JPS6339307B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for coagulation-sedimentation treatment of wastewater containing COD, and particularly to an improved method for reusing coagulation-sediment sludge as a flocculant. Conventionally, when treating river water that mainly contains clay-based suspended solids, such as water treatment, sulfuric acid is added to the precipitated sludge generated during coagulation and sedimentation treatment by adding sulfuric acid as a coagulant. The method of adding aluminum hydroxide (Al(OH) ₃ ) to Al ³⁺ ions and reusing this solution as a flocculant is widely known and is also practiced on a commercial brand scale. . However, COD,
When an acid is added to the sludge generated as a result of coagulating and removing organic substances such as BOD components to dissolve Al(OH) ₃ and trying to reuse it as a flocculant, the COD and BOD components that were painstakingly coagulated and removed will also re-elute. Since these are added to the raw water, the process of flocculation and sedimentation (or flotation) becomes meaningless as it is not overturned, so acid dissolution and reuse of flocculation and sedimentation sludge, as in water treatment, is not actually implemented. It was impossible. The present invention makes it possible to reuse the sludge generated from the process of coagulating and removing COD and BOD components as a coagulant again without causing the above-mentioned pre-shielding, and at the same time, it is possible to reuse the sludge generated from the process of coagulating and removing COD and BOD components. The purpose of this invention is to provide an innovative method that can substantially eliminate the problem of agglomerated sludge having poor concentration and dewatering properties. That is, the present invention involves coagulating and separating COD-containing wastewater using a coagulant containing at least one of iron salts, aluminum salts, and copper salts, and then adding mineral acid and at least hydrogen peroxide or ozone to the resulting coagulated sludge. An oxidizing agent containing the sludge is added and stirred to oxidize and decompose the organic matter in the flocculating sludge and dissolve the metal hydroxide, and the dissolved solution is reused as the metal salt flocculant in the flocculation separation step. This is a coagulation and precipitation treatment method. To summarize the gist of the present invention, it is essential to use a metal salt as a flocculant.
As the metal salt, iron salts such as ferric chloride and ferrous sulfate, aluminum salts such as aluminum sulfate and polyaluminum chloride, or copper salts can be used alone or in combination, but iron salts is most preferred. Note that an organic polymer flocculant can also be used in combination. A mineral acid such as sulfuric acid and the above-mentioned oxidizing agent are added to the metal hydroxide produced as a result of using a metal salt as a flocculant, and the COD and BOD components in the flocculated sludge are removed under acidic conditions (preferably in the pH range of 2 to 4). Chemically oxidized and decomposed. For example, it is known that H ₂ O ₂ as the oxidizing agent exhibits strong oxidizing power by generating hydroxyl radicals, i.e., . The most distinctive feature of this method is that it uses the metal in the agglomerated sludge, which is mainly composed of oxides, to cause this reaction, which has never been seen before. This is the reason why the use of a metal salt as a flocculant in the present invention is an important requirement in the previous section. Note that oxygen, chlorine, or the like can also be used together as the oxidizing agent. Simultaneously with or after oxidizing the metal in the flocculated sludge, add a mineral acid such as sulfuric acid to lower the pH to below 2, for example, in the case of ferric hydroxide, Fe(OH) ₃ +3H ⁺ →Fe After it is completely dissolved into Fe ³⁺ ions through the reaction of ³⁺ + 3H ₂ O, this is reused as a flocculant. The important point here is that the COD components in the flocculated sludge are decomposed into CO ₂ and H ₂ O by the chemical oxidation treatment using a combination of metal and oxidizing agent as described in the section above, so they cannot be used as flocculants. be reused
Even if the dissolved solution containing Fe ³⁺ ions is recycled to raw water, it does not lead to a vicious cycle of COD components, as is the case with the conventional method of simply dissolving coagulated sludge with acid. To explain the embodiment of the present invention with reference to the drawings, COD-containing wastewater 1 such as biologically treated water such as sewage and human waste is treated with ferric chloride 3 in a flocculation tank 2 (this is necessary only at the start of normal operation). After flocculation, an organic polymer flocculant 4 such as an anionic polymer is usually added, and the sludge is separated into flocculated sludge 6 and treated water 7 in a settling basin 5. The flocculated sludge 6 is a sludge whose main component is Fe(OH) ₃ and which contains a COD component and a chromaticity component which have been coagulated and removed. Next, the flocculated sludge 6 is further concentrated using a sixer (not shown) if necessary, and then mineral acid 9 and hydrogen peroxide 10 are added in the first stirring tank 8, and the mixture is heated under acidic conditions of pH 2 to 4 for the required time. (Usually, it takes about several hours to a day) After stirring, the next second
In the stirring tank 11, mineral acid 9 is further added and the pH is increased.
Set to 2 or less (preferably around PH1), Fe
After the (OH) ₃ is dissolved into Fe ³⁺ ions, it is recycled into the waste water 1 through the return pipe 12 and reused as a flocculant. In addition, the pH of stirring tank 8 should be set to 1.5~
If it is set to about 2.5, the second stirring tank 11 can be omitted. If the wastewater 1 contains inorganic SS components, if no sludge is removed at all, a significant accumulation of SS will occur in the treatment system, which is undesirable.
It is necessary to remove the sludge from the sludge pipe 13. In addition, if phosphoric acid is contained in the wastewater 1, it will precipitate as FePO ₄ , but in the first stirring tank 8, it will be converted into the following reaction FePO ₄ ↓ + 3H ⁺ →Fe ³⁺ +H ₃ PO ₄ . Therefore, in order to solve the problem that phosphoric acid is eluted, it is necessary to use coagulation and sedimentation treated water 7 or sedimentation tank 5.
The inflow water contains slaked ash, calcium chloride, etc.
After adding the Ca ²⁺ ion source 14, in the catalytic dephosphorization (also called crystallization dephosphorization) step 15, which utilizes crystallization on the surface of a granular solid such as phosphate rock or bone char, which has already been applied by the present applicant. A method for removing phosphorus may be used. As described above, the present invention utilizes metal components, that is, metal ions, in a metal salt flocculant such as iron salt, which has been added for the initial coagulation treatment of wastewater, as a treatment agent for Fuenton treatment, and COD in the flocculated sludge. After the BOD component is subjected to oxidative decomposition treatment, the metal ions are further reused as a flocculant for the flocculation treatment.According to the present invention, the following industrially important You can make a profit. (1) Even when COD and raw water containing it are targeted, it is possible to reuse the flocculated sludge by dissolving it with mineral acid without creating a shield that will lead to a vicious cycle of COD components, unlike the conventional method of simply dissolving flocculated sludge with acid. As a result, the amount of coagulated sludge discharged from the system is extremely small. (2) As a result, the generation of flocculated sludge containing metal hydroxides with poor concentration and dewatering properties can be virtually eliminated, so sludge treatment (dewatering, incineration)
The process is significantly streamlined. (3) Since the metal hydroxide in the flocculated sludge is used as the metal ion required for chemical oxidation with hydrogen peroxide or ozone, there is no need to separately add metal salts, and operating costs are significantly reduced. be done. Next, examples of the present invention will be described. Example: Using activated sludge treated sewage water as raw water,
After adding 50 mg of iron (as FeCl ₃ ) and 1.0 mg of anionic polymer (Acofrock A720) to form a floc, the sedimentation tank (sedimentation separation speed 50 mm/
The flocs were separated at 1 min) to obtain flocculation-sedimentation treated water and sedimentation sludge. The water quality of the raw water and coagulation-sedimentation treated water was as shown in Table 1.

[Table] Next, concentrate the precipitated sludge with a thickener.
After obtaining a concentrated sludge with _an SS concentration of 32 g/sludge and adjusting sulfuric acid to pH 3, _{H 2 O 2} (peroxide Hydrogen) was added and stirred for 15 hours. After this, add more sulfuric acid to make the pH 1.5.
Adjust the Fe(CH) ₃ sludge to
It was dissolved in Fe ³⁺ ions and reused as a flocculant added to raw water. As a result, flocs with good sedimentation and dewatering properties were formed, similar to when ferric chloride was added as a new flocculant, and treated water with the quality shown in Table 2 was obtained. In addition, COD in the liquid that is reused as a flocculant
was several mg/, confirming that the oxidative decomposition of Fe and H ₂ O ₂ was carried out effectively.

【table】 [Brief explanation of the drawing]

The drawings are system explanatory diagrams showing embodiments of the present invention. 1...Wastewater, 2...Coagulation tank, 3...Second chloride
Iron, 4... Organic polymer flocculant, 5... Sedimentation tank, 6
...Agglomerated sludge, 7 ... Treated water, 8 ... First stirring tank, 9 ... Mineral acid, 10 ... Hydrogen peroxide, 11 ...
... Second stirring tank, 12 ... Return pipe, 13 ... Sludge removal pipe, 14 ... Ca ²⁺ ion source, 15 ... Catalytic dephosphorization process.

Claims (1)

[Claims] 1. Wastewater containing COD is treated with iron salts, aluminum salts,
After flocculating and separating using a flocculant containing at least one copper salt, mineral acid and
An oxidizing agent containing at least hydrogen peroxide or ozone is added and stirred to dissolve the agglomerated sludge.
A coagulation-sedimentation treatment method characterized by oxidatively decomposing COD and BOD and dissolving metal hydroxide, and reusing the dissolved solution as the metal salt flocculant in the coagulation separation step. 2. The coagulation-sedimentation treatment method according to claim 1, wherein the separated water from the coagulation and separation step is treated with a catalytic dephosphorization step and discharged after having been previously added with a calcium ion source.