Document Type

Thesis - University Access Only

Award Date

1994

Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology

First Advisor

William R. Gibbons

Abstract

Ethanol can be produced from a variety of carbohydrate containing compounds (sugar, starch, and lignocellulosics). Of these materials, lignocellulosic substrates such as municipal solid waste (MSW) offer the best outlook for future ethanol production due to their abundance and availability, cellulosic content, and low cost. The main factor limiting commercialization of cellulose to ethanol processes has been the low efficiencies and high costs of various pretreatments and enzymes. Much research and development effort has been expended worldwide during the past 15 years to overcome these obstacles. The objective of this study was to evaluate commercially available technology and enzymes to determine how efficiently they could convert MSW to ethanol. In order to maximize ethanol production from MSW, several parameters were first investigated. Shake-flask trials were used to evaluate a range of relatively mild physical and chemical pretreatments to determine which would provide the best conversion of lignocellulosic materials. Solka floe and newspaper were used as substrates to evaluate NaOH, H202, NH4, and H2SO4 chemical pretreatments, followed by simultaneous saccharification and fermentation (SSF) using commercially available enzymes and a 5% (v/v) inoculum of Saccharomyces cerevisiae. It was determined that the 12% (w/v) NaOH pretreatment (4-5 h, 80°C) was best, achieving a 0.44 g ethanol/100 g substrate yield, 0.008 g/L/h productivity, and a maximum ethanol concentration of 0.20 g/L. Physical pretreatment was then evaluated using newspaper ground in a high speed Retsch mill using screen sizes of 0.5, 1.0, and 2.0 mm, followed by NaOH pretreatment and SSF using a combination of two commercially available enzymes and a 5% (v/v) inoculum of S. cerevisiae. The 1.0 mm screen size was optimal, showing average ethanol yields of 0.22 - 0.26 g/100 g, an ethanol productivity of 0.07 g/L/h, and maximum ethanol concentrations from 0.22 - 0.26 g/L. However, it was later determined that blending of separated MSW gave comparable results. Following the pretreatment investigations, a variety of commercially available and experimental enzymes were evaluated using newspaper, paper pulp, computer paper, and MSW as substrates. Fermentation trials were conducted in shake-flasks using both batch and fed-batch modes. In addition to the variety of enzymes, Tween 20 surfactant was used in certain trials to enhance enzymatic hydrolysis (by facilitating enzyme movement between substrate molecules). The combination of two experimental enzymes from Genencor (standard+ 8-glucosidase) with the Tween 20 addition was determined to be the optimal combination for use with blended and NaOH-pretreated MSW. These results were then used for scale-up in the Bioflo III fermenter to provide controlled fermentation (30°C, pH 3.9 - 4.5, 500 rpm) and simulation of pilot plant conditions. As determined from the shake-flask trials, a 5% (v/v) inoculum of S. cerevisiae, -1.54 ml std/100 g dry substrate, -4.0 ml B-glu/100 g dry substrate, and -1.54 - 1.92 ml Tween 20/100 g dry substrate were added to NaOH-pretreated MSW in the 5 L Bioflo III fermentor (30°C, pH 3.9 - 4.0, 500 rpm). Experiments were performed using either initially wet or initially dry MSW in equal volumes. Dried, pretreated MSW was also added during fermentation (fed-batch fermentations). The initially wet MSW provided the best results in the fed-batch mode, having achieved an average ethanol yield of 6.72 g/100 g, average ethanol productivity of 0.10 g/L/h, and an average maximum ethanol concentration of 11.70 g/L. The results obtained in this study indicate that the currently available enzymes and technology evaluated are not sufficiently efficient to permit commercialization of a cellulose-to-ethanol process. Therefore, further research is needed in the areas of cellulase enzymes, pretreatment, and strain improvement. If advances can be made in one or preferably all of these areas, production of ethanol from MSW may be feasible in the near future.

Library of Congress Subject Headings

Refuse and refuse disposal
Alcohol as fuel
Refuse as fuel

Format

application/pdf

Publisher

South Dakota State University

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Rights Statement

In Copyright