JP2017136570A - Water treatment method, and water treatment system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water treatment method and a water treatment system for removing calcium in raw water and efficiently obtaining reusable treated water.SOLUTION: A water treatment method includes a reaction step for obtaining a reaction liquid containing calcium carbonate and not containing a coagulant from raw water, a membrane separation step for separating the reaction liquid into a permeate and a concentrated liquid by a membrane filter 15a, and a concentrated-liquid discharging step for discharging a part of the concentrated liquid to the outside. In the reaction step, at least a part of the concentrated liquid obtained in the membrane separation step is recycled and mixed with the reaction liquid, and the reaction liquid is alkalinized. The minimum particle diameter of the calcium carbonate in the reaction liquid is controlled to be larger than the average pore diameter of the membrane filter 15a.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a water treatment method and a water treatment system for removing treated calcium from raw water containing calcium such as cooling tower blow waste water and obtaining treated water that can be reused as cooling tower cooling water, boiler replenishing water, and the like.

  Conventionally, a cooling tower has been used as a device for cooling cooling water used in building air conditioning and air conditioning equipment. Cooling Tower Blowdown (hereinafter referred to as “CTB”) wastewater discharged from this cooling tower contains a high concentration of calcium contained in the cooling water, so cooling the cooling tower again In order to use as water or boiler make-up water, it is necessary to remove calcium contained in the CTB waste water.

  Therefore, as a method of obtaining treated water by removing calcium contained in CTB wastewater, a method of adding a flocculant and filtering with an MF membrane or a UF membrane is known (see Patent Literature 1 and Patent Literature 2).

Japanese Patent Laid-Open No. 10-5563 JP 2003-300069 A

  However, the methods described in Patent Document 1 and Patent Document 2 require a large amount of a flocculant and a coagulant, which not only has a problem of chemical cost, but also increases the amount of sludge generated by these chemicals. Since these sludges are usually landfilled as industrial waste, they are not only an environmental burden, but also cost disposal.

  In Patent Document 1, studies are made to suppress a decrease in flux (permeated water amount per unit area) of the UF membrane or MF membrane by limiting the concentration of the concentrate and the type of the flocculant. However, the flocculant itself also increases the viscosity of the concentrate when it is concentrated and adheres to the membrane surface cake layer, causing so-called fouling (membrane clogging), and fundamentally lowering the flux. It cannot be prevented. As a result, in order to improve the reduced flux, it is necessary to frequently wash or replace the membrane with chemicals, and the operating rate of the equipment is low, so that the efficiency is low and the cost of chemicals and membranes is high.

  This invention is made | formed in view of the said situation, and it aims at providing the water treatment method and water treatment system for removing the calcium in raw | natural water and obtaining the treated water which can be reused efficiently. To do.

  The inventors have studied these problems earnestly, and obtained by supplying raw water containing calcium to the reaction process and supplying an alkaline substance to the raw water in the reaction process without supplying a flocculant. It was found that the reaction solution containing calcium carbonate was separated into a permeate and a concentrated solution by membrane separation, and at least a part of the concentrated solution was circulated and mixed with the reaction solution to efficiently remove calcium.

  That is, the water treatment method according to the present invention is a water treatment method for removing calcium from raw water, a reaction step for obtaining a reaction solution containing calcium carbonate and not containing a flocculant from raw water, and allowing the reaction solution to pass through a filtration membrane. A membrane separation step for separating the liquid into a concentrate and a concentrate discharge step for discharging a part of the concentrate out of the system. In the reaction step, at least a part of the concentrate obtained in the membrane separation step Is mixed with the reaction solution, the reaction solution is adjusted to be alkaline, and the minimum particle size of calcium carbonate in the reaction solution is controlled to be larger than the average pore size of the filtration membrane. The phrase “not containing the flocculant” includes not only the case where the content of the flocculant is completely zero, but also the case where the flocculant contains a small amount such that the effect of the flocculant is not substantially recognized.

  As a result of intensive studies, the inventors have paid attention to the size of solid matter (calcium carbonate) in the liquid to be filtered and the pore size of the filtration membrane, and without using a flocculant, the size of the solid matter> the filtration membrane. It was found that an ideal membrane filtration state in which fouling does not occur fundamentally can be maintained if the “pore diameter” is used. And earnestly examined about the conditions for maintaining this ideal membrane filtration state.

  That is, in the water treatment method according to the present invention, at least a part of the concentrate obtained in the membrane separation step is circulated and mixed with the reaction solution in the reaction step. Thereby, in the reaction step, the calcium carbonate particles in the concentrated solution become seed crystals and a crystallization effect is expressed, and calcium carbonate in the reaction solution is precipitated on the outer surface of the seed crystals, and the calcium carbonate in the reaction solution The particle size of the becomes larger. In the water treatment method according to the present invention, this crystallization effect is utilized, and the minimum particle diameter of calcium carbonate in the reaction solution is controlled to be larger than the average pore diameter of the filtration membrane. As a result, it is possible to fundamentally prevent a decrease in flux due to pore blockage, and it is possible to stably maintain a high flux. Moreover, in the water treatment method according to the present invention, the flux is prevented from being lowered because no flocculant is used. As a result, according to the water treatment method of the present invention, it is possible to omit frequent membrane cleaning and membrane replacement required due to a decrease in flux, and to efficiently obtain reusable treated water.

  In the reaction step of the water treatment method according to the present invention, at least a part of the concentrated liquid is circulated and mixed with the reaction liquid so that the calcium carbonate concentration in the reaction liquid is within a predetermined range, and the concentrated liquid is discharged. In the step, it is preferable that a part of the concentrate is discharged out of the system so that the calcium carbonate concentration in the concentrate obtained in the membrane separation step is within a predetermined range.

  According to this water treatment method, the calcium carbonate concentration in the reaction solution and the calcium carbonate concentration in the concentrate obtained in the membrane separation step are determined so that the minimum particle size of calcium carbonate in the reaction solution is smaller than the average pore size of the filtration membrane. Also, it is set within a predetermined range necessary for increasing the size. As a result, the minimum particle size of calcium carbonate in the reaction solution can be controlled to be larger than the average pore size of the filtration membrane.

  In the reaction step of the water treatment method according to the present invention, at least a part of the concentrated solution is circulated and mixed with the reaction solution so that the calcium carbonate concentration in the reaction solution is 0.1% or more. In the discharge step, a part of the concentrate is discharged out of the system so that the calcium carbonate concentration in the concentrate obtained in the membrane separation step is 0.1% to 15%, and the average pore diameter of the filtration membrane Is preferably 0.04 μm or more and 1.0 μm or less.

  According to this water treatment method, the calcium carbonate concentration in the reaction is 0.1% or more, and the calcium carbonate concentration in the concentrate obtained in the membrane separation step is 0.1% or more. The reaction solution can be maintained in a crystallization environment. Moreover, since the calcium carbonate density | concentration in the concentrate obtained at the membrane separation process shall be 15% or less, the increase in the operating cost resulting from a fluid deterioration can be prevented. Furthermore, by setting the average pore diameter of the filtration membrane to 1.0 μm or less, not only calcium precipitates but also fine floating substances and organic substances contained in the raw water permeate through the hollow fiber membrane and leak to the treated water side. FI (fouling index), which is an index of the quality of water supplied to the RO membrane, when the treated water that can be reused as cooling tower cooling water or boiler replenishment water is provided. ) Or good water with very low SDI (silt density index) can be supplied to the RO membrane. Moreover, the filtration water which permeate | transmits a hollow fiber membrane with little energy can be obtained because the average pore diameter of a filtration membrane shall be 0.04 micrometer or more. That is, if the average pore diameter of the filtration membrane is less than 0.04 μm, the filtration resistance increases, so the pump for pumping to the filtration membrane increases, the equipment cost increases, and the running cost of electricity becomes enormous. .

  In the membrane separation step of the water treatment method according to the present invention, it is preferable to use an internal pressure filtration method using a hollow fiber membrane as the filtration membrane. In this internal pressure filtration method, the membrane surface flow velocity on the inner surface of the hollow fiber membrane can be stably maintained high, so that turbidity is hardly attached to the membrane surface. For this reason, it is suitable for calcium carbonate concentration.

  In the water treatment method according to the present invention, it is preferable to maintain the pH of the reaction solution at 9 or more and 13 or less in the reaction step. If the pH of the reaction solution is low, it takes time for the calcium precipitation reaction, so the raw water cannot be treated in a short time. Moreover, since the quantity of the alkaline substance required for pH adjustment will increase when the pH of a reaction liquid is high, a running cost becomes high. Therefore, it is preferable to set within the pH range. In this case, for example, by using an alkaline substance, the pH of the raw water can be adjusted to alkaline, and for example, an aqueous sodium hydroxide solution can be used as the alkaline substance.

  Moreover, it is preferable that the water treatment method according to the present invention further includes a carbon dioxide supply step of supplying carbon dioxide to the reaction solution. By supplying carbon dioxide to the reaction solution, calcium carbonate can be efficiently generated in the reaction solution. In particular, when the carbonic acid component contained in the raw water is small, it is necessary to sufficiently replenish the carbonic acid component to react with calcium to produce calcium carbonate, but by supplying carbon dioxide to the reaction solution, Calcium carbonate can be generated efficiently.

  In the water treatment method according to the present invention, the carbon dioxide supplied to the reaction liquid is preferably carbon dioxide in the flue gas discharged from the combustion device of the thermal power generator. As the exhaust gas, for example, exhaust gas discharged from a gas turbine engine, a gas engine, a combustion apparatus of a refuse incinerator, or a combustion apparatus of a thermal power plant can be used. Carbon dioxide contained in the flue gas of the combustion device is considered to be a causative agent of global warming, and the carbon dioxide contained in the flue gas reacts with calcium contained in the raw water to fix it as calcium carbonate. As a result, the absolute amount of carbon dioxide emitted can be reduced, and not only can it contribute to the suppression of global warming, but also the supply cost of carbon dioxide can be reduced.

  In the water treatment method according to the present invention, it is preferable to use calcium carbonate contained in the concentrated liquid discharged in the concentrated liquid discharging step as a desulfurizing agent for removing sulfur components from the flue gas of the combustion apparatus.

  As described above, the methods described in Patent Document 1 and Patent Document 2 have a problem in that the amount of sludge generated is increased and the disposal cost is increased. Here, since the main component of the sludge obtained from raw water containing calcium is calcium carbonate, if this purity is high, desulfurization agent for the purpose of removing sulfur components from the cement raw material, the exhaust gas of the thermal power generator, etc. Industrial use is possible. However, when impurities such as a flocculant and a coagulant are included, it cannot be used for these purposes, and therefore, treatment as industrial waste has become indispensable.

  On the other hand, in the water treatment method according to the present invention, the main component of the concentrate discharged from the concentrate discharge step is calcium carbonate, which is concentrated without using a flocculant, so the purity of calcium carbonate is Extremely expensive. For this reason, it is possible to utilize the obtained calcium carbonate as a desulfurization reaction agent which removes a sulfur component from the flue gas of the combustion apparatus of a thermal power generator. In the case of an ordinary thermal power generator boiler combustion device, the calcium carbonate in the concentrate is supplied to the flue gas desulfurization device in a liquid state, so that the sulfur component in the flue gas reacts with the calcium carbonate to produce calcium sulfate (gypsum). ) And can be desulfurized. Also, in the case of a circulating fluidized bed boiler combustion device, the calcium carbonate in the concentrate is dehydrated to form a cake, and this cake is mixed with the coal of the fuel and supplied, so that the sulfur component in the coal fuel Can be reacted with calcium carbonate and converted to calcium sulfate for desulfurization. In any combustion apparatus, the supply cost of calcium carbonate for desulfurization can be reduced.

  The water treatment system according to the present invention is a water treatment system that removes calcium from raw water, a reaction part that stores a reaction liquid that contains calcium carbonate and does not contain a flocculant, and raw water that supplies raw water to the reaction part A supply unit, an alkali supply unit that supplies an alkaline substance to the reaction solution in the reaction unit, a membrane separation unit that separates the reaction solution into a permeate and a concentrate through a filtration membrane, and discharges a part of the concentrate to the outside of the system A concentrated liquid discharge part, a circulation line connecting the concentrating side of the membrane separation part and the reaction part, and supplying at least a part of the concentrate obtained in the membrane separation part into the reaction part, and calcium carbonate in the reaction liquid And a control unit that controls the minimum particle size of the filter to be larger than the average pore size of the filtration membrane.

  According to this water treatment system, similarly to the above water treatment method, by utilizing the crystallization effect, the minimum particle diameter of calcium carbonate in the reaction solution is controlled to be larger than the average pore diameter of the filtration membrane. Thus, it is possible to fundamentally prevent the flux from being reduced due to pore blockage, and to maintain the flux stably and high.

  According to the present invention, it is possible to provide a water treatment method and a water treatment system for removing calcium in raw water and efficiently obtaining reusable treated water.

It is a figure showing typically the water treatment system concerning one embodiment of the present invention. It is a graph which shows the relationship between the calcium carbonate density | concentration in a reaction tank, and a calcium removal rate. It is a graph which shows the relationship between pH in a reaction tank, and a calcium removal rate. It is a graph which shows the difference in the calcium removal rate before and behind injecting carbon dioxide. It is explanatory drawing for demonstrating the minimum particle diameter of a calcium carbonate. It is a graph which shows the relationship between the density | concentration of a calcium carbonate concentrate, and a viscosity. It is a figure which shows typically the water treatment system of Example 1. FIG. It is a figure which shows the water treatment system of Example 2 typically. It is a figure which shows the water treatment system of Example 3 typically. It is a figure which shows typically the water treatment system of a comparative example. It is a graph which shows the difference in the filtration performance of an Example and a comparative example.

  Hereinafter, a water treatment system and a water treatment method according to an embodiment of the present invention will be described with reference to the drawings.

  The water treatment system which concerns on embodiment is an apparatus aiming at removing calcium from raw | natural water, and this embodiment demonstrates the apparatus and method which process CTB wastewater as raw | natural water as an example. In this embodiment, the treatment target is CTB wastewater, but the raw water to be treated is not limited to CTB wastewater.

  CTB waste water is waste water used as cooling water for cooling towers. When this CTB wastewater is used again as cooling water, the calcium content contained in the cooling water is concentrated. Therefore, it is necessary to remove calcium contained in the CTB wastewater and prevent calcium from precipitating in the cooling tower. There is. Conventionally, calcium is removed by adding alkali and flocculants and coagulants to CTB wastewater, and precipitating calcium by the agglomeration effect, adsorption effect, coprecipitation effect, etc. by these agents, followed by solid-liquid separation. Thus, a technique for obtaining treated water in which calcium is reduced to a very low concentration is common. However, the residual concentration of calcium in CTB wastewater is not only very high, but also phosphoric acid and phosphonic acid systems for suppressing the precipitation (calcium scale) of calcium contained in the cooling tower and the cooling water circulation system. Since precipitation of calcium is suppressed by the influence of precipitation inhibitors (scale inhibitors, scale dispersants) such as polymers, it is difficult to remove calcium in CTB wastewater with a high removal rate. When removing at a high removal rate, it is necessary to add a large amount of alkali and a chemical such as a flocculant and a coagulant to the CTB wastewater, which increases the cost of the chemical. In addition, the amount of sludge generated by these chemicals increases and disposal costs increase. In contrast, the water treatment system according to the present embodiment efficiently removes calcium in CTB wastewater with a high removal rate without adding chemicals other than alkali such as a flocculant and a coagulant, and cools the treated water. It can be reused as cooling water for the tower.

  FIG. 1 is a diagram schematically showing a water treatment system according to an embodiment of the present invention. As shown in FIG. 1, the water treatment system 10 includes a reaction tank (reaction unit) 14 that stores a reaction liquid that contains calcium carbonate and does not contain a flocculant, and raw water supply that supplies CTB waste water (raw water) to the reaction tank 14. Part 11, an alkali supply part 12 for supplying an alkaline substance to the reaction liquid in the reaction tank 14, a carbon dioxide supply part 13 for supplying carbon dioxide to the reaction tank 14, and the reaction liquid is concentrated with a permeate by a filtration membrane 15a. A membrane filtration device (membrane separation unit) 15 that separates into a liquid, a concentration side of the membrane filtration device 15 and the reaction tank 14 are connected, and at least a part of the concentrated liquid separated by the membrane filtration device 15 is reacted into the reaction tank 14. The main configuration includes a circulation line 16 that is supplied to the inside, a post-processing device 17 that is made of a reverse osmosis membrane and the like, and a concentrate discharge section 18 that discharges a part of the concentrate to the outside of the system. The alkali supply part (pH adjustment part) 12 supplies sodium hydroxide aqueous solution to a reaction liquid as an alkaline substance, adjusts the pH of a reaction liquid to 9 or more and 13 or less, and maintains the state.

  The reaction tank 14 has a receiving port 14a for receiving the CTB waste water supplied from the raw water supply unit 11, a discharge port 14b for discharging the reaction solution, and an introduction port 14c for introducing the concentrated solution from the circulation line 16. doing. The reaction tank 14 is connected to the reaction liquid feed pipe 19 at the discharge port 14 b and communicates with the membrane filtration device 15 via the reaction liquid feed pipe 19. The membrane filtration device 15 is connected to the post-treatment device 17 via the treated water transfer line 20 on the secondary side (treatment side) Ab, and the discharge line 21 is connected to the primary side (treated side) Aa. To the concentrate discharge section 18 and to the reaction tank 14 via the circulation line 16 branched from the discharge line 21.

  In addition, the water treatment system 10 includes a concentration detection unit 23 that detects a calcium carbonate concentration in the concentrate obtained by the membrane filtration device 15, a discharge amount adjustment unit 25 that adjusts the discharge amount of the concentrate discharge unit 18, On the discharge line 21. Further, the water treatment system 10 has a circulation amount adjustment unit 24 on the circulation line 16 for adjusting the flow rate of the concentrate supplied from the circulation line 16 to the reaction tank 14. The remaining concentrated liquid that is not supplied to the reaction tank 14 is sent to the primary side of the membrane filtration device 15. Further, the water treatment system 10 includes a control unit 27 connected to the concentration detection unit 23, the discharge amount adjustment unit 25, and the circulation amount adjustment unit 24. The control unit 27 is a computer including, for example, a CPU (Central Processing Unit). The control unit 27 controls the discharge amount adjusting unit 25 to adjust the discharge amount of the concentrate discharge unit 18 and the circulation amount adjusting unit 24. The flow rate of the concentrate supplied to the reaction tank 14 from the circulation line 16 is controlled. Note that an SS (Suspended Solid) concentration meter such as an infrared scattered light measurement method or an ultrasonic method is used for the concentration detection unit 23 that detects the calcium carbonate concentration in the concentrate.

  The water treatment method performed in the water treatment system 10 having such a configuration includes a reaction step of obtaining a reaction solution containing calcium carbonate and not containing a flocculant from CTB wastewater (raw water), and the reaction solution is filtered by a filtration membrane 15a. And a concentrated liquid discharging process for discharging a part of the concentrated liquid out of the system. In the reaction step, at least a part of the concentrate obtained in the membrane separation step is circulated and mixed with the reaction solution, and the reaction solution is adjusted to be alkaline. The water treatment method may further include a carbon dioxide supply step for supplying carbon dioxide to the reaction solution.

  In the reaction tank 14, the reaction solution is adjusted to be alkaline by the alkaline substance supplied from the alkali supply unit 12 (reaction process). Furthermore, at least a part of the concentrated liquid containing calcium carbonate separated by the membrane filtration device 15 is circulated through the circulation line 16 and mixed with the reaction tank 14 (reaction process). As a result, the calcium carbonate particles in the concentrated solution become seed crystals, and it is possible to develop a crystallization effect, and calcium carbonate is precipitated on the outer surface of the seed crystals without adding a chemical such as a flocculant or a coagulant. It is possible to make the environment easy to do. At this time, the calcium carbonate concentration in the reaction tank 14 is preferably 0.1% or more and 2.5% or less, and more preferably 0.5% or more and 2.5% or less. That is, in the water treatment system 10, the control unit 27 controls the circulation amount adjustment unit 24 so that the calcium carbonate concentration in the reaction solution is within a predetermined range (0.1% or more). It is preferable to control the supply amount of the concentrate supplied from 16 to the reaction tank 14, in other words, in the reaction step, the calcium carbonate concentration in the reaction solution is within a predetermined range (0.1% or more). Thus, it is preferable to circulate at least a part of the concentrated solution and mix it with the reaction solution.

  This knowledge has been derived based on the results of experiments by the inventors, and details thereof will be described using the results of the beaker test. The beaker in this beaker test corresponds to the reaction tank 14 in the embodiment shown in FIG. 1, and the addition of calcium carbonate in this beaker test means that the concentrated solution concentrated by the membrane filtration device 15 shown in FIG. The addition of sodium hydroxide corresponds to the circulation to the reaction tank 14 by the circulation line 16 and corresponds to the alkali supply unit 12 shown in FIG.

  In a state in which 1 liter of CTB waste water having the water quality shown in Table 1 was put in each of 9 beakers, sodium hydroxide was added to 9 beakers, and the pH in all 9 beakers was adjusted to 11. In this initial state, the calcium removal rate in each beaker was determined, and all the beakers had a calcium removal rate of about 35%. Next, commercially available calcium carbonate reagent (average particle size 0.8 μm) was placed in 9 beakers, 0%, 0.1%, 0.3%, 0.5%, 1%, 2%, 3%, respectively. 4% and 5% by weight were added and stirred. Ten minutes after the start of stirring by adding calcium carbonate, the stirring was stopped. FIG. 2 shows the result of calculating the calcium removal rate in each beaker in this post-stirring state (called “post-stirring state”). From FIG. 2, when no calcium carbonate was added to the beaker (reaction vessel 14) in the initial state (0%), the calcium removal rate was about 35%, which was very low. It can be seen that the removal rate was dramatically improved by adding calcium. Furthermore, it turns out that it becomes the environment where calcium carbonate precipitates more easily by increasing the quantity which adds calcium carbonate. Here, the calcium removal rate exceeds 80% when the calcium carbonate addition amount is 0.5%, reaches about 90% when the addition amount is 2.5%, and the calcium removal rate is improved even if the addition amount is further increased. Not done. Therefore, the calcium carbonate concentration in the reaction tank 14 (beaker) is preferably 0.1% to 2.5%, and more preferably 0.5% to 2.5%. More specifically, as will be described later, the calcium carbonate concentration in the reaction vessel 14 is preferably 0.1% or more and 15% or less, and more preferably 0.1% or more and 2.5% or less. Is more preferably 0.5% or more and 2.5% or less.

In addition, the measuring method of the said calcium removal rate is as follows.

Calcium removal rate [%] = (1-residual calcium concentration in reaction solution (after filtration of 0.45 μm) / calcium concentration in raw water (after filtration of 0.45 μm)) × 100
For the measurement of the calcium removal rate, a filter having an aperture of 0.45 μm was used. This corresponds to the membrane filtration device 15 in the embodiment shown in FIG. In addition, when this filter is used, it has been confirmed that the same result as that of a filtration membrane having an average pore diameter of 0.04 μm or more and 1.0 μm or less used in the membrane filtration device 15 can be obtained.

  As described above, the CTB wastewater contains a precipitation inhibitor (a scale inhibitor, a scale dispersant) for suppressing the precipitation of calcium in the cooling tower and the circulating system of the cooling water. In the initial state in which is added only, the reaction in which primary crystal nuclei of calcium carbonate are generated from calcium ions and carbonate ions is hindered by the precipitation inhibitor. As a result, calcium carbonate does not easily precipitate, and the calcium removal rate is considered to be as low as about 35%. On the other hand, when calcium carbonate particles are added to a beaker, these particles become seed crystals in the crystallization phenomenon, and calcium carbonate is precipitated on the outer surface of the seed crystals, so that the calcium removal rate is considered to be high.

Here, the pH of the reaction solution in the reaction step will also be described. In order to remove calcium in the form of calcium carbonate from raw water containing calcium, the form of carbonic acid present in the reaction solution needs to be carbonate ion CO ₃ ²⁻ . Carbonate has an overwhelmingly high abundance ratio in the form of bicarbonate ions HCO ₃ ⁻ around pH 8, and carbonate ions CO ₃ ²⁻ are hardly present. Therefore, at pH 8, calcium can hardly be removed. Since the abundance ratio of hydrogen carbonate ion to carbonate ion at pH 9 is about HCO ₃ ⁻ / CO ₃ ²⁻ = 95/5, when it is adjusted to pH 9, carbonate ions and calcium ions react slightly to precipitate calcium carbonate. The Furthermore, at pH 9.5, HCO ₃ ⁻ / CO ₃ ²⁻ = 90/10 (the existence ratio of CO ₃ ²⁻ is twice that of pH 9). Therefore, a practical condition for removing calcium is at least pH 9 .5 or more is preferable. By increasing the pH, the abundance ratio of carbonate ions gradually increases, so that the calcium removal rate increases. The higher the pH, the higher the calcium removal rate, but the improvement in the removal rate becomes dull from around pH 12. Therefore, considering the drug cost, it is not preferable to operate at a pH of 13 or higher.

  Moreover, in order to perform crystallization efficiently, it is necessary to perform reaction in a metastable region. This is because in the unstable region, a reaction in which primary crystal nuclei are generated is given priority over crystallization (precipitation on the particle surface). In general, the unstable region is a region having a pH higher than that of the metastable region. For the reasons described above, the pH of the reaction solution is preferably pH 13 or less, more preferably pH 12.5 or less. is there.

  That is, the pH of the reaction solution is preferably 9 or more and 13 or less, and more preferably 9.5 or more and pH 12.5 or less. In addition, as a temperature of a reaction liquid, 5 to 45 degreeC is a suitable temperature.

  This will be described in detail using the results of the beaker test. The beaker in this beaker test corresponds to the reaction tank 14 in the embodiment shown in FIG.

  Solid water of 2% sodium hydroxide and commercially available calcium carbonate reagent (average particle size 0.8 μm) in 4 beakers with 1 liter each of the water quality CTB waste water shown in Table 1 above. In addition, the pH of each beaker was adjusted to 8, 9, 10, 11, and 13. FIG. 3 shows the result of stirring in this state and obtaining the calcium removal rate in each beaker.

  As described above, the removal rate at pH 8 is zero, the removal rate of calcium is about 10% at pH 9, and it can be seen that the removal rate is gradually improved above this.

  Next, by supplying carbon dioxide into the reaction tank 4 containing the CTB wastewater, it is considered that an environment in which calcium is more likely to precipitate is obtained. As described above, the CTB wastewater contains a precipitation inhibitor (a scale inhibitor, a scale dispersant) for suppressing the precipitation of calcium in the cooling tower and the circulating system of the cooling water. If only is added, there is a possibility that the precipitation inhibitor inhibits the reaction that the primary crystal nuclei of calcium carbonate are generated from calcium ions and carbonate ions. On the other hand, since carbon dioxide is easily dissolved in the reaction tank 4 in an alkaline environment, by supplying carbon dioxide, a carbonate ion concentration sufficiently excessive to the calcium ion concentration is dissolved in the reaction solution. However, it is considered that calcium carbonate is likely to precipitate.

  This knowledge has been derived based on the results of experiments by the inventors, and details thereof will be described using the results of the beaker test. A beaker in the present beaker test corresponds to the reaction tank 14 in the embodiment shown in FIG. 1, and injecting carbon dioxide in the present beaker test corresponds to the carbon dioxide supply unit 13. That is, the water treatment method of the present embodiment may further include a carbon dioxide supply step for supplying carbon dioxide to the reaction solution.

  In a state where 1 liter of the CTB waste water having the water quality shown in Table 1 was put in four beakers, sodium hydroxide was added to the four beakers, and the pH of each beaker was adjusted to 9, 10, 11, 13. . FIG. 4 shows the result of obtaining the calcium removal rate in each beaker in this initial state (referred to as “initial state”). In the beaker test, the calcium carbonate reagent is not added to the beaker in order to clarify the single effect of carbon dioxide injection.

Immediately after adding sodium hydroxide, carbon dioxide gas was injected into each beaker at a rate of 20 cc / min. During the process of injecting carbon dioxide, the pH of each beaker is maintained at the initial value by adding sodium hydroxide to the beaker because carbon dioxide dissolves in the waste water and has a property of lowering the pH. Thus, the amount of sodium hydroxide added was continuously adjusted. Twenty minutes after the start of carbon dioxide injection, carbon dioxide and sodium hydroxide injection were stopped (referred to as the “post-CO ₂ injection state”). FIG. 4 shows the result of obtaining the calcium removal rate in each beaker in the state after the CO ₂ injection. (Plot marked with “●”) CTB wastewater contains a precipitation inhibitor to suppress calcium precipitation in the cooling tower and cooling water circulation system, so only sodium hydroxide was added. In the initial state, calcium carbonate does not easily precipitate and the calcium removal rate is low. On the other hand, in the state after CO ₂ injection, since the precipitation reaction of calcium carbonate proceeds by CO ₂ injection, the removal rate of calcium becomes high at any pH. That is, from the result of the beaker test shown in FIG. 4, the reaction solution is adjusted to be alkaline in the beaker (reaction tank 14) and further supplied with carbon dioxide, so that the reaction solution has a sufficient excess with respect to the calcium ion concentration. Therefore, it is considered that calcium carbonate easily precipitates against the calcium precipitation inhibitor contained in the CTB wastewater. That is, it turns out that it becomes the environment where calcium carbonate precipitates more easily without adding chemicals such as a flocculant and a coagulant.

  As described above, the water treatment system 10 is separated by the membrane filtration device (membrane separation unit) 15 that obtains a permeate from which the calcium is removed by membrane separation of the reaction solution obtained from the reaction tank 14. A circulation line 16 for supplying at least a part of the concentrated liquid to the reaction tank 14 and circulating the concentrated liquid.

  In addition, the water treatment system 10 applies the reaction liquid feed pipe 19 that forms the forward path connecting the reaction tank 14 and the membrane filtration apparatus 15, the circulation line 16 that forms the return path, and the pressure of the reaction liquid to add the membrane filtration apparatus 15. A pressure pump (not shown) is provided. The reaction tank 14 is connected to the receiving line 14 a that receives the CTB waste water (raw water) obtained from the raw water supply unit 11, the discharge port 14 b that is connected to the reaction liquid feed pipe 19 that is the outgoing path of the circulation line 16, and the circulation line 16. An inlet 14c.

  A discharge line 21 communicating with the concentrate discharge unit 18 is connected to the circulation line 16. Further, a treated water transfer line 20 from which treated water from which calcium-containing precipitates are removed is connected to the secondary side (treated side) Ab of the filtration membrane 15a of the membrane filtration device 15, and the treated water transfer line is connected thereto. Reference numeral 20 denotes a post-processing device 17. The post-processing device 17 includes a single-stage or multi-stage reverse osmosis membrane (hereinafter, RO membrane), a nanofiltration membrane (hereinafter, NF membrane), and the like. The membrane filtration device 15 in the previous stage can remove most of the calcium components as well as fine suspended substances in the raw water. More specifically, when an RO membrane is provided, FI (fouling index) or SDI (silt density index), which is an index of water quality supplied to the RO membrane, is extremely small (SDI, FI ≦ 3 or less). Good water can be supplied to the RO membrane. Further, since calcium ions contained in the raw water are removed by the filtration membrane 15a in the form of calcium carbonate particles, the risk of calcium scale in the RO membrane is extremely reduced, and the water recovery rate of the RO membrane is increased. Can do. The treated water treated by the post-treatment device 17 can be used as cooling water for the cooling tower or as makeup water for the boiler.

  According to the water treatment system 10 described above, the reaction step of supplying the alkaline substance to the reaction solution without supplying the flocculant to generate the reaction solution, and the membrane for separating the reaction solution into the permeate and the concentrated solution A water treatment method that includes a separation step and circulates the reaction solution on the side to be membrane-separated to concentrate the precipitate, and circulates at least a part of the concentrate and mixes it with the reaction vessel 14.

  In this embodiment, the membrane filtration apparatus 15 is applied as a membrane separation part. The membrane filtration device 15 is an internal pressure type filtration device having a hollow fiber membrane (hereinafter referred to as “filtration membrane”) in the casing, and the inside of the hollow portion (inside the hollow fiber) of the filtration membrane 15a is the primary side (treated). Side) Aa, and the outside of the hollow portion of the filtration membrane 15a (outside of the hollow fiber) becomes the secondary side (processing side) Ab. The average pore diameter of the filtration membrane 15a is preferably 0.04 μm or more and 1.0 μm or less, and more preferably 0.08 μm or more and 0.6 μm or less. Moreover, in the filtration apparatus of a hollow fiber membrane, it is preferable that the flow rate which flows through a hollow part shall be 0.5-3.0 m / s.

In addition, the average pore diameter of the filtration membrane 15a is calculated | required as follows. Latex particles whose average particle diameter is known in advance are diluted with a 0.5% SDS (sodium dodecyl sulfonate) aqueous solution to prepare a suspension (feed solution) having a latex concentration of 0.01%. The suspension is supplied to the filtration membrane, the permeate is collected, and the latex rejection is determined from the respective concentrations of the suspension and permeate by the following formula.
Latex rejection [%] = (1-permeate concentration / suspension concentration) × 100
The permeate concentration and the suspension concentration may be obtained by measuring the absorbance using an absorptiometer.

  Obtaining the blocking rate for latex particles having five or more different average particle sizes, plotting the relationship between the average particle size and blocking rate of the latex particles, and determining the latex average particle size when the latex blocking rate reached 90% or more. The average pore diameter of the filtration membrane is used.

  The larger the average pore diameter of the filtration membrane 15a, the larger the permeation amount of the filtration membrane 15a. On the other hand, the fine particles of calcium carbonate precipitated in the reaction tank 14 are liable to leak to the treated water side, and the pore portion of the membrane In addition, there is a problem that the fine particles are clogged and the chemical cleaning cost is increased. Conversely, if the average pore diameter of the filtration membrane 15a is reduced, it becomes difficult for the fine particles to permeate and the risk of clogging the fine particles into the pore portions of the membrane is reduced, but on the other hand, the water permeability of the filtration membrane 15a There is a problem that the facility cost and the energy cost of the filtration membrane 15a are increased.

  The crystallization effect not only improves the calcium removal rate, but also greatly contributes to the stability of the membrane flux in the membrane filtration device 15. That is, the calcium carbonate particles in the concentrated solution become seed crystals and repeat the crystallization phenomenon. This crystallization phenomenon not only increases the precipitation (removal rate) of calcium carbonate, but also increases the particle size of calcium carbonate. Therefore, even if the average pore size of the filtration membrane 15a is increased by controlling the particle size of the calcium carbonate so that “the particle size of the calcium carbonate> the average pore size of the membrane”, the pores are blocked. It is possible to fundamentally prevent a decrease in membrane flux due to. Further, it is possible to fundamentally prevent the calcium carbonate particles from leaking to the permeate side. In other words, the membrane flux can be stably maintained high and the removal rate of calcium carbonate can be maintained high without using a chemical such as a flocculant or a coagulant in the prior art. That is, in the water treatment method of the present invention, fouling occurs by controlling the minimum particle diameter of calcium carbonate in the reaction solution (the definition will be described later) larger than the average pore diameter of the filtration membrane 15a. Can be fundamentally prevented. Moreover, in order to implement | achieve this, in the water treatment system 10 of this invention, the control part 27 controls the minimum particle diameter of the calcium carbonate in a reaction liquid larger than the average pore diameter of the filtration membrane 15a.

  The inventors made extensive studies on a specific particle size control method and obtained the following knowledge. The concentrate discharge unit 18 is appropriately adjusted so that the concentration of calcium carbonate in the concentrate at the outlet of the membrane filtration device 15 is preferably 0.1% or more, more preferably 0.5% or more, and the maximum concentration is 15% or less. The concentrated liquid is extracted from the liquid, and at least a part of the concentrated liquid is circulated to the reaction tank 14 and mixed with the reaction liquid. And by controlling the calcium carbonate concentration at the outlet of the membrane filtration device 15, the calcium carbonate concentration in the reaction tank 14 is preferably 0.1% or more and 15% or less, more preferably 0.1% or more and 2.5%. Hereinafter, the particle diameter of the calcium carbonate obtained by the crystallization effect can be controlled by adjusting to 0.5% or more and 2.5% or less more preferably. Here, the upper limit value of the calcium carbonate concentration in the reaction tank 14 is set to 15% when the concentrated liquid of 15%, which is the maximum concentration, is circulated from the membrane filtration device 15 to the reaction tank 14 with respect to the amount of raw water. This is because if the amount of circulation is sufficiently large, the calcium carbonate concentration in the reaction tank 14 will also be 15%, the same as the concentration of the concentrate.

  That is, since the calcium carbonate concentration in the concentrate obtained by the membrane filtration device 15 (membrane separation step) and the minimum particle size of calcium carbonate in the reaction solution have a correlation, the water of this embodiment is used. In the treatment method, the concentration of calcium carbonate in the concentrate obtained by the membrane filtration device 15 is within a predetermined range (0.1% to 15%) in the concentrate discharge unit 18 (concentrate discharge step). Thus, it is preferable to discharge a part of the concentrated liquid out of the system. In other words, in the water treatment system 10 of the present embodiment, the control unit 27 controls the discharge amount adjustment unit 25 so that the calcium carbonate concentration in the concentrated liquid detected by the concentration detection unit 23 is within a predetermined range ( It is preferable to control the discharge amount of the concentrate discharge section 18 so that it is within 0.1% or more and 15% or less.

  As described above, when at least a part of the concentrated solution containing calcium carbonate is not mixed into the reaction vessel 14, that is, when the calcium carbonate reagent added to the reaction vessel 14 is 0% in the beaker test result shown in FIG. The crystallization effect does not appear. In this case, calcium ions in the CTB wastewater react to precipitate calcium carbonate particles as primary crystal nuclei. The calcium ion concentration in CTB wastewater is generally about 40 to 400 mg / L. Using this CTB wastewater, primary crystal nuclei were precipitated by a beaker test under an environment of pH 9 or more and 13 or less, and the minimum particle diameter of calcium carbonate was examined. The result was about 0.022 to 0.445 μm.

  Here, “the minimum particle diameter of calcium carbonate” will be described with reference to FIG. The particle size distribution of calcium carbonate is usually in the form of a normal distribution. In the present embodiment, when the average particle diameter is represented by A and the standard deviation is represented by σ, the “minimum particle diameter of calcium carbonate” is defined as (A-2σ). In addition, although defined as (A-2σ) in the present embodiment, it may be defined as (A-σ) or (A-3σ) depending on the operating conditions.

  Examples of the method for measuring the particle size distribution of calcium carbonate include an image analysis method, a laser diffraction / scattering light method, and an electric resistance method, but it is preferable to use a laser diffraction / scattering light method with a wide measurement range.

  According to the probability statistical theory, the ratio of the particle diameter from the section (A-2σ) to (A + 2σ) in the particle size distribution is 95.45%. That is, the ratio of the calcium carbonate particles having a diameter smaller than (A-2σ) is about 2.3% ((100% −95.45%) ÷ 2), so the average pore diameter of the filtration membrane 15a is small. If it is smaller than (A-2σ), the influence of the blockage of the membrane is substantially negligible and negligible during the filtration operation, and no practical problem occurs. Furthermore, since the ratio of the particle diameter of the section (A-3σ) to (A + 3σ) is 99.73%, the ratio of calcium carbonate particles having a diameter smaller than (A-3σ) is about 0.1% ((100 % −99.73%) ÷ 2), which is a very small amount. Therefore, if the average pore diameter of the filtration membrane is smaller than (A-3σ), a more preferable filtration operation state is obtained.

  Here, calcium carbonate having a minimum particle size of 0.022 μm (the lower limit value of the above-mentioned primary crystal nucleus minimum particle size range of about 0.022 to 0.445 μm) is 0.04 μm (the average fineness of the above-mentioned filtration membrane). A specific method will be described with respect to an example of filtration through a filter membrane having an average pore diameter in the pore diameter range of 0.04 to 1.0 μm.

  When using a filtration membrane having an average pore size of 0.04 μm (a filtration membrane having a minimum filtration pore size with a small filtration resistance and capable of obtaining filtered water with less energy), a minimum particle size of 0.022 μm or more Calcium carbonate particles of less than 0.04 μm are clogged by clogging. If the crystal grain size with a minimum particle size of 0.022 μm is grown at least twice due to the crystallization effect, the particle size can exceed 0.04 μm, and “minimum particle size of calcium carbonate> The relationship of “average pore diameter of membrane” is established, and the membrane filtration operation can be stably continued. This can be realized by the following principle.

  When calcium ions in CTB wastewater react to become calcium carbonate, if there are sufficiently many seed crystals, the crystallization reaction tends to proceed more preferentially than the primary crystal nucleus formation reaction. Generally, when a supersaturated solute (calcium ion in the present invention) is precipitated, the crystallization reaction proceeds with a supersaturation degree smaller than that of the primary crystal nucleation reaction. proceed. Here, the case where the seed crystals are sufficiently large means that the calcium carbonate concentration in the reaction vessel 14 is at least 0.1%, as is clear from the result of the beaker test regarding the calcium carbonate removal rate described above (FIG. 2). Above, preferably 0.5% or more, and by always maintaining this condition, it is possible to maintain the crystallization environment.

At the initial stage of operation, the particle diameter of primary crystal nuclei generated in a state where no seed crystal of calcium carbonate is present in the reaction tank 4 is Di (μm), and the calcium carbonate concentration obtained from raw water in the reaction tank 4 is Ci ( mg / L). Here, the calcium carbonate concentration obtained from the raw water in the reaction tank 14 is the increase in concentration due to the calcium carbonate newly generated by the reaction of calcium ions brought into the reaction tank 14 as raw water in the reaction tank 14. means. More specifically, it is the sum of increments of calcium carbonate newly crystallized on the surface of the seed crystal mixed in the reaction tank 14 as a concentrate and increments of calcium carbonate newly precipitated as primary crystal nuclei. Further, while supplying the CTB waste water to the reaction tank 14, the calcium carbonate concentration in the reaction tank 14 is set to Cr = 0.5% (5000 mg / L) so that the inside of the reaction tank 14 is always a crystallization environment, and the membrane It is assumed that the calcium carbonate concentration at the outlet of the filtration device 15 is Cm (mg / L) and Cm ≧ Cr, and the operation is continued for a sufficiently long time to reach a steady state. (Operating while maintaining this condition for a sufficiently long time while discharging the concentrate from the concentrate discharge section 18) The crystallization environment is maintained in the reaction tank 14, so that the calcium ions in the CTB wastewater Assuming that all of the obtained calcium carbonate has grown crystals greatly by the crystallization reaction (the primary crystal nucleus formation reaction does not occur), the primary crystal nucleus generated in the initial stage of operation is Cm / Ci (times). Will grow to a volume of. For example, if Di = 0.022 μm, Ci = 500 mg / L, and Cm = 5000 mg / L, Cm / Ci = 10 (times) and the particle size is Di × (Cm / Ci) ^(1/3) = 0.022 μm × 2.2 times = 0.047 μm, which is larger than the average pore diameter of the filtration membrane 15a of 0.04 μm.

  From the above examination results, when Cm / Ci is at least 10 times or more, 0.022 μm of the primary crystal nucleus having the minimum size is filtered with an average pore diameter of 0.04 μm (filter having the minimum pore size of the average size). Film). However, even if the crystallization environment is maintained in the reaction vessel 4, in reality, there is a possibility that the formation reaction of the primary crystal nuclei may proceed simultaneously with the crystallization reaction although it is extremely small. In addition, some of the calcium ions in the raw water may remain unreacted without reacting with calcium carbonate. Considering these, it is preferable to set Cm / Ci to 20 times or more practically.

As described above, the calcium ion concentration in CTB wastewater is generally about 40 to 400 mg / L. Assuming that raw water (CTB wastewater) having a calcium ion concentration of 40 mg / L is supplied to the reaction tank 14 and assuming that all of the calcium ions have reacted with calcium carbonate, from the molecular weight ratio of Ca and CaCO ₃ , Ci = 100 mg / L. When Cm / Ci ≧ 10 (times), Cm ≧ Ci × 10 = 100 × 10 = 1000 mg / L (0.1%). Since Cm / Ci ≧ 20 (times), Cm ≧ Ci × 20 = 100 × 20 = 2000 mg / L (0.2%). That is, the calcium carbonate concentration Cm of the concentrated liquid at the outlet of the membrane filtration device 15 is preferably 0.1% or more, and more preferably 0.2% or more.

  Next, calcium carbonate having a minimum particle size of 0.445 μm (upper limit value of the above-mentioned primary crystal nucleus minimum particle size range of 0.022 to 0.445 μm) is changed to 1.0 μm (average fineness of the above-mentioned filtration membrane). A specific method will be described for an example of filtration through a filtration membrane having an average pore diameter in the pore diameter range of 0.04 to 1.0 μm.

  When using a filtration membrane having an average pore diameter of 1.0 μm (a filtration membrane having a maximum average pore diameter that can obtain filtered water with low filtration resistance and low energy), a minimum particle diameter of 0.445 μm or more Calcium carbonate particles of less than 1.0 μm are clogged by clogging. If the crystal grain size with a minimum particle size of 0.445 μm is grown at least 2.3 times or more due to the crystallization effect, the particle size can exceed 1.0 μm. By satisfying the relationship of “diameter> average pore diameter of membrane”, the membrane filtration operation can be stably continued. This can be realized by the following principle.

For example, assuming that raw water (CTB wastewater) having a calcium ion concentration of 80 mg / L is supplied to the reaction tank 14 and that all of the calcium ions have reacted to calcium carbonate, from the molecular weight ratio of Ca and CaCO ₃ , Ci = 200 mg / L. While continuing to supply CTB wastewater to the reaction tank 14 under these conditions, the calcium carbonate concentration at the outlet of the membrane filtration device 15 is Cm = 1.0% (10000 mg / L), and the calcium carbonate concentration in the reaction tank 14 is Cr = 0. It is assumed that the concentration is adjusted to 5% (5000 mg / L) (the amount by which the concentrated liquid is circulated to the reaction tank 14), the operation is sufficiently long, and a steady state is reached.

  In the initial stage of operation, since the seed crystal of calcium carbonate does not exist in the reaction tank 14, primary crystal nuclei of calcium carbonate having a minimum particle diameter Di = 0.445 μm from the calcium ions in the CTB wastewater in the reaction tank 14. Assume that it was created. Thereafter, calcium ions brought into the reaction vessel 14 as CTB wastewater react on the surface of primary crystal nuclei generated in the initial stage of operation due to the crystallization effect to gradually grow crystals.

  Immediately after the start of operation, since there are few calcium carbonate particles as seed crystals in the reaction vessel 14, a new primary crystal nucleus formation reaction proceeds simultaneously with the crystal growth reaction by crystallization, but the operation is performed for a sufficiently long time. In the state, the calcium carbonate concentration in the reaction tank 14 is Cr = 0.5%, and the environment is sufficiently rich in seed crystals.

A steady state was reached with the calcium carbonate concentration at the outlet of the membrane filtration device 15 being Cm = 10000 mg / L (1.0%) and the calcium carbonate concentration in the reaction vessel 14 being Cr = 5000 mg / L (0.5%). At that time, all of the calcium carbonate of Ci = 200 mg / L obtained by reacting with calcium ions in the CTB wastewater was converted into calcium carbonate by the crystallization reaction, and the crystal grew greatly (the primary crystal nucleus formation reaction was Assuming that this does not occur, the primary crystal nuclei generated at the beginning of operation have grown to a volume of Cm / Ci = 10000 mg / L ÷ 200 mg / L = 50 times. That is, when Di = 0.445 μm, Ci = 200 mg / L, and Cm = 10000 mg / L, Cm / Ci = 50 (times) and the particle size is Di × (Cm / Ci) ^(1/3) = 0. .445 μm × 3.7 times = 1.65 μm, and it is possible to continue a stable filtration operation without clogging of pores even with a maximum-size filter membrane having an average pore diameter of 1.0 μm (described above).

  In addition, calcium carbonate particles having a minimum particle diameter larger than the average pore diameter of the membrane can be introduced as seed crystals in the reaction tank 14 at the initial start-up of the water treatment system 10. For example, at the initial stage of start-up of the water treatment system 10, the carbonic acid in the reaction vessel 14 is seeded with calcium carbonate particles having a minimum particle size (eg, 1.1 μm or more) larger than the average pore size (eg, 1.0 μm) of the membrane. When the calcium carbonate concentration Cm = 1.0% and the calcium carbonate concentration Cm = 2.0% of the concentrated liquid at the outlet of the membrane filtration device 15 are charged and the operation is performed while maintaining this condition, the concentrated liquid is obtained. The minimum particle size of the calcium carbonate particles in the membrane does not fall below the average pore size of 1.0 μm due to the crystallization effect, so that it is possible to fundamentally prevent a decrease in membrane flux due to pore clogging, and an always stable membrane. The filtration operation can be continued.

  As described above, if the calcium carbonate concentration at the outlet of the membrane filtration device 15 is increased, the time during which the calcium carbonate particles are exposed to the crystallization environment in the water treatment system 10 becomes longer (or the opportunity increases). The particle size of calcium carbonate can be controlled to be larger than the average pore size of the filtration membrane 15a by the deposition effect.

  In addition, when the calcium carbonate concentration at the outlet of the membrane filtration device 15 is increased, when the calcium carbonate concentrate extracted from the concentrate discharge section 18 is dehydrated and used as a flue gas desulfurization agent, The size can be reduced, and the equipment cost, operating power cost, and installation space of the dehydrator can be reduced.

  However, as shown in FIG. 6, the viscosity of the calcium carbonate concentrate tends to rapidly increase from the vicinity where the calcium carbonate concentration exceeds 15% and the fluidity tends to deteriorate. When an internal pressure filtration method using a hollow fiber membrane is used as the membrane filtration device 15, the pressure loss inside the hollow fiber increases in proportion to the viscosity. The power of the pump, that is, the operating cost increases rapidly. Therefore, the concentrate may be appropriately extracted from the concentrate discharge unit 18 so that the concentration of calcium carbonate in the concentrate at the outlet of the membrane filtration device 15 is at most 15%.

  In this embodiment, since the calcium removal rate is extremely high, a post-treatment device 17 such as an RO membrane or an NF membrane is provided in the subsequent stage to obtain treated water that can be reused for cooling tower cooling water or boiler makeup water. , The burden on the post-treatment device 17 is reduced, and the final treatment efficiency (water recovery rate) can be improved.

  Further, in the present embodiment, the membrane filtration device 15 is used as the membrane separation unit, and as a result, compared to, for example, the coagulation sedimentation method, a sedimentation tank is unnecessary, which is advantageous from the viewpoint of space saving. There is also an advantage that there is no worry about floating sludge outflow (carry over) from the sedimentation tank.

  In the above embodiment, the amount of raw water supplied to the reaction tank 14 (known operating conditions), the concentration of calcium carbonate in the concentrate (detected by the concentration detection unit 23), and the supply from the circulation line 16 to the reaction tank 14 The concentration of calcium carbonate in the reaction liquid is indirectly estimated from the supply amount of the concentrated liquid (adjusted by the circulation amount adjusting unit 24) and the concentration of calcium carbonate in the reaction liquid is within a predetermined range (0.1 % Or more), however, a concentration detector for detecting the calcium carbonate concentration in the reaction solution may be further installed in the reaction tank 14 or on the piping at the outlet of the reaction tank 14. .

  Moreover, in this embodiment, although the internal pressure type membrane filtration apparatus provided with the hollow fiber membrane in the casing was used as the membrane filtration apparatus 15, the membrane filtration apparatus 15 is used for a long time under a high pH condition. It is required to have the durability of As a hollow fiber membrane satisfying such requirements, and having excellent scratch resistance and capable of stably producing a concentrated liquid containing a high concentration of calcium carbonate, International Publication No. 2007/119850 is disclosed. It is desirable to use the described porous membrane.

That is, a porous film containing a polymer mainly composed of PVDF (poly (vinylidene fluoride)) resin, and the degree of crystallinity of the PVDF resin in the porous film is 50% or more and 90% or less. And the value obtained by multiplying the crystallinity of the PVDF resin in the porous film by the specific surface area of the film is 300 (% · m ² / g) or more and 2000 (% · m ² / g) or less. It is desirable to use

  Here, the PVDF resin means a homopolymer of vinylidene fluoride or a copolymer containing 50% or more of vinylidene fluoride in a molar ratio. The PVDF resin is preferably a homopolymer from the viewpoint of excellent strength. When the PVDF resin is a copolymer, other copolymer monomers to be copolymerized with the vinylidene fluoride monomer can be appropriately selected from known ones, and are not particularly limited. Fluorine monomers and chlorine monomers can be preferably used. The weight average molecular weight (Mw) of the PVDF resin is not particularly limited, but is preferably 100,000 or more and 1,000,000 or less, more preferably 150,000 or more and 500,000 or less.

  The porous membrane contains PVDF resin as the main component of the polymer component. Here, “containing as a main component” means containing 50% by weight or more in terms of solid content of the polymer component. The porous membrane is not particularly limited, but preferably contains 80% by weight or more and 99.99% by weight or less of PVDF resin as a main component of the polymer component, and 90% by weight or more and 99% by weight. % Or less is more preferable. On the other hand, the porous membrane may contain other polymer components. The other polymer component is not particularly limited, but is preferably compatible with PVDF resin. For example, a fluorine-based resin exhibiting high chemical resistance like PVDF resin is preferably used. it can. Moreover, you may use hydrophilic resin like the polyethylene glycol mentioned later as another polymer component.

As described above, the porous film has a crystallinity of PVDF resin constituting the film of 50% or more and 90% or less, and a value obtained by multiplying the crystallinity by the specific surface area of the porous film. Is 300 (% · m ² / g) or more and 2000 (% · m ² / g) or less.

  Here, when the degree of crystallinity of the PVDF resin is less than 50%, the rigidity of the membrane is low and the film is deformed by the filtration pressure, and thus is not suitable for filtration. It is presumed that the deterioration of the PVDF resin due to the concentrated solution having a high pH is caused by an amorphous part exhibiting flexibility. Therefore, when the degree of crystallinity of PVDF resin exceeds 90% and the amorphous part becomes relatively small, the entire porous part becomes brittle and breaks when the amorphous part is decomposed and deteriorated by the concentrated solution having a high pH. It becomes easy. On the other hand, if the specific surface area of the porous membrane is too small, the water permeability decreases, so it is not suitable for filtration applications. On the other hand, if it becomes large, the water permeability improves, but the area that comes into contact with the high pH reaction liquid increases. As a result, the resistance to the reaction solution decreases. From these findings, it is desirable that the value obtained by multiplying the specific surface area of the membrane by the degree of crystallinity is in the above range as a porous membrane excellent in water permeability and resistance to a high pH reaction solution.

  In addition, the manufacturing method of the international publication 2007/119850 can be used for the manufacturing method of the said porous membrane. Moreover, also about the measuring method of a crystallinity degree and a specific surface area, the measuring method as described in international publication 2007/119850 can be used.

  Hereinafter, the present invention will be described in more detail using examples and comparative examples. FIGS. 7 to 9 are diagrams schematically showing the water treatment system of the example, FIG. 10 is a diagram schematically showing the water treatment system of the comparative example, and FIG. 11 shows the difference in filtration performance between the example and the comparative example. It is a graph to show. The scope of the present invention is not limited to these examples.

  First, Example 1 shown in FIG. 7 will be described. As shown in FIG. 7, the water treatment system 30 according to Example 1 includes a reaction tank (reaction unit) 34 that stores a reaction liquid that contains calcium carbonate and does not contain a flocculant, and CTB waste water (raw water) in the reaction tank 34. The raw water supply unit 31 for supplying the reaction solution, the alkali supply unit 32 for supplying an alkaline substance to the reaction solution in the reaction vessel 34, the carbon dioxide supply unit 33 for supplying carbon dioxide to the reaction vessel 34, and the reaction solution through the filtration membrane 35a. The membrane separation part 35 that separates the permeate and the concentrated liquid by the above, the concentration side of the membrane separation part 35 and the reaction tank 34 are connected, and at least a part of the concentrated liquid separated by the membrane separation part 35 is connected to the reaction tank 34. And a circulation line 36 for supplying the inside.

  The reaction tank 34 is connected to a pressure feed pump 42 by a reaction liquid feed pipe 39. The membrane separation unit 35 is connected to the post-treatment device via the treated water transfer line 40 on the secondary side (treatment side) of the filtration membrane 35a, and via the discharge line 41 on the primary side (side to be treated). In addition to being communicated to the concentrate discharge section, it is also communicated to the reaction tank 34 via a circulation line 36 branched from the discharge line 41. The circulation line 36 includes a pipe 36 a that supplies the concentrated liquid into the reaction tank 34 and a pipe 36 b that supplies the remaining concentrated liquid to the upstream side of the pressure feed pump 42.

  In addition, the water treatment system 30 includes a concentration detection unit 43 that detects the calcium carbonate concentration in the concentrate obtained by the membrane separation unit 35, and a discharge amount adjustment unit 45 that adjusts the discharge amount of the concentrate discharge unit. A circulation amount adjusting unit 44 is provided on the circulation line 36 for detecting the flow rate of the concentrate supplied from the circulation line 36 (pipe 36a) to the reaction tank 34 and adjusting the flow rate. Yes. Further, the water treatment system 30 includes a control unit 47 connected to the concentration detection unit 43, the discharge amount adjustment unit 45, and the circulation amount adjustment unit 44. The control unit 47 is a computer that includes, for example, a CPU (Central Processing Unit), and controls the discharge amount adjusting unit 45 to adjust the discharge amount of the concentrate discharge unit, and also controls the circulation amount adjusting unit 44. Then, the flow rate of the concentrate supplied from the circulation line 36 to the reaction vessel 34 is adjusted. In addition, as the carbon dioxide supplied in the carbon dioxide supply part 33, the carbon dioxide in the flue gas discharged | emitted from the combustion apparatus of a thermal power generator can be used. Moreover, the calcium carbonate contained in the concentrate discharged | emitted by a concentrate discharge part can be used as a desulfurization agent which removes a sulfur component from the flue gas of the said combustion apparatus.

  The water treatment method performed in the water treatment system 30 includes a reaction step of obtaining a reaction solution containing calcium carbonate and not containing a flocculant from CTB waste water (raw water), and the reaction solution is separated into a permeate and a concentrated solution by a filtration membrane 35a. A membrane separation step for separating the concentrated solution and a concentrated solution discharging step for discharging a part of the concentrated solution out of the system. In the reaction step, at least a part of the concentrate obtained in the membrane separation step is circulated and mixed with the reaction solution, and the reaction solution is adjusted to be alkaline. The water treatment method may further include a carbon dioxide supply step for supplying carbon dioxide to the reaction solution.

Hereinafter, the water treatment system 30 shown in FIG. 7 will be described more specifically using experimental results from a beaker scale test apparatus. The beaker in this beaker scale test corresponds to the reaction tank 34 in Example 1 shown in FIG. 7, and the addition of calcium carbonate in this beaker test is the concentration concentrated by the membrane separation unit 35 shown in FIG. This corresponds to circulating the liquid through the circulation line 3 to the reaction vessel 34. The volume of the reaction vessel 34 in the reaction step of FIG. 7 is 1 liter, and the CTB waste water shown in Table 1 is continuously supplied as raw water, and the pH is adjusted to 9.5 to 12 while adding sodium hydroxide to the reaction vessel 34. Adjustments were made to be as follows. Further, as the filtration membrane 35a in the membrane separation step, an internal pressure type hollow fiber microfiltration membrane having an average pore diameter of 0.45 μm and a yarn inner diameter of 1.1 mm was used. Carbon dioxide gas (shown as “(CO ₂ )”) in FIG. 7 is not injected in this beaker test, but as described above, when the carbonic acid component in the raw water is insufficient for reaction with calcium, It is preferable to perform the injection.

  FIG. 7 shows an embodiment of the present invention, and the present invention is not limited to FIG. 7 as long as it does not exceed the gist thereof. The embodiment of FIGS. The gist and effect are the same.

  For example, the water treatment system 30B according to Example 2 in FIG. 8 is obtained by replacing the reaction tank 34 in the reaction step in FIG. 7 with a reaction tube (reaction unit) 48 having the same volume (= reaction time). The gist and effects of the invention are the same. That is, the reaction unit in the present invention may be the reaction tank 34 or the reaction tube 48. Since other configurations are the same as those of the first embodiment, the description thereof is omitted.

  Further, in Example 1 of FIG. 7, the concentrated liquid that has passed through the hollow portion of the internal pressure filtration membrane in the membrane separation step is directly returned to the suction side of the pressure pump 42, whereas in FIG. In the water treatment system 30 </ b> C according to the third embodiment, a circulation tank 49 for circulation is provided on the upstream side of the pressure feed pump 42. Example 3 is different from Example 1 in that the concentrated liquid is disadvantageous by the amount of kinetic energy when returning to the pumping pump 42, but regardless of the presence or absence of the circulation tank 49, the gist of the present invention in the present invention. The effect is the same. In addition, when providing the circulation tank 49, it is preferable to provide the circulation tank 49 which has a residence time of less than 1 hour in consideration of the installation space. Since other configurations are the same as those of the first embodiment, the description thereof is omitted.

  That is, in the three examples of FIGS. 7 to 9, the amount of raw water supplied to the reaction step (the amount supplied by the raw water supply unit 31), the reaction time in the reaction step, and the filtration membrane 35a in the membrane separation step (from the reaction step) If the output (= discharge amount) of the pressure feed pump 42 that sends the mixed liquid (mixed with the reaction liquid and the concentrated liquid from the filtration membrane) is operated under the same conditions, the amount of permeate discharged from the filtration membrane 35a, the filtration membrane outlet The amount of the concentrated liquid returned to the reaction step (the flow rate in the circulation rate adjusting unit 44) is the same in the three embodiments, and the gist and the effect of the present invention are also the same. Therefore, the first embodiment of FIG. 7 will be described in detail below as a representative of the first to third embodiments of FIGS.

  While supplying 50 cc of raw water from the raw water supply unit 31 to the reaction tank 34 (volume 1 liter) in the reaction process, the pumping pump 42 upstream of the membrane separation unit 35 (filtration membrane 35a) in the membrane separation process is started. The output (= discharge amount) of the pressure feed pump 42 was adjusted so that the permeate was discharged at a filtration rate of 50 cc per minute.

  The calcium component in the raw water is precipitated as calcium carbonate in the reaction tank 34 in which sodium hydroxide is supplied from the alkali supply unit 32 to make an alkaline environment. The reaction liquid containing calcium carbonate particles precipitated in the reaction tank 34 is introduced into the hollow fiber of the internal pressure type filtration membrane 35a via the pressure feed pump 42 and separated into solid and liquid (membrane separation step).

  At the beginning of the operation, the permeated liquid amount of the filtration membrane is 50 cc per minute, and the concentrated liquid flowing out from the outlet of the filtered film flows again through the circulation line 36 toward the upstream side of the pressure pump 42, but a part of this concentrated liquid ( 50 cc per minute, which is equal to the amount supplied by the raw water supply unit 31), and is circulated to the reaction tank 34 through the pipe 36 a (this is realized by the control unit 47 controlling the circulation amount adjustment unit 44). The remaining concentrated liquid was circulated by sending it to the pressure feed pump 42 side through the pipe 36b. The concentrated liquid circulated in the reaction vessel 34 contains calcium carbonate particles, and these particles exhibit a crystallization effect as seed crystals in the reaction vessel 34.

  Note that at the beginning of the above operation, when the crystallization effect in the reaction vessel 34 is not manifested, the minimum particle diameter of the calcium carbonate particles (primary crystal nuclei) is 0.19 μm, and the average pore diameter of the filtration membrane 35a is 0. It was smaller than 45 μm.

  The particle size distribution of the calcium carbonate particles was measured using a measuring instrument LA-920 (manufactured by Horiba, Ltd.) based on the principle of laser diffraction / scattered light method. In the measurement, 1000 ml of the sample was used, and a dispersion medium hexametaphosphate Na 1% solution was added as a pretreatment for the purpose of making the sample uniform. The average particle diameter A and the standard deviation σ in the measured particle size distribution of the calcium carbonate particles were determined, and the value of (A-2σ) was calculated as the minimum particle diameter of Example 1.

  Moreover, the average pore diameter of the filtration membrane 35a was calculated | required as follows. Monodisperse latexes (STADEX from JSR) with latex particle sizes of 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, and 0.5 μm were each added to 0.5% SDS (sodium dodecyl sulfonate) aqueous solution. To prepare a suspension having a latex concentration of 0.01%. 100 mL of the suspension was placed in a beaker and supplied to the hollow fiber membrane having an effective length of about 21 cm with a tube pump at a pressure of 100 kPa from the inner surface, and the permeate was obtained by permeating the hollow fiber membrane. The suspension and the permeate were sampled and the absorbance was measured using a spectrophotometer (UV-mini 1240 manufactured by Shimadzu Corporation) to obtain the concentration. When the relationship between the latex rejection rate and the latex average particle size was determined from the concentrations of the permeate and the suspension, the latex average particle size when the latex rejection rate reached 90% or more was 0.45 μm. From this result, the average pore diameter of the filtration membrane 35a was set to 0.45 μm.

  Under these conditions, the raw water supply and membrane filtration operation are continued, and the operation is continued without discharging the concentrate from the concentrate discharge process until the calcium carbonate concentration in the concentrate at the outlet of the filtration membrane reaches 2.5%. did. When the concentration of calcium carbonate in the concentrated liquid at the outlet of the filtration membrane reached 2.5%, the concentration of calcium carbonate in the reaction tank 34 was about 1.3%.

  When the calcium carbonate concentration in the concentrate at the filtration membrane outlet in the membrane separation process reaches 2.5%, the filtration membrane 35a is replaced with a new one, and the output of the pressure pump 42 is initially filtered at 50 cc per minute. After that, the permeate was adjusted so that the permeated liquid was discharged. Thereafter, the output (= discharge amount) of the pressure feed pump 42 was kept constant, and the operation was resumed.

  On the other hand, the raw water continues to be supplied from the raw water supply unit 31 while being adjusted so as to be equivalent to the permeate exiting from the filtration membrane 35a (50 cc per minute at the beginning of the operation, gradually decreasing with time). The backwashing operation of the filtration membrane 35a was not performed. In addition, the concentrated liquid was returned to the reaction tank 34 (constant at 50 cc / min) even after the operation was resumed after replacing with a new membrane. At the beginning of the resumption of operation, the calcium carbonate concentration in the reaction vessel 34 was about 1.3%. This is because the control unit 47 controls the circulation amount adjusting unit 44 so that the calcium carbonate concentration in the reaction solution in the reaction tank 34 is within a predetermined range (here, about 1.3%). This is realized by controlling the supply amount of the concentrate supplied from the circulation line 36 into the reaction tank 34 (reaction process).

  Further, the amount of concentrated liquid discharged from the concentrated liquid discharging step was adjusted so that the concentration of calcium carbonate in the concentrated liquid at the outlet of the filtration membrane was always maintained at 2.5%. The discharge amount of the concentrate was about 0.34 cc per minute. This is because the control unit 47 controls the discharge amount adjustment unit 45 so that the calcium carbonate concentration in the concentrated liquid detected by the concentration detection unit 43 is within a predetermined range (here, 2.5%). Further, it is realized by controlling the discharge amount of the concentrate discharge section (concentrate discharge process).

FIG. 11 shows the relationship between filtration time and filtration performance after switching to a new membrane and restarting operation. Here, the filtration performance means that the difference between the circulating water side average pressure and the permeate side pressure passing through the hollow portion of the filtration membrane 35a is converted to 100 kPa, and the circulating water side temperature is 25 ° C. The unit is LMH (L / m ² / Hr).

Further, the rate of decrease in film flux in filtration time (200 minutes = 3.33Hr) ^{was 39LMH 2 (L / m 2 /} Hr 2).

  When the membrane filtration operation was resumed after replacement with a new membrane, the minimum particle size of calcium carbonate contained in the concentrate at the outlet of the filtration membrane was 0.88 μm, which exceeded the average pore size of 0.45 μm of the filtration membrane 35a. Thus, in the water treatment system 30 of Example 1, the control part 47 controls the minimum particle diameter of the calcium carbonate in a reaction liquid larger than the average pore diameter of the filtration membrane 35a. The reason why the filtration rate does not decrease with time is that the calcium carbonate particles having a minimum particle diameter of 0.88 μm are not blocked by the pores of the filtration membrane (average pore diameter of 0.45 μm), and the filtration continues stably. It is thought that it was because of. In addition, the calcium concentration in the permeate exiting from the filtration membrane 35a was 16 mg / L, and the removal rate of calcium in the raw water was 81%.

Furthermore, since the concentrate discharged from the concentrate discharge process does not contain impurities such as flocculants and contains only pure calcium carbonate, the sludge obtained from this concentrate is burned. It can be used as a desulfurizing agent for removing sulfur components from the flue gas of the apparatus.
(Comparative example)
The configuration of the water treatment system 60 according to Comparative Example 1 shown in FIG. 10 is basically the same as that of Example 1 shown in FIG. 7. The difference from Example 1 is that a flocculant is added in the reaction tank 34. It is only a point provided with the flocculant addition part 50 to do. That is, the volume of the reaction tank 34 in FIG. 10 is 1 liter, and the CTB waste water shown in Table 1 is continuously supplied as raw water, and a flocculant (FeCl ₃ ) is added to the reaction tank 34 at 50 mg / L (Fe conversion) and water. The pH was adjusted to 9.5 or more and 12 or less while adding sodium oxide. Further, as the filtration membrane 35a, an internal pressure type hollow fiber microfiltration membrane having an average pore diameter of 0.45 μm and a yarn inner diameter of 1.1 mm was used. Carbon dioxide gas (shown as “(CO ₂ )”) in FIG. 10 is not injected in this beaker test. However, as described above, when the carbonic acid component in the raw water is insufficient for the reaction with calcium, It is preferable to perform the injection.

  While supplying 50 cc of raw water to the reaction tank 34 (volume: 1 liter) in the reaction process, the pumping pump 42 upstream of the filtration membrane 35a in the membrane separation process is started, and the permeate is filtered at a filtration rate of 50 cc per minute. The output (= discharge amount) of the pressure-feed pump 42 was adjusted so as to exit.

  The calcium component in the raw water is precipitated as calcium carbonate in the reaction tank 34 supplied with sodium hydroxide to be in an alkaline environment. The reaction liquid containing calcium carbonate particles precipitated in the reaction tank 34 is introduced into the hollow fiber of the internal pressure filtration membrane through the pressure feed pump 42 in the membrane separation step, and is separated into solid and liquid.

  At the beginning of operation, the amount of permeated liquid through the filtration membrane is 50 cc / min, and the concentrated liquid flowing out from the outlet of the filtering film flows again toward the upstream side of the pumping pump 42. 50 cc / min.) Was branched halfway and circulated to the reaction tank 34, and the remaining concentrated liquid was sent to the pressure pump 42 side and circulated. The concentrated liquid circulated in the reaction vessel 34 contains calcium carbonate particles, and these particles exhibit a crystallization effect as seed crystals in the reaction vessel 34 as in Example 1. The concentrated liquid also contains a flocculant added in the reaction vessel 34.

  Under these conditions, the raw water supply and the membrane filtration operation are continued, and the concentration from the concentrate discharge step is continued until the SS concentration (concentration of the mixture of calcium carbonate and coagulant) in the concentrate at the outlet of the filtration membrane becomes 2.5% The operation was continued without draining the liquid. When the SS concentration in the concentrate at the outlet of the filtration membrane became 2.5%, the SS concentration in the reaction vessel 34 was about 1.3%.

  When the calcium carbonate concentration in the concentrate at the filtration membrane outlet in the membrane separation process reaches 2.5%, the filtration membrane 35a is replaced with a new one, and the output of the pressure pump 42 is initially filtered at 50 cc per minute. After that, the permeate was adjusted so that the permeated liquid was discharged. Thereafter, the output (= discharge amount) of the pressure feed pump 42 was kept constant, and the operation was resumed.

  On the other hand, the raw water continues to be supplied while being adjusted so as to be equivalent to the permeate flowing out from the filtration membrane 35a (50 cc per minute at the beginning of operation, gradually decreasing with time). No backwash operation was performed. In addition, the concentrated liquid was returned to the reaction tank 34 (constant at 50 cc / min) even after the operation was resumed after replacing with a new membrane. At the beginning of the resumption of operation, the SS concentration in the reaction vessel 34 was about 1.3%.

  Furthermore, the concentrated liquid discharge amount from the concentrated liquid discharge process was adjusted so that the SS concentration in the concentrated liquid at the outlet of the filtration membrane always maintained 2.5%. The amount of concentrated liquid discharged was about 0.53 cc per minute.

FIG. 11 shows the relationship between filtration time and filtration performance after switching to a new membrane and restarting operation. (Plot marked with “◇”) The rate of decrease in membrane flux at the filtration time (200 minutes = 3.33 Hr) was 153 LMH ² (L / m ² / Hr ² ), which was 39 LMH ² (L / m ² / Hr ² ).

  When the membrane filtration operation is restarted after replacing with a new membrane, the minimum particle size of SS (floating solids: mixture of calcium carbonate and coagulant reactant) contained in the concentrate at the filtration membrane outlet is 3.4 μm or more. The average pore diameter of the membrane 35a was much larger than 0.45 μm. This is because the calcium carbonate particles are aggregated by the aggregating agent to increase the particle diameter. In the relationship between the particle diameter in the liquid to be filtered and the pore diameter of the filtration membrane 35a, the filtration should be continued stably, but the amount of filtration gradually decreases with time. The reason for this is that the flocculant reactant (iron oxide component) contained in the SS is concentrated to increase the viscosity of the concentrate and fouling (film clogging) adhering to the film surface cake layer occurs. Conceivable.

  When the content of calcium carbonate contained in 2.5% SS (mixture of calcium carbonate and flocculant reactant) at the membrane filtration outlet was measured, about 60% of the total SS and the remaining 40% were flocculant reactant. I found out that This confirms that a large amount of the flocculant reactant contained in the concentrate increases the viscosity of the concentrate, which causes membrane fouling.

  Further, as described above, the SS in the concentrate discharged from the concentrate discharge step contains about 40% of a flocculant reactant (iron oxide component and mainly calcium carbonate impurities). The sludge obtained from the concentrated liquid cannot be used as a desulfurization agent for removing sulfur components from the flue gas of the combustion apparatus.

  Furthermore, the amount of concentrated liquid discharged from the concentrated liquid discharging step was 0.34 cc / min (concentration 2.5%) in Example 1 whereas the comparative example was 0.53 cc / min (concentration 2.5%). Therefore, the amount is increased by about 60%, and it must be treated as industrial waste, which increases the environmental load and enormous disposal costs.

  Furthermore, the calcium concentration in the permeate discharged from the filtration membrane 35a was 19 mg / L, and the removal rate of calcium in the raw water was 78%, which was the same as in Example 1.

  As described in detail above, according to the present invention, without using a chemical such as a flocculant or a coagulant, the membrane flux of the filtration membrane in the membrane separation process is stably maintained high, and at a low cost, Calcium can be removed efficiently. In addition, since the membrane flux of the filtration membrane is stabilized, the frequency of chemical cleaning can be reduced, the chemical cost for chemical cleaning can be reduced, and the number of preliminary membranes can be reduced. Furthermore, since the device operating rate is improved, the capacity of the wastewater storage tank can be reduced, and the equipment cost is reduced. In addition, the calcium carbonate concentrate concentrated in the membrane separation process and discharged out of the system from the concentrate discharge section does not contain any impurities such as flocculants and coagulants. Industrial utilization such as desulfurization agent for removing sulfur components from the machine smoke is possible.

10, 30, 30B, 30C Water treatment system 11, 31 Raw water supply unit 12, 32 Alkali supply unit 13, 33 Carbon dioxide supply unit 14 Reaction tank 15 Membrane filtration device (membrane separation unit)
15a, 35a Filtration membrane 16, 36 Circulation line 17 Post-processing device 18 Concentrate discharge unit 23, 43 Concentration detection unit 24, 44 Circulation amount adjustment unit 25, 45 Discharge amount adjustment unit 25 Circulating water side temperature 27, 47 Control unit 34 Reaction tank (reaction unit)
35 Membrane separation part 48 Reaction tube (reaction part)

Claims (16)

A water treatment method for removing calcium from raw water,
A reaction step of obtaining a reaction solution containing calcium carbonate and not containing a flocculant from the raw water;
A membrane separation step of separating the reaction solution into a permeate and a concentrate by a filtration membrane;
A concentrate discharging step of discharging a part of the concentrate out of the system,
In the reaction step, at least a part of the concentrated solution obtained in the membrane separation step is circulated and mixed with the reaction solution, and the reaction solution is adjusted to be alkaline.
A water treatment method for controlling a minimum particle size of calcium carbonate in the reaction solution to be larger than an average pore size of the filtration membrane. In the reaction step, at least part of the concentrated solution is circulated and mixed with the reaction solution so that the calcium carbonate concentration in the reaction solution is within a predetermined range,
The said concentrate discharge process WHEREIN: A part of said concentrate is discharged | emitted out of the system so that the calcium carbonate density | concentration in the said concentrate obtained in the said membrane separation process may become in a predetermined range. Water treatment method. In the reaction step, at least a part of the concentrated liquid is circulated and mixed with the reaction liquid so that the calcium carbonate concentration in the reaction liquid is 0.1% or more,
In the concentrate discharge step, a part of the concentrate is discharged out of the system so that the calcium carbonate concentration in the concentrate obtained in the membrane separation step is 0.1% or more and 15% or less,
The water treatment method according to claim 1 or 2, wherein an average pore diameter of the filtration membrane is 0.04 µm or more and 1.0 µm or less.   The water treatment method according to any one of claims 1 to 3, wherein the membrane separation step uses an internal pressure filtration method using a hollow fiber membrane as the filtration membrane.   The water treatment method according to any one of claims 1 to 4, wherein in the reaction step, the pH of the reaction solution is maintained at 9 or more and 13 or less.   The water treatment method according to any one of claims 1 to 5, further comprising a carbon dioxide supply step of supplying carbon dioxide to the reaction solution.   The water treatment method according to claim 6, wherein the carbon dioxide is carbon dioxide in flue gas discharged from a combustion device of a thermal power generator.   The water according to any one of claims 1 to 7, wherein calcium carbonate contained in the concentrate discharged in the concentrate discharge step is used as a desulfurizing agent for removing sulfur components from the flue gas of the combustion apparatus. Processing method. A water treatment system for removing calcium from raw water,
A reaction part for storing a reaction liquid containing calcium carbonate and not containing a flocculant;
A raw water supply unit for supplying the raw water to the reaction unit;
An alkali supply unit for supplying an alkaline substance to the reaction solution in the reaction unit;
A membrane separation part for separating the reaction solution into a permeate and a concentrate by a filtration membrane;
A concentrated liquid discharger for discharging a part of the concentrated liquid out of the system;
A circulation line that connects the concentration side of the membrane separation unit and the reaction unit, and supplies at least a part of the concentrate obtained in the membrane separation unit into the reaction unit;
A water treatment system comprising: a control unit that controls a minimum particle diameter of calcium carbonate in the reaction solution to be larger than an average pore diameter of the filtration membrane. The water treatment system further includes a concentration detection unit that detects a calcium carbonate concentration in the concentrate obtained by the membrane separation unit,
The control unit controls the supply amount of the concentrate supplied from the circulation line into the reaction unit so that the calcium carbonate concentration in the reaction solution in the reaction unit is within a predetermined range;
The water treatment system of Claim 9 which controls the discharge | emission amount of the said concentrate discharge part so that the calcium carbonate density | concentration in the said concentrate detected in the said density | concentration detection part may be in the predetermined range. The control unit controls the supply amount of the concentrate supplied from the circulation line into the reaction unit so that the calcium carbonate concentration in the reaction solution in the reaction unit is 0.1% or more,
Controlling the discharge amount of the concentrate discharge section so that the calcium carbonate concentration in the concentrate detected by the concentration detection section is 0.1% or more and 15% or less,
The water treatment system according to claim 10, wherein an average pore diameter of the filtration membrane is 0.04 μm or more and 1.0 μm or less.   The water treatment system according to any one of claims 9 to 11, wherein the membrane separation unit uses an internal pressure filtration method using a hollow fiber membrane as the filtration membrane.   The water treatment system according to any one of claims 9 to 12, wherein the pH of the reaction solution in the reaction unit is maintained at 9 or more and 13 or less.   The water treatment system according to any one of claims 9 to 13, further comprising a carbon dioxide supply unit that supplies carbon dioxide to the reaction unit.   The water treatment system according to claim 14, wherein the carbon dioxide is carbon dioxide in flue gas discharged from a combustion device of a thermal power generator.   The water as described in any one of Claims 9-15 which uses calcium carbonate contained in the said concentrate discharged | emitted by the said concentrate discharge part as a desulfurization agent which removes a sulfur component from the flue gas of a combustion apparatus. Processing system.