Paper Mill Sludge as a Source of Sugars for Use in the Production of Bioethanol and Isoprene

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

doi:10.3390/en13184662

Abstract

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.

Highlights

Woody biomass is the largest source of renewable biomass and is traditionally used as feedstock for biofuel production [1,2]
Sugar solutions for fermentation were produced using the above method except that the solids content was increased to 15%, the volume was increased to 1 L and the hydrolysis was completed in a 3 L bioreactor (New Brunswick, NJ, USA) at 50 ◦ C at 120 rpm until sugar level in the solution reached a constant value
Washing the same mill had numbers that were identical, 12.97 and 12.65 respectively this indicates that calcium carbonate was not the only filler in this sludge

Summary

Introduction

Woody biomass is the largest source of renewable biomass and is traditionally used as feedstock for biofuel production [1,2]. During the wood digestion process millions of tons of both primary and secondary sludge are annually discarded in the USA; most is disposed of in landfills or incinerated at substantial cost to the industry. The cost to dispose of sludge will be different for each facility depending on the percent solids in the sludge and landfill fees [3,4,5]. Energy recovery efficiency depends on the nature of the sludge type and the deployed energy recovery technology. In Wisconsin, a ballpark figure is US $30 per dry ton to dispose of sludge in a landfill. For a large mill that produces 100 tons of sludge per day this cost could be as high as $1 million per year for disposal, Energies 2020, 13, 4662; doi:10.3390/en13184662 www.mdpi.com/journal/energies

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