JPS6044997B2 - How to treat water containing phosphates - 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 attracting attention as a cause of eutrophication, and its removal has become an urgent issue. Phosphate, which causes eutrophication, is contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.
It exists in the form of inorganic phosphates such as orthophosphates and condensed phosphates, and organic phosphates. As a method for removing such phosphates, a method has been proposed in which water containing phosphates is brought into contact with crystalline species containing calcium phosphate, such as phosphate rock, in the presence of calcium ions (
DissertationAbstractsInte
rnational9Vol, 3O9No, 129Pa
rtI, page 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. , processing efficiency will also be significantly improved. However, with this method, the removal rate of phosphates containing organic substances and other impurities in the crystal seeds is poor, especially when natural ores such as phosphate rock are used as the crystal seeds. Furthermore, it is recognized that the removal ability of phosphate rock itself varies widely depending on the place of production, and even if the crystal type has a high removal rate, slime etc. will adhere to the surface during the continuous treatment and the activity will decrease. and the removal rate decreases. An object of the present invention is to provide a method for treating water containing phosphate, which can improve such a low removal rate and maintain a high removal rate. The present invention is a method for treating water containing phosphate, which is characterized by bringing crystal seeds containing calcium phosphate treated with a chlorine agent into contact with water containing phosphate.

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.

As can be seen from equation (1), in order to increase the removal rate of phosphate, it is necessary to allow the reaction to proceed in the right direction, and at the same time, it is necessary to keep the surface of the crystal seeds clean and maintain high activity. be.

For this purpose, in the present invention, crystal seeds containing calcium phosphate treated with a chlorine agent are brought into contact with water containing phosphate.

A chlorine agent is an agent that generates free chlorine in water, and examples of the agents that can be used include chlorine gas, chlorine water, sodium hypochlorite, white powder, and high-grade white powder. As a crystal species containing calcium phosphate, hydroxyapatite [
Ca5(O)()(PO4)3], fluoroapatite [Ca5(F)(PO4)3] or tricalcium phosphate [
Crystal seeds containing calcium phosphate such as [Ca3(PO4)2] can be used, and natural phosphate rock contains these calcium phosphates as a main component and is suitable as a crystal seed.

Furthermore, crystal seeds in which calcium phosphate is precipitated on the surface of offshore materials such as sand 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, hydroxyapa. In tight-forming systems, the use of hydroxyapatite facilitates the precipitation of new crystals and increases the efficiency of phosphate removal. When crystal seeds containing calcium phosphate with low removal ability are used for crystallization treatment, the crystal seeds are first treated with chlorine. Treat with a chemical. For this purpose, the crystal seeds are immersed in an aqueous solution in which a chlorine agent is dissolved, and the two are brought into contact in a stationary or fluidized bed manner. In this way, in order to improve the removal rate of crystal species with a low removal rate, the material is treated with a chlorine agent in advance and then subjected to phosphate removal. In this case, the concentration of the chlorine agent is not particularly limited, but the higher the concentration, the more effective it is, and the effective chlorine is generally 0.0001 to 10%. However, even if the concentration is low, there is a method that has the same effect when brought into contact with a large amount of liquid. Thus, the chlorinated crystal seeds with a low removal rate or the unchlorinated crystal seeds with a high removal rate are used to remove phosphate. To remove phosphate using these crystal seeds containing calcium phosphate, the crystal seeds are brought into contact with water containing phosphate in the presence of calcium ions. As a result, calcium phosphate produced by the above formula (1) is precipitated on the surface of the crystal seed, the crystal grows, and the phosphate in the water is removed. In this case, even if the crystal seeds originally have a low removal ability, the organic matter and other impurities attached to the surface of the crystal seeds are removed by the chlorine treatment and the surface is activated, so the phosphate removal rate by the crystal seeds is high, and the treatment The concentration of phosphoric acid in the solution can be lowered. Calcium ions and hydroxyl 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; however, 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. The range should be such that these do not occur. That is, the amount of calcium ions and hydroxide ions is (1
) The conditions are such that hydroxyapatite is produced at a concentration higher than the solubility of hydroxyapatite produced in formula (2) and 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. 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 door floor, but in a metastable region lower than the supersolubility, new crystals precipitate where the crystal seeds do not exist. New crystals are precipitated and the crystals grow, but no precipitate is formed. In addition, crystals do not precipitate in systems lower than the solubility. The amount of hydroxyapatite produced is controlled by the phosphate ion concentration, calcium ion concentration, and pH of the reaction system.

The amount of calcium ions and the pH value that bring the amount of produced hydroxyapatite within the metastable range can be determined experimentally by changing these values for each reaction system. The approximate range is when phosphate ions are 50 ppm or less, and calcium ions are 10 to 100 ppm. ．． The pH is about 6 to 100 ml, but it varies depending on each condition. The method of contacting the water containing the phosphate with the crystal seeds containing calcium phosphate may be a fixed bed method or a fluidized bed method.

A fixed bed type is preferable when turbidity removal is also performed at the same time, and a fluidized bed type is suitable when turbidity removal is not required. The smaller the size of the crystal seeds, the larger the surface area and the easier the formation of new crystals, 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, it is packed with crystal seeds with a particle size of 9 to 35 mesh, and the SVl
5 (1/Hr) in an upward or downward flow. In the case of water flow treatment in a downward flow, it is advantageous to pre-treat the raw water to remove impurities in order to avoid the adhesion of impurities to the surface of the crystal seeds. If phosphates in water are removed using crystal seeds containing calcium phosphate in this way, the surface of the crystal seeds becomes contaminated during water flow and is likely to become clogged, so operation must be periodically interrupted and backwashed. The crystal seed bed is spread and washed by washing, and impurities adhering to the surface are peeled off. By backwashing, among the crystal seeds in the packed bed, those with a large particle size settle downward, while those with a small particle size move upward. As the particles come into contact with each other during this movement, most of the impurities and precipitates on the crystal seeds are peeled off, but a small amount of impurities and precipitates are not removed by contact alone, and they remain even during the next operation. If the coating remains, the activity of the crystal seeds decreases and the phosphorus removal rate decreases. Therefore, by adding a chlorine agent during backwashing, organic substances and other impurities are chemically removed and the activity is restored.

As for the method of adding a chlorine agent, if the chlorine agent is added to the backwash water during backwashing and backwashing is performed, performance can be improved with a simple operation. Of course, in addition to this, a chlorine agent may be added directly to the packed bed. The chlorine agent added to the backwash water at this time is 1 to 50 ppm as available chlorine, preferably 5
- About 10 ppm. Furthermore, after first injecting backwash water into the packed bed to wash the crystal particles, the packed bed may be filled with water containing a chlorine agent, and this state may be maintained for a certain period of time.

The conditions at this time can be the same as those for immersion described above.
It is not necessary to add chlorine agent every time during backwashing.
Just do it at the appropriate time.

The backwash water may be treated water or raw water, and the flow rate of the backwash water is 20~
The backwashing time is approximately 80 rn/Hr, and the backwashing time is approximately 5 to 6 times depending on the degree of activity reduction of crystal seeds. After backwashing, the packed bed is filled with small-sized, highly active crystal seeds at the top and large-sized, low-activity crystal seeds at the bottom, so the flow direction of the raw water is made to flow upward. This is preferable because the suspended solids are removed in the lower part and the phosphate is almost completely removed by the microcrystalline seed group in the upper part near the outlet.

Crystal seeds that have become large in size due to long-term operation are extracted and disposed of, or crushed and reused. Example 1 150 mL of Florida phosphate rock having a particle size of 16 to 32 mesh, which has poor phosphate removal performance, was immersed in 200 ml of an aqueous sodium hypochlorite solution containing 5% available chlorine for 20 hours.

After washing with tap water, it was filled into a cylinder with an inner diameter of 25 wrIn. Average phosphate ion 10ppmNC0D14ppm
．． ．． Calcium chloride aqueous solution was added to secondary treated sewage water containing 10 to 30 ppm of SS so that the calcium concentration was 40 ppm, and the pH was adjusted to 9 with sodium hydroxide aqueous solution.
The treatment was carried out by passing water in an upward flow through the packed bed of crystal seeds at a flow rate of S12 hr-1.

In addition, the crystal seed bed is heated once a day for 15 minutes, L■30rT1/H.
Backwashed with R. On the other hand, as a comparative example, phosphate rock that was not treated with chlorine was used and treated with water under the same conditions.

As a result, the phosphate removal performance of chlorinated phosphate rock was significantly higher than that of untreated rock at the initial stage of water flow, but it decreased to almost the same level as untreated rock after about a week. did. Therefore, after 7 days, the sample was treated with chlorine again under the same conditions and water was passed through it, and it showed higher removal performance than the sample without treatment. The results are shown in Table 1. Example 2 Phosphate ion 1
0-18ppm,. C0D10~20ppm1SS10
Add an aqueous calcium chloride solution to the secondary treated sewage water containing ~30 ppm so that the concentration is about 40 ppm as calcium, and further adjust the pH to 9 with an aqueous sodium hydroxide solution.
The material was passed through a cylinder with an inner diameter of 3 mm filled with 300 mt of Jordanian phosphate rock having a particle size of 16 to 32 mesh and having a good removal ability, by passing it in an upward flow at an S nozzle of r-1 to perform a deep filtration treatment.

To remove trapped turbidity, backwash water containing 8 ppm of sodium hypochlorite is added once a day to approximately 30 nl of water in an upward flow.
A backwashing operation was performed by passing water through the powder three times at a flow rate of /Hr. For comparison, an experiment was also conducted under the same conditions in which backwashing was performed using backwash water without the addition of chlorine. The results are shown in Table 2. From the results in Table 2, there is no difference until 15 days later due to the adsorption phenomenon.
It is observed that there is a difference in activity after the 21st day. As described above, the present invention can improve the removal performance of crystal species with low phosphate removal performance, and can restore the removal performance of crystal species whose removal performance has decreased due to use.

Claims (1)

[Scope of Claims] 1. A method for treating water containing phosphate, which comprises bringing crystal seeds containing calcium phosphate treated with a chlorine agent into contact with water containing phosphate. 2. The method for treating water containing phosphates according to claim 1, wherein the crystal seeds containing calcium phosphate are treated with a chlorine agent during backwashing. 3. The method for treating water containing phosphates according to claim 1, wherein the chlorine agent is chlorine gas, chlorine water, sodium hypochlorite, salami powder, or highly salami powder. 4. The method for treating water containing phosphate according to any one of claims 1 to 3, wherein the crystal seeds containing calcium phosphate contain hydroxyapatite, fluoroapatite, or tricalcium phosphate. 5. The method for treating water containing phosphate according to any one of claims 1 to 2, wherein the crystal seed containing calcium phosphate is phosphate rock.