Method for treating cyanuric acid-containing wastewater

Abstract

To provide a method which enables effective separation and removal of cyanuric acid from wastewater containing cyanuric acid, causing the wastewater to be reusable treated water.SOLUTION: A treatment method of wastewater containing cyanuric acid and a chloride ion includes a step of bringing the wastewater into contact with an OH-type II-type strongly basic ion-exchange resin, preferably adjusting a pH of the wastewater to be treated to 7 or more before bringing the wastewater into contact with the II-type strongly basic ion-exchange resin, to thereby cause the cyanuric acid contained in the wastewater to be adsorbed on the II-type strongly basic ion-exchange resin.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for treating cyanuric acid-containing wastewater. [Background technology]

[0002] Cyanuric acid, in the form of chlorinated cyanuric acid, is mainly used for disinfecting swimming pools and other similar facilities. Chlorinated cyanuric acid is more stable than sodium hypochlorite and undergoes almost no self-decomposition during storage, which is why it is widely used in the aforementioned applications.

[0003] Chlorinated cyanuric acid, when dissolved in water, produces hypochlorous acid. This hypochlorous acid's disinfectant properties are used for disinfecting swimming pools and other facilities. However, cyanuric acid is produced along with hypochlorous acid. Since cyanuric acid is stable in water, continuous disinfection through water circulation in pools and other facilities leads to its accumulation and increased concentration. High concentrations of cyanuric acid inhibit the disinfectant action of hypochlorous acid and lower the water's pH. Therefore, a portion of the water is usually drained and replaced with fresh water to prevent cyanuric acid buildup. Thus, if cyanuric acid could be removed and the water reused, a reduction in the amount of fresh water used could be expected.

[0004] Methods for removing cyanuric acid from water include those disclosed in Patent Documents 1 to 3, for example. Patent Document 1, for instance, discloses a decomposition method using microorganisms. The decomposition method using microorganisms is considered suitable when the treated water is to be discharged. However, as mentioned above, when the treated water is to be recycled and reused, there are problems in terms of complete separation of microorganisms, odor control, and miniaturization of equipment, making it difficult to say that it is a suitable method.

[0005] Patent documents 2 and 3 disclose a method for adsorbing cyanuric acid onto an anion exchange resin, but they do not specify the detailed conditions. Furthermore, an elution step is always required after adsorption, and since the eluent will contain a high concentration of cyanuric acid, its treatment is also necessary. These documents do not describe or suggest the treatment itself, nor the conditions required in the adsorption and elution steps to facilitate the treatment. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6260987 [Patent Document 2] Chinese Patent No. 101565246 Specification [Patent Document 3] U.S. Patent No. 9950940 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] This invention was proposed in view of the above circumstances, and aims to provide a method that can effectively separate and remove cyanuric acid from wastewater containing cyanuric acid, thereby making the wastewater reusable treated water. [Means for solving the problem]

[0008] As a result of diligent research, the inventors discovered that cyanuric acid can be effectively separated and removed by contacting wastewater containing cyanuric acid with an OH-type II strongly basic ion exchange resin, thus completing the present invention.

[0009] (1) The first invention of the present invention is a method for treating wastewater containing cyanuric acid and chloride ions, which includes a step of adsorbing the cyanuric acid contained in the wastewater onto a type II strong basic ion exchange resin of OH type by bringing the wastewater into contact with the type II strong basic ion exchange resin. It is a method for treating cyanuric acid-containing wastewater.

[0010] (2) The second invention of the present invention is, in the first invention, a method for treating cyanuric acid-containing wastewater, in which the pH of the wastewater to be treated is adjusted to 7 or more in advance and then brought into contact with the type II strong basic ion exchange resin.

[0011] (3) The third invention of the present invention is, in the first or second invention, further includes a step of eluting the cyanuric acid from the type II strong basic ion exchange resin adsorbed with the cyanuric acid, and performing the elution treatment using a caustic soda solution with a concentration of 2.0 mol / L or more. It is a method for treating cyanuric acid-containing wastewater.

Advantages of the Invention

[0012] According to the present invention, cyanuric acid can be effectively separated and removed from the wastewater, and the wastewater can be made into reusable treated water.

Embodiments for Carrying out the Invention

[0013] Hereinafter, specific embodiments of the present invention (hereinafter also referred to as "the present embodiments") will be described in detail. Note that the present invention is not limited to the following embodiments, and various changes can be made without changing the gist of the present invention.

[0014] ≪Method for Treating Cyanuric Acid-Containing Wastewater≫ The method according to the present embodiments is a method for treating wastewater containing cyanuric acid.

[0015] As mentioned above, cyanuric acid, in the form of chlorinated cyanuric acid, is used for disinfecting pool water and other similar applications. As shown in the chemical formula below, when chlorinated cyanuric acid is dissolved in water, it generates hypochlorous acid, exhibiting a disinfecting effect, and also generates and accumulates stable cyanuric acid (isocyanuric acid) in the water. Furthermore, chloride ions derived from chlorinated cyanuric acid coexist with cyanuric acid in that water.

[0016] [ka]

[0017] When cyanuric acid accumulates in water, its pH decreases. Furthermore, the accumulation of cyanuric acid inhibits the disinfection reaction by hypochlorous acid. Therefore, by removing cyanuric acid from the water (wastewater) after disinfection operations such as adding chlorinated cyanuric acid to pool water, etc., that is, wastewater containing at least cyanuric acid and chloride ions, it is possible to obtain reusable water that suppresses the decrease in pH and allows for the disinfection action of hypochlorous acid.

[0018] Specifically, the method according to this embodiment is characterized by including a step (adsorption step) in which the wastewater to be treated is brought into contact with an OH-type type II strong basic ion exchange resin, thereby adsorbing the cyanuric acid contained in the wastewater onto the type II strong basic ion exchange resin.

[0019] (1) Adsorption using OH-type type II strongly basic ion exchange resin (adsorption process) [Regarding the types of ion exchange resins and their adsorption reactions] Here, the acid dissociation constants (pKa) of cyanuric acid are 6.88, 11.4, and 13.5, and it does not dissociate unless the pH is higher than neutral, meaning it cannot exist as an anion. The water to be treated, such as swimming pool water, is neutral, but it is undesirable for the pH to decrease during the ion exchange process. The ionic form of a typical strong basic anion exchange resin is the Cl type. Therefore, for example, when cyanurate ions are adsorbed onto a strong basic ion exchange resin according to [Equation 1] below, as the concentration of cyanurate ions in the solution gradually decreases, the equilibrium of the reaction shown in [Equation 2] below shifts to the left side, and the pH of the treated water decreases. In the following equations, "R" represents the resin.

[0020] R-Cl+H2C3N3O3 - → R-H2C3N3O3+Cl - ...[Formula 1] H2C3N3O3 - +H + ⇔ H3C3N3O3...[Formula 2]

[0021] As mentioned above, the dissociation (ionization) of cyanuric acid becomes less likely when the pH decreases, so it is preferable to adjust the pH of the wastewater to be treated to 7 or higher beforehand. Furthermore, it is preferable to adjust the pH of the wastewater to 8 or higher, and more preferably to 10 or higher. Considering the dissociation constant of cyanuric acid, if the pH of the wastewater containing cyanuric acid is 7 or higher, 50% or more of the cyanuric acid will be dissociated and ionized, and if the pH is 8 or higher, 90% or more of the cyanuric acid will be dissociated and ionized. If the proportion of undissociated cyanuric acid is low, the Cl adsorbed on the strong basic anion exchange resin (Cl type) will be ionized. - However, when the reaction in which cyanuric acid is replaced occurs based on [Equation 1] above, the pH decrease based on [Equation 2] above becomes less likely. And when the pH is less likely to decrease, the ionized state of cyanuric acid is maintained.

[0022] As will be explained in more detail later, adjusting the pH of the wastewater, that is, adjusting the pH of the wastewater to 7 or higher in advance, suppresses the decrease in the pH of the wastewater, maintains the ionized state of cyanuric acid, and allows for more efficient adsorption and removal by the strongly basic anion exchange resin. This effect is also present and preferable when using the "OH-type" strongly basic ion exchange resin used in the method according to this embodiment.

[0023] Anion exchange resins are classified into three types based on differences in their functional groups: Type I strongly basic anion exchange resins, Type II strongly basic anion exchange resins, and weakly basic anion exchange resins. Among these, weakly basic anion exchange resins have an usable pH range of acidic to neutral, in which case cyanuric acid itself hardly dissociates. Therefore, it is difficult to adsorb cyanuric acid onto the resin, or even if adsorption occurs, the adsorption capacity is small. Consequently, in treatments to adsorb and remove cyanuric acid from wastewater, it is necessary to use strongly basic anion exchange resins.

[0024] The wastewater (solution) to be treated contains both cyanuric acid and chloride ions. Cyanuric acid has a higher selectivity to strongly basic anion exchange resin than chloride ions. However, when using type I strongly basic anion exchange resin, the difference in selectivity is small. Furthermore, when the adsorption of cyanuric acid reaches the break point, both cyanuric acid and chloride ions are adsorbed together, resulting in a smaller adsorption capacity based on cyanuric acid. Therefore, by using type II strongly basic anion exchange resin, the adsorption selectivity for cyanuric acid can be increased, and the adsorption capacity can be increased.

[0025] Furthermore, the method according to this embodiment is characterized by the use of a type II strongly basic anion exchange resin which is of the "OH type".

[0026] Using a type II strongly basic anion exchange resin, which is of the OH type, results in a greater difference in selectivity between chloride ions and cyanuric acid compared to a type I strongly basic anion exchange resin. The reason for this is not entirely clear, but the following reasons are possible.

[0027] That is, by treating the wastewater using an OH-type strongly basic anion exchange resin of Type II, cyanuric acid and chloride ions in the wastewater are adsorbed onto the resin, while hydroxide ions are desorbed (detached) from the resin side. In the strongly basic anion exchange resin of Type II, the selectivity of hydroxide ions is relatively high compared to that of the strongly basic anion exchange resin of Type I. Therefore, the difference in selectivity between chloride ions and hydroxide ions becomes smaller. And when the concentration of hydroxide ions on the solution side increases due to the desorption of hydroxide ions from the resin, it is speculated that the once-adsorbed chloride ions will desorb, and cyanuric acid will be adsorbed onto that site. By using an OH-type strongly basic anion exchange resin of Type II, it is considered that the adsorption capacity based on cyanuric acid increases due to such a phenomenon.

[0028] In addition, when using an OH-type strongly basic anion exchange resin of Type II, when cyanuric acid reaches the breakthrough point, since there are few adsorbed chloride ions, the amount of elution agent consumed for chloride ions during elution can be reduced, enabling more efficient elution. From this point as well, it is preferable to use an OH-type strongly basic anion exchange resin of Type II.

[0029] More specifically, when using an OH-type strongly basic anion exchange resin of Type II, an adsorption reaction as shown in the following reaction formula occurs, and the pH of the wastewater (solution) changes. In the following formula, "H3Cy" represents cyanuric acid (undissociated cyanuric acid), "H2Cy - " represents cyanurate ion, and "R" represents the resin respectively. H2Cy - +R-OH → OH - +R-H2Cy ···[Formula 3] Cl - +R-OH → OH - +R-Cl ···[Formula 4] H3Cy → H2Cy-+H + ···[Formula 5] H2Cy - +R-Cl → Cl -+R-H2Cy...[Formula 6]

[0030] First, as shown in [Equation 3] above, by contacting the wastewater containing cyanuric acid with an OH-type II strongly basic anion exchange resin, the resin is converted to cyanurate ions (H2Cy - ) is adsorbed, and hydroxide ions (OH) are released from the resin. - An exchange reaction occurs in which ) is eliminated. - As the detachment occurs, the pH of the wastewater rises.

[0031] Furthermore, as shown in [Equation 4] above, chloride ions (Cl) coexist in the wastewater. - ) is adsorbed onto the resin, and OH is released from the resin. - An exchange reaction occurs in which OH is eliminated. - As the detachment of the substance occurs, the pH of the wastewater rises further.

[0032] On the other hand, as shown in [Equation 5] above, H2Cy shown in [Equation 3] - The adsorption of the resin, and the increase in the pH of the wastewater as shown in [Equation 3] and [Equation 4], cause the dissociation of undissociated cyanuric acid (H3Cy). As a result, H2Cy, which is the target of adsorption in the wastewater, is dissociated. - The amount increases. Furthermore, the protons (H) from H3Cy at this time increase. + The dissociation of ) gradually leads to a decrease in the pH of the wastewater.

[0033] Then, as shown in [Equation 6] above, the Cl once adsorbed onto the resin - ([Equation 4]) and H2Cy in the wastewater - An exchange reaction occurs with H2Cy in the wastewater. - Further adsorption will occur. Note that the exchange reaction shown in [Equation 6] is more likely to occur when the pH of the wastewater is high.

[0034] In this way, by using an OH-type type II strongly basic anion exchange resin, H2Cy -As the substance is adsorbed onto the resin, the pH of the wastewater rises from the initial stages of adsorption, increasing the adsorption capacity based on cyanuric acid, and allowing for more efficient and effective separation and removal of cyanuric acid.

[0035] In contrast, when using a Cl-type type II strong basic anion exchange resin, the pH of the wastewater rapidly decreases due to the reactions in [Equation 5] and [Equation 6] above. Therefore, compared to an OH-type type II strong basic anion exchange resin, the adsorption capacity based on cyanuric acid is smaller, and cyanuric acid in the wastewater cannot be sufficiently separated and removed.

[0036] [Preparation of anion exchange resin and adsorption operation] As described above, in the method according to this embodiment, an OH-type type II strongly basic anion exchange resin is used as the anion exchange resin for adsorbing cyanuric acid. The OH-type type II strongly basic anion exchange resin is not particularly limited, and commercially available resins can be used.

[0037] Alternatively, a Cl-type type II strongly basic anion exchange resin may be prepared, and then a pretreatment such as passing a caustic soda solution through the resin may be performed to prepare an OH-type type II strongly basic anion exchange resin.

[0038] The method of contacting the wastewater to be treated, which contains cyanuric acid, with an OH-type II strongly basic anion exchange resin, i.e., the adsorption treatment method, is not particularly limited. For example, a method can be used in which a column is filled with resin and the wastewater to be treated is passed through the column.

[0039] [Regarding pH adjustment of wastewater to be treated] As described above, it is preferable to adjust the pH of the wastewater to be treated to 7 or higher before contacting it with the OH-type II strongly basic anion exchange resin. Preferably, it should be adjusted to 8 or higher, and more preferably to 10 or higher.

[0040] When the pH of wastewater containing cyanuric acid is 7 or higher, the dissociation (ionization) of cyanuric acid progresses, and the ionized state of cyanuric acid is maintained. As a result, the reaction shown in [Equation 3] above proceeds more efficiently, and cyanuric acid can be effectively adsorbed.

[0041] Furthermore, pH adjustment of the wastewater to be treated may be carried out in accordance with the progress of the adsorption reaction of cyanuric acid onto the OH-type II strongly basic anion exchange resin. As shown in [Equation 5] above, H2Cy in [Equation 3] - Adsorption of the resin and the increase in pH of the wastewater in [Equation 3] and [Equation 4] cause the dissociation of undissociated cyanuric acid (H3Cy), at which time protons (H) are released from H3Cy. + As the dissociation of ) progresses, the pH of the wastewater gradually decreases.

[0042] Therefore, the pH of the wastewater is continuously measured to monitor the pH as the reaction progresses, and if a decrease in pH is observed, the pH of the wastewater is adjusted to maintain a level of 7 or higher. This increases the adsorption capacity based on cyanuric acid, allowing for more efficient adsorption of cyanuric acid.

[0043] The method for adjusting the pH of wastewater is not particularly limited. For example, it can be done by adding a pH adjusting agent such as caustic soda (sodium hydroxide).

[0044] (2) Elution of cyanuric acid from ion exchange resin (elution process) In the method according to this embodiment, after the adsorption step described above, a step of eluting cyanuric acid from the type II strong basic ion exchange resin on which cyanuric acid has been adsorbed (elution step) may be included. By eluting cyanuric acid from the type II strong basic ion exchange resin in this way, the type II strong basic ion exchange resin can be reused.

[0045] Here, after eluting the cyanuric acid adsorbed from the type II strongly basic ion exchange resin, the resulting eluent (post-elution solution) is neutralized to precipitate the cyanuric acid. At this time, the cyanuric acid concentration in the supernatant of the precipitated solution is uniquely determined by the solubility of cyanuric acid. From the perspective of wastewater treatment, it is necessary to minimize the amount of cyanuric acid transferred to the wastewater. Therefore, it is required to minimize the amount of wastewater, i.e., the amount of supernatant water, as much as possible.

[0046] Therefore, in the process of eluting from the type II strongly basic ion exchange resin (in the elution step), it is preferable to maximize the cyanuric acid concentration in the resulting eluent and minimize the amount of eluent.

[0047] In the elution process, a caustic soda solution is used to elute cyanuric acid from the type II strongly basic ion exchange resin. In solutions containing cyanuric acid, solubility is low in acidic to neutral solutions and high in alkaline solutions. Therefore, to increase the concentration of cyanuric acid in the eluent obtained by eluting cyanuric acid, it is best to elute with an alkaline solution, and among these, the use of a caustic soda solution is preferable. By eluting with a caustic soda solution, the ionic form of the resin becomes the OH type, which can then be directly transferred to the adsorption process and reused. Note that in order to use the Cl type, it is necessary to convert the ionic form from OH to Cl by passing a NaCl solution through it.

[0048] Furthermore, to increase the cyanuric acid concentration in the eluent, a higher concentration of the caustic soda solution used for elution is desirable. Specifically, it is preferable to use a caustic soda solution with a concentration of 2.0 mol / L or higher. A caustic soda solution with a concentration of 2.5 mol / L or higher is even more preferable. Although it is possible to eluate cyanuric acid even with a concentration of less than 2.0 mol / L, the cyanuric acid concentration in the eluent will be lower.

[0049] When the eluent obtained through this elution process is neutralized, cyanuric acid precipitates. This precipitate is filtered and separated, and the cyanuric acid precipitate is recovered and reused, while the filtrate is discharged. Because the amount of filtrate is minimized, the amount of cyanuric acid discharged is extremely small. [Examples]

[0050] The following describes specific embodiments of the present invention. However, the scope of the present invention is not limited to any of the embodiments described below.

[0051] <<Verification of the adsorption of cyanuric acid onto anion exchange resin>> [Example 1] The wastewater to be treated, containing 200 mg / L of cyanuric acid and 100 mg / L of chloride ions and adjusted to pH 7, was passed through a column packed with 60 ml of OH-type II strongly basic ion exchange resin at a rate of 20 ml / min. The point at which the cyanuric acid concentration of the column outlet water exceeded 30 mg / L was defined as the breakpoint, which occurred at 280 BV (= cumulative water flow rate / amount of packed resin).

[0052] At the point of rupture, the flow of wastewater into the column was stopped, and elution was performed by passing a 2.5 mol / L caustic soda solution through it. Of the obtained eluents, the solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 28,000 mg / L.

[0053] Hydrochloric acid was added to the fractionated eluent to adjust the pH to 7, causing cyanuric acid to precipitate. The precipitate was filtered to obtain the filtrate. The cyanuric acid concentration in the filtrate was 1100 mg / L.

[0054] [Example 2] The test was conducted under the same conditions as in Example 1, except that the pH of the wastewater to be treated was pre-adjusted to 8. The break point was 300 BV (= cumulative water flow rate / amount of filling resin).

[0055] At the point of rupture, the flow of wastewater into the column was stopped, and elution was performed by passing a 2.5 mol / L caustic soda solution through it. Of the obtained eluents, solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 30,000 mg / L.

[0056] [Example 3] The test was conducted under the same conditions as in Example 1, except that the pH of the wastewater to be treated was pre-adjusted to 10. The break point was 400 BV (= cumulative water flow rate / amount of filling resin).

[0057] At the point of rupture, the flow of wastewater into the column was stopped, and elution was performed by passing a 2.5 mol / L caustic soda solution through it. Of the obtained eluents, solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 40,000 mg / L.

[0058] [Comparative Example 1] The test was conducted under the same conditions as in Example 1, except that a type I strongly basic anion exchange resin was used. The break point was 160 BV.

[0059] At the point of rupture, the flow of wastewater into the column was stopped, and elution was performed by passing a 2.5 mol / L caustic soda solution through it. Of the obtained eluents, solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 15,000 mg / L.

[0060] [Comparative Example 2] The test was conducted under the same conditions as in Example 1, except that a weak-base anion exchange resin was used. The fracture point was 70 BV.

[0061] At the point of rupture, the flow of wastewater into the column was stopped, and elution was performed by passing a 2.5 mol / L caustic soda solution through it. Of the obtained eluents, the solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 6500 mg / L.

[0062] [Comparative Example 3] The test was conducted under the same conditions as in Example 1, except that a Cl-type type II strongly basic anion exchange resin was used. The break point was 20 BV.

[0063] At the point of rupture, the flow of wastewater into the column was stopped, and elution was performed by passing a 2.5 mol / L caustic soda solution through it. Of the obtained eluents, the solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 1700 mg / L.

[0064] The results above confirm that cyanuric acid contained in wastewater can be selectively adsorbed and separated. Furthermore, it was found that by using a type II strongly basic ion exchange resin in the OH form, the adsorption capacity of cyanuric acid can be increased, allowing cyanuric acid to be adsorbed onto the resin efficiently and effectively and removed from wastewater.

[0065] <<Verification of the elution of cyanuric acid adsorbed on an anion exchange resin>> [Example 4] The test was conducted under the same conditions as in Example 1, except that a 2.0 mol / L caustic soda solution was used to elute the cyanuric acid adsorbed on the anion exchange resin.

[0066] Of the obtained eluents, solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 24,000 mg / L.

[0067] [Example 5] The test was conducted under the same conditions as in Example 1, except that a 1.0 mol / L caustic soda solution was used to elute the cyanuric acid adsorbed on the anion exchange resin.

[0068] Of the obtained eluents, solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 12,000 mg / L.

[0069] [Example 6] The test was conducted under the same conditions as in Example 1, except that a 0.5 mol / L caustic soda solution was used to elute the cyanuric acid adsorbed on the anion exchange resin.

[0070] Of the obtained eluents, solutions with a cyanuric acid concentration of 500 mg / L or higher were fractionated and collected, and the cyanuric acid concentration was found to be 6000 mg / L.

[0071] From the results of Example 1 and Examples 4-5, it was found that when eluting cyanuric acid adsorbed on an anion exchange resin, using a caustic soda solution with a concentration of 2.0 mol / L or higher is preferable because it effectively increases the concentration of cyanuric acid in the resulting eluent.

Claims

1. A method for treating wastewater containing cyanuric acid and chloride ions, The process includes contacting the wastewater with an OH-type II strongly basic ion exchange resin having selective adsorption properties for cyanuric acid under conditions of pH 7 or higher where chloride ions are present, thereby adsorbing the cyanuric acid contained in the wastewater onto the type II strongly basic ion exchange resin. A method for treating wastewater containing cyanuric acid.

2. The process further includes a step of eluting the cyanuric acid from a type II strongly basic ion exchange resin on which the cyanuric acid has been adsorbed, The elution process is performed using a caustic soda solution with a concentration of 2.0 mol / L or higher. A method for treating cyanuric acid-containing wastewater according to claim 1.

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