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The Use of Final Clarifier Models in Understanding and Anticipating Performance Under Operational Extremes S. L. JENNINGS, Associate Bucher and Willis Salina, Kansas C. P. L. GRADY, JR., Associate Professor School of Civil Engineering Purdue University West Lafayette, Indiana INTRODUCTION When an engineer embarks upon the design of an activated sludge treatment plant he must determine four key elements: the volume of the aeration basin, the concentration of mixed liquor suspended solids (MLSS) in that basin, the surface area of the final settler, and the rate of the return sludge flow. Usually the decisions that are made concerning these elements are influenced strongly by guidelines or accepted standards of practice (I). For example, the volume of the aeration basin may be chosen to give a mean hydraulic residence time of around six hours. The concentration of mixed liquor suspended solids may next be chosen by considering a process loading factor which reflects the amenability of the waste to biological treatment. Then the area of the final settler might be determined through the use of a surface overflow rate while the return sludge flow rate may be estimated by considering the anticipated sludge volume index. Although the design procedure outlined can lead to reasonable decisions it suffers from several potential weaknesses. First, the primary consideration in aeration basin design is the product of mean hydraulic residence time and MLSS concentration implying that considerable freedom is available to the designer in the choice of either of those elements (2). Second, activated sludge settlers can be limited by thickening capacity rather than by clarification ability, but overflow rates are based solely upon the latter (3). Third, the sludge volume index does not necessarily bear any relationship to the return sludge concentration and therefore could be misleading if used to size the return sludge system (4). Rather, the rate of return sludge influences markedly the performance of the final settlerand should be considered in the actual sizing of that operation (3). Last, the procedure ignores the interactions between the aeration chamber and the settler by assuming that the two can be designed independently. The last potential weakness is perhaps the most important because in seeking to account for aerator - settler interactions an engineer must consider all of the other points outlined. In addition he will be able to see how cost trade-offs are made and ultimately arrive at a design that achieves its objective at least cost—the desired goal of all design. Because a realization of operational interactions is dependent upon a knowledge of the performance of the individual operations, only recently has it been possible for the design engineer to attempt to minimize the cost of the aerator - settler combination. Descriptions of these performance interrelationships and examples of the effects that they have upon process costs have been presented by several authors in the form of graphs, but with relatively little discussion of the techniques employed to develop them (5,6,7). The intent of this paper is to consider those interrelationships in more detail, showing how they govern the cost of the initial facility and how they must be anticipated if proper process operation is to be achieved. This will be accomplished through a review of current research and the synthesis of that research into a general computational procedure. Finally a method will be presented whereby the design engineer can prepare operational charts which will aid plant operators in anticipating settler performance under operational extremes. 221
Object Description
Purdue Identification Number | ETRIWC197219 |
Title | Use of final clarifier models in understanding and anticipating performance under operational extremes |
Author |
Jennings, S. L. Grady, C. P. Leslie, 1938- |
Date of Original | 1972 |
Conference Title | Proceedings of the 27th Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://earchives.lib.purdue.edu/u?/engext,20246 |
Extent of Original | p. 221-241 |
Series | Engineering extension series no. 141 |
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 | 2009-06-08 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page0221 |
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 | The Use of Final Clarifier Models in Understanding and Anticipating Performance Under Operational Extremes S. L. JENNINGS, Associate Bucher and Willis Salina, Kansas C. P. L. GRADY, JR., Associate Professor School of Civil Engineering Purdue University West Lafayette, Indiana INTRODUCTION When an engineer embarks upon the design of an activated sludge treatment plant he must determine four key elements: the volume of the aeration basin, the concentration of mixed liquor suspended solids (MLSS) in that basin, the surface area of the final settler, and the rate of the return sludge flow. Usually the decisions that are made concerning these elements are influenced strongly by guidelines or accepted standards of practice (I). For example, the volume of the aeration basin may be chosen to give a mean hydraulic residence time of around six hours. The concentration of mixed liquor suspended solids may next be chosen by considering a process loading factor which reflects the amenability of the waste to biological treatment. Then the area of the final settler might be determined through the use of a surface overflow rate while the return sludge flow rate may be estimated by considering the anticipated sludge volume index. Although the design procedure outlined can lead to reasonable decisions it suffers from several potential weaknesses. First, the primary consideration in aeration basin design is the product of mean hydraulic residence time and MLSS concentration implying that considerable freedom is available to the designer in the choice of either of those elements (2). Second, activated sludge settlers can be limited by thickening capacity rather than by clarification ability, but overflow rates are based solely upon the latter (3). Third, the sludge volume index does not necessarily bear any relationship to the return sludge concentration and therefore could be misleading if used to size the return sludge system (4). Rather, the rate of return sludge influences markedly the performance of the final settlerand should be considered in the actual sizing of that operation (3). Last, the procedure ignores the interactions between the aeration chamber and the settler by assuming that the two can be designed independently. The last potential weakness is perhaps the most important because in seeking to account for aerator - settler interactions an engineer must consider all of the other points outlined. In addition he will be able to see how cost trade-offs are made and ultimately arrive at a design that achieves its objective at least cost—the desired goal of all design. Because a realization of operational interactions is dependent upon a knowledge of the performance of the individual operations, only recently has it been possible for the design engineer to attempt to minimize the cost of the aerator - settler combination. Descriptions of these performance interrelationships and examples of the effects that they have upon process costs have been presented by several authors in the form of graphs, but with relatively little discussion of the techniques employed to develop them (5,6,7). The intent of this paper is to consider those interrelationships in more detail, showing how they govern the cost of the initial facility and how they must be anticipated if proper process operation is to be achieved. This will be accomplished through a review of current research and the synthesis of that research into a general computational procedure. Finally a method will be presented whereby the design engineer can prepare operational charts which will aid plant operators in anticipating settler performance under operational extremes. 221 |
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