JPH0251678B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating water containing phosphate;
Specifically, the present invention relates to a treatment method using a crystallization dephosphorization method.

In recent years, the problem of eutrophication in closed water bodies such as lakes, marshes, and inner bays has become significant, and countermeasures are urgently needed. One of the causes of eutrophication is phosphates present in water, and as nitrogen treatment is not being done adequately, research is underway on how to remove phosphates. This phosphate is a detergent builder,
It is widely used in fertilizers, etc., and is present in domestic and industrial wastewater in the form of inorganic phosphates such as orthophosphates and condensed phosphates, and organic phosphates.

As methods for removing phosphates, the coagulation sedimentation method using aluminum sulfate soil, the activated sludge method, or a combination of these methods have been used in experimental plants and actual plants and have been recognized to be effective. It is easy to operate and does not generate sludge.
The crystallization dephosphorization method, which has high processing efficiency, is attracting attention.

The crystallization dephosphorization method involves contacting water containing phosphate with a crystal species containing calcium phosphate such as hydroxyapatite in the presence of calcium ions.
This is a method in which phosphate ions in water are converted into calcium phosphate and crystallized on the surface of a crystal seed, and is thought to proceed mainly through the following reaction.

5Ca ²⁺ +3HPO ^2- ₄ +40H ^- →Ca ₅ (OH) (PO ₄ ) ₃ +3H ₂ O ...(1) However, when removing phosphorus with this method, magnesium ions, sulfate ions, or bicarbonate are present in the target water. There was a problem in that when ions and the like were contained above a certain concentration, the phosphorus removal efficiency decreased. On the other hand, when removing phosphorus by precipitating hydroxyapatite, we confirmed that adding fluoride ions to the reaction system improves the dephosphorization effect, and we also focused on the promotion effect of fluoride ions on the crystallization and dephosphorization reaction. As a result of experiments, we confirmed that adding fluorine ions is effective when removing phosphorus from water containing magnesium ions and phosphates by crystallization, and we developed a new method for removing phosphorus. Proposed. However, when treating water by adding fluorine ions, if the amount of fluorine added is controlled at an appropriate concentration relative to the content of phosphates, the concentration of fluorine ions remaining in the treated water will be lower than practical. However, when the amount of fluorine ions added temporarily increases due to a decrease in the phosphate concentration in the target water, the disadvantage of residual fluoride ions in the treated water becomes I confirmed that there is.

The present invention has been made to eliminate the drawbacks of the conventional methods as described above, and is capable of efficiently removing phosphates even in the presence of inhibitors such as magnesium, and moreover, It is an object of the present invention to provide a phosphorus removal method that does not cause the attendant problem of residual fluorine ions. Another object of the present invention is to provide a phosphorus removal method that can stably obtain treated water with an extremely low concentration of phosphorus compared to the phosphorus concentration of treated water obtained by conventional phosphorus removal methods.

That is, in the method for treating water containing phosphate of the present invention, water containing phosphate is treated with crystals containing calcium phosphate in the presence of calcium ions and fluorine ions and under conditions of pH 6 or higher. After the contact with the seeds, the method is further brought into contact with a fluorine adsorbent.

The present invention will be explained in more detail below.

It is thought that by adding fluorine ions, phosphate in water generates fluoroapatite, which has even lower solubility than hydroxyapatite, through the reaction shown in equation (2) below.

5Ca ²⁺ +3HPO ^2- ₄ +F ^- +3OH ^- →Ca ₅ F (PO ₄ ) ₃ +3H ₂ O ......(2) Therefore, the phosphorus removal efficiency is higher than when phosphorus is removed by generating hydroxyapatite. Not only does this improve the quality of treated water, but even if the target water (raw water) contains inhibitory substances such as magnesium ions, it is less susceptible to the effects and stable treated water quality can be obtained.

In reality, in addition to the formation of fluoroapatite, hydroxyapatite as shown in formula (1) above also occurs on the surface of the crystal seed, but in any case, for the efficient crystallization reaction to proceed, calcium ions and fluorine ions are present, and it is necessary to bring the raw water into contact with the crystal seeds under conditions where the pH is 6 or higher.

FIG. 1 is a flow sheet showing an embodiment of the present invention, in which raw water containing phosphate ions passes through line 10, where calcium ions, fluorine ions, and further hydroxide ions are added as necessary to form crystal seeds. It is introduced into the packed bed 12 and subjected to dephosphorization treatment. This crystallization reaction is carried out in the metastable region (region in which new crystals do not precipitate when no crystal seeds exist) as described in JP-A No. 56-28694, and the Ca ²⁺ ions and alkaline agents are added.

The crystal seed filling layer 12 is filled with crystal seeds containing calcium phosphate such as hydroxyapatite, fluoroapatite, tricalcium phosphate, and phosphate rock, and these crystal seeds themselves may be filled as granules, and It may also be filled in a state in which it is trapped in a suitable support layer as a powder.

The water treated by the crystal seed packed bed 12 contains some phosphorus ions and surplus fluorine ions and calcium ions, and then this water is passed through the fluorine adsorbent packed bed 14. The fluorine adsorbent packed layer 14 is filled with a material exhibiting adsorption ability for fluorine, and specific examples thereof include activated alumina, chelate resin, activated magnesia-based adsorbent, bone char, and the like. Fluorine ions in the water are removed by passing through the fluorine adsorbent packed bed 14, but if a fluorine adsorbent that also exhibits adsorption ability for phosphorus is used, the fluorine ions can be removed by the crystal seed packed bed 12. This method is more preferable because it allows removal of unused phosphate. Among these, activated alumina is a particularly preferred fluorine adsorbent because it has the ability to adsorb phosphorus and can be easily regenerated as described later. The effluent water from the fluorine adsorbent packed bed 14 has extremely reduced phosphate concentration and fluorine ions, and is discharged from the system through a line 16.

As the dephosphorization process progresses, the fluorine adsorbent packed bed 14 adsorbs fluorine ions or even phosphates, and gradually loses its adsorption ability. Therefore, the fluorine adsorbent is regenerated at an appropriate time. The regeneration method is selected as appropriate depending on the type of adsorbent, but fluorine ions are recovered from the recycled waste liquid obtained at this time and used as fluorine ions that should be present in the raw water during the crystallization reaction. It is preferable.

FIG. 1 illustrates a preferred embodiment in which activated alumina is used as the fluorine adsorbent. Activated alumina that has adsorbed fluorine is treated with an alkaline agent, e.g., after draining the layer.
Supply NaOH and Ca(OH ₂ ) ₂ to obtain regenerated waste liquid. As the alkaline agent, it is preferable to use one with higher solubility, such as NaOH, than Ca(OH) ₂ . Note that the regeneration time can be determined, for example, by measuring the fluorine ion concentration in the treated water. During the regeneration process, the flow of raw water will be stopped, but depending on the amount of raw water, a fluorine adsorbent packed bed may be provided separately and switched during regeneration to continue water flow operation. In large-scale sewage treatment plants, it is difficult to stop operation, so it is better to provide a separate bed filled with adsorbent.

The alkaline agent is passed through once, and the recycled waste liquid discharged is stored in the reaction tank 18. When the regeneration is completed, the supply of the alkaline agent is stopped, and after extrusion with water, the water flow to the fluorine adsorbent packed bed 14 is restarted.

A calcium agent is added to the reaction tank 18 storing recycled waste liquid containing fluorine ions and phosphorus ions while stirring, and the pH is adjusted to 9 or higher. As a result, a calcium phosphate-based precipitate is generated. When the pH of the liquid is less than 9, calcium fluoride begins to form, which is not preferable because the amount of fluorine ions that can be recycled is reduced. Therefore, when using a neutral salt as a calcium agent, it is necessary to use an alkaline agent together, and from this point of view, it is preferable to use Ca(OH) ₂ as the calcium agent.

The reaction liquid obtained in the reaction tank 18 is sent to the precipitation tank 20 to separate the precipitate, and then the supernatant liquid is added to the raw water line as part of the source of F ^- , Ca ²⁺ , and OH ^- to be added to the raw water. send it back to On the other hand, the separated calcium phosphate sludge can be used as it is as fertilizer or as a reactivating agent for reactivating crystal seeds.

In addition, in a small-scale facility, a batch-type reaction and precipitation layer can be used in one tank without providing a reaction tank and a precipitation tank separately. Furthermore, the separation of the liquid phase containing F ^- ions and the like from the calcium phosphate precipitate is not limited to precipitation separation, and may be performed by any appropriate solid-liquid separation means.

According to the present invention, not only can dephosphorization treatment be carried out efficiently by the presence of fluoride ions, but also, for example, the phosphate concentration in raw water can be reduced, and the set fluorine ion supply amount can become excessive as a result. However, the fluorine ions remaining in the crystallization-dephosphorized water are removed by contact with the fluorine adsorbent in the subsequent stage, so the fluorine ion concentration in the final treated water is below a level that does not pose a practical problem. becomes. The same applies when the amount of fluorine ions added increases abnormally due to troubles such as operational problems.

Example: In an alkaline column with an inner diameter of 30 mm and a length of 500 mm,
Jordanian phosphate rock with a particle size of 16 to 32 mesh (magnesium ion concentration 40 to 50 mg/, phosphorus concentration 1 to
Filled with 150 ml of phosphorus (which was passed through water for about 1 year to remove phosphorus from secondary treated sewage water containing 2 mg of phosphorus),
Phosphorus-containing synthetic water (phosphorus concentration: 1 mg/, total alkalinity: approximately 100 mg/, magnesium ion concentration;
At the column inlet, add an aqueous solution of calcium chloride, sodium hydroxide, and sodium fluoride to the column inlet to adjust the calcium ion concentration to 80-85 mg/, and the pH to 8.8-85 mg/.
9.0, the fluorine ion concentration was set to approximately 1 mg/hr, water was passed through a packed bed of phosphate rock at a flow rate of 300 ml/hr, and the treated water was then passed through a bed with an inner diameter of 30 mm and a length of
A 500 mm acrylic column was filled with 150 ml of activated alumina with a particle size of 16 to 32 mesh.
Water was passed through at the same flow rate. As a result of water treatment for 30 days, the phosphorus concentration in the final treated water was 0.1
mg/or less, and the fluorine ion concentration was less than 0.5 mg/, allowing stable processing.

After 30 days, the flow of water to the activated alumina layer was stopped, and after draining the water, an aqueous NaOH solution was passed through the layer at SV=2, and the regenerated waste liquid was collected in a container. Subsequently, Ca(OH) ₂ was added to adjust the pH to 10, and the resulting precipitate was separated. The fluoride ion concentration in the supernatant liquid is approximately 950mg/
It was hot. This shows that the supernatant liquid contains a sufficient amount of F ^- as a fluorine ion source.

After the above experiment, the amount of fluorine ions added was increased to 1
The experiment was continued for an additional 20 days to determine the effect of varying the dose in the range ~15 mg/day over several hours. As a result, it was confirmed that even in this case, the phosphorus concentration in the final treated water was 0.1 mg/or less, and the fluorine ion concentration was 0.5 mg/or less, making it possible to continue the treatment.

Comparative Example An experiment was conducted under the same conditions as in Example 1, except that the contact treatment operation to the packed bed of activated alumina was not performed.

When the amount of fluoride ion added was 1 mg/, the treatment result was that the phosphorus concentration in the treated water was 0.25.
~0.1mg/, and the concentration of fluorine ion is 0.5
mg/or less.

On the other hand, when the amount of fluorine ions added was varied in the range of 1 to 15 mg/, the phosphoric acid in the treated water was 0.1 to 0.2 mg/; In the following cases, it was confirmed that 1 mg/or more of fluoride ions remained in the treated water.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing an embodiment of the present invention. 12...Crystal seed packed bed, 14...Fluorine adsorbent packed bed, 18...Reaction tank, 20...Settling tank.

Claims (1)

[Claims] 1. After bringing water containing phosphate into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and fluorine ions and under conditions of pH 6 or higher, fluorine adsorption is further carried out. A method for treating water containing phosphates, the method comprising contacting water with a phosphate agent. 2. The treatment method according to claim 1, wherein the fluorine adsorbent is at least one selected from the group consisting of activated alumina, chelate resin, activated magnesia adsorbent, and bone char. 3. The treatment method according to claim 1 or 2, wherein at least a portion of the fluorine ions are fluorine ions recovered from recycled waste liquid of a fluorine adsorbent. 4. At least a portion of the fluorine ions are present in the liquid phase obtained by adding a calcium agent to the recycled waste liquid obtained by contacting the fluorine adsorbent with an alkali, adjusting the pH to 9 or higher, and separating it from the generated precipitate. The treatment method according to claim 1 or 2, wherein the fluorine ion is fluorine ion. 5. The treatment method according to claim 4, wherein the fluorine adsorbent is activated alumina.