JPS6351076B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for treating steel surface treatment wastewater with a pH of 5.0 or less with activated sludge.

[Prior art]

When manufacturing cold-rolled steel sheets or surface-treated steel sheets by galvanizing, tin-plating, etc. in steel works, pickling treatment is performed with sulfuric acid or hydrochloric acid to remove scale, dirt, oxide films, acids, etc. from the surface of the steel sheets. In addition to this, pickling treatment is often used to clean steel plates.

These steel materials are pickled using a pickling bath containing hydrochloric acid, sulfuric acid, or the like with a concentration of about 3 to 20%. If the pickling bath is used for more than a certain period of time, its pickling ability decreases, so it must be discarded. Further, after pickling the steel material, it is washed with a large amount of water to remove the remaining pickling solution. Furthermore, in many cases, an inhibitor containing an organic compound as a main component is added to the pickling bath in order to dissolve only the scale of the steel material and to suppress dissolution of the base iron by acid as much as possible. For this reason, the process of pickling steel materials generates waste water that has a low pH and contains ferrous ions and organic compounds.

In addition to these pickling wastewaters, in steel works, exhaust gas similar to the pickling process wastewater is also discharged from the manufacturing process of galvanized steel sheets, tin-plated steel sheets, and other galvanized steel sheets. In addition to iron ions, this wastewater contains zinc, tin, chromium, and other substances used in the plating process.
Contains plating additives whose main components are metal ions and organic compounds such as. This plating additive is added to the plating bath in order to obtain good plating properties, and is mainly composed of an organic compound different from the above-mentioned pickling inhibitor.

Therefore, wastewater discharged from steel surface treatment processes such as pickling and plating at steel plants, or from other steel-related factories, has a very low pH and contains zinc, tin, and other metals in addition to iron ions. Contains ions and organic compounds.

Note that most of the iron ions contained in these surface treatment wastewaters are ferrous (Fe ²⁺ ) ions.

When discharging these wastewaters into public waters,
Metal ions such as iron, zinc, and tin, organic compounds indicated by COD, and pH are removed or adjusted to below environmental regulation values before being discharged.

The conventional method for this is as follows.

First, an alkali coagulation precipitation method is mainly applied to remove metal ions. In other words, these wastewaters have a pH of about 2.0 to 3.0, and the metal ions mentioned above are almost completely dissolved, so in order to remove these metal ions, a large amount of alkaline agent must be added to the wastewater. The pH of the waste water is raised and maintained at 9 to 10, and the metal ions are precipitated and removed as hydroxides. However, ferrous salts remain dissolved in waste water unless the pH is raised to 9.5 or higher, and even if hydroxides of ferrous salts are generated at pH 9.5 or higher, ferrous hydroxide will precipitate. Since the speed is slow, large-capacity equipment is required to separate the ferrous hydroxide by sedimentation.

Therefore, in order to solve these problems, we added alkaline agents such as calcium hydroxide and calcium carbonate to the wastewater containing ferrous salts to maintain the pH at 9 to 10, and then blown a large amount of air for a long period of time. Ferrous oxide is oxidized to ferric hydroxide. The reason for this is that ferric hydroxide has a solubility of less than 0.1 mg/kg in the pH range of 5 to 11, which is extremely low compared to ferrous hydroxide, and this floc has good sedimentation properties, so it cannot be treated. This is because there is less runoff into the water.

The treated wastewater is then subjected to solid-liquid separation in a sludge settling tank, and the supernatant water is discharged after its pH is neutralized to regulated values using sulfuric acid, hydrochloric acid, etc. The iron, zinc, tin and other metal ions contained in the treated water are all 0.1mg/
or less, and can fully satisfy the regulation value.

However, even if the above-mentioned metal ions are precipitated as hydroxides and ferrous hydroxide is further oxidized to ferric hydroxide, the pickling inhibitor and plating additive contained in the wastewater Organic compounds such as are not removed sufficiently. Therefore, in some cases, to remove these organic compounds,
Furthermore, a separate treatment such as chemical oxidation treatment using hydrogen peroxide, hypochlorite, etc., or activated carbon adsorption method may be performed.

In this way, conventional wastewater treatment at steel plants is a multi-stage process that includes metal ion precipitation, oxidation of ferrous hydroxide to ferric hydroxide, and organic compound removal. The problem is that there are too many.

In addition, other problems with conventional processing methods include:
Since calcium compounds such as calcium hydroxide and calcium carbonate are used in the neutralization coagulation and precipitation treatment, a large amount of sludge containing a large amount of calcium compounds and metal hydroxides such as iron, zinc, and tin is generated. Furthermore, since the sludge contains various metal oxides, its effective utilization is subject to many restrictions. On the other hand, a biochemical method of oxidizing ferrous ions in wastewater to ferric ions, a method using so-called iron-oxidizing bacteria,
Special Publication No. 47-38981, Special Publication No. 47-38981, Special Publication No.
No. 57-44393, Special Publication No. 18559, Special Publication No. 1855-
Publication No. 22345 is known. However, these target wastewaters are mine, coal mine, smelting wastewater, and industrial wastewater similar to these wastewaters. These wastewaters do not contain organic compounds, and are different from methods that use bacteria to treat wastewater that contains organic compounds and ferrous ions, such as steel-related wastewater.
In addition, in the above-mentioned known technology, the first
The oxidation conditions, such as the amount of aeration air required to oxidize iron ions to ferric ions, have not been disclosed.

[Purpose of the invention]

The present invention aims to solve the above-mentioned problems and is a new treatment method for ferrous salts in wastewater, and a new treatment method for treating ferrous salts in wastewater and at the same time oxidizing and decomposing coexisting organic matter. .

[Structure and operation of the invention]

The gist of the present invention is to install an aeration device and a redox potential measuring device in a pit that collects wastewater with a pH of 5.0 or less that contains at least water-soluble ferrous ions and organic compounds discharged from steel-related factories. , Aeration is carried out using the redox potential in the pit as an indicator, and the microorganisms living in the pit are multiplied.
A biochemical treatment method for steel-related wastewater, which uses this microorganism to oxidize ferrous ions to ferric ions and simultaneously decomposes coexisting organic compounds, and at least Contains water-soluble ferrous ions and organic compounds
Slime or sludge containing microorganisms generated in a pit that collects wastewater with a pH of 5.0 or less is collected and placed in the aeration tank of a separate aerobic activated sludge treatment equipment, and the oxidation-reduction potential of the aeration tank is +700 mV.
Ferrous ions in the wastewater are replaced with ferric iron by passing steel-related wastewater with a pH of 5.0 or less, in which ferrous ions and organic compounds coexist, while performing aeration so that the pH is above (based on the hydrogen electrode). This is a biochemical treatment method for steel-related wastewater that is characterized by oxidizing it into ions and simultaneously decomposing coexisting organic compounds.

First, an experiment conducted by the present inventor will be described.

When chemically oxidizing ferrous ions contained in pickling wastewater, pickling wastewater, etc. using the conventional method described above, in order to perform the process smoothly, it is necessary to mix various types of wastewater and make it uniform. Water collection pit (the larger the capacity, the better the various types of wastewater will be mixed and homogenized)
It is normal to install The residence time of the waste water in this pit is approximately 10 minutes to over 1 hour.

The inventors focused on the agar-like slime that adhered to the side wall of this pit, collected this slime, put it in the aeration tank of an aerobic activated sludge experimental device, aerated it, and then let it flow into the pit. As a result of conducting an experiment on activated sludge treatment by supplying wastewater to the aeration tank, we found that
The treatment time is about 1 hour, and the wastewater contains about
500 mg/ferrous ion to ferric ion
It is oxidized with an oxidation rate of 99.9% or more, and organic compounds such as pickling inhibitors and pickling additives, which contain about 30 mg/total organic carbon (TOC), are
It was found that the amount of TOC removed was less than 5 mg. It has also become clear that the sludge deposited at the bottom of this pit has a similar effect.

From these experimental results, it was found that the agar-like slime or sludge contains bacteria that oxidizes ferrous ions contained in the wastewater to ferric ions, and organic compounds used as pickling inhibitors, plating additives, etc. It is presumed that there are bacteria that oxidize and decompose it.

The present invention has been made based on such new findings.

The inventors were able to treat steel-related wastewater, such as wastewater from pickling of steel and wastewater from plating of steel, by using slices or sludge containing naturally occurring bacteria.

This will be explained below using examples.

Example 1 In a pit 1 for collecting wastewater discharged from pickling and plating processes related to steel, a diffuser pipe 6 for aeration is installed at the bottom as shown in Fig. 1, and an oxidation-reduction potential measuring device is installed at the top of the pit. As, redox potential sensor 1
1. An oxidation-reduction potential meter 12 and a recorder 13 were installed to form a water-flowing pit. In Fig. 1, 2 and 3 are weirs, 4 is a water collection pipe, 5 is a pump, 7 is a surplus sludge extraction pump, 8 is a sludge discharge pipe, and 9
1 is a water pump, and 10 is a drainage pipe.

When aeration was carried out while water was flowing through the water passage pit, slime, sludge, etc. increased in large quantities, and the oxidation-reduction potential increased. As the redox potential increased, the removal rate of ferrous ions and/or coexisting organic compounds in the wastewater increased. When the redox potential is about +700 mV or higher (based on the hydrogen electrode), the removal rate of these is approximately 100 mV.
%, most of the ferrous ions are oxidized to ferric ions, and for example, the ferrous ion concentration at the outlet of the water collection pit is less than 1 mg/%, and most of the organic compounds are oxidized and decomposed, and the concentration is the same as that of the total organic The amount of carbon was less than 3mg/.

Furthermore, if this kind of treatment is carried out for a long period of time, the sludge concentration in the water collection pit will increase to over 200,000 mg/kg.
It has been found that aeration alone does not result in sufficient stirring, resulting in a decrease in treatment efficiency, and that the optimum sludge concentration is approximately 30,000 mg/ to 100,000 mg/.

When the sludge concentration reaches 100,000 mg/or more, the sludge is removed by the sludge removal pump 7 shown in Fig. 1, and the supernatant water from which the sludge has been separated is extracted by the water supply pump 9 shown in Fig. 1 and sent to a neutralization tank (not shown). The pH was adjusted to 4 to 5 using caustic soda, calcium carbonate, calcium hydroxide, etc. to neutralize the oxidized ferric ions, and a sludge containing ferric hydroxide as a main component was recovered. When this is fired, iron oxide with an iron content of about 50% or more is obtained, which can be used for pigments, etc.

Example 2 If air is blown into a collection pit for wastewater discharged from steel-related pickling and plating processes while water is flowing through it, a large amount of sludge will be generated over a period of about 1 to 2 months. This sludge is collected and put into the aeration tank 15 of the aerobic activated sludge treatment equipment 14 shown in FIG. Washing-related wastewater and plating-related wastewater were passed through, and air or oxygen was aerated using the oxidation-reduction potential as an index. As a result, ferrous ions contained in the waste water were oxidized to ferric ions, and organic compounds such as pickling inhibitors and plating additives were also oxidized and decomposed.

When the sludge grown in the water collection pit is placed in an aeration tank for aerobic activated sludge treatment and the wastewater is passed through, the oxidation-reduction potential of the aeration tank is initially +550 mV.
~600mV (based on hydrogen electrode), which is low, but for about 10 days~
One month later, the redox potential rose to over +700mV, and the sludge concentration in the aeration tank reached over 200,000mg/. As described above, when the sludge concentration became high, the treatment efficiency decreased. And the sludge concentration in the aeration tank is 10,000
mg/~100,000 mg/, preferably 50,000 mg/~10
It was found that the treatment efficiency was highest when the amount was in the range of 10,000 mg/.

The above-mentioned pickling-related wastewater and plating-related wastewater are mixed in a water collection pit, and the wastewater is pumped to the aeration tank 1 by a pump 5.
5, the oxidation-reduction potential of the aeration tank and 800
Adjust the amount of wastewater inflow and/or the amount of air aeration so that the voltage is ±25 mV (hydrogen electrode standard), and water is passed so that the residence time in the aeration tank is about 1 hour.
The influence of aeration was blocked, the sludge was decomposed, and the supernatant water was discharged using the pump 9.

As a result, the pH was 2.5 and the ferrous ion concentration was approximately 500.
The mixed wastewater had an organic compound concentration of about 50 mg/ as total organic carbon, a ferrous ion concentration of 1 mg/or less, and a total organic carbon concentration of 3.
The wastewater amounted to less than mg/mg.

In addition, this wastewater was treated with caustic soda and
PH4 due to calcium carbonate, calcium hydroxide, etc.
By adjusting the temperature to ~5, sludge containing ferric hydroxide as a main component could be recovered. When this is fired, iron oxide with an iron content of approximately 50% or more is obtained, which can be used for pigments, etc.

〔Effect of the invention〕

As described above, according to the method of the present invention, slime and sludge, which are aggregates of bacteria living in pits that collect steel-related wastewater such as wastewater from pickling of steel and wastewater from plating of steel, can be removed. This method can oxidize ferrous ions in wastewater to ferric ions, which can be precipitated and coagulated at a low pH of about 5, making it possible to carry out the treatment in a shorter time than with conventional methods. Since organic compounds can be treated as well as ferric ions, this method has the effect of simplifying the process compared to conventional methods, and since sludge containing only ferric ions is produced, the sludge can be used effectively. It's also effective.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a water flow pit for implementing the method of the present invention, and FIG. 2 is a diagram showing an example of an aerobic activated sludge treatment apparatus for implementing the method of the present invention. 1 is a water collection pit, 2 and 3 are weirs, 4 is a water collection pipe, 5 is a water collection pump, 6 is a diffuser pipe for air aeration,
7 is a pump for removing excess sludge, 8 is a pipe for removing excess sludge, 9 is a water supply pump, 10 is a drainage pipe, 11 is a redox potential sensor, 12 is a redox potential meter, and 13 is a redox potential meter. Recorder for continuous recording, 14 is aerobic activated sludge treatment equipment, 1
5 is an aeration tank.

Claims (1)

[Scope of Claims] 1. An aeration device and an oxidation-reduction potential measuring device are installed in a pit that collects wastewater with a pH of 5.0 or less that contains at least water-soluble ferrous ions and organic compounds discharged from a steel-related factory. Then, aeration is carried out using the redox potential inside the pit as an indicator, and the microorganisms living in the pit are multiplied. At the same time, the microorganisms are used to oxidize ferrous ions to ferric ions, and at the same time, the coexisting organic A biochemical treatment method for steel-related wastewater that also decomposes compounds. 2. Collect slime or sludge containing microorganisms generated in a pit that collects wastewater with a pH of 5.0 or less that contains at least water-soluble ferrous ions and organic compounds discharged from steel-related factories, and collect it in a separate pit. The first sludge was placed in the aeration tank of the aerobic activated sludge treatment equipment, and while aeration was performed so that the oxidation-reduction potential of the aeration tank was +700 mV or higher (based on the hydrogen electrode).
A steel that is characterized by passing steel-related wastewater with a pH of 5.0 or lower in which iron ions and organic compounds coexist to oxidize the ferrous ions in the wastewater to ferric ions and simultaneously decompose the coexisting organic compounds. Biochemical treatment methods for associated wastewater.