JP2011050843A - Method of and system for desalinating water to be treated - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of and a system for desalinating water to be treated capable of reducing the amount of an oxidizer used for sterilization of the water when desalinating the sterilized water by filtration with a reverse osmosis membrane. <P>SOLUTION: The method of desalinating the water to be treated includes a process of sterilizing the water by adding the oxidizer to the water, a process of filtering the sterilized water with a reverse osmosis membrane, and a process of adjusting the pH value of the water by adding a pH adjusting agent to the water prior to the sterilizing process. The desalination system includes an oxidizer adding means adding the oxidizer to the water, a reverse osmosis membrane filtering means provided in the downstream of the oxidizer adding means in relation to the flow of the water and filtering the water for desalinating, and a pH adjusting agent adding means provided in the upstream of the oxidizer adding means in relation to the flow of the water and adding the pH adjusting agent to the water. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method and a system for desalinating water to be treated such as brine or seawater, and in particular, a method and a system for desalination by pretreating the water to be treated (sterilization, etc.) and then filtering with a reverse osmosis membrane. It is about.

  Conventionally, a membrane separation method (reverse osmosis method) using a reverse osmosis membrane (RO membrane) is known as a general method for obtaining fresh water from treated water such as brine or seawater.

  And in the seawater desalination system using the reverse osmosis method, as a pretreatment for preventing the occurrence of early clogging of the reverse osmosis membrane with respect to the seawater as the treated water, an oxidizing agent such as sodium hypochlorite After performing sterilization treatment using water, flocculation treatment of suspended substances in seawater using a coagulant such as ferric chloride, and sand filtration to remove the aggregated suspended substances from seawater, The pH of the water is adjusted to, for example, 6 to 6.7, and the pretreated seawater having the adjusted pH is passed through the reverse osmosis membrane to desalinate the seawater.

Here, in general, undissociated hypochlorous acid (HClO) has a bactericidal effect about 100 times that of hypochlorite ion (ClO ⁻ ), so that an oxidizing agent such as sodium hypochlorite has a low pH. It is known that a stronger sterilizing power is exhibited under conditions (conditions where the amount of undissociated hypochlorous acid is large). However, in the conventional seawater desalination system, the pH of seawater is adjusted immediately before passing through the reverse osmosis membrane, and the pH is not adjusted when the oxidant is added to seawater. The added sodium hypochlorite could not have a sufficient bactericidal effect. Therefore, in the conventional seawater desalination system, it was necessary to add a large amount of an oxidizing agent in order to sterilize seawater.

  Therefore, when water to be treated is sterilized and then filtered through a reverse osmosis membrane for desalination, the amount of the oxidizing agent used for sterilization of the water to be treated can be reduced, and the desalination method and water to be treated are treated. There was a need to develop a water desalination system.

This invention aims to solve the above-mentioned problem advantageously, and the desalination method of the water to be treated of the present invention is a sterilization step of sterilizing the water to be treated by adding an oxidizing agent to the water to be treated. And a reverse osmosis membrane filtration step of filtering the treated water that has undergone the sterilization step with a reverse osmosis membrane, and the pH adjuster is added to the treated water before the sterilization step. And a pH adjusting step of adjusting the pH of the water to be treated. In the present invention, the “oxidant” refers to a substance capable of generating hypochlorous acid (HClO) in the water to be treated, such as chlorine, hypochlorous acid or a salt thereof, and the like.
As described above, if the pH of the water to be treated is adjusted in advance and then the oxidant is added to sterilize the water to be treated, the oxidant is sterilized under pH conditions that can exert a stronger sterilizing power. Therefore, the amount of the oxidizing agent used when the water to be treated is desalinated can be reduced.

  Here, the desalination method of the present invention includes a coagulation step in which a coagulant is added to the water to be treated to coagulate suspended substances in the water to be treated, and the aggregate formed in the coagulation step is separated from the water to be treated. It is preferable to further include an aggregate separation step to be performed after the sterilization step and before the reverse osmosis membrane filtration step. In this way, if the suspended substances are separated from the water to be treated after sterilization, the occurrence of clogging of the reverse osmosis membrane can be suppressed, and an oxidant is added to the water to be treated. This is because the time (residence time) from the filtration to the reverse osmosis membrane becomes longer, so that the water to be treated can be sufficiently sterilized.

  In the desalination method of the present invention, the pH of the water to be treated is preferably 6.0 to 7.5 in the pH adjustment step. This is because if the pH of the water to be treated is 7.5 or less, the oxidizing agent can exert a great sterilizing effect in the subsequent sterilization step. On the other hand, from the viewpoint of reducing chemical costs necessary to prevent scaling in the reverse osmosis membrane and to adjust the pH (neutralization) of the treated water (fresh water) after filtration of the reverse osmosis membrane, the treated water filtered through the reverse osmosis membrane The pH range is preferably 6.0 to 7.0. When the pH of the water to be treated is less than 6.0 before the sterilization step, the amount of the pH adjuster (acid and alkali agent) used This increases the cost. Note that when the water to be treated is seawater, the seawater has a buffering action and the pH is unlikely to be lowered, so that the cost is particularly increased to make the pH of the water to be treated less than 6.0.

In the desalination method of the present invention, it is preferable that the water to be treated contains bromide ions. When bromide ions are contained in the water to be treated, hypochlorous acid generated by the addition of an oxidizing agent to the water to be treated and bromide ions react as shown in the following reaction formula (I), resulting in a pH of 9 This is because hypobromite (HBrO) that can exhibit a high bactericidal power even in a high pH range is produced.
HClO + Br ⁻ → HBrO + Cl ⁻ (I)

Further, the desalination system of the present invention includes an oxidant addition means for adding an oxidant to the water to be treated, and a downstream of the oxidant addition means as viewed in the direction of treating the water to be treated from the upstream side to the downstream side. A desalination system for water to be treated comprising a reverse osmosis membrane filtration means for filtering and desalinating the water to be treated, as viewed in the direction of treating the water to be treated from the upstream side to the downstream side. Further, it is characterized by further comprising a pH adjuster adding means for adding a pH adjuster to the water to be treated upstream of the oxidizing agent adding means. In the present invention, the “oxidant” refers to a substance capable of generating hypochlorous acid (HClO) in the water to be treated, such as chlorine, hypochlorous acid or a salt thereof, and the like.
Thus, if the oxidizing agent adding means is provided downstream of the pH adjusting means, the water to be treated can be sterilized under pH conditions where the oxidizing agent can exert a stronger sterilizing power. The amount of the oxidizing agent used in can be reduced.

  And the desalination system of this invention was formed by the addition of the flocculant between the said oxidizing agent addition means and the said reverse osmosis membrane filtration means, and the flocculant addition means which adds a flocculant to to-be-processed water. It is preferable to further comprise a separating means for separating the aggregate from the water to be treated. In this way, if the suspended substance is separated from the water to be treated after the sterilization treatment using the oxidizing agent, the occurrence of clogging of the reverse osmosis membrane can be suppressed, and the water to be treated is also treated. This is because the time (residence time) from the addition of the oxidant to the filtration with the reverse osmosis membrane becomes longer, so that the water to be treated can be sufficiently sterilized.

  ADVANTAGE OF THE INVENTION According to this invention, the desalination method of the to-be-processed water which can reduce the quantity of the oxidizing agent used for the sterilization of the to-be-processed water in the desalination process which filters with a reverse osmosis membrane after sterilizing to-be-processed water and obtains fresh water And the desalination system of to-be-processed water can be provided.

It is explanatory drawing which shows the seawater desalination system which concerns on this invention. It is a graph which shows the influence which pH has on hypochlorous acid abundance ratio and hypobromite abundance ratio. It is explanatory drawing for demonstrating the washing | cleaning method of the filtration means of a seawater desalination system. It is explanatory drawing explaining the structure of the ceramic membrane used for the seawater desalination system shown in FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Here, an example of the desalination system of the present invention shown in FIG. 1 is a seawater desalination system 100 for producing seawater by desalinating seawater as water to be treated. Is sterilized and then filtered through a reverse osmosis membrane. In the desalination system of the present invention, fresh water may be produced using brine or the like as treated water.

  Here, the seawater desalination system 100 shown in FIG. 1 includes a water intake tower 10 installed on the seabed, a sterilizer 20 for sterilizing seawater taken from the sea SW via the water intake tower 10, and a seawater desalination system 100. A coagulation filtration device 40 that is provided on the downstream side of the sterilization device 20 with respect to the flow of seawater in the water and agglomerates the suspended substances in the seawater sterilized by the sterilization device 20 and separates and removes them from the seawater; And a reverse osmosis membrane filtration device 80 that is provided downstream of the agglomeration filtration device 40 and filters the seawater that has been sterilized and removed of suspended substances through a reverse osmosis membrane 81 to produce fresh water.

  The seawater desalination system 100 is provided with a filter (aperture 200 μm) 31 for removing contaminants from seawater between the sterilizer 20 and the coagulation filtration device 40. Further, a reducing agent addition means 51 for adding a reducing agent such as sodium bisulfite to seawater to remove residual chlorine present in the seawater between the flocculation filtration device 40 and the reverse osmosis membrane filtration device 80, In addition, a high-pressure pump 71 for pressurizing the water added with the reducing agent and supplying the water to the reverse osmosis membrane filtration device 80 is sequentially provided.

  The sterilizer 20 has a pH adjuster addition means 21 for adding a pH adjuster such as hydrochloric acid or sulfuric acid to seawater (pH = about 8) taken through the intake tower 10 and the flow of seawater. an oxidant addition means 22 for adding an oxidant such as chlorine, sodium hypochlorite or calcium hypochlorite to the pH-adjusted seawater provided downstream of the pH adjuster addition means 21; ing.

  Here, as the pH adjuster, it is preferable to use sulfuric acid that is inexpensive and easy to handle. In the sterilizer 20, the seawater flowing in the pipe or the seawater stored in the pH adjusting tank (not shown) is used. The pH of the seawater is adjusted to 5.5 to 7.5, preferably 6.0 to 7.0 by adding sulfuric acid with a chemical injection pump (not shown) as the pH adjusting agent adding means 21. The pH of seawater is adjusted by measuring the pH of seawater to which a pH adjuster has been added with a pH meter (not shown) and feedback-controlling the chemical injection pump using the pH value of the seawater measured with the pH meter. Can be performed.

  Moreover, sodium hypochlorite which is easy to handle can be used as the oxidizer, and in the sterilizer 20, seawater flowing in the pipe, or seawater stored in a pH adjustment tank or a sterilization tank (not shown). On the other hand, a sodium hypochlorite solution (concentration: 10% by mass) is added by a chemical injection pump (not shown) as the oxidant addition means 22 so that the effective chlorine concentration becomes 1.0 mg / L.

Here, in the sterilizer 20, since sodium hypochlorite is added under the condition that the pH is 5.5 to 7.5, as is clear from the chemical equilibrium formula (II) below, Most of the added sodium hypochlorite becomes hypochlorous acid (HClO), which has stronger sterilizing power than hypochlorite ions.
NaClO + H ⁺ ＨＣｌ HClO + Na ⁺ (II)

In addition, since about 65 mg / L of bromide ions are contained in seawater, hypochlorous acid formed by adding sodium hypochlorite to seawater, as shown in the following reaction formula (I), Hypobromite (HBrO) that can exhibit high bactericidal power even in a high pH range of about pH 9 is produced.
HClO + Br ⁻ → HBrO + Cl ⁻ (I)

  Therefore, in the sterilizer 20, since sodium hypochlorite is added after adjusting the pH of seawater to 5.5-7.5, seawater is effectively used using hypochlorous acid and hypochlorous acid. Can be sterilized.

In addition, it is because the dissociation constant of hypobromous acid has a small dissociation constant of hypobromite that hypobromite can exhibit a high bactericidal power even at a higher pH compared with hypochlorous acid. The relationship between the pH and the abundance ratio of hypochlorous acid (ratio of undissociated HClO: [HClO] / ([HClO] + [ClO ⁻ ])) and the abundance ratio of pH and hypobromite (ratio of undissociated HBrO) : [HBrO] / ([HBrO] + [BrO ⁻ ])).

  The flocculation filtration device 40 is provided on the downstream side of the flocculating agent addition means 41 with respect to the seawater sterilized by the sterilization device 20 and the flocculating agent addition means 41 for adding the flocculating agent to the flow of seawater. And a filtering means 42 as a separating means for separating the agglomerates formed by the addition of the flocculant from the seawater.

  Here, as the flocculant, it is preferable to use a substance capable of aggregating suspended substances contained in seawater, for example, ferric chloride. In the aggregation filtration device 40, seawater flowing in the pipe, or A ferric chloride solution is added to seawater stored in a coagulation tank (not shown) by a chemical injection pump (not shown) as the coagulant addition means 41 so that the iron concentration becomes 1 mg / L. In addition, from the viewpoint of effectively aggregating suspended substances, the pH of seawater when adding ferric chloride may be about 6.5. Therefore, in the seawater desalination system 100, the pH of the seawater may be optionally adjusted by the pH adjuster adding means 21 so that the pH of the seawater at the time of adding the flocculant becomes 6.5, or the addition of the flocculant The pH of the seawater may be adjusted again immediately before.

  As the filtration means 42, a membrane filtration device equipped with an ultrafiltration membrane or a microfiltration membrane, a sand filtration device or the like can be used, but it has excellent mechanical strength and chemical resistance, and can be filtered at a high flux. It is preferable to use a filtration device provided with a simple ceramic membrane (pore diameter 0.1 to 10 μm).

  And in the coagulation filtration apparatus 40, the suspended substance in seawater which causes the clogging of the reverse osmosis membrane 81 provided in the downstream is isolate | separated from seawater. In the seawater desalination system 100, since the seawater is effectively sterilized by the sterilization apparatus 20, the filtration means 42 of the coagulation filtration apparatus 40 is unlikely to be clogged with living organisms.

  The reverse osmosis membrane filtration device 80 uses a known reverse osmosis membrane 81, and the seawater is adjusted to the pH arbitrarily, and then the reverse osmosis membrane filtration device 80 uses fresh water that has passed through the reverse osmosis membrane 81. The concentrated water that has not permeated the reverse osmosis membrane 81 is separated. And the fresh water which permeate | transmitted the reverse osmosis membrane 81 is used as drinking water etc., after performing pH adjustment by NaOH and the sterilization process by chlorine arbitrarily. On the other hand, the high-pressure concentrated water that has not permeated the reverse osmosis membrane 81 is treated as waste water after energy is recovered by the power recovery turbine 72. In the seawater desalination system 100, since the seawater is effectively sterilized by the sterilizer 20, the reverse osmosis membrane 81 is unlikely to be clogged with living organisms.

  And according to such a seawater desalination system 100, seawater can be desalinated using a reverse osmosis membrane, reducing the quantity of the oxidizing agent used for disinfection of the seawater which is to-be-processed water. In this seawater desalination system 100, since the oxidizing agent is added immediately after the seawater is taken, the seawater is sufficiently sterilized while flowing through the seawater desalination system 100. Therefore, algae and the like hardly propagate in the seawater desalination system 100. In this seawater desalination system 100, the adjustment of pH and the addition of an oxidizing agent were performed immediately after the intake of seawater. However, in the desalination system of the present invention, the adjustment of the pH and the addition of an oxidizing agent were performed using a reverse osmosis membrane filtration device. It can be performed at an arbitrary position on the upstream side.

  Here, in the seawater desalination system 100, sodium hypochlorite is added as an oxidizing agent after adjusting the pH, and the seawater is effectively sterilized, so clogging of the filtering means 42 is unlikely to occur. In order to prevent or eliminate clogging, the means 42 preferably performs cleaning such as backflow cleaning (hereinafter simply referred to as “cleaning”) periodically or when the differential pressure increases.

  Therefore, an example of a cleaning method in the case of using a ceramic membrane as the filtering means of the seawater desalination system will be described below with reference to FIG. As the ceramic film, a monolithic ceramic film 91 having a lotus root-like hole 97 into which water flows as shown in FIG. In the ceramic membrane 91, the water that flows into the holes 97 is filtered and flows out from the outer surface 98.

  As shown in FIG. 3, in this seawater desalination system, the ceramic membrane 91 is accommodated in a container 90, and the ceramic membrane 91 and the container 90 constitute a ceramic membrane module. In addition, when manufacturing fresh water from seawater using the seawater desalination system provided with this ceramic membrane module, first, seawater that has been sterilized and agglomerated is converted into a lotus-like hole in the ceramic membrane 91 by a pump 92. The water is sent and the seawater is filtered in a dead end format or a cross flow format, and the obtained filtrate is stored in the ceramic membrane filtration water tank 93. Then, filtered water (seawater) stored in the ceramic membrane filtration water tank 93 is supplied to the reverse osmosis membrane filtration device 80 using the high-pressure pump 71, filtered through the reverse osmosis membrane 81, and permeated through the reverse osmosis membrane 81. The fresh water and the concentrated water that has not permeated the reverse osmosis membrane 81 are separated. The obtained fresh water is stored in the treated water tank 82, and the concentrated water is discarded.

  The ceramic film 91 can be cleaned as follows, for example. First, regardless of the occurrence of clogging of the ceramic membrane 91 or an increase in the membrane differential pressure, as a cleaning method for periodically cleaning the ceramic membrane 91, (1) filtration stored in the ceramic membrane filtration water tank 93 is performed. Water (seawater) is supplied into the container 90 by the backwash pump 96, and filtered water (in the direction from the outer surface 98 side of the ceramic membrane 91 toward the lotus root hole 97) in the direction opposite to that during normal filtration ( (2) The filtered water (seawater) stored in the ceramic membrane filtration water tank 93 is sent to the backwash water pressurization tank 95 by the transfer pump 94, and the backwash water pressurization tank 95 is supplied. For example, a method of supplying high-pressure (300 kPa) filtered water (seawater) in a direction opposite to that during normal filtration by supplying pressurized air to the water and pressurizing the filtered water (seawater) can be considered. In addition, the water which flowed into the lotus root hole 97 of the ceramic membrane at the time of cleaning is alternately discharged from only one side of the ceramic membrane 91 using a valve (not shown) (in order of right and left in FIG. 3). Alternatively, it may be discharged from both of the ceramic films 91 at the same time.

  Here, about 0.1 to 1.0 mg / L of an oxidizing agent such as sodium hypochlorite used when sterilizing seawater remains in the filtered water used when cleaning the ceramic membrane 91. preferable. This is because the cleaning effect of the ceramic membrane is improved when sodium hypochlorite remains in the filtered water. In the cleaning methods (1) and (2) above, cleaning is performed using filtered water (seawater) obtained by filtering seawater with the ceramic membrane 91. However, the ceramic membrane 91 is separately cleaned. Further, fresh water such as tap water and industrial water is filtered through the ceramic membrane 91 using the pump 92 and stored in the ceramic membrane filtered water tank 93, and the obtained filtered water (fresh water) may be used. . In this way, since the pipe is replaced with fresh water during the filtration of fresh water, especially when performing immersion cleaning of the ceramic membrane 91 in the chemical during cleaning, compared with the case where seawater is used. It is possible to prevent the pipe from being corroded during cleaning.

  As a cleaning method for cleaning the ceramic film 91 when the film differential pressure of the ceramic film 91 rises, water obtained by adding NaClO or HCl to fresh water stored in the treated water tank 82 is used as a backwash pump. There is a method in which water is passed through the ceramic membrane 91 in the direction opposite to that during normal filtration using 96, and then the ceramic membrane 91 is immersed for a predetermined time. In this way, compared with the case where the chemical is added to the seawater filtered by the ceramic membrane 91 and used for cleaning, the cleaning effect by the addition of the chemical can be enhanced, and the carbonate of the chemical by the addition of the chemical can be increased. Scaling can be suppressed. In addition, since the ceramic film | membrane 91 is excellent in chemical resistance, the effective chlorine density | concentration at the time of washing | cleaning can be 50 mg / L-1000 mg / L, for example, and pH can be 1-13, for example.

  Washing when the membrane differential pressure of the ceramic membrane 91 is increased may be performed using filtered seawater. Chemicals added to the water used for washing include chlorine, sulfuric acid, sodium hydroxide, excess Hydrogen oxide or the like can also be used. In addition, the cleaning performed by adding a chemical can be performed not only when the film differential pressure increases but also periodically. Thus, if it wash | cleans regularly, it can filter by the ceramic membrane 91 with a high flux.

  The ceramic film cleaning method as described above can be performed by opening and closing a valve (not shown) provided at an appropriate position to form a water flow path used for cleaning. This cleaning method can also be used as a cleaning method in the case where a ceramic membrane is used for a seawater filtration device for cleaning fish and shellfish installed in a fish landing site.

  ADVANTAGE OF THE INVENTION According to this invention, in the desalination process which sterilizes to-be-processed water and filters with a reverse osmosis membrane and obtains fresh water, the quantity of the oxidizing agent used for the sterilization of to-be-processed water can be reduced.

DESCRIPTION OF SYMBOLS 10 Water intake tower 20 Sterilizer 21 pH adjuster addition means 22 Oxidant addition means 31 Filter 40 Coagulation filtration apparatus 41 Coagulant addition means 42 Filtration means 51 Reducing agent addition means 71 High pressure pump 72 Power recovery turbine 80 Reverse osmosis membrane filtration apparatus 81 Reverse osmosis membrane 82 Treated water tank 90 Container 91 Ceramic membrane 92 Pump 93 Ceramic membrane filtered water tank 94 Transfer pump 95 Backwash water pressurizing tank 96 Backwash pump 97 Hole 98 Outer surface 100 Seawater desalination system

Claims (6)

A desalination method of water to be treated comprising a sterilization step of sterilizing the water to be treated by adding an oxidizing agent to the water to be treated and a reverse osmosis membrane filtration step of filtering the water to be treated after the sterilization step with a reverse osmosis membrane Because
A desalination method for water to be treated, further comprising a pH adjustment step of adjusting the pH of the water to be treated by adding a pH adjuster to the water to be treated before the sterilization step. A coagulation step of adding a flocculant to the water to be treated to aggregate suspended substances in the water to be treated;
The aggregate separation step of separating the aggregate formed in the aggregation step from the water to be treated is further included after the sterilization step and before the reverse osmosis membrane filtration step. 2. The desalination method according to 1.   The desalination method according to claim 1 or 2, wherein the pH of the water to be treated is adjusted to 6.0 to 7.5 in the pH adjusting step.   The desalination method according to claim 1, wherein the water to be treated contains bromide ions. An oxidant addition means for adding an oxidant to the water to be treated and a downstream side of the oxidant addition means as viewed in the direction of treating the water to be treated from the upstream side to the downstream side. A desalination system for treated water comprising reverse osmosis membrane filtration means for desalination,
It is characterized by further comprising a pH adjuster addition means for adding a pH adjuster to the treated water on the upstream side of the oxidant addition means when viewed in the direction of treating the treated water from the upstream side to the downstream side. A desalination system for treated water.   The desalination system includes a flocculant addition means for adding a flocculant to the water to be treated and an agglomerate formed by the addition of the flocculant between the oxidant addition means and the reverse osmosis membrane filtration means. The desalination system according to claim 5, further comprising separation means for separating from the treated water.

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