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EFFECTS AND FATE OF SELECTED HEAVY METALS IN ANAEROBIC DIGESTION Thomas D. Hayes, Graduate Student Thomas L. Theis, Assistant Professor Department of Civil Engineering University of Notre Dame Notre Dame, Indiana 46556 INTRODUCTION In recent years a great deal of interest has been generated over the distribution and effects of influent wastewater heavy metals in conventional wastewater treatment. Of the various process components of conventional sewage treatment, anaerobic digestion seems to be particularly vulnerable to excessively high heavy metal loadings to the treatment plant [ 1 ]. The widespread use of anaerobic digestion in the treatment of sewage sludges has underscored the importance of understanding the various control problems stemming from excessively high heavy metal concentrations in treatment plant sludges fed to anaerobic digesters. This study was undertaken to observe the effects of heavy metals on bench-scale anaerobic digesters and to determine the distribution of these metals among various forms in the digester relevant to digester performance. PREVIOUS WORK Early investigations conducted in the 1920s and 1930s produced three important observations on the effects of cations on anaerobic processes: toxicity, stimulation and antagonism. In general, it was found that all cations could produce toxicity in any organism at some level but the relative toxicity of the cations varied when toxicities were compared on a weight-weight basis. Rudolphs and Zeller [2] investigated the effects of 1000 mg/1 of various sulfates on the anaerobic digestion of sewage sludge and found heavy metals more toxic than the light metal ions. Early investigations also showed that the toxicity of a cation varied in the presence of other cations in the medium. This ability of one cation to decrease the toxicity of another was termed "antagonism" [3]. Stimulatory effects of cations at concentrations below those where toxicity was produced were also recognized by these early investigators. Hotchkiss [4], for example, found that very low concentrations of extremely toxic cations, such as mercury and lead, could produce a stimulatory effect in anaerobic digestion. More recently, Barth, Ettenger, Stones, Salotto and McDermott [ 1 ] have summarized the results of research conducted over a ten-year period on the effects of continuous and pulse feeding of heavy metals, including zinc, chromium (VI), nickel and copper, on anaerobic digesters but only in limited terms of pH and gas production. Mass balances conducted by these researchers showed that a significant amount of heavy metals was removed from the bulk solution in anaerobic digestion, but no effort was made to investigate removal mechanisms or the ultimate chemical forms assumed by these metals. Significantly, it was found that the anaerobic digesters receiving wasted secondary sludges were far more sensitive to elevated heavy metal concentrations in the treatment plant influent than were the activated sludge systems. This was primarily due to the fact that the heavy metals were removed from solution in secondary treatment and were concentrated in the aerobic sludges. Barth et al. found that such removals (as high as 50 to 60%) resulted in high levels of heavy metals in digester feed sludges which were 6 to 37 times the metal concentrations of the treatment plant influent [ 1 ]. Other researchers have noted the same phenomenon with mercury, cadmium, lead, manganese and iron [5-9). Data collated from various sources in the literature indicate a general relationship between influent sewage heavy metal concentrations and metal concentrations found in the combined sludges of municipal treatment plants as shown in Figure 1 for chromium, copper, nickel and zinc [7, 10, 11]. 157
Object Description
Purdue Identification Number | ETRIWC197616 |
Title | Effects and fate of selected heavy metals in anaerobic digestion |
Author |
Hayes, Thomas D. Theis, Thomas L. |
Date of Original | 1976 |
Conference Title | Proceedings of the 31st Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,27048 |
Extent of Original | p. 157-173 |
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 | 2009-07-07 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 157 |
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 | EFFECTS AND FATE OF SELECTED HEAVY METALS IN ANAEROBIC DIGESTION Thomas D. Hayes, Graduate Student Thomas L. Theis, Assistant Professor Department of Civil Engineering University of Notre Dame Notre Dame, Indiana 46556 INTRODUCTION In recent years a great deal of interest has been generated over the distribution and effects of influent wastewater heavy metals in conventional wastewater treatment. Of the various process components of conventional sewage treatment, anaerobic digestion seems to be particularly vulnerable to excessively high heavy metal loadings to the treatment plant [ 1 ]. The widespread use of anaerobic digestion in the treatment of sewage sludges has underscored the importance of understanding the various control problems stemming from excessively high heavy metal concentrations in treatment plant sludges fed to anaerobic digesters. This study was undertaken to observe the effects of heavy metals on bench-scale anaerobic digesters and to determine the distribution of these metals among various forms in the digester relevant to digester performance. PREVIOUS WORK Early investigations conducted in the 1920s and 1930s produced three important observations on the effects of cations on anaerobic processes: toxicity, stimulation and antagonism. In general, it was found that all cations could produce toxicity in any organism at some level but the relative toxicity of the cations varied when toxicities were compared on a weight-weight basis. Rudolphs and Zeller [2] investigated the effects of 1000 mg/1 of various sulfates on the anaerobic digestion of sewage sludge and found heavy metals more toxic than the light metal ions. Early investigations also showed that the toxicity of a cation varied in the presence of other cations in the medium. This ability of one cation to decrease the toxicity of another was termed "antagonism" [3]. Stimulatory effects of cations at concentrations below those where toxicity was produced were also recognized by these early investigators. Hotchkiss [4], for example, found that very low concentrations of extremely toxic cations, such as mercury and lead, could produce a stimulatory effect in anaerobic digestion. More recently, Barth, Ettenger, Stones, Salotto and McDermott [ 1 ] have summarized the results of research conducted over a ten-year period on the effects of continuous and pulse feeding of heavy metals, including zinc, chromium (VI), nickel and copper, on anaerobic digesters but only in limited terms of pH and gas production. Mass balances conducted by these researchers showed that a significant amount of heavy metals was removed from the bulk solution in anaerobic digestion, but no effort was made to investigate removal mechanisms or the ultimate chemical forms assumed by these metals. Significantly, it was found that the anaerobic digesters receiving wasted secondary sludges were far more sensitive to elevated heavy metal concentrations in the treatment plant influent than were the activated sludge systems. This was primarily due to the fact that the heavy metals were removed from solution in secondary treatment and were concentrated in the aerobic sludges. Barth et al. found that such removals (as high as 50 to 60%) resulted in high levels of heavy metals in digester feed sludges which were 6 to 37 times the metal concentrations of the treatment plant influent [ 1 ]. Other researchers have noted the same phenomenon with mercury, cadmium, lead, manganese and iron [5-9). Data collated from various sources in the literature indicate a general relationship between influent sewage heavy metal concentrations and metal concentrations found in the combined sludges of municipal treatment plants as shown in Figure 1 for chromium, copper, nickel and zinc [7, 10, 11]. 157 |
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