JPH0315514B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a dephosphorization method. 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 is present in domestic and industrial wastewater in the form of inorganic phosphates such as orthophosphates and condensed phosphates, and organic phosphates. There is. As methods for removing phosphates from water, the flocculation method using sulfuric acid, activated sludge method, or a combination of these methods have been used in experimental plants and experimental plants and have been recognized to be effective, but in recent years The crystallization dephosphorization method is attracting attention because it is easier to operate, does not generate sludge, and has high processing efficiency. The crystallization dephosphorization method involves contacting water containing phosphate with a crystal species containing calcium phosphate such as hydroxyapatite in the presence of calcium ions.
This is a method in which phosphate ions in water are converted into calcium phosphate and crystallized on the surface of crystal seeds. However, even with the crystallization dephosphorization method, the expected phosphorus removal effect may not always be obtained, and the inventors have conducted various studies over many years to determine the cause of this, and have found that aluminum compounds in the water to be treated are The inventors have discovered that dephosphorization is significantly inhibited, leading to the present invention. That is, in the present invention, aluminum compounds are removed in advance from treated water containing phosphates and aluminum compounds, and then the treated water is added to crystal seeds containing calcium phosphate at a pH of 6 or higher and in the presence of calcium ions. This is a dephosphorization method characterized by removing phosphates from the water to be treated by bringing the water into contact with the water. At human waste treatment plants and sewage treatment plants, when human waste and sewage are treated by biological treatment such as activated sludge treatment,
Before or after biological treatment, or by adding aluminum sulfate as a flocculant to the biological treatment tank, flocculation treatment may be performed to remove as much of the SS and COD components of wastewater as possible. In this case, all of the added aluminum salt cannot be precipitated as aluminum hydroxide unless the PH of the treated water is properly controlled, and some of it leaks into the primary treated water. It was discovered that such leaked aluminum compounds could not be removed by the biological treatment process, and that if this water was then subjected to normal crystallization treatment, the phosphorus removal rate would deteriorate. Furthermore, as seen in Example 1, which will be described later, we also found that the rate of fixation of phosphorus to the crystal seeds becomes poor, and when the crystal seeds are backwashed, the removed phosphorus crystals peel off from the crystal seeds. It was. If the material is peeled off, the advantage of not requiring sludge treatment in the crystallization process is lost, and the peeled material must be treated separately. It has also become clear that if aluminum compounds are present in the target water, it becomes difficult to lower the pH of the treated water after crystallization treatment, necessitating the use of a large amount of neutralizing agent. The crystallization treatment is carried out at a pH of 6 or higher, but the adverse effects of aluminum compounds become particularly noticeable at a pH of 9 or higher. Conventionally, there has been no knowledge that aluminum salts have these adverse effects in crystallization treatment, and this was revealed for the first time by the present inventors. Therefore, in the present invention, the aluminum compound is removed before the crystallization treatment to eliminate factors that adversely affect the crystallization treatment. Examples of methods for removing aluminum compounds include the following methods. (b) A method in which the pH is adjusted to 2.0 to 9.5, preferably 4.0 to 7.0, and the solubility of the aluminum compound is minimized to form a colloid, followed by filtration. (For filtration, gravity filtration or pressure filtration using sand or anthracite as material, or membrane separation using a porous membrane (e.g. membrane filter) can be used when the water to be treated is not a large amount.) Ion exchange Method of removing with resin. (Efficiency can be improved by adding an alkaline agent for crystallization to the water to be treated in advance to increase the pH and ionize the aluminum compound before contacting it with the resin. If a strongly acidic cation exchange resin is used, only aluminum can be ion-exchanged selectively. (Preferably.) In addition, various methods can be applied, such as a combination of methods A and B above. The water from which the aluminum compound has been removed in this way is passed through a packed bed consisting of crystal seeds and the like to undergo crystallization and dephosphorization treatment. 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 ²⁺ +7OH ^- +3H ₂ PO ₄ ^- →Ca ₅ (OH) (PO ₄ ) ₃
+6H ₂ O...(1) As can be seen from equation (1), in order to increase the removal rate of phosphate, it is necessary to advance the reaction to the right, and at the same time, in order to crystallize the produced calcium phosphate, It is necessary to keep the surface of the crystal seeds clean to maintain high activity. Therefore, in the present invention, water passing treatment is performed in the presence of calcium ions and under conditions of pH 6 or higher. In order for the reaction to proceed, it is desirable to have a large amount of calcium ions and hydroxide ions present in the reaction system, but if they are present in excess, fine precipitates such as calcium phosphate will occur in areas other than the crystal seeds, and the crystal seeds, etc. It may clog the packed bed consisting of water and reduce the water flow treatment efficiency. In equation (1), the calcium ion concentration and pH are such that the concentration of the produced calcium phosphate is higher than the solubility and lower than the supersolubility (the concentration at which crystals begin to precipitate when no crystal seeds exist in the reaction system).
In the region of , that is, the metastable region, the calcium phosphate produced precipitates on the surface of the crystal seeds and no fine precipitates are produced. However, calcium ions and/or
Alternatively, in an unstable region where the pH is high and the produced calcium phosphate exceeds supersolubility, calcium phosphate precipitates as fine precipitates. Therefore, the range of the metastable region varies depending on the concentration of phosphate ions contained in the raw water, and the lower the phosphate ion concentration, the wider it becomes. Therefore, when the phosphate ion concentration is high, increasing the calcium ion concentration and/or PH creates an unstable region and tends to generate precipitates, but when the phosphate ion concentration is low, the calcium ion concentration and/or pH increases. Even if the pH is increased, crystallization can occur in the metastable region, so the reaction rate can be increased without forming a precipitate. Calcium ions are made to exist in the raw water passed through the packed bed so that the reaction of formula (1) above proceeds. If calcium ions are absent or insufficient in the raw water, add a calcium agent from the outside. The pH of the reaction system is 6 or more, usually 6 to 12, and an alkali agent is added if necessary. In this way, the concentration of calcium ions present in the water to be treated and
Adjust the pH to be within the metastable range. The packed bed is filled with crystal seeds containing calcium phosphate such as hydroxyapatite, fluoroapatite, and tricharcoal phosphate, and these crystal seeds themselves may form the packed bed as granules, and It may also 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 neutralized by known means if necessary, and then discharged. According to the present invention, since the aluminum compound is removed before crystallization dephosphorization, problems such as a decrease in the phosphorus removal rate and a decrease in the fixation rate of phosphorus to crystal seeds do not occur.
Furthermore, the advantages of the crystallization dephosphorization method are fully utilized, and a high phosphorus removal effect can be reliably obtained. Example 1 In an acrylic column A with an inner diameter of 30 mm and a length of 500 mm,
Phosphate rock (particle size 0.5-1.0
mm) was filled with 130ml. Phosphorus concentration 1mg/, total alkalinity 100mg/, calcium ion concentration 45
mg/, sodium hydroxide solution was added to the synthetic water adjusted to have an aluminum ion concentration of 1 mg/, and the pH was ^9.5 .
Water was passed in an upward flow for 40 hours. The phosphorus concentration of the treated water that has been dephosphorized in this way and
PH was measured. In addition, after water flowing for 40 hours, the packed bed of phosphate rock crystal seeds was washed, and the phosphorus weight in the backwash waste water was determined. Based on the relationship with the phosphorus weight removed in the packed bed during water flowing, the following formula was calculated. The crystallization fixation rate of phosphorus was determined. Crystallization fixation rate [%] = (1-
Weight of phosphorus in backwash wastewater/weight of phosphorus removed in the packed bed during water flow) × 100 To analyze the phosphorus concentration of backwash wastewater, add hydrochloric acid to the backwash wastewater to bring the pH to 1 or less, and then The solution was filtered. Separately, as a column (B), an experiment was conducted under the same conditions as column (A) except that aluminum ions were not added. Both results are shown in Table-1.

[Table] When aluminum ions are included, the phosphorus concentration in the treated water is higher than when aluminum ions are not included, and the phosphorus crystallization fixation rate is also lower. The pH value also shows a high value. Example 2 An experiment was conducted under the same conditions as Example 1, except that the water flow time was 90 hours. The results are shown in Table 2, where the case corresponding to column (A) is designated as column (C), and the case corresponding to column (B) is designated as column (D).

[Table] Results similar to those in Example 1 were obtained. Example 3 An acrylic column (E) with an inner diameter of 30 mm and a length of 500 mm was filled with sand. Phosphorus concentration 1 mg/, total alkalinity
Synthetic water adjusted to 100mg/, calcium ion concentration 45mg/, and aluminum ion concentration 1mg/ to which an aqueous sodium hydroxide solution was added so as to have a pH of 9, was passed through the column (E) in an upward flow at SV2hr ^-1. I watered it. Furthermore, a sodium hydroxide aqueous solution was added to this treated water to make the pH 9.5, and this was poured into an acrylic column (F) with an inner diameter of 30 mm and a length of 500 mm, containing the same phosphate rock as in the column (A) of Example 1. In a layer filled with 130ml of
Water was passed in an upward flow for 90 hours at SV2hr ^-1 . The results are shown in Table-3.

[Table] Column (E) hardly removed phosphorus, but sufficiently removed aluminum ions.
Next, the treated water after removing aluminum ions is passed through the column.
It was confirmed that phosphorus could be effectively removed by passing water through (F).

Claims (1)

[Claims] 1. After removing aluminum compounds in advance from water to be treated containing phosphates and aluminum compounds, the water to be treated is treated with crystal seeds containing calcium phosphate at a pH of 6 or higher and in the presence of calcium ions. A dephosphorization method characterized by removing phosphates from water to be treated by bringing water into contact with the water. 2. The method according to claim 1, wherein the aluminum compound is removed using a cationic exchange resin. 3. The method according to claim 1, wherein the aluminum compound is removed by passing the aluminum compound under pH 2.0 to 9.5.