Wastewater treatment method

Abstract

To provide a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater, which suppresses membrane blockage to achieve high efficiency.SOLUTION: The present invention relates to a wastewater treatment method including: a step of adding a chelating agent having a weight-average molecular weight of 2000 or less to heavy metal-containing wastewater to provide heavy metal-containing sludge; and a filtration step of separating and removing the heavy metal-containing sludge.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a wastewater treatment method. Specifically, the present invention relates to a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater.

Background Art

[0002] In order to protect water resources, water treatment is required to remove impurity components from various waters such as tap water, wastewater, sewage, and polluted water to obtain purified water. For example, heavy metal components contained in the wastewater of the metal industry, non-ferrous metal primary smelting and refining industry, non-ferrous metal secondary smelting and refining industry, electroplating industry, and fishery food manufacturing industry, such as cadmium which is the causative substance of itai-itai disease, may affect human health. Therefore, environmental standards and emission standards regarding heavy metals have been established in various countries including United Nations agencies and Japan. Cadmium has been strengthened to a drainage standard of 0.03 mg / L or less under the Water Pollution Control Law in response to the setting of the tolerable intake by United Nations agencies (Non-Patent Document 1).

[0003] As a method for treating heavy metal-containing wastewater, a treatment method using a chelating agent is known. For example, Patent Document 1 discloses a method for determining the required addition amount of a chelating heavy metal treatment agent for removing heavy metal components in heavy metal-containing wastewater by adding a chelating heavy metal treatment agent to the heavy metal-containing wastewater. In this method, the pH of the heavy metal-containing wastewater is adjusted to near neutral (pH 6 to 8) while suppressing the pH fluctuation range within ±0.5, and the chelating heavy metal treatment agent is added to the wastewater. The addition amount of this chelating heavy metal treatment agent and the change amount of the oxidation-reduction potential of the wastewater before and after the addition of this chelating heavy metal treatment agent are measured, and based on this measurement result, the required addition amount is determined. The method is characterized in that the addition amount of the chelating heavy metal treatment agent when the change amount of the oxidation-reduction potential is maximized with respect to the addition amount of the chelating heavy metal treatment agent is set as the required addition amount. Further, Patent Document 2 describes a water treatment system having a primary treatment means for biologically treating raw water, which is fishery processing wastewater, to obtain treated water with a COD of 160 mg / L or less, a means for adding an inorganic flocculant and a chelating agent to the treated water disposed downstream of the primary treatment means, and a means for performing a physical treatment of filtration using a microfiltration membrane or an ultrafiltration membrane on the treated water disposed downstream of the means for adding the inorganic flocculant and the chelating agent, and having a means for adjusting the pH of the treated water to 6 to 8 downstream of the primary treatment means and upstream of the means for performing the physical treatment.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, chelating agents containing polymer components may cause membrane fouling in the filtration process carried out downstream, which has been a problem.

[0007] Therefore, in order to solve such problems of the prior art, the present inventors have conducted studies for the purpose of providing an efficient wastewater treatment method with suppressed membrane fouling in a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater.

Means for Solving the Problems

[0008] Examples of specific embodiments of the present invention are shown below.

[0009] [1] A step of adding a chelating agent having a weight average molecular weight of 2000 or less to heavy metal-containing wastewater to obtain heavy metal-containing sludge, and A wastewater treatment method including a filtration treatment step of separating and removing the heavy metal-containing sludge. [2] The wastewater treatment method according to [1], wherein the filtration treatment step is a filtration treatment step using a microfiltration membrane or an ultrafiltration membrane. [3] The wastewater treatment method according to [2], wherein the microfiltration membrane or ultrafiltration membrane is regularly washed with a cleaning liquid containing at least one selected from the group consisting of citric acid, hypochlorous acid, and hydrochloric acid. [4] The wastewater treatment method according to any one of [1] to [3], wherein the heavy metal-containing wastewater contains cadmium. [5] The wastewater treatment method according to any one of [1] to [4], wherein the heavy metal-containing wastewater is flue gas desulfurization wastewater from a thermal power plant. [6] The wastewater treatment method according to any one of [1] to [5], wherein the chelating agent is piperazine-based dithiocarbamate.

Effects of the Invention

[0010] According to the present invention, in a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater, membrane fouling can be suppressed. Thereby, while suppressing the cost of the wastewater treatment method, wastewater treatment can be efficiently performed.

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] Hereinafter, the present invention will be described in detail. The description of the constituent elements described below may be made based on typical embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0013] (Wastewater Treatment Method) The present embodiment relates to a wastewater treatment method including a step of adding a chelating agent having a weight average molecular weight of 2000 or less to heavy metal-containing wastewater to obtain heavy metal-containing sludge, and a filtration treatment step of separating and removing the heavy metal-containing sludge. In the present embodiment, the chelating agent is an agent for collecting heavy metals contained in wastewater, and purified water can be obtained by appropriately removing the chelating agent that has collected heavy metals.

[0014] In the wastewater treatment method of the present embodiment, since a chelating agent having a weight average molecular weight of 2000 or less is added to the heavy metal-containing wastewater, it is possible to suppress the membrane from being fouled in the filtration treatment performed downstream. Thereby, while suppressing the cost of the wastewater treatment method, the wastewater treatment efficiency can be increased.

[0015] Also, in the wastewater treatment method of this embodiment, even when using wastewater containing heavy metals at a high concentration, the impurity concentration of the finally obtained purified water can be made very low. In particular, even when using heavy metal-containing wastewater containing cadmium at a high concentration (0.1 mg / L or more), it is possible to surely obtain purified water with a cadmium concentration less than the discharge regulation value (0.03 mg / L) of the Water Pollution Control Law.

[0016] The weight average molecular weight of the chelating agent is preferably 2000 or less, more preferably 1500 or less, still more preferably 1000 or less, even more preferably 700 or less, and particularly preferably 500 or less. In addition, the weight average molecular weight of the chelating agent is preferably 100 or more, and more preferably 200 or more. By setting the weight average molecular weight of the chelating agent within the above range, it is possible to more effectively suppress the clogging of the membrane in the filtration process performed downstream. Thereby, while suppressing the cost of the wastewater treatment method, the wastewater treatment efficiency can be increased. The weight average molecular weight of the chelating agent can be measured by gel permeation chromatography (GPC) method. Here, the weight average molecular weight refers to a value converted using standard PEG / PEO measured by gel permeation chromatography (GPC method). <GPC Conditions> · Equipment: HLC-8320GPC (manufactured by Tosoh Corporation) · Detector: RI detector polarity (+) · Column: TSKgel guardcolumn SuperAW-H (4.6 mm I.D. × 3.5 cm) + TSKgel SuperAWM-H (6.0 mm I.D. × 15 cm) × 2 (manufactured by Tosoh Corporation) · Eluent: Dimethylformamide (DMF) + 10 mM-LiBr + 30 mM-Triethylamine · Flow rate: 0.6 L / min · Temperature: 40 °C

[0017] Examples of the chelating agent used in this embodiment include dialkyldithiocarbamic acid, piperazine, piperazine-based dithiocarbamic acid, cycloalkyldithiocarbamic acid, dithiocarbamic acid, pyrrolidine dithiocarbamic acid, and salts thereof. Since these compounds and their salts have complex-forming functional groups, they can capture heavy metals contained in wastewater. Among them, the chelating agent is preferably at least one selected from piperazine, piperazine-based dithiocarbamic acid, and piperazine-based dithiocarbamate, and particularly preferably a piperazine-based dithiocarbamate.

[0018] This embodiment may relate to a wastewater treatment apparatus for implementing a wastewater treatment method. FIG. 1 is a schematic diagram showing an example of a wastewater treatment apparatus (wastewater treatment system) for implementing the wastewater treatment method of this embodiment. The wastewater treatment apparatus shown in FIG. 1 preferably includes means for adding a chelating agent to the wastewater. In the pretreatment tank, it is preferable to provide means for measuring the quality of the wastewater (for example, the concentration of heavy metals such as cadmium in the wastewater). Accordingly, the addition amount of the chelating agent in the means for adding the chelating agent is calculated according to the cadmium concentration measured there.

[0019] In the pretreatment tank or upstream thereof, preliminary treatment (primary treatment means) can be performed so as to obtain the maximum addition effect of the chelating agent. For example, when the heavy metal-containing wastewater contains a large amount of protein and oil and fat, problems such as inhibiting the reaction between the chelating agent and the heavy metal may occur. In the pretreatment tank, it is preferable to remove components such as protein and oil and fat, for example. In this specification, the wastewater that has undergone preliminary treatment (primary treatment means) is also referred to as treated water. When the primary treatment means is not provided, the quality of the heavy metal-containing wastewater and the wastewater upstream of the pretreatment tank referred to in FIG. 1 is the same.

[0020] The wastewater treatment apparatus is provided with a filtration treatment step downstream of the pretreatment tank. In the filtration treatment step, the heavy metal-containing sludge obtained in the step of adding the chelating agent is separated and removed.

[0021] The wastewater treatment device may include means (not shown, for example, an arbitrary pump, etc.) for returning at least a part of the treated water to the pretreatment layer as recycled treated water. When the cadmium concentration and the concentrations of other components (for example, COD, BOD, SS concentration, turbidity, etc.) of the treated water are not less than a predetermined value, at least a part of the treated water may be returned to the pretreatment layer as recycled treated water. Further, at least a part of the treated water that has passed through the filtration treatment step may be returned to the pretreatment layer as recycled treated water.

[0022] <Heavy metal-containing wastewater> Examples of heavy metal-containing wastewater include, for example, industrial wastewater (such as wastewater discharged from fishery processing plants, automobile factories, non-ferrous metal industries, plating factories, semiconductor factories, etc.), thermal power plant wastewater, sewage, and the like. In the present embodiment, it is preferable that the heavy metal-containing wastewater to be treated is wastewater containing cadmium, but in addition to cadmium, heavy metals such as copper (Cu), zinc (Zn), tin (Sn), nickel (Ni), and lead (Pb) may be contained.

[0023] Examples of heavy metal-containing wastewater include, for example, fishery processing wastewater. Fishery processing wastewater is wastewater discharged from fishery processing establishments and mainly contains organic substances and heavy metals derived from fish, shellfish, and fish eggs. In particular, since cadmium is contained in the midgut gland of scallops, the liver of squid, and the internal organs of crabs, wastewater containing cadmium is discharged from fishery processing plants that process these mollusks and crustaceans. Fishery processing wastewater usually has a BOD concentration of 10 to 5000 mg / L, a COD concentration of 30 to 3000 mg / L, a salt concentration of 0 to 20%, a cadmium concentration of 0 to 3 mg / L, a zinc concentration of 0 to 30 mg / L, and an iron concentration of 0 to 30 mg / L.

[0024] In addition, in this embodiment, it is preferable to use the wastewater from a thermal power plant as the heavy metal-containing wastewater, and it is particularly suitable as a method for treating the flue gas desulfurization wastewater of a thermal power plant containing components such as COD, heavy metals, and fluorine. The flue gas desulfurization wastewater of a thermal power plant is wastewater containing a large amount of cadmium, and particularly in summer, the wastewater temperature rises to about 50°C. The flue gas desulfurization wastewater of a thermal power plant usually has a COD concentration of 10 - 300 mg / L, a cadmium concentration of 0.1 - 0.5 mg / L, and a fluorine concentration of 10 - 2000 mg / L.

[0025] When using the flue gas desulfurization wastewater of a thermal power plant as the heavy metal-containing wastewater, the difference between the maximum water temperature and the minimum water temperature of the heavy metal-containing wastewater during the year is preferably 10°C or more, and the maximum water temperature during the year is preferably 40°C or more. In the wastewater treatment method of this embodiment, even when the difference between the maximum water temperature and the minimum water temperature of the heavy metal-containing wastewater during the year is 10°C or more and the maximum water temperature during the year is 40°C or more, the cadmium concentration in the wastewater remains almost constant throughout the year. Therefore, the addition amount of the chelating agent can be controlled to an appropriate amount, and the control of the addition amount of the chelating agent in the wastewater treatment process is easy. For this reason, the cost of the wastewater treatment method can be suppressed throughout the year, and the wastewater treatment efficiency can be more effectively enhanced.

[0026] The amount of heavy metal-containing wastewater that can be treated in the wastewater treatment method of this embodiment is not particularly limited, and it is possible to treat a large amount of heavy metal-containing wastewater. Specifically, it is also possible to treat 500 m 3 or more of heavy metal-containing wastewater per day.

[0027] <Primary treatment process> When the heavy metal-containing wastewater contains organic substances (e.g., proteins and fats), it may inhibit the reaction between the chelating agent and heavy metals, and as a result, there is a concern that the addition amount of the chelating agent may increase. In addition, when performing membrane treatment or the like described later, organic substances adhere to the membrane surface, preventing filtration, increasing the viscosity of the heavy metal-containing wastewater, increasing the passing resistance of the filtration membrane, increasing the frequency of backwashing of the membrane, or increasing the required membrane area, causing various problems. Therefore, in the wastewater treatment method of this embodiment, primary treatment (pretreatment) may be performed before adding a chelating agent to the heavy metal-containing wastewater. Such a primary treatment step can be carried out by any one of physicochemical treatments such as coagulation sedimentation treatment and pressurized flotation treatment, and biological treatments such as activated sludge treatment, biological membrane filtration treatment, biological membrane treatment, and anaerobic biological treatment, or a combination of multiple treatment methods. By selecting and combining appropriate treatment methods, treatment with excellent cost-effectiveness can be achieved.

[0028] <Step of adding a chelating agent> In the step of adding a chelating agent, a chelating agent with a weight average molecular weight of 2000 or less is added to the heavy metal-containing wastewater or the treated water obtained through the primary treatment step. By adding a chelating agent to the heavy metal-containing wastewater or the treated water obtained through the primary treatment step, heavy metal-containing sludge can be obtained.

[0029] In the step of adding a chelating agent, after adding the chelating agent, it is preferable to mix well for an appropriate mixing time.

[0030] In the step of adding a chelating agent, an inorganic flocculant may be added in addition to the chelating agent. The timing of adding the inorganic flocculant is preferably before the addition of the chelating agent. Examples of the inorganic flocculant include PAC (polyaluminum chloride), aluminum-based flocculants such as aluminum sulfate, polyiron, ferrous chloride, or ferric chloride. Examples of the polymer flocculant include anionic polymer flocculants, cationic polymer flocculants, nonionic polymer flocculants, etc. More specifically, polyacrylamide, 2-acryloylamino-2-methylpropanesulfonic acid (AMPS), etc. can be mentioned. By adding an inorganic flocculant or a polymer flocculant, heavy metals other than cadmium and other components can also be effectively removed. When the step of separating heavy metals described later is a filtration treatment step using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane), it is preferable not to add a polymer flocculant from the viewpoint of suppressing membrane fouling.

[0031] In the step of adding a chelating agent, the pH of the heavy metal-containing wastewater or the treated water obtained through the primary treatment step may be adjusted as necessary. For example, as shown in FIG. 1, means 14 for adjusting the pH may be provided. As shown in FIG. 1, the means 14 for adjusting the pH may be provided simultaneously with the means 13 for adding the chelating agent, or may be provided upstream or downstream of the means 13 for adding the chelating agent. Examples of the pH adjuster include sodium hydroxide, calcium hydroxide, etc.

[0032] The pH of the heavy metal-containing wastewater or the treated water obtained through the primary treatment step in the step of adding a chelating agent is preferably from 4 to 10, more preferably from 5 to 9, and even more preferably from 6 to 8. By setting the pH of the heavy metal-containing wastewater or the treated water obtained through the primary treatment step within the above range, the collection effect of heavy metals by the chelating agent can be enhanced.

[0033] <Filtration treatment step> In the wastewater treatment method of this embodiment, it is preferable to further have a filtration treatment step after the step of adding a chelating agent. In the filtration treatment step, the heavy metal-containing sludge obtained in the step of adding a chelating agent is separated and removed.

[0034] The filtration treatment step is preferably a filtration treatment step using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane). By adopting a filtration treatment step using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane), bacteria such as Escherichia coli can be completely removed from the heavy metal-containing wastewater, and it is also possible to obtain sterile water. In addition, it is possible to suppress the outflow of insolubilized heavy metals into the treated water due to floating depending on the floc formation state. The membrane form is a hollow fiber or a flat membrane, and the heavy metal-containing wastewater is treated by a cross-flow method or a dead-end method. From the viewpoint of energy saving, it is preferable to adopt the dead-end method.

[0035] The purified water 3 obtained through the above-mentioned filtration treatment step using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane) can be directly treated with a nanofiltration membrane (NF membrane) or a reverse osmosis membrane (RO membrane), and desalted water can be easily obtained.

[0036] In this embodiment, it is preferable that the microfiltration membrane or the ultrafiltration membrane is regularly washed with a cleaning liquid containing at least one selected from the group consisting of citric acid, hypochlorous acid, and hydrochloric acid. For example, when the filtration method is the dead-end method, it is desirable to wash the membrane surface at a frequency of once every 5 to 120 minutes, more preferably once every 10 to 60 minutes, and most preferably once every 20 to 40 minutes. Although there is no particular limitation on the cleaning device, since the cleaning waste liquid containing acid, hypochlorous acid, etc. may have an adverse effect on cadmium capture, it is preferable to separate the cleaning device from the wastewater treatment device in terms of line.

[0037] <Cleaning Step> In the cleaning process, as physical cleaning, it is preferable to perform a process of cleaning the membrane surface or backwashing. The cleaning frequency is preferably once every 5 minutes to 120 minutes, more preferably once every 10 minutes to 60 minutes, and even more preferably once every 20 minutes to 40 minutes. As chemical cleaning, the filtration membrane may be chemically cleaned with at least one selected from the group consisting of citric acid, hypochlorous acid, and hydrochloric acid. In this case, the cleaning frequency preferably includes a frequency of once or more per 24 hours, more preferably once every 1 hour to 48 hours, even more preferably once every 6 hours to 36 hours, and particularly preferably once every 12 hours to 24 hours. The chemical concentration may be appropriately set according to the chemical selected, and it is preferably carried out at about 0.1 to 0.5% by mass from the viewpoint of not damaging the filtration membrane.

Examples

[0038] The features of the present invention will be further specifically described below by giving examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be appropriately changed as long as they do not deviate from the gist of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0039] (Example) Using the heavy metal-containing wastewater discharged from a thermal power plant as raw water, the liquid temperature was adjusted to 25°C. The wastewater was continuously treated for 15 days according to the treatment flow shown in Figure 1, and the pressure difference between the membrane inlet and the membrane outlet was measured over time. In the mixing tank in Figure 1, OJI-FLOCK CH-140 (weight average molecular weight 315) manufactured by Oji Engineering Co., Ltd. was added as a chelating agent to a concentration of 20 mg / L.

[0040] (Comparative Example) Except that the chelating agent was changed to Epofloc L-1 (weight average molecular weight 3400) manufactured by Miyoshi Oil & Fat Co., Ltd., the wastewater was continuously treated for 15 days in the same manner as in the example, and the pressure difference between the membrane inlet and the membrane outlet was measured over time.

[0041] From the measurement results, the differential pressure fluctuations (kPa) on the first day (filter membrane outlet pressure - filter membrane inlet pressure) (unit: kPa) and the differential pressure fluctuations (kPa) on the 15th day (filter membrane outlet pressure - filter membrane inlet pressure) (unit: kPa) were determined, and the differential pressure fluctuation range was calculated by finding the difference between the differential pressure fluctuations (kPa) on the 15th day and the differential pressure fluctuations (kPa) on the first day. The differential pressure fluctuation range was as shown in Table 1. Also, physical cleaning was carried out once every 40 minutes, chemical cleaning consisting of citric acid, hypochlorous acid, and hydrochloric acid was carried out once every 24 hours, and the measurement was carried out in the same manner as in the example except that the wastewater was continuously treated for 80 days. As a result, stable operation for a long period was possible without exceeding the differential pressure fluctuation range in Table 1.

[0042]

Table 1

Claims

1. A step of adding a chelating agent having a weight average molecular weight of 2000 or less to the heavy metal-containing wastewater to obtain heavy metal-containing sludge; And a filtration treatment step of separating and removing the heavy metal-containing sludge. A wastewater treatment method.

2. The wastewater treatment method according to claim 1, wherein the filtration treatment step is a filtration treatment step using a microfiltration membrane or an ultrafiltration membrane.

3. The wastewater treatment method according to claim 2, wherein the microfiltration membrane or the ultrafiltration membrane is regularly washed with a cleaning solution containing at least one selected from the group consisting of citric acid, hypochlorous acid, and hydrochloric acid.

4. The wastewater treatment method according to claim 1, wherein the heavy metal-containing wastewater contains cadmium.

5. The wastewater treatment method according to claim 4, wherein the heavy metal-containing wastewater is flue gas desulfurization wastewater from a thermal power plant.

6. The wastewater treatment method according to claim 1, wherein the chelating agent is piperazine-based dithiocarbamate.

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