JPH022628B2 - - Google Patents

Description

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

As a method for removing phosphates from water, a method has been proposed in which water containing phosphates 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. , processing efficiency will also be significantly improved.

As such a crystallization method, there is a method in which raw water is passed through a packed bed of crystal seeds in an upward or downward flow and brought into contact with the bed. In this method, phosphorus is removed by crystallization reaction when water is passed, and SS components are also removed by overaction. The layers need to be developed and cleaned. However, when such a cleaning operation is performed, the packing state of crystal seeds changes, and particles with a surface with a high phosphorus bonding ratio on the inlet side of the raw water (lower part in the case of upward flowing water) are absorbed into the treated water. Outflow side (upper part in case of upward flowing water)
There is a problem in that the removal effect of phosphates is temporarily reduced because the removal of phosphates may occur.

In order to solve these problems, a plurality of dephosphorization towers filled with crystal seeds are installed and water is passed in series. In this method, in order to prevent deterioration of the treated water due to changes in the filling state during packed bed cleaning, the first column is always used as the first column to introduce raw water for treatment, and the final column is always used as the final column in terms of polishing. used.

However, in such a method, although it is possible to eliminate the negative effects of cleaning the packed bed, there is a tendency for the treatment effect of the first column to gradually deteriorate, and the concentration of treated water in the final column also tends to increase accordingly. There is a problem that it becomes difficult to maintain stable treatment effects over a long period of time.

This invention is intended to solve the above-mentioned problems in the prior art. By installing a plurality of dephosphorization towers filled with crystal seeds containing calcium phosphate, and changing the water flow order, the crystal seeds can be removed. The object of the present invention is to provide a method for treating water containing phosphate, which can perform crystallization while activating phosphate, and can perform stable treatment.

In this invention, water containing phosphate is in the presence of calcium ions and under conditions of pH 6 or higher,
In the method of removing phosphate by bringing it into contact with crystal seeds containing calcium phosphate, raw water is passed in series through multiple dephosphorization towers filled with crystal seeds to perform crystallization. This is a method for treating water containing phosphate, which is characterized in that the dephosphorization tower is connected to the second stage or later in the next step and water is passed through the dephosphorization tower.

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
It is usually expressed by the following equation.

5Ca ²⁺ +3HPO ₄ ^2- +4OH ^- →Ca ₅ (OH) (PO ₄ ) ₃ +3H ₂ O...(1) As can be seen from equation (1), in order to increase the phosphate removal rate, the reaction It is necessary to advance the crystal species to the right, and at the same time, it is necessary to increase the activity of the crystal seeds. One method for increasing the activity of crystal seeds is to increase the weight ratio of calcium concentration to phosphorus concentration (Ca/P) in water.

By the way, if two dephosphorization towers filled with crystal seeds are installed and crystallization is performed by passing water through the series, Ca/P will be higher at the second stage inlet than at the first stage inlet, and 2
It was found that the higher the number of rows, the better the conditions for activation. As an example, raw water with a phosphorus concentration of 2 mg/ is fed to the first and second dephosphorization towers, respectively.
When performing crystallization with upward flowing water at SV2hr ^-1
Ca/P was 34.1 at the first stage entrance, but it became 156.5 at the second stage entrance. When there are three or more dephosphorization towers, Ca/P becomes higher in the later stages.

In this way, when water is passed through multiple dephosphorization towers in series, Ca/P is higher in the later stages, which is a condition for more activation. When a dephosphorization tower is connected to the second stage or later in the next process, the crystal seeds in that dephosphorization tower are activated, and the phosphates leaking from the new first stage dephosphorization tower are removed and used as a polisher. Function.

When changing the water flow order of the dephosphorization tower, if the packed bed is washed by upward flow, the filling state may change as described above and the treatment effect may decrease, but this may be avoided. To avoid this, the treated water quality can always be maintained within the target value by switching the water flow order at a time when the quality of the treated water in the first stage does not become higher than the target water quality. Furthermore, as will be described later, when switching, the first stage dephosphorization tower is connected to the second stage and subsequent stages after stopping the water flow for a predetermined time in the next step, thereby promoting activation and stabilizing the treatment effect.

The above dephosphorization tower is filled with crystal seeds containing calcium phosphate to form a packed bed, and raw water is passed through the plurality of dephosphorization towers in series, and the water passage order is connected so as to be switchable. . The grain size of the crystal seeds is 5 to 35 mesh, and crystallization is carried out by passing water in an upward or downward flow at SV1 to 5 hr ^-1 .

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 material 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, in a system that produces hydroxyapatite, the use of hydroxyapatite facilitates the precipitation of new crystals and efficiently removes phosphates.
Removal rate increases.

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 set to be higher than the solubility of the 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 supersolubility, new crystals precipitate where crystal seeds do not exist, forming fine precipitates.
Although bed clogging occurs, in the metastable region below supersolubility, new crystals precipitate on top of the crystal seeds, and only crystals grow and no precipitates are formed. In addition, crystals do not precipitate in systems lower than the solubility.

The amount of hydroxyapatite produced depends on the phosphate ion concentration, calcium ion concentration, and
Controlled by PH. The amount of calcium ions and PH value that bring the amount of hydroxyapatite produced within the metastable range can be determined experimentally by changing these values for each reaction system. The approximate range is phosphate ion 50mg/or less, calcium ion 10-100mg/, PH 6
~12, but varies depending on each condition.

If the crystal seed surface becomes contaminated or clogged during water flow, periodically perform upward flow cleaning to expand and clean the crystal seed bed and peel off impurities attached to the surface. is desirable. Water flow conditions during cleaning include a flow rate of approximately 20 to 80 m/hr, and a cleaning time of approximately 5 to 60 minutes. By cleaning such crystal seeds, SS components are removed, and water flow can be restarted.

As I was repeatedly washing and running water,
Since the phosphorus removal effect of the first-stage dephosphorization tower decreases, the water flow order is changed during operation. The water flow order after switching is to introduce raw water into the second and subsequent dephosphorization towers as the new first stage, and connect the old first stage to the second and subsequent stages.

The drawing is a system diagram showing the switching method, and 1, 2,
3 indicates the first, second, and n-th dephosphorization towers, respectively.
When operating with two towers as the first step, raw water pipe 4
After raw water is supplied to the first dephosphorization tower 1 and passed through in an upward flow, it is supplied to the second dephosphorization tower 2 from the connecting pipe 5, and treated water is obtained from the treated water pipe 6. When operating with the n-th tower, the water is further supplied to the n-th dephosphorization tower 3 from the pipe 7 and treated water is obtained from the treated water pipe 8.

Next, in the second step, in the case of two towers, raw water is supplied from the raw water pipe 9 to the second dephosphorization tower 2, and the connecting pipe 1
0 to the first dephosphorization tower 1 to obtain treated water from the treated water pipe 11. For n-tower operation, raw water pipe 12
The raw water is supplied to the n-th dephosphorization tower 3, and the connecting pipe 13
The water is supplied from the water to the second dephosphorization tower 2 and further from the connecting pipe 10 to the first dephosphorization tower 1, and the treated water is obtained from the treated water pipe 11. In the case of n-tower operation, in the second step, the second dephosphorization tower 2, the n-th dephosphorization tower 3, the first dephosphorization tower 1
The water may be passed in this order, or in the order of the second dephosphorization tower 2, the first dephosphorization tower 1, and the n-th dephosphorization tower 3.

Next, when the activity of the second dephosphorization tower 2 or the n-th dephosphorization tower 3, which is the first stage, decreases in the second step, these dephosphorization towers are replaced with the second and subsequent stages in the next third step. Connect and run water. In this way, the order of water flow is sequentially changed to perform the dephosphorization operation.

The above-mentioned switching of the water flow order may be performed independently of cleaning the packed bed, or may be performed simultaneously. The water flow order is switched at regular intervals (by a timer, etc.) using the phosphorus concentration in the treated water of the first-stage dephosphorization tower as an index.

The timing for switching is selected at a time when the phosphorus concentration in the first-stage treated water does not exceed the target value. For example, in the above case, the upper limit of the phosphorus concentration in the first stage treated water should be set to 0.4 to 0.5.
mg/, it can be reduced to about 0.3 mg/by passing water through the second stage. Even if the water flow order is changed in the second step and the first stage is connected to the final stage, the final treated water has a phosphorus concentration of 0.5 mg/
It can be maintained within the target value.

As mentioned above, after the activity of the first stage dephosphorization tower decreases, if it is connected to the second stage or later in the next process, the crystal seeds of the former first stage dephosphorization tower connected to the second stage or later is activated by treated water with increased Ca/P, and also functions as a polisher for advanced treatment and can remove leaking phosphates.

Since the second and subsequent dephosphorization towers are activated when switching is performed, phosphates can be sufficiently removed with just one tower. For example, in the case of the above example, the second stage treated water has a phosphorus concentration of 0.3 mg/
It is possible to obtain treated water with a lower phosphorus concentration by performing crystallization in the column than by connecting the first-stage dephosphorization column later. For this reason, the next process can be started using only the second and subsequent dephosphorization towers, and while these towers alone can process to a low phosphorus concentration, the former first stage dephosphorization tower can be activated to some extent by other means. If the activated dephosphorization tower is connected to the second stage or later, deterioration of the treated water at the time of switching can be prevented and stable treatment can be performed.

As a means for such activation, stopping the water flow to the first-stage dephosphorization tower, especially draining the water, is effective. When the first-stage dephosphorization tower, whose activity level has decreased, is disconnected from the treatment system and the water flow is stopped, calcium ions in the water are incorporated into the crystals, activating the crystal surfaces. This tendency is especially noticeable when draining.

The purpose of draining is to reduce the water adhering to the crystal seeds and increase the phosphorus and calcium ion concentrations in the adhering water.There is no need to completely dry the seeds, just dry them to the point where they are slightly sticky to the touch. As a rough guide, it is desirable to dry the crystal seed surface to a moisture content of about 20%. Of course, drying may be performed to a higher or lower moisture content.

Although the method of draining is not limited, generally the crystal seeds are drained and the water is evaporated. If the tank is large-scale, change the water flow, drain the water, clean as necessary, open the lid of the tank, and let it dry naturally. In this case, the time required for drying varies depending on the layer height, but is approximately one week or more on a standard scale. Forced ventilation may be applied to the packed bed to facilitate drying.
Furthermore, if an economical heat source such as waste heat is available, it is preferable to introduce hot air into the tank because drying can be accomplished in 2 to 3 hours. In both cases, within the crystal seed layer
If there is SS, etc., it is desirable to wash it immediately after stopping the water flow, and then dry it. In addition, in the case of small to medium scale, the above method is possible, but in addition to this, it is also possible to take out the crystal seeds in the tank, spread them out and air dry them.

As mentioned above, when the water flow is stopped or water is drained, the crystal seeds are activated to some extent, so if this dephosphorization tower is connected to the second stage or later, activation will proceed further due to the treated water with high Ca/P content. With,
It is possible to remove phosphates leaking from the first-stage dephosphorization tower and perform stable high-level treatment.

Instead of or in addition to the activation means described above, other activation means may be employed, for example, a method in which the packed bed is brought into contact with a solution containing a chlorine agent or a calcium compound during or after cleaning.

As explained above, according to the present invention, a plurality of dephosphorization towers are provided and water is passed through the series while changing the water flow order, so that the activity of the first stage dephosphorization tower is reduced. However, it can be activated by switching, and stable phosphate removal can be achieved over a long period of time. Moreover, if the first stage is activated at the time of switching and then the second stage and subsequent stages are connected, deterioration of the treated water quality at the time of switching can be prevented and stable treatment can be continued.

Example 1 Particle size 0.5-1.0mm used for about 1 year to remove phosphate from secondary treated sewage water with phosphorus concentration 1-2mg/
Two dephosphorization towers were installed in which 150ml of phosphorous ore was packed into an acrylic resin column with an inner diameter of 30mm and a length of 500mm.The phosphorus concentration was 2mg/, and the total alkalinity was approximately 100mg/.
Calcium chloride and sodium hydroxide aqueous solution were added to the water containing water at the entrance of the first tower to adjust the calcium ion concentration to 65 to 75 mg/, and the pH to 8.7 to 9.0, and then to the dephosphorization tower of the two towers. Each
Water is passed through the series in an upward flow with SV2hr ^-1 , and once in two
Crystallization was performed while washing the packed bed several times.

In the first step, when the phosphorus concentration of the treated water in the first tower reached 0.45 mg/, the order of water passage between the first and second towers was switched and the process moved to the second step. The two-week average value of phosphorus concentration in the final treated water in the second process is 0.3
mg/or less. In the second step, when the phosphorus concentration of the treated water in the first tower reaches 0.45 mg/, and the phosphorus concentration of the final treated water reaches 0.30 mg/, the water flow order between the first tower and the second tower is switched and the third step begins. The average phosphorus concentration in the final treated water for two weeks was 0.3.
mg/or less. Thereafter, when the phosphorus concentration in the treated water in the first column reached 0.45 mg/l, the water flow order was changed and the treatment was continued, and similar stable results were obtained.

Comparative Example In the first step of Example 1, when the phosphorus concentration of the treated water in the first column reached 0.45 mg/, water flow was continued in the same manner as in the second step of Example 1 without changing the water flow order. As a result, the average value of the final treated water over two weeks was 0.42mg/.

Example 2 After carrying out the first step in the same manner as in Example 1, the raw water was passed only to the second dephosphorization tower and the second step was started, and the water flow to the first tower was stopped and the chlorine agent was not used. After washing, drain the water and leave the lid open for one week to dry and activate the crystal seed surface to a moisture content of 20%.
After that, the treated water from the second tower is transferred to the activated first tower.
When water was passed through the tower, the phosphorus concentration in the final treated water was
The average phosphorus concentration for two weeks was approximately 0.16 mg/.

From the above results, activation is performed by switching the water flow order, and it is possible to remove phosphates over a long period of time.Also, by stopping the water flow when switching, the deterioration of the treated water quality can be prevented at the time of switching. , it can be seen that stable processing is possible.

[Brief explanation of drawings]

The drawing is a system diagram showing the switching of the water flow order in the present invention, and 1, 2, and 3 are the first, the first, and the third, respectively.
This is the second and n-th dephosphorization tower.

Claims (1)

[Claims] 1. A method for removing phosphate by bringing water containing phosphate into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 6 or higher, Raw water is passed in series through multiple dephosphorization towers filled with seeds to perform crystallization.
A method for treating water containing phosphates, characterized in that a first stage dephosphorization tower into which raw water is introduced is connected to a second stage or later stages in the next step and water is passed therethrough. 2. A method for treating water containing phosphates according to claim 1, wherein in the next step, the first stage dephosphorization tower is connected to the second stage and subsequent stages after water flow is stopped for a predetermined period of time. . 3. The method for treating water containing phosphates according to claim 2, wherein water is drained from the first stage dephosphorization tower while water flow is stopped. 4. The method for treating water containing phosphates according to claim 3, wherein the draining is performed to dry the water content of the surface of the crystal seeds to 20% or less. 5. The water containing phosphate according to any one of claims 1 to 4, wherein the crystal seeds containing calcium phosphate are one or more selected from hydroxyapatite, fluoroapatite, and tricalcium phosphate. processing method. 6. Claims 1 to 5, wherein the crystal species containing calcium phosphate are phosphate rock or bone charcoal.
A method for treating water containing phosphates according to any of paragraphs.