JPS59177191A - Treatment of phosphate-containing water - Google Patents

Abstract

PURPOSE:To remove phosphoric ion from waste water at a low cost, by stripping waste water containing phosphoric ion, and bringing it into contact with the crystal seeds of calcium phosphate in the presence of Ca. CONSTITUTION:Phosphate-contg. raw water 11, e.g. treated water of night soil, sewage or industrial waste water, is adjusted to a pH of 3-5 by the addition of an acid 13 such as sulfuric acid. Said raw water is introduced into a decarboxylation column 15 and stripped therein by releasing air from a lower side. CO2- contg. stripping gas is discharged through a line 17, while the decarboxylated raw water is withdrawn through a line 19. Said raw water is adjusted to a pH above 6 by the addition of an alkaline Ca agent 21 such as slaked lime. Thereafter, the raw water is sent into a crystallization dephosphorizer column 23 and brought into contact with crystal seeds containing calcium phosphate, e.g. hydroxyapatite, fluoroapatite or trilime phosphate, charged in the column to crystallizedly dephosphorize phosphoric ion as calcium phosphate on the surfaces of said crystal seeds. The dephosphorized raw water is introduced into a neutralizer column 27, neutralized therein with the stripping gas 17 and then drained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating water containing phosphate, and more particularly, to a method for treating water by crystallization dephosphorization.

In recent years, the problem of eutrophication in closed water bodies such as lakes, marshes, and inner bays has become serious, and countermeasures are urgently needed. One of the causes of eutrophication is phosphates present in water, and research is underway to remove phosphates. This phosphate is widely used in detergent builders, fertilizers, etc., and exists in the form of inorganic phosphates such as IJ phosphate condensed phosphates and organic phosphates in domestic wastewater, industrial wastewater, etc. are doing.

Methods for removing phosphates from water include the flocculation method using sulfuric acid bread, the activated sludge method, and a combination of these methods, which have been used in experimental plants and actual plants and have been shown to be effective. However, in recent years, crystallization dephosphorization has attracted attention because it is easier to operate, does not generate sludge, and has high processing efficiency.

In the crystallization dephosphorization method, water containing phosphate is brought into contact with crystal seeds containing hydroxyabatite calcium phosphate in the presence of calcium ions, and the phosphate ions in the water are converted into calcium phosphate onto the surface of the crystal seeds. It is a method of crystallization, and is thought to proceed primarily through the reaction of formula (1).

5Ca"-1-3HPOj-+40H--+ Ca,
(OH) (PO4)! + +3H20””” (1)
In this way, the presence of Ca ions is essential in the crystallization dephosphorization method, but on the other hand, when the alkalinity of raw water (water targeted for dephosphorization treatment) is high, Ca"+ HCO3- + OH- → CaC0,↓+H20 ...As shown in song (2), precipitation of carbonate scale occurs, so it is necessary to perform decarboxylation treatment as a pretreatment.

In addition, as can be estimated from the above equation (1), the higher the hydroxyl ion concentration, the higher the reaction rate, so it is desirable to treat at a high pU within the metastable region where phosphate aggregation does not occur. . However, if the treatment is carried out at a high pH suitable for the crystallization reaction, the PU of the treated water will also be high and it cannot be discharged as it is, and countermeasures such as post-neutralization treatment will be required, which is complicated and costly. It was a disadvantage.

As a result of intensive study to solve the above-mentioned problems inherent in the crystallization dephosphorization method, the present inventor decarboxylated the raw water and neutralized the treated water with the carbon dioxide gas obtained thereby, We found a way to solve the above problems all at once,
Based on this knowledge, we have completed the present invention.

That is, the method for treating water containing all phosphates of the present invention includes (a) a first step of acidifying and stripping water containing phosphates; (b) treating water in the first step; a second step of contacting with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of 1116 or higher; (e) this second step treated water is further discharged from the first step; The method is characterized by comprising a third step of bringing the method into contact with a stripping gas.

Figure ii is a flow sheet showing an embodiment of the present invention.O Raw water containing phosphate is introduced from line 11, and an acid agent 13 such as sulfuric acid is added to make it acidic, preferably pl! 3～
5 and sent to the decarboxylation tower 15. Decarboxylation tower 15
Then, the air from the lower part F: is degassed and stripped, and the stripping gas containing carbon dioxide is discharged from the line 17, while the raw water that has been decarboxylated is passed through the line 19.
is sent to the next process.

For stripping the carbon dioxide gas, any suitable method such as heating or reduced pressure may be employed, but as mentioned above, aeration is preferably used.

The raw water to be treated in the present invention is preferably one with high alkalinity, from the viewpoint of the necessity of decarboxylation treatment to prevent carbon dioxide scale and the effective use of recovered carbon dioxide as a neutralizing agent. - Effective for raw water with alkalinity of 200 m9/II or more.

An alkaline agent and a calcium agent 21 are added to the decarboxylated raw water, adjusted to 986 or higher, and supplied to the crystallization dephosphorization tower 23.

For the addition of an alkali agent and a Ca agent, it is preferable to use something that is both an alkaline agent and a Ca agent, such as slaked lime, but it is also possible to add a water-soluble calcium salt such as calcium chloride and an alkaline agent. .

As can be seen from the above equation (1), it is desirable to have large amounts of calcium ions and hydroxide ions present in the reaction system in order to advance the crystallization reaction, but if these are present in excess, calcium phosphate will be present in areas other than the crystal seeds. This may cause fine precipitates such as, etc., which may clog the packed bed and reduce the water flow treatment efficiency. In a region of calcium ion concentration and pH such that the concentration of calcium phosphate produced is higher than its solubility and lower than its supersolubility (the concentration at which crystals begin to precipitate in the absence of crystal seeds in the reaction system), that is, in the metastable region. In this case, the produced calcium phosphate precipitates on the surface of the crystal seed and fineness 1a is not produced, but in the unstable region where calcium ions and/or pH become high and the produced calcium phosphate exceeds supersolubility,
Calcium phosphate precipitates as a fine precipitate. The range of this metastable region varies depending on the concentration of phosphate ions contained in the raw water, and the lower the 6 degrees of phosphate ion, the wider it becomes.

Therefore, when the phosphate ion concentration is high, I) [1
If the pH is raised, it tends to become unstable and precipitates are likely to form, or if the phosphate ion is 6 degrees or lower, it is possible to crystallize in the metastable region even if the pH is increased, so precipitation is not possible. It is possible to speed up the crystallization reaction without producing it. In any case, it is preferable in terms of dephosphorization efficiency to perform the crystallization dephosphorization treatment at a high pi within the metastable region.

However, if the crystallization reaction is carried out at a high pH value, the pn of the crystallized and dephosphorized water will naturally increase and will no longer match the discharge water base, so countermeasures such as post-neutralization treatment will be required. . Conventionally, post-neutralization increases chemical costs and makes maintenance complicated, so conditions were set so that the pII of the treated water would fall within the discharge standard, even though the crystallization dephosphorization effect would decrease. Crystallization and dephosphorization treatment was carried out. On the other hand, in the present invention, since the stripping gas (exhaust gas) from the decarbonation tower 5 can be used as a neutralizing agent, the pH value is set to a correspondingly higher value, and the decarboxylated raw water and phosphorus salt are contained. The crystal seeds can be brought into contact with each other, and the dephosphorization efficiency can be increased.

The crystallization dephosphorization tower 23 is filled with crystal seeds containing calcium phosphate such as hydroxyapatite, fluoroanozotite, and tricalcium phosphate, and these crystal seeds themselves may form a packed bed as granules. Alternatively, it may be filled in a state in which it is trapped in a suitable support layer as a powder.

Furthermore, the method of contacting the raw water with the crystal seeds may be either a fixed bed method or a fluidized bed method.

The dephosphorized water is sent from the line 25 to the post-neutralization tower 27, where it comes into contact with the stripping gas supplied from the decarbonation tower 15 through the line 17, and is neutralized by the carbon dioxide contained in this stripping gas. The water is treated with water and drained through line 29. The amount of carbon dioxide gas generated in the decarboxylation process is not excessive compared to the acid content required to control the pH of the treated water, so there is no need for pH control in the post-neutralization tower, and the process is highly stable. .

General gas absorption equipment can be used as the post-neutralization equipment, and typical examples include packed towers (honeycomb, crushed stone, etc.).
Filled with raschig rings, terrarets, etc. ), plate towers or cascade devices, spray towers or scrubbers, bubble columns, bubble stirring devices, etc.

According to the present invention, decarboxylation treatment is performed as a pretreatment for crystallization dephosphorization treatment, and the waste residue containing carbon dioxide generated here is absorbed into the crystallization dephosphorization treated water as a neutralizing agent. Crystallization dephosphorization treatment can be performed at pH value,
Furthermore, the pH of the treated effluent water can be kept within the positive standard without using a separate neutralizing agent.

The treatment method of the present invention is suitable for raw water with high alkalinity, such as bed-processed water, highly hard sewage, and industrial wastewater.

Example: Phosphate 3-strength (as p) containing M-Alkalinity 30
0 Add sulfuric acid to the synthetic water of rry7i and pl! 3.
5, and decarboxylation was performed by aeration with air in a decarboxylation tower.

As a result, treated water with an alkalinity of about 40 mg/l was obtained.

Next, slaked lime and calcium chloride were added to this treated water to adjust the pH to 9.0 and the amount of calcium to be 362 ψ/l, and then 1 ounce of phosphate rock with a particle size of 16 to 32 mesh was added.
5V=2 (hr-') in a fixed bed packed with 50ml
Water was passed at a flow rate of . As a result, the phosphorus concentration (' 4 Tn
Treated water with a pH of 7/1 was obtained.

Next, when this treated water was brought into contact with the stripping gas discharged from the decarboxylation tower, the pH became 84.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing an embodiment of the present invention. 15...Decarboxylation tower 23...Crystallization dephosphorization tower 27
... Post-neutralization tower patent applicant Kurita Industries Co., Ltd.

Claims (1)

[Claims] 1. A first step of stripping water containing phosphate under acidic conditions, and stripping the first step treated water in the presence of calcium ions and under conditions of 9116 or higher. and a third step of contacting the second step treated water with a stripping gas discharged from the first step. How to treat water. 2. The processing method according to claim 1, wherein the stripping is performed by aeration. 3. The treatment method according to claim 1 or 2, wherein the crystal seed containing calcium phosphate is at least one selected from the group consisting of hydroxyapatite, fluoroapatite, and tricalcium phosphate.