JPS6230836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating water containing phosphate to remove phosphate.

In recent years, the progress of eutrophication in closed water bodies such as lakes, marshes, and inner bays has become a serious problem.
Phosphate present in water has been highlighted as a cause of eutrophication, and its removal has been raised as an urgent issue. Phosphates, which cause eutrophication, are contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.; It exists in the form of phosphate.

As a method to remove such phosphates, water containing phosphates is treated in the presence of calcium ions,
A method has been proposed in which contact with crystal species containing calcium phosphate, such as phosphate rock (Dissertation
Abstracts International, Vol.33, No.12, Part
I, pages 5878-B, etc.). This method removes phosphate ions contained in water by converting them into calcium phosphate, such as hydroxyapatite, and crystallizing them into crystal seeds. , has been attracting particular attention in recent years because it has significantly improved processing efficiency.

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⁺ +7OH ^- +3H ₂ PO ₄ ^- →Ca ₅ (OH) (PO ₄ ) ₃ +6H ₂ O...(1) As can be seen from equation (1), this reaction produces hydroxyapatite and converts it into phosphate. In order to increase the removal efficiency of phosphates, it is necessary to increase the concentration of calcium ions or increase the pH. If alkalinity above a certain level exists in raw water, precipitation of calcium carbonate is likely to occur. There was a tendency for the crystal surface to become contaminated. There was also the problem that the resulting treated water had to be reprocessed before being discharged.

This invention is intended to improve the conventional method as described above, and it is possible to further increase the removal rate of phosphate by passing water through a packed bed of fluorite or cryolite prior to crystallization. The purpose of the present invention is to provide a method for treating water containing phosphate.

This invention involves passing water containing phosphate through a packed bed of fluorite or cryolite, and then bringing it into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and an alkaline agent to effect crystallization. A method for treating water containing phosphate, characterized by:

In the present invention, fluoride ions are eluted from the raw water by passing water through a packed bed of fluorite, etc., and crystallization is performed in the presence of fluoride ions.
Fluoroapatite, which has a lower solubility than hydroxyapatite, is crystallized, thereby making it possible to remove phosphates from raw water at a high removal rate. The reaction in this case also varies depending on the reaction conditions, but a typical example is one according to the following formula.

5Ca ²⁺ +F ^- +6OH ^- +3H ₂ PO ₄ ^- →Ca ₅ F(PO ₄ ) ₃ +6H ₂ O...(2) As can be seen from equation (2), in the above reaction, the fluoride ions present in the reaction system are It is desirable that it be around the reaction equivalent. In other words, if it is present in excess, fluoride ions will leak into the treated water and cause secondary pollution, and if it is extremely insufficient, the desired reaction shown in equation (2) will be difficult to occur, making it difficult to obtain a significant effect. .

As a method for adding fluoride ions to raw water that does not contain fluoride ions, in the present invention, raw water is passed through a packed bed of fluorite or the like, thereby dissolving fluoride ions to the concentration necessary for the reaction, This makes it possible to remove fluoride ions with a high removal rate.

Fluorite is a mineral mainly composed of CaF ₂ and cryolite is a mineral mainly composed of Na ₃ AlF ₆ , both of which have low solubility and are suitable for the treatment of the present invention. These minerals may be used alone or in combination to form a packed bed, and raw water is passed through and brought into contact with these minerals.

The method of contact between the raw water and the packed bed may be either a fixed bed water flow system or a fluidized bed water flow system. When treating water that contains suspended matter, if you use a fixed bed water flow system filled with fluorine-containing minerals that are about the size of the filter media used in normal filtration, suspended matter in the water can be removed by filtration. Since the effect of suspended matter in the subsequent crystallization step is negligible, a more stable processing effect is achieved. Particularly in the crystallization process, when the crystal seeds containing calcium phosphate are brought into contact with the fixed bed water flow method, the capture of suspended matter in the crystal seed packed bed can be ignored, so the frequency of backwashing of the crystal seed packed bed is reduced. It requires less and more stable processing effects can be obtained. In other words, when the crystal seed packed bed is backwashed, the packed bed is disturbed, and even if the treatment is restarted, the water quality will be affected for several hours and the quality of the treated water will deteriorate, but if the frequency of backwashing is infrequent, it will be less This results in better processing efficiency.

However, if the concentration of suspended solids is low, such as when raw water is filtered in a pretreatment stage, a fluidized bed water flow system may be adopted. Further, the water flow direction is not particularly limited, and may be either an upward flow or a downward flow.
The flow rate during water flow is determined by the flow rate that can elute the necessary fluoride ions, and is determined by the particle size of the packed bed, the bed height, and the pH of the raw water, but when filled with fluorite with a particle size of 16 to 32 mesh, It should be about 1 to 10hr ^-1 . The eluted fluoride ion concentration is approximately 0.5 to 10 mg/. If it exceeds 10mg/, it may remain in the treated water depending on the phosphate content, but if it is within the above range, the eluted fluoride ions will become fluoroapatite and be removed by crystallization. ,
No new fluoride ion removal equipment is required.

There is no limit to the pH of the raw water, and it may be within a range that allows fluoride ions to be eluted, but if the raw water has a high pH, it is preferable to adjust the pH to 3 to 6 by adding an acid in advance.
Acid addition treatment can be incorporated into the main treatment process as necessary, taking into account the water quality of the target wastewater and the target phosphate concentration of the treated water. If the pH of the raw water is too low, excessive fluoride ions will be eluted, necessitating post-treatment.

Raw water, which has been passed through a packed bed of fluorite or the like to elute fluoride ions, is brought into contact with crystal seeds containing calcium phosphate in the presence of a calcium agent and an alkaline agent to undergo crystallization. When adding a calcium agent or an alkaline agent, it must be added after water is passed through the packed bed, and even if it is added before water is passed, it will not be very effective. Furthermore, a method of filling fluorite, etc. and calcium phosphate crystal seeds in the same layer is hardly effective.

Crystallization is carried out by the reaction represented by equation (2) above, but as can be seen from equation (2), in order to increase the removal rate of phosphate, it is necessary to advance the reaction to the right. For this purpose, a necessary amount of calcium agent or alkaline agent must be added. However, compared to the case where fluoride ions are not present, the amount of fluoride ions added can be relatively small, so that the resulting treated water can be discharged without the need for further treatment.

If too large amounts of calcium or alkaline agents are added to raw water, fine precipitates will form in places other than crystal seeds, and impurities such as calcium carbonate will be generated when raw water with high alkalinity is used. Therefore, it should be set within a range where these do not occur. That is, the amounts of calcium ions and hydroxide ions are set to be higher than the solubility of the fluoroapatite produced in equation (2), but the conditions are such that fluoroapatite is produced at a concentration lower than supersolubility, that is, 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.

Calcium and PH values that keep the amount of fluoroapatite within the metastable range can be determined experimentally by changing these values for each reaction system, but the approximate range is: Calcium ion is 10-100mg/, PH is 6
It is about ~10.

Calcium agents used in this invention include calcium hydroxide and calcium chloride; alkaline agents include sodium hydroxide, potassium hydroxide,
Examples include calcium hydroxide.

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 ) ₂ Crystal seeds containing calcium phosphate, such as . Furthermore, crystal seeds in which calcium phosphate is precipitated on the surface of a filter medium such as sand can also be used. The crystal seed is preferably one whose main component is fluoroapatite, which is the same type as calcium phosphate produced by the reaction. The use of fluoroapatite facilitates the precipitation of new crystals,
Phosphate removal is performed efficiently and the removal rate increases.

The method of contacting the water containing phosphate with the crystal seeds may be either a fixed bed method or a fluidized bed method, but if the packed bed of fluorite, etc. in the previous stage is a fluidized bed, the fixed bed method is used to remove suspended matter at the same time. If the packed bed of fluorite or the like is a fixed bed, a fluidized bed type may be used. Preferably, by using a fixed bed packed bed of fluorite or the like to elute fluoride ions and remove suspended matter at the same time, the particle size of the crystal seeds filled in the crystal seed packed bed can be made smaller, and the above-mentioned The number of times the crystal seeds are backwashed can also be reduced, as a result of which treated water with more stable quality can be obtained.

The smaller the size of the crystal seeds, the larger the specific 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 meshes 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 upwardly or downwardly at a flow rate of SV1 to 10 hr ^-1 to precipitate fluoroapatite crystals. In addition, when the first step is a fluidized bed, in the second step, suspended matter can be captured in the lower layer with large particle size and crystallization can be performed in the upper layer with small particle size and high activity. . Similarly, when passing water in a downward flow, in order to avoid adhesion of suspended matter to the crystal seed surface, a filter medium with a smaller specific gravity and larger particle size than the crystal seeds is layered on top of the crystal seed fixed layer. Preferably, suspended matter is removed by a filter medium. If the surface of the crystal seeds becomes contaminated or clogged during water flow, periodically backwash the crystal seeds using an upward flow to develop and clean the crystal seeds.
It is desirable to remove impurities attached to the surface.

Water flow conditions during backwashing include a flow rate of 20 to 80 m/
The backwashing time is about 5 to 60 minutes.

By contacting the crystal seeds as described above, fluoroapatite, which has extremely low solubility, crystallizes on the crystal seeds through the reaction represented by formula (2) above, resulting in extremely improved treated water quality. At the same time, almost no fluoride ions remain in the treated water.

As described above, according to the present invention, by passing water through a packed bed of fluorite or cryolite prior to crystallization, phosphates can be removed at a high removal rate with a simple operation, and phosphates in treated water can be removed with a high removal rate. The acid salt concentration can be made extremely low.

Next, examples of the present invention will be described.

Example 1 In an acrylic column with an inner diameter of 3.0 cm and a length of 100 cm,
Filled with 300ml of fluorite adjusted to a particle size of 16 to 32 mesh (packed layer 1), phosphorus concentration 3.2mg/, pH 7.2
The secondary treated sewage water was transferred to the packed bed 1 at a flow rate of 2.5hr ^-1 .
When water was treated using an upward flow fixed bed water flow method,
The F ^- concentration in the treated water of packed bed 1 was 1 mg/. A calcium chloride aqueous solution and a sodium hydroxide aqueous solution were continuously injected into the treated water to make the calcium concentration about 40 mg/ and the pH about 9, and phosphate rock adjusted to a particle size of 16 to 32 mesh was placed in the same column as packed bed 1.
When water was passed through the packed bed 2 filled with 300 ml using an upward flow fixed bed water flow method at a flow rate of 2.5 hr ^-1 , the average value of the phosphorus concentration in the treated water was 0.45 mg /
A treated water quality of 0.5mg/or less was obtained. Furthermore, as a result of analyzing the fluorine ion concentration in the treated water, no fluorine ions were detected. Such treatment lasted about half a year.

Example 2 The same treatment as in Example 1 was carried out except that hydrochloric acid was added to the raw water to adjust the pH to 5. The phosphorus concentration of the treated water was 0.31 mg/, and the pH was 8.31.

Comparative Example 1 Calcium chloride aqueous solution and sodium hydroxide aqueous solution were continuously injected into the same secondary treated sewage water used in the experiment of Example 1 to bring the calcium concentration to about 40%.
When water was passed through a packed bed of phosphate rock filled in the same manner as packed bed 2 of Example 1 with a pH of about 9 and a flow rate of 2.5 hr ^-1 , the average phosphorus concentration of the treated water was The value was 0.65 mg/. It can be seen from this that the removal rate is better when crystallization is performed in the presence of fluoride ions.

Comparative Example 2 Calcium chloride aqueous solution and sodium hydroxide aqueous solution were continuously injected into the same secondary treated sewage water as that used in the experiment of Example 1 to bring the calcium concentration to about 40%.
mg/, after setting the pH to about 9, the packed bed 1 of Example 1
When water was passed in series at a flow rate of 2.5 hr ^-1 through a fluorite packed bed and a phosphorite packed bed filled in the same manner as packed bed 2, the average value of the phosphorus concentration in the treated water was 0.67 mg/ It was hot. This shows that the quality of treated water deteriorates when calcium ions or alkaline agents are added before water is passed through the fluorite packed bed.

Example 3 Calcium chloride aqueous solution and sodium hydroxide aqueous solution were continuously injected into the same secondary treated sewage water used in the experiment of Example 1 to bring the calcium concentration to about 40%.
mg/, 240ml of phosphate rock adjusted to 16-32 mesh after adjusting the pH to about 9, and 60ml of fluorite with the same particle size.
ml mixed packed bed (the packed column was the same as in Example 1) was treated with water at a flow rate of 2.5 hr ^-1 in the same manner, and the average value of the phosphorus concentration in the treated water was 0.82 mg/ml.
It was hot. This shows that the phosphate removal rate does not improve even if water is passed through a packed bed containing a mixed bed of fluorite and phosphate rock.

From the above results, it can be seen that the treatment results of the examples show extremely high phosphate removal effects compared to the comparative examples.

Claims (1)

[Claims] 1 Water containing phosphate is passed through a packed bed of fluorite or cryolite, and then brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and an alkaline agent to form crystals. 1. A method for treating water containing phosphate, characterized by carrying out analysis. 2. A method for treating water containing phosphates according to claim 1, wherein water is passed through a packed bed of fluorite or cryolite using a fixed bed water flow system. 3. The phosphate-containing water according to claim 1 or 2, wherein the pH of the phosphate-containing water is adjusted to 3 to 6, and the water is passed through a packed bed of fluorite or cryolite. Processing method. 4. Phosphate according to any one of claims 1 to 3, wherein water is passed through a packed bed of fluorite or cryolite, and crystallization is carried out at a fluoride ion concentration of 0.5 to 10 mg/. water treatment methods including; 5. The method for treating water containing phosphate according to any one of claims 1 to 4, wherein the crystal seeds containing calcium phosphate are hydroxyapatite, fluoroapatite, or tricalcium phosphate.