News Column

"Washing Method for Separation Membrane Module" in Patent Application Approval Process

May 27, 2014



By a News Reporter-Staff News Editor at Life Science Weekly -- A patent application by the inventor Ikeda, Keiichi (Otsu-shi, JP), filed on May 9, 2012, was made available online on May 15, 2014, according to news reporting originating from Washington, D.C., by NewsRx correspondents (see also Toray Industries, Inc.).

This patent application is assigned to Toray Industries, Inc.

The following quote was obtained by the news editors from the background information supplied by the inventors: "Since a membrane separation method has characteristic features such as energy saving, space saving, and an improvement in filtrate quality, use of the method is continued to spread in various fields. For example, there may be mentioned an application of a microfiltration membrane and an ultrafiltration membrane to a water treatment process of producing industrial water or tap water from river water, groundwater, and water obtained by sewage treatment and to a pretreatment in a reverse osmosis membrane treatment process for seawater desalination. Furthermore, in the course of membrane treatment thereof, active carbon may be sometimes added to raw water or the like for the purpose of removing soluble organic matters (Patent Document 1).

"When the membrane filtration of raw water is continued, amounts of humic substances, proteins derived from microorganisms, and the like deposited on the surfaces of the membranes and in the membrane pores increase with an increase in an amount of filtrate, whereby a drop in filtrate flow rate or a rise in transmembrane pressure has become a problem.

"Under these circumstances, physical cleaning methods such as an air scrubbing of vibrating membranes with air bubbles introduced to the membrane primary side (raw water side) and bringing the membranes into contact with one another, thereby scraping off the substances attached to the membrane surfaces, and a backwashing of flowing under pressure a membrane filtrate or clarified water from the membrane secondary side (filtrate side) to the membrane primary side in a direction reverse to the filtration through the membrane and removing the contaminants attached to the membrane surfaces and in membrane pores, have been put to practical use (Patent Documents 2, 3, and 4).

"For the purpose of further enhancing cleaning effect, for example, a method of adding sodium hypochlorite to backwashing water and a method of using an ozone-containing water as backwashing water have been proposed (Patent Documents 5 and 6). The oxidizing agents have an effect of decomposing and removing organic matters, such as humic substances and proteins derived from microorganisms, having been attached on the membrane surfaces and in the membrane pores.

"In addition, at the backwashing, a method of once discharging water on the membrane primary side in the separation membrane module and performing backwashing with discharging backwashing discharge water has been proposed (Patent Document 7).

"However, in the case of membrane filtration of raw water containing particles having high hardness, particularly particles harder than the separation membrane, such as powdered activated carbon or the like, there is a problem that the particles having high hardness exfoliated from the membrane surface collide with the membrane surface and thus abrade it by performing the air scrubbing, whereby filtration performance is degraded. Moreover, in the case where only the backwashing is performed without performing the air scrubbing, the particles having high hardness are not sufficiently exfoliated from the membrane surface and a large amount thereof are accumulated, so that there is a problem that cake filtration resistance derived from the particles having high hardness (filtration resistance based on the Ruth's filtration expression described in Non-Patent Document 1, which is expressed by Rc=.alpha.c (cake average filtration specific resistance).times.We (cake deposition quantity per unit membrane area)) increases and transmembrane pressure steeply increases. Furthermore, even when sodium hypochlorite is added to the backwashing water or the ozone-containing water is used as the backwashing water, there is a problem that the chemicals are consumed by the powdered activated carbon and hence an effect of decomposing and removing the membrane-attached organic matters is decreased."

In addition to the background information obtained for this patent application, NewsRx journalists also obtained the inventor's summary information for this patent application: "Under these circumstances, the applicant of the present application devised a method for cleaning a separation membrane module in which, after the completion of filtration, backwashing discharge water in a separation membrane module is discharged while performing backwashing after water on the membrane primary side in the separation membrane module is discharged outside the system, subsequently the membrane primary side in the separation membrane module is filled with water and air scrubbing is performed, and then the water on the membrane primary side in the separation membrane module is discharged outside the system.

"However, in the case where membrane filtration is performed after powdered activated carbon and an inorganic flocculant are added to raw water and then the whole is mixed and stirred for the purpose of adsorbing and removing low-molecular-weight organic matters having a fractional molecular weight of 1,500 Da or less by powdered activated carbon and simultaneously removing high-molecular-weight organic matters having a fractional molecular weight of more than 1,500 Da by a flocculation treatment through the inorganic flocculant injection, when the above cleaning method is performed, the following problems have arisen even when backwashing discharge water in the separation membrane module is discharged while performing backwashing after the water on the membrane primary side in the separation membrane module is discharged outside the system. That is, in the case where the inorganic flocculant is injected in a large amount, flocculation flocks containing the powdered activated carbon are not sufficiently exfoliated from membrane surfaces. Moreover, since a part of the exfoliated flocculation flocks also have a large particle diameter, they are prone to remain at void parts on the membrane primary side in the separation membrane module, so that they are difficultly discharged outside the system. Therefore, the flocculation flocks containing the powdered activated carbon exfoliated from the membrane surfaces collide with the membrane surfaces during subsequent air scrubbing, thereby abrading the surfaces, so that there is a problem of deterioration in filtration performance.

"Even when flux of the backwashing is increased or backwashing time is extended for the purpose of solving these problems, the cleaning effect is small and there is a problem that water recovery ratio drops. Moreover, in the case where the air flow rate for the air scrubbing is diminished or the air scrubbing time is decreased, the abrasion of the membrane surfaces can be suppressed but the powdered activated carbon is not sufficiently exfoliated from the membrane surfaces and is accumulated in a large amount. Therefore, there are problems that the cake filtration resistance derived from the flocculation flocks containing the powdered activated carbon increases and the transmembrane pressure rapidly rises.

"The present invention makes it possible to enable stable operation under a low transmembrane pressure over a long period of time through effective reduction of the abrasion of the membrane surfaces by the particles having high hardness during the air scrubbing and, in the case where a filtration step is successively preformed, through suppression of the cake filtration resistance derived from the flocculation flocks containing the particles having high hardness on the membrane surfaces, by making the flocculation flocks containing the particles having high hardness easy to foliate and making the exfoliated flocculation flocks easy to be discharged outside the system, in the case where backwashing discharge water in the separation membrane module is discharged while performing backwashing, in a method for cleaning a separation membrane module in which raw water containing particles having hardness higher than that of the separation membrane is mixed and stirred with an inorganic flocculant and then filtration is performed through the separation membrane.

"For the purpose of solving the above problems, the method for cleaning a separation membrane module of the invention has any of the following characteristic features.

"(1) A method for cleaning a separation membrane module in which filtration is performed through a separation membrane after mixing and stirring raw water containing particles having hardness higher than that of the separation membrane with an inorganic flocculant, the method including:

"(a) discharging water on a membrane primary side in the separation membrane module outside the system after completion of the filtration;

"then (b) filling the membrane primary side in the separation membrane module with water containing a chelating agent for a certain period of time;

"subsequently discharging the water containing the chelating agent on the membrane primary side in the separation membrane module outside the system; and

"then (d) discharging backwashing discharge water in the separation membrane module while performing backwashing in which backwashing water is transferred from a membrane secondary side to the membrane primary side of the separation membrane module.

"(2) A method for cleaning a separation membrane module in which filtration is performed through a separation membrane after mixing and stirring raw water containing particles having hardness higher than that of the separation membrane with an inorganic flocculant, the method including:

"(e) injecting a chelating agent to a membrane primary side in the separation membrane module in the course of the filtration;

"subsequently (f) discharging water containing the chelating agent in the separation membrane module outside the system after completion of the filtration; and

"then (d) discharging backwashing discharge water in the separation membrane module while performing backwashing in which backwashing water is transferred from a membrane secondary side to the membrane primary side of the separation membrane module.

"(3) A method for cleaning a separation membrane module in which filtration is performed through a separation membrane after mixing and stirring raw water containing particles having hardness higher than that of the separation membrane with an inorganic flocculant, the method including:

"(a) discharging water on a membrane primary side in the separation membrane module outside the system after completion of the filtration; and

"then (g) discharging backwashing discharge water in the separation membrane module while performing backwashing in which water containing a chelating agent is transferred from a membrane secondary side to the membrane primary side of the separation membrane module.

"(4) The method for cleaning a separation membrane module according to any of (1) to (3), in which, following any of the steps (d) and (g), (h) air scrubbing is performed while water is fed to the membrane primary side in the separation membrane module or after the membrane primary side in the separation membrane module is filled with water. (5) The method for cleaning a separation membrane module according to (4), in which (i) the water on the membrane primary side in the separation membrane module is discharged outside the system after performing the air scrubbing. (6) The method for cleaning a separation membrane module according to (4) or (5), in which the water to be fed to the membrane primary side in the separation membrane module in the step (h) is at least one of the backwashing water, the raw water and flocculation water obtained by mixing and stirring the raw water and the inorganic flocculant. (7) The method for cleaning a separation membrane module according to any of (1) to (6), in which pH of the water containing the chelating agent in any of steps (b), (e) and (g) is 5 or more. (8) The method for cleaning a separation membrane module according to any of (1) to (7), in which the water on the membrane primary side in the separation membrane module is discharged outside the system until a water level on the membrane primary side in the separation membrane module reaches one third or less of a length of the separation membrane, in at least one step of the steps (a), and (f). (9) The method for cleaning a separation membrane module according to (8), in which a whole quantity of the water on the membrane primary side in the separation membrane module is discharged in at least one step of the steps (a), and (f). (10) The method for cleaning a separation membrane module according to any of (1) to (9), in which a flow rate of the backwashing is controlled so that a water level on the membrane primary side in the separation membrane module is kept to be one third or less of a length of the separation membrane, in any of the steps (d) and (g).

"According to the method for cleaning a separation membrane module of an embodiment of the invention, in a method for cleaning a separation membrane module in which raw water containing particles having hardness higher than that of the separation membrane is mixed and stirred with an inorganic flocculant and then filtration is performed through the separation membrane, a chelating agent forms a chelate complex with the inorganic flocculant. Therefore, even when flux for backwashing is not increased or the water quantity for backwashing is not increased by extending the backwashing time, the flocculation flocks are easily exfoliated from the membrane surfaces during the backwashing and also the flocculation flocks tend to be broken. As a result, the exfoliated flocculation flocks are easily discharged outside the system without remaining in void parts of the membrane primary side in the separation membrane module and also the abrasion of the membrane surfaces by the particles having higher hardness during air scrubbing can be efficiently reduced. Furthermore, at the time of successively performing a filtration step, the cake filtration resistance derived from the flocculation flocks containing the particles having high hardness on the membrane surfaces is suppressed and hence stable operation at a low transmembrane pressure over a long period of time is enabled.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is an apparatus schematic flow chart showing one example of an apparatus for treating water to which the method for cleaning the invention is applied."

URL and more information on this patent application, see: Ikeda, Keiichi. Washing Method for Separation Membrane Module. Filed May 9, 2012 and posted May 15, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=5781&p=116&f=G&l=50&d=PG01&S1=20140508.PD.&OS=PD/20140508&RS=PD/20140508

Keywords for this news article include: Chelating Agents, Toray Industries Inc.

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Source: Life Science Weekly