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A Dynamic Model of the Anaerobic Digestion Process JOHN F. ANDREWS, Professor Environmental Systems Engineering Clemson University Clemson, South Carolina INTRODUCTION Most Mathematical models currently in use for the description of biological processes are steady state models and therefore cannot be used to predict process performance during start-up operations or under transient conditions resulting from changes in process loading. Dynamic models can be used to make these predictions and the importance of such models is realized when one considers, for example, the problems of starting up anaerobic digesters, the frequency of digester failure, and the effects of bulking on the activated sludge process. A failing digester or bulking activated sludge unit is definitely not at steady state ! Dynamic models are also useful in obtaining a quantitative measure of process stability which can be of considerable importance in selecting a process for a specific application. Most sanitary engineers will accept the statement that the trickling filter process is more stable than the activated sludge process but would be hard pressed to express this quantitatively. Although the anaerobic digestion process has many advantages such as a low production of waste sludge, low power requirements for operation, and production of a useful product, methane, it has a poor record with respect to process stability as evidenced by the many reports of "sour" or failing digesters. This has led to the increased use of more expensive processes such as aerobic digestion and wet combustion. Many studies have been made to determine the causes of digester failure and techniques for the prevention of failure. Prominent among the many causes of failure mentioned in the literature are the effects of high volatile acid concentration and low pH on the methane bacteria. Although most workers agree that a pH less than 6. 5 is detrimental to the process there have been considerable differences of opinion concerning the effect of high concentrations of volatile acids. One group, as exemplified by the work of McCarty (1), feels that volatile acids are inhibitory to the methane bacteria only in an indirect way through a reduction in pH and that this inhibition can be relieved by maintaining the pH near neutrality. Another group, as exemplified by the work of Buswell (2), maintains that volatile acid concentrations above 2, 000 to 3, 000 mg/1 are inhibitory ir- regardless of pH and that this inhibition can be relieved by reducing the organic load or diluting the reactor contents. The model that will be presented here resolves this conflict by considering the unionized fraction of the volatile acids as the inhibiting agent. Since the concentration of unionized acid is a function of pH and total acid concentration both are therefore of importance. The model also shows that inhibition can be relieved by maintaining the pH near neutrality or reducing the organic loading, or diluting the reactor contents. Most current steady state models of the anaerobic digestion process are based on the relationship between limiting substrate concentration and growth rate as proposed by Monod (3). James (4) nas pointed out that in biological processes a wide variety of substances may act as limiting substrates. Stewart (51 and Agardy, - 285 -
Object Description
Purdue Identification Number | ETRIWC196825 |
Title | Dynamic model of the anaerobic digestion process |
Author | Andrews, John F. |
Date of Original | 1968 |
Conference Title | Proceedings of the 23rd Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://earchives.lib.purdue.edu/u?/engext,15314 |
Extent of Original | p. 285-310 |
Series |
Engineering extension series no. 132 Engineering bulletin v. 53, no. 2 |
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 |
Format | JP2 |
Date Digitized | 2009-05-20 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 285 |
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 |
Format | JP2 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Transcript | A Dynamic Model of the Anaerobic Digestion Process JOHN F. ANDREWS, Professor Environmental Systems Engineering Clemson University Clemson, South Carolina INTRODUCTION Most Mathematical models currently in use for the description of biological processes are steady state models and therefore cannot be used to predict process performance during start-up operations or under transient conditions resulting from changes in process loading. Dynamic models can be used to make these predictions and the importance of such models is realized when one considers, for example, the problems of starting up anaerobic digesters, the frequency of digester failure, and the effects of bulking on the activated sludge process. A failing digester or bulking activated sludge unit is definitely not at steady state ! Dynamic models are also useful in obtaining a quantitative measure of process stability which can be of considerable importance in selecting a process for a specific application. Most sanitary engineers will accept the statement that the trickling filter process is more stable than the activated sludge process but would be hard pressed to express this quantitatively. Although the anaerobic digestion process has many advantages such as a low production of waste sludge, low power requirements for operation, and production of a useful product, methane, it has a poor record with respect to process stability as evidenced by the many reports of "sour" or failing digesters. This has led to the increased use of more expensive processes such as aerobic digestion and wet combustion. Many studies have been made to determine the causes of digester failure and techniques for the prevention of failure. Prominent among the many causes of failure mentioned in the literature are the effects of high volatile acid concentration and low pH on the methane bacteria. Although most workers agree that a pH less than 6. 5 is detrimental to the process there have been considerable differences of opinion concerning the effect of high concentrations of volatile acids. One group, as exemplified by the work of McCarty (1), feels that volatile acids are inhibitory to the methane bacteria only in an indirect way through a reduction in pH and that this inhibition can be relieved by maintaining the pH near neutrality. Another group, as exemplified by the work of Buswell (2), maintains that volatile acid concentrations above 2, 000 to 3, 000 mg/1 are inhibitory ir- regardless of pH and that this inhibition can be relieved by reducing the organic load or diluting the reactor contents. The model that will be presented here resolves this conflict by considering the unionized fraction of the volatile acids as the inhibiting agent. Since the concentration of unionized acid is a function of pH and total acid concentration both are therefore of importance. The model also shows that inhibition can be relieved by maintaining the pH near neutrality or reducing the organic loading, or diluting the reactor contents. Most current steady state models of the anaerobic digestion process are based on the relationship between limiting substrate concentration and growth rate as proposed by Monod (3). James (4) nas pointed out that in biological processes a wide variety of substances may act as limiting substrates. Stewart (51 and Agardy, - 285 - |
Resolution | 300 ppi |
Color Depth | 8 bit |
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