JP2946481B2 - Method for removing chemical oxygen demand from wastewater by electrolysis and oxidation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a method for removing chemical oxygen demand (COD) from wastewater by electrolysis and oxidization, and more particularly to a method for adding ferrous ions generated in an electrolysis tank. The present invention relates to a method for oxidizing organic pollutants using hydrogen peroxide to remove chemical oxygen demand in wastewater.

[0002]

BACKGROUND OF THE INVENTION In order to meet stringent environmental protection regulations, effluents discharged from factories are required to reduce the chemical oxygen demand to a significant extent. To reduce the chemical oxygen demand of wastewater, the method known as Fenton's method has been widely used as a viable method. According to the Fenton's method, hydrogen peroxide and ferrous ions are added to wastewater, so that organic pollutants contained in the wastewater are free hydroxyl groups (OH) generated by the reaction between hydrogen peroxide and ferrous ions. ).

[0003]

However, in actual use, Fenton's method is not satisfactory, and its disadvantages are summarized as follows. 1. For example, it is necessary to add chemical reactants such as hydrogen peroxide, ferrous ions, acids and alkalis, and the implementation of this method is costly. 2. The addition of ferrous ions to the wastewater produces a significant amount of ferric hydroxide (Fe (OH) ₃ ) sludge, which must be further processed, and The environment is increasingly polluted because of

[0004] It is therefore an object of the present invention to remove chemical oxygen demand from wastewater, which can significantly reduce processing costs and the production of iron hydroxide sludge.
It is to provide a method .

[0005]

In order to achieve the above object, the present invention provides a first pH adjusting step of supplying wastewater to a pH adjusting tank and adjusting the pH value of the wastewater to 2 to 6.
The discharged wastewater from the pH adjustment tank is introduced into an electrolysis tank including an anode formed of an iron metal material and a cathode formed of a metal material selected from iron, stainless steel, nickel, zinc, and lead. An oxidizing agent adding step of adding a predetermined amount of hydrogen peroxide; and holding a discharge water discharged from the pH adjusting tank in the electrolytic tank for a predetermined residence time to obtain a current density of 50 to 500 A / m 2 and 3
An electrolysis step of electrolyzing and oxidizing at a voltage of １５15 V, a stabilization step of introducing discharge water discharged from the electrolysis tank into a stabilization tank, and holding the discharge water for a predetermined residence time; Adjusting the pH value of the effluent discharged from the tank to 6 to 9 to form a second pH for forming an iron hydroxide precipitate;
The present invention proposes a method for removing chemical oxygen demand from wastewater by electrolysis and oxidation, which comprises an adjustment step and a precipitate separation step of separating the iron hydroxide precipitate from wastewater in the second pH adjustment step.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, first, first P
The pH value of the wastewater is adjusted to 2 to 6 in the pH adjusting tank in the H adjusting step.
Adjust to If the pH value is less than 2, the elution of iron ions at the anode in the electrolysis step becomes unstable, and if it exceeds 6, the reducibility of iron ions at the cathode deteriorates. It becomes a problem.

[0007] The electrolysis tank includes an anode formed of an iron metal material and a cathode formed of a metal material selected from iron, stainless steel, nickel, zinc and lead. A predetermined amount of hydrogen peroxide is added. Further, the electrolytic treatment is performed by retaining the discharged wastewater for a predetermined time. That is, by performing electrolysis using the discharged wastewater as an electrolytic solution, ferrous ions are generated from the anode. The hydrogen peroxide and the ferrous ions generated by the electrolysis oxidize the organic matter in the effluent, thereby removing the chemical oxygen demand of the effluent and removing the second oxygen generated by the oxidation. The iron ions are reduced to ferrous ions at the cathode, and are again used for oxidizing organic substances.

[0008] Since only the reducing action of ferric ions is carried out at the cathode in the electrolytic tank, the consumption of the material does not matter, and in addition to the iron metal material (iron steel material), nickel, zinc, lead or the like is used. A readily available metal material can be selected and used. The residence time of the discharged wastewater in this electrolysis step is preferably 10 to 60 minutes, and if it is less than 10 minutes, the oxidation reaction of organic substances may be insufficient. Will be limited.

In the present invention, all the reactions of the reduction reaction occurring at the cathode of the electrolysis tank and the oxidation reaction occurring at the anode and the oxidation reaction occurring in the aqueous phase can be expressed as follows. Anode: Fe → Fe ²⁺ + 2e ⁻ (1) H ₂ O → 2H ⁺⁺ ／ O ₂ + 2e ⁻ (2) Organic matter → CO ₂ + H ₂ O (Organic matter is directly oxidized at the anode) (3) Cathode: H _{2 ^{O + e - → 1 / 2H}} 2 + OH - (4) Fe 3+ + e - → Fe 2+ (5) aqueous ^{_{phase: H 2 O 2 + Fe 2+}} + organics → _{H 2} O + CO 2 (or organic ^{acid) + Fe} 3+ ⁽⁶⁾ anode surface : Fe + 2Fe ³⁺ → 3Fe ²⁺ (7)

That is, at the anode, the iron metal of the anode is electrolyzed to ferrous ions (Fe ²⁺ ) by the reaction (1), and at the cathode, the second metal formed in the aqueous solution phase by the reaction (6) is reacted. The iron ion (Fe ³⁺ ) reacts with (5) to form F
It is reduced to e ²⁺ and is preferably recycled. The organic matter in the wastewater is converted into F by the reaction (6) in the aqueous solution phase.
It reacts with e ²⁺ ions and added hydrogen peroxide to be oxidized to produce CO ₂ .

As shown in the reaction (1), ferrous ions (Fe ²⁺ ) are generated at the anode by electrolysis.
There is no need to add a solution containing or forming ²⁺ ions, thus reducing operating costs for wastewater treatment. If waste iron or waste rope is used as the material of the anode, the material cost can be reduced, and the operating cost for performing the wastewater treatment of the present invention can be further reduced. Furthermore, ferric ions (F
e ³⁺ ), as shown in reaction (5),
Since two Fe ³⁺ ions are reduced to ²⁺ ions, when one Fe ²⁺ ion is generated at the anode, two Fe ³⁺ ions are converted to Fe ^{2+ ions.}
Reduced to ions. As a result, the Fe ³⁺ ions on the anode surface generated in the reaction (6) are reduced to Fe ²⁺ ions at the cathode and reused. Further, as shown in reaction (7), since the ^{Fe 3+} ions are reduced to ^{Fe 2+} ions by autooxidation and reduction reactions of Fe and ^{Fe 3+} ions,
Can reduce the amount of Fe ^{3+, Fe 3+} and 3 (OH) ^- generation with less hydroxide by reaction of iron (Fe _{(OH) 3)} and. Therefore, the consumption of iron metal material of the anode, the consumption of electricity for electrolysis and the amount of sludge generated can be significantly reduced by the method of the present invention.

Further, hydrogen peroxide is directly added to the tank for electrolysis in order to react with ferrous ions (Fe ²⁺ ) generated by electrolysis and organic substances in the wastewater, and the hydrogen peroxide is obtained by the oxidation reaction of the organic substances. Since the ferric ion (Fe ³⁺ ) is directly reduced to Fe ²⁺ ion at the cathode, the oxidizing efficiency of hydrogen peroxide is improved, and the amount of hydrogen peroxide added to the wastewater and the operating cost are reduced. . In addition, as in the reaction (3), a part of the organic substances contained in the wastewater can be removed by direct oxidation of the organic substances at the anode.

[0013] A stabilization step can be inserted between the electrolysis step of the present invention and the second pH adjustment step. In this stabilization step, discharged water discharged from the tank for electrolysis is introduced into the stabilization tank, and the discharged water is retained preferably for 10 to 60 minutes. In this stabilization step, unreacted hydrogen peroxide in the electrolysis step By further oxidizing organic pollutants with ferrous ions, the chemical oxygen demand in the wastewater can be reduced. If the retention time is less than 10 minutes, the oxidation of unreacted organic matter is insufficient, and if it exceeds 60 minutes, the working efficiency is reduced.

In order to complete the electrolysis and oxidation reaction in the electrolysis tank, a part of the wastewater flowing into the stabilization tank is recirculated to the pH adjustment tank in the first pH adjustment step, so that unreacted H ₂ O ₂ And the effective use of the cathode generated Fe ²⁺ can be achieved. The ratio of recirculated effluent to this effluent is preferably in the range of 0.5 to 10. If the recirculation ratio is less than 0.5, the circulation effect is small, and if it exceeds 10, the effect is not changed much and power consumption becomes remarkable.

In the second pH adjusting step, an alkaline solution is added to adjust the pH to 6-9. When the pH value falls below 6, the formation of ferric hydroxide (Fe (OH) ₃ )
^If the removal efficiency of ³⁺ becomes poor and exceeds 9, the formation of precipitates becomes unnecessarily large, and ferrous hydroxide (Fe
(OH) ₂ ) is undesirably mixed. Further, in the precipitate forming step, in order to separate these iron hydroxides generated in the second pH adjusting step, it is preferable to add a polymer coagulant to coagulate the precipitates of the iron hydroxide to form flocs. Form. Agglomerated flocs can be easily removed by sedimentation or flotation. When the flotation method is used for separation, a polymer flocculant can be added via a pipeline without using a flocculation tank.

According to the invention, the electrolysis has a current density of 50
５００500 A / m 2 and a voltage of 3 to 15 V are preferable.
g of hydrogen peroxide is added to the tank for electrolysis. When the current density is less than 50 A / m ² and the voltage is less than 3 V, the current efficiency is reduced. When the current density is more than 500 A / m ² and the voltage is more than 15 V, the oxidation reaction of organic substances becomes insufficient. On the other hand, if the addition ratio of hydrogen peroxide to the wastewater is less than 50 mg, the oxidation reaction of organic substances is insufficient, and if it exceeds 500 mg, the effect does not change much, waste of material is large, and the working environment is deteriorated.

Referring to FIG. 1, a preferred embodiment of an apparatus for implementing the method of the present invention will be described. Wastewater from a petrochemical industry, a chemical factory, a paper mill, a rubber factory, a dyeing factory, or the like is introduced into the first pH adjustment tank 1 as treated wastewater. Acid solution or alkali solution is pumped to the first pH by pump 11.
It is added to the regulating tank 1 and the pH value of the waste water is adjusted to meet the requirements of the subsequent electrolysis and oxidation reactions. First p
The H adjustment tank 1 is provided with a stirrer 13 and a pH meter 12.

The pH-adjusted waste water is then introduced into the electrolytic tank 2 by a pipeline. The electrolysis tank 2 includes a cathode 8 and an anode 9, and a stable and sufficient DC current is supplied to the tank 8 from a power supply 7. The cathode 8 is made of iron,
The anode 9 is made of an iron metal material such as stainless steel, nickel, zinc, or lead, and the anode 9 is made of an iron metal material, preferably waste iron or waste steel. Hydrogen peroxide (H ₂ O ₂ ) is added to the tank for electrolysis 2 by the pump 21.

Next, the discharged wastewater from the electrolysis tank 2 is introduced into the stabilization tank 3, and is retained in the stabilization tank 3 for a predetermined time so that unreacted hydrogen peroxide from the electrolysis tank 2 and the first The iron ions are further reacted with the organic matter in the effluent. The stabilizing tank 3 is also provided with a stirring device 31. In addition, a part of the drainage discharged from the stabilization tank 3 is recirculated to the first pH adjustment tank 1 by the pump 32.

The discharged wastewater from the stabilizing tank 3 into which the recirculated wastewater is diverted is introduced into the second pH adjusting tank 4 where the alkaline liquid is added by the pump 41. The pH value of the discharged wastewater is adjusted. After the alkali solution is added, a precipitate of ferric hydroxide (Fe (OH) ₃ ) is formed in the second pH adjustment tank 4. The second pH adjusting tank 4 is also provided with a stirring device 42 and a pH meter 43.

Next, the waste water discharged from the second pH adjusting tank 4 is continuously introduced into the coagulation tank 5, and the coagulation tank 5 is also provided with a stirrer 52. The wastewater in which the iron hydroxide particles having a small particle size are suspended is slowly stirred in the coagulation tank 5 and at the same time, the polymer coagulant is pumped.
And flocculate the iron hydroxide particles to obtain flocs. The effluent containing iron hydroxide flocs from the flocculation tank 5 is introduced into the settling tank 6 via a pipeline to separate the sludge layer from the effluent with reduced chemical oxygen demand. That is, this sludge is extracted from the tank bottom by the pump and discarded as waste.

[0022]

EXAMPLE In this example, the effect of the method of the present invention on the removal of the chemical oxygen demand by treating the wastewater collected from petrochemical industry, dyeing factory, chemical industry factory, rubber factory, paper mill and paper product factory. Tested. These wastewaters were treated by the apparatus shown in the flow chart of FIG. 1, and the conditions and results are shown in Table 1 below. The sign of drainage in the table indicates the drainage from the following collection points. A Petrochemical industry area B Dyeing factory C Paper product factory D Chemical industry factory (1) E Chemical industry factory (2) F Paper mill G Rubber factory

[0023]

[Table 1]

[Table 1]

From the results in Table 1 , according to the method of the present invention,
When the chemical oxygen demand (COD) of the effluent is less than 200 mg / l, and more than that, the removal rate exceeds 60%, and the chemical oxygen demand of the final effluent is reduced by at least Taiwan. It is evident that the COD emission standard can be reduced to 100 mg / l or less, which is the standard for COD emission. Further, the amount of sludge generated by this treatment was remarkably reduced to 1/3 to 1/2 that of the conventional method in which the same wastewater was oxidized with a potassium peroxide oxidizing agent.

The present invention can be better understood with reference to the above preferred embodiments, the above examples and the accompanying drawings. Further, the above-mentioned embodiment is to explain the present invention more completely, and does not limit the scope of the invention since various changes and modifications can be easily made by those skilled in the art.

[0025]

As is apparent from the above description, according to the present invention, the chemical oxygen demand can be effectively removed with a small amount of the oxidizing agent added, and the amount of sludge generated is small.
There is an effect that the processing cost can be reduced. From the electrolysis process
By recirculating the discharged wastewater to the first pH adjustment step, preferably at a ratio of 0.5 to 10, further in the electrolysis step and the stabilization step, by retaining the discharged wastewater for 10 to 60 minutes, Effective utilization of unreacted hydrogen peroxide and ferrous ions in the electrolysis step can be achieved, and an effect is obtained that work management in the electrolysis step becomes easy. In the electrolytic process, the current density in the range of 50~500A / m ^2, since the hold voltage in the range of 3～15V, successfully to produce a ferrous ion, efficient use of hydrogen peroxide It has the effect that it can be achieved. In addition, organic substances are directly
It has a remarkable effect of becoming

[Brief description of the drawings]

FIG. 1 is a flow chart of an apparatus for removing a chemical oxygen demand from wastewater according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 First pH adjustment tank 2 Electrolysis tank 3 Stabilization tank 4 Second pH adjustment tank 5 Coagulation tank 6 Sedimentation tank 7 Power supply 8 Cathode 9 Anode 11, 21, 32, 41, 51 Pump 12, 43 pH meter 13, 31 , 42,52 Stirring device

Continuation of the front page (56) References JP-A-4-349997 (JP, A) JP-A-6-106173 (JP, A) JP-A-6-182362 (JP, A) JP-A-7-241574 (JP) , A) Japanese Patent Publication No. 3-31120 (JP, B2) Japanese Patent Publication No. 4-52916 (JP, B2) (58) Field surveyed (Int. Cl. ⁶ , DB name) C02F 1/72

Claims (1)

(57) [Claims] 1. A first pH adjustment step of supplying wastewater to a pH adjustment tank and adjusting the pH value of the wastewater to 2 to 6, and a discharge water discharged from the pH adjustment tank is formed of a ferrous metal material. An oxidizing agent adding step of introducing an anode and iron, stainless steel, nickel, zinc and lead into an electrolytic tank having a cathode formed of a metal material selected from among zinc and lead, and adding a predetermined amount of hydrogen peroxide; In the tank for electrolysis, the discharged water discharged from the pH adjusting tank is held for a predetermined residence time to be 50 to 50.
An electrolysis step of electrolyzing and oxidizing at a current density of 0 A / m 2 and a voltage of 3 to 15 V; introducing discharge water discharged from the electrolysis tank into a stabilization tank, and holding the discharge water for a predetermined residence time A stabilizing step, a second pH adjusting step of adjusting the pH value of the discharged effluent from the stabilizing tank to 6 to 9 to form an iron hydroxide precipitate, and a discharged effluent of the second pH adjusting step. And separating the iron hydroxide precipitate from the wastewater. 2. A method for removing chemical oxygen demand from wastewater by electrolysis and oxidation.