Method and apparatus for treating high-hardness water

Abstract

To provide a treatment method and device capable of effectively preventing scaling and fouling of a reverse osmosis membrane when treating high hardness water containing calcium and magnesium such as wet desulfurization waste water, and reducing replacement frequency of the reverse osmosis membrane and reducing operation cost.SOLUTION: A high hardness water treatment method includes the following: adding alkali metal hydroxide A and sodium carbonate B to high hardness water W containing calcium and magnesium so that the pH of the high hardness water becomes 9.5-11.5; removing a precipitate S obtained thereby; adding an inorganic acid H and a ferrous flocculant F to a liquid obtained by removing the precipitate so that the pH of the liquid becomes 2.5-4.5; removing flocks S' and S'' obtained thereby; applying separation treatment by a reverse osmosis membrane 9 to a liquid obtained by removing the flocks to separate permeated water P and concentrated water C; and applying treatment 10 of evaporating to dryness to the concentrated water to obtain a dried solid D.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for treating high-hardness water containing calcium and magnesium, such as wet desulfurization effluent, and more particularly to an apparatus and method for treating high-hardness water using a reverse osmosis membrane, which can effectively prevent scaling and fouling of the reverse osmosis membrane, reduce the frequency of reverse osmosis membrane replacement, and reduce operating costs. [Background technology]

[0002] High-hardness water, such as wastewater from flue gas desulfurization systems (hereinafter referred to as "desulfurization wastewater"), contains various elements and must be treated to reduce its content to below environmental regulation values. One known treatment method involves evaporating the desulfurization wastewater to recover the solids. This method requires a large amount of thermal energy for evaporation. To reduce the thermal energy required for evaporation, it has been proposed to concentrate the desulfurization wastewater using a reverse osmosis membrane.

[0003] For example, Patent Document 1 discloses a method for treating desulfurization effluent, which is characterized by subjecting desulfurization effluent to a precipitation treatment, passing the resulting treated liquid through a reverse osmosis membrane to concentrate it and obtain purified water, and further concentrating the concentrated water obtained by passing it through the reverse osmosis membrane using an evaporator, and then drying it to obtain a dry solid.

[0004] Patent Document 2 discloses a wastewater treatment method in which the wastewater, after separating sulfur oxides from flue gas as gypsum, is adjusted to a pH of at least 5.0 and then treated using a reverse osmosis membrane. Patent Document 2 suggests that, to prevent calcium sulfate scale formation, prior to pH adjustment, alkaline agents such as alkali carbonates, alkali phosphates, and caustic alkalis can be added; coagulation and precipitation treatment can be performed using a coagulant; metal ions such as calcium and magnesium ions can be separated and removed using a weakly or strongly acidic ion exchange resin; or crystallization of metal ions can be inhibited by adding an ion-blocking agent such as sodium carboxymethylcellulose (CMC), sodium carboxymethylcellulose (CMS), or EDTA salt. Patent Document 2 also suggests that, to prevent calcium carbonate scale formation, after metal ion separation or crystallization inhibition treatment, the pH can be adjusted to 3.5 to 5.0, carbonates can be separated using an aeration tower, and carbon dioxide can be released.

[0005] Patent Document 3 discloses a method for treating wastewater from a wet flue gas desulfurization system, which comprises: a reaction step in which wastewater from a wet flue gas desulfurization system is subjected to alkali treatment and softening treatment to precipitate heavy metals, fluorine, calcium, magnesium, and the like as solids; a first solid-liquid separation step in which the solids produced in the reaction step are separated from the wastewater; a second solid-liquid separation step in which the pH of the wastewater treated in the first solid-liquid separation step is adjusted and fine solids contained in the wastewater are separated using a microfiltration membrane; and a salts separation step in which salts dissolved in the wastewater treated in the second solid-liquid separation step are separated using a reverse osmosis membrane, and which is characterized in that the water recovered in the salts separation step is circulated to the flue gas desulfurization system for reuse.

[0006] Patent Document 4 discloses a method for treating wastewater containing calcium and sulfuric acid, characterized by adding sodium carbonate to wastewater containing calcium and sulfuric acid to be treated, thereby precipitating the calcium contained in the wastewater as calcium carbonate crystals, and then concentrating the wastewater by boiling and evaporating it using indirect heating while it still contains the calcium carbonate crystals; drying the concentrated wastewater that has been boiled and evaporated while it still contains the calcium carbonate crystals; and supplying the wastewater containing calcium and sulfuric acid to be treated to a reverse osmosis membrane module using a reverse osmosis membrane as a pretreatment before adding sodium carbonate to it, and separating it into permeate and non-permeate, and supplying the non-permeate to the point where sodium carbonate is added. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 10-85742 [Patent Document 2] Japanese Patent Application Publication No. 51-93789 [Patent Document 3] Japanese Patent Application Publication No. 10-137540 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-305541 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide an apparatus and method for treating high-hardness water containing calcium and magnesium, such as wet desulfurization effluent, using a reverse osmosis membrane, which can effectively prevent scaling and fouling of the reverse osmosis membrane, reduce the frequency of reverse osmosis membrane replacement, and reduce operating costs. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention has been completed, including the following aspects.

[0010] [1] Adding an alkali metal hydroxide or a water-soluble alkaline earth metal hydroxide and an alkali metal carbonate to high-hardness water containing calcium and magnesium so that the pH is 9.5 to 11.5, and removing the resulting precipitate; Add an inorganic acid and an iron-based coagulant to the liquid from which the precipitates have been removed so that the pH is 2.5 to 4.5, and remove the resulting flocs. To the liquid from which flocs have been removed by adding an inorganic acid and an iron-based coagulant, an alkali metal hydroxide is added so that the pH becomes 6.5 to 7.5, and the resulting flocs are removed. the The liquid from which the flocs have been removed is subjected to a separation process using a reverse osmosis membrane to separate it into permeate and concentrated water, The concentrated water is evaporated to dryness to obtain a dry solid. How to treat high hardness water.

[0012] [ 2 ] Reactive crystallizer, first flocculation and sedimentation device, Second coagulation and sedimentation device, A reverse osmosis membrane device and an evaporation and drying device are included. The reactive crystallizer is equipped with a mechanism for adding an alkali metal hydroxide or a water-soluble alkaline earth metal hydroxide and an alkali metal carbonate to high-hardness water containing calcium and magnesium so that the pH is adjusted to 9.5 to 11.5, and for promoting the removal of the precipitate obtained thereby; The first coagulation settling device is equipped with a mechanism for adding an inorganic acid and an iron-based coagulant to the liquid from which precipitates have been removed so that the pH is adjusted to 2.5 to 4.5, thereby promoting the removal of the resulting flocs; The second coagulation settling device is equipped with a mechanism for adding an alkali metal hydroxide to the liquid from which flocs have been removed by adding an inorganic acid and an iron-based coagulant so that the pH is 6.5 to 7.5, thereby promoting the removal of the resulting flocs; The reverse osmosis membrane device is equipped with a reverse osmosis membrane for promoting separation of the liquid from which the flocs have been removed into permeate and concentrated water, The evaporator includes a mechanism for facilitating evaporation of liquid from the concentrate to form a dry solid. High hardness water treatment equipment. [Effects of the Invention]

[0014] The present invention High hardness waterThe treatment method and treatment device can effectively prevent scaling and fouling of the reverse osmosis membrane, reduce the frequency of replacement of the reverse osmosis membrane, and reduce operating costs. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram illustrating an example of a wet desulfurization device. [Figure 2] 1 is a diagram showing an example of a high-hardness water treatment device of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The method for treating high-hardness water of the present invention comprises a reactive crystallization step, a first coagulation-sedimentation step, a membrane separation step, an evaporation-to-dryness step, and, optionally, a second coagulation-sedimentation step.The high-hardness water treatment device of the present invention comprises a reactive crystallization apparatus, a first coagulation-sedimentation apparatus, a reverse osmosis membrane apparatus, an evaporation-to-dryness apparatus, and, optionally, a second coagulation-sedimentation apparatus.The high-hardness water treatment device of the present invention has flow paths between each apparatus so that the liquid to be treated flows through the reactive crystallization apparatus, the first coagulation-sedimentation apparatus, the optional second coagulation-sedimentation apparatus, and the reverse osmosis membrane apparatus in this order.A flow path is also provided for supplying concentrated water from the reverse osmosis membrane apparatus to the evaporation-to-dryness apparatus.

[0017] Examples of high-hardness water containing calcium and magnesium include desulfurization wastewater, seawater, cooling tower blowdown wastewater, landfill leachate, groundwater, and mine wastewater. From the viewpoint of large treatment volumes, the present invention is preferably applied to desulfurization wastewater. The high-hardness water applicable to the present invention typically has a hardness of 60 mg / L or more, preferably 180 mg / L or more, more preferably 357 mg / L or more, even more preferably 1000 mg / L or more, even more preferably 3000 mg / L or more, and most preferably 5000 mg / L or more. Note that hardness is calculated by converting the amounts of calcium and magnesium into the amount of calcium carbonate (CaCO3) and can be calculated using the following formula: Hardness [mg / L] = (Calcium [mg / L] x 2.5) + (Magnesium [mg / L] x 4.1)

[0018] Flue gas desulfurization is performed by passing the combustion exhaust gas through a wet flue gas desulfurization system. Representative wet flue gas desulfurization methods include the lime-gypsum method, the magnesium hydroxide method, and the soda method. The wet flue gas desulfurization system used in the present invention includes a gas absorption system 15 that can contact the combustion exhaust gas with an aqueous slurry containing limestone (CaCO), slaked lime (Ca(OH)), quicklime (CaO), etc., and further includes a gypsum separation and removal system 26. This gas absorption system can be a packed tower, a wetted-wall tower, a spray tower, etc. In this gas absorption system, sulfur oxides in the combustion exhaust gas react with the limestone, slaked lime, or quicklime in the aqueous slurry to form calcium sulfite, which is then oxidized to calcium sulfate (gypsum). In the gypsum separation and removal system, gypsum is separated and removed from the gypsum slurry (aqueous slurry containing calcium) discharged from the gas absorption system. This separation and removal of gypsum results in the discharge of gypsum dehydration filtrate (aqueous solution containing calcium). Calcium, magnesium, silica, and other elements contained in high-hardness water, such as desulfurization wastewater, solidify and form scale. Furthermore, in desulfurization equipment, for example, water is recycled, and high-hardness water often has high COD and BOD, which makes it prone to fouling in reverse osmosis membranes.

[0019] In the reactive crystallization step, an alkali metal hydroxide or a water-soluble alkaline earth metal hydroxide A and an alkali metal carbonate B are added to high-hardness water, and the resulting precipitate S is removed. A reactive crystallization apparatus I for promoting the progress of the reactive crystallization step includes a reaction tank 1, a solid-liquid separation device (e.g., a settling tank 2), a supply flow path for the alkali metal hydroxide or the water-soluble alkaline earth metal hydroxide A to the reaction tank 1, a supply flow path for the alkali metal carbonate B to the reaction tank 1, a supply flow path for the high-hardness water to the reaction tank 1, a flow path for transferring the liquid from the reaction tank 1 to the solid-liquid separation device, and a flow path for discharging the liquid from the solid-liquid separation device. Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, etc. Among these, sodium hydroxide is preferred. Examples of the water-soluble alkaline earth metal hydroxide include calcium hydroxide, strontium hydroxide, etc. Among these, calcium hydroxide is preferred. Examples of alkali metal carbonates include sodium carbonate and potassium carbonate, with sodium carbonate being preferred. The alkali metal hydroxide or water-soluble alkaline earth metal hydroxide A and the alkali metal carbonate B can be added in the form of powder, granules, flakes, or an aqueous solution.

[0020] The alkali metal hydroxide or the water-soluble alkaline earth metal hydroxide and the alkali metal carbonate are added in amounts such that the pH of the high-hardness water becomes 9.5 to 11.5. The alkali metal hydroxide or water-soluble alkaline earth metal hydroxide A and the alkali metal carbonate B may be added simultaneously or sequentially. The order of addition of the alkali metal hydroxide or water-soluble alkaline earth metal hydroxide A and the alkali metal carbonate B is not particularly limited, and either may be added first. The addition of alkali metal hydroxide or water-soluble alkaline earth metal hydroxide mainly precipitates magnesium contained in high-hardness water as magnesium hydroxide. It also promotes the precipitation of heavy metals, such as Si and Al, contained in high-hardness water. On the other hand, the addition of alkali metal carbonate mainly precipitates calcium contained in high-hardness water as calcium carbonate. The solubility of calcium carbonate and magnesium hydroxide decreases as the liquid temperature increases, making precipitation easier. However, as the liquid temperature increases, convection is more likely to occur due to the influence of the surrounding environment, which may reduce the efficiency of gravity-type solid-liquid separation.

[0021] The precipitate is removed using a solid-liquid separator. Examples of solid-liquid separators include decanters (overflow type, skimming type, etc.), filter cloth bags, screw presses, roller presses, rotary drum screens, belt screens, vibrating screens, multi-plate wave filters, vacuum dehydrators, pressure dehydrators, belt presses, cyclone-type solid-liquid separators (liquid cyclones), centrifugal thickeners, and multi-disk dehydrators. A portion of the removed precipitate may be returned to the reaction vessel 1 as seed crystals. The addition of seed crystals increases the size of the precipitate, facilitating solid-liquid separation. The precipitate slurry obtained by solid-liquid separation can be evaporated to dryness together with the concentrated water.

[0022] The liquid from which the precipitates have been removed often contains suspended fine particles (soot, gypsum, the above-mentioned precipitates, etc.) that were not removed in the reactive crystallization step. Furthermore, the liquid may contain compounds that are highly soluble at high pH. Therefore, the first coagulation and precipitation step is carried out.

[0023] In the first coagulation-sedimentation step, inorganic acid H and iron-based coagulant F are added to the liquid from which precipitates have been removed, and the resulting first flocs S' are removed. The first coagulation-sedimentation device II, which promotes the progress of the first coagulation-sedimentation step, includes a reaction tank 3, a solid-liquid separation device (e.g., settling tank 4), a supply flow path for the inorganic acid to the reaction tank 3, a supply flow path for the iron-based coagulant to the reaction tank 3, a supply flow path for the liquid from which precipitates have been removed to the reaction tank 3, a flow path for transferring the liquid from the reaction tank 3 to the solid-liquid separation device, and a flow path for discharging the liquid from the solid-liquid separation device. In a device in which the reaction tank 3 and the solid-liquid separation device are integrated, the flow path for transferring the liquid from the reaction tank 3 to the solid-liquid separation device can be omitted. Furthermore, since the first flocs formed in the first coagulation-sedimentation step can be removed together with the second flocs by the solid-liquid separation device in the subsequent second coagulation-sedimentation step, the solid-liquid separation device (e.g., settling tank 4) provided in the first coagulation-sedimentation device may be omitted.

[0024] Examples of inorganic acids include hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid. Of these, hydrochloric acid is preferred. Examples of iron-based flocculants include ferric chloride, polyferric sulfate, and polysilica iron. Of these, ferric chloride is preferred. Inorganic acid H and iron-based flocculant F can be added in the form of powder, granules, flakes, dispersion, or aqueous solution.

[0025] The inorganic acid and the iron-based flocculant are added in amounts that adjust the pH of the liquid to 2.5 to 4.5. The inorganic acid and the iron-based flocculant may be added simultaneously or sequentially. The order in which the inorganic acid and the iron-based flocculant are added is not particularly limited, and either may be added first. However, it is preferable to add the inorganic acid first from the viewpoint of flocculating substances that precipitate under acidic conditions. This causes the fine particles to flocculate, forming first flocs. The first flocs are removed using a solid-liquid separator. Examples of the solid-liquid separator include the same ones as those described above. A portion of the removed first flocs may be returned to the reaction tank 3 as seed crystals. The liquid from which the first flocs have been removed can be sent as raw water to the membrane separation step.

[0026] In the first flocculation and sedimentation step, if necessary, a polymer flocculant G may be added to the liquid from which the precipitate has been removed. Examples of polymer flocculants include anionic polymer flocculants, nonionic polymer flocculants, cationic polymer flocculants, and amphoteric polymer flocculants. The polymer flocculant G can be added in the form of powder, granules, flakes, dispersion, or aqueous solution. The addition of the polymer flocculant may promote coarsening of the first flocs.

[0027] The liquid from which the first flocs have been removed may contain compounds that are highly soluble at low pH. Furthermore, fine particles that could not be removed in the first coagulation and sedimentation step may remain suspended in the liquid. In such cases, a second coagulation and sedimentation step (IV) may be carried out.

[0028] In the second coagulation and sedimentation step, an alkali metal hydroxide N is added to the liquid from which the first flocs have been removed, and the resulting second flocs S" are removed. The second coagulation and sedimentation device III, which promotes the progress of the second coagulation and sedimentation step, includes a reaction tank 5 and a solid-liquid separation device (e.g., a settling tank 6). It includes a supply flow path for the alkali metal hydroxide to the reaction tank 5, a supply flow path for the liquid from which the first flocs have been removed to the reaction tank 5, a flow path for transferring the liquid from the reaction tank 5 to the solid-liquid separation device, and a flow path for discharging the liquid from the solid-liquid separation device. In an apparatus in which the reaction tank 5 and the solid-liquid separation device are integrated, the flow path for transferring the liquid from the reaction tank to the solid-liquid separation device can be omitted.

[0029] Examples of the alkali metal hydroxide N used in the second coagulation and precipitation step include sodium hydroxide and potassium hydroxide. Of these, sodium hydroxide is preferred. The alkali metal hydroxide N can be added in the form of powder, granules, flakes, or an aqueous solution. The alkali metal hydroxide is added in an amount that makes the pH of the liquid 6.5 to 7.5. This causes the fine particles to coagulate and form second flocs. The second flocs are removed using a solid-liquid separator. Examples of the solid-liquid separator include the same ones as those described above. A portion of the removed second flocs may be returned to the reaction tank 5 as seed crystals.

[0030] In the present invention, in order to remove the second flocs, solid-liquid separation by filtration using a multi-layer filtration membrane, an ultrafiltration membrane, filter sand 7, or the like may be performed instead of or in addition to the solid-liquid separator (e.g., settling tank 6) in the second coagulation and sedimentation step. This may prevent the reverse osmosis membrane from fouling. Backwashing can be performed when the efficiency of the filtration step decreases. The backwash wastewater can be subjected to solid-liquid separation in a settling tank as necessary, and the supernatant water can be sent to the first coagulation and sedimentation step. The solid slurry obtained by backwashing can be evaporated to dryness together with the concentrated water. The liquid from which the second flocs have been removed can be sent as raw water to the membrane separation step.

[0031] Next, a membrane separation step is carried out. In the membrane separation step, raw water (the liquid from which the first flocs have been removed or the liquid from which the second flocs have been removed) is subjected to a separation treatment using a reverse osmosis membrane 9 to separate it into permeate P and concentrate C. The reverse osmosis membrane device IV, which promotes the progress of the membrane separation step, includes a reverse osmosis membrane, a raw water pressurizing device or a permeate suction device, a flow path for supplying raw water to the reverse osmosis membrane, a flow path for discharging concentrate water from the reverse osmosis membrane, and a flow path for discharging permeate water from the reverse osmosis membrane. The amount of water permeating the reverse osmosis membrane decreases as the water temperature decreases, and the differential pressure required to obtain the same amount of water increases. Conversely, as the water temperature increases, the amount of water permeating increases, but the salt rejection rate decreases. Therefore, when performing separation treatment using the reverse osmosis membrane, the temperature of the liquid is preferably adjusted to between 0°C and 85°C, more preferably between 0°C and 45°C, and even more preferably between 5°C and 35°C. To adjust the temperature of the liquid, a temperature control device such as a cooler, cooling tower 8, or heater may be installed, for example, between the second coagulation sedimentation device and the reverse osmosis membrane device, although the installation location is not particularly limited.

[0032] Examples of reverse osmosis membranes include membranes made of cellulose acetate, membranes made of aromatic polyamide, membranes made of polyvinyl alcohol, membranes made of polysulfone, etc. The reverse osmosis membrane is not limited by its structure, and examples thereof include hollow fiber membranes, spiral membranes, and tubular membranes.

[0033] The reverse osmosis membrane is preferably incorporated into a membrane module. A casing-type module is constructed by accommodating a membrane element, which is an integral combination of a reverse osmosis membrane, its support, and a flow path material, in a casing. Examples of casing-type modules include pleated modules, spiral modules, monolith modules, tubular modules, and hollow fiber modules.

[0034] The reverse osmosis membrane uses calcium ions (Ca 2+ ), sodium ions (Na + ), potassium ions (K + ), iron ions (Fe 3+), cations such as sulfate ions (SO4 2- ), chloride ions (Cl - It does not allow anions such as lead (Pb), mercury (Hg), cadmium (Cd), or arsenic (As) to pass through. The water that passes through the reverse osmosis membrane (permeate) has almost all of the impurities contained in high-hardness water removed. If necessary, the permeate can be subjected to sterilization, final filtering, ion exchange, or other processes to make it meet the desired water quality standards. The permeate can be reused in wet flue gas desulfurization, etc.

[0035] The concentrated water is evaporated to dryness to form a dry solid D. For the evaporation to dryness, a known evaporation to dryness apparatus V, such as an evaporator / drier 10, can be used. The evaporated water can be condensed and reused in wet flue gas desulfurization or the like. Valuable components can be separated and recovered from the dry solid as needed, and the residue can be disposed of.

[0036] The high-hardness water treatment device of the present invention can be modified in structure, shape, arrangement, etc., within the scope that does not contradict the spirit of the present invention, and components, mechanisms, etc. that were used in conventional technology can also be added, and it can be understood that such modified or added embodiments fall within the technical scope of the present invention.

[0037] The effects of the present invention will be shown by examples and comparative examples.

[0038] (Evaluation method) The following measurements were carried out with reference to the measurement methods described in ASTM D4189, JIS K 3802, or JP-A-2012-213676. 500 ml of clear water (e.g., reverse osmosis membrane permeate water) was subjected to suction filtration at -67 kPa using a new membrane filter with a pore size of 0.45 μm. The time T0 required for the suction filtration was measured. Using the same membrane filter used to measure time T0, 500 ml of test liquid (raw water) was subjected to suction filtration at -67 kPa. The time required for suction filtration, T1, was measured. Using the same membrane filter used to measure time T1, 500 ml of test liquid (raw water) was subjected to suction filtration at -67 kPa. The time required for suction filtration, T2, was measured. The ratio T1 / T0 is an index of the degree of contamination of the test liquid (raw water) by water-soluble polymers. The ratio T2 / T1 is an index of the degree of contamination of the test liquid (raw water) by fine particles. The lower these values, the less likely the raw water is to cause fouling or scaling in the reverse osmosis membrane.

[0039] Example 1 High hardness water (Ca 2+ =about 5400mg / l, Mg 2+ =about 400mg / l, SO 4- (=approximately 900 mg / L, room temperature [JIS Z 8703]) and sodium hydroxide and sodium carbonate were added to adjust the pH to 10.5. The precipitate was removed by decantation. Next, hydrochloric acid was added to the liquid from which the precipitate had been removed to adjust the pH to 7. Next, an iron-based coagulant (mainly composed of ferric chloride) was added to adjust the pH to 3. The resulting flocs were removed by decantation (first floc removal). Next, sodium hydroxide was added to the liquid after flocculation to adjust the pH to 7. The formed flocs were removed by decantation (second flocculation). The liquid after the second flocculation had a T1 / T0 ratio of 1.04 and a T2 / T1 ratio of 1.03. It can be assumed that the liquid after the second flocculation (raw water) is less likely to develop fouling or scaling when subjected to separation treatment using a reverse osmosis membrane.

[0040] Comparative Example 1 High hardness water (Ca 2+ =about 5400mg / l, Mg 2+ =about 400mg / l, SO 4- (=approximately 900 mg / L, room temperature [JIS Z 8703]) and sodium hydroxide and sodium carbonate were added to adjust the pH to 10.5. The precipitate was removed by decantation. Next, an iron-based coagulant (mainly composed of ferric chloride) was added to the liquid from which the precipitates had been removed, and the pH was adjusted to 9. The resulting flocs were removed by decantation (first floc removal). Next, hydrochloric acid was added to the liquid after flocculation to adjust the pH to 7. The formed flocs were removed by decantation (second flocculation). The liquid after the second flocculation had a T1 / T0 ratio of 3.28 and a T2 / T1 ratio of 1.19. It can be assumed that the liquid after the second flocculation (raw water) is prone to fouling and scaling when subjected to separation treatment using a reverse osmosis membrane.

[0041] Example 2 A dry solid was obtained in the same manner as in Example 1, except that a polymer flocculant was added at the same time as the iron-based flocculant. There was no change in pH due to the addition of the polymer flocculant. The ratio T1 / T0 was 1.03. The ratio T2 / T1 was 1.03.

[0042] From the above results, it is clear that the present invention can effectively prevent scaling and fouling of reverse osmosis membranes, thereby reducing the frequency of reverse osmosis membrane replacement and reducing operating costs. [Explanation of symbols]

[0043] 1: Reaction tank for reaction crystallization 2: Settling tank for reaction crystallization 3: First coagulation and sedimentation reaction tank 4: First coagulation and sedimentation tank 5: Second coagulation and sedimentation reaction tank 6: Second coagulation and sedimentation settling tank 7: Filter 8: Cooling tower 9: Reverse osmosis membrane 10: Evaporation / Dryer 11: High hardness water treatment equipment I: Reaction Crystallizer II: First coagulation and sedimentation unit III: Second coagulation and sedimentation device IV: Reverse osmosis membrane device V: Evaporation and drying apparatus W: High hardness water A: Alkali metal hydroxide or water-soluble alkaline earth metal hydroxide B: Alkali metal carbonate H: Inorganic acid F: Iron-based flocculant G: Polymer flocculant N: alkali metal hydroxide P: Permeated water C: Concentrated water D: Dry solids S: Precipitate S': First flock S”: Second flock 15: Wet flue gas desulfurization equipment (gas absorption equipment) 16: Chimney 17: Flue 25: Plaster 26: Gypsum separator (dehydrator) 27: Gypsum dehydration filtrate tank 28: Desulfurization drainage tank 29: Calcium carbonate slurry preparation tank

Claims

1. adding an alkali metal hydroxide or a water-soluble alkaline earth metal hydroxide and an alkali metal carbonate to high-hardness water containing calcium and magnesium so that the pH is 9.5 to 11.5, and removing the resulting precipitate; adding an inorganic acid and an iron-based coagulant to the liquid from which the precipitates have been removed so that the pH is 2.5 to 4.5, and removing the resulting flocs; Adding an alkali metal hydroxide to the liquid from which flocs have been removed by adding an inorganic acid and an iron-based coagulant so that the pH is 6.5 to 7.5, and removing the resulting flocs; The liquid from which the flocs have been removed is subjected to a separation process using a reverse osmosis membrane to separate it into permeate and concentrated water, The concentrated water is evaporated to dryness to obtain a dry solid. How to treat high hardness water.

2. The apparatus includes a reactive crystallizer, a first coagulation / sedimentation device, a second coagulation / sedimentation device, a reverse osmosis membrane device, and an evaporation / drying device; The reactive crystallizer comprises a mechanism for adding an alkali metal hydroxide or a water-soluble alkaline earth metal hydroxide and an alkali metal carbonate to high-hardness water containing calcium and magnesium so as to adjust the pH to 9.5 to 11.5, and for promoting removal of the precipitate obtained thereby; the first coagulation settling device is equipped with a mechanism for adding an inorganic acid and an iron-based coagulant to the liquid from which precipitates have been removed so that the pH is adjusted to 2.5 to 4.5, and for promoting the removal of the resulting flocs; The second coagulation settling device is equipped with a mechanism for adding an alkali metal hydroxide to the liquid from which flocs have been removed by adding an inorganic acid and an iron-based coagulant so as to adjust the pH to 6.5 to 7.5, thereby promoting the removal of the resulting flocs; The reverse osmosis membrane device is equipped with a reverse osmosis membrane for promoting separation of the liquid from which the flocs have been removed into permeate and concentrated water, The evaporator includes a mechanism for facilitating evaporation of liquid from the concentrate to form a dry solid. High hardness water treatment equipment.

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