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20 SIMULATION STUDIES OF THE TRANSIENT RESPONSE OF ACTIVATED SLUDGE SYSTEMS TO BIODEGRADABLE INHIBITORY SHOCK LOADS Ivonne Santiago, Process Engineer RMT, Inc. Greenville, South Carolina 29607 C. P. Leslie Grady, Jr., Professor Environmental Systems Engineering Clemson University Clemson, South Carolina 29634 INTRODUCTION During the last ten years there has been increasing interest in the fate and impact of inhibitory organic compounds in the activated sludge process. Although many such compounds are biodegradable at low concentrations, they may be inhibitory to their own biodegradation or to the biodegradation of normal, biogenic, organic matter when their concentration is high. Nevertheless, as long as their input to the activated sludge process can be held constant, their biodegradation will usually proceed without problems, maintaining their concentration low, and allowing uninhibited process performance. Problems arise, however, during the nonconstant input of inhibitory compounds because large shock loads can result in transient increases in their concentration that are severe enough to disrupt treatment efficiency. Thus, an important question facing the design engineer is how to minimize the impact of shock loads. The most effective way to minimize shock load impact is to use equalization. However, because total equalization is not economically feasible, consideration must also be given to bioreactor design, and it is generally accepted that completely mixed activated sludge systems are more resistant to upset by shock loads than other configurations because they offer more dampening. Unfortunately, completely mixed basins tend to produce bulking sludges.1 Plug flow reactors, on the other hand, are much less prone to this problem but are thought to offer less protection against shock loads. Thus, an important question is whether completely mixed basins really offer more protection against inhibitory shock loads. If they do not, then it may be possible to use plug flow reactors, thereby improving sludge settling without sacrificing protection against shock loads. The purpose of this study was to compare the performance of completely mixed and plug flow activated sludge systems when subjected to shock loads of inhibitory organic matter. Because of the large number of possible cases that could exist and because little guidance was available in the literature, simulation was used to conduct the study. Although the models used were very simple, it was felt that the results would serve as a stimulus for experimental study. Such was the case when a similar modeling study found that the response of a completely mixed activated sludge system to a quantitative shock load of a biogenic substrate was independent of the hydraulic retention time (HRT) and determined solely by the solids retention time (SRT).2 That finding was subsequently verified experimentally 3 MODELS USED The completely mixed activated sludge process was modeled as a single continuous stirred tank reactor (CSTR) with cell recycle. The plug flow reactor (PFR) system was modeled as six equal- volume CSTRs in series, with all cell recycle and all feed entering the first tank because that configuration represents well the hydraulic regime found in most full-scale "plug flow" systems.4 Both systems were configured as Garrett5 flow schemes in which sludge was wasted directly from the aeration basin. In the case of the PFR, sludge was wasted equally from each reactor. Both configurations received the same feed at a constant flow rate, both had the same system volume, and both had a recycle ratio of 0.5. Finally, the settlers in both systems were considered to be perfect separation points, with all 44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 191
Object Description
Purdue Identification Number | ETRIWC198920 |
Title | Simulation studies of the transient response of activated sludge systems to biodegradable inhibitory shock loads |
Author |
Santiago, Ivonne Grady, C. P. Leslie, 1938- |
Date of Original | 1989 |
Conference Title | Proceedings of the 44th Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,40757 |
Extent of Original | p. 191-198 |
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 | 2009-08-18 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 191 |
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 | 20 SIMULATION STUDIES OF THE TRANSIENT RESPONSE OF ACTIVATED SLUDGE SYSTEMS TO BIODEGRADABLE INHIBITORY SHOCK LOADS Ivonne Santiago, Process Engineer RMT, Inc. Greenville, South Carolina 29607 C. P. Leslie Grady, Jr., Professor Environmental Systems Engineering Clemson University Clemson, South Carolina 29634 INTRODUCTION During the last ten years there has been increasing interest in the fate and impact of inhibitory organic compounds in the activated sludge process. Although many such compounds are biodegradable at low concentrations, they may be inhibitory to their own biodegradation or to the biodegradation of normal, biogenic, organic matter when their concentration is high. Nevertheless, as long as their input to the activated sludge process can be held constant, their biodegradation will usually proceed without problems, maintaining their concentration low, and allowing uninhibited process performance. Problems arise, however, during the nonconstant input of inhibitory compounds because large shock loads can result in transient increases in their concentration that are severe enough to disrupt treatment efficiency. Thus, an important question facing the design engineer is how to minimize the impact of shock loads. The most effective way to minimize shock load impact is to use equalization. However, because total equalization is not economically feasible, consideration must also be given to bioreactor design, and it is generally accepted that completely mixed activated sludge systems are more resistant to upset by shock loads than other configurations because they offer more dampening. Unfortunately, completely mixed basins tend to produce bulking sludges.1 Plug flow reactors, on the other hand, are much less prone to this problem but are thought to offer less protection against shock loads. Thus, an important question is whether completely mixed basins really offer more protection against inhibitory shock loads. If they do not, then it may be possible to use plug flow reactors, thereby improving sludge settling without sacrificing protection against shock loads. The purpose of this study was to compare the performance of completely mixed and plug flow activated sludge systems when subjected to shock loads of inhibitory organic matter. Because of the large number of possible cases that could exist and because little guidance was available in the literature, simulation was used to conduct the study. Although the models used were very simple, it was felt that the results would serve as a stimulus for experimental study. Such was the case when a similar modeling study found that the response of a completely mixed activated sludge system to a quantitative shock load of a biogenic substrate was independent of the hydraulic retention time (HRT) and determined solely by the solids retention time (SRT).2 That finding was subsequently verified experimentally 3 MODELS USED The completely mixed activated sludge process was modeled as a single continuous stirred tank reactor (CSTR) with cell recycle. The plug flow reactor (PFR) system was modeled as six equal- volume CSTRs in series, with all cell recycle and all feed entering the first tank because that configuration represents well the hydraulic regime found in most full-scale "plug flow" systems.4 Both systems were configured as Garrett5 flow schemes in which sludge was wasted directly from the aeration basin. In the case of the PFR, sludge was wasted equally from each reactor. Both configurations received the same feed at a constant flow rate, both had the same system volume, and both had a recycle ratio of 0.5. Finally, the settlers in both systems were considered to be perfect separation points, with all 44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 191 |
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Color Depth | 8 bit |
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