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The exploitation of municipal solid waste (MSW) and related waste paper streams in the production of bioalcohol

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Disciplines
  • Biology
  • Design
  • Engineering
  • Medicine

Abstract

An organic fraction from municipal solid waste (MSW) comprised 38.9% (w/w) glucose (cellulose and starch) indicating its potential as a substrate for bioalcohol production. Microscopy indicated that the fraction was rich in waste paper fibres. Much paper waste comes from shredded office paper (50.4% w/w glucose) which is unrecyclable because of poor fibre length. This, and microbiological hazards associated with the use of MSW led to its choice as model substrate for study. Saccharification of shredded paper waste was optimised by selection of Accellerase® and additional beta-glucosidase enabling digestion of 99.27% of cellulose. Sequential batch-addition of substrate permitted substrate “concentrations” equivalent to 25-30% (w/v). Saccharification was enhanced by detergent, but reduced by the presence of alcohols at over 3-4% (v/v). Steam explosion of paper slightly enhanced saccharification. However, the approach was rejected due to high energy cost, production of fermentation inhibitors at high severities, and lack of clear benefit regarding ethanol yield. Interestingly, levels of inhibitors were low compared to other pre-treated substrates and addition of paper to other substrates greatly reduced their own production of inhibitors during pre-treatment (wheat straw 60%, filter paper 95%). Larger pilot-scale (1.5-5 L) operations involved developing the batch-addition regime with a high-shear stirring capacity vessel. Additions equating to final substrate concentrations of ~65% (w/v) were achieved (from an initial 5% w/v) and facilitated high ethanol concentrations (11.6% v/v) with minimal enzyme input (3.7 FPU/g substrate). Thermal tolerance of a range of yeast strains was investigated by developing a rapid screening approach with liquid-handling robotics. This identified strains able to endure temperatures up to 40°C. Evolutionary engineering may improve tolerances to temperatures nearer to enzyme optimums (50°C). Some previously unused strains exhibited superior growth to referenced industrial strains. The above findings were integrated into a process design along with recommendations for further enhancement.

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