JP2007260638A - Water treatment method using reverse osmosis membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water treatment method using a reverse osmosis membrane by which deterioration in the performance of the reverse osmosis membrane can be prevented without the addition of excessive components such as acid. <P>SOLUTION: In the water treatment method using the reverse osmosis membrane, while the feeding operation of the water to be treated to the reverse osmosis membrane is continued, the state of the water to be treated is alternately changed between an aerobic state and an anaerobic state per prescribed time, and by bringing the water to be treated into the anaerobic state, the propagation of aerobic microorganisms in the water to be treated is suppressed, and also, by bringing the water to be treated into the aerobic state, the propagation of anaerobic microorganisms in the water to be treated is suppressed, thus the propagation of both anaerobic microorganisms and aerobic microorganisms is alternately suppressed, and the amount of deposit of both microorganisms to the reverse osmosis membrane is reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a water treatment method using a reverse osmosis membrane, and more particularly to a method used when desalinating seawater, brine, or the like.

  BACKGROUND ART A desalination apparatus using a reverse osmosis membrane is used in various fields such as desalination of seawater and brine, production of pure water, wastewater treatment, and the like. The reverse osmosis membrane used in such a desalination apparatus is a semi-permeable membrane that allows only water in treated water such as seawater and brine to be treated to pass, and does not allow other components to pass through. As a result, dirt accumulates on the membrane. As the membrane contamination progresses, various problems such as deterioration of permeate water quality, increase in operating pressure, and increase in membrane module differential pressure occur.

  Film fouling has various factors such as particulate matter accumulation, organic matter adhesion, and microbial growth. Among these factors, contamination by microorganisms is a big problem, and various sterilization methods and operation methods for preventing this have been studied.

  The most common sterilization method is injection of a chlorine-based disinfectant. However, the chlorine-based disinfectant promotes deterioration of the reverse osmosis membrane by chlorine injection, and the membrane deterioration is particularly remarkable when the reverse osmosis membrane is a polyamide-based membrane. For this reason, it is necessary to reduce free chlorine in the treated water by injecting a reducing agent such as sodium hydrogen sulfate into the treated water before the treated water is supplied to the reverse osmosis membrane.

  However, it is already known that it is difficult to completely kill microorganisms in the treated water with a chlorine-based disinfectant. By treating the disinfectant with a reducing agent injection, the microorganisms are reactivated or the system is reactivated. When microorganisms are mixed from the outside, the microorganisms propagate on the reverse osmosis membrane, which may deteriorate the performance of the membrane.

  As another sterilization method, a method by ultraviolet irradiation is also known, but in this method, the sterilization effect is only the portion irradiated with ultraviolet rays, and since there is no continuity in sterilization power, sterilization becomes insufficient, There is a possibility that microorganisms adhere to the film, and further microorganisms accumulate on the film.

  If the performance of the membrane deteriorates, the performance of the membrane can be recovered by immersing or circulating cleaning with caustic soda or sodium hydrogen sulfate after once stopping the operation of the desalination apparatus. However, in order to avoid a decrease in the amount of treated water due to the operation stop of the desalination apparatus, a method capable of cleaning and sterilizing the membrane while the apparatus is operated is desired.

  As such a method, a sterilization method in which sodium hydrogen sulfate is injected intermittently has been developed. However, this method cannot perform sufficient sterilization, increases resistance to chemicals such as sodium hydrogen sulfate, and gradually deteriorates the membrane performance.

  Further, in Patent Documents 1 and 2, instead of sterilization with sodium hydrogen sulfate as described above, the pH of treated water is set to 4 or lower, or the pH of treated water is set to 2.7 to 4 for the first time. And a method of performing the second treatment to reduce the pH to 2.6 or less.

However, in this method, it is necessary to add an acid in order to make the treated water strongly acidic to weakly acidic, and further, an extra operation such as neutralizing the acidic water after the membrane treatment is required.
JP 2000-237555 A JP 2000-354744 A

  The present invention has been made in view of the above circumstances, and provides a water treatment method using a reverse osmosis membrane that can prevent performance deterioration of the reverse osmosis membrane without adding an extra component such as an acid. With the goal.

  In the present invention, it is difficult to completely kill microorganisms present in the water to be treated with a bactericide or the like, but in a constant environment, utilizing the advantage of the species of microorganisms suitable for the state, It has been found that the amount of microorganisms attached to the reverse osmosis membrane can be reduced, and the present invention has been completed.

  That is, in the water treatment method using the reverse osmosis membrane of the present invention, the state of the water to be treated is changed every predetermined time in an aerobic state and an anaerobic state while continuing the supply operation of the water to be treated to the reverse osmosis membrane. By alternately switching and making the treated water anaerobic, the growth of aerobic microorganisms in the treated water is suppressed, and by making the treated water aerobic, the anaerobic microorganisms in the treated water By suppressing the growth, the growth of both anaerobic microorganisms and aerobic microorganisms is suppressed, and the amount of both microorganisms attached to the reverse osmosis membrane is reduced.

  In the method of the present invention, the aerobic state and the anaerobic state are preferably switched every 1 to 72 hours.

  In the method of the present invention, switching between the aerobic state and the anaerobic state is preferably performed by changing the injection amount of the reducing agent.

  In the method of the present invention, it is preferable that the water to be treated is brought into an anaerobic state by setting its oxidation-reduction potential to 0 mV or less, and brought into an aerobic state by making the oxidation-reduction potential to 240 mV or less.

  In the water treatment method using the reverse osmosis membrane of the present invention, the state of the water to be treated is alternately switched between the aerobic state and the anaerobic state at predetermined time intervals while continuing the supply operation of the water to be treated to the reverse osmosis membrane. . Thereby, while the state of to-be-processed water is an anaerobic state, since the growth of anaerobic microorganisms becomes dominant, the growth of an aerobic microorganism is prevented. On the other hand, while the water to be treated is in an aerobic state, the growth of aerobic microorganisms is dominant, so that the growth of anaerobic microorganisms is hindered. By repeating such an anaerobic state and an aerobic state every predetermined time, the growth of both anaerobic microorganisms and aerobic microorganisms in the water to be treated is hindered. For this reason, both microorganisms adhere to the reverse osmosis membrane. The amount can be reduced. In the present invention, it is not necessary to add an extra component such as an acid, and naturally, it is not necessary to perform an extra operation such as neutralizing the treated water that has become acidic after permeating the reverse osmosis membrane.

  Hereinafter, the water treatment method using the reverse osmosis membrane of the present invention will be described in detail based on the drawings.

  FIG. 1 is a flow sheet for explaining a method for producing fresh water from seawater or brine using a reverse osmosis membrane according to the present invention. Note that the method for producing fresh water described with reference to FIG. 1 is merely an example of a water treatment method using the reverse osmosis membrane of the present invention, and the method of the present invention can be used for the production of pure water, waste water treatment, etc. It can be applied in various fields.

  When treated water to be desalinated such as seawater and brine is taken into the water storage tank (1), first, microorganisms contained in the treated water are pretreated as a membrane permeation process for producing fresh water. Exclusion processing is performed. For this purpose, a disinfectant such as sodium hypochlorite (NaClO) is injected into the water tank (1) from the disinfectant injection device (2).

  Next, the water to be treated is introduced into the filtration tank (3), where filtration treatment such as coagulation sedimentation, coagulation filtration, precision membrane filtration, and ultrafiltration is performed to remove the precipitate.

Subsequently, the water to be treated is sent to the dechlorination tank (4). In the dechlorination tank (4), a reducing agent such as sodium hydrogen sulfate (NaHSO ₃ ) is injected from the reducing agent injection device (5) to obtain dechlorinated water to be treated. In addition, a pH adjusting agent such as sulfuric acid (H ₂ SO ₄ ) or a scale inhibitor is injected into the water to be treated in the dechlorination tank (4) by another chemical injection device (not shown).

  The dechlorinated water to be treated is passed through the high pressure pump (7) after fine particles, precipitates, etc. are removed by filtration with the cartridge filter (6) so that the drive of the high pressure pump described later is not damaged. Here, the pressure is increased to about 6 MPa, and the pressure-treated water is supplied to the reverse osmosis membrane (8).

  The various chemicals and pretreatment methods and numerical values mentioned here may be appropriately selected and changed, and an optimum combination is adopted as necessary.

  As the reverse osmosis membrane (8), any material may be used as long as the water to be treated can be desalted. Examples thereof include those composed of cellulose acetate-based materials and polyamide-based materials. Among these, what is particularly effective in the method of the present invention is composed of a polyamide-based material.

  In the method of the present invention, before the water to be treated is supplied to the reverse osmosis membrane (8), the water to be treated is brought into an aerobic state or an anaerobic state, and the aerobic state and the anaerobic state are alternately switched every predetermined time. Like that.

  Any method may be used to make the water to be treated aerobic or anaerobic, but a particularly convenient method for easily switching the state of the water to be treated is, for example, a reducing agent. Addition of sodium hydrogen sulfate, sodium thiosulfate and the like can be mentioned.

  That is, the reducing agent injection device (5) is installed to inject a reducing agent in order to dechlorinate the water to be treated. In addition to the amount of reducing agent required for this dechlorination, If the amount of reducing agent necessary to bring the treated water into an aerobic state or reduced state is injected, it is not necessary to separately provide a configuration for changing the state of the treated water.

  In this way, by injecting the reducing agent into the water to be treated while adjusting the amount of the reducing agent added from the reducing agent injection device (5), the water to be treated can be easily switched between the anaerobic state and the aerobic state. Here, the anaerobic state is based on setting the redox potential of the water to be treated to 0 mV or less, and the aerobic state is based on controlling the redox potential of the water to be treated to about 240 mV. Such a redox potential of the water to be treated is measured by, for example, a redox potential meter.

  Switching between the aerobic state and the anaerobic state of the treated water activates the other microorganism so that the growth of either the aerobic microorganism or the anaerobic microorganism can be sufficiently suppressed, and the aerobic microorganism or the anaerobic microorganism Every time when the microorganisms are activated too much and the performance of the reverse osmosis membrane does not deteriorate, preferably every 1 to 72 hours, for example, as shown in FIG. Do. The time for maintaining the aerobic state and the time for maintaining the anaerobic state may be the same as shown in FIG. 2, but depending on the ratio of anaerobic microorganisms and aerobic microorganisms present in the water to be treated. You may change suitably.

  The water to be treated discharged from the reverse osmosis membrane is desalinated, and is used after being treated with the added reducing agent if necessary.

  FIG. 3 shows that the water to be treated was chlorinated when the anaerobic state (0 mV) and the aerobic state (240 mV) were alternately switched every predetermined time (48 hours) (solid line). It is a graph showing the relationship between the elapsed time (days) and the increase rate (%) of the pressure difference of a membrane osmosis membrane about the case where treated water is supplied to a reverse osmosis membrane as it is (dashed line).

  As apparent from the graph of FIG. 3, in the method of the present invention, the increase in the differential pressure increase ratio is suppressed as compared with the conventional method, and the growth of both anaerobic microorganisms and aerobic microorganisms is suppressed. As a result, the performance deterioration of the reverse osmosis membrane is suppressed.

It is a flow sheet explaining the fresh water production | generation method using the reverse osmosis membrane which is an example of the water treatment method of this invention. It is a graph which shows the illustration in the case of changing the aerobic state and the anaerobic state of to-be-processed water by changing a redox potential by reducing agent addition. When the water to be treated is switched between an anaerobic state (0 mV) and an aerobic state (240 mV) alternately every predetermined time (48 hours) (solid line), and the chlorinated water to be treated is directly used as a reverse osmosis membrane. It is a graph showing the relationship between the elapsed time (days) and the rate of increase in the differential pressure of the membrane permeable membrane (%) with respect to the case of being supplied (wave line)

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Water storage tank 2 Bactericidal agent injection apparatus 3 Filtration tank 4 Dechlorination tank 5 Reducing agent injection apparatus 6 Cartridge filter 7 High pressure pump 8 Reverse osmosis membrane

Claims (4)

  By continuously supplying the water to be treated to the reverse osmosis membrane, the state of the water to be treated is alternately switched between an aerobic state and an anaerobic state every predetermined time, thereby making the water to be treated anaerobic. By suppressing the growth of anaerobic microorganisms in the treated water and suppressing the growth of anaerobic microorganisms in the treated water by bringing the treated water into an aerobic state, A water treatment method using a reverse osmosis membrane, which suppresses both growths from each other and reduces the amount of both microorganisms attached to the reverse osmosis membrane.   The water treatment method using a reverse osmosis membrane according to claim 1, wherein switching between an aerobic state and an anaerobic state is performed every 1 to 72 hours.   The water treatment method using a reverse osmosis membrane according to claim 1 or 2, wherein switching between the aerobic state and the anaerobic state is performed by changing an injection amount of the reducing agent. The reverse osmosis membrane according to claim 3, wherein the water to be treated is rendered anaerobic by setting its oxidation-reduction potential to 0 mV or less, and is rendered to an aerobic condition by setting the oxidation-reduction potential to 240 mV or less. Water treatment method.

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