JPS5926190A - Phosphate-contg. water disposal - Google Patents

Abstract

PURPOSE:To perform the stable removal of phosphates over a long time, while recovering the power of dephsphorization, by dewatering calcium phosphate- contg. crystal seeds which was brought into contact with phosphate-contg. waste water for the removal of phosphates from it to activate them. CONSTITUTION:Phosphate-contg. water of a pH above 6 is circulated as an upward or downward stream through a stationary bed packed with crystal seeds having a particle size of 9-35 meshes and containing calcium phsphate such as hydroxyl appatite at a speed of SV 1-5hr<-1> to precipitate the crystal of calcium phosphate and to remove phsphates from the water. When the activity of said crystal seeds is deteriorated during the continuation of said treatment, the crystal seeds are activated by dewatering. Said dewatering is generally performed by dewatering the crystal seeds and evaporating a water part. In case of large- sized arrangements, the supply of water is suspended, and the stationary bed is dewatered, backwashed at need and then spontaneously dried for about a week.

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. 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 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 operating method is much simpler than the conventional agglomeration method, and the processing efficiency is also much improved.

However, this method has a problem in that the presence of macrounsium ions and organic substances in the raw water gradually inhibits the crystallization reaction, inactivates the surface of the crystal seeds, and lowers the dephosphorization performance.

This invention is intended to solve the above-mentioned conventional problems, and by interrupting the water flow to the crystal seed layer during the treatment and draining the crystal seeds, the crystal seeds are activated and removed. The object of the present invention is to provide a method for treating water containing phosphates, which can restore phosphorus performance and remove phosphates stably over a long period of time.

In this invention, water containing phosphate is adjusted to a pH of 6 or higher,
and a method for removing phosphate by bringing it into contact with crystal seeds containing calcium phosphate in the presence of calcium ions, which comprises activating the crystal seeds used for removing phosphate by draining them. This is a method for treating water containing salt.

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”+RoHpo;-+40H”as(OHXPO
4) 3+3 H2o ・= (11 As can be seen from equation (1), in order to increase the removal rate of phosphate, it is necessary to make the reaction proceed to the right, and at the same time maintain the activity of the crystal species high. There is a need to.

For crystal seeders containing calcium phosphate, calcium phosphates such as hydroxyapatite [Ca1 ('0H) (POJ3), fluoroapatite [, Ca5 (F) (PO4) 3] or tricalcium phosphate [C23 (PO4)2] are used. Crystal seeds containing these minerals can be used, and natural phosphate rock or bone char is mainly composed of calcium phosphate.
Suitable as a crystal seed. It is also possible to use crystal seeds in which calcium phosphate is precipitated on the surface of off-shore materials such as sand. 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 crystallization conditions are the same as those of the conventional method, and when the crystallization is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and at a pH of 6 or more, the calcium phosphate produced by the old method is 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 hydroxide 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, if the concentration is higher than the supersolubility, new crystals will precipitate where crystal seeds do not exist, forming fine precipitates.
Although clogging of the hearth occurs, in the metastable region lower than supersolubility, new crystals are precipitated on top of the crystal seeds, and no precipitate is formed as the crystals grow. Also, in systems with lower solubility, crystals do not precipitate.

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 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 that when the phosphate ion concentration is 50 to 100 cm/min or less, the calcium ion content is 10 to 100 cm/min, and the pH is about 6 to 12, but it varies depending on the respective conditions.

The method of contacting the water containing phosphate with the crystal seeds containing calcium phosphate may be a fixed bed method or a fluidized bed method. The smaller the size of the crystal seeds, the larger the surface area and the easier it is 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.

If the grain size is too large, the specific surface area per unit filling amount will be small, so particles of about 9 to 000 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, fill with crystal seeds with a particle size of 9 to 35 mesunyu.

Water is passed upwardly or downwardly at a flow rate of SV1 to 5 hr-' to precipitate calcium phosphate crystals.

If the crystal seed surface becomes contaminated or clogged during water flow, periodically perform backwashing using upward flow to expand and clean the crystal seed bed and peel off impurities attached to the surface. This is desirable. The water flow conditions during backwashing are as follows:
The speed is about 0 to 80 m/hr, and the backwashing time is about 5 to 60 minutes.

If the activity of the crystal seeds decreases while such treatment continues, the dephosphorization performance will not recover even if the above-mentioned backwashing is performed. Particularly in systems where magnesium ions and organic substances are present, the activity is significantly reduced and the dephosphorization performance is significantly reduced. In the present invention, by draining the crystal seeds containing calcium phosphate that have been subjected to phosphate removal in this way, they are activated and the phosphorus performance is restored.

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 2%. 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 it is a large scale, stop the water flow, or if you have a spare crystal seed tank, switch the water flow, drain the water, backwash if necessary, open the tank lid, and dry naturally. let 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.

Further, 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 completed in 2 to 3 hours. In any case, if there is 88 minutes in the crystal seed layer, it is desirable to perform backwashing after stopping the water flow and then drying. In addition, in the case of small to medium scale, the above method is possible, but in addition to this method, it is also possible to take out the crystal seeds in the tank, spread them out and air dry them.

Activation by draining water may be performed when the phosphor performance deteriorates due to continued crystallization, but it can be performed periodically or irregularly when the equipment is stopped, etc., to maintain the activation that is constantly immersed. It may be maintained. In these cases, if multiple tanks are used and activation is performed alternately, the treatment can be continued without stopping water flow. The crystal seeds to be activated may be those that have been subjected to processing in another device. The frequency of activation can be arbitrarily determined based on the transition of dephosphorization performance of crystal seeds, the target level of treated water quality, etc.

When draining water as described above, in the process of stopping the water flow, draining water, and evaporating, the calcium phosphate crystals precipitated on the surface of the crystal seeds grow, progressing to a crystalline state similar to a new crystal seed, and the surface of the crystal seeds grows. The attached water evaporates, the phosphorus concentration and calcium ion concentration in the attached water increase, the degree of supersaturation of calcium phosphate increases, and highly activated crystal nuclei precipitate on the surface of the crystal seeds, which activates the crystal seeds. It is speculated that it will be sold separately, but the details are unknown.

When the raw water contains magnesium ions, the precipitation reaction of calcium phosphate on the surface of the crystal seeds and the growth of the crystals are inhibited, and the form of the calcium phosphate on the surface of the crystal seeds changes to a crystal form other than hydroxyapatite. If it contains organic matter, the organic matter will adhere and the dephosphorization performance will decrease, but in either case, it will be activated by draining and the dephosphorization performance will be restored. The same applies when the performance deteriorates due to causes other than magnesium and organic substances.

Note that, if necessary, during or after backwashing, it may be brought into contact with a solution containing a chlorine agent or a calcium compound in order to further activate the crystal seeds.

In this way, the crystal seeds which are activated by draining and whose dephosphorization performance has been restored can be subjected to crystallization again, and after filling with water, water can be passed through the reactor according to the conventional operating method to perform crystallization.

Note that in the above description, other performance deterioration prevention means and activation means may be used in combination.

As described above, according to the present invention, the crystal seeds used for phosphate removal are drained and activated, thereby achieving an economical and effective method without using chemicals or the like. This has the effect of restoring and improving the dephosphorization performance of crystal seeds and allowing stable phosphate removal over a long period of time.

Next, practical examples and comparative examples of the present invention will be explained.

Comparative example 1 Macunium ion 40-60m? /, sulfate ion 1
50-251]+++r/no, 1 used for about 9 months to remove phosphates from secondary treated sewage water containing phosphorus 1-/no
150 rhe of phosphate rock crystal seeds of 6 to 32 sizes were prepared with an inner diameter of 3 CTn and a length of 50 mm (1711 (17) 7 mm).
Calcium chloride and sodium hydroxide were added to the raw water packed in a fat column, with a phosphorus concentration of 1 m2/i, total alkali 'J iL1' + 100 my/i, and a machinenium ion concentration of 50/i. by injecting an aqueous solution of.

Calcium ion concentration 80-85■/no, pH 8.8-
After adjusting to 90, 5V2hr-' was applied to the crystal seed packed bed.
Continuous water flow treatment was carried out at a flow rate of .

The phosphorus concentration of the treated water on the 66th day after the start of water flow is shown in the graph of the drawing. - Phosphorus concentration in treated water during this period is 06~
It was 0.5 rq/no.

Comparative Example 2 The same crystal seeds as in Comparative Example 1 were drained, left as they were at room temperature for 60 days, and subjected to water passage treatment under the same conditions as in Comparative Example 1. The processing results are also written on the club of the drawing.

The phosphorus concentration in the treated water was 0.2 mg on the 10th day after the start of water flow.
The phosphorus concentration gradually increased after that, or was lower than that of Comparative Example 1 until about 30 days.

The same crystal seeds as in Example Comparative Example 1.2 were filtered at room temperature with water drained.
The sample was left to stand for several days, and water was passed under the same conditions as in Comparative Example 1. The processing results are also shown in the graph of the drawing.

The phosphorus concentration in the treated water remains approximately 0.01% for about 15 days after the start of water flow.
5~o, 2 rrq/no, and showed a higher phosphorus removal rate than the comparative Shio 1.2. After aging, the IJ concentration gradually increased, but stable IJ removal performance was exhibited, and treated water with stable water quality and lower a degree than Comparative Example 1.2 could be obtained.

From the above results, it can be seen that by draining the crystal seeds, the phosphorus performance can be restored and stable treatment can be achieved over a long period of time.

[Brief explanation of drawings]

The drawing is a graph showing the results of Comparative Example 1.2 and Example. Agent Patent Attorney Sei Yanagihara

Claims (3)

[Claims] (1) A method of removing phosphate by bringing water containing phosphate into contact with crystal seeds containing calcium phosphate at a pH of 6 or higher and in the presence of calcium ions,
A method for treating water containing phosphates, characterized by draining and activating crystal seeds used for phosphate removal. (2) The method for treating water containing phosphates according to claim 1, wherein the draining is to dry the water content of the surface of the crystal seeds to 20% or less. (3) Calcium phosphate is hydroxyapatite. A method for treating water containing phosphate according to claim 1 or 2, which is fluoroapatite or trilime phosphate.