JPH0714512B2 - Treatment method of wastewater containing heavy metals - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for treating heavy metal-containing wastewater, and particularly to efficiently remove heavy metals from heavy metal-containing wastewater to obtain treated water of excellent treated water quality. The present invention relates to a method for treating heavy metal-containing wastewater.

[Prior Art] Conventionally, as a method for treating heavy metal-containing wastewater, a method of adding an alkali such as calcium hydroxide to precipitate and separate heavy metals in the form of hydroxide, or adding an iron salt, an aluminum salt, a magnesium salt, etc. Then, a method of co-precipitating is known. For example, JP-A-61-192386 discloses a method of adding a magnesium salt and performing solid-liquid separation at pH 10-12.

Further, as a method for separating the produced insoluble matter, precipitation, filtration, membrane separation using an ultrafiltration membrane or a reverse osmosis membrane, etc. are known. For example, Japanese Examined Patent Publication No. 55-33953 discloses a separation method using an ultrafiltration membrane.

Good treated water cannot be obtained only by the coagulation-sedimentation method. In order to clear the strict effluent standard, a method of filtering with a filter after coagulation and sedimentation is generally adopted.

However, this method usually requires backwashing once or twice per day, and when a chelating agent coexists in wastewater, heavy metals leak from the filter and the treatment effect deteriorates. There was a problem. Further, there is a drawback that the installation area of the sedimentation tank is required and the maintenance of the apparatus is very complicated.

As a method for solving such a problem, the pore size is 0.1 to 1
Attempts have been made to apply a method of membrane separation with a microfiltration membrane (MF membrane) of about μm.

[Problems to be Solved by the Invention] However, the treatment with the MF membrane has drawbacks such as a low filtration rate and easy clogging of the membrane.

There are various causes for this, for example, the following may be mentioned. Membrane filtration of the pH-neutralized coagulation-sedimentation-treated water using dithiocarbamate-based heavy metal scavenger or sodium sulfide causes bacteria to occur on the membrane surface and causes membrane clogging. On the other hand, when the alkaline coagulation sedimentation method that uses calcium salt and iron salt together, which is difficult to generate bacteria, is adopted, the filtration rate decreases due to membrane clogging due to CaCO ₃ scale generation on the membrane surface or colloidal heavy metal hydroxide. Happens.

Therefore, it was practically difficult to apply the MF membrane in the conventional method.

The present invention solves the above-mentioned conventional problems and eliminates wastewater containing heavy metals.
The present invention relates to a treatment method for heavy metal-containing wastewater, which can be efficiently treated by applying a membrane treatment with an MF membrane to obtain treated water of excellent water quality.

[Problems to be Solved by the Invention] A method for treating heavy metal-containing wastewater according to the present invention is a method in which a magnesium compound is added to heavy metal-containing wastewater, pH is adjusted to alkaline, and the produced insoluble matter is subjected to a microfiltration membrane (MF membrane). It is characterized in that the membrane is separated by.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a system diagram showing an example of implementation of the present invention. In the figure, 1 is a heavy metal-containing wastewater introduction pipe 11 and a chemical supply pipe 12
PH adjusting tank equipped with 2; circulation tank 3; MF equipped with MF membrane 3a
Membrane separator 4, backwash water tank, 5 pH meter, 6 circulation pump, these are connected by pipes 13, 14, 15. 16
Is a drainage pipe for treated water.

In this embodiment, first, the heavy metal-containing wastewater is introduced into the pH adjusting tank 1 through the introduction pipe 11, and the magnesium compound (for example, Mg salt) and the pH adjusting agent are supplied through the supply pipe 12 to adjust the pH. It is adjusted to be alkaline and the heavy metal-containing wastewater is subjected to coagulation treatment. The coagulated water is supplied to the MF membrane separation device 3 through the circulation tank 2 and the power of the pump 6 through the pipe 13.

In the MF membrane separator 3, the treated water that has passed through the MF membrane 3a is pipe 1
5, it is discharged to the outside of the system through the backwash water tank 4 and the discharge pipe 16.
On the other hand, the concentrated water of the MF membrane separator is circulated to the circulation tank 2 via the pipe 14.

In the present invention, the heavy metal-containing wastewater to be treated is a wastewater containing heavy metal ions, a heavy metal complex of a heavy metal and a chelating agent, and examples thereof include plating wastewater. Heavy metals include copper, zinc, nickel, cadmium, manganese, lead and iron. Further, as the chelating agent, citric acid, tartaric acid, gluconic acid, malonic acid, EDTA,
Organic acids such as NTA, or amines such as triethanolamine. Generally, many wastewaters containing heavy metal complexes are acidic, but in the present invention, the pH of the wastewater to be treated is not particularly limited.

As magnesium compounds added to these wastewaters,
There are magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium chloride, magnesium sulfate and the like, and any compound containing magnesium such as magnesium hydroxide or magnesium chloride which is a by-product of salt water purification and salt production may be used. These magnesium compounds can be dissolved by adding solid ones such as magnesium oxide and magnesium hydroxide when the raw water is acidic, but magnesium chloride when the raw water is neutral or alkaline,
It is preferable to add a soluble salt such as magnesium sulfate.

The treatment method of the present invention comprises adding the above-mentioned magnesium compound to heavy metal-containing wastewater and adjusting the pH to alkaline, preferably pH 10 to 10.
The pH is adjusted to 12, more preferably pH 10.5-11.5. When the pH is less than 10, the insoluble product generation efficiency is low, and when the pH is more than 12, the insoluble product is dispersed and a large amount of a pH adjusting agent is required, which is disadvantageous. Here, examples of the pH adjusting agent used for pH adjustment include alkaline agents such as sodium hydroxide and slaked lime.

In this way, by adding a magnesium compound to the heavy metal-containing wastewater to adjust the pH to alkaline, the heavy metal becomes insoluble as a hydroxide, and excess magnesium becomes insoluble as a hydroxide. Therefore, this insoluble matter is MF
By performing membrane separation with a membrane, treated water of extremely high quality can be obtained.

The addition amount of the magnesium compound is appropriately a reaction equivalent amount of the magnesium compound and the heavy metal or a 2- to 3-fold excess amount thereof, and is appropriately determined according to the heavy metal concentration of the heavy metal-containing wastewater used as raw water. Since the amount of heavy metals in the raw water varies, it is preferable to add it in excess to react. The magnesium compound added in excess precipitates as sludge, but by adjusting the pH of the precipitated sludge to 9-10,
70-80% of the precipitated magnesium can be dissolved and recovered,
Can be reused.

In the present invention, the material of the MF film is not particularly limited,
There are also no particular restrictions on the operating conditions. The pore size of the MF membrane is preferably about 0.1 to 1 μm from the viewpoint of improving treatment efficiency and treated water quality. In addition, there is no particular limitation on the membrane separation method using the MF membrane, either a total volume type or a cross flow type may be used, but when the turbidity concentration of the inflow water to the membrane separation device is high, It is preferable to adopt a cross-flow type in which raw water flows in parallel. According to the cross flow formula,
It is possible to reduce the clogging of the membrane even in the case of raw water having a high suspended matter concentration.

When carrying out the method of the present invention, a cake layer of insoluble matter is formed on the MF membrane surface by passing water to the MF membrane separator,
The filtration rate decreases with time. Therefore, the treated water is supplied from the backwash water tank 4 from the permeation side of the membrane to the MF membrane separator 3 at regular intervals.
It is preferable to return it to and carry out backwashing.

[Operation] When the heavy metal-containing wastewater is highly treated, iron salt, aluminum salt and the like are usually added as a coprecipitating agent in the coagulation-precipitation step, and the precipitation-treated water is further filtered. In this case, iron salt
The Fe _x (OH) _y ^{(3x-y) +} inorganic polymer and the aluminum salt form Al _x (OH) _y ^{(3x-y) +} inorganic polymer, which acts as a coprecipitant.

In contrast, in the magnesium compound used in the present invention, although ^{_{Mg x (OH) y (2x}} -y) + the inorganic polymer is a co-precipitant, ^{_{Mg x (OH) y (2x}} -y) + inorganic Since the polymer has a shorter cross-linking length than other inorganic polymers, sludge (insoluble matter) having low viscosity can be obtained. For this reason, the sludge dewatering efficiency is increased, and the water content of the dehydrated cake is lowered.

On the other hand, in the membrane separation by the MF membrane, the filtration rate is affected by the viscosity of the sludge, and when the viscosity increases, the filtration resistance increases, and as a result, the filtration rate decreases, but according to the method of the present invention, as described above, A low-viscosity sludge is formed and a high filtration rate is obtained. Further, since the sludge has low tackiness, backwashing can be easily performed.

By the way, the magnesium compound is extremely effective also for a heavy metal complex. That is, by adding a magnesium compound and adjusting the pH to be alkaline, the heavy metal complex is efficiently decomposed and insolubilized by the following reaction mechanism.

X · M + Mg ²⁺ → X · Mg + M ²⁺ X · Mg + M ²⁺ ＋2 (OH) ⁻ → X · Mg + M (OH) ₂ ↓ (X: chelating agent, M: heavy metal) From the above, the method of the present invention According to the method, the heavy metal complex can be efficiently treated, and treated water having extremely high water quality can be obtained.

[Examples] Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

Example 1 Plating factory wastewater (pH3.0, COD20ppm, Cu15.9ppm, Fe1.1pp
The MgSO ₄ · 7H ₂ O to m) by 100ppm added as Mg, pH with NaOH
11, and the treatment was performed by the method shown in FIG. The MF membrane used was a tube with a pore size of 0.2μ. Operating conditions are circulation flow rate 8.5 l / min, sludge concentration 8000 ppm, membrane inlet pressure 2 kg / c
m ²

The processing results are shown in Table 1.

As is clear from Table 1, the amount of filtered water is stable,
The quality of the treated water is excellent, with both Cu and Fe being less than 0.1 ppm.

Comparative Example _{1 MgSO 4 · 7H 2 O FeCl} 3 · 6H 2 O 100ppm instead of (as Fe)
The same processes as in Example 1 were carried out except that was used, and the results are shown in Table 1.

As is clear from Table 1, compared with the Mg salt agglomeration method, the iron salt method was slightly inferior in treated water and the filtered water amount was about 1/3.

In addition, in this case, the amount of filtered water in the iron salt method was low, in addition to the difference in the filtration characteristics of the inorganic polymer, a small amount of colloidal heavy metal remained in the iron salt method, but it is presumed that this blocked the membrane. It

Comparative Example 2 For the same wastewater as in Example 1, MgSO ₄ .7H ₂ O was used as Mg to obtain 100
ppm at pH 11, or, at 100ppm of FeCl ₃ · 6H ₂ O as Fe pH 1
1, or after adjusting the pH to 11 with NaOH, add 1 each polymer flocculant
ppm was added to produce flocs, and the filtrate was analyzed by filtering with filter paper No. 5A. The results are shown in Table 2.

As is clear from Table 2, in the case of the Mg salt method, the treated water after coagulation and sedimentation is the same as the MF membrane treated water by the filtration method, but the iron salt method is inferior to the MF membrane treated water. Since Cu remains in the NaOH method, it is presumed that a dispersant or a chelating agent is present in the sample water.

Example 2 Electro-deposition coating wastewater (pH 6.1, COD 138 ppm, Zn 67.1 ppm, Cu 2.89p
pm, Ni 8.44ppm, Fe 4.41ppm) to _{MgSO 4 · 7H 2 O 200ppm (} Mg
) Was added to adjust the pH to 11 with NaOH, and the mixture was treated by the method shown in FIG. The specifications of the membrane and the operating conditions were the same as in Example 1. However, the sludge concentration was 10,000 ppm.

The results are shown in Table 3.

As shown in Table 3, the amount of filtered water is stable and
Zn, Cu, Ni and Fe were 0.1 ppm or less.

Comparative Example 3 Wastewater was coagulated with FeCl ₃ 6H ₂ O 200ppm (as Fe) (pH 11)
The same operation as in Example 2 was performed except that the above was performed.

The results are shown in Table 3.

As shown in Table 3, the quality of treated water in the iron salt agglomeration method was slightly worse than that in the Mg salt agglomeration method, and the amount of filtered water was about 1/2.

Comparative Example 4 The same operation as in Example 2 was performed except that the Mg salt was not added and the wastewater was adjusted to pH 11.4 with NaOH. The results are shown in Table 3.

As shown in Table 3, the treated water quality of the NaOH method was poorer than that of the Mg salt agglomerated salt method, and the amount of filtered water was about 1/3.

[Effects of the Invention] As described above in detail, according to the method for treating heavy metal-containing wastewater of the present invention, maintenance of the device is easier than the method of filtering with a filter after coagulation and sedimentation.

No sedimentation basin is required, and the equipment installation area is reduced by about 40%.

Due to the excellent agglomeration effect of the Mg-based inorganic polymer, an insolubilized product with good dehydration and separability can be obtained.

As a result, clogging of the MF membrane is prevented and the filtration rate is increased. Also, the amount of filtered water (the amount of treated water) is large.

 Treatment of heavy metal complexes is also possible.

The effects such as
It is possible to efficiently and highly collect highly treated water of extremely high quality in a large amount and quickly.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of implementation of the present invention. 1 ... pH adjusting tank, 2 ... Circulating tank, 3 ... MF membrane separator,
4 ... Backwash water tank.

Claims (1)

[Claims] 1. A method for treating heavy metal-containing wastewater, which comprises adding a magnesium compound to heavy metal-containing wastewater, adjusting the pH to alkaline, and subjecting the produced insoluble matter to membrane separation with a microfiltration membrane.