System for maintaining a clean skein of hollow fibers while filtering suspended solids

• US 5,944,997 A
• Filed: 02/17/1998
• Issued: 08/31/1999
• Est. Priority Date: 08/11/1995
• Status: Expired due to Fees

• Current Assignee: Zenon Environmental Incorporated
• Original Assignee: Zenon Environmental Incorporated
• Inventors: Behmann, Henry, Pedersen, Steven Kristian
• Primary Examiner(s): FORTUNA, ANA M
• Application Number:
  US09/024,417
• Time in Patent Office: 560 Days
• Field of Search: 210/636, 210/500.23, 210/321.69, 210/321.8, 210/321.89, 210/321.78, 210/321.79, 210/257.2, 210/650, 210/195.2, 261/124, 261/126, 261/96, 261/DIG. 70
• US Class Current: 210/636
• CPC Class Codes: B01D 2321/02 Forward flushing
• CPC Class Codes: B01D 2321/12 Use of permeate
• CPC Class Codes: B01D 2321/185 Aeration
• CPC Class Codes: B01D 61/18 Apparatus therefor
• CPC Class Codes: B01D 61/20 Accessories; Auxiliary operations
• CPC Class Codes: B01D 63/021 Manufacturing thereof
• CPC Class Codes: B01D 63/022 Encapsulating hollow fibres
• CPC Class Codes: B01D 63/0221 using a mould
• CPC Class Codes: B01D 63/023 Encapsulating materials
• CPC Class Codes: B01D 63/0231 using supporting structures, e.g. filaments for weaving mats
• CPC Class Codes: B01D 63/026 Wafer type modules or flat-surface type modules
• CPC Class Codes: B01D 63/043 with separate tube sheets
• CPC Class Codes: B01D 65/02 Membrane cleaning or sterilisation ; Membrane regeneration
• CPC Class Codes: B01D 65/08 Prevention of membrane fouling or of concentration polarisation
• CPC Class Codes: C02F 1/444 by ultrafiltration or microfiltration
• CPC Class Codes: Y10S 261/70 Sewage aerators; diffusers

Abstract

The outer surfaces of hollow fiber membranes in a skein, or a bank or cassette of stacked skeins, are kept essentially free of solids by maintaining a substantially uniform flow of a stream of coarse bubbles through each of plural orifices in an aerator disposed beneath the fibers. Aerators for skeins used in microfiltration of wastewater in a tank are periodically flushed by injecting flushing air into permeate returned to the tank over a short period. The air is injected into returning permeate for less than 1 minute, preferably less than 20 sec so as to produce discrete masses of water which are flowed laterally through the body of the aerator, cleaning its walls and maintaining the orifices essentially free from plugging.

Claims

1. In a method for maintaining production of permeate at equilibrium flux, in a skein submerged in a solids-containing substrate held in a tank, continuously, by scrubbing hollow fiber membranes ("fibers") of said skein with bubbles generated from scrubbing gas flowed into an aerator, the improvement comprising,

supplying said scrubbing gas to said aerator'"'"'s air plenum at a pressure greater than the liquid head between the surface of substrate in the tank and the surface of the substrate in the plenum;

maintaining a substantially uniform flow of a stream of said bubbles through each of plural orifices in lateral walls of said aerator, in which stream the majority are coarse bubbles having an effective diameter in excess of 3 mm and rise at a velocity in the range from about 5 ft/sec to 10 ft/sec (1.5 m/sec to 3 m/sec);

periodically returning permeate to said aerator through a conduit means connected thereto;

injecting a desired amount of said flushing gas into said permeate in said conduit means within an injection period of less than 10 min so as to flush said aerator with a two-phase mixture of flushing gas and permeate flowing laterally through said aerator, in a volumetric ratio of flushing gas;

permeate in the range from about 1;

2 to about 25;

1, said gas in said mixture being at elevated pressure, above that of permeate flowing in said permeate line, and in addition, sufficient to overcome a hydrostatic head above said aerator, so as to provide discrete dispersed masses of permeate which impinge against internal walls of said aerator;

maintaining flow of said two-phase mixture for less than 10 min; and

, returning loosened solid deposits from within said aerator into said substrate; and

, re-establishing flow of said scrubbing gas.

2. The method of claim 1 wherein periodic flushing is effected, repetitively, after a period in the range from about 4 hr to about 72 hr;

said flushing gas is introduced into said plenum at a pressure in the range from about 5 cm to about 50 cm above the hydrostatic head exerted at the lowest orifices in the walls of an open-bottom the aerator, and for a closed-bottom aerator, said pressure is increased by the height of the water-seal leg; and

, prior to said flushing, periodically, without interrupting filtration, returning permeate to said vessel through a permeate line connected to said aerator.

3. The method of claim 2 comprising,

sensing the pressure of permeate supplied;

sensing the pressure of said flushing gas supplied; and

,

mixing said injected flushing gas and said permeate prior to entering said aerator.

4. The method of claim 2 wherein said scrubbing gas is air supplied at a pressure in the range from greater than 1 atm to 1.5 atm (atmospheres).

5. The method of claim 4 wherein the ratio of cross-sectional area of the orifice(s) of the inlet nozzle(s):

cross-sectional area of the air plenum is in the range from about 1;

1 to about 1;

5 for a closed-bottom aerator; and

, from about 1;

5 to about 1;

25 for an open-bottom aerator.

6. The method of claim 4 wherein said flushing gas is injected for a period of less than 1 min, and the velocity of said mixture at the inlet orifice is in the range from about 20 to about 60 ft/sec.

7. A system for maintaining a skein submerged in a tank, and operating at equilibrium flux to produce permeate from a solids-containing substrate continuously, by scrubbing fibers of said skein with an inert scrubbing gas in the form of coarse bubbles generated in an aerator, said system comprising in combination,

a skein and a coarse bubble aerator disposed beneath said skein said aerator having an inlet for said scrubbing gas and flushing fluid;

means for supplying said aerator with said scrubbing gas at a predetermined pressure sufficient to maintain a substantially uniform flow of a stream of said bubbles through each of plural orifices in lateral walls of said aerator, in which stream the majority are coarse bubbles having an effective diameter in excess of 3 mm and rise at a velocity in the range from about 5 ft/sec to 10 ft/sec (1.5 m/sec to 3 m/sec);

means for returning said permeate to said aerator, upstream and exteriorly thereof;

means for injecting flushing gas into said means for returning said permeate;

said means for returning said permeate being in selective open communication with said means for supplying said aerator with said flushing gas;

means to control flow of said returning permeate for a chosen period in the range from 5 seconds to 10 min; and

,

means to control the amount of said flushing gas injected into said permeate within a chosen period of time less than 10 min, independently, so as to provide said flushing fluid as discrete dispersed masses of permeate which impinge laterally against internal walls of said aerator.

8. The system of claim 7 wherein lateral direction of said mixture is effected with orifice means in said inlet conduit having an effective diameter sufficient to provide a velocity of said mixture in the range from 20 ft/sec to about 60 ft/sec.