Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1999-08-30
2001-02-27
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
active
06193890
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to economical microfiltration of solids-contaminated wastewater with a skein of hollow fiber membranes (“fibers” for brevity) by maintaining the outer surfaces of the fibers clean. If fibers are not continuously scrubbed with streams of relatively fast-moving “coarse” bubbles, they are quickly fouled. Coarse bubbles range in effective diameter from about 3 mm to about 35 mm, most preferably from 15 mm to 25 mm, measured within a distance of from 1 cm to 50 cm from the through-passages generating them; bubbles smaller than 3 mm in diameter are referred to as “fine ” bubbles; fine bubbles, which rise more slowly than coarse, are efficient to transfer oxygen but inefficient to provide effective scrubbing. Any fine bubbles present are incidental and unintentional. A “skein” refers to plural arrays of fibers held together in at least one stack of arrays. The arrays may be linear, circular, or randomly oriented within a stack. Scrubbing fiber surfaces, and not aeration, is the primary purpose of coarse bubbles. Scrubbing relies upon a substantially uniform upward flow of the bubbles of inert gas generated in a “scrubbing aerator” which remains submerged and continuously operational in a substrate while the skein is withdrawing permeate. Fouling of the aerator impairs uniformity of such flow, and such non-uniformity results in fouled fibers and inefficient microfiltration. By “inert” is meant “chemically unreactive with either the deposits to be removed, or the liquid from which the deposits were derived”. The terms “aerator” or “aerator means” (sometimes referred to as a “diffuser”) refer to a device designed to supply inert gas as predominantly coarse bubbles under specified operating conditions; the terms are used for convenience because the inert gas used for scrubbing fibers is typically air; and because the air is expelled through each of a multiplicity of suitably large orifices in the aerator it is referred to as a “coarse bubble aerator”. Typically the substrate is contaminated water, or domestic or industrial waste (hereafter “wastewater” for brevity) containing organic solids including microorganisms. Other typical substrates are organic liquids in which the suspended solids are substantially insoluble. The streams of air, issuing from a multiplicity of orifices in an aerator, together form a column of coarse bubbles having sufficient energy to scrub the outer surfaces of the fibers.
THE PROBLEM
Semipermeable membranes having a high flux are not economical filtration means if their surfaces are quickly fouled during operation. Solids, particularly of organic matter, have a high affinity for the surface of such membranes. To date, the best way to minimize fouling of a skein is to scrub the fibers continuously with streams of coarse bubbles rising at a specified rate. It is essential that aeration of a skein be substantially uniform and that the majority of the bubbles, preferably substantially all of them, remain coarse throughout their travel toward the surface of the substrate. By “substantially uniform” is meant that each unit volume within a rising column of bubbles, contains on average, approximately the same number of coarse bubbles as an adjacent unit volume within the column. By “majority of the bubbles” is meant that more than 50% by volume of the scrubbing gas is in the form of coarse bubbles. Such coarse bubble aerators are used to produce streams of fast-rising coarse bubbles, in columns or curtains, within and around arrays of fibers forming the skein. But the aerator itself is quickly fouled by deposits which build up on the inner surfaces of walls of the aerator, plugging orifices in those walls; even partial plugging produces a hindered flow of the stream of the bubbles, which hindered flow is also non-uniform, and results in fouling of the fibers. Fouling of a scrubbing aerator occurs far more easily than in a conventional aerator for municipal wastewater treatment (“municipal aerator”) because the total suspended solids concentration in a membrane bioreactor system is typically twice as high as that in a conventional wastewater treatment plant aeration basin. It was believed that not only was the uniform flow of bubbles required, but also that essentially all the orifices be kept open continuously during operation of the skein. Keeping aerators clean by the method disclosed herein is only feasible where returning deposited solids (foulant) into the substrate is acceptable. Though the method disclosed herein may be used to maintain clean orifices in any aerator, there is no overriding reason for using it where uniformity of the flow of coarse bubbles is not of critical importance and the aerator is designed specifically for the purpose at hand.
It was only recently recognized that stopping scrubbing of the skein with bubbles for even an hour, causes unacceptable fouling of the skein; therefore it was believed that an essentially continuous uniform flow of a column of bubbles not be substantially diminished, even for an hour during operation of the skein, because of the proclivity of the aerator's body to become internally fouled with solid deposits from the liquid substrate. Prior to the teaching of Ser. No. 09/024,417, due to the criticality of maintaining an essentially continuous uniform flow of scrubbing air while a skein was operating, aerators for skeins were best kept clean by removing them from the substrate, and thoroughly cleaning them. Then, typically, a high velocity jet of water is used, both internally and externally, optionally with brushing the surfaces. Alternatively, the fouled aerator is immediately replaced with a pre-cleaned one. Prior art methods of injecting high pressure air (referred to as “flushing air”) or water, sporadically, to flush deposited solids from the internal surfaces of an aerator, have been successful where maintaining aeration in municipal aerators was the goal, but there is nothing to suggest that such aerators generated bubbles which would provide a scrubbing function for a filtration device immersed in the substrate.
BACKGROUND OF THE INVENTION
Filtering a substrate through a skein of fibers has long been known in the art. Irrespective of the configuration of the skein, whether arcuate or vertical; and if vertical, whether in a rectangular or cyclindrical configuration, to date, the emphasis has been on maintaining the outer surfaces of the fibers clean by backwashing them (through their lumens) frequently, either with permeate, or clean water when the substrate is aqueous. Because backwashing with substrate would foul the lumens of the fibers, use of substrate as flushing liquid was never considered. Ser. No. 09/024,417 taught that the most effective flushing fluid was a mixture of gas (typically air) and a flushing liquid such as permeate, clean water or substrate, preferably fresh, clean substrate. Except for a brief period before flushing was commenced, flushing liquid was introduced into the aerator and soon thereafter the two-phase mixture was introduced so that it provided scrubbing gas to the skein essentially continuously.
Since it was believed that the continuous flow of scrubbing bubbles was required to be maintained, interrupting the flow of scrubbing bubbles for a short period as little as 5 min, was not considered a viable option. This belief proved untenable as it was subsequently found that interrupting aerating gas for as much as 10 min, and flushing the aerator with flushing liquid alone during the interruption, preferably for less than 5 min, effectively cleaned the aerator provided it was designed for the purpose.
An orifice through which air exits the lateral walls of an aerator, becomes clogged because solids penetrate the interior of the aerator and are deposited on its inner walls forming a sludge or slime which gradually accumulates on the periphery of the orifice. As air continuously passes through the orifice, the sludge is dried, providing a base for the accumulation of yet more sludge which, in turn, is dried, until the orifice is sub
Behmann Henry
Pedersen Steven Kristian
Fortuna Ana
Lobo Alfred D.
Zenon Environmental Inc.
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