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21 THE EFFECTS OF MASS TRANSFER ON LANDFILL STABILIZATION RATES James J. Noble, Associate Professor of Chemical Engineering Thelma Nunez-McNally, Chemical Research Engineer Berrin Tansel, Environmental Research Engineer Center for Environmental Management Tufts University Medford, Massachusetts 02155 INTRODUCTION Rates of municipal solid waste (MSW) decomposition and attendant sanitary landfill stabilization are often seen to be quite slow. Indeed it is not uncommon to qualitatively gauge the extent of biodegradation in landfills by the legibility of buried newspapers. These may be recovered essentially intact after 20-30 years. Recent archaeological excavations of sanitary landfills by Rathje1 have also shown that when 10 year old rubbish is exhumed, it may appear to be mummified or "entombed". The long time scales for landfill stabilization stand in marked contrast to the fairly short time scales for decomposition demonstrated by the myriad of published papers on biomass conversion and the extracellular enzymatic hydrolysis of cellulosic materials in the laboratory. Three important reasons that landfills decompose so slowly are: 1) lack of sufficient moisture for optimal enzyme activity; 2) imbalance between the production of organic acids by acid-forming bacteria and the conversion of these acids into methane by methanogenic bacteria; and 3) mass transfer limitations in the landfill environment. The main purpose of this chapter is to speculate on the role of mass transfer limitations in landfills and to discuss possible consequences. It is argued that under certain circumstances mass transfer is the rate controlling step in landfill stabilization and that detailed knowledge of metabolic pathways for the microorganisms may not be always necessary for predictive purposes. This work is part of a larger study on landfill dynamics at Tufts University, Center for Environmental Management, which seeks to develop a first-round systems model for landfill behavior. At the outset it is important to clarify our view that the rate limiting step for biodegradation in a landfill may change as a function of landfill age. Thus one may argue that since MSW includes a small readily soluble fraction, the initial rate limiting step might be methane formation because of insufficient time to develop viable methanogen populations. To fix ideas better, the readily biodegradable material might comprise 5% of the MSW and convert completely to methane in 2 to 3 years. This behavior may well be that observed in lysimeter studies of several years duration.2 Our focus here, however, is the next phase of degradation when the more recalcitrant and insoluble lignocellulosic material would become the most biodegradable remaining substrate. In this latter period, cellulose hydrolysis then becomes rate limiting with the resulting 15-50 year degradation time scales. This view has been discussed in a recent experimental bench study of landfill gas production enhancement.3 In what follows we review the role of moisture content in landfill biodegradation and describe the microporous structure of lignocellulosic materials. The latter greatly affects moisture distribution and mass transfer in the landfill. Lastly some estimates of the time scales for biodegradation based on mass transfer control are calculated. This analysis follows obvious parallels with heterogeneous catalysis in chemical engineering systems. THE ROLE OF LANDFILL MOISTURE By way of review, the primary environmental variables affecting landfill stabilization rates are:4 1. Moisture content 2. pH 3. Nutrient content 43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 153
Object Description
Purdue Identification Number | ETRIWC198821 |
Title | Effects of mass transfer on landfill stabilization rates |
Author |
Noble, James J. Nunez-McNally, Thelma Tansel, Berrin |
Date of Original | 1988 |
Conference Title | Proceedings of the 43rd Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,39828 |
Extent of Original | p. 153-160 |
Collection Title | Engineering Technical Reports Collection, Purdue University |
Repository | Purdue University Libraries |
Rights Statement | Digital object copyright Purdue University. All rights reserved. |
Language | eng |
Type (DCMI) | text |
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Capture Device | Fujitsu fi-5650C |
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Description
Title | page 153 |
Collection Title | Engineering Technical Reports Collection, Purdue University |
Repository | Purdue University Libraries |
Rights Statement | Digital copyright Purdue University. All rights reserved. |
Language | eng |
Type (DCMI) | text |
Format | JP2 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Transcript | 21 THE EFFECTS OF MASS TRANSFER ON LANDFILL STABILIZATION RATES James J. Noble, Associate Professor of Chemical Engineering Thelma Nunez-McNally, Chemical Research Engineer Berrin Tansel, Environmental Research Engineer Center for Environmental Management Tufts University Medford, Massachusetts 02155 INTRODUCTION Rates of municipal solid waste (MSW) decomposition and attendant sanitary landfill stabilization are often seen to be quite slow. Indeed it is not uncommon to qualitatively gauge the extent of biodegradation in landfills by the legibility of buried newspapers. These may be recovered essentially intact after 20-30 years. Recent archaeological excavations of sanitary landfills by Rathje1 have also shown that when 10 year old rubbish is exhumed, it may appear to be mummified or "entombed". The long time scales for landfill stabilization stand in marked contrast to the fairly short time scales for decomposition demonstrated by the myriad of published papers on biomass conversion and the extracellular enzymatic hydrolysis of cellulosic materials in the laboratory. Three important reasons that landfills decompose so slowly are: 1) lack of sufficient moisture for optimal enzyme activity; 2) imbalance between the production of organic acids by acid-forming bacteria and the conversion of these acids into methane by methanogenic bacteria; and 3) mass transfer limitations in the landfill environment. The main purpose of this chapter is to speculate on the role of mass transfer limitations in landfills and to discuss possible consequences. It is argued that under certain circumstances mass transfer is the rate controlling step in landfill stabilization and that detailed knowledge of metabolic pathways for the microorganisms may not be always necessary for predictive purposes. This work is part of a larger study on landfill dynamics at Tufts University, Center for Environmental Management, which seeks to develop a first-round systems model for landfill behavior. At the outset it is important to clarify our view that the rate limiting step for biodegradation in a landfill may change as a function of landfill age. Thus one may argue that since MSW includes a small readily soluble fraction, the initial rate limiting step might be methane formation because of insufficient time to develop viable methanogen populations. To fix ideas better, the readily biodegradable material might comprise 5% of the MSW and convert completely to methane in 2 to 3 years. This behavior may well be that observed in lysimeter studies of several years duration.2 Our focus here, however, is the next phase of degradation when the more recalcitrant and insoluble lignocellulosic material would become the most biodegradable remaining substrate. In this latter period, cellulose hydrolysis then becomes rate limiting with the resulting 15-50 year degradation time scales. This view has been discussed in a recent experimental bench study of landfill gas production enhancement.3 In what follows we review the role of moisture content in landfill biodegradation and describe the microporous structure of lignocellulosic materials. The latter greatly affects moisture distribution and mass transfer in the landfill. Lastly some estimates of the time scales for biodegradation based on mass transfer control are calculated. This analysis follows obvious parallels with heterogeneous catalysis in chemical engineering systems. THE ROLE OF LANDFILL MOISTURE By way of review, the primary environmental variables affecting landfill stabilization rates are:4 1. Moisture content 2. pH 3. Nutrient content 43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 153 |
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Color Depth | 8 bit |
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