JPS6341638B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for treating wastewater containing COD, particularly to an improvement in a chemical oxidation method in which iron salts and hydrogen peroxide (H ₂ O ₂ ) are added. [Prior Art] Conventionally, methods such as a coagulation sedimentation method, a chemical oxidation method, and an adsorption method are known in order to remove persistent COD that cannot be removed even by biological treatment such as activated sludge. In particular, as a method of chemical oxidation, the principle of Fuenton's reagent (a method in which iron salt and H ₂ O ₂ are made to react under acidic conditions), which has long been known as a unique oxidizing agent in the field of organic chemistry, has been applied to water treatment. Considerable attempts have been made to apply it to the removal of non-biodegradable COD components in the field. In addition, in order to reuse the sludge mainly composed of ferric hydroxide produced as a result of this Fuenton treatment and reduce the amount of sludge generated, mineral acid is added to the sludge to dissolve the ferric hydroxide. A method is known in which iron is converted into ferric ions and recycled into raw water. [Problems to be Solved by the Invention] However, the inventor's follow-up experiments have revealed that the method of reusing the sludge produced in the chemical oxidation method causes the following serious problems. Ta. In other words, in the conventional so-called Fenton treatment, in the sludge mainly composed of ferric hydroxide produced as a result of the neutralization step after the oxidation reaction, the COD components that have not undergone oxidative decomposition are adsorbed to the ferric hydroxide flocs. (If this adsorbed organic substance is expressed as Org, and Fe(OH) ₃・Org indicates the state in which Org is adsorbed to ferric hydroxide), acid (H ⁺ ) is added. When ferric hydroxide is dissolved in ionic form, Fe(OH) ₃・Org+3H ⁺ →Fe ³⁺ +Org+3H ₂ O causes Org to be redissolved and transferred to the liquid side, resulting in the acid solution being converted into raw water. It was recognized that if recycled and reused, Org would be added at the same time as Fe ³⁺ , which would eventually lead to a decrease in the COD removal rate. The present invention effectively solves the above problems,
The purpose of this invention is to provide a method in which the reduction in COD removal rate is extremely small even when Fe ³⁺ is recycled. _[ Means for Solving the Problems] _The present invention provides Fe ( In addition to solid-liquid separation of SS mainly composed of OH) ₃ , mineral acid is added to the separated sludge to dissolve Fe(OH) ₃ , and H ₂ O ₂ is added to the solution and stirred to remove undecomposed sludge. This method of treating COD-containing wastewater is characterized in that the COD component is oxidized and decomposed and then added to the COD-containing wastewater. [Function] Next, the function of the present invention will be explained with reference to a drawing showing one embodiment of the present invention.
H ₂ O ₂ 3 and an iron salt 4 such as ferrous sulfate (FeSO ₄ ) are added and stirred for a predetermined period of time under acidic conditions, usually at a pH of 2 to 4. In this case, it is also possible to use H ₂ O ₂ and ozone (O ₃ ) in combination. Note that the stirring time in the first stirring tank 2 varies depending on the raw water COD concentration, water temperature, etc., so the stirring time may be determined by experiment. The reaction liquid 5, which has completed the oxidation reaction in the first stirring tank 2, flows into the second stirring tank 6, and by adding an alkaline agent 7 (a magnesium-based alkaline agent such as magnesium hydroxide or magnesium oxide),
Neutralized to pH 5 to 8, ferric hydroxide (Fe
After precipitating a precipitate mainly consisting of (OH) ₃ ),
If necessary, a polymer flocculant 17 is added, and the water is separated into treated water 9 and sludge 10 in a settling device 8. Conventionally, only caustic soda or slaked lime has been used as the type of alkaline agent 7 used in the neutralization process after oxidation treatment (Fuenton treatment) with H ₂ O ₂ 3 and iron salt 4, but the inventor's experimental results According to the study, it was recognized that the ferric hydroxide produced was extremely bulky, fluffy, and fluffy, and had a serious drawback of poor concentrating properties.In order to solve this drawback, magnesium-based We have found that neutralization by alkaline agents is highly preferred. This is one of the important points of the present invention. Therefore, the sludge 10 has extremely good thickening properties and does not require a thickener unlike the conventional method, so it is introduced into the third stirring tank 11 as it is, and the sludge 10 is dissolved by adding sulfuric acid 12 as a mineral acid. Next, in the fourth stirring tank 14, H ₂ O ₂ 13 as the oxidizing agent is added. The process of dissolving the sludge 10 and adding it to the solution
The oxidative decomposition reaction of organic matter (Org) by the addition of H ₂ O ₂ 13 is also the gist of the present invention, and to explain its function in detail, the sludge 10 converts organic matter that has not been oxidized into ferric hydroxide. Since the sludge 10 is in an adsorbed state, when acid (H ⁺ ) is added to it in the third stirring tank 11, the sludge 10 is dissolved by the reaction Fe(OH) ₃・Org+3H ⁺ →Fe ³⁺ +Org+3H ₂ O. do. Next, H ₂ O ₂ 13 is added in the fourth stirring tank 14 to oxidize and decompose the organic matter (Org) eluted from the sludge 10 using Fe ³⁺ eluted from the sludge 10 by the above reaction. . In this way, using Fe ³⁺ of Fe(OH) ₃ in sludge 10, H ₂ O ₂
The most important point of the present invention is to enhance the oxidizing power of No. 13, and it is a technical idea unprecedented in the prior art. As a result, most of the organic matter contained in the sludge 10 is oxidized and decomposed in the fourth stirring tank 14, and as a result is recycled to the wastewater 1 and added to the returned liquid 15 (recovered Fe ³⁺ -containing liquid). Since the COD component is very low, it cannot be used like the conventional method.
It does not cause accumulation of COD components and decrease in COD removal rate. As a result, the amount of fresh iron salt 4 added can be significantly reduced, and the amount of sludge discharged outside the system is also significantly reduced. Note that if the sludge drain pipe 16 does not drain the sludge at all when the wastewater 1 contains SS components, the SS components will accumulate in the system, so in order to prevent this, a part of the sludge 10 is drained. It was prepared for getting muddy. [Example] Examples of the present invention will be described below. An experiment was conducted using a wastewater treatment plant in accordance with the flow sheet of the drawing cited in the description of the above embodiment, using treated water obtained by treating human waste through a biological nitrification and denitrification process as raw water (COD-containing wastewater 1). (1) Raw water quality

【table】 (2) Fenton oxidation conditions (first stirring tank 2)

[Table] (3) Neutralization conditions (second stirring tank 6) Neutralization was performed to pH 6.0 using magnesium hydroxide. (4) Precipitated sludge 10 concentration 150g/ On the other hand, when NaOH is used as an alkaline agent for neutralization, a gel-like precipitate is formed and the concentration is extremely poor, making it impossible to concentrate the sludge to a concentration of 10g/or more. Nakatsuta. Next, 10 precipitated sludge with an SS concentration of 150 g/
was introduced into the third stirring tank 11, the pH was adjusted to 1.2 by adding H ₂ SO _4, and the mixture was stirred for 2 hours. As a result, most of the precipitated sludge 10 was dissolved. The COD concentration in this solution was 700 mg/. Next, this solution is led to the fourth stirring tank 14 and H ₂ O ₂ is added at 1500 ml.
As a result of adding mg/24 hours of stirring at pH 2.5, the COD in the solution decreased to 85 mg/.
As a result of recycling the dissolved solution after this oxidative decomposition into the stirring tank 2 and using it, the amount of fresh iron salt (FeSO ₄ .7H ₂ O) consumed could be reduced to 300 mg/. (5) Water quality of treated water The quality of treated water at the beginning of the above wastewater treatment is as shown in Table 3, while the quality of treated water after one month of continuous operation is as shown in Table 4, which is the same as that at the beginning of operation. In comparison, the decrease in COD removal rate was very small. Thus, according to the present invention, it has been confirmed that the precipitated sludge can be recycled and used many times over a considerable period of time.

【table】

〔Effect of the invention〕

With the configuration of the present invention as described above, the following industrially important benefits can be obtained. Since the COD components in the separated sludge can be decomposed rationally, even if Fe(OH) ₃ in the sludge is dissolved in acid and reused, the COD removal rate will decrease less than in conventional methods. Since the addition of fresh iron salts is significantly less,
The amount of sludge generated is significantly reduced and the sludge treatment process is streamlined. As a result of adopting a new idea of neutralizing the Fenton treatment liquid with a magnesium-based alkaline agent as an alkaline agent, the sedimentation and concentration properties of the sludge are extremely good, and the sludge concentration is high. The amount of addition will be reduced.

[Brief explanation of the drawing]

The drawing is a system explanatory diagram showing one embodiment of the present invention. 1... COD-containing wastewater, 2... First stirring tank, 3
...H ₂ O ₂ , 4 ... iron salt, 5 ... reaction solution, 6 ...
2nd stirring tank, 7... Alkaline agent, 8... Precipitation device, 9... Treated water, 10... Sludge, 11...
Third stirring tank, 12...Sulfuric acid, 13...H ₂ O ₂ , 1
4... Fourth stirring tank, 15... Return liquid, 16... Sludge removal pipe, 17... Polymer flocculant.

Claims (1)

[Claims] 1 Mainly Fe(OH) ₃ produced by adding iron salts and H ₂ O ₂ to COD-containing wastewater and reacting under acidic conditions, then adding a magnesium-based alkaline agent.
At the same time as solid-liquid separation of SS, mineral acid is added to the separated sludge to dissolve Fe(OH) ₃ and the dissolved liquid is
A method for treating COD-containing wastewater, which comprises adding H ₂ O ₂ and stirring to oxidize and decompose undecomposed COD components, and then adding the mixture to the COD-containing wastewater.