JPS5931394B2 - How to treat water containing phosphates - Google Patents

Description

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

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.

Phosphates, which cause eutrophication, are contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.; It exists in the form of phosphate.

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 (D 1ssertationAbstra
cts international, Vol, 33
, N O, 12.

Part I, pages 5878-8, etc.).

The Kono method removes phosphate ions contained in water by converting them into calcium phosphate, such as hydroxyapatite, and crystallizing them into crystal seeds. , has been attracting particular attention in recent years because it has significantly improved processing efficiency.

However, in this method, when crystal seeds are packed using a fixed bed packing method and water containing phosphate is passed through the packed bed, suspended matter in the raw water becomes trapped in the packed bed over the course of operation time. The crystal seeds gradually enlarge due to the crystallization phenomenon of calcium phosphate, which induces mutual blocking phenomenon. The seed layer must be backwashed.

However, when the crystal seeds are backwashed with raw water, as is commonly done in the filtration field, the phosphate ion concentration in the treated water does not decrease for a certain period of time immediately after water flow operation resumes, and the treated water is discharged as is. It often happens that it is not possible to do so.

This invention was developed with the aim of solving these drawbacks, and by backwashing crystal seeds by using backwash water under specific conditions, as a result, immediately after restarting water flow operation, The purpose of this study is to propose a method for treating water containing phosphate, which yields treated water that contains almost no phosphate ions.

That is, the present invention involves bringing treated water containing phosphates to which a calcium agent and/or an alkaline agent has been added into contact with crystal seeds containing calcium phosphate in the presence of calcium ions, to achieve treatment that contains almost no phosphates. The method for obtaining water is characterized in that the crystal seeds are backwashed using backwash water having a specific supersaturation degree (ratio of supersaturation degree to phosphate ion concentration) or more of the water to be treated. This is a method for treating water containing phosphates.

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"+70H+3H2P04-Ca5(OH)G(
)4) 36H20...(1) As seen from equation (1), in order to increase the removal rate of phosphate, it is necessary to make the reaction proceed to the right. must add the required amount of calcium or alkaline agents.

The amount of calcium agent or alkaline agent to be added to the raw water should be in excess of the reaction equivalent expressed by equation (1), but if too large a amount is added, fine precipitates may form in places other than the crystal seeds. Furthermore, since impurities such as calcium carbonate may be generated, the temperature should be within a range where these are not generated.

That is, the amounts of calcium ions and hydroxide ions are
The conditions are such that hydroxyapatite has a concentration higher than the solubility (but lower than supersolubility) of the hydroxyapatite produced in formula (1), that is, a concentration in the metastable range.

Here, supersolubility is the concentration at which nodules begin to precipitate when no crystal seeds are present in the reaction system.

Even when operating within such a metastable region, the mutual blocking phenomenon of seeds as described above cannot be avoided. It is necessary to keep the level high.

In this invention, backwash water having a specific supersaturation degree higher than that of the water to be treated is used as the backwash water.

This point will be explained in further detail.

In general, the degree of supersaturation for average secondary sewage treatment water is as follows (
2) is approximately expressed by the equation.

K=logP+logCa+0.65pH...-(
2) (K is the degree of supersaturation, P and Ca are the phosphorus and calcium concentrations (7IIIi/l), respectively) Adding a calcium agent and/or an alkaline agent to raw water containing phosphates means that the water to be treated is When it comes into contact with a crystal seed containing calcium phosphate, its stability as a solution is destroyed and it begins to crystallize as hydroxyapatite on the surface of the crystal seed.

As is clear from equation (2), the degree of supersaturation is a function of calcium ions, phosphate ions, and pH in water, but when considering leakage of phosphate ions after backwashing,
Among these factors, it was found that the concentration of phosphorus and acid ions plays a particularly large role, and the ratio of the degree of supersaturation to the concentration of phosphate ions in water (K/P) is defined as the specific supersaturation degree.
By backwashing the crystal seeds with backwash water that has a specific supersaturation level higher than the specific supersaturation level of the water to be treated, we have succeeded in preventing phosphate ions from leaking into the treated water after backwashing. .

That is, the present invention calculates the ratio of K and P (K/P) determined by equation (2), and uses backwash water having a specific supersaturation degree greater than that value of the water to be treated.

When raw water backwashing, which has conventionally been commonly used in the filtration field, is applied to the crystallization method of the present invention, the raw water containing phosphate comes into contact with the crystal seeds, and at that time, the phosphate in the raw water becomes supersaturated. Because of its low carbon content, it does not crystallize and is simply adsorbed to the crystal seeds mainly physicochemically.

Under such conditions, calcium supplements and (
Or) When the crystal seeds are brought into contact with the water to be treated to which an alkaline agent has been added, the phosphate ions in the water to be treated are mainly crystallized on the surface of the crystal seeds from the inlet of the crystal seeds toward the center of the layer. Although it is removed, the phosphate ion concentration in the treated water gradually decreases (then, due to chemical equilibrium, due to the desorption phenomenon of the phosphate ions adsorbed in the previous backwashing process, conversely, the concentration of phosphate ions in the treated water decreases). This results in significant amounts of phosphate ions leaking into the treated water discharged from the seed bed outlet.

On the other hand, when water having a specific supersaturation higher than the specific supersaturation of the water to be treated is used as backwash water when backwashing crystal seeds as in the present invention, Physical and chemical adsorption of phosphate ions only results in the formation of fine calcium phosphate crystals in water or crystallization on the surface of crystal seeds.

Such fine crystals are easily discharged from the bed during backwashing.

In addition, even after the hydroxyapatite formed on the surface of the crystal seeds due to crystallization comes into contact with the treated water, which has extremely low phosphate ions, it is difficult to re-elute compared to the physicochemically adsorbed phosphate ions. The possibility of this is extremely small.

The backwash water having a specific supersaturation level higher than the specific supersaturation level of the water to be treated used in this invention includes: - Treated water, tap water, or industrial water; - Treated water, tap water, industrial water, or raw water with a calcium agent. and/or those to which an alkaline agent is added.

In the latter case, the amount of alkaline agent and/or calcium agent to be added needs to be set to at least the specific supersaturation degree of the water to be treated under crystallization operating conditions, as can be seen above. However, there is no particular upper limit, and if a large amount is added, fine crystals of calcium phosphate will be produced in the water.

In particular, when added to treated water, it is preferable to add the calcium agent and/or alkaline agent so that the specific supersaturation degree is 10 times or more higher than the specific supersaturation degree of the water to be treated, as this further improves the quality of the treated water.

On the other hand, when using raw water as backwash water, it is possible to use the raw water itself with calcium and/or alkaline agents added to it, which simplifies operation and piping, and eliminates the need for a backwash tank. At the same time, the processing capacity also increases, which is preferable.

Crystal seeds containing calcium phosphate include hydroxyapatite [Ca5(oH)(Po4)3] and fluoroapatite [Ca5(F)(PO4)3) Mataha! , J.
Crystal seeds containing calcium phosphate such as tricalcium phosphate [Ca3(Po4)2] can be used, and natural phosphate rock has 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 a filter medium 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, efficiently removes phosphate, and increases the removal rate.

The grain size of the crystal seeds is usually 9 to 35 mesh.

Calcium agents used in this invention include calcium hydroxide and calcium chloride, and alkaline agents include sodium hydroxide, potassium hydroxide, calcium hydroxide, and the like.

Water flow conditions during backwashing include a flow rate of 20 to 80 m/hr.
The backwashing time is about 5 to 60 minutes.

In addition, as mentioned earlier, the normal water flow operation conditions are as follows.
It is assumed to be within the metastable region.

Calcium and pH values that bring the amount of hydroxyapatite within the metastable range were determined experimentally by changing these values for each reaction system.

However, the approximate range is that if the phosphate ion is less than 50%, the calcium ion is less than 10%.
100■/lz pH is about 6-12.

Moreover, the water flow rate (SV) is about 1 to 5 (t/hr).

As described above, according to the present invention, crystal seeds can be efficiently backwashed without worrying about leakage of phosphate ions into the treated water after backwashing.

Example pH 7.40, phosphate ion concentration 3.0-3.4 mfI
/l (as P), calcium agent O726.0711q
/l (1:) A calcium chloride aqueous solution was added to the secondary sewage treatment water so that the concentration of calcium ions was approximately 407 IIfI/l, and the pH was further adjusted to 9 with a sodium hydroxide aqueous solution, and phosphate rock with a particle size of 16 to 32 mesh was added. A column with an inner diameter of 30 m filled with
Water was passed at a flow rate of .

Once a day, the following backwash water was used for 10 minutes to backwash in an upward flow at a flow rate of 50 m/hr, and then after the backwash water was withdrawn, the water flow operation was performed again. It is shown in Table-1.

Case 1 Treated water〃 2 Treated water with a predetermined amount of slaked lime (metastable range) 〃・3 Raw water with a predetermined amount of slaked lime added (unstable range) Also, for comparison, the following backwash water Table 1 also applies when using
Also listed in

From these results, in Case 4 with low specific supersaturation, there was a phenomenon in which a relatively high concentration of phosphate ions leaked into the treated water for a long period of time after the end of backwashing. In cases 1 to 3, which are examples, 1,
No such phenomenon was observed, and the phosphate ion concentration (as P) in the treated water was 0.5 during the 200-day continuous water flow period.
~/ or less, and a stable treatment effect could be maintained.

Claims (1)

[Scope of Claims] 1. Water to be treated containing phosphates to which a calcium agent and/or an alkaline agent has been added is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions so that the water contains almost no phosphates. In the method for obtaining treated water with no water, the crystal seeds are backwashed using backwash water having a specific supersaturation degree (ratio of supersaturation degree to phosphate ion concentration) or higher than the specific supersaturation degree (ratio of supersaturation degree to phosphate ion concentration) of the water to be treated. A method for treating water containing phosphate, characterized by: 2. The method according to claim 1, wherein the backwash water is treated water. 3. The method according to claim 1, wherein the backwash water is raw water or treated water to which a calcium agent and/or an alkaline agent has been added.