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.
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