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

• US 6,193,890 B1
• Filed: 08/30/1999
• Issued: 02/27/2001
• Est. Priority Date: 08/11/1995
• Status: Expired due to Fees

• Current Assignee: Zenon Technology Partnership
• Original Assignee: Zenon Environmental Incorporated (Veolia Environnement SA)
• Inventors: Behmann, Henry, Pedersen, Steven Kristian
• Primary Examiner(s): FORTUNA, ANA M
• Application Number:
  US09/386,575
• Time in Patent Office: 547 Days
• Field of Search: 210/636, 210/321.69, 210/195.2, 210/650, 210/257.2, 210/321.89, 210/321.79, 210/321.8, 261/DIG.70, 261/124, 261/126, 261/96, 261/87, 261/121.1
• US Class Current: 210/636
• CPC Class Codes: B01D 2315/06 Submerged-type; Immersion type
• CPC Class Codes: B01D 2321/02 Forward flushing
• CPC Class Codes: B01D 2321/04 Backflushing
• 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 63/022 Encapsulating hollow fibres
• CPC Class Codes: B01D 63/023 Encapsulating materials
• 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

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 when their air orifices are fouled, by injecting a flushing fluid into a fouled aerator. The flushing fluid may be a two-phase mixture of flushing liquid and air, or flushing liquid only. The flushing liquid may be permeate, clean water or wastewater (substrate). When air is injected into the flushing liquid flowed to an aerator for less than 5 min, preferably less than 20 sec, the flushing fluid produces discrete masses of liquid which are flowed laterally through the body of the aerator, cleaning its walls and maintaining the orifices essentially free from plugging. When only flushing liquid is flowed through the fouled aerator for less than 5 min, preferably less than 20 sec, the flushing liquid is highly effective to keep the air orifices clean.

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'"'"'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 so as to maintain a substantially uniform flow of a stream of said bubbles through each of plural air orifices in lateral walls of said aerator, in which stream the majority are coarse bubbles have 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), the improvement comprising,

interrupting scrubbing gas while said aerator is flushed with a flushing liquid only;

periodically injecting flushing liquid into said aerator through a conduit means connected thereto within an injection period of less than 5 min so as to flush said aerator'"'"'s plenum with a single phase of flushing liquid flowing laterally therethrough in a mass flow sufficient to cleanse fouled air orifices in said aerator;

maintaining flow of said flushing liquid for less than 5 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 said mass flow is in the range from 10 to 80 liters/min;

said aerator has a cross-sectional area in the range from 3 to 20 sq cm; and

said air orifices in said aerator each have a diameter in the range from 3 mm to 10 mm, provided the total combined cross-sectional area of the orifices is less than 70% of the cross-sectional area of said plenum in the vertical plane at right angles to said plenum'"'"'s longitudinal axis; and

the trough of said aerator is provided with drainage holes the central axis of each being in the central vertical plane through the diameter of said aerator.

3. The method of claim 1 wherein said aerator comprises a tubular body having closed ends and plural air orifices spaced along the length of said body, each pair of orifices being angularly positioned so that said pair of orifices are in mirror image relationship with each other, each air orifice being angulated in the range from about 20 to 50 measured from the transverse central horizontal axis of said body, and the trough of said body being provided with drainage holes, the central axis of each being in the central vertical plane through the diameter of said tubular body.

4. 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 a coarse bubble aerator disposed beneath said skein, said aerator having an inlet for said scrubbing gas and flushing liquid, wherein said scrubbing gas is supplied 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), said system comprising in combination,

means for interrupting scrubbing gas while said aerator is flushed with a flushing liquid only;

means for flowing said flushing liquid to said aerator, upstream and exteriorly thereof; and

,

means to control mass flow of said flushing liquid for a chosen period in the range from 5 seconds to 5 min.

5. The system of claim 4 wherein said flushing liquid is directed laterally through said plenum, said mass flow is in the range from 10 to 80 liters/min;

said aerator has a cross-sectional area in the range from 3 to 20 sq cm;

said orifices in said aerator each have a diameter in the range from 3 mm to 10 mm, provided the total combined cross-sectional area of the orifices is less than 70% of the cross-sectional area of said plenum in the vertical plane at right angles to said plenum'"'"'s longitudinal axis; and

the trough of said aerator is provided with drainage holes the central axis of each being in the central vertical plane through the diameter of said aerator.

6. 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,

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),

the improvement comprising,

interrupting scrubbing gas while said aerator is flushed with a flushing liquid only;

periodically injecting flushing liquid into said aerator through a conduit means connected thereto;

injecting a desired amount of flushing gas into said permeate in said conduit means within an injection period of less than 5 min so as to flush said aerator with a two-phase mixture of flushing gas and flushing liquid flowing laterally through said aerator, in a volumetric ratio of flushing gas;

flushing liquid 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 a flushing liquid 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 5 min;

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

, re-establishing flow of said scrubbing gas.

7. The method of claim 6 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 flushing fluid to said vessel through a line connected to said aerator.

8. The method of claim 7 comprising,

sensing the pressure of flushing fluid supplied;

sensing the pressure of said flushing gas supplied; and

,

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

9. The method of claim 8 wherein said scrubbing gas is air supplied at a pressure in the range from greater than 1 atm to 1.5 atm (atmospheres), and said flushing fluid is substrate.

10. The method of claim 8 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.

11. 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 liquid;

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 interrupting scrubbing gas while said aerator is flushed with a flushing liquid only;

means for flowing said flushing liquid to said aerator;

means for injecting flushing gas into said means for flowing said flushing liquid;

said means for flowing said flushing liquid being in selective open communication

with said means for supplying said aerator with said flushing gas;

means to control flow of said flushing liquid for a chosen period in the range from 5 seconds to 5 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 flushing fluid as discrete dispersed masses of permeate which impinge laterally against internal walls of said aerator.