JP5374497B2 - Pretreatment device and pretreatment method for high hardness raw water membrane filtration - Google Patents

Description

  The present invention relates to a pretreatment device provided in a preceding stage of a reverse osmosis membrane or the like and a pretreatment method using the device when high hardness raw water is subjected to membrane filtration with a reverse osmosis membrane or the like.

  Conventionally, a membrane separation method using a reverse osmosis membrane is known as a general method for separating water molecules from water having high hardness (high hardness raw water) such as seawater and brine. Here, the reverse osmosis membrane is a membrane that selectively permeates only water molecules in the water to be treated and does not permeate impurities other than water such as ions and salts. Therefore, according to the membrane separation method, the water to be treated is separated into permeated water that has permeated through the reverse osmosis membrane and concentrated water that has not permeated through the reverse osmosis membrane.

  However, when water molecules are separated from high-hardness raw water using the membrane separation method, calcium, magnesium, etc. contained in the high-hardness raw water that is the water to be treated are concentrated in the concentrated water. There is a problem that the salt and the magnesium salt are deposited, the scale adheres to the reverse osmosis membrane, and the processing efficiency decreases.

  As a method for preventing trouble due to scale adhesion to such a reverse osmosis membrane, a method of previously removing calcium and magnesium in the water to be treated is known. Specifically, as a method for removing calcium and magnesium, an ion exchange method using a cation exchange resin, a precipitation method in which an alkali agent is injected to precipitate calcium and magnesium, and an alkali in the presence of seed crystals. There is known a crystallization method in which an agent is injected to precipitate calcium and magnesium on a seed crystal. Among these, the ion exchange method has a problem that it is expensive and difficult to adopt.

The precipitation method is also referred to as an alkali coagulation method. In the alkali coagulation method, calcium and magnesium dissolved in the water to be treated are increased by injecting an alkali agent to increase the pH of the water to be treated. Precipitate as a sparingly soluble hydroxide or carbonate. Here, in the alkali coagulation method in which calcium hydroxide (Ca (OH) ₂ ), sodium carbonate (Na ₂ CO ₃ ), caustic soda (NaOH), or the like is added as an alkaline agent, the pH of the water to be treated is alkaline. It fluctuates according to the injection amount of the agent. Further, the concentration ratio between hydrogen carbonate ions (HCO ₃ ⁻ ) and carbonate ions (CO ₃ ^{2 −} ) in the water to be treated is determined based on the equilibrium equation shown in (I) below. And in the said alkali coagulation method, calcium ion (Ca ^<2+> ) in to-be-processed water is shown to Formula (II)-(III) and (IV) according to the pH of to-be-processed water, and the presence form of carbonic acid. In this way, a carbonate is formed and precipitated as hardly soluble calcium carbonate (CaCO ₃ ) and removed. Similarly, magnesium is precipitated and removed as hardly soluble magnesium carbonate (MgCO ₃ ).
^{_{HCO 3 - ⇔H + + CO 3}} 2- ····· (I)
Ca ²⁺ + 2HCO ₃ ⁻ ⇔Ca (HCO ₃ ) ₂ (II)
Ca (HCO ₃ ) ₂ + Ca (OH) ₂ ⇔2CaCO ₃ ↓ + 2H ₂ O (III)
Ca ²⁺ + CO ₃ ²⁻ ⇔CaCO ₃ ↓ (IV)

Theoretically, in order to completely precipitate calcium and magnesium in the water to be treated by the above reaction, calcium ions and equimolar carbonate ions in the water to be treated are required. However, the pH dependence of the abundance ratio of hydrogen carbonate ions (HCO ₃ ⁻ ) relative to the total alkali (CO ₂ + HCO ₃ ⁻ + CO ₃ ²⁻ ) in the aqueous solution is shown in FIG. The abundance ratio (molar ratio) between the hydrogen carbonate ions depends on the pH. Therefore, for example, when the water to be treated is seawater, the abundance ratio of carbonate ions, that is, the ratio of the carbonate ion amount to the total alkali amount is about 0.5% in seawater having a pH of about 8.0, and most of them It exists as bicarbonate ions. On the other hand, in order to effectively remove calcium ions and magnesium ions in the water to be treated by the above reaction, the abundance ratio of carbonate ions in the water to be treated needs to be about 50%. Therefore, in order to effectively remove calcium ions and magnesium ions by setting the abundance ratio of carbonate ions in seawater to about 50%, it is necessary to adjust the pH of seawater to 10 or more. However, as shown in FIG. 2, seawater exhibits a buffering effect near pH 10.0. Therefore, in order to adjust the pH of seawater to 10 or more, it is necessary to add a large amount of an alkaline agent such as caustic soda, which is very expensive. That is, by adjusting the pH to 10 or more, a means for precipitating calcium ions and magnesium ions in seawater as calcium carbonate and magnesium carbonate and suppressing scale formation in the subsequent reverse osmosis membrane is not realistic.

  In the alkali coagulation method, when an alkali agent is added to the water to be treated, calcium carbonate and magnesium carbonate are immediately precipitated by the reaction, but the precipitated calcium carbonate and magnesium carbonate have small particle sizes and are not easily precipitated. Therefore, the precipitated calcium carbonate and magnesium carbonate are mixed as suspended fine particles in the treated water, which may cause clogging of the reverse osmosis membrane. Furthermore, calcium carbonate and magnesium carbonate are liable to be fixed once precipitated, causing a problem of scale adhesion to the entire membrane separation apparatus.

  On the other hand, in Japanese Patent Laid-Open No. 2000-24673, core particles (pellets) obtained by adding core particles to be crystallized nuclei in the water to be treated, crystallizing and laminating calcium carbonate on the core particles, and growing and enlarging them. The problem is solved by removing suspended fine particles mixed in the treated water after crystallization using a coagulating device and a sand filtration device separately provided.

  However, in the above-described conventional technique in which core particles that serve as crystallization nuclei are added to the water to be treated, two apparatuses, a crystallization reaction apparatus and an agglomeration reaction apparatus, are required, and the treatment process becomes complicated. There was a problem that the apparatus became large. Further, in the above prior art, when there is a water treatment process using a microfiltration membrane or an ultrafiltration membrane after the pretreatment step, particles and precipitates must be removed from the treated water in the pretreatment step in advance. Further, there is a problem that the subsequent microfiltration membrane or ultrafiltration membrane may cause membrane clogging, and there is a problem that cleaning operation at the time of membrane clogging is difficult.

  An object of the present invention is to provide a pretreatment device for high-hardness raw water membrane filtration that can effectively suppress scale formation of a reverse osmosis membrane in the subsequent stage, has a low running cost, and has a simple configuration. Moreover, it is providing the pre-processing method of the high hardness raw | natural water membrane filtration using the apparatus.

The pretreatment device for high hardness raw water membrane filtration of the present invention is a pretreatment device for high hardness raw water membrane filtration in which a crystallization reaction tank having a two-stage structure is arranged in the previous stage of the high hardness raw water membrane filtration device, The crystallization reaction tank composed of this two-stage structure is connected to the first tank into which high-hardness raw water having a total hardness concentration of 300 mg / L or more flows, and the water in the tank of the first tank is at the lower end. A second tank that is injected from above and pushed upward by an upward flow, and the first tank is provided with an alkali injection means to serve as an alkali contact tank, and a flocculant injection means is provided above the second tank. And a stirrer, and a lower part of the second tank is provided with an accumulation part of sediment that settles from the upper part and a mud discharge means. Here, in this pretreatment apparatus, in the first tank of the crystallization reaction tank, for example, high-hardness raw water and an alkaline agent are brought into contact with each other under a flowing stirring condition, and viscous cloudy substances (calcium carbonate, magnesium carbonate, etc.) are generated. Precipitate. This cloudy substance is led to the second tank as agglomerated nuclei, and in the second tank, flock formation by agglutination reaction and crystallization reaction using the flock as a crystallization nucleus occur. Therefore, this pretreatment apparatus does not require a dedicated apparatus for agglomeration, crystallization and precipitation. That is, the process is simplified and the pretreatment device for high hardness raw water membrane filtration can be downsized.

  Here, the pretreatment device for high hardness raw water membrane filtration of the present invention includes a partition plate provided with a plurality of nozzles protruding into the second tank between the first tank and the second tank. Is preferred. It occurs immediately after injecting the alkaline agent into the high-hardness raw water in the first tank by providing a partition plate having a plurality of nozzles protruding into the second tank between the first tank and the second tank. This is because the cloudy substance can be maintained in a gel state and can be introduced into the second tank while efficiently floating.

The pretreatment method for high hardness raw water membrane filtration of the present invention is a pretreatment method for high hardness raw water membrane filtration using the above-described apparatus, and has a pH of 8 or more and a total hardness concentration of 300 mg / L. (CaCO ₃ equivalent concentration) or higher hardness raw water is introduced into the first tank of the crystallization reaction tank and subjected to alkali contact by flow stirring, and the raw water subjected to alkali contact in the first tank is the first tank. A step of introducing from the lower part of the two tanks to carry out the agglomeration reaction at the upper part of the second tank, and aggregating the agglomerates generated in the agglomeration reaction as crystallization core particles, And a step of discharging the sediment precipitated in the accumulation part to the outside of the second tank. Thus, high hardness raw water having a pH of 8 or higher and a total hardness concentration of 300 mg / L (CaCO ₃ equivalent concentration) or higher is introduced into the first tank of the crystallization reaction tank, By bringing it into contact with an alkaline agent, a viscous cloudy substance (calcium carbonate, magnesium carbonate, etc.) can be precipitated. Then, the water to be treated in which the white turbid substance is suspended is introduced from the lower part of the second tank, and pushed upward to the upper part of the second tank, and the flocculant is added from the upper part to add organic matter in the treated water. Can also be agglomerated. Here, the aggregates enlarged by the addition of the flocculant settle down toward the lower part of the second tank, and a crystallization reaction occurs using the aggregates as crystallization nuclei in the process. As a result, it is possible to reduce organic substances by coagulation precipitation and to remove calcium and magnesium by crystallization (that is, to reduce scale formation). Further, according to the present invention, when the water to be treated is seawater, the scale formation of the reverse osmosis membrane is effective without using a means of adjusting the pH to a high alkali of 10 or higher to increase the carbonate ion abundance ratio. Can be prevented. In addition, water flow stirring refers to stirring using the flow (water flow) itself of water flowing into the apparatus, not mechanical stirring using a stirrer or the like. Specifically, by setting the cross-sectional area of the tank with respect to the design inflow of raw water and alkaline agent so that the Reynolds number, which is an indicator of hydraulic disturbance in the tank, is 10,000 or more, Can be realized. If it does in this way, the pre-processing method which can perform the pre-processing of high hardness raw | natural water membrane filtration with the apparatus of a simple structure which does not use a stirring apparatus can be provided.

It is a graph which shows the relationship between the carbonate ion abundance ratio in aqueous solution, and pH. It is a graph which shows pH change at the time of adding an alkaline agent to seawater. It is explanatory drawing of the 1st Embodiment of this invention. It is explanatory drawing of the 2nd Embodiment of this invention.

(Embodiment 1)
FIG. 3 shows a first embodiment of the pretreatment apparatus and the pretreatment method of the present invention. The pretreatment device for high hardness raw water membrane filtration according to the present invention is a high hardness raw water in which a column-type crystallization reaction tank 3 having a two-stage structure is arranged in front of a filtration membrane 12 made of a microfiltration membrane or an ultrafiltration membrane. A pretreatment device for membrane filtration, the crystallization reaction tank 3 having a two-stage structure includes a first tank 1 into which high-hardness raw water flows and water in the tank of the first tank 1 connected to the upper part thereof. The first tank 1 is provided with an alkali injection means 5 to serve as an alkali contact tank, and a flocculant injection means 9 is provided above the second tank 2. And a stirrer 10, and a sediment collecting portion and a mud discharging means 11 that are sedimented from the upper portion are provided in the lower part of the second tank 2. And the reverse osmosis membrane which is not illustrated is provided in the back | latter stage of this pretreatment apparatus, and the treated water which flows out from a pretreatment apparatus is processed with a reverse osmosis membrane.

A raw water inlet 4 is formed at the lowermost part of the crystallization reaction tank 3, from which high hardness raw water having a total hardness concentration of 300 mg / L (CaCO ₃ equivalent concentration) or more flows. Here, in the present specification, high-hardness raw water refers to water having a total hardness concentration of 300 mg / L (CaCO ₃ equivalent concentration) or more, and the total hardness concentration is determined by using ICP emission spectroscopy in accordance with JIS K0101. The total amount of calcium and magnesium in the water can be calculated. Therefore, for example, seawater, brine, etc. correspond to high hardness raw water.

  The first tank 1 of the crystallization reaction tank 3 is provided with alkali injection means 5 comprising an alkali agent injection pump and an alkali agent injection port (not shown). Then, an alkali agent is injected into the first tank 1 using the alkali injection means 5. For example, when seawater is used as the raw water, the pH of the seawater is around 8.0, but NaOH is injected from the alkali injection means 5 of the first tank 1 so that the pH of the seawater is about 9.5 to 9.8. adjust. Here, the alkaline agent is not particularly limited to NaOH as long as the pH of the raw water is adjusted to about 9.5 to 9.8. When seawater is guided to the first tank 1 as raw water (treated water) and an alkaline agent is injected therein, a gel-like cloudy substance is generated. This cloudy substance is a carbonate mainly composed of calcium and magnesium, and has a high viscosity.

  The first tank 1 of the crystallization reaction tank 3 is a water flow stirring tank, and the raw water flowing into the first tank 1 and the alkaline agent injected from the alkali injection means 5 are brought into gentle contact with each other by the water flow. . In addition, a water flow stirring tank can be made into the tank of a shape and a magnitude | size which the inflow raw | natural water flows with a turbulent flow, for example.

  Here, in general, exopolysaccharides (hereinafter referred to as EPS) derived from plankton metabolites present in seawater and brine are highly sticky and cause biofouling of membranes. It is. The biofouling-causing substance is a highly sticky substance that adheres to the surface of the filtration membrane 12 provided at the rear stage of the crystallization reaction tank 3 and causes biofouling. The long-term continuous operation of the pre-treatment device becomes impossible. For this reason, it is calculated | required to remove these from to-be-processed water and to suppress biofouling. On the other hand, in this invention, it is thought that the said biofouling causative substance in raw | natural water is also adsorbed and removed by making the said cloudy substance float by water flow stirring. Therefore, according to the present invention, not only calcium and magnesium but also biofouling-causing substances can be effectively removed from the water to be treated.

  The treated water that has been subjected to alkali treatment in the first tank 1 and has become cloudy is introduced into the second tank 2 from the inlet 7 of the partition plate 6 provided between the first tank 1 and the second tank 2. In order to prevent the precipitate generated in the second layer 2 from flowing back to the first tank, that is, to efficiently float the precipitate, the partition plate 6 preferably has a structure including a plurality of protruding nozzles 8. Here, the partition plate 6 can be manufactured, for example, by providing a plurality of holes in the plate and joining the short tube to the holes to form the introduction port 7. And in the crystallization reaction tank 3, the part on this partition plate 6 becomes an accumulation part.

  The second tank 2 is an upward flow tank in which an upward flow is formed in the tank, and the treated water introduced from the first tank 1 is pushed out to the upper part of the second tank 2 by the upward flow. A flocculant injection means 9 and a stirrer 10 are provided in the upper part of the second tank 2. The cloudy substance generated by the alkali contact in the first tank 1 is coagulated by the upward flow of the second tank 2. These agglomerates are those in which calcium carbonate or magnesium carbonate is mixed as suspended fine particles in the water to be treated, and have a small particle size and are difficult to precipitate. This coagulated product causes clogging of the subsequent microfiltration membrane and ultrafiltration membrane, and thus needs to be removed. In the second tank 2, the suspended fine particles serve as agglomeration nuclei and react with the aggregating agent injected from the injection means 9 to form a floc, but the water to be treated is subjected to alkali treatment in the first tank 1. Since it has an alkaline property of pH 9.5 to 9.8, it is necessary to select an aggregating agent that exhibits an aggregating effect even under alkaline conditions. For example, it is preferable to use ferric chloride as the flocculant of the present invention.

  In addition, a stirrer 10 is also provided in the upper part of the second tank 2, so that the aggregation reaction is efficiently promoted. In the floc formation process by the agglomeration reaction, scale components, SS (floating substances), etc. in the water to be treated are taken into the floc, and the enlarged floc counteracts the upward flow in the second tank 2 to be second It sinks to the bottom of the tank 2.

  Since the water to be treated in the second tank 2 is alkaline with a pH of 9.0 to 9.8 as described above, the flocs themselves crystallize in the process of the flocs sinking to the lower part of the second tank 2. The crystallization reaction proceeds as a nucleus. For this reason, according to the present invention, calcium and magnesium contained in the water to be treated in the second tank 2 can be crystallized without separately adding core particles for promoting the crystallization reaction. It is possible to avoid the problem of film clogging by fine particles, the problem of enlargement of the crystallizer and complicated processes.

  In addition, as shown in FIG. 1, since the carbonate ion abundance ratio in the aqueous solution depends on the pH, it is necessary to increase the pH to 10 or more in order to completely precipitate calcium and magnesium contained in the water to be treated. become. However, as shown in FIG. 2, when the water to be treated is seawater, an equilibrium point exists near pH 10, so that a large amount of an alkaline agent is required to achieve a high pH, from an economic viewpoint. It is not preferable. On the other hand, according to the present invention, the entire structure does not increase the pH to 10 or more, and the water to be treated and the flock are in the pH region where the concentration of bicarbonate ions is large under the equilibrium reaction of carbonate ions and bicarbonate ions. Is effectively brought into solution equilibrium with calcium and magnesium in the water to be treated, and scaling formation can be reduced.

  In addition, the aggregate which settled in the lower part of the 2nd tank 2 can be discharged | emitted intermittently or continuously from the mud outlet as the mud means 11 provided in the lower part of the 2nd tank 2. FIG. Specifically, it can be appropriately discharged by intermittently opening or closing a gate valve provided on the rear side of the mud outlet or continuously opening it. In addition, waste mud may be performed using a water head difference, and may be performed using a drainage pump. The treated water from which the scale component has been removed is discharged from the upper part of the second tank 2. And since this process water has the density | concentration of the scale component reduced, the frequency which a scale generate | occur | produces between the crystallization reaction tank 3 and the filtration membrane 12 is reduced.

(Embodiment 2)
In the pretreatment step performed in the two-stage crystallization reaction tank constituting the present invention, it is not intended to completely remove scale components such as calcium ions in the raw water, and the scale components do not precipitate (resulting in supersaturation). It is also intended to effectively remove a causative substance of biofouling that has not been removed by a conventional pretreatment apparatus. Therefore, as shown in FIG. 4, as a second embodiment of the present invention, pretreatment for the step of further removing scale components such as calcium ions contained in the treated water discharged from the upper part of the second tank 2. It is also possible to provide a device.

  In the second embodiment of the present invention shown in FIG. 4, a flocculation / mixing tank 13 is provided at the subsequent stage of the crystallization reaction tank 3 used in the first embodiment, and a ceramic filter membrane 14 is further provided at the subsequent stage. Thus, pretreatment of high-hardness raw water is performed.

  In the second embodiment, the pH of the raw water is adjusted to 9.0 or higher, and a part of calcium and magnesium in the raw water and biofouling-causing substances are removed in the crystallization reaction tank 3. In the agglomeration mixing tank 13, a flocculant exhibiting an agglomeration effect in an environment having a pH of 9.0 or more, for example, ferric chloride is added, and then filtered through the ceramic membrane 14, so that the amount of calcium in the raw water is reduced. About 10% of calcium is further removed from the pretreated water.

In the second embodiment, since the biofouling substance is removed in the crystallization reaction tank 3 in the front stage of the agglomeration and mixing tank 13, biofouling in the ceramic film 14 in the rear stage is suppressed, and continuous operation for a long time is possible. It becomes. Further, by using a ceramic membrane 14 having high strength, acid resistance and alkali resistance properties as a filtration membrane, filtration with a high flux of 6 m ³ / m ² / day or more is possible. Furthermore, since the ceramic membrane can be washed with a strong acid at a pH of 2 or less, continuous operation for a long time using the strong acid washing is possible.

[Example]
As a pre-treatment device for reverse osmosis membrane for removing calcium ions from seawater, the pre-treatment device of FIG. 3 was used, and the pre-treatment method according to the present invention was used for pre-treatment under the following conditions. Even when performed continuously, no problems such as increased membrane pressure or breakage in the reverse osmosis membrane were observed.
(Operating conditions)
To-be-treated water 10m < ³ > / day Alkaline agent injection volume NaOH 50-100mg / L
Addition amount of flocculant 1-6 mg / L of FeCl ₃
Permeated water volume 9m ³ / day or more

[Comparative example]
On the other hand, as a pretreatment device for reverse osmosis membrane, instead of the crystallization reaction tank of the present invention, a conventional precipitation apparatus using an alkali agent (a first tank comprising an alkali agent injection means and a stirrer, a flocculant injection means, In the case of using a second tank equipped with a stirrer and a device in which a membrane filtration device is sequentially connected), the microfiltration membrane has an increase in membrane differential pressure reaching 100 KPa in 14 days, Membrane cleaning was also difficult.

Claims (3)

A pretreatment device for high-hardness raw water membrane filtration in which a crystallization reaction tank having a two-stage structure is arranged in front of the membrane filtration device for high-hardness raw water,
The crystallization reaction tank having the two-stage structure is connected to the first tank into which the high hardness raw water having a total hardness concentration of 300 mg / L or more flows, and the water in the tank of the first tank is the bottom surface. A second tank of the upward flow type injected from
The first tank is provided with an alkali injection means to form an alkali contact tank,
The upper part of the second tank is provided with a flocculant injection means and a stirrer, and the lower part of the second tank is provided with an accumulation part of sediment that settles from the upper part and a drainage means. A pre-treatment device for high hardness raw water membrane filtration.   The partition for a high hardness raw water membrane according to claim 1, further comprising a partition plate provided with a plurality of nozzles protruding into the second tank between the first tank and the second tank. Processing equipment. A pretreatment method for high-hardness raw water membrane filtration using the apparatus according to claim 1 or 2 ,
a step of introducing high hardness raw water having a pH of 8 or more and a total hardness concentration of 300 mg / L or more into the first tank of the crystallization reaction tank and performing alkali contact by water stirring;
A step of introducing raw water that has been subjected to alkali contact in the first tank from the lower part of the second tank and performing an agglomeration reaction in the upper part of the second tank;
A crystallization step in which the agglomerate produced by the agglomeration reaction is allowed to proceed as a crystallization core particle and the crystallization reaction is allowed to settle at the bottom of the second tank;
Draining the sediment precipitated in the accumulation part to the outside of the second tank;
A pretreatment method for high-hardness raw water membrane filtration, comprising:

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