Paper mill sludge as a source of sugars for use in the production of bioethanol and isoprene

Shona M. Duncan, Malek Alkasrawi, Raghu Gurram, Fares Almomani, Amy E. Wiberley-Bradford, Eric Singsaas

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


Paper mill sludge (PMS) solids are predominantly comprised of cellulosic fibers and fillers rejected during the pulping or paper making process. Most sludges are dewatered and discharged into landfills or land spread at a cost to the mill; creating large economic and environmental burdens. This lignocellulosic residual stream can be used as a source of sugars for microbial fermentation to renewable chemicals. The aim of this study was to determine the possibility of converting mill sludge to sugars and then fermentation to either isoprene or ethanol. Chemical analysis indicated that the cellulosic fiber composition between 28 to 68% and hemicellulose content ranged from 8.4 to 10.7%. Calcium carbonate concentration in the sludge ranged from 0.4 to 34%. Sludge samples were enzyme hydrolyzed to convert cellulose fibers to glucose, percent conversion ranged from 10.5 to 98%. Calcium carbonate present with the sludge resulted in low hydrolysis rates; washing of sludge with hydrochloric acid to neutralize the calcium carbonate, increased hydrolysis rates by 50 to 88%. The production of isoprene “very low” (190 to 470 nmol) because the isoprene yields were little. Using an industrial yeast strain for fermentation of the sludge sugars obtained from all sludge samples, the maximum conversion efficiency was achieved with productivity ranging from 0.18 to 1.64 g L−1 h−1. Our data demonstrates that PMS can be converted into sugars that can be fermented to renewable chemicals for industry.

Original languageEnglish (US)
Article number4662
Issue number18
StatePublished - Sep 2020

Bibliographical note

Funding Information:
Acknowledgments: Thank you to all the staff at the four mills for providing us with sludge samples and information on mill process for this study. The authors are grateful to Donald Guay, Senior Research Scientist, NewPage Corporation Research & Development for his expertise and guidance. This research is sponsored as a part of an Economic Development Incentive Grant from the University of Wisconsin System.

Publisher Copyright:
© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (


  • Biofuels
  • Calcium carbonate
  • Hydrolysis
  • Lignocellulosic biomass
  • Sludge
  • Sugars


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