NL1002995C2 - Waste liquor treatment to reduce chemical oxygen demand - Google Patents

Abstract

Removal of the chemical oxygen demand (COD) of waste liquor by electrolysis and oxidation comprises (a) passing the waste liquor into a vessel where it is adjusted to pH 2-6; (b) passing the discharge from this vessel to an electrolysis/oxidation vessel with a steel or iron anode and an iron, stainless steel, nickel, zinc or lead cathode and adding a suitable amount of hydrogen peroxide; (c) carrying out electrolysis and oxidation for a first residence time; (d) adjusting the discharge from this vessel to pH 6-9 to precipitate iron hydroxide; and (e) separating the precipitate. Also claimed is the apparatus used, comprising the vessels used in these stages and a unit for separating the precipitate.

Description

4

Method and device for reducing the chemical oxygen consumption of waste water by electrolysis and oxidation

The invention relates to a method and apparatus for reducing the chemical oxygen demand (COD) of waste water by means of electrolysis and oxidation. In particular, it relates to a process using the ferrous ions produced in an electrolysis / oxidation tank and added hydrogen peroxide to oxidize the organic contaminants and reduce the COD of wastewater.

In order to comply with strict environmental protection laws, the COD of discharged wastewater from companies must be significantly reduced. A workable method, known as the Fenton method, is widely used to reduce the COD of wastewater. According to the Fenton method, hydrogen peroxide and ferrous ions are added to the wastewater, so that organic contaminants present in the wastewater by free hydroxyl radicals (OH) / which are produced in the reaction between hydrogen peroxide and the ferrous ion, are oxidized. However, in practical applications, the Fenton process is not satisfactory and the drawbacks thereof are summarized below: 1. The necessity of adding chemical reagents such as hydrogen peroxide, ferrous ions, acids and bases makes the operation expensive according to this process.

2. When ferrous ions are added to wastewater, a significant amount of iron hydroxide (Fe (OH) j) sludge is created. This necessitates further treatment of the sludge. The treatment again pollutes the environment.

Accordingly, it is an object of the present invention to provide a method for reducing the COD of wastewater, which can significantly reduce the operating costs and production of iron hydroxide sludge.

1002995 2

The object of the invention is achieved by making direct use of ferrous ions produced by electrolysis and oxidation of the organic pollutants in waste water.

More specifically, the method comprises (a) supplying wastewater to a tank for adjusting the pH, and adjusting the pH value of the wastewater to 2-6; (b) introducing the effluent from the tank for adjusting the pH into an electrolysis / oxidation tank 10 comprising an anode and a cathode and adding an appropriate amount of hydrogen peroxide to the electrolysis / oxidation tank, the anode being steel or iron and the cathode comprises a metal selected from iron, stainless steel, nickel, zinc and lead; (c) electrolyzing and oxidizing the effluent from the tank to adjust the pH and hold the effluent for an appropriate residence time; (d) adjusting the pH of the electrolysis / oxidation tank effluent to 6-9 to form an iron hydroxide precipitate; and (e) separating the iron hydroxide precipitate.

The present invention also includes an apparatus for performing the above method. The device comprises a first tank for adjusting the pH to which the waste water can be supplied and in which the pH-25 value of the waste water can be adjusted to 2-6; (b) an electrolysis / oxidation tank into which the effluent from the first pH adjustment tank can be introduced and held for an initial residence time, and into which an appropriate amount of hydrogen peroxide can be added, with an anode and a cathode, the anode comprising steel or iron and the cathode comprising a metal selected from iron, stainless steel, nickel, zinc and lead; (c) a pH adjustment tank to which the effluent from the electrolysis / oxidation tank can be supplied and in which it can be adjusted to pH 6-9 to form an iron hydroxide precipitate; and (d) means for separating the iron hydroxide precipitate.

The invention can be better understood with reference to the preferred embodiments, examples and drawing 1002995 3 in which the figure shows a preferred embodiment of the method and the device according to the invention.

According to the present invention, the reactions occurring in the electrolysis / oxidation tank including the reduction / oxidation reactions that take place on the cathode / anode and the oxidation reaction that takes place in the aqueous solution can be represented as follows:

Anode:

Fe - * Fe2 + + 2e '(1) 10 H20 - »2H + + 1/2 02 + 2e' (2)

Organic matter - * CO2 + H20 (organic matter is oxidized directly on the anode) (3)

Cathode: H20 + e * - 1/2 H2 + OH '(4) 15 Fe3 + + e' - * Fe2 * (5)

Aqueous solution: H2o2 + Fe2 * + organic matter - * H20 + C02 (or organic acids) + Fe3 + (6)

Anode surface: 20 Fe + 2Fe3 + - 3Fe2 + (7)

According to reaction (1), the iron metal of the anode is electrolyzed at the anode to Fe3 * 1 ions and at the cathode, according to reaction (5), Fe3 * ions produced in reaction (6) are reduced under recirculation. In the aqueous solution, according to reaction (6), organic matter in waste water is oxidized on reaction with Fe2 * ions and added hydrogen peroxide. At the anode surface, an auto-oxidation-reduction reaction of iron metal occurs according to reaction (7).

According to reaction (1), ferrous ions are produced at the anode and therefore no added ferrous ion-containing or producing solutions are required and therefore the operating costs for the treatment can be reduced. The anode material may also be scrap iron or scrap steel to further reduce the operating costs of performing the wastewater treatment of the invention. In addition, Fe3 * ions are reduced to Fe2 * ions at the cathode, as represented by reaction (5), and when an Fe2 + ion is produced at anode 1002995 4, two Fe3 + ions are reduced to Fe2 + ions so that the Fe3 + ions produced in reaction (6) are reduced to Fe2 + ions for reuse. Fe3 + ions are also reduced to Fe2 + ions by the auto oxidation-reduction reaction of Fe and Fe3 + ions, as indicated by reaction equation (7). Therefore, the consumption of iron as the anode material, the electricity for electrolysis and the amount of sludge produced can be significantly reduced by the method of the invention.

Furthermore, the oxidation efficiency of hydrogen peroxide is improved, since the hydrogen peroxide is added directly to the electrolysis / oxidation tank to react with ferrous ions and organic matter produced in wastewater produced by electrolysis, and the Fe3 + produced in this reaction. ions are reduced to ferrous ions directly on the cathode, and the amount of hydrogen peroxide to be added to wastewater and operating costs are reduced. Note that the oxidation that takes place at the anode can also remove some of the organic matter present in the waste water.

According to the present invention, a stabilization step between step (c) and step (d) can be included. This stabilizing step includes introducing a drain stream from the electrolysis / oxidation tank to a stabilizing tank and holding the drain stream therein for 10-60 minutes to allow for a further reaction between unreacted hydrogen peroxide and the ferrous ions. As a result, the organic pollutants can be further oxidized and the COD of wastewater can be reduced.

To separate the iron hydroxide produced, polymers are added to flocculate the iron hydroxide precipitate. The flocculated flocks can be easily removed using sedimentation or flotation techniques. If a separation flotation technique is used, the polymers can be added via lines without using a flocculation tank.

1002995 5

To complete the electrolysis / oxidation reactions, some of the waste water flowing into the stabilization tank is returned to the tank to adjust the pH and the ratio of recycled waste water to waste water is preferably 0.5 to 10.

According to the present invention, the electrolysis is preferably performed at a current density of 100-500 A / m2 and a voltage of 3-15 V, and 50-500 mg of hydrogen peroxide per liter of waste water is added to the electrolysis / oxidation tank.

Reference is now made to the figure where a preferred embodiment of the device for carrying out the method according to the invention is shown. Wastewater, such as wastewater from petrochemical companies, chemical companies, paper mills, rubber mills or paint mills, is introduced into a first tank 1 for adjusting the pH. Acids or bases are added to the pH adjustment tank 1 using a pump 11 to adjust the pH of the wastewater to pH 2-6 to meet the requirements for subsequent electrolysis / oxy- data responses. The first tank 1 for adjusting the pH is equipped with a stirrer 13 and a pH meter 12.

The wastewater whose pH has been adjusted is then introduced via a pipe into an electrolysis / oxy-25 data tank 2. The electrolysis / oxidation tank 2 has a cathode 8 and an anode 9 to which sufficient and stable direct current is supplied from a power source 7. Note that the cathode 8 is made of iron, nickel, stainless steel, zinc or lead, and the anode 9 is made of scrap iron or scrap steel 30. Hydrogen peroxide (HjOj) is added to the electrolysis / oxidation tank 2 using a pump 21.

The waste water discharged from the electrolysis / oxidation tank 2 is then placed in a stabilization tank 3 in which the waste water is kept for 10-60 minutes to allow a further reaction of unreacted hydrogen peroxide and the ferrous ions of the electrolysis / oxidation tank 2 . The stabilization tank 3 is also equipped with a stirrer 31. Note that part of the waste water discharged from the stabilization tank 3 is returned by a pump 32 1002995 6 to the first tank 1 for adjusting the pH value.

Another part of the waste water discharged from the stabilization tank 3 is introduced into a second pH adjustment tank 4 to which base 41 is added using a pump to adjust the pH value of the discharged waste stream to pH 6-9. bring. After base is added, a Fe (OH)} precipitate forms in the second tank 4 to adjust the pH. The second tank 4 for adjusting the pH is also equipped with a stirrer 42 and a pH meter 43.

The waste water discharged from the second tank 4 for adjusting the pH is then continuously introduced into a flocculation tank 5, which is also equipped with a stirrer 52. The waste water containing small size suspended iron hydroxide particles is slowly introduced into the flocculation tank 5. stirred, while polymers are added using a pump 51 to flocculate the iron hydroxide particles.

The effluent water containing iron hydroxide flakes is then introduced via pipeline into a settling tank 6 and a layer of sludge is separated from a reduced COD effluent. The sludge is removed as waste via a pump.

The following examples serve to further illustrate the invention without limiting its scope, since numerous modifications and variations will be apparent to those skilled in the art.

EXAMPLE

In this example, wastewater from petrochemical, paint, chemical, rubber, paper and paper product plants was treated to investigate the effect of the method of the invention on the COD. The wastewater was treated with a device as shown in Figure 1 and the conditions and results are summarized in Table 1 below.

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H H --4 0 «TlA r-4 4J φ - -t k - ® · 4J C B -4 A Vl φ J <- -

Φ_Φ CA —4 Ό Η V φ C φ Η CN C

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* Η φ e>, s e8 5 vu> Φ 0 -> <0 -4 C 4J Q φ JQ 4-1 «44 * ΗΛΛ 0 £ _ -4 Β Ο —4 Α Τ-ΙΤΊ e

Smm Γ · Ij Ό Φ Β ü JQ -44-4-4 φ Φ Φ> -η® φ -η 4J 0 φ C 4-> Vi Vi J * «ti £ Έ - - Η Β> -4 -4 0 φ ΟΦΧΐΐ Φ Φ «» θ '<4->> ι k 4-> 3 k O' 3 φ φ —4 u Ε Φ - θ 'Ή Η φ Ί4 4-J - A. φ Ό Δ Λ Vi

5 £ Ο Ο C τ-ι Ο O-r-iC Β Η 4-1 t-Ηθ XI

ϊ A 0> Ê -4 -4 Vl> -4 A k C C k k JS f A

H>> 4 Φ 0 CH 4J O 'rl> Φ O' Φ I 0 ft U Ο Ή A 4J O '- 0 CJ3J <3 J3 OE 0 A Vl Β B Vi> Q Β Φ Vl 14 A Vl® Vl Vl Q> Vl him φ-Ι-4 a iMp c 0Φ44 ft φ η φ ® p <£ e φ σ, ΙΠ -4 ε e —4

Aj o h t * 0 cn cn ^ φ h ^ O ιί — 4 cv e k ft φ φ ft te-, 11 = .- 4-. I. I., i -.1 | . | || 4 Φ a J- j; a

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1002995 8

From the results of Table 1 it can be deduced that using the method and apparatus according to the invention the CODs of the final effluent streams were reduced below 100 mg / l regardless of whether the wastewater had a COD of more or less than 200 mg / 1.

1002995

Claims (13)

A method for reducing the chemical oxygen demand of wastewater by electrolysis and oxidation, comprising the following steps: (a) adding wastewater to a tank for adjusting the pH to the pH value of the wastewater to 2 -6 to bring; (b) introducing a drain stream from the tank for adjusting the pH into an electrolysis / oxidation tank comprising an anode and a cathode and adding an appropriate amount of hydrogen peroxide to the electrolysis / oxidation tank, the anode being steel or iron and the cathode comprises a metal selected from iron, stainless steel, nickel, zinc and lead; (c) electrolyzing and oxidizing the effluent from the tank to adjust the pH and retaining the effluent for a first residence time; (d) adjusting the pH of an electrolysis / oxidation tank effluent to 6-9 to form an iron hydroxide precipitate; (e) separating the iron hydroxide precipitate. 2. A method according to claim 1, further comprising a stabilization step between step (c) and step (d), wherein said stabilization step introduces an effluent from the electrolysis / oxidation tank into a stabilization tank and retains it there the effluent during a second residence time. 3. The method of claim 1 or 2, wherein said step (e) comprises adding polymers to flocculate the iron hydroxide precipitate and removing the flakes. 4. A method according to any one of the preceding claims, wherein a part of the waste water flowing in the stabilization tank is recycled to the tank for adjusting the pH of step (a). 1002995 The method of claim 4, wherein the ratio of the recycled waste water to the waste water ranges from 0.5 to 10. A method according to any preceding claim, wherein the first residence time in step (c) is 10-60 minutes. The method of any one of claims 2-6, wherein the second residence time in the stabilization tank is 10-60 minutes. 8. Method according to any one of the preceding claims, wherein in step (c) an electric current density of 50- 500 A / m2 and a voltage of 3-15 V are applied. A method according to any one of the preceding claims, wherein in step (b) the amount of hydrogen peroxide added is 50-500 mg / liter waste water. 10. Device for reducing the chemical oxygen demand of waste water by electrolysis and oxidation, comprising: (a) a first tank (1) for adjusting the pH to which the waste water can be supplied and in which the pH value of the wastewater can be adjusted up to 2-6; (b) an electrolysis / oxidation tank (2) into which the effluent from the first tank (1) for adjusting the pH can be introduced and held for an initial residence time, and an appropriate amount of hydrogen peroxide can be added, with a anode (9) and a cathode (8), the anode (9) comprising steel or iron and the cathode (8) comprising a metal selected from iron, stainless steel, nickel, zinc and lead; (c) a second tank (4) for adjusting the pH to which an electrolysis / oxidation tank effluent can be applied and adjusted to pH 6-9 to form an iron hydroxide precipitate; (d) means for separating the iron hydroxide precipitate. The method according to claim 10, further comprising a stabilization tank (3) disposed between the electrolysis / oxidation tank (2) and the second tank (4) for adjusting the pH, for retaining from the electrolysis lysis. oxidation tank discharged waste water from the electrolysis / oxidation tank (2) for a second residence time. 12. Device according to claim 10 or 11, wherein said iron hydroxide precipitating agent comprises a flocculation tank (5) for flocculating the iron hydroxide precipitate and a sedimentation tank (6) for removing the flakes. Apparatus according to any one of claims 10-12, further comprising means for returning a portion of the waste water flowing into the stabilizing tank (3) to the first tank (1) for adjusting the pH. 1002995