WO2007064831A1

WO2007064831A1 - Membrane flushing system

Info

Publication number: WO2007064831A1
Application number: PCT/US2006/045912
Authority: WO
Inventors: Samuel Mason; Christopher L. Hansen
Original assignee: Kinetico Incorporated
Priority date: 2005-12-02
Filing date: 2006-12-01
Publication date: 2007-06-07

Abstract

A filtration membrane is cleaned by passing permeate from the permeate side of the membrane to the concentrate side of the membrane under a controlled positive pressure. This technique targets scale that forms at the boundary layer of the membrane providing an efficient and effective cleaning of the membrane.

Description

Membrane Flushing System

Technical Field

The present invention relates generally to fluid treatment systems and in particular to a method and apparatus for purifying water using the reverse osmosis principle.

Background

Various method and apparatus are known for purifying solvents, particularly water. One such method utilizes the principle of reverse osmosis to reduce or eliminate the quantity of dissolved solids in a liquid. According to the reverse osmosis principle, a semi permeable membrane is used separate the solvent from the dissolved solids. For example, in purifying water, a membrane is selected that exhibits greater permeability to water than the dissolved solids carried by the water. Raw feed water is applied to the membrane at a pressure generally greater than the osmotic pressure of the water. Under pressure, water passes through the membrane, leaving behind the dissolved solids. The liquid passing thorough the membrane is generally termed "permeate" whereas the liquid remaining on the input side of the membrane is generally termed "concentrate" and is usually discarded to drain. The ration of the permeate volume produced to the total volume water used is the water recovery.

Since the concentration of solutes increases on the concentrate side of the membrane during the reverse osmosis process, precipitation of one or more of the dissolved solids can occur. This precipitation can cause plugging of the membrane thus lowering the efficiency of the process. At high influent calcium and magnesium concentration and high water recovery, hardness scale forms on the concentrate side of the membrane. Scale formation begins in the boundary layer next to the membrane, where the concentration is highest and the velocity is effectively zero. This scale is difficult to clean. In some systems, plugging or compaction of the membrane is compensated for by increasing the pressure of feed water. In other systems, the feed water is fed at a relatively high flow rate to cause turbulence in the vicinity of the membrane. Those employing this arrangement believe that the turbulence prevents the solids from adhering to the membrane. With this method, however, a rather large quantity of feed water is discharged as concentrate.

U.S. Patent No. 4,629,568 to Ellis, III, discloses a membrane flushing system in which during a flush cycle, the concentrate line is opened, lessening the pressure on the membrane and permeate is routed to the concentrate side of the filter unit. In this manner, the side of the membrane that has a high amount of dissolved solids is flushed with purified water while the pressure on membrane is relieved, serving to cleanse the membrane.

Summary of the Invention

Periodically reversing the flow across a membrane to cause permeate to flow back across the membrane dilutes water at the boundary layer to loosen or inhibit the formation of dissolved solids on the membrane's surface and effectively cleans the membrane. Accordingly, a filtration membrane element having a concentrate side where unfiltered fluid is present and a permeate side to which permeate fluid that has been treated by the filtration membrane passes is cleaned. Permeate fluid is passed through the filtration membrane element from the permeate side to the concentrate side.

Brief Description of the Drawings

Figures 1 and 2 are schematic illustrations of a water filtration system constructed in accordance with one embodiment of the present invention; and

Figure 3 is a graph showing the results of a test that was conducted to demonstrate the efficacy of the present invention.

Detailed Description

During the service cycle of a membrane water filtration system such as a reverse osmosis filtration system, the highest concentration of dissolved solids in the concentrate stream is in the stationary boundary layer right next to the membrane's surface on the concentrate side. While flushing the concentrate side of the membrane removes some of the solids built up on the membrane, it may not be very effective at removing the solids right next to the membrane surface. The solids in the stationary boundary layer remain after a flush, degrading the filtering ability of the membrane.

Figure 1 shows a reverse osmosis water filtration system 10 that can perform a reverse permeate flush cycle to clean the concentrate side of the membrane surface. As shown in Figure 1, during normal water treatment unsoftened feed water passes through a feed valve 17 to a pressurizing pump 16. The pressurized feed water is fed into a membrane filter housing 12 that has a concentrate side 14 and a permeate side 13 that are separated by a reverse osmosis membrane element. It will be recognized by one of ordinary skill in the art that other types of membrane filter elements, such as a nanofilter element, can benefit from the described reverse permeate flush cleaning system. A portion of the concentrate is recycled to the pump 16 with the rest of the concentrate passing to the drain through a concentrate regulator 19. The volume of concentrate sent to the drain is controlled by the regulator 19 to maintain high water recovery. Permeate that has been treated by the membrane element flows through a three way valve 25 to a storage tank or tap for use.

Figure 2 shows the reverse osmosis water filtration system 10 during a reverse permeate flush cleaning cycle. The inlet valve 17 is closed to isolate the system from the feed water. A rinse drain valve 27 opens to bleed residual pressure from the filter housing 12. The three way valve 25 is actuated to route pressurized permeate to the permeate side 13 of the membrane element under a controlled positive pressure. A permeate pressure regulator 21 controls the positive pressure of the reverse flow to limit the pressure to a level that will not cause damage to the reverse osmosis membrane, such as five p.s.i. or less. When the flush cycle is complete, the three way valve 25 returns to its original position, the rinse drain valve 27 closes and the feed valve 17 is opened so that as feed water is admitted to the reverse osmosis water filtration system, permeate again flows out of the permeate line. The flow of permeate backwards through the membrane dilutes and rinses away the boundary layer of scale more effectively than rinsing from the concentrate side. In addition, the reduction of concentration of solids in the boundary layer due to the dilution of that layer by the permeate flush is believed to reduce the tendency for solids to pass through the membrane from the concentrate side to the permeate side during periods of no permeate production.

Reverse permeate flush membrane cleaning has many advantages. It is believed that less flush water will be needed because the flow of permeate targets the area of highest concentration of potential scale forming solids. Because the membrane can be more effectively cleaned using less flush water, the overall recovery rate of the filtration system can be improved.

Experimentation has confirmed the efficacy of the present invention. A test demonstrating the effectiveness of the disclosed rinsing apparatus and method of the present invention was conducted. The results are shown in Fig. 3. The test was conducted as follows. A membrane was installed in a water treatment test unit. The membrane was conditioned and an operational baseline was established using softened water as the infeed to the unit. Referring to -A-

Fig. 3, the feed to the test unit was then changed to hard water. Three test runs labeled generally as 50, 52, 54 are shown in Fig. 3. In each test run, hard water was fed through the unit for ten minutes and was follows by a sixty second reverse permeate rinse (indicated by the reference character 60). As seen in Fig. 3, approximately nine service/rinse cycles were conducted. Over time, the flow rate degradated from approximately 500 CL/MIN to approximately 300 CL/MIN. At that point in the test, a longer reverse permeate rinse was initiated that lasted for approximately 11 minutes and is indicated by the reference character 62. At the conclusion of the longer reverse permeate rinse, the flow rate returned to 500 CL/MIN and the second test run 52 was commenced. At the conclusion of the second test run, an extended permeate rinse 62 was also completed and again the flow rate returned to 500 CL/MIN.

Although the invention has been described with a certain degree of particularity, it is understood that those skilled in the art can make various changes to the invention without departing from the spirit or scope of the invention as hereinafter claimed.

Claims

CIaimsWe claim:

1. A method that cleans a filtration membrane element having a concentrate side where unfiltered fluid is present and a permeate side to which permeate fluid that has been treated by the filtration membrane passes, the method comprising passing permeate fluid through the filtration membrane element from the permeate side to the concentrate side.

2. The method of claim 1 comprising draining fluid from the concentrate side when the permeate is passed from the permeate side to the concentrate side to flush to membrane.

3. The method of claim 1 comprising controlling a pressure at which the permeate fluid is passed through the filtration membrane.