JPH0315515B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for removing phosphate from water containing phosphate by crystallization. As a method to remove phosphates contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.
A method has been proposed in which water containing phosphate is brought into contact with a crystal species containing calcium phosphate, such as phosphate rock, in the presence of calcium ions. This method removes phosphate ions contained in water by converting them into calcium phosphate such as hydroxyapatite and crystallizing them into crystal seeds. The processing efficiency is also significantly improved. However, in this method, since the crystallization is carried out in the presence of calcium ions and hydroxyl ions in excess of the reaction equivalent, there was a problem in that the excess calcium ions and hydroxide ions leaked into the treated water. Regarding calcium ions, the calcium utilization rate of the added calcium agent is usually less than 30%, and the remainder flows into the treated water, resulting in increased hardness of the treated water. Regarding hydroxyl ions, when treated at a pH of 8.5 to 9.0, the pH of the treated water will be 8.4 to 9.0.
8.8, which in some cases exceeded the wastewater standards. This invention is intended to solve the above-mentioned problems. After crystallization, treatment with an H-type ion exchanger lowers the calcium ion concentration in the final treated water and also lowers the pH. The purpose of the present invention is to provide a dephosphorization method that can reduce the amount of calcium and recover the calcium content and reuse it for crystallization. In this invention, water containing phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 6 or higher, and then brought into contact with an H-type ion exchanger, and the ion exchanger is regenerated. This dephosphorization method is characterized by repeated use. The reaction that occurs when water containing phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions varies depending on the reaction conditions, but is usually expressed by the following formula. 5Ca ² +3HPO ^2- +4OH ^- →Ca ₅ (OH) (PO ₄ ) ₃ +
3H ₂ O...(1) As can be seen from equation (1), in order to increase the removal rate of phosphate, the reaction must proceed to the right.
At the same time, it is necessary to keep the surface of the crystal seeds clean to maintain high activity. Crystal species containing calcium phosphate include hydroxyapatite [Ca ₅ (OH) (PO ₄ ) ₃ ], fluoroapatite [Ca ₅ (F)) (PO ₄ ) ₃ ], or tricalcium phosphate [Ca ₃ (PO ₄ )]. Crystal seeds containing calcium phosphate such as _[2 ] can be used, and natural phosphate rock or bone char contains these calcium phosphates as a main component and is suitable as a crystal seed. Furthermore, crystal seeds in which calcium phosphate is precipitated, supported, or captured on the surface of granular materials such as sand or calcium carbonate can also be used. The crystal seed is preferably one whose main component is calcium phosphate of the same type as the calcium phosphate produced by the reaction. For example, in a system that produces hydroxyapatite, the use of hydroxyapatite facilitates the precipitation of new crystals, efficiently removes phosphate, and increases the removal rate. The raw water to be crystallized by contacting with the crystal seeds containing calcium phosphate as described above is water containing phosphate, and includes secondary treated water such as sewage, human waste, and industrial wastewater. The crystallization conditions are the same as those of the conventional method, and when it is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 6 or higher, the calcium phosphate produced by the above formula (1) becomes a crystal seed. Crystals precipitate on the surface and grow, removing phosphate ions from the water. Calcium ions and hydroxide ions to be present in water do not need to be added from the outside if they are present in the raw water from the beginning, but if they are not present in the raw water or are insufficient, they are added from the outside. The amount added should be in excess of the reaction equivalent, but if too much is added, fine precipitates may precipitate in places other than the crystal seeds, and impurities such as calcium carbonate may be generated, so these should not be generated. should be within the range. That is, the amounts of calcium ions and hydroxyl ions are higher than the solubility of hydroxyapatite produced in equation (1), but lower than the supersolubility, that is, the conditions are such that hydroxyapatite is produced at a concentration in the metastable range. . Here, supersolubility is the concentration at which crystals begin to precipitate when no crystal seeds are present in the reaction system. In other words, at a concentration higher than the supersolubility, new crystals precipitate where there are no crystal seeds, forming fine precipitates and clogging the bed, but in a metastable region lower than the supersolubility, new crystals precipitate on top of the crystal seeds. No precipitate is formed, only crystals precipitate and grow. In addition, crystals do not precipitate in systems lower than the solubility. The amount of hydroxyapatite produced is mainly controlled by the phosphate ion concentration, calcium ion concentration, and pH of the reaction system. The amount of calcium ions and PH value that bring hydroxyapatite within the metastable range can be determined experimentally by changing these values for each reaction system. The approximate range is when phosphate ion is less than 50mg/
Calcium ion 10-100mg/, PH 6-12
However, it varies depending on each condition. When targeting secondary treated sewage water with a phosphorus concentration of 2 to 5 mg, calcium ion concentration of 40 to 80
It is practical to process at a pH of about 8.5 to 9. The method of contacting the raw water containing phosphate with the crystal seeds containing calcium phosphate may be a fixed bed type or a fluidized bed type. The smaller the size of the crystal seeds, the larger the surface area, which makes it easier for new crystals to precipitate, but if the size of the seeds is too small, it will be difficult to contact or separate the crystal seeds from water. Furthermore, if the particle size is too large, the specific surface area per unit filling amount will be small, so particles of about 9 to 300 mesh are usually used. The larger ones are suitable for fixed beds, and the smaller ones are suitable for fluidized beds. In the case of a fixed bed, crystal seeds with a particle size of 9 to 35 mesh are packed, and water is passed upward or downward at a flow rate of SV1 to 5 hr ^-1 to precipitate calcium phosphate crystals. If the crystal seed surface becomes contaminated or clogged during water flow, periodically perform upward flow cleaning (backwashing) to expand and clean the crystal seed bed and remove impurities attached to the surface. It is desirable to peel off the
Water flow conditions during backwashing include a flow rate of 20 to 80 m/hr.
The cleaning time is approximately 5 to 60 minutes. If the crystallization operation as described above is continued, the activity of the crystal seeds may decrease and the dephosphorization performance may decrease. In such cases, a concentrated slaked lime solution,
The dephosphorization performance can be restored by contacting with a chlorine agent-containing solution (chlorine water, sodium hypochlorite, dry powder, highly processed dry powder solution, etc.) or an aqueous alkaline solution, etc. to perform a reactivation treatment. The calcium utilization rate in the above crystallization treatment is usually about 27% of the added calcium agent, and the calcium ion content of the crystallized water obtained is usually 40 to 45 mg/, and the pH is 8.4 to 8.8. . Therefore, in the present invention, this crystallized water is further brought into contact with an H-type ion exchanger to perform an ion exchange treatment. As the ion exchanger, those commonly used as ion exchangers can be used, such as zeolites and ion exchange resins. Zeolites may be natural or synthetic. The ion exchange resin may be strongly acidic, medium or weakly acidic. Particle sizes of about 9 to 300 mesh can be used. In order to convert the ion exchanger into the H-form, it is brought into contact with an acid such as hydrochloric acid or sulfuric acid through a normal regeneration operation to convert it into the H-form. The ion exchanger does not have to be entirely in the H form, and may contain a portion of the Na form or the like. The method of contact between the treated water subjected to crystallization and the H-type ion exchanger may be either a fixed bed type or a fluidized bed type, and the water flow direction can be arbitrarily selected, such as upward flow or downward flow. Further, the amount of ion exchanger to be used, particle size, flow rate during contact, etc. can be arbitrarily selected depending on the exchange capacity of the ion exchanger, processing conditions, etc. When the crystallized water is brought into contact with the H-type ion exchanger, the calcium ions contained in the crystallized water are
According to equation (2), hydrogen ions on the ion exchanger surface are replaced. Ca ²⁺ +Z-2H→Z-Ca+2H ²⁺ (2) In this way, the calcium ions in the crystallized water are captured on the surface of the ion exchanger, while the hydrogen ion concentration increases, resulting in a decrease in pH. The final treated water has a calcium ion concentration of 2 to 35 mg/, and a pH of 6.8 to 8.3, and can be discharged as is, but may be subjected to post-treatment such as sterilization. If the ion exchange treatment of the crystallized water is continued, the ion exchange capacity decreases and exchange adsorption of calcium ions is no longer performed, so regeneration is performed to restore the ion exchange capacity. Regeneration can be performed when the pH or calcium ion concentration of the final water reaches a predetermined value or higher. When regenerating by adjusting the pH of the final treated water, if the calcium ion exchange capacity of the ion exchanger is low, calcium ions may leak into the final treated water after a certain period of time. From the viewpoint of calcium recovery, when the calcium exchange capacity of the ion exchanger is approached, the process can be interrupted and regeneration can be started. Regeneration can be performed in the same manner as when the ion exchanger was first converted to the H form.
It can be made into an H shape again. The ion exchanger thus regenerated into H-form can be again subjected to the ion exchange treatment of the crystallized water. In this way, the ion exchanger can be reused and used repeatedly. Regeneration of the ion exchanger generates recycled waste liquid, which contains a large amount of calcium ions, so adding this to raw water can increase the calcium ion concentration, which can lead to crystallization. Crystallization can be efficiently performed without discharging necessary calcium ions out of the system. In this case, the recycled waste liquid is acidic, so if an alkaline agent such as lime, which is a calcium agent, is further added, the undissolved lime added to the raw water will be dissolved by the acidic recycled waste liquid. As a result, the amount of calcium ions increases, and the crystallization reaction is promoted without increasing the pH during crystallization. Since recycled waste liquid is generated all at once, it is desirable to store it in a storage tank and add it to raw water little by little. Furthermore, when adding recycled waste liquid to raw water, the amount of newly added calcium agent may be reduced depending on the amount of calcium ions contained. In addition, in the above operation, before crystallization, suspended matter in the raw water may be removed by filtration, or pretreatment for promoting crystallization may be performed. Crystallization can be performed by combining multiple crystallization towers.
In this case, different types of crystal seeds may be used for each crystallization tower, and other treatments may be used in conjunction with the crystallization. Ion exchange treatment can also be performed using a combination of a plurality of ion exchange towers, and in this case also a different type of ion exchanger can be used for each ion exchange tower. According to the present invention, after crystallization, the treatment is performed with an H-type ion exchanger, so that the concentration of calcium ions in the final treated water can be lowered and the pH
Moreover, the recycled waste liquid can be used as a calcium agent for crystallization, and crystallization can be carried out efficiently. Next, an example will be explained. Example 1 Phosphorus concentration 2 mg/, total alkalinity approximately 100 mg/
150 ml of phosphate rock with a particle size of 0.5 to 1.0 mm was used as crystal seed for dephosphorization of raw water for about 6 months, and the inner diameter was 30 mm.
It was packed into a 500 mm long acrylic resin column to serve as a dephosphorization tower. In addition, in the same column, particle size 0.5~
It was filled with 1.0 mm clinoptilolite natural zerolite, and was regenerated into H-form by flowing 0.1N hydrochloric acid aqueous solution in a downward flow of 750 ml/hr for 4 hours to form an ion exchange tower. and phosphorus concentration 2 mg/, total alkalinity approximately 100
After adjusting the calcium ion concentration to about 45 mg/hr and pH 8.8 to 9.0 by continuously adding a calcium chloride aqueous solution and a sodium hydroxide aqueous solution to synthetic water of Water was passed through the phosphorus tower to perform crystallization, and then the entire amount of the treated water was passed upwardly to the ion exchange tower at the same flow rate to perform ion exchange treatment. Table 1 shows the quality of the treated water in the 9-day water treatment. Next, 0.1N was added to the ion exchange tower after water treatment for 9 days.
When the ion exchanger was regenerated by passing an aqueous hydrochloric acid solution through it in a downward flow of 750 ml/hr for 2 hours,
Calcium ion concentration in recycled waste liquid is 449mg/
It was hot. Subsequently, recycled waste liquid is further added to the synthetic water to which calcium chloride aqueous solution and sodium hydroxide aqueous solution have been added, so that the calcium ion concentration is approximately
55 mg/, pH 8.8 to 9, water was passed through the dephosphorization tower to perform crystallization in the same manner as above, and the crystallized water was passed through the regenerated ion exchange tower to perform ion exchange treatment. Table 2 shows the quality of the treated water in the 9-day water treatment.

【table】

[Table] Comparative Example Table 3 shows the quality of treated water that was crystallized under the same conditions as those before regeneration in Example 1 and was not subjected to ion exchange treatment.

[Table] Example 2 In Example 1, 20 mg of calcium chloride was added as calcium to the synthetic water (simulating actual sewage), and a slaked lime aqueous solution was added to the synthetic water as an alkaline agent and a calcium agent at pH 9.0. The treatment was carried out in the same manner as in Example 1, except that the addition amount was as follows. In this case, the amount of slaked lime added was approximately 40 mg/(22 mg/as calcium). After a predetermined period of time, the zeolite was regenerated in the same manner as in Example 1 to obtain a regenerated waste liquid. Next, this recycled waste liquid was added to synthetic water at a ratio of 22 ml/-synthetic water, and then slaked lime aqueous solution was added to make the pH
Adjusted to 8.6-8.8. At this time, the calcium ion concentration in the synthetic water was 50 to 55 mg/. As a result, treated water having almost the same water quality as in Table 2 was obtained. However, the pH was about 0.2 lower than in Table 2. The following results show that according to the dephosphorization method of the present invention, the calcium ion concentration and PH of the final treated water can be lowered than in the comparative example.
Note that it is presumed that the reason why the phosphorus concentration in the example is lower than that in the comparative example is due to the temporary adsorption power of zeolite.

Claims (1)

[Claims] 1 Water containing phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 6 or higher, and then brought into contact with an H-type ion exchanger to perform ion exchange. This dephosphorization method is characterized by the fact that the body is regenerated and used repeatedly. 2. The dephosphorization method according to claim 1, wherein the ion exchanger is a zeolite or an ion exchange resin. 3. The dephosphorization method according to claim 1 or 2, wherein the ion exchanger is regenerated by contacting it with hydrochloric acid or sulfuric acid. 4 The waste liquid generated by regeneration of the ion exchanger is
The dephosphorization method according to any one of claims 1 to 3, which is added to water containing phosphate. 5. The dephosphorization method according to any one of claims 1 to 4, wherein the calcium ions are generated from lime.