JP4368159B2 - Method for treating wastewater containing phosphate - Google Patents

Description

  The present invention relates to a method for removing phosphorus in wastewater containing phosphate (phosphate-containing wastewater), and more specifically, phosphoric acid is removed from wastewater containing phosphate but having a low ammonium ion concentration. The present invention relates to a crystallization dephosphorization method for precipitation and recovery as magnesium crystals.

  Conventionally, as a method for treating phosphate-containing wastewater, iron compounds, aluminum compounds or calcium compounds are added to phosphate-containing wastewater, and phosphorus in phosphate-containing wastewater is hardly soluble iron salt, aluminum salt or calcium salt. There is a method of coagulating and precipitating and solid-liquid separation (see Non-Patent Document 1, for example).

  However, these agglomerated sludges are often very poor in dewaterability, and there is a problem that the dewatering treatment of the generated sludge requires manual labor. In addition, dewatered sludge and sludge are disposed of in landfills or incinerated, and then disposed of in landfills.However, dewatering and incineration require a large amount of money and excessive equipment, and in recent years problems such as dioxin generation have occurred. Therefore, incineration disposal has become difficult, and a treatment method that reduces the amount of sludge as much as possible has been desired.

  On the other hand, in Japan, phosphorus ore is often used as a raw material for fertilizer, but there is almost no domestic phosphorus resource and it depends on imports. Phosphate is made from phosphate ore, which is a mineral resource, but high-quality phosphate ore like the one currently used is expected to be depleted in the future. Under such circumstances, it is necessary to treat phosphorus-containing wastewater such as sewage as a phosphorus resource, and there has been a demand for a treatment method that can recover and effectively utilize phosphorus resources in the treatment of phosphate-containing wastewater.

  Therefore, in recent years, as a technique applying the crystallization method, a method of adding magnesium salt to phosphate-containing waste water to precipitate magnesium ammonium phosphate (MAP) particles has been developed (for example, Non-Patent Document 2, (See Patent Documents 1 and 2). In this method, phosphate in wastewater containing phosphate, ammonium ions and magnesium ions react equimolarly to precipitate and grow magnesium ammonium phosphate particles, resulting in good dehydration and extremely easy handling. In this, phosphorus is removed from phosphate-containing wastewater and recovered as phosphorus resources.

  In order to precipitate magnesium ammonium phosphate (MAP) particles by adding a magnesium salt to a phosphate-containing wastewater, the phosphate-containing wastewater preferably contains ammonium ions. Therefore, as a method that applies crystallization, magnesium salt is added to phosphate-containing wastewater, and magnesium ammonium phosphate (MAP) particles are precipitated and recovered. It was limited to wastewater containing both of these in high concentrations, and was not applied to wastewater that required the addition of ammonium ions. The reason for this is that ammonium ion is a causative substance that brings eutrophication to sea areas and lakes as well as phosphate, so it is intended for wastewater treatment that is intended primarily for the treatment of substances that promote eutrophication. If it is not desirable to add eutrophication substances such as ammonium ions to the crystallization reactor, and if ammonium ions are added to the crystallization reactor for wastewater treatment, running costs will increase as a chemical cost. Increase.

  Examples of limited applications include wastewater from the production line of phosphoric acid fertilizer, anaerobic digested sludge treatment wastewater at a sewage treatment plant, and the like. These were all wastewaters containing both phosphate and ammonium ions at high concentrations. Specifically, conventionally, a magnesium ammonium phosphate crystallization method has been used as a wastewater treatment method including a phosphorus concentration of 100 mg / L or more and an ammoniacal nitrogen concentration of 200 mg / L or more.

  On the other hand, unlike the crystallization method in which magnesium ammonium phosphate particles are precipitated, a method of recovering phosphorus by treating low-concentration phosphorus-containing wastewater by the crystallization method has also been developed (see, for example, Patent Documents 3 and 4). ). The crystals recovered and used here are crystals called hydroxyapatite, which is a wastewater treatment method that applies the fact that phosphate and calcium ions instantaneously crystallize in an extremely low concentration region.

  The solubility product of hydroxyapatite is very small compared to the solubility product of magnesium ammonium phosphate. Therefore, this reaction is suitable for recovering phosphorus by treating low-concentration phosphorus-containing wastewater by a crystallization method. Specifically, it is suitable for a wastewater treatment method containing phosphorus with a phosphorus concentration of 10 mg / L or less. When this method was carried out with sewage having a phosphorus concentration of several tens of mg / L or more, the formation reaction of hydroxyapatite occurred instantaneously and its control was very difficult.

  As another method for removing phosphorus in water, a method for removing phosphorus by adding a basic magnesium salt as a flocculant to water containing phosphorus has been proposed (see, for example, Patent Document 5). In this method, a phosphorus compound that is hardly soluble in water is produced and the phosphorus concentration in the water is lowered, but there is a problem that a slurry that is difficult to handle is generated.

There has also been proposed a crystallization dephosphorization method in which phosphorus in waste water containing phosphorus of several tens mg / L as a phosphorus concentration is precipitated and separated as magnesium phosphate compound crystals that are easy to handle (for example, Patent Documents 6 and 7). reference). However, since it is difficult to control the crystallization reaction of the magnesium phosphate compound, a fixed bed filtration apparatus for filtering the fine floc flowing out from the fluidized bed reaction tank has been an essential component.
"Phosphorus Removal Design Manual" published by Japan Sewerage Association, October 1988, p.7-9 Sugimori, Ito, Nakamura “Recovery of phosphorus from sludge treated return water” 32nd Sewerage Research Presentation, p.400-402 JP 62-262789 A Japanese Unexamined Patent Publication No. 63-200908 JP 54-25516 A JP 54-25749 A Japanese Laid-Open Patent Publication No. 57-130587 JP 2001-47065 A JP 2001-149951 A

  As described above, the method for removing and recovering phosphorus from waste water containing phosphate by crystallization method is that the concentration of phosphorus is 100 mg / L or more in waste water containing both phosphate and ammonium ions at a high concentration. A treatment method has been put into practical use for wastewater containing 200 mg / L or more as the nitrogen concentration or for wastewater containing 10 mg / L or less of phosphorus as the phosphorus concentration. However, an appropriate crystallization method was being sought for wastewater containing phosphorus with a phosphorus concentration of several tens mg / L.

  This invention is made | formed in view of such the present condition, The objective is to isolate | separate and collect | recover phosphorus components efficiently from the waste_water | drain containing a phosphate, and provides the processing method useful for circulation utilization of phosphorus resources. There is.

As a result of intensive studies to solve the above problems, the present inventors have converted phosphorus in waste water containing phosphate as magnesium phosphate (Mg ₃ (PO ₄ ) ₂ .XH ₂ 0) in the presence of magnesium ions. In the removal method, magnesium ammonium phosphate (MgNH ₄ PO ₄ .6H ₂ 0) crystal is used as a seed crystal, and a magnesium agent and an alkali agent are added so as to be metastable with respect to magnesium phosphate. As a result, it was found that magnesium phosphate crystals can be efficiently precipitated and recovered, and the present invention has been achieved.

That is, the present invention relates to a method for removing phosphate from a wastewater containing phosphate as magnesium phosphate (Mg ₃ (PO ₄ ) ₂ .xH ₂ 0) in the presence of magnesium ions. Into the crystallization reaction tank for precipitating, magnesium ammonium phosphate (MgNH ₄ PO ₄ .6H ₂ 0) is charged as a seed crystal, and a magnesium agent and an alkali agent are added and fluidized to precipitate the magnesium phosphate crystal. A method for treating wastewater containing phosphate, characterized in that it is collected, preferably, magnesium ammonium phosphate is a magnesium ammonium phosphate crystal having a particle diameter of 0.1 to 1 mm, The filling amount of magnesium ammonium phosphate is 10 to 50 w / v% with respect to the crystallization reaction tank capacity.

  According to the method for treating wastewater containing phosphate of the present invention, phosphate is contained, but phosphate is stably recovered from wastewater having a low ammonium ion concentration in a high-purity state called magnesium phosphate crystals. The separated and recovered crystals can also be effectively used as fertilizers and soil improvers.

  Examples of the wastewater containing phosphate to which the treatment method of the present invention can be applied include wastewater containing phosphate but having a low ammonium ion concentration, such as concentrated separation water of septic tank sludge and concentrated separation water of sewage surplus sludge. The phosphate concentration in the waste water is particularly preferably 10 to 50 mg / L.

  In the present invention, it is essential to use magnesium ammonium phosphate as a seed crystal. Magnesium ammonium phosphate has a larger specific surface area as the particle size is smaller, and the effect as a seed crystal is larger. However, when it is actually filled in a crystallization reaction tank and used, the particle size is too small with treated water. It will leak. Accordingly, the particle size is preferably 0.1 mm or more. Conversely, magnesium ammonium phosphate particles having a particle diameter exceeding 1 mm are not preferable because excessive energy is required to flow in the crystallization reaction tank. In the present invention, aeration is appropriate as a method for maintaining the flow in the crystallization tank. However, in order to flow a seed crystal having a large particle size, the electricity cost required for the blower operation accounts for a proportion of the running cost. It is expensive and not economical. The magnesium ammonium phosphate is preferably a crystal. The reason why the magnesium ammonium phosphate crystal is suitable as the seed crystal is that the magnesium ammonium phosphate crystal is less soluble in water in the alkaline region and is more stable in the crystallization reaction tank than the magnesium phosphate crystal. In addition, it has a crystal composition more similar to magnesium phosphate crystals than powdered activated carbon and zeolite, and can grow such that the magnesium phosphate crystals cover the surface of the magnesium ammonium phosphate crystals.

  Such magnesium ammonium phosphate can be purchased in small quantities, purchased from magnesium ammonium phosphate recovery equipment that is actually operating in several domestic sewerage treatment facilities or purchased from fertilizer manufacturers. It can be obtained by purchasing as a reagent.

  In this invention, it is preferable that the filling amount of magnesium ammonium phosphate is 10-50 w / v% with respect to the capacity | capacitance of a crystallization reaction tank. There are two effects of the initial filling condition of the seed crystal on the crystallization reaction tank. One is the size of the place where magnesium phosphate is deposited, and the other is the reaction time in the crystallization reaction tank of phosphate-containing wastewater. These two points are contradictory indicators, and when one is increased, the other is decreased. Therefore, although the point that balances these two important points is set for each target wastewater, the present inventors have determined that the initial filling amount of the magnesium ammonium phosphate crystal is relative to the crystallization reaction tank capacity. It has been found that 10-50 w / v% is suitable. More preferably, it is 30-50 w / v%, Most preferably, it is 30-40 w / v%. Filling with magnesium ammonium phosphate may be performed when the crystallization reaction is started, and once the crystallization reaction is started, no additional addition is required.

  As the magnesium agent, it is necessary to use a magnesium salt having high solubility, and magnesium chloride, magnesium sulfate, magnesium nitrate and the like are suitable. Those having low solubility such as magnesium hydroxide are not suitable because they require a long treatment time. As the magnesium agent, a magnesium salt dissolved in an appropriate concentration is added from the upper part of the crystallization reaction tank. The higher the concentration of magnesium salt added, the lower the cost associated with the addition, but the higher the concentration that precipitates in the piping, the lower the operability, so generally it dissolves at a concentration of about 80% of the solubility at room temperature. And use. Since the reaction molar ratio of magnesium and phosphoric acid is 3: 2, the addition amount of the magnesium agent is preferably 1.5 times or more with respect to the phosphate.

  The reaction pH depends on the phosphate concentration of the target water, but when the phosphate concentration is 10 to 20 mg / L, it is desirable that pH = 10 to 10.5. The use of sodium hydroxide solution is preferred as the alkaline agent, but excessive addition of alkaline agent is not preferred because it promotes the formation of magnesium hydroxide.

In the present invention, while the crystallization reaction is in progress, the inside of the crystallization reaction tank is preferably stirred and in a fluid state. Precipitation of magnesium phosphate can be prevented from occurring locally, and it can be stably deposited on the charged seed crystal. For this purpose, for example, air may be supplied from the bottom of the crystallization reaction tank. An appropriate amount of air is 30 to 100 m ³ / m ² .

  Magnesium phosphate produced in the crystallization reaction tank is withdrawn in a slurry form together with water from the reaction tank and separated and recovered by a screen. As the screen used here, a general-purpose flat plate-type wedge wire screen or drum-type wedge wire screen is suitable.

  Hereinafter, the present invention will be further described with reference to the drawings. FIG. 1 is a schematic view of an example of an apparatus that can suitably perform the processing method of the present invention. 1 is a crystallization reaction tank, 2 is a raw water supply pipe for supplying water to be treated to the crystallization reaction tank, 3 is a magnesium supply pipe for supplying a magnesium solution, and 4 is a pH sensor for measuring the pH in the crystallization reaction tank. is there. Reference numeral 5 denotes an alkali supply pipe for supplying an alkaline solution when the pH value in the crystallization reaction tank measured by the pH sensor 4 is lower than a predetermined pH value. 6 is an air blowing pipe for stirring the inside of the crystallization reaction tank, and 7 is an outflow pipe for treated water after the crystallization reaction. 8 is an inlet for introducing seed crystals into the crystallization reaction tank.

  When wastewater containing phosphate is treated in the apparatus of FIG. 1, first, water to be treated is supplied to the crystallization reaction tank 1. While blowing air from the air blowing pipe 6 and stirring and mixing the inside of the tank, a predetermined amount of seed crystal is introduced. Thereafter, the magnesium salt is supplied while adjusting the pH in the tank, and the crystallization reaction is started. After a predetermined reaction time, the water to be treated is continuously supplied. Once the crystallization reaction is started in this way, the magnesium phosphate crystals will grow sequentially, so there is no need to add additional magnesium ammonium phosphate, but crystallization is required for maintenance of the crystallization reactor. When the reaction vessel is emptied, the above procedure may be performed again.

  As the structure of the crystallization reaction tank, a conventionally used structure having a crystallization reaction part and a precipitation part can be preferably used (for example, JP-A-8-24875 and JP-A-8-192168). ).

  The wastewater containing phosphate was treated using the apparatus shown in FIG. The reaction part volume of the crystallization reaction tank 1 is 7L. The drainage flow rate was 340 L / day. The properties of the phosphate-containing wastewater used are as follows.

Water temperature …… 18 ℃, pH …… 5.2
TP …… 53 ～ 61mg / L
PO ₄ -P …… 53-60mg / L
Mg …… 100mg / L
A seed crystal having a particle size of 0.5 to 0.83 mm was used, and the filling rate was 15% with respect to the volume of the crystallization reaction tank. The pH value in the crystallization reaction tank was set to 10 with 0.1 mol / L sodium hydroxide. The magnesium agent was added at 100 mg / L with magnesium chloride.
FIG. 3 shows the phosphorus concentration in the treated water when treated for one week.

  FIG. 4 shows the treatment results when no seed crystal is added as a comparative example and when powdered activated carbon is added as a seed crystal. The particle diameter of the powdered activated carbon is 0.3 to 0.5 mm, and the filling rate is 10% with respect to the crystallization reaction tank capacity. The pH value in the crystallization reaction tank was set to 9 as in the example. Compared with the case where magnesium ammonium phosphate particles were used as seed crystals, the quality of the treated water was unstable, and good treated water could not be obtained even after one week.

It is the schematic which shows an example of the apparatus with which the processing method of this invention is used suitably. 1 is a schematic diagram of an apparatus used in Example 1. FIG. It is a figure which shows the phosphorus density | concentration of the treated water when it processes with the processing method of this invention. It is a figure which shows the phosphorus density | concentration of the treated water when processed by the conventional method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Crystallization reaction tank 2 Raw water supply pipe 3 Magnesium supply pipe 4 pH sensor 5 Alkali supply pipe 6 Air blowing pipe 7 Treated water outflow pipe 8 Seed crystal inlet

Claims (3)

Crystallization reaction for precipitating magnesium phosphate crystals in a method of removing phosphate from waste water containing phosphate as magnesium phosphate (Mg ₃ (PO ₄ ) ₂ .xH ₂ 0) in the presence of magnesium ions The tank is filled with magnesium ammonium phosphate (MgNH ₄ PO ₄ .6H ₂ 0) as a seed crystal, and magnesium phosphate crystals are precipitated and recovered by adding a magnesium agent and an alkali agent, and collecting them. A method for treating wastewater containing phosphate. The method for treating waste water containing phosphate according to claim 1, wherein the magnesium ammonium phosphate is a magnesium ammonium phosphate crystal having a particle diameter of 0.1 to 1 mm. The method for treating wastewater containing phosphate according to claim 1 or 2, wherein a filling amount of magnesium ammonium phosphate is 10 to 50 w / v% with respect to a crystallization reaction tank capacity.

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