JP2002145607A - Apparatus and method of recovering phosphorus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus and method for recovering phosphorus, in which the granulation reaction time in a magnesium ammonium phosphate granulation tower and the retention time in a granulation reaction vessel are reduced and the concentration of phosphorus in treated water is decreased to a concentration allowable to discharge. SOLUTION: The apparatus for recovering phosphorus possesses a reverse osmosis membrane device in a front stage and/or post stage of the granulation tower for recovering phosphate ion in water to be treated as the crystal of magnesium ammonium phosphate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a magnesium ammonium phosphate granulating tower, which is used for concentrating phosphate ions in water to be treated in a former stage of a granulating tower and / or phosphoric acid in granulated treated water in a latter stage of a granulating tower. This is a phosphorus recovery device characterized by providing a reverse osmosis membrane device for removing acid ions. Phosphate ion enrichment method that can shorten the granulation reaction time and residence time and pass the treated water that has been subjected to granulation dephosphorization through a reverse osmosis membrane to further reduce phosphate ions into natural water of treated water The present invention relates to a method for recovering phosphorus which can be discharged.

[0002]

BACKGROUND ART Phosphorus is an essential resource for modern agriculture,
Japan mostly relies on imports. However, in recent years, there has been a concern about a decrease in the amount of phosphate ore resources. The available years of phosphorus ore estimated from recoverable reserves and annual consumption vary depending on the researchers, but it is reported that the minimum is 50 to 100 years. Technology, environmental technology, vol.
6, pp. 24-28 (1998)). Phosphorus is said to be a valuable resource like oil. Phosphorus is a factor in eutrophication, which is a particular problem in closed waters, and it is required to remove phosphate ions from wastewater.

[0003] As described above, removal and recovery of phosphate ions in wastewater will be indispensable in the future from the viewpoint of resource and environmental conservation. Phosphate ion removal technology includes coagulation sedimentation in which metal ions such as aluminum salts and iron salts are reacted with phosphate ions to precipitate and remove them as phosphorus compounds. There are a crystallization method for precipitating phosphorus (contact dephosphorization method) and a biological dephosphorization method utilizing the excessive phosphorus uptake action of microorganisms. However, each of these processing processes has the following disadvantages. That is, the coagulation sedimentation method has a high running cost due to a large amount of generated sludge and a large amount of chemicals used, and it is difficult to isolate and recover phosphorus in separated sludge,
The crystallization method requires a pretreatment such as a decarbonation tank and sand filtration, which increases equipment costs. The biological dephosphorization method has a lower removal performance than physical and chemical treatments, so the installation area of the equipment increases. That is their disadvantage.

In view of such circumstances, in recent years, a magnesium compound has been added to a waste liquid containing ammonium ions and phosphate ions, and the pH of the waste water has been adjusted to 8 or more. Technology that effectively removes the resulting solid particles of magnesium ammonium phosphate (hereinafter referred to as M
(AP method) is attracting attention. That is, Japanese Patent Laid-Open No. 1-11
No. 9392 discloses that a magnesium compound is added to waste water containing ammonium ions and phosphate ions, the pH is adjusted to 8 or more, the waste water is stirred by aeration, and fine crystals of magnesium ammonium phosphate are generated. The suspended solids contained therein and the fine crystals of magnesium ammonium phosphate are separated and the suspended solids are discharged out of the system, and the waste water containing the fine crystals of magnesium ammonium phosphate is continuously aerated while stirring by aeration. An apparatus and method for supplying and forming solid particles of magnesium ammonium phosphate as fine crystal nuclei of the magnesium ammonium phosphate are described.

[0005] Drugs used in the MAP method are inexpensive,
No waste is generated, and the generated particles do not contain harmful substances, so they can be reused as slow-release fertilizers. Is less likely to cause secondary contamination. Therefore, MA
The P method is considered to be more effective in terms of phosphorus recovery than other phosphate ion removal processes.

[0006]

However, in a granulation dephosphorization apparatus, magnesium ammonium phosphate (MA) is used.
P) utilizing the chemical reaction of formation (formula (1) below)
The reaction rate is affected by the stirring speed, pressure, suspended crystal concentration, etc. in the reaction tank in addition to the pH and temperature. For example, the solubility product K when the reaction reaches an equilibrium state (Equation (2) below) ) Is about 1 × 10 ^-14 (mol / l) under conditions of pH 9.0, temperature 20 ° C. and 1 atm, and has a disadvantage that the reaction rate is extremely slow.

[0007] _{^{Mg 2+ + NH 4 + + HPO}} 4 2+ OH - + 6H 2 O → MgNH 4 PO 4 · 6H 2 O + H 2 O (1) K = [Mg ^2+] [NH ₄ ^+] [HPO ₄ ^2-] [OH ^-] (2) for this reason, in the conventional MAP method, the granulation of magnesium ammonium phosphate crystals and takes a very long time of 1-2 weeks, efficiency is very poor. In addition, MAP
The concentration of phosphate ions in the treated water by the method is often several mg to several tens mg / l, which exceeds the wastewater standard determined based on the Water Pollution Control Law, and the phosphate ion concentration is reduced to 0 by another treatment. It was necessary to lower it to about 0.5 mg / l or less, and it was difficult to discharge the treated water into natural water as it was.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems, to enhance the efficiency of MAP formation, to increase the granulation reaction time of the magnesium ammonium phosphate granulation tower and the residence time in the granulation reaction tank. Provided are a phosphorus recovery apparatus and a phosphorus recovery method capable of shortening the concentration and reducing the phosphorus concentration of treated water to a concentration capable of being discharged.

[0009]

In order to achieve the above object, the present invention employs the following constitution. That is, the present invention provides a "phosphorus recovery device provided with a reverse osmosis membrane device at the front and / or the rear of a granulation tower for recovering phosphate ions in the water to be treated as magnesium ammonium phosphate crystals." In the phosphorus recovery method for recovering phosphate ions in the water to be treated as crystals of magnesium ammonium phosphate, (1) phosphate ions in the water to be treated are concentrated by a reverse osmosis membrane before the granulation of the magnesium ammonium phosphate crystals. And (2) filtering the treated water having a reduced phosphate ion concentration through a reverse osmosis membrane by granulation of magnesium ammonium phosphate crystals, and further reducing the phosphate ion concentration. Method for recovering phosphorus. "

[0010]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 shows an example of the flow of the phosphorus recovery apparatus of the present invention. The water to be treated in the magnesium ammonium phosphate granulation tower 1 is supplied under pressure to a reverse osmosis membrane module (reverse osmosis membrane device) 2. The water to be treated, which is supplied under pressure to the reverse osmosis membrane module 2, is subjected to sand filtration, activated carbon filtration, security filter, precision filtration,
Pretreatment such as ultrafiltration may be performed. For example, in the case of the water to be treated which has a large amount of turbid components, it is effective to perform the pretreatment to reduce impurities flowing into the magnesium ammonium phosphate granulation tower 1 from the viewpoint of increasing the reaction efficiency of MAP. Reverse osmosis membrane module (reverse osmosis membrane device)
The membrane permeated water of No. 2 is discharged to natural water as it is.

Here, the reverse osmosis membrane is a membrane capable of separating impurities at an ion level, and in the present invention, it is sufficient that the rejection of phosphate ions is high. Such materials include cellulose acetate and polyamide. In the present invention, those having a phosphate ion removal rate of 90% or more are preferable, and those having a removal rate of 95% or more are more preferable. Applicable.

Although there are a hollow fiber and a spiral type in the membrane module form, any of the membrane module forms may be used in the present invention. The water to be treated concentrated by the reverse osmosis membrane module 2 is supplied to the magnesium ammonium phosphate granulation tower 1
Flows into. Magnesium ammonium phosphate granulation tower 1
In addition to the concentrated water to be treated, a magnesium compound to be reacted with phosphate ions and an alkaline agent for adjusting the pH of the water to be treated to alkaline flow from the tanks 3 and 4, respectively. Magnesium compounds [Mg ^2+] / [PO ₄ ^3-]
Is preferably 1 or more and 2 or less, and a magnesium compound is more preferably [M
It is desirable to add such that the molar ratio of g ²⁺ ] / [PO ₄ ^3- ] becomes 1: 1 and to add the alkaline agent so that the pH becomes 8.0 or more and 9.0 or less.

Here, the present inventors have determined that phosphate ion concentration and M
As a result of intensive studies on the relationship between the AP generation rate, it was found that there was a correlation between the phosphate ion concentration and the MAP generation rate, and that the higher the phosphate ion, the faster the MAP generation rate. In other words, the mainstream of the ammonium ammonium phosphate granulation tower 1 is an upward-flow type column apparatus composed of a reaction section located at the center of the tower and a separation section located at the top of the tower.
First, the gas supplied from the stirring blower 6 is blown from the lowermost part of the magnesium ammonium phosphate granulation tower 1 to thereby mix the water to be treated containing phosphate ions, the magnesium compound, and the alkali agent in the reaction section. The reaction of equation (1) occurs. Another stirring method is a paddle.

As the magnesium compound, magnesium chloride, magnesium nitrate, magnesium sulfate and the like are used. As the alkali agent, sodium hydroxide, potassium hydroxide or the like is used. When the reaction of the formula (1) occurs, fine crystals of magnesium ammonium phosphate are generated, and when the water to be treated is continuously supplied, the fine crystals grow,
The particle size increases and solid particles are formed. Since the reaction of the formula (1) depends on the solubility product, the pH is not less than 8.0 and not more than 9.0.
In the following, the phosphate ion concentration of the treated water in the magnesium ammonium phosphate granulation tower 1 is theoretically about 30 mg / l, and is preferably 50 mg / l or more in order for the reaction of the formula (1) to proceed. The formed solid particles descend by gravity and are discharged intermittently or continuously to the drum type screen 7. Drum type screen 7 sorts magnesium ammonium phosphate according to the difference in particle size, magnesium ammonium phosphate having a large particle size is dried by drying hopper 8, and magnesium ammonium phosphate having a small particle size is granulated with magnesium ammonium phosphate. Returned to Tower 1.

Magnesium ammonium phosphate granulation tower 1
The treated water and the solid particles of magnesium ammonium phosphate are separated from each other in the separation section located at the top, and only the treated water is discharged from the upper portion of the interface.

The discharged treated water is supplied under pressure to a reverse osmosis membrane module (reverse osmosis membrane device) 5. The treated water of the magnesium ammonium phosphate granulation tower 1 is not pretreated because it is clearer than the water to be treated, but depending on the water quality, sand filtration, activated carbon filtration, security filter, microfiltration,
Pretreatment such as ultrafiltration may be performed. The permeated water of the reverse osmosis membrane module 5 is discharged to natural water as it is, and the water that has not been permeated and concentrated flows into the magnesium ammonium phosphate granulation column 1.

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples.

[0018]

EXAMPLE 1 FIG. 2 shows a phosphorus recovery apparatus operated. The water to be treated was supplied under pressure to the reverse osmosis membrane module 2, and the water to be treated concentrated by the reverse osmosis membrane module 2 flowed into the magnesium ammonium phosphate granulation tower 1. In addition to the water to be treated, a magnesium compound to be reacted with phosphate ions and an alkaline agent for adjusting the pH of the water to be treated to alkaline were flowed into the magnesium ammonium phosphate granulating tower 1 from the tanks 3 and 4, respectively. Magnesium compounds (M
The molar ratio of g ^2+] / [PO ₄ ^3-] is 1: is added to a 1, the alkaline agent is added to a pH of 8.0 to 9.0. This time, magnesium chloride was used as a magnesium compound, and sodium hydroxide was used as an alkali agent.

The water to be treated was industrial wastewater (average phosphate ion concentration: 35 mg / l, average nitrogen concentration: 103 mg / l). The loading amount of phosphate ions in the magnesium ammonium phosphate granulation tower 1 was 20 kg / d. The operation of the reverse osmosis membrane module 2 is performed at a pressure of 15 kgf / cm ² and a recovery rate of 6
The cross-linking aromatic element was used as a cross-linked aromatic element, and a polyamide reverse osmosis membrane module was used as the membrane module. The phosphate ion concentration in the concentrated water was about 87 mg / l, and the average particle size of magnesium ammonium phosphate in the magnesium ammonium phosphate granulation tower 1 was 2.0 mm in 120 hours. The recovery rate of phosphate ions by this recovery device was about 90%.

Comparative Example 1 Water to be treated into the magnesium ammonium phosphate granulation tower 1 flowed into the magnesium ammonium phosphate granulation tower 1 without being supplied to the reverse osmosis membrane module 2 under pressure. The water to be treated was the same industrial wastewater as in Example 1 (average phosphate ion concentration: 35 mg / l, average nitrogen concentration: 103 mg / l). As a result, the average particle size of magnesium ammonium phosphate in the magnesium ammonium phosphate granulation tower 1 was 2.
It took 350 hours to reach 0 mm. The recovery of phosphate ions was about 73%.

Example 2 FIG. 3 shows a phosphorus recovery apparatus in which the operation was performed. The treated water of the magnesium ammonium phosphate granulation tower 1 was supplied under pressure to the reverse osmosis membrane module 5, and the concentrated water was returned to the magnesium ammonium phosphate granulation tower 1. In addition to the water to be treated, a magnesium compound to be reacted with phosphate ions and an alkaline agent for adjusting the pH of the water to be treated to alkaline were flowed into the magnesium ammonium phosphate granulating tower 1 from the tanks 3 and 4, respectively. Magnesium compounds (M
The molar ratio of g ^2+] / [PO ₄ ^3-] is 1: is added to a 1, the alkaline agent is added to a pH of 8.0 to 9.0. This time, magnesium chloride was used as a magnesium compound, and sodium hydroxide was used as an alkali agent.

The treated water used was the treated water of Example 1 (average daily phosphate ion concentration: 11 mg / l). The reverse osmosis membrane module 5 of the treated water was operated at a constant linear flow rate of 1.2 m / d and a recovery rate of 80%, and a polyamide reverse osmosis membrane module was used as the membrane module. As a result, the phosphate ion concentration of the permeated water was about 0.01 mg / l.
It was possible to discharge into natural water.

Comparative Example 2 Since the treated water of the magnesium ammonium phosphate granulation tower 1 was not supplied under pressure to the reverse osmosis membrane module 5, the average daily TP of the treated water remained at 11 mg / l, It was not possible to release it.

[0024]

According to the present invention, in a magnesium ammonium phosphate granulation tower, a reverse osmosis membrane device for concentrating phosphorus in the water to be treated at a stage preceding the granulation tower or a phosphorus concentration in a granulation treatment water at a stage subsequent to the granulation tower. By installing a reverse osmosis membrane device for lowering the concentration, the granulation reaction time and the residence time in the granulation reaction tank can be shortened as compared with the conventional case, and the phosphorus concentration of the treated water can be reduced to a concentration that can be discharged.

[Brief description of the drawings]

FIG. 1 is a flowchart showing an example of a granulation tower of magnesium ammonium phosphate of the present invention.

FIG. 2 is a flowchart showing an example of a granulation tower of magnesium ammonium phosphate of Example 1.

FIG. 3 is a flowchart showing an example of a granulation tower of magnesium ammonium phosphate in Example 2.

[Explanation of symbols]

 1: Magnesium ammonium phosphate granulation tower 2, 5: Reverse osmosis membrane module 3: Magnesium tank 4: Alkaline tank 6: Stirring blower 7: Drum tube screen 8: Drying hopper

Claims (4)

[Claims] An apparatus for recovering phosphorus, comprising a reverse osmosis membrane device upstream and / or downstream of a granulation tower for recovering phosphate ions in treated water as crystals of magnesium ammonium phosphate. 2. A phosphorus recovery method for recovering phosphate ions in water to be treated as magnesium ammonium phosphate crystals, wherein one or both of the following (1) and (2) are simultaneously performed. How to recover phosphorus. (1) Prior to granulation of magnesium ammonium phosphate crystals, phosphate ions in the water to be treated are concentrated by a reverse osmosis membrane. (2) The treated water having a reduced phosphate ion concentration due to granulation of magnesium ammonium phosphate crystals is filtered through a reverse osmosis membrane to further reduce the phosphate ion concentration. 3. The phosphate ion concentration in the water to be treated is 50 mg.
3. The method for recovering phosphorus according to claim 2, wherein the concentration is increased to not less than / l. 4. The phosphorus recovery according to claim 2 or 3, wherein the concentrated water of the reverse osmosis membrane according to claim 2 (2) is returned to a granulation tower of magnesium ammonium phosphate crystals. Method.