In situ cleaning system for fouled membranes

• US 5,403,479 A
• Filed: 12/20/1993
• Issued: 04/04/1995
• Est. Priority Date: 12/20/1993
• Status: Expired due to Fees

• Current Assignee: Zenon Environmental Incorporated
• Original Assignee: Zenon Environmental Incorporated
• Inventors: Smith, Bradley M., Goodboy, Kenneth P., Deutschmann, Ake A.
• Primary Examiner(s): Dawson, Robert A.
• Assistant Examiner(s): Fortuna, Ana M.
• Application Number:
  US08/170,053
• Time in Patent Office: 470 Days
• Field of Search: 210/636, 210/108, 210/195.2, 210/257.2, 210/791, 210/321.69, 210/333.9, 210/106, 210/321.8, 210/321.89, 210/321.87, 210/321.88, 210/500.23
• US Class Current: 210/321.69
• CPC Class Codes: B01D 2321/168 Use of other chemical agents
• CPC Class Codes: B01D 65/02 Membrane cleaning or sterilisation ; Membrane regeneration

Abstract

A method and cleaning system is disclosed for cleaning the outer surface of a fouled microfiltration (MF) or ultrafiltration (UF) semipermeable hollow fiber membrane after its initial stable transmembrane flux has been decreased to an unacceptably low level. The method is specifically applicable to any fiber used to withdraw purified water from dirty water, particularly water containing organic matter including beneficial bacteria and/or undesirable inorganic salts, where the viability of the bacteria population is to be maintained. The membrane is cleaned by flowing a cleaning fluid, preferably a biocidal oxidative electrolyte having an oxidizing anion and an associated cation through the clean, permeate-side of the membrane, at low pressure no more than the bubble pressure breakthrough, usually <300 kPa (30 psig) for a MF or UF fiber. Such low pressure is sufficient to diffuse the electrolyte through both, the pores of the membrane and a fouling film which typically includes a biofilm accumulated on the fibers'"'"' outer surface, but not enough electrolyte flows through the membrane to kill numerically more than 20% of the living bacteria in the dirty water. This limitation can be met only if the cleaning period is brief. This period is only long enough to oxidize organic matter within the pores and kill essentially all bacteria in the biofilm. Preferably less than 5% of the bacteria population is decimated. As diffusion takes place, pores are again opened, both in the wall of the fiber and through the biofilm, and when the fibers are returned to normal operation, the restored flux is equal to at least 70% of the initial stable flux.

Claims

1. In a system for withdrawing permeate from a multicomponent liquid substrate having particulate matter and a population of beneficial aerobic bacteria suspended therein, with a gas-scrubbed assembly comprising a frameless array of hollow fiber membranes in combination with a gas-distribution means, said system comprising, a reservoir containing a volume of at least 100 liters of said substrate from which a permeate is to be withdrawn;

a pair of headers adapted to be mounted in spaced-apart relationship within said substrate without being confined in a modular shell, a first header having terminal end portions of a multiplicity of hollow fibers secured therein, and a second header having opposed terminal end portions of said hollow fibers secured therein, essentially all ends of said hollow fibers being open so as to discharge permeate through said headers, at least one header being disposed below a horizontal plane through the horizontal center plane of said one header;

said hollow fibers formed from a material selected from the group consisting of an inorganic material and an organic synthetic resinous material, and swayably buoyantly deployed as a skein in a body of said substrate, said hollow fibers together having an outer surface area in excess of 10 m², each fiber having a length >

0.5 m and sufficiently greater than the direct distance between said first and second headers, so as to present, when said skein is deployed, a generally arcuate configuration above a plane through the horizontal center-line of a headers;

permeate collection means for collecting said permeate;

means for mounting said spaced-apart headers in open fluid communication with said permeate collection means;

means for withdrawing said permeate; and

, said gas-distribution means disposed within a zone beneath said skein, and adapted to generate bubbles which flow upwardly through said skein, whereby said hollow fibers are kept awash in bubbles and resist the build-up of said particulate matter on the surfaces of said hollow fibers on which is generated a biofilm clogging pores of said membrane, yet affords an initial stable transmembrane flux which decreases during each withdrawal period as a function of time by at least 20%, the improvement comprising,

(i) a container containing an aqueous solution of a biocidal oxidative electrolyte;

(ii) a conduit having a pump means in fluid connection with said fibers to flow said electrolyte in laminar flow through the lumens of said hollow fibers to permeate through said outer surfaces and said biofilm, at a pressure no higher than said membrane'"'"'s bubble breakthrough pressure, but enough pressure to diffuse said solution through said pores and said biofilm, for a period sufficient to oxidize organic matter within said pores and in said biofilm so as to form a random distribution of pores through said biofilm and provide a restored flux equal to at least 70% of said initial stable flux;

(iii) conduit means operatively placing said permeate collection means in selectively open flow communication with said container; and

,

(iv) valve means operatively connected in said conduit means to alternately withdraw permeate from said collection means, and to recirculate said electrolyte through said hollow fibers.

2. The system of claim 1 wherein said bubble pressure breakthrough is no more than about 300 kPa (30 psig).