JPH10244279A - Method for removing nitrogen and phosphorus in water to be treated using iron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable simultaneous nitrogen treatment and phosphorus removal at a low expense by a method in which a nitrate ion and a nitrite ion contained in raw water are contacted with iron to be reduced into nitrogen gas and an ammonium ion. SOLUTION: Raw water contains nitrogen oxide in the forms of nitric acid, nitrous acid, etc., and phosphorus. The nitrogen oxide exists as ions or in the form which can be converted into ions, and phosphorus exists as ions such as a phosphate ion. By contacting the raw water with iron, the removal of nitrogen and phosphorus is attempted. The contact is preferably carried out at pH 7.0 or less. When the surface of iron is oxidized remarkably, washing with acid and surface pretreatment such as polishing are recommended. The iron can be in the shape of powder, particles, wool, foil, sheets, chips, etc., insofar as an enough surface area is obtained to attain an intended treatment speed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for removing nitrate nitrogen, nitrite nitrogen and phosphorus contained in water to be treated.

[0002]

2. Description of the Related Art In order to cope with the eutrophication phenomenon that is becoming more serious in closed water bodies, there is an increasing need to remove nitrogen and phosphorus from wastewater. Has been developed. The pH of this wastewater is 3-10
It is about. When attention is paid to nitrogen oxides represented by nitric acid and nitrous acid among the nitrogen compounds, the conventional methods for removing these can be classified as follows.

1-a. Biological nitrogen method A biological method using so-called nitrate respiration, in which a denitrifying bacterium oxidizes an organic substance which is a hydrogen donor using molecular oxygen in nitrate ions under anaerobic conditions. 1-b. Physicochemical nitrogen removal method A physicochemical method such as an ion exchange treatment using an anion exchange resin or a catalytic reduction treatment using a hydrogenation catalyst represented by palladium on palladium.

2-a. Biological phosphorus removal method Biological treatment that removes phosphorus by taking up phosphorus into bacterial cells and sludge using the function of excessive intake of phosphorus by microorganisms and separating treated water from bacterial cells and sludge. Method. 2-b. Physicochemical phosphorus removal method Phosphorus compound mainly composed of orthophosphoric acid in the water to be treated is aggregated as a hardly soluble phosphorus compound by adding an aluminum-based or iron-based coagulant such as sodium nitrate or ferric chloride, The water to be treated is brought into contact with an iron contact filter medium under aerobic conditions to form iron phosphate, which is a hardly soluble salt, by the action of iron ions eluted from the iron contact filter medium. After that, a physicochemical method such as a method of separating the precipitate.

[0005]

1-a is a method for removing biological nitrogen, but this method has the following problems.

[0006] 1. Since an organic substance serving as a hydrogen donor is required, when treating the inorganic treated water, it is necessary to add an organic substance typified by methanol from outside the system,
It costs medicine. 2. Since the biological denitrification reaction rate is low, it is necessary to increase the volume of the reaction tank or take a long reaction time for complete treatment, and the practically applicable nitrogen concentration in the treated water is limited. There is. 3. Since the activity of microorganisms is remarkably reduced in regions other than the neutral region, acidic water to be treated cannot be treated as it is, and it is necessary to adjust the pH of the water to be treated to an optimum range, which requires a high chemical cost. 4. Since it is difficult for organisms to respond and react quickly to sudden changes in load and changes in inflow water quality, the treatment results may not be stable.

[0007] These problems are attributable to the sole use of biological functions.

[0008] 1-b is a method for removing physicochemical nitrogen. The removal by adsorption using an anion exchange resin is not a fundamental denitrification, but a concentrated nitrogen recovered by a regeneration operation of an ion exchange resin. There are problems such as the necessity of secondary treatment of the regeneration waste liquid containing oxides, the necessity of a large amount of acid and other chemicals for the regeneration of the resin, and the high treatment cost. In addition, treatment using a catalyst involves problems such as the need for expensive catalysts, the use of dangerous hydrogen gas, and the need for high-temperature and high-pressure equipment. There is also a problem that the equipment becomes difficult.

2-a is a method for removing biological phosphorus, but this method has the following problems.

[0010] 1. The reaction of excessive intake of phosphorus by microorganisms is greatly affected by the BOD concentration in the water to be treated, the anaerobic degree in the reaction tank, and the like, so that it is difficult to perform a stable treatment. 2. Since there is a limit to the amount of phosphoric acid that can be incorporated into an organism, there is a limit to the amount of phosphorus that can be removed at a given biomass. 3. Since living organisms re-release phosphorus taken in due to changes in ambient oxygen concentration and other conditions, it is necessary to handle excess sludge so that it does not release phosphorus again.Depending on conditions such as the processing capacity of sludge treatment equipment, sludge Phosphorus may return to water systems.

[0011] 2-b is a physicochemical phosphorus removal method, but there is a problem that adding a flocculant is not economical because the cost of the flocculant as a chemical is high. In addition, the method using an iron filter medium requires the use of a mechanism in which a local concentration cell is formed by a combination of an iron material and dissolved oxygen to elute iron ions, so that it is necessary to perform aeration. There is a problem that power equipment for aeration and operation costs are required.

In recent years, a biological nitrogen / phosphorus simultaneous removal method combining a biological nitrogen removal method classified as 1-a and a biological phosphorus removal method classified as 2-a has been developed. However, this method has a problem that the operation management of the processing equipment becomes difficult because complicated operations are required.

The present invention has been made in order to solve the above-mentioned problems, and a physicochemical treatment method not depending on a biological function can perform a phosphorus removal treatment simultaneously with a nitrogen treatment. Another object of the present invention is to provide a method for treating water to be treated which has low operating costs.

[0014]

The present invention attains the above object by the following means.

First, nitrate ions and nitrite ions contained in the water to be treated are reduced to nitrogen gas or ammonium ions by contact with iron.

Second, the contact treatment between the water to be treated and iron is carried out at pH
By performing the treatment under the condition of 7.0 or less, nitrogen and phosphorus in the water to be treated are treated.

The present inventors have assiduously studied and found that nitrogen oxide ions are rapidly reduced to nitrogen gas or ammonium ions by direct reaction with iron, which is a metal having a low redox potential. A method for selecting the form of the reaction product to be converted and controlling the reaction rate by maintaining the water to be treated at a predetermined pH between neutral and acidic. Established.

Further, iron ionized by donating electrons to reduce nitrogen oxide ions reacts with phosphorus coexisting in the water to be treated to form a hardly soluble salt. Can remove phosphorus in the water to be treated, and some of the phosphorus in the water to be treated is adsorbed on iron hydroxide particles resulting from the reaction between iron ions and hydroxide ions, a secondary reaction in this method. Is removed from the water to be treated. In other words, unlike existing methods that supply iron ions for phosphorus removal, iron ions that accompany nitrogen removal operations are used, so this is a very rational and energy-saving method. is there.

One of the major features of the method of the present invention is that the reaction p
By controlling H, the reaction rate can be controlled. Since the rate of reaction between nitrogen oxide ions and iron is extremely low at a pH higher than neutral, a practical processing rate can be obtained at a pH of 7 or less. In the case of wastewater containing nitrate ions discharged from the metal surface washing process, etc., since the wastewater itself is acidic, the chemical cost for acidification is reduced, and this method is a more economical treatment method. .

Another major feature of the method of the present invention is that the reaction product also changes with the pH, and at pH 2, only ammonium ions are formed, and as the pH approaches from the acidic region to the neutral region, the reaction to nitrogen gas is started. It is a point that a desired form of the reaction product can be obtained by controlling the pH as the reaction condition based on the experimental fact that the conversion becomes large.

Utilizing such properties, it is possible to carry out nitrogen removal treatment of water to be treated containing nitrogen oxide ions of various properties and concentrations using metallic iron, and also to remove phosphorus. Can be achieved.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The water to be treated contains nitrogen oxides and phosphorus. The nitrogen oxides are nitrate nitrogen, nitrite nitrogen and the like, and exist in the water to be treated in the form of ions or ions. Phosphorus exists as ions in the water to be treated such as phosphoric acid. These concentrations are not particularly limited.
About 100 mg / l, preferably about 10 to 100 mg / l, nitrite nitrogen is about 1 to 1000, preferably 1 to 1000
About 50 mg / l, phosphoric acid phosphorus is about 0.1 to 100, preferably about 1 to 10 mg / l.

Iron is a metal and can usually be used as it is. However, when the surface is oxidized remarkably, it is better to perform pretreatment such as cleaning with an acid or polishing of the surface.

As for the shape of iron, powdery, granular, wool-like, foil-like, plate-like, or iron shavings such as cutting shavings can be used as long as they can provide a surface area capable of achieving a target processing speed. is there.

The amount of iron supplied is 3 in molar ratio to the amount of nitrogen to be treated.
About 10 to 10 times, preferably about 4 to 5 times is appropriate.

The iron can be used by any means as long as the two are in good contact. For example, by passing the water to be treated into a porous container filled with iron,
Iron may be added to the water to be treated and stirred as shown in the drawing. The supply time of iron may be a continuous supply when the water to be treated is rich and the consumption of iron is remarkable, and may be an intermittent supply if the consumption is small.

The pH of the water to be treated in the removal treatment of the present invention is 7.0 or less, preferably about pH 1.0 to 7.0, more preferably about pH 3 to 5, and particularly preferably pH 3 to 4.
It is about.

Since the pH of the water to be treated increases due to the reduction of the nitrogen oxide ions, the reaction rate decreases. Therefore, the pH of the water to be treated can be adjusted to give the desired product.
Although it is necessary to maintain H, if the concentration of nitrogen oxide ions in the water to be treated is small, it is not necessary to continuously control the pH because the rise in pH is small.

It is preferable to perform stirring to increase the reaction rate between iron and nitrogen oxide. This is particularly effective when iron is in a powder state, a wool form, a foil form, a plate form, a cutting waste, or the like, and easily becomes a suspended state by stirring. On the other hand, when the fluid does not flow much even when agitated in the form of granules, plates, blocks, or the like, it is possible to achieve the object by circulating the water to be treated in a shape in which the voids increase in the state of iron accumulation. .

The iron ions remaining after the removal of nitrate ions, nitrite ions or phosphate ions from the water to be treated are precipitated as iron hydroxide. P of treated water by this method
Regarding H, in order to achieve the normal discharge standard of pH 5.8 to 8.6, alkali is added to adjust the pH to 5.8 to 8.6.
It may be adjusted to 8.6, but if the supply of the acid added to maintain the predetermined pH is stopped,
Since the pH gradually rises to reach the neutral range, it is sufficient to leave the pH until it reaches an appropriate value.

[0031]

EXAMPLE An experiment related to the method of the present invention was conducted using an experimental apparatus as shown in FIG. Using 10 g of iron powder without an oxide film on the surface having a particle size of 80 mesh, a sample water 0.5 containing both 30 mM as an initial concentration of nitrate ion and nitrite ion and 5 mM as an initial concentration of phosphoric acid phosphorus.
Table 1 and Table 2 show the results of reacting L with various pH conditions at a water temperature of 25 ° C.

[0032]

[Table 1]

[0033]

[Table 2]

After the above treatment, the concentration of phosphorus in the supernatant water obtained by standing for 15 minutes was measured by a molybdenum blue (ascorbic acid) absorption spectrophotometer, and no phosphorus was detected.

[0035]

As described above, according to the method of the present invention, nitrogen oxide ions can be reduced by a simple operation of contacting iron and water to be treated under acidic conditions without depending on biological functions. And completely remove the phosphorus component.

While the biological nitrogen removal rate per unit volume of the reaction vessel is generally 0.1 to 0.5 kg · N / m ³ / day, the present processing method has a maximum of 130 kN / m ³ / day.
g ・ N / m ³ / day (when nitrite ions are treated at pH 2), and at least 0.2 kg ・ N / m ³ / day (when nitrate ions are treated at pH 5), a remarkably high processing speed can be obtained. ing. Since the contact area can be further increased by controlling the amount and shape of the iron material to be brought into contact, the method of the present invention can be a significantly more compact process than the biological treatment process, and the nitrogen and
It is possible to achieve a simultaneous phosphorus removal process.

The present method can cope with extremely high concentration of treated water which cannot be practically treated by biological treatment without dilution, and can be treated even under acidic conditions of pH 5 or less, where biological treatment is practically impossible. It is. Regarding the treatment temperature, it has been confirmed by experiments that the higher the temperature, the more desirable the method is, and at a water temperature of 40 ° C. where biological treatment becomes difficult, the treatment speed becomes about twice the value shown in Tables 1 and 2. I have.

In particular, regarding the reduction of nitrite ions, a high conversion rate to nitrogen gas is obtained, so that a high degree of denitrification treatment can be achieved by a single-stage treatment process.

[Brief description of the drawings]

FIG. 1 is a flow sheet of a removal processing apparatus used in an embodiment of the present application.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kei Baba 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor's Bureau Toshiaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Tatsuo Takechi 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (2)

[Claims] 1. A method for removing nitrogen and phosphorus, comprising contacting treated water containing nitrogen oxides and phosphorus with iron. 2. The contact treatment between water to be treated and iron is performed at pH 7.0.
2. The method for removing nitrogen and phosphorus according to claim 1, wherein the method is performed under the following conditions.