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36 THE INFLUENCE OF WATER QUALITY ON BUBBLELESS MEMBRANE AERATION PERFORMANCE Vincent T. Vander Top, Graduate Research Assistant Michael J. Semmens, Associate Professor Department of Civil Engineering University of Minnesota Minneapolis, Minnesota 55455 INTRODUCTION The use of gas-permeable hollow-fiber membranes for water and wastewater aeration is a novel concept. In this process, pressurized, pure oxygen flows inside the fiber lumen, while water flows outside the membrane creating a high oxygen concentration gradient for the dissolution of oxygen across the interface without the formation of bubbles. Pressurized oxygen in the fiber produces a saturation concentration which is orders of magnitude higher than standard saturation conditions. Thus, the membrane aerator can oversaturate water with oxygen. High transfer rates are also aided by high surface areas. Small diameter fibers can provide large amounts of surface area per volume of water.1 Various configurations of this process have been studied.2"8 In early field tests on membrane aeration, it was found that the oxygen transfer was better than that observed in clean water tests.9 This effect may have been caused by the high organic content of the water in which the aerators were tested. Surfactants in small concentrations have been demonstrated to improve the performance of membrane aeration.7 This paper explores the effect of organic surfactants on the oxygen transfer performance of a membrane aerator. BACKGROUND Membrane Types Microporous membranes made from hydrophobic materials such as polypropylene (PP) and polyethylene (PE) are ideal for gas transfer into water. The pores in the membrane do not wet, but remain dry and gas-filled allowing for the rapid transport of oxygen through the membrane by gaseous diffusion. As a result, the membranes provide little resistance to gas transfer. Instead, the transfer is controlled by the liquid film resistance. Unfortunately, porous membranes are limited to use at low gas pressures as bubbles form at high pressures. Electron scanned micrographs (ESMs) indicate that the porosity (interfacial area per fiber area) of the PP membrane is 30% to 40% while the PE membrane is approximately 3% to 5%. Elevated gas pressures lead to the formation and release of bubbles from the micropores. The pore dimensions determine the gas pressure required to form bubbles with larger pores bubbling at lower pressures. The PP membrane pore size distribution is wider than that of the PE membrane. The manufacturer of the PP (Hoechst Celanese, Charlotte NC) reports pore dimensions of approximately 0.05 x 0.15 microns; however, ESMs suggest pore diameters as large as 1 micron. ESMs of the PE fiber reveal a more uniform pore population with a diameter of about 0.2 microns. A composite membrane (CM) provides a nonporous polyurethane layer (1 micron thick) between two porous polyethylene layers. Oxygen must dissolve into and diffuse through the nonporous layer which increases the effective resistance of the membrane. The cost of increased membrane resistance is offset by the ability to operate at higher gas pressures. ESMs reveal a CM porosity between that of PP and PE, and the pore diameters are relatively constant as in the PE membrane. Thus the CM has a high porosity and can be operated at high pressures. 47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 317
Object Description
Purdue Identification Number | ETRIWC199236 |
Title | Influence of water quality on bubbleless membrane aeration performance |
Author |
Vander Top, Vincent T. Semmens, M. J. |
Date of Original | 1992 |
Conference Title | Proceedings of the 47th Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,43678 |
Extent of Original | p. 317-326 |
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 | 2009-12-10 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 317 |
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 | 36 THE INFLUENCE OF WATER QUALITY ON BUBBLELESS MEMBRANE AERATION PERFORMANCE Vincent T. Vander Top, Graduate Research Assistant Michael J. Semmens, Associate Professor Department of Civil Engineering University of Minnesota Minneapolis, Minnesota 55455 INTRODUCTION The use of gas-permeable hollow-fiber membranes for water and wastewater aeration is a novel concept. In this process, pressurized, pure oxygen flows inside the fiber lumen, while water flows outside the membrane creating a high oxygen concentration gradient for the dissolution of oxygen across the interface without the formation of bubbles. Pressurized oxygen in the fiber produces a saturation concentration which is orders of magnitude higher than standard saturation conditions. Thus, the membrane aerator can oversaturate water with oxygen. High transfer rates are also aided by high surface areas. Small diameter fibers can provide large amounts of surface area per volume of water.1 Various configurations of this process have been studied.2"8 In early field tests on membrane aeration, it was found that the oxygen transfer was better than that observed in clean water tests.9 This effect may have been caused by the high organic content of the water in which the aerators were tested. Surfactants in small concentrations have been demonstrated to improve the performance of membrane aeration.7 This paper explores the effect of organic surfactants on the oxygen transfer performance of a membrane aerator. BACKGROUND Membrane Types Microporous membranes made from hydrophobic materials such as polypropylene (PP) and polyethylene (PE) are ideal for gas transfer into water. The pores in the membrane do not wet, but remain dry and gas-filled allowing for the rapid transport of oxygen through the membrane by gaseous diffusion. As a result, the membranes provide little resistance to gas transfer. Instead, the transfer is controlled by the liquid film resistance. Unfortunately, porous membranes are limited to use at low gas pressures as bubbles form at high pressures. Electron scanned micrographs (ESMs) indicate that the porosity (interfacial area per fiber area) of the PP membrane is 30% to 40% while the PE membrane is approximately 3% to 5%. Elevated gas pressures lead to the formation and release of bubbles from the micropores. The pore dimensions determine the gas pressure required to form bubbles with larger pores bubbling at lower pressures. The PP membrane pore size distribution is wider than that of the PE membrane. The manufacturer of the PP (Hoechst Celanese, Charlotte NC) reports pore dimensions of approximately 0.05 x 0.15 microns; however, ESMs suggest pore diameters as large as 1 micron. ESMs of the PE fiber reveal a more uniform pore population with a diameter of about 0.2 microns. A composite membrane (CM) provides a nonporous polyurethane layer (1 micron thick) between two porous polyethylene layers. Oxygen must dissolve into and diffuse through the nonporous layer which increases the effective resistance of the membrane. The cost of increased membrane resistance is offset by the ability to operate at higher gas pressures. ESMs reveal a CM porosity between that of PP and PE, and the pore diameters are relatively constant as in the PE membrane. Thus the CM has a high porosity and can be operated at high pressures. 47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 317 |
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