Water treatment method and water treatment apparatus

Abstract

To provide a water treatment method that efficiently removes silica and calcium from water containing silica and calcium.SOLUTION: A method for treating water containing silica and calcium includes a treatment process in which an acidic coagulant for aggregating silica, a basic coagulant for aggregating calcium, and water coexist in a treatment tank to aggregate the silica and calcium in the water.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method and apparatus for treating water containing silica and calcium. [Background technology]

[0002] Tap water sources contain calcium, silica, and other elements. For example, Japan is an island nation, and water veins such as rivers and groundwater are short, and little components are dissolved from the soil, so the hardness of tap water sources is about half that of Europe (about 200 mg / L), but it still contains about 50 mg / L of calcium. In addition, Japan's soil, which has many volcanoes, contains a lot of silicic acid, such as granite and quartz trachyte, and the silica in tap water sources is about twice that of Europe (about 50 mg / L). Therefore, wastewater (such as factory wastewater) that uses tap water sources also contains several to several tens of ppm of silica and calcium ions.

[0003] When using reverse osmosis (RO) membranes to concentrate and remove dissolved components from wastewater containing calcium, silica, etc., for reuse, silica and calcium can become insoluble and precipitate, causing clogging of the membrane (scaling), making treatment impossible.

[0004] For example, a method of removing silica as a fluorine compound has been proposed as seen in Patent Document 1. However, a method using a toxic substance such as a fluorine compound cannot be said to be versatile. In addition, since there are currently no practical adsorbents or chemicals that can safely and efficiently remove silica, it is common to add a large amount of acidic flocculant to wastewater to flocculate and separate silica as a solid.

[0005] As for calcium removal, a method in which carbon dioxide is blown into wastewater to precipitate and separate calcium carbonate is known as an inexpensive method. However, this method requires a gas containing carbon dioxide at a concentration of several percent, and its range of application is limited. For this reason, calcium in wastewater is generally separated by precipitating it as calcium carbonate by adding carbonate, which is a basic flocculant. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-136648 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, the general method for removing silica from wastewater is to add a large amount of acidic coagulant to the wastewater to coagulate and separate the silica as a solid. The water in which the silica has been coagulated becomes acidic due to the large amount of acidic coagulant. To neutralize the water so that it meets the wastewater standards, a large amount of alkaline neutralizing agent (such as calcium hydroxide) is required.

[0008] On the other hand, calcium in wastewater is generally separated by precipitating it as calcium carbonate by adding a carbonate salt, which is a basic coagulant. In order to coagulate calcium with a basic coagulant, a high pH is preferable, and an agent (such as calcium hydroxide) is added in addition to the basic coagulant. The water in which calcium has been coagulated is alkaline due to the basic coagulant and alkaline agent, so a large amount of an acid neutralizer (such as hydrochloric acid) is required to neutralize the water so that it meets the wastewater standards.

[0009] Thus, to remove silica and calcium from wastewater, it is necessary to use large amounts of acidic coagulants, basic coagulants, alkaline agents, and acids. Using a large amount of coagulant not only increases the cost of water treatment, but also increases the salt concentration in the water, which leads to an increase in the operating pressure when concentrating the water using an RO membrane, for example.

[0010] The main object of the present disclosure is to provide a water treatment method for efficiently removing silica and calcium from water containing silica and calcium. Another object of the present disclosure is to provide a water treatment device that can be suitably used in the water treatment method. [Means for solving the problem]

[0011] The inventors of the present disclosure have conducted extensive research to solve the above problems. As a result, they have found that in a method for treating water containing silica and calcium, when an acidic flocculant for flocculating silica, a basic flocculant for flocculating calcium, and water are allowed to coexist in the same treatment tank to flocculate the silica and calcium in the water, the silica and calcium can be efficiently removed.

[0012] The present disclosure has been completed based on these findings and through further investigations. That is, the present disclosure provides the invention of the following aspects. Item 1. A method for treating water containing silica and calcium, comprising the steps of: A water treatment method comprising a treatment step of causing an acidic flocculant that flocculates silica, a basic flocculant that flocculates calcium, and water to coexist in a treatment tank, thereby flocculating the silica and calcium in the water. Item 2. The water treatment method according to Item 1, wherein the water subjected to the treatment step has a silica content of 0.02 ppm or more and a calcium content of 20 ppm or more. Item 3. The water treatment method according to Item 1 or 2, wherein a molar ratio of the acidic flocculant to silica (acidic flocculant / silica) is adjusted to a range of 15 to 30 in the treatment step. Item 4. The water treatment method according to any one of Items 1 to 3, wherein a molar ratio of the basic flocculant to calcium (basic flocculant / calcium) is adjusted to a range of 15 to 25 in the treatment step. Item 5. The water treatment method according to any one of Items 1 to 4, wherein in the treatment step, a molar ratio of the acidic flocculant to the basic flocculant (acidic flocculant / basic flocculant) is adjusted to a range of 0.5 to 0.6. Item 6. The water treatment method according to any one of Items 1 to 5, wherein in the treatment step, the pH of the water is adjusted to a range of 6.0 to 9.0. Item 7. The water treatment method according to any one of Items 1 to 6, wherein in the treatment step, a calcium coagulation step is carried out in the treatment tank by mixing the basic coagulant with water, and subsequently, a silica coagulation step is carried out in the treatment tank by mixing the acidic coagulant with water. Item 8. The water treatment method according to any one of Items 1 to 7, further comprising a water neutralization step after the treatment step. Item 9. The water treatment method according to any one of Items 1 to 8, wherein the water to be subjected to the treatment step further contains ammonium ions. Item 10. The water treatment method according to Item 9, further comprising a step of recovering ammonium ions from the water after the treatment step. Item 11. A water treatment device used in a method for treating water containing silica and calcium, A water treatment device comprising a treatment tank in which an acidic flocculant that flocculates silica, a basic flocculant that flocculates calcium, and water are allowed to coexist. Effect of the Invention

[0013] According to the present disclosure, it is possible to provide a water treatment method for efficiently removing silica and calcium from water containing silica and calcium. Furthermore, according to the present disclosure, it is also possible to provide a water treatment device that can be suitably used in the water treatment method.

[0014] The water treatment method and water treatment device disclosed herein enable simultaneous removal of silica and calcium from water (single-tank treatment), and the synergistic effect of the acidic flocculant and the basic flocculant enables efficient removal of silica and calcium.

[0015] In addition, in conventional silica removal and calcium removal, since silica removal and calcium removal are performed in separate treatment tanks, a large amount of neutralizing agent is required between silica removal and calcium removal, and a large amount of alkaline agent is also used in calcium removal. In contrast, in the water treatment method disclosed herein, an acidic flocculant and a basic flocculant are used in combination in the same treatment tank, so the amount of neutralizing agent (acidic neutralizing agent or alkaline neutralizing agent) used in the subsequent neutralization step can be reduced. [Brief description of the drawings]

[0016] [Figure 1] 1 is a graph showing the effect of removing Si by iron salt (acidic flocculant) in the method of Comparative Example 1. [Diagram 2] 1 is a graph showing the Ca removal effect by soda ash (basic flocculant) in the method of Comparative Example 1. [Diagram 3] 1 is a graph showing the Si and Ca removal effect by using iron salt (acidic flocculant) and soda ash (basic flocculant) in the same treatment tank (simultaneous addition of iron salt and soda ash) in the method of Example 1. [Figure 4] 1 is a graph showing the relationship between the mixed molar ratio of iron salt (acidic flocculant) and soda ash (basic flocculant) and the pH of water in the method of Example 1. [Diagram 5] 1 is a graph showing the effect of prior addition of soda ash (basic flocculant) in the method of Example 2 (compared to simultaneous addition of iron salt and soda ash). [Figure 6] 1 is a graph showing the effect of water pH on TN removal rate (ammonia recovery rate) in Example 3. [Figure 7] FIG. 1 is a schematic diagram for explaining a water treatment method according to the present disclosure. [Figure 8] FIG. 1 is a schematic diagram for explaining a water treatment method according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The water treatment method of the present disclosure is a method for treating water containing silica and calcium, and is characterized by comprising a treatment step in which an acidic flocculant for flocculating silica, a basic flocculant for flocculating calcium, and water are allowed to coexist in a treatment tank, and the silica and calcium in the water are flocculated. By comprising this configuration, the water treatment method of the present disclosure can efficiently remove silica and calcium from water containing silica and calcium.

[0018] The water treatment device disclosed herein is used in a method for treating water containing silica and calcium, and is characterized by comprising a treatment tank in which an acidic flocculant for flocculating silica, a basic flocculant for flocculating calcium, and water are allowed to coexist. By being provided with this configuration, the water treatment device disclosed herein can efficiently remove silica and calcium from water containing silica and calcium.

[0019] The water treatment method and water treatment device disclosed herein will be described in detail below.

[0020] In this specification, a numerical value connected with "~" means a numerical range including the numerical values ​​before and after "~" as the lower limit and upper limit. When multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and upper limit value can be selected and connected with "~".

[0021] 1. Water treatment method The water treatment method disclosed herein is a method for treating water containing silica and calcium. Specifically, the method is a method for removing silica and calcium contained in the water to reduce the concentrations of silica and calcium in the water.

[0022] The water treatment method disclosed herein includes a treatment step of coexisting an acidic flocculant that flocculates silica, a basic flocculant that flocculates calcium, and water in a treatment tank to flocculate the silica and calcium in the water.

[0023] 7 and 8 are schematic diagrams for explaining the water treatment method of the present disclosure. In FIG. 7 and FIG. 8, water to be treated (raw water) is stored in a raw water tank 5. 7 and 8, an acidic flocculant tank 2 storing an acidic flocculant and a basic flocculant tank 3 storing a basic flocculant are connected to the treatment tank 1. Water, an acidic flocculant, and a basic flocculant are sent to the treatment tank 1, and in the treatment tank 1, the acidic flocculant, the basic flocculant, and the water are mixed to produce treated water.

[0024] In the water treatment method shown in FIG. 7, a pH adjuster tank 4 is further connected to the treatment tank 1, and a pH adjuster is sent to the treatment tank 1 as required to adjust the pH.

[0025] On the other hand, in the water treatment method shown in Fig. 8, a neutralization treatment tank 7 is connected after the treatment tank 1 (where the water travels). A pH adjuster tank 4 is connected to the neutralization treatment tank 7. In the treatment tank 1, treatment water in which an acidic coagulant, a basic coagulant, and water are mixed is sent to the neutralization treatment tank 7, and in the neutralization treatment tank 7, a pH adjuster is further sent to the neutralization treatment tank 7 as necessary to adjust the pH.

[0026] The pH adjuster can be used for the purpose of adjusting the pH of the treated water to near neutral (i.e., for the purpose of performing a water neutralization process), adjusting the pH of the treated water to alkaline or acidic and promoting the aggregation of calcium or silica, etc.

[0027] In Fig. 7, a settling tank 6 is connected after the treatment tank 1 (where the water flows). Also, in Fig. 8, a settling tank 6 is connected after the neutralization treatment tank 7 (where the water flows). The treated water adjusted in the treatment tank 1 is sent to the settling tank 6 after pH adjustment as necessary. In the settling tank 6, the aggregated silica and aggregated calcium contained in the treated water are precipitated, and the aggregated silica and aggregated calcium are separated from the treated water.

[0028] The water to be treated by the water treatment method of the present disclosure is not particularly limited as long as it contains silica and calcium, and examples thereof include wastewater (industrial wastewater, sewage, leachate from final disposal sites, etc.), seawater, river water, and groundwater.

[0029] The water treatment methods shown in FIG. 7 and FIG. 8 are each an example of the water treatment method of the present disclosure, and the water treatment method of the present disclosure may include a treatment step in which an acidic flocculant for flocculating silica, a basic flocculant for flocculating calcium, and water are allowed to coexist in a treatment tank to flocculate silica and calcium in the water. As long as the effects of the present disclosure are achieved, for example, before and after the treatment tank 1, in addition to the raw water tank 5, neutralization treatment tank 7, and sedimentation tank 6, any tank, device, etc. may be provided as necessary. For example, as described later, a step of recovering ammonium ions may be provided after the treatment tank 1 (where the water travels), and a tank, device, etc. for carrying out the step may be provided. In addition, the treatment tank 1 may further include a storage tank for storing components to be mixed into the treatment tank 1 as necessary.

[0030] It is preferable to provide an agitator in the treatment tank 1, neutralization tank 7, etc., for the purpose of increasing the mixing efficiency of the treated water. It is also preferable to provide a pH adjuster that measures the pH of the water in the treatment tank 1, neutralization tank 7, etc. Water, treated water, acidic flocculant, basic flocculant, pH adjuster, etc. can be moved between each tank through piping using a pump.

[0031] The silica content of the water subjected to the treatment step is, for example, 0.02 ppm or more, 2 ppm or more, 5 ppm or more, etc. (specifically, for example, 5 ppm or more for sewage, 0.02 ppm or more for seawater, and 2 ppm or more for factory (iron and steel) wastewater), and, for example, 12 ppm or less, 7 ppm or less, or 5 ppm or less, etc. (specifically, 12 ppm or less for sewage, 5 ppm or less for seawater, and 7 ppm or less for factory (iron and steel) wastewater), and the range of the silica content is, for example, 0.02 to 12 ppm. The calcium content of the water subjected to the treatment step is, for example, 20 ppm or more, 140 ppm or more, 400 ppm or more, etc. (specifically, for example, 20 ppm or more for sewage, 400 ppm or more for seawater, and 140 ppm or more for factory (iron and steel) wastewater), and for example, 600 ppm or less, 180 ppm or less, 40 ppm or less, etc. (specifically, 40 ppm or less for sewage, 600 ppm or less for seawater, and 180 ppm or less for factory (iron and steel) wastewater), and the calcium content range is, for example, 20 to 600 ppm.

[0032] Furthermore, when ammonium ions are contained in the water to be subjected to the treatment step, the ammonium ion content is, for example, 2 ppm or more, 4 ppm or more, 25 ppm or more, etc. (specifically, 25 ppm or more for sewage, 2 ppm or more for seawater, 4 ppm or more for factory (iron and steel) wastewater, etc.), and, for example, 40 ppm or less, 15 ppm or less, 6 ppm or less, etc. (specifically, 40 ppm or less for sewage, 15 ppm or less for seawater, 6 ppm or less for factory (iron and steel) wastewater, etc.), and the ammonium ion content ranges from 2 to 40 ppm, for example.

[0033] The acidic flocculant for flocculating silica is not particularly limited as long as it provides the effects of the present invention, and examples of the acidic flocculant include iron-based flocculants such as ferric chloride (FeCl3), aluminum-based flocculants such as aluminum sulfate (Al2(SO4)3), and polymer flocculants such as polyacrylamide-based nonionic flocculants. The acidic flocculant used in the water treatment method of the present disclosure may be one type or two or more types.

[0034] The basic flocculant that flocculates calcium is not particularly limited as long as it provides the effects of the present invention, and examples of the basic flocculant include alkali metal carbonates such as sodium carbonate (Na2CO3) and magnesium carbonate (Na2CO3); polymer flocculants such as polyacrylamide anions; etc. The basic flocculant used in the water treatment method of the present disclosure may be one type or two or more types.

[0035] In order to more suitably exert the effects of the present invention, in the treatment step, the molar ratio of the acidic flocculant to silica (acidic flocculant / silica) is preferably adjusted to 1 or more, more preferably 5 or more, and even more preferably in the range of 15 to 30.

[0036] In addition, from the viewpoint of more suitably exerting the effects of the present invention, in the treatment process, the molar ratio of the basic flocculant to calcium (basic flocculant / calcium) is adjusted to preferably 1 or more, more preferably 2 or more, and even more preferably in the range of 15 to 25.

[0037] In addition, from the viewpoint of more suitably exerting the effects of the present disclosure, in the treatment process, the molar ratio of the acidic flocculant to the basic flocculant (acidic flocculant / basic flocculant) is preferably adjusted to the range of 0.3 to 0.8, more preferably 0.4 to 0.7, and even more preferably 0.5 to 0.6.

[0038] In addition, from the viewpoint of more suitably exerting the effects of the present disclosure, the pH of the water is preferably adjusted to a range of 6.0 to 9.0, more preferably 6.5 to 8.5, and even more preferably 7.0 to 8.2 in the treatment step. The higher the pH, the more efficiently calcium flocculation by the basic flocculant proceeds, so the pH is preferably 7.0 or more. On the other hand, silica flocculation by the acidic flocculant proceeds easily under neutral conditions, so the pH is preferably 7.0 or more and 8.0 or less. In the water treatment method of the present disclosure, the pH of the water in the treatment step is changed over time depending on the contents of silica and calcium contained in the water, etc., so that silica and calcium can be flocculated more efficiently. The pH of the water can be adjusted by the timing and amount of mixing the acidic flocculant and the basic flocculant with the water, and further by using a pH adjuster.

[0039] For example, in the treatment process, a basic flocculant and water can be mixed to carry out a calcium flocculation process in the treatment tank, and then an acidic flocculant and water can be mixed to carry out a silica flocculation process in the treatment tank. In this case, calcium flocculation is given priority while keeping the pH of the water at 7.0 or higher (alkaline side), and then an acidic flocculant is added to carry out silica flocculation. Conversely, an acidic flocculant and water can be mixed to carry out a silica flocculation process in the treatment tank, and then a basic flocculant and water can be mixed to carry out a calcium flocculation process in the treatment tank.

[0040] Furthermore, for example, when a step of removing (recovering) ammonium ions from the water is further provided after the treatment step, it is preferable to carry out the treatment step while maintaining the pH of the water at 8.2 or less, since the ammonium ions in the water are likely to volatilize as ammonia when the pH exceeds 8.2.

[0041] Examples of pH adjusters include alkalis such as sodium hydroxide, potassium hydroxide, and magnesium hydroxide, and acids such as hydrochloric acid, sulfuric acid, and nitric acid. Among these, sodium hydroxide, potassium hydroxide, hydrochloric acid, and nitric acid, which do not produce solids by neutralization, are preferred from the viewpoint of suppressing the generation of scale from the treated water. In addition, as the pH adjuster, a basic flocculant or an acidic flocculant can be used.

[0042] The silica content of the water (treated water) after being subjected to the treatment process is, for example, 2.4 ppm or less, 1.4 ppm or less, 1.0 ppm or less, etc. (specifically, 2.4 ppm or less for sewage, 1.0 ppm or less for seawater, 1.4 ppm or less for factory (steel) wastewater, etc.), and, for example, 0.004 ppm or more, 0.4 ppm or more, 1.0 ppm or more, etc. (specifically, 1.0 ppm or more for sewage, 0.004 ppm or more for seawater, 0.4 ppm or more for factory (steel) wastewater, etc.), and the range of the silica content is, for example, 0.004 to 2.4 ppm. The calcium content of the water (treated water) after being subjected to the treatment process is, for example, 120 ppm or less, 36 ppm or less, 8 ppm or less, etc. (specifically, 8 ppm or less for sewage, 120 ppm or less for seawater, and 36 ppm or less for factory (iron and steel) wastewater), or, for example, 4 ppm or more, 28 ppm or more, 80 ppm or more, etc. (specifically, 4 ppm or more for sewage, 80 ppm or more for seawater, and 28 ppm or more for factory (iron and steel) wastewater), and the calcium content range is, for example, 4 to 120 ppm.

[0043] As described above, in the present disclosure, the water to be subjected to the treatment step may have an ammonium ion content of, for example, 2 ppm or more, 4 ppm or more, 25 ppm or more, etc. (specifically, 25 ppm or more for sewage, 2 ppm or more for seawater, 4 ppm or more for factory (steel) wastewater, etc.), or may have an ammonium ion content of, for example, 40 ppm or less, 15 ppm or less, 6 ppm or less, etc. (specifically, 40 ppm or less for sewage, 15 ppm or less for seawater, 6 ppm or less for factory (steel) wastewater, etc.). In the water treatment method of the present disclosure, the silica content and calcium content in the treated water are suitably reduced. Therefore, even if the treated water is highly concentrated using an RO membrane or the like (for example, concentrated until the ammonium ion concentration is 400 ppm or more), the silica content in the concentrated liquid can be reduced to, for example, 50 ppm or less, and the calcium content can be reduced to, for example, 200 ppm or less, and the generation of scale caused by silica and calcium can be suppressed. For this reason, by removing ammonium ions from treated water having a high ammonium ion concentration of, for example, 400 ppm or more, it is possible to efficiently recover ammonium ions (ammonia). Therefore, in the water treatment method of the present disclosure, a step of removing ammonium ions from water (treated water) can be preferably provided after the treatment step.

[0044] Ammonium ions can be removed from water (treated water) by a known method, such as heating water with a pH of more than 8.2 to volatilize it as ammonia gas.

[0045] 2.Water treatment equipment The water treatment device disclosed herein is used in a method for treating water containing silica and calcium. The water treatment device disclosed herein includes a treatment tank in which an acidic flocculant that flocculates silica, a basic flocculant that flocculates calcium, and water are allowed to coexist.

[0046] The water treatment device of the present disclosure can be suitably used in the water treatment method of the present disclosure described above in the section "1. Water treatment method." Details of the acidic flocculant that flocculates silica, the basic flocculant that flocculates calcium, water, the treatment tank, etc. are as described above in the section "1. Water treatment method."

[0047] As described above, the water treatment device of the present disclosure may have, in addition to the treatment tank 1, a raw water tank 5, an acidic coagulant tank 2, a basic coagulant tank 3, a pH adjuster tank 4, a neutralization treatment tank 7, a sedimentation tank 6, etc. (Figures 7 and 8). EXAMPLES

[0048] The present disclosure will be described more specifically below with reference to examples, but the present disclosure is not limited to these examples.

[0049] [Experimental Method] The experimental method of water treatment in the following examples and comparative examples is as follows. 50 ml of raw water was put into a 100 ml glass beaker, and a flocculant and a neutralizing agent (pH adjuster) were added while stirring with a magnetic stirrer (RS-6DN, rotor φ7×20, manufactured by AS ONE Corporation), and the mixture was reacted for 20 minutes at 600 rpm rapid stirring and 20 minutes at 50 rpm slow stirring. The flocculants used were iron salt (Ferric chloride hexahydrate special grade reagent, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) as an acidic flocculant that flocculates silica in the water, and soda ash (sodium carbonate anhydrous special grade reagent, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) as a basic flocculant that flocculates calcium in the water, each adjusted to a concentration of 2N. In addition, hydrochloric acid (36% special grade reagent, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) and sodium hydroxide (granular special grade reagent, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) were used as pH adjusters, each adjusted to a concentration of 2N. The pH was adjusted by adding a pH adjuster with a micropipette while checking the value of a pH indicator (IM-32P, manufactured by Toa DKK Co., Ltd.). The components contained in the solution after the agglutination reaction were quantified by filtering the supernatant obtained by centrifuging at 6000 rpm for 10 minutes with a tabletop centrifuge (CT6E, manufactured by Hitachi Koki Co., Ltd.) through a 0.45 μm filter (DISMIC, manufactured by Advantec Co., Ltd.) after filtering with an ICP emission spectrometer (ICPE-9800, manufactured by Shimadzu Corporation) after filtering.

[0050] <Comparative Example 1> In Comparative Example 1, the relationship between the amount of coagulant added and the removal rate was examined when removing Si with iron salt and Ca with soda ash as a conventional method. The results of the component analysis of the sample used (chemical plant wastewater) are shown in Table 1.

[0051] [Table 1]

[0052] A specified molar ratio of iron salt was added to 50 ml of the sample while stirring. Since the pH value decreased with the addition of the coagulant, the pH value was adjusted to about 7.5 with sodium hydroxide. The results are shown in Figure 1.

[0053] To remove 80% of silicon using iron salt alone, an Fe / Si molar ratio of 15 or more was required. In this case, the Ca removal rate was about 30%, and the amount of 2N NaOH required for neutralization was about 2.5% of the raw water volume.

[0054] On the other hand, when removing Ca with soda ash, the pH rises with the addition of soda ash, so the pH was adjusted to about 7.5 with hydrochloric acid. In the normal soda-lime method, the pH is adjusted to 10 or more, but in Comparative Example 1, the pH was adjusted to about 7.5 to match the coagulation conditions of iron salt. The results are shown in Figure 2.

[0055] At a pH of about 7.5, no significant coagulation effect was expected from soda ash, and even at a CO3 / Fe molar ratio of 20, the Ca and Si removal rates were approximately 20% and 10%, respectively.

[0056] <Example 1> In Example 1, after the addition of iron salt in Comparative Example 1, pH adjustment was further performed using soda ash. The results are shown in Figure 3. With iron salt-NaOH, 80% of Si and 30% of Ca were removed at an Fe / Si molar ratio of 15 or more, but with iron salt-Na2CO3, 85% of Si and 75% of Ca could be removed at a molar ratio of 13, and simultaneous removal of Si and Ca was possible. Since the Ca removal effect was small with soda ash alone at a neutral pH, a synergistic effect was observed in which an acidic flocculant that flocculates silica, a basic flocculant that flocculates calcium, and water were allowed to coexist in the same treatment tank to flocculate silica and calcium in the water, and the simultaneous addition of iron salt and soda ash.

[0057] In addition, with iron salt-NaOH, it was necessary to add about 2.5% NaOH to the raw water for neutralization, but no additional pH adjuster was required in Example 1. The mixed molar ratio (Fe / CO3) of iron salt and soda ash in Example 1 was 0.5 to 0.6. Figure 4 shows the relationship between the mixed ratio of iron salt and soda ash at the same 2N concentration and the pH.

[0058] The mixing molar ratio for a pH of 7-8, which is the normal pH range for the coagulation reaction of iron salt, is 0.5-0.65, and when the raw water Si concentration is high (large amount of iron salt added) and the Fe / CO3 molar ratio exceeds 0.65, the pH becomes acidic, so additional alkali (NaOH, etc.) is added to maintain the pH at an appropriate level. Conversely, when the raw water Ca concentration is high (large amount of soda ash added) and the Fe / CO3 molar ratio falls below 0.5, the pH becomes alkaline, so additional acid (hydrochloric acid, etc.) is added to maintain the pH at an appropriate level.

[0059] <Example 2> Generally, polyvalent metals (Al, Ca, etc.) tend to precipitate as hydroxides under high pH conditions, so soda ash was added first to maintain the pH at 8 or above and promote the coagulation reaction of polyvalent metals for 10 minutes, after which iron salt was injected and the reaction continued for another 10 minutes. The conditions for adding the coagulant were that soda ash was added under conditions of a CO3 / Ca ratio of 8.4, and 10 minutes later, iron salt was added so that the pH was approximately 7.5. The results are shown in Figure 5. The removal rate of polyvalent metals was improved by adding soda ash first.

[0060] <Example 3> In the case of an ammonia recovery process, if soda ash is added in advance and the pH rises, the ammonium ions in the wastewater turn into ammonia and begin to disperse, resulting in a decrease in the recovery rate of ammonia. Therefore, the relationship between the pH of the coagulation tank and the TN removal rate when the industrial wastewater is coagulated was investigated. The pH of the coagulation tank was changed by changing the amount of soda ash and iron salt added. When the raw water shown in Table 1 was used as the test raw water, the Ca molar concentration (5 mmol / L) in the raw water was about 2.5 times higher than the Si (2 mmol / L), so the amount of soda ash added was large and the pH became slightly alkaline. When soda ash and iron salt were added as coagulants so that the molar ratios of CO3 / Ca and Fe / Si to scale components were 10 and 15, respectively, the pH of the coagulation tank was 7.4. To change the pH of the coagulation tank, the molar ratio of iron salt added was decreased in increments of 0.5 from 15 to 14.5 and 14.0. When the CO3 / Ca and Fe / Si molar ratios were finally set at 10 and 10, respectively, as the amount of flocculant added, the pH of the flocculation tank rose to 8.5. Figure 6 shows the relationship between the pH of the flocculation tank and the TN removal rate. It can be seen that the TN removal rate (ammonia recovery rate) is affected by the pH, and that to ensure an ammonia recovery rate of 95% or more (removal rate of 5% or less), the pH must be kept below 8.2. Therefore, if the pH exceeds 8.2 after the first addition of soda ash, it is necessary to add iron salt to control the pH so that it does not exceed 8.2. The flocculation reaction of polyvalent metals is promoted under conditions where the pH does not exceed 8.2, and the remaining iron salt is added in the second tank, and the pH is adjusted to a pH suitable for the flocculation reaction with additional neutralizing agent. If the pH still exceeds 8.2 even after adding iron salt in the first tank (treatment tank), it is preferable to add acid to the first tank to promote coagulation under conditions where the pH does not exceed 8.2, and then add acid to the second tank (neutralization treatment tank) to adjust the pH to an appropriate level so as to maintain the effluent water quality before discharging the water. [Explanation of symbols]

[0061] 1 Treatment tank 2. Acid coagulant tank 3. Basic coagulant tank 4 pH adjuster tank 5 Raw Water Tank 6 Sedimentation tank 7 Neutralization tank 10 Water treatment equipment

Claims

1. A method for treating water containing silica and calcium, A water treatment method comprising a treatment step of coagulating silica with an acidic coagulant, calcium with a basic coagulant, and water in a treatment tank, thereby coagulating the silica and calcium in the water.

2. The water treatment method according to claim 1, wherein the water subjected to the treatment step has a silica content of 0.02 ppm or more and a calcium content of 20 ppm or more.

3. The water treatment method according to claim 1 or 2, wherein in the treatment step, the molar ratio of the acidic coagulant to silica (acidic coagulant / silica) is adjusted to a range of 1 to 30.

4. The water treatment method according to claim 1 or 2, wherein in the processing step, the molar ratio of the basic coagulant to calcium (basic coagulant / calcium) is adjusted to a range of 1 to 25.

5. The water treatment method according to claim 1 or 2, wherein in the treatment step, the molar ratio of the acidic coagulant to the basic coagulant (acidic coagulant / basic coagulant) is adjusted to a range of 0.5 to 0.

6.

6. The water treatment method according to claim 1 or 2, wherein in the treatment step, the pH of the water is adjusted to a range of 6.0 to 9.

0.

7. The water treatment method according to claim 1 or 2, wherein in the treatment step, the basic coagulant and water are mixed to perform a calcium coagulation step in the treatment tank, and subsequently, the acidic coagulant and water are mixed to perform a silica coagulation step in the treatment tank.

8. The water treatment method according to claim 1 or 2, further comprising a water neutralization step after the processing step.

9. The water treatment method according to claim 1 or 2, wherein the water subjected to the treatment step further contains ammonium ions.

10. The water treatment method according to claim 9, further comprising a step of recovering ammonium ions from water after the processing step.

11. A water treatment apparatus used in a method for treating water containing silica and calcium, A water treatment device equipped with a treatment tank that contains an acidic coagulant for agglomerating silica, a basic coagulant for agglomerating calcium, and water.

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