JP4438402B2 - Dephosphorization method - Google Patents

Description

  The present invention relates to a dephosphorization method, and in particular, dephosphorization that efficiently removes phosphorus in phosphorus-containing wastewater such as sewage and domestic wastewater treated water such as agricultural settlement wastewater as calcium phosphate by fluidized bed crystallization dephosphorization reaction. Regarding the method.

Biological treatment methods, coagulation-precipitation methods, adsorption methods, crystallization methods, etc. are known as methods for removing phosphorus from wastewater-containing wastewater such as sewage and domestic wastewater treatment water, but the final product Dephosphorization method by crystallization, which has a high recyclability, and has the smallest volume of phosphorus-containing material discharged out of the system and less liquid content (adhered liquid content) compared to biological treatment methods and coagulation sedimentation methods Is attracting attention. The crystallization method is a method of removing phosphorus in the raw water as, for example, calcium phosphate crystals by the following reaction by adding a calcium compound to the raw water and passing it through a fixed bed or a fluidized bed of seed crystals. 2-333435).
5Ca ²⁺ + 3HPO ₄ ²⁻ + 4OH ⁻ → Ca ₅ (OH) (PO ₄ ) ₃ + 3H ₂ O

  As seed crystals used in such a crystallization method, those mainly composed of bone charcoal and phosphorus ore have been used, but since bone charcoal and phosphorus ore vary greatly in particle size, When used, it is necessary to prepare the particle diameters in advance and to clean the surface, which requires a large number of manufacturing steps and increases the manufacturing cost. In addition, phosphate ore cannot obtain a stable dephosphorization effect due to differences in chemical composition and physical properties depending on the place of origin, and is hardly produced in Japan and may be depleted in the future. On the other hand, bone charcoal itself has a high cost, and there is a problem that the production cost of seed crystals becomes high.

As a seed crystal having no such problem, having a uniform particle size, readily available at low cost, and having stable dephosphorization performance, the present applicant previously obtained a seed crystal obtained by precipitating calcium phosphate on the surface of calcium carbonate. Proposed (Japanese Patent Laid-Open No. 2003-181469). If this seed crystal,
-Since calcium phosphate is precipitated on the surface of calcium carbonate having a certain particle size at the distribution stage, the particle size can be made uniform.
Calcium carbonate is inexpensive in itself and can be easily obtained in the future in the future, so that the cost of seed crystals can be reduced (for example, the cost can be reduced to 1/10 or less of conventional seed crystals such as phosphate ore). Can be reduced).
-A stable dephosphorization effect can be obtained without causing variations in chemical composition and physical properties.
-The seed crystals used for dephosphorization can be used effectively as phosphorus fertilizers.
It is effective for improving the dephosphorization efficiency and reducing the processing cost.
Japanese Patent Publication No. 2-3334 JP 2003-181469 A

As described above, the crystallization dephosphorization method includes a fixed bed type crystallization method using a fixed bed of seed crystals and a fluidized bed type crystallization method using a fluidized bed of seed crystals. In the method of crystal crystallization, the treatment capacity is low, and it can only be treated with SV2h ^-1 or less as a water flow SV. Moreover, since the fixed bed is clogged with floating substances, it is necessary to periodically backwash the fixed bed. There is a problem of wastewater treatment. In contrast, the fluidized bed crystallization method does not have such a problem, but removes phosphorus so that the processing speed can be further increased in order to reduce the size of the reaction tank and that it can fully meet the discharge regulation. It is desired to further improve the rate.

  Accordingly, the present invention provides a dephosphorization method for removing phosphorus in phosphorus-containing wastewater by a fluidized bed crystallization method, and capable of removing phosphorus to a low concentration at a high treatment rate. The purpose is to do.

In the dephosphorization method of the present invention, raw water containing phosphoric acid is passed in an upward flow through a reaction tank filled with seed crystals formed by precipitating calcium phosphate on the surface of calcium carbonate, and the seed is introduced into the reaction tank. In the dephosphorization method of forming a fluidized bed of crystals and removing phosphorus in raw water as calcium phosphate crystals in the reaction tank by adding a calcium compound and / or an alkali agent, a cylinder having both ends opened in the reaction tank, The cylinder is provided so that the axial direction of the cylinder is the vertical direction, and at least the lower part of the cylinder is positioned in the fluidized bed, and downflow forming means is provided for forming a downward flow in the cylinder. The crystal grain size is in the range of 0.2 to 0.5 mm, the raw water flow LV in the reaction tank is 20 to 25 m / h, the raw water flow SV in the reaction tank is 5 to 10 h ⁻¹ , The phosphorus concentration (TP) of the inflow water into the reaction tank was 6 mg / By setting it to L or less, treated water having a phosphorus concentration (TP) of 1 mg / L or less is obtained.

According to the present invention, since raw water can be processed at a high speed of LV 20 to 25 m / h, the reaction vessel can be remarkably reduced in size compared to the conventional method, and the reaction vessel can be used with respect to the amount of treated water. The number can be reduced. For example, since the raw water LV of the conventional fluidized bed crystallization method is generally about 15 m / h, one reaction tank is required for the amount of treated water of 1800 m ³ / d. By increasing the pressure to 1, the reaction tank is sufficient for the treated water amount of 3000 m ³ / d.

  In the fluidized bed crystallization method, when the raw water LV is raised in this way, the seed crystal may flow out of the reaction vessel. In the present invention, calcium carbonate having a true specific gravity of about 2.7 is used as the parent crystal. And by slightly increasing the particle size to 0.2 to 0.5 mm, the expansion rate of the fluidized bed is suppressed and the seed crystals are prevented from flowing out at a high LV.

Further, when the seed crystal particle size is increased, the specific surface area of the seed crystal is decreased, the contact efficiency with the raw water is decreased, and the dephosphorization reaction rate is decreased. For this reason, in this invention, by providing the cylinder for tower | column circulation in a reaction tank, raw water SV is made comparatively low with 5-10h < ^-1 >, and the crystallization reaction rate is improved and made uniform.

  And in this invention, while controlling the particle diameter of seed crystals and raw water LV and SV in this way, TP of the inflow water to a reaction tank is controlled to 3-5 mg / L, so that TP 1 mg / L L or less high quality treated water is obtained.

  In the present invention, a downward flow forming means such as an underwater stirrer is provided for circulation in the reaction tank. As described above, the reaction tank can be downsized by increasing the processing speed. The counterflow forming means can also be reduced in size.

  In the dephosphorization process using a fluidized bed crystallization method, in order to develop seed crystals and form a fluidized bed, a part of the treated water is circulated in the reaction tank in an upward flow. In the invention, the treated water may be circulated. However, in the case where a sufficient fluidized bed can be formed with only raw water, circulation of treated water is not necessarily required.

  In the present invention, the seed crystal filling amount is 20 to 40% by volume with respect to the reaction vessel volume, and the fluidized bed seed crystal development rate (ratio of fluid bed height to seed crystal filling height (%) ) Is preferably 200 to 220%.

  In addition, when the calcium compound and / or alkali agent is injected into the reaction vessel, the crystallization reaction rate is improved and uniformized by injecting the calcium compound and / or alkali agent at a plurality of locations in the reaction vessel. It is preferable to plan.

  According to the present invention, a uniform and efficient crystallization reaction can be performed in a reaction tank, and phosphorus in phosphorus-containing waste water can be removed at a high treatment rate to a low concentration.

  Hereinafter, embodiments of the dephosphorization method of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a system diagram of a dephosphorization apparatus showing an embodiment of the dephosphorization method of the present invention.

In the following, the case where calcium hydroxide (Ca (OH) ₂ ) is used as the calcium compound is exemplified, but the calcium compound used in the present invention is not limited to Ca (OH) ₂ , such as calcium chloride. Other calcium compounds can also be used. However, it is desirable to use Ca (OH) ₂ as the calcium compound because it can also function as an alkaline agent.

  In the method of FIG. 1, raw water (phosphorus-containing wastewater) such as sewage secondary treated water introduced from the pipe 11 to the raw water tank 1 is passed through the pipes 12 and 13 by the raw water pump 1P, along with the circulating water from the pipe 16. It introduce | transduces into the reaction tank 2 from the tank lower part.

  This reaction tank 2 has an inlet at the lower part of the tank, an outlet for treated water at the upper part of the tank, and a seed crystal fluidized bed F is formed in the tank.

  A strainer 8 is provided in the lower part of the reaction tank 2 to allow the liquid introduced from the pipe 13 to uniformly flow into the fluidized bed F. Both ends of the reaction tank 2 are open. A cylinder (hereinafter referred to as “draft tube”) 9 is provided such that its axial direction is the vertical direction, and at least the lower part of the draft tube 9 is located in the fluidized bed F. An underwater stirrer 10 as a downflow forming means for forming a downward flow in the draft tube 9 is provided in the upper portion of the draft tube 9.

Further, the reaction vessel 2 in the fluidized bed F, Ca _(OH) pipe 14A supplies via a pipe 14 having a pump 3P of Ca _{(OH) 2} from ₂ reservoir 3, 14B are inserted. The pipes 14A and 14B are preferably provided with a plurality of supply ports so that Ca (OH) ₂ can be uniformly supplied to a plurality of locations in the fluidized bed F. Incidentally, Ca _{(OH) 2} in the Ca _{(OH) 2} storage tank 3, handling, it is preferable to use an aqueous solution of about 5 to 10 wt% concentration of the slaked lime powder was dissolved in water.

  The liquid introduced into the lower part of the reaction tank 2 through the pipe 13 causes a crystallization reaction in the fluidized bed F while flowing upward in the reaction tank 2, and phosphorus in the liquid is removed as calcium phosphate. The In this way, part of the water that has been crystallized and dephosphorized and reached the upper part of the reaction tank 2 flows downward in the draft tube 9 by the underwater stirrer 10 and then rises again in the fluidized bed F to reach the tank. It circulates inside, and the remainder is extracted from the pipe 15. Part of the extracted treated water is circulated through the circulating water tank 4 to the lower part of the reaction tank 2 through the pipes 16 and 13 having the circulation pump 4P, and the remainder is fed to the neutralization tank 5 through the pipe 17.

The crystallization reaction in the reaction tank 2 is preferably performed at pH 9 to 11, particularly pH 9.5 to 10.5, and the Ca (OH) ₂ supply pipe so that the pH in the reaction tank 2 falls within this range. The flow rates of 14A and 14B are controlled in conjunction with a pH meter (not shown) provided in the reaction tank 2.

The amount of calcium ions is preferably about 3 to 6 times the weight of phosphorus to be removed. Therefore, the amount of Ca (OH) ₂ added so that the amount of calcium ions is within the above pH range. To control.

To the neutralization tank 5, the H ₂ SO ₄ aqueous solution of the H ₂ SO ₄ storage tank 6 is added from a pipe 18 having a pump 6P, and the inflow water from the circulation water tank 4 is neutralized to pH neutral, and then the treated water tank. 7 and discharged from the pipe 19 to the outside of the system. The neutralizing acid is not limited to H ₂ SO ₄ , and may be other mineral acids such as HCl.

  In the present invention, as a seed crystal for forming the fluidized bed F in the reaction tank 2, a crystal in which calcium phosphate is precipitated on the surface of calcium carbonate by a method as described in JP-A No. 2003-181469 is used. The seed crystal has a particle size in the range of 0.2 to 0.5 mm. If the seed crystal particle size is less than 0.2 mm, the seed crystal may flow out of the reaction vessel 2 due to high LV water flow, and if it exceeds 0.5 mm, the specific surface area becomes too small and the reaction efficiency decreases. At the same time, the seed crystal is developed by the upward flow in the reaction tank, and a good fluidized bed F cannot be formed.

Therefore, in the present invention, a seed crystal in which calcium phosphate is precipitated to about 2-3 mg-P / g-CaCO _{3 on} the surface of calcium carbonate having a uniform particle size in the range of 0.2 to 0.5 mm is used. The seed crystal is appropriately replenished and pulled out, and the seed crystal in the reaction vessel is treated so that the particle size is in the range of 0.2 to 0.5 mm.

Further, the filling rate of the seed crystals varies depending on the raw water LV in the reaction tank, but it is 25 to 40% by volume with respect to the reaction tank volume, and the seed crystal development rate of the fluidized bed F is 200 to 220% (ie, FIG. in 1, it is preferable to perform processing so that the height _{H 2} of the fluidized bed F against seed filling height _{H 1} is _{H = (2~2.2) × H 1} ). When the filling rate of the seed crystals is less than the above range, the dephosphorization performance of the reaction tank is lowered. When the filling rate is large, the seed crystals cannot be sufficiently developed to form a good fluidized bed. There is also a risk of seed crystals flowing out. Further, if the development rate of the seed bed in the fluidized bed is lower than the above range, a fluidized bed excellent in contact efficiency between the liquid and the seed crystal cannot be formed, and if it is high, the reaction efficiency is lowered. There is a possibility that seed crystals may flow out of the base.

Further, in the present invention, the flow of raw water LV and SV in the reaction tank 2 is LV = 20 to 25 m / h, respectively.
SV = 5-10h ^-1
And

  When the flow rate LV of raw water is less than 20 m / h, seed crystals having a large particle size cannot be sufficiently developed, and a good fluidized bed cannot be formed. When the LV of the influent water exceeds 25 m / h, the seed crystal develops with a small grain size of 220%, further exceeds 250%, and flows out of the reaction tank 2. Therefore, the flow rate LV of the raw water is 20 to 25 m / h.

In addition, if the raw water flow SV is less than 5 h ⁻¹ , the required seed crystal filling amount is large and uneconomical. When the raw water passing SV exceeds 10 h ⁻¹ , the treated water TP concentration becomes unstable. In particular, when the water temperature is 10 ° C. or lower, the dephosphorization performance is lowered, and the treated water TP concentration is 1 mg / L. It is not preferable because it exceeds the range. Accordingly, the flow rate SV of the raw water depends on the water temperature, but when the water temperature is 10 ° C. or lower, the SV is preferably low, for example, about 5 to 7 h ⁻¹ .

  In addition, when the TP concentration of the inflowing water in the reaction tank 2 (that is, the water flowing into the reaction tank 2 from the pipe 13 (total with the raw water circulating water in FIG. 1)) exceeds 6 mg / L, the present invention Under the water flow conditions, it becomes difficult to obtain treated water having a TP concentration of 1 mg / L or less. Therefore, the TP concentration of the influent water is set to a range of 6 mg / L or less.

The TP concentration of the inflow water to the reaction tank 2 can be adjusted by adjusting the amount of circulating water. However, in the present invention, the circulation of the treated water is not necessarily required, and only the raw water is passed. If the seed bed can be sufficiently developed in the reaction tank 2 to form the fluidized bed F, the treatment water need not be circulated. Usually, about 30 to 50% of the treated water is circulated, so that the inflow water LV of the reaction tank 2 summed with the raw water and the circulating water is 20 to 25 m / h, and the SV is 5 to 10 h ^−1. It is necessary to make the treated water TP concentration 1 mg / L or less.

  In the present invention, the water temperature in the reaction vessel is not particularly limited as long as it is 9 ° C. or higher.

  In the present invention, by adopting a combination of the above treatment conditions, phosphorus-containing wastewater having a phosphorus concentration of several mg / L can be efficiently treated to quickly reduce phosphorus to a low concentration of TP concentration of 1 mg / L or less. Can be removed. Then, phosphorus can be recovered at a high recovery rate as calcium phosphate crystals with little variation in particle size, which can be effectively used as a fertilizer.

  The method shown in FIG. 1 is an example of an embodiment of the present invention, and the present invention is not limited to the illustrated method as long as the gist of the present invention is not exceeded.

For example, the raw water may be introduced into the reaction tank after the pH is adjusted by adding Ca (OH) ₂ in the pH adjustment tank before being introduced into the reaction tank. Further, as described above, the circulation of the treated water is not necessarily required, and may be discharged out of the system after neutralization as it is.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
Sewage secondary treated water (average PO ₄ -P concentration: 5 mg / L) was treated under the following conditions by a dephosphorization apparatus (reaction tank diameter 750 mm) shown in FIG. As the calcium compound, a 5% by weight aqueous solution of slaked lime (special grade) was used.
Treated water volume: 260 m ³ / d
Raw water LV in reaction tank: 25 m / h
Raw water in reaction tank SV: 10 h ^-1
Circulating water volume: 0% of treated water (without treated water circulation)
Inflow water LV of reaction tank: 25 m / h
Inflow water SV of reaction tank: 10h ^-1
TP concentration of inflow water of reaction tank: 4 to 5 mg / L
Seed crystal: particle diameter of about 3 mg-P / g-CaCO ₃ precipitated calcium phosphate on the surface
0.25-0.4 mm seed crystal filling rate: 40% by volume of reaction vessel volume
Seed development rate of fluidized bed: 200-220%
pH: 10 to 10.5
CaCO ₃ addition amount: 25 mg-Ca / L
Water temperature: 11-13 ° C

As a result, treated water with TP: 0.9 to 0.95 mg / L and PO ₄ -P: 0.2 to 0.4 mg / L could be stably obtained.

  The dephosphorization method of the present invention is suitable for dephosphorization treatment of phosphorus-containing wastewater discharged in domestic wastewater treatment such as sewage and agricultural settlement wastewater.

It is a systematic diagram which shows embodiment of the dephosphorization method of this invention.

Explanation of symbols

1 raw water tank 2 reactor 3 Ca _{(OH) 2} storage tank 4 circulation water tank 5 neutralization tank 6 H ₂ SO ₄ tank 7 treatment water tank 8 strainer 9 draft tube 10 water stirrer F fluidized bed

Claims (5)

Raw water containing phosphoric acid is passed upwardly into a reaction tank filled with seed crystals formed by depositing calcium phosphate on the surface of calcium carbonate to form a fluidized bed of the seed crystals in the reaction tank, In the dephosphorization method of removing phosphorus in raw water as calcium phosphate crystals in the reaction vessel by adding a calcium compound and / or an alkali agent,
The reaction vessel is provided with a cylinder open at both ends so that the axial direction of the cylinder is the vertical direction and at least the lower part of the cylinder is located in the fluidized bed, and a downward flow is formed in the cylinder. Providing a downward flow forming means for
The seed crystal grain size is in the range of 0.2 to 0.5 mm,
The raw water flow LV in the reaction tank is 20 to 25 m / h, the raw water flow SV in the reaction tank is 5 to 10 h ⁻¹ , and the phosphorus concentration of inflow water into the reaction tank (TP) A dephosphorization method characterized in that treated water having a phosphorus concentration (TP) of 1 mg / L or less is obtained by adjusting the concentration to 6 mg / L or less.   The dephosphorization method according to claim 1, wherein a part of the treated water flowing out of the reaction tank is circulated in the reaction tank in an upward flow.   3. The dephosphorization method according to claim 1, wherein a filling amount of the seed crystal is 25 to 40% by volume with respect to the reaction tank volume.   The dephosphorization method according to any one of claims 1 to 3, wherein a seed crystal development rate of the fluidized bed is 200 to 220%.   The method according to any one of claims 1 to 4, wherein the calcium compound and / or the alkali agent is injected into the reaction vessel, and the calcium compound and / or the alkali agent is injected at a plurality of locations in the reaction vessel. A dephosphorization method.

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