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REMOVAL OF CADMIUM AT HIGH AND LOW DOSAGES BY AN EXTENDED AERATION PROCESS William L. Lowe, Research Fellow Anthony F. Gaudy, Jr., Professor Department of Civil Engineering University of Delaware Newark, Delaware 19716 INTRODUCTION The search for ways to minimize the production of excess sludge and safely dispose of that which is produced continues to be of great interest in the pollution control field. Although fraught with many problems, disposal on land is attractive in many instances and offers a means to approach the ideal situation, i.e., direct recycling of organic matter for fertilizer value. However, the biomass generated in the treatment process has been shown to possess the capability of accumulating conservative toxic pollutants such as heavy metals and, for obvious reasons, this property tends to negate the practice of land disposal. The heavy metals may be routed to the animal food chain or, under acidic conditions, they can be solubilized and become active contaminants of surface and groundwater [1]. In fact, municipal sludge has been shown to release a significant portion of its heavy metal loading upon acidification [2]. The fact that biomass can remove and concentrate heavy metals from solution and/or colloidal suspension can be helpful because the sludge is, in effect, a gratuitously- manufactured sorbent for ridding the waste, at least partially, of its heavy metal load. Thus, there are possibilities of developing biological processes for removal of heavy metals from wastes and such processes have been suggested by several workers [3,4,5]. An extended aeration process, with modifications based on our laboratory studies, seems particularly effective for heavy metal removal. For over a decade, a portion of the research effort in our laboratories has been concerned with research germane to minimization of excess sludge production via concurrent wastewater purification and autodigestion of biomass, i.e., the extended aeration process [6,7,8,9]. One of the process modifications arising from this work was the periodic acid hydrolysis of small portions of the sludge to assist the autodigestion process and to control suspended solids levels in the system. Also, recently, we have found that modification of the flow scheme of an extended aeration plant, so that the return sludge is taken from a separate aerobic digester rather than directly from clarifier underflow, permits closer control and more stable operation even without the benefits of periodic acid hydrolysis [10], In Figure 1, this flow scheme is compared to the more common modes (A and B) of operation for extended aeration plants. It is apparent that the suggested modification permits one to operate an extended aeration plant with low overall specific growth rate (high mean cell retention time, 9C) but also allows the option of using a faster growth rate (lower sludge age) in the purification reactor. A bench scale pilot plant study using this flow scheme has shown it to have promise for very stable operation with no sludge wasting and with some savings in volume of reaction fluid under aeration [10]. Also, preliminary cost estimates show a decided savings in initial and O/M costs over conventional plants [11]. In addition, we have recently reported results of studies in which the hydrolytically-assisted extended aeration process was employed for removal and recovery of heavy metals concurrently with wastewater purification [12]. Button and Gaudy employed an internal recycle system, like that shown in the top portion of Figure 1, and dosed it with increasing amounts of cadmium (0.05 mg/1 -» 1.0 mg/1) added as Cd(NO,)2. It was found that the system could operate for a significant period of time without leakage of the dosed cadmium and that the accumulation of cadmium-laden biomass had little adverse effect on wastewater purification. It was also shown that acid hydrolysis of a portion of the cadmium-laden sludge solubilized both the cadmium and the biomass. Addition of sodium sulfide caused precipitation of the cadmium but not the solubilized 431
Object Description
Purdue Identification Number | ETRIWC198543 |
Title | Removal of cadmium at high and low dosages by an extended aeration process |
Author |
Lowe, William L. Gaudy, Anthony F. |
Date of Original | 1985 |
Conference Title | Proceedings of the 40th Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,36131 |
Extent of Original | p. 431-442 |
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 | 2009-07-15 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 431 |
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 | REMOVAL OF CADMIUM AT HIGH AND LOW DOSAGES BY AN EXTENDED AERATION PROCESS William L. Lowe, Research Fellow Anthony F. Gaudy, Jr., Professor Department of Civil Engineering University of Delaware Newark, Delaware 19716 INTRODUCTION The search for ways to minimize the production of excess sludge and safely dispose of that which is produced continues to be of great interest in the pollution control field. Although fraught with many problems, disposal on land is attractive in many instances and offers a means to approach the ideal situation, i.e., direct recycling of organic matter for fertilizer value. However, the biomass generated in the treatment process has been shown to possess the capability of accumulating conservative toxic pollutants such as heavy metals and, for obvious reasons, this property tends to negate the practice of land disposal. The heavy metals may be routed to the animal food chain or, under acidic conditions, they can be solubilized and become active contaminants of surface and groundwater [1]. In fact, municipal sludge has been shown to release a significant portion of its heavy metal loading upon acidification [2]. The fact that biomass can remove and concentrate heavy metals from solution and/or colloidal suspension can be helpful because the sludge is, in effect, a gratuitously- manufactured sorbent for ridding the waste, at least partially, of its heavy metal load. Thus, there are possibilities of developing biological processes for removal of heavy metals from wastes and such processes have been suggested by several workers [3,4,5]. An extended aeration process, with modifications based on our laboratory studies, seems particularly effective for heavy metal removal. For over a decade, a portion of the research effort in our laboratories has been concerned with research germane to minimization of excess sludge production via concurrent wastewater purification and autodigestion of biomass, i.e., the extended aeration process [6,7,8,9]. One of the process modifications arising from this work was the periodic acid hydrolysis of small portions of the sludge to assist the autodigestion process and to control suspended solids levels in the system. Also, recently, we have found that modification of the flow scheme of an extended aeration plant, so that the return sludge is taken from a separate aerobic digester rather than directly from clarifier underflow, permits closer control and more stable operation even without the benefits of periodic acid hydrolysis [10], In Figure 1, this flow scheme is compared to the more common modes (A and B) of operation for extended aeration plants. It is apparent that the suggested modification permits one to operate an extended aeration plant with low overall specific growth rate (high mean cell retention time, 9C) but also allows the option of using a faster growth rate (lower sludge age) in the purification reactor. A bench scale pilot plant study using this flow scheme has shown it to have promise for very stable operation with no sludge wasting and with some savings in volume of reaction fluid under aeration [10]. Also, preliminary cost estimates show a decided savings in initial and O/M costs over conventional plants [11]. In addition, we have recently reported results of studies in which the hydrolytically-assisted extended aeration process was employed for removal and recovery of heavy metals concurrently with wastewater purification [12]. Button and Gaudy employed an internal recycle system, like that shown in the top portion of Figure 1, and dosed it with increasing amounts of cadmium (0.05 mg/1 -» 1.0 mg/1) added as Cd(NO,)2. It was found that the system could operate for a significant period of time without leakage of the dosed cadmium and that the accumulation of cadmium-laden biomass had little adverse effect on wastewater purification. It was also shown that acid hydrolysis of a portion of the cadmium-laden sludge solubilized both the cadmium and the biomass. Addition of sodium sulfide caused precipitation of the cadmium but not the solubilized 431 |
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