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Activated Sludge Thickening by Dissolved Air Flotation G. A. ETTELT, Chemical Engineer The Metropolitan Sanitary District of Greater Chicago Chicago, Illinois INTRODUCTION Sludge thickening became a major and urgent research problem at the Chicago Southwest plant when the Zimmermann Wet Oxidation (Zimpro) Process was first considered for the ultimate disposal of activated sludge. In order to maintain this combustion process on an energy basis, the sludge must contain a minimum COD of 30 gms per liter, whereas a COD of 60 gms per liter would enable the process to be self-sustaining. This corresponds to the approximate activated sludge solids concentration of three and five per cent, respectively. Increases in the sludge handling capability of the combustion system would also be provided by the attainment of more thickened sludge. In addition, the performance of the digestor units, presently under construction, would benefit from the greater feed solids. Thickening of activated sludge by settling is not adequate since concentrations of only 1.2 to 2.5 per cent solids are obtained. If we utilize 50 per cent by weight of preliminary sludge, five per cent solids are possible. However, the activated sludge handling capacity is then decreased notably. Preliminary sludge disposal is not of primary concern in our present operation. Moreover, the supply of preliminary solids would be limited accordingly. Obviously, another means of solids thickening is needed and attention was directed to the flotation technique. Flotation process has long been employed in industry, especially in mining and refineries for two-phase separation. Generally, the process is applied to systems where there is a low concentration of insoluble or immiscible particles suspended in a bulk liquid. Either the suspended particles are quite small (nearly colloidal) or they have a density comparable to that of the bulk liquid. Air bubbles are introduced into the system to combine with the particles resulting in an aggregate with a density sufficiently less than the bulk density to effect flotation. MECHANICAL FLOTATION The mechanical flotation process, also described as dispersed air flotation, utilizes air bubbles formed either by agitation, e.g., cavitational action of impellers, or by passage of compressed air through screens or diffusers. Bubbles usually are relatively large compared to the solids (1), producing heavy foaming at the surface. From experiments conducted with the Denver Laboratory mechanical flotation cell (2), maximum solids concentration of only 2.8 per cent was obtained. Recovery of the total solids in the feed as floated solids ranged from 40 to 80 per cent. However, these results were achieved with the use of collector and frother agents and at high air input. The gain in solids thickening by this process over settling was not significant. DISSOLVED AIR FLOTATION Interest in dissolved air flotation process followed from literature reports on laboratory experiments with sewage sludges showing greater solids thickening by - 210
Object Description
Purdue Identification Number | ETRIWC196421 |
Title | Activated sludge thickening by dissolved air flotation |
Author | Ettelt, G. A. (Gregory A.) |
Date of Original | 1964 |
Conference Title | Proceedings of the nineteenth Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://earchives.lib.purdue.edu/u?/engext,11114 |
Extent of Original | p. 210-244 |
Series |
Engineering extension series no. 117 Engineering bulletin v. 49, no. 1(a)-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 | 2009-05-19 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 210 |
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 | Activated Sludge Thickening by Dissolved Air Flotation G. A. ETTELT, Chemical Engineer The Metropolitan Sanitary District of Greater Chicago Chicago, Illinois INTRODUCTION Sludge thickening became a major and urgent research problem at the Chicago Southwest plant when the Zimmermann Wet Oxidation (Zimpro) Process was first considered for the ultimate disposal of activated sludge. In order to maintain this combustion process on an energy basis, the sludge must contain a minimum COD of 30 gms per liter, whereas a COD of 60 gms per liter would enable the process to be self-sustaining. This corresponds to the approximate activated sludge solids concentration of three and five per cent, respectively. Increases in the sludge handling capability of the combustion system would also be provided by the attainment of more thickened sludge. In addition, the performance of the digestor units, presently under construction, would benefit from the greater feed solids. Thickening of activated sludge by settling is not adequate since concentrations of only 1.2 to 2.5 per cent solids are obtained. If we utilize 50 per cent by weight of preliminary sludge, five per cent solids are possible. However, the activated sludge handling capacity is then decreased notably. Preliminary sludge disposal is not of primary concern in our present operation. Moreover, the supply of preliminary solids would be limited accordingly. Obviously, another means of solids thickening is needed and attention was directed to the flotation technique. Flotation process has long been employed in industry, especially in mining and refineries for two-phase separation. Generally, the process is applied to systems where there is a low concentration of insoluble or immiscible particles suspended in a bulk liquid. Either the suspended particles are quite small (nearly colloidal) or they have a density comparable to that of the bulk liquid. Air bubbles are introduced into the system to combine with the particles resulting in an aggregate with a density sufficiently less than the bulk density to effect flotation. MECHANICAL FLOTATION The mechanical flotation process, also described as dispersed air flotation, utilizes air bubbles formed either by agitation, e.g., cavitational action of impellers, or by passage of compressed air through screens or diffusers. Bubbles usually are relatively large compared to the solids (1), producing heavy foaming at the surface. From experiments conducted with the Denver Laboratory mechanical flotation cell (2), maximum solids concentration of only 2.8 per cent was obtained. Recovery of the total solids in the feed as floated solids ranged from 40 to 80 per cent. However, these results were achieved with the use of collector and frother agents and at high air input. The gain in solids thickening by this process over settling was not significant. DISSOLVED AIR FLOTATION Interest in dissolved air flotation process followed from literature reports on laboratory experiments with sewage sludges showing greater solids thickening by - 210 |
Resolution | 300 ppi |
Color Depth | 8 bit |
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