolerance of the microorganism in the case of SSCF The ethanol yield in fedbatch operation decreased to 68 for SSCF and 70 for SSF The highsolids condition in the bioreactor appears to create adverse effects on the cellulase reactionThe authors wish to acknowledge the financial support provided by Masada LLC Alabama Center for Renewable Resources Engineering and Laboratory of Biofuels and Biomaterials Auburn University AL USA They are grateful to Boise Paper Jackson AL USA for providing paper mill sludges and technical information and to Danisco Genencor Palo Alto CA USA for providing cellulase enzymes

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