JPS6345271B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating surface-treated steel wastewater with activated sludge.

In steel works, when producing cold-rolled steel sheets or surface-treated steel sheets by galvanizing, tin plating, etc., pickling treatment is performed with sulfuric acid or hydrochloric acid to remove scale, dirt, oxide film, rust, etc. from the surface of the steel sheets. In addition to this, pickling treatment is often used to clean steel materials.

Pickling of these steel materials is carried out using hydrochloric acid, sulfuric acid, etc. with a concentration of about 3 to 20%, but when these pickling baths are used for more than a certain period of time, their pickling ability decreases and they 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, in these pickling processes, 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, wastewater 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 metal ions such as zinc, tin, chromium, nickel, cobalt, and manganese used in the plating process, as well as plating additives whose main components are organic compounds. 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 has a very low pH and contains zinc, tin, other metal ions, and organic compounds in addition to iron ions. are doing. Note that most of the iron ions contained in these surface treatment wastewaters are ferrous (Fe ⁺⁺ ) ions.

When discharging these wastewaters into public waters, iron,
Metal ions such as zinc and tin, organic compounds indicated by COD, and pH are removed or adjusted to below environmental regulation values before discharge. As a method for removing these metal ions, an alkali coagulation precipitation method has conventionally been mainly applied. That is, these wastewaters have a pH of about 2.0 to 3.0, and the metal ions mentioned above are almost completely dissolved. Therefore, in order to remove these metal ions, a large amount of alkaline agent is added to the waste water, the pH of the waste water is maintained at 9 to 10, and the metal ions are precipitated as hydroxides. However, the hydroxide of ferrous salt remains dissolved in the wastewater unless the pH is raised to 9.5 or higher, and even if the PH is raised to 9.5 or higher to form a precipitate, ferrous hydroxide has a slow sedimentation rate. Therefore, a sedimentation separation tank for ferrous hydroxide requires large-capacity equipment.

In order to solve these problems, we add alkaline agents such as calcium hydroxide and calcium carbonate to the wastewater containing these ferrous salts to maintain the pH at 9 to 10, and then blow a large amount of air over a long period of time. Ferrous hydroxide is oxidized to ferric hydroxide. This ferric hydroxide has a low solubility in the pH range of 5 to 11.
The solubility is 0.1 mg/or less, which is very low compared to ferrous hydroxide, and since this block has good sedimentation properties, there is little leakage into the treated water.

The aforementioned surface-treated wastewater from steel plants has a pH of 9 to 10 due to calcium hydroxide, calcium carbonate, etc.
Adjust and maintain the level, blow in a large amount of air,
Afterwards, these wastewaters are subjected to solid-liquid separation in a sludge settling tank, and the supernatant water is treated with sulfuric acid, hydrochloric acid, etc.
It is discharged after neutralizing the pH to the regulated value. By performing such treatment, iron, zinc, tin, and other metal ions in the treated water are all removed to 0.1 mg/or less, which fully satisfies the regulatory values.

However, this processing method also includes many problems. For example, organic compounds such as pickling inhibitors and plating additives contained in the aforementioned wastewater are not sufficiently removed by this neutralization coagulation-sedimentation method. Therefore, most of these organic compounds flow directly into the neutralized coagulation-sedimentation treated water and account for a large portion of the COD of the treated water. Therefore, in order to remove these organic compounds, chemical oxidation treatment using hydrogen peroxide, hypochlorite, etc. or activated carbon adsorption method must be used to remove these organic substances from wastewater. need to be done.

Another problem is that a large amount of sludge is generated by the neutralization, coagulation and sedimentation treatment of this wastewater. When calcium compounds such as calcium hydroxide and calcium carbonate are used for neutralization, this sludge contains a large amount of calcium compounds and metal hydroxides such as iron, zinc, and tin. In addition, there are many restrictions on the effective use of sludge.

The inventors have invented a new treatment method that solves these problems when treating the aforementioned wastewater. This method uses iron oxidizing bacteria to oxidize ferrous salts in wastewater to ferric salts, and then simultaneously oxidizes and decomposes organic substances such as pickling inhibitors or plating additives that coexist with the ferrous salts. be.

Generally, iron oxidizing bacteria are used to oxidize ferrous salts contained in mine drainage, which has a very low pH of 1 to 3, into ferric salts in an aerobic atmosphere.
However, when treating wastewater with these oxidizing bacteria,
Mine wastewater, industrial wastewater, etc. that do not contain organic compounds oxidize ferrous salts in the wastewater to ferric salts relatively easily, but when ferrous salts and organic compounds coexist in this wastewater, iron oxidation occurs. The function of bacteria is inhibited by organic compounds, and the oxidation rate of ferrous salts decreases. For example, when wastewater in which ferrous salts and glucose coexist is treated with iron oxidizing bacteria, the oxidation rate of ferrous salts decreases significantly, and ferrous salts may not be detected in the treated water. Are known. For this reason, wastewater in which ferrous salts and organic compounds coexist is treated by an activated sludge method using iron oxidizing bacteria, and the ferrous ions in the treated water are reduced to 1 mg/less or less, and the amount of coexisting organic compounds is reduced. There are no examples of parts being disassembled.

In addition, when oxidizing ferrous ions in mine drainage using the aforementioned iron oxidizing bacteria, air aeration is performed by coexisting activated sludge of iron oxidizing bacteria and wastewater containing ferrous ions in an aeration tank for activated sludge treatment. However, the management conditions at this time, especially the indicators that determine the amount of aeration, have not been clarified.

According to the inventors' research, in the case of wastewater that does not contain organic compounds, if sufficient aeration is performed, almost 100% of the ferrous ions will be oxidized, and the ferrous ions in the treated water will be less than 1 mg/kg. . However, when organic compounds coexist, the amount of aeration is simply
Even using conventional control methods such as dissolved oxygen concentration, the oxidation rate of ferrous iron decreases.

As described above, when ferrous ions and organic compounds coexist in wastewater, it is almost impossible to treat the wastewater using the activated sludge method using oxidizing bacteria, which is almost impossible. The activated sludge method, which uses bacteria, was applied to mine and industrial wastewater, which contains almost no organic compounds. The present invention is an improved method of such a conventional method, and in order to oxidize ferrous ions and organic compounds in acidic wastewater in the same way, it is treated by an aerobic activated sludge treatment containing iron-oxidizing bacteria. The method is characterized by maintaining the relationship between organic compound concentration and activated sludge concentration under specific conditions during activated sludge treatment, and in addition, managing the amount of aeration during activated sludge treatment using the redox potential as an index.

Next, the present invention will be explained in detail.

The present invention aims to treat wastewater containing ferrous iron and organic compounds by an activated sludge treatment method using iron oxidizing bacteria.
This method oxidizes almost 100% of iron to ferric iron, and also simultaneously oxidizes and decomposes coexisting organic compounds.

First, ferrous iron coexisting in wastewater is easily oxidized by iron oxidizing bacteria unless organic compounds coexist, but as mentioned above, there is a problem in that the oxidation rate decreases when organic compounds coexist. In order to solve this problem, the inventors investigated the concentration of organic compounds in wastewater and the activated sludge concentration in the aeration tank for activated sludge treatment (hereinafter referred to as
As a result of focusing on the relationship with MLSS (abbreviated as MLSS),
Even if ferrous ions and organic compounds coexist in wastewater, if the relationship between the organic compound concentration and MLSS is maintained under certain conditions, ferrous ions will be almost completely oxidized, and ferrous It was found that the ion concentration was less than 1 mg/mg.

In other words, the inventors found that when a certain amount of organic compounds is present in MLSS, iron-oxidizing bacteria preferentially oxidize organic compounds over ferrous iron, and therefore It was found that some parts remained unoxidized. Based on this, the inventors believe that such problems can be solved if there is sufficient MLSS relative to the amount of organic compounds in the wastewater, and the ferrous ion concentration, organic compound concentration (total organic carbon concentration
(abbreviated as TOC) and MLSS, and the relationship between treated water quality was investigated. As a result, when iron-oxidizing bacteria treat ferrous ions in wastewater that coexists with organic compounds, the sludge concentration in the aeration tank (hereinafter abbreviated as MLSS concentration) and the ferrous ions in the supplied wastewater coexist. TOC
Organic compound concentration indicated by (TOC)
If the relationship between the concentration and the abbreviation) is on the straight line A shown in Figure 1 or below the straight line A, the ferrous ions are oxidized, and the ferrous ions in the water treated with iron-oxidizing bacteria are
Experiments have revealed that it is less than mg/mg/mg.

Therefore, from the results shown in Figure 1, in order to oxidize the ferrous ions in the wastewater where organic compounds coexist by iron-oxidizing bacteria and reduce the ferrous ions in the treated water to 1 mg/or less, it is necessary to increase the TOC of the supplied wastewater. It became clear that the relationship between the sludge concentration and the sludge concentration in the aeration tank should be maintained as shown in equation (1) below.

Furthermore, from Figure 1, when ferrous ions in wastewater containing organic compounds are oxidized to ferric ions by iron-oxidizing bacteria, in order to reduce the ferrous ions in water treated with iron-oxidizing bacteria to 1 mg/or less, , it is necessary to maintain the sludge concentration in the aeration tank at approximately 20,000g/or more.
On the other hand, the upper limit of the sludge concentration is approximately 150,000 mg/ml due to the necessity of transporting the sludge mixture using a pump. Therefore, when oxidizing ferrous ions in wastewater where ferrous ions and organic compounds coexist by iron-oxidizing bacteria, the limit is about 150,000 mg/kg when pumping a mixed solution of sludge containing iron-oxidizing bacteria. Therefore, the concentration of organic compounds in the wastewater to be treated is (1)
From the formula, a TOC of 940mg/or less is desirable.

TOC≦0.007×MLSS−110 ……(1) Note that TOC and MLSS in equation (1) are respectively
Expressed in mg/.

That is, the activated sludge treatment of the present invention using iron-oxidizing bacteria can reduce ferrous ions and organic compounds in wastewater by adjusting the activated sludge concentration and/or organic compound concentration to maintain the conditions expressed by equation (1) It has been found that even if the compounds coexist, both components can be almost completely decomposed, and the amount of ferrous ions and organic compounds in the treated water can be reduced to 1 mg/or less.

In addition, activated sludge in the aeration tank for activated sludge treatment using iron-oxidizing bacteria has a heating reduction amount of about 20 to 30%, and the heating residue is an inorganic compound containing iron oxide as a main component. Therefore, activated sludge of iron oxidizing bacteria is
It has become clear that iron-oxidizing bacteria adhere to a carrier containing iron oxide as a main component, and the MLSS in the present invention includes these inorganic compounds.

Next, the amount of air supplied to the aeration tank for activated sludge treatment should be controlled using the ORP of the aeration tank as an index. The reaction in which ferrous ions in wastewater are oxidized by iron-oxidizing bacteria is a reaction in which ferrous ions adsorbed to iron-oxidizing bacteria are absorbed by oxygen taken into the cells of iron-oxidizing bacteria through respiration. This reaction is likely to be oxidized to form ferric ions.
It is easily maintained that ORP is involved.

Therefore, as a result of examining the aeration amount using the ORP of the aeration tank as an index, we found that if ORP is maintained at +750 to 850 mV (based on the hydrogen electrode), the treatment time will be 1 to 1.
In a short period of 4 hours, ferrous ions are converted to ferric ions by almost 100% (ferrous ion concentration of treated water is 1mg/100%).
below) is oxidized. Furthermore, if the MLSS and TOC are maintained in the relationship shown in equation (1) above, and the ORP of the aeration tank is maintained within the above range, the organic compounds that coexist with ferrous ions will be reduced to approximately 100% (TOC of 1 mg of treated water). /
below) will be decomposed.

In this way, when treating wastewater in which ferrous ions and organic compounds coexist by an aerobic activated sludge method using iron oxidizing bacteria, it is most preferable to The relationship with the concentration of organic compounds (total organic carbon: expressed as TOC) is maintained under the condition of TOC ≦ 0.007 × MLSS − 110, and the ORP of the aeration tank is +750 to 850 mV (based on the hydrogen electrode).
When aeration is carried out to achieve this, almost 100% of ferrous ions are oxidized to ferric ions, and almost 100% of organic compounds are also decomposed.

Next, by performing the treatment under the above conditions, the ferrous salts contained in the wastewater are oxidized, so that the aerobic activated sludge treated water contains only ferric salts as iron ions. . Therefore, the removal of metal ions such as ferric ions, tin ions, zinc ions, etc. contained in wastewater cannot be easily achieved using the pH-adjusted coagulation-sedimentation method used in general wastewater treatment. be.

In other words, the wastewater treated by iron oxidizing bacteria in the aeration tank is a secondary iron salt with good sedimentation properties.
Since it only contains iron salts, activated sludge is separated by sedimentation in an activated sludge settling tank, and the pH of the supernatant water is adjusted with an alkaline agent to easily recover ferric ions, zinc ions, etc. This supernatant water is
It has a reddish-brown color with a pH of 2 to 3, and is almost transparent. First, the recovery of ferric compounds from this treated water includes caustic soda, soda carbonate, calcium carbonate,
When an alkaline agent such as calcium hydroxide is added to adjust the pH of the wastewater to about 4.0 to 5.0, reddish-brown ferric hydroxide precipitates. Next, ferric hydroxide is separated in a sedimentation separation tank, and the colorless and transparent supernatant water is
In the pH adjustment tank, add the above alkaline agent to adjust the pH.
When adjusted to 8.0 to 9.0, zinc hydroxide and other precipitates are generated, and the precipitates are sent to a sedimentation separation tank to recover zinc hydroxide. When the pH of the supernatant water after recovering zinc hydroxide is readjusted to the discharge regulation value using sulfuric acid, hydrochloric acid, etc., no heavy metals are detected in the treated water, and it is completely rendered harmless and becomes a public water area. can be discharged.

In addition, if water-soluble divalent tin ions are present in the wastewater, these tin ions are oxidized by iron-oxidizing bacteria during dirty sludge treatment to form insoluble tetravalent tin compounds, which accumulate in activated sludge. However, it does not flow into the treated water. Furthermore, even if metal salts such as tin compounds and zinc compounds are present, the functions of iron-oxidizing bacteria are hardly inhibited.

Next, examples of the present invention will be described.

Wastewater mainly consisting of rins water from the pickling process of cold rolling mills of steel mills, galvanized iron plates and tinplate manufacturing plants was treated by the activated sludge method using iron oxidizing bacteria.

This wastewater has a pH of 2.0 to 2.5 and approximately ferrous ions.
500-1000mg/, organic compound approximately 10-20mg/
(converted as TOC), zinc ion 5-10mg/,
Each contains 20 to 50 mg of tin ion. The COD of this wastewater is 100 to 150 mg/
The organic compounds are inhibitors used when pickling steel sheets and additives used in tinning baths.

Approximately 20,000mg of activated sludge of iron oxidizing bacteria is collected from this wastewater.
The above-mentioned wastewater is supplied to the aeration tank (conditions that satisfy formula (1) above) so that the residence time is approximately 2 hours, and the ORP of the aeration tank is ±800±25mV (based on hydrogen electrode). Activated sludge treatment with aeration was performed.

Note that when this wastewater is treated, iron oxidizing bacteria proliferate, and the MLSS in the aeration tank becomes 20,000 mg/or more. Therefore, the proliferated sludge is extracted as surplus sludge in the sludge sedimentation separation tank next to the aeration tank. When this excess sludge is burned, a red powder whose main component is Fe ₂ O ₃ (over 40% iron) is obtained.

When caustic soda water is added to the supernatant water of the sludge sedimentation separation tank to adjust the pH to 4.5 to 5.0, a red precipitate is produced. When this red precipitate is collected and burned, the iron content is approximately
A red powder whose main component is more than 67% Fe ₂ O ₃ is obtained.

Next, when calcium hydroxide is added to the supernatant water, which has a pH of 4.5 to 5.0, and the pH is adjusted to 8.0 to 9.0, a white precipitate is formed. The wastewater from which this precipitate was separated was adjusted to pH 7.0±0.5 with hydrochloric acid, resulting in colorless and transparent wastewater.

This wastewater has a pH of 7.0±0.5, metal ions such as iron, zinc, and tin less than 1 mg/day, and organic compounds of 1 to 2
mg/(as TOC) or less, COD is 5 mg/or less.

The red powder containing Fe ₂ O ₃ as a main component obtained by burning red precipitate generated during pH adjustment of excess sludge and biologically oxidized water can be used as a pigment valve.

Furthermore, when the ORP of the aeration tank becomes approximately +700 mV or less (hydrogen electrode standard), the ferrous ion concentration of the treated water will decrease.
1 mg/ or more, and the lower the ORP, the higher the ferrous ion concentration in the treated water tends to be, and the oxidation rate of ferrous ions tends to decrease.

In addition, when the activated sludge concentration in the aeration tank was lowered and the relationship between the TOC (10 to 20 mg/) of the supplied raw water and the activated sludge concentration (MLSS) in the aeration tank was set to TOC > 0.007 x MLSS - 110, organic compounds decomposes almost completely, but the oxidation rate of ferrous ions decreases. For example, when the raw water supplied in this example was used to reduce the MLSS to about 10,000 ppm, the oxidation rate of ferrous ions decreased to about 90%,
Activated sludge treated water contains 50-100mg/ferrous ion
has also started to be detected.

In this way, wastewater in which ferrous ions and organic compounds coexist is treated by aerobic activated sludge treatment using iron oxidizing bacteria, oxidizing ferrous ions to ferric ions, and converting the treated water into ferrous ions. In order to keep the iron ion concentration below 1mg/ and to almost completely decompose organic compounds, maintain the relationship between TOC of the feed water and MLSS of the aeration tank to be TOC≦0.007×MLSS−110, and further,
It is necessary to control the ORP of the aeration tank to +750-850mV.

[Brief explanation of the drawing]

Figure 1 shows that when wastewater in which ferrous ions and organic compounds coexist is oxidized by iron-oxidizing bacteria to ferric ions, the first
It is a graph showing the relationship between the organic compound concentration (TOC) and the sludge concentration in the aeration tank (MLSS) for iron ions to be 1 mg/or less.

Claims (1)

[Scope of Claims] 1. Acidic steel surface treatment wastewater containing at least iron salts mainly ferrous salts and organic compounds is treated by an aerobic activated sludge treatment containing iron oxidizing bacteria; The relationship between total organic carbon concentration (TOC) of wastewater supplied to activated sludge treatment and activated sludge concentration (MLSS) in the aeration tank of aerobic activated sludge treatment is expressed by the following formula: TOC (mg/) ≦ 0.007 × MLSS (mg/ )-110 (However, ferrous ion concentration ≦5000mg/) After activated sludge treatment, the metal ions in the treated water are removed by a conventional method.Biochemistry of surface treatment wastewater for steel materials processing method. 2. The method according to claim 1, for treating surface treatment wastewater containing metal ions used in metal plating in addition to iron salts. 3. The method according to claim 1 or 2, wherein the amount of aeration supplied to the aeration tank for activated sludge treatment is managed using the oxidation-reduction potential of the aeration tank as an index. 4. The method according to claim 3, wherein the redox potential is maintained at +750 to +850 mV.