JP2002273456A - Dephosphorizing method and device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dephosphorizing method and a device therefore by which the generating quantity of fine crystals is small even in an unstable region and crystallization can be executed in a short time in a high removing ratio by using a small quantity of crystal seeds and at a high crystallization speed. SOLUTION: In the dephosphorizing method in which a phosphor-containing water is brought into contact with a fluidized bed of the crystal seeds in the presence of calcium ions, and phosphor is removed by crystallization, the fluidized bed of the crystal seeds is formed by making water to be treated pass in a crystallization tank upward, an alkali and/or a calcium compound are added to the fluidized bed to cause reaction and a reaction liquid in the unstable region is generated, the reaction liquid in the unstable region is brought into contact with the crystal seeds to crystallize crystal of calcium phosphate on its surface to obtain treated water having 9.5 to 12 pH, <=1 mg/l calcium ion concentration and <=1 mg/l total phosphor concentration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for removing phosphorus by crystallization by bringing phosphorus-containing water into contact with crystal seeds in the presence of calcium ions.

[0002]

2. Description of the Related Art As a method of dephosphorization by crystallization, water to be treated is supplied to a lower part of a crystallization tank filled with crystal seeds such as calcium phosphate, and treated water is taken out from an upper part. Of the crystal seed by forming a fluidized bed of crystal seeds by circulating the water in the lower part of the tube and passing water upward. This method is a method of adding an alkali and / or a calcium compound to water to be treated, thereby precipitating phosphorus in the water to be treated as calcium phosphate such as hydroxyapatite on crystal seeds and removing phosphorus.

In such a method, when a reaction solution obtained by adding an alkali and / or calcium compound to the water to be treated and brought into contact with a crystal seed, the pH of the reaction solution rapidly drops due to precipitation of crystals, In order to prevent the reaction from stopping at the upper part of the fluidized bed,
A method has been proposed in which crystallization is carried out by injecting an alkali into a fluidized bed so as to maintain the temperature (see JP-A-60-168587).
issue). In this method, attention is paid only to pH, and an alkaline agent is added in a fluidized bed, while a calcium compound is added to circulating water.

However, in such a conventional method, crystallization can be performed by injecting an alkali so as to maintain the pH of the treated water at 8.5 to 11.
If it is 5 or more, although the phosphorus removal rate is high, there is a disadvantage that a large amount of SS is generated.
It is said that by adjusting the range of H9 to 9.5, the generation of SS can be reduced and the phosphorus removal rate can be increased. However, in such a pH range, the crystallization rate is lower than that in the case of pH 9.5 to 12, and it is necessary to lengthen the crystallization time in order to remove phosphorus at a high removal rate.

In general, the crystallization of phosphorus-containing water has a stable range,
It is classified into a metastable region and an unstable region. The stable region is a region where crystals do not precipitate, the metastable region is a region where crystals are precipitated on the surface of a crystal existing in the system, and the unstable region is a region where fine crystals are formed even in a portion where no crystal exists in the system. When the phosphorus concentration, calcium concentration and / or pH in the phosphorus-containing water are high, the region becomes unstable, and when these are low, the region becomes stable, and when it is intermediate, the region becomes metastable. In an unstable region, a large amount of fine crystals are generated as SS and flow out into treated water, which is not preferable. Therefore, crystallization is performed in a metastable region.

In JP-A-60-168587, crystallization is carried out in the metastable region of pH 9 to 9.5 where the amount of generated SS is small.
It is assumed that S occurs. However, when crystallization is performed in such a metastable region, the amount of fine crystals generated as SS is small, but the crystallization speed is low as described above, and the crystallization time is required to perform dephosphorization at a high removal rate. It needs to be longer. In addition, there is a problem that it is necessary to decarbonate the water to be treated in order to prevent the precipitation of calcium carbonate.

[0007]

An object of the present invention is to produce a small amount of fine crystals even in an unstable region, to increase the crystallization speed with a small amount of crystal seeds, and to carry out crystallization with a high removal rate in a short time. To provide a method and apparatus for dephosphorization.

[0008]

The present invention is the following dephosphorization method and apparatus. (1) A method for removing phosphorus by crystallization by bringing phosphorus-containing water into contact with a fluidized bed of crystal seeds in the presence of calcium ions, wherein the water to be treated is passed upward in a crystallization tank. To form a fluidized bed of crystal seeds,
9.5-12, calcium ion concentration 20-100mg
/ L, an alkali and / or calcium compound is added to the fluidized bed and reacted so that the total phosphorus concentration becomes 1 mg / l or less, to form a reaction solution in an unstable region, and the reaction solution in the unstable region is used as a crystal seed. A dephosphorization method in which calcium phosphate crystals are precipitated on the surface by contact. (2) The total phosphorus concentration of the phosphorus-containing water supplied to the crystallization tank is 1
The method according to the above (1), wherein the amount is 0 mg / l or less. (3) An apparatus for removing phosphorus by crystallization by bringing phosphorus-containing water into contact with a fluidized bed of crystal seeds in the presence of calcium ions. A crystallization tank forming a fluidized bed, a treated water supply path for supplying treated water to a lower part of the crystallization tank, a treated water extraction path for taking out treated water from an upper part of the crystallization tank, and an alkali and And / or adding a calcium compound to cause a reaction to generate a reaction solution in an unstable region;
~ 12, calcium ion concentration 20 ~ 100mg / l,
A dephosphorization device comprising: a control device for controlling an injection amount of an alkali and / or calcium compound so that a total phosphorus concentration is 1 mg / l or less.

The phosphorus-containing water to be treated in the present invention is water containing phosphorus which can be removed by crystallization, and water containing phosphorus in the form of orthophosphoric acid is generally suitable. Polyphosphoric acid or other water containing phosphorus may be used as long as it reacts with alkali and / or calcium to precipitate calcium phosphate-based crystals during the course of the treatment. The phosphorus content is not particularly limited, but when mixed with treated water and passed through, the dissolved phosphorus concentration of the mixed solution is 10%.
It is preferable that the phosphorus content is such that the phosphorus concentration is not more than mg / l, preferably not more than 5 mg / l, and more preferably not more than 3 mg / l. Such phosphorus-containing water includes sewage, night soil, industrial wastewater, and other wastewater.
The next treated water is mentioned.

In the present invention, crystallization is performed by bringing such phosphorus-containing water into contact with a fluidized bed of crystal seeds. In this case, it is possible to supply only the water to be treated to the crystallization tank and to form a fluidized bed by providing a stirrer or a diffuser tube in the crystallization tank, but the phosphorus-containing water supplied to the crystallization tank can be formed. Since the phosphorus concentration can be adjusted, it is preferable to form the mixture by flowing a mixed liquid of the water to be treated and the treated water upward in the crystallization tank. The crystallization tank may be the same as the conventional one, and generally a cylindrical one such as a cylindrical one may be used as a vertical one. The crystallization tank can be connected to the treated water supply path at the bottom and the treated water take-out path at the top, and the treated water can be circulated upward to form a fluidized bed. Then, it is preferable that a part of the treated water is circulated, mixed with the water to be treated, and the mixed liquid flows upward to form a fluidized bed.

The crystal seeds may be any as long as they can precipitate the crystals that precipitate during crystallization, but it is preferable to use crystal seeds composed of the same type of crystals so that the crystals can be easily precipitated. The crystals that precipitate during crystallization are the composition of the water to be treated and the form of phosphorus, the type of alkali and / or calcium compound to be added,
Although it varies depending on the amount and the like, it is a calcium phosphate crystal and includes hydroxyapatite, fluorapatite, tricalcium phosphate and the like. For this reason, the crystal seeds forming these fluidized beds also include those containing these crystals. Generally, it is preferable to use phosphorus ore, bone charcoal and the like, but crystal seeds increased in other treatment plants may be used. The average grain size of the crystals is 0.1 to 1 mm, preferably 0.15 to 0.5 mm.
A 5 mm one is used. The filling amount of the crystal seed may be any amount necessary for the treatment, and can be arbitrarily determined according to the target value.
To 5 m, preferably 1 to 2 m.

In the present invention, in order to form a fluidized bed in the crystallization tank, it is preferable that the water to be treated is mixed with a part of the treated water and the mixed liquid is passed upward. Is 5: 1 to 1:20 by volume ratio, preferably 2: 1 to 1
1: 4. The water to be treated and the treated water are preferably mixed and then introduced into the crystallization tank, but may be introduced into the crystallization tank and mixed. When the water to be treated and the treated water are mixed and then introduced into a crystallization tank, the mixed liquid is preferably mixed so as to be in a stable region or a metastable region, and the mixed solution has a pH of 8 to 9.5. , Preferably 8.5-9.2, calcium concentration 20-80 mg / l, preferably 30-40.
It is preferably mg / l.

The upward flow velocity of the mixture is LV 5 to 20 m / h
r, preferably 10 to 15 m / hr, for filling amount at rest
1 to 30 hours as SV^-1, Preferably 6 to 20
hr ^-1It can be. Fluidized bed deployment rate is 20-2
00%, preferably 30-100%
You. After addition of alkali and / or calcium compounds
The contact time between the reaction solution and the crystal seed is 2 to 60 minutes, preferably
It can be about 3 to 10 minutes.

In the present invention, an alkali and / or a calcium compound is added to the fluidized bed from the chemical injection section and allowed to react while the water to be treated is flowing upward under such conditions.
A reaction solution in an unstable region is generated, and calcium phosphate crystals are precipitated on the surface of the crystal seed. Examples of the alkali include an alkali hydroxide such as sodium hydroxide and calcium hydroxide, and examples of the calcium compound include a calcium salt such as calcium chloride.If calcium hydroxide is used, the alkali and the calcium compound may be used together. It is preferable. The alkali and / or calcium compound is added in such an amount that the reaction solution can be brought into an unstable range, and further added in such an amount that crystallization is completed in the whole process of raising the fluidized bed.

The alkali and / or calcium compound may be injected all at once during the upward flow of the fluidized bed, or may be injected in portions, but in any case, the reaction liquid is in an unstable region. Added. In this case, the pH and / or calcium concentration and the phosphorus concentration of the treated water are measured at the upper part of the crystallization tank, and the treated water has a pH of 9.5 to 12, preferably 9.7 to 11, and a calcium ion concentration of 20 to 100 m.
g / l, preferably 20-80 mg / l, and alkali and / or calcium compounds are added so that the total phosphorus concentration is 1 mg / l or less, preferably 0.6 mg / l or less.

Generally, in the unstable region, fine crystals precipitate as a precipitate in a portion where no crystal seed is present, and a large amount of SS is generated and flows out into treated water. For this reason, crystallization in the unstable region has been conventionally avoided. On the other hand, in the present invention, in the process of passing the liquid to be treated upward in the fluidized bed,
9.5-12, calcium ion concentration 20-100mg
/ L, the alkali and / or calcium compound is added to the fluidized bed so that the total phosphorus concentration becomes 1 mg / l or less, and the reaction is performed. Do. In this case, in the fluidized bed, the crystal seed and the reaction solution repeatedly contact with each other at a high frequency, so that the supersaturated portion always comes into contact with the crystal seed, so that the crystal is deposited on the crystal seed surface, and the crystal seed is deposited. The number of fine crystals generated by precipitation in a portion having no particle is reduced.

In the present invention, it is preferable that the liquid to be treated immediately before the unstable region is repeatedly brought into contact with the crystal seed in the fluidized bed in a stable region or a metastable region. By adding a calcium compound to the unstable region at once to precipitate crystals, it is possible to prevent the precipitation of fine crystals in a portion having no crystal seed. The concentration of phosphorus in the liquid to be treated in the stable region or the metastable region is preferably as low as possible, particularly preferably in the above-mentioned concentration range. In addition, the generation of fine crystals can be reduced. For this reason, in the present invention, the water to be treated is mixed with the treated water so as to have a low phosphorus concentration before coming into contact with the alkali and / or calcium compound. The water to be treated and the treated water before mixing preferably do not contain an alkali and / or calcium compound, but may be added with an alkali and / or calcium compound as long as a stable or metastable region can be maintained before and after mixing. Good.

The unstable region means that the degree of supersaturation is high, so that the rate of crystal precipitation is higher than that of the metastable region, so that the crystallization speed is high. ,
Phosphoric acid removal rate can be increased. In this case, the processing efficiency can be increased by increasing the ratio of the liquid to be processed to the circulating processing liquid. Further, since dephosphorization can be performed by short-time contact, the filling height of the crystal seed to be used can be reduced.

[0019]

As described above, according to the present invention, alkali and / or calcium are adjusted so that the pH of the treated water, the calcium ion concentration and the total phosphorus concentration become predetermined values while passing the liquid to be treated upward. Since crystallization is performed by adding a compound, the amount of fine crystals generated is small even in the unstable region, the crystallization speed is increased with a small amount of crystal seeds, and the crystallization is performed with high removal rate in a short time Can be.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a flowchart of the dephosphorization apparatus of the embodiment. In FIG. 1, reference numeral 1 denotes a crystallization tank, in which a fluidized bed 2 is formed by filling a crystal seed made of phosphate ore. A treated water supply path 3 communicates with the lower part of the crystallization tank 1, a treated water extraction path 4 communicates with the upper part, and a circulation path 5 communicates with the treated water supply path 3 from the upper part. A chemical injection passage 6 communicates with a portion slightly raised from the lower part of the fluidized bed 2, and a treated water extraction passage 4 and a dilution water passage 7 communicate with the passage.

In the dephosphorization method using the above apparatus, the water to be treated is supplied to the crystallization tank 1 from the water supply passage 3 by the pump P1, and a part of the treated water is supplied from the circulation passage 5 to be mixed on the way. Then, the mixed solution is allowed to flow in an upward flow in a stable region or a metastable region to develop crystal seeds, thereby forming a fluidized bed 2. Then, an alkali and / or calcium compound is supplied from the chemical injection path 6 by the pump P3, and a part of the treated water is mixed and diluted by the pump P4 from the dilution water path 7 on the way.
And reacted with phosphorus in the mixed solution to make the reaction solution an unstable region and perform crystallization. The degree of supersaturation increases due to the unstable region, and calcium phosphate-based crystals precipitate. However, in the fluidized bed 2, the contact between the crystal seed and the reaction solution is frequently repeated, so that the crystal precipitates on the crystal seed. Crystallization is performed with high efficiency. The treated water from which phosphorus has been removed by crystallization is taken out from the treated water take-out passage 4. In this case, the pH of the processing solution is detected by the pH meter pH, and the control device 8 controls the injection amount of the pump P3 so as to keep the pH value at a predetermined value, thereby performing stable processing. When calcium hydroxide is used as the calcium compound, the concentration of calcium ion in the treated water can be controlled at the same time by controlling the injection amount of calcium hydroxide so as to be in a predetermined pH range.

[0022]

Embodiments of the present invention will be described below. Example 1 As the water to be treated (raw water), the total phosphorus concentration was 2 to 3.3 mg / l.
The sewage secondary treatment water was crystallized by the apparatus shown in FIG. 1 to remove phosphorus. Average grain size 0.15 to 0.3 mm as crystal seed
, The mixing ratio (volume ratio) of the water to be treated and the circulating water is 2: 1, the upward flow velocity LV is 10 m / hr, and the expansion rate is 50.
%, SV 15 hr ^-1 to form a fluidized bed, and calcium hydroxide as an alkali and / or calcium compound was injected so that the treated water had a pH of 10. At this time, the pH of the mixed solution was about 9 when the calcium ion concentration of the treated water was around 50 mg / l.

Comparative Example 1 Calcium hydroxide and calcium chloride were added to the water to be treated (raw water) used in Example 1 to adjust the pH to 9.0, and the reaction solution was adjusted to a phosphorus having an average particle size of 0.5 to 1.0 mm. Crystallization was performed by passing water through the fixed bed filled with ore at SV1 hr ^-1 .

FIG. 2 shows the total phosphorus concentration (TP [mg / l]) of the treated water and the raw water of Example 1 and Comparative Example 1.
From FIG. 2, in the treatment using the fixed bed of the comparative example, the phosphorus concentration of the treated water is increased, whereas in the treatment using the fluidized bed of the first embodiment, the treated water is treated without performing pretreatment such as decarboxylation. It can be seen that stable treatment with a total phosphorus concentration of 0.1 mg / l or less has been performed over a long period of time.

Example 2 Using water to be treated having a phosphorus concentration of 3 mg / l, crystallization was performed at an SV of 10 hr ^-1 using the apparatus shown in FIG. Slaked lime was added as an alkali and / or calcium compound so that the calcium concentration in the treated water became 15, 25, 40, or 60 mg / l, and the pH was adjusted with hydrochloric acid or sodium hydroxide. Soluble phosphorus concentration of treated water _(PO 4 -P) shown in FIG. FIG. 3 shows that good results can be obtained at pH 9.5 to 12, particularly pH 9.7 to 11.

Example 3 Using a liquid to be treated having a soluble phosphorus concentration of 15 mg / l, and changing the mixing ratio with circulating water in the apparatus shown in FIG.
Changing the solubility of phosphorus concentration of the mixed liquid _(PO 4 _{-P in),} L
Water was passed at V10 m / hr and SV10 hr ^-1 , and hydrated lime was added to adjust the pH to 10 to carry out crystallization. Microcrystalline phosphorus concentration of treated water (Particulate P), the solubility of phosphorus concentration _{(PO 4 -P out)} and the total phosphorus concentration (T-P) FIG 4 (a), (b) , shown in (c). FIG.
It can be seen from the graph that the treated water can be treated to a phosphorus concentration of 1 mg / l or less at a phosphorus concentration of 10 mg / l or less, and that an excellent phosphorus removal effect can be obtained particularly at a phosphorus concentration of 3 mg / l or less.

[Brief description of the drawings]

FIG. 1 is a flowchart of a dephosphorization apparatus of an embodiment.

FIG. 2 is a graph showing the results of Example 1 and Comparative Example 1.

FIG. 3 is a graph showing the results of Example 2.

FIGS. 4A, 4B and 4C are graphs showing the results of Example 3. FIGS.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 crystallization tank 2 fluidized bed 3 treated water supply path 4 treated water extraction path 5 circulation path 6 chemical injection path 7 dilution water path 8 controller

Claims (3)

[Claims] 1. A method for removing phosphorus by crystallization by bringing phosphorus-containing water into contact with a fluidized bed of crystal seeds in the presence of calcium ions, wherein the water to be treated is upwardly flowing in a crystallization tank. A fluidized bed of crystal seeds is formed by passing water, and the treated water has a pH of 9.5 to 12, and a calcium ion concentration of 20.
An alkali and / or calcium compound is added to the fluidized bed so that the total phosphorus concentration is not more than 100 mg / l and the total phosphorus concentration is 1 mg / l or less, and a reaction solution in an unstable region is generated. A dephosphorization method in which calcium phosphate crystals are precipitated on the surface by contact with a seed. 2. The method according to claim 1, wherein the total phosphorus concentration of the phosphorus-containing water supplied to the crystallization tank is 10 mg / l or less. 3. An apparatus for removing phosphorus by crystallization by bringing phosphorous-containing water into contact with a fluidized bed of crystal seeds in the presence of calcium ions, wherein the water to be treated is passed in an upward flow to crystallize. A crystallization tank that forms a fluidized bed of seeds, a treated water supply path that supplies treated water to the lower part of the crystallization tank, a treated water extraction path that extracts treated water from the upper part of the crystallization tank, and a fluidized bed A chemical injection route for adding and reacting an alkali and / or calcium compound to form a reaction solution in an unstable region;
A dephosphorization device comprising a control device for controlling the injection amount of an alkali and / or calcium compound so that the total phosphorus concentration is １００100 mg / l and the total phosphorus concentration is 1 mg / l or less.