Wastewater treatment method

Abstract

To provide a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater, which enables addition of a suitable amount of a chelating agent.SOLUTION: The present invention relates to a wastewater treatment method which includes a step of adding a chelating agent to heavy metal-containing wastewater, wherein the addition amount of the chelating agent is determined in accordance with an approximate straight line as a function of the temperature of the heavy metal-containing wastewater and the addition amount of the chelating agent for the target removal rate (%) calculated by the following formula. The target removal rate (%)=(Cadmium level in wastewater (mg / L)-0.02) / (Cadmium level in wastewater (mg / L))×100.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, in response to the setting of the tolerable intake amount by United Nations agencies, the wastewater standard of the Water Pollution Control Law has been strengthened to 0.03 mg / L or less (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 6-8) 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. Based on this measurement result, a method for determining the required addition amount is provided, wherein the addition amount of the chelating heavy metal treatment agent at which the change amount of the oxidation-reduction potential becomes maximum with respect to the addition amount of the chelating heavy metal treatment agent is defined 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, which is 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, since chelating agents are generally expensive, there is a problem that the cost of water treatment increases. In addition, when an excessive amount of chelating agent is added, there is also a concern that the wastewater treatment efficiency deteriorates. Generally, since dithiocarbamate-based chelating agents generate carbon disulfide and decompose, chelating agents tend to be added in excess of the minimum required amount, and with regard to suppressing the excess addition amount, satisfactory results have not necessarily been obtained at present.

[0007] Therefore, in order to solve such problems of the prior art, the present inventors have advanced studies with the aim of providing a wastewater treatment method capable of adding an appropriate amount of chelating agent 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 wastewater treatment method including a step of adding a chelating agent to heavy metal-containing wastewater, wherein the addition amount of the chelating agent is determined based on a function of the temperature of the heavy metal-containing wastewater and the addition amount of the chelating agent that gives a target removal rate (%) calculated by the following formula. Target removal rate (%) = ((cadmium concentration in wastewater (mg / L) - 0.02) / cadmium concentration in wastewater (mg / L)) × 100 [2] The wastewater treatment method according to [1], wherein when the optimum addition amounts calculated by the following procedure are plotted for each temperature of the heavy metal-containing wastewater and an approximate straight line is obtained, the slope of the approximate straight line is 0.5 or less: (Calculation of the optimum addition amount) While keeping the temperature of the heavy metal-containing wastewater constant, the chelating agent is added in each addition amount, and the addition amount when the removal rate calculated by measuring the cadmium concentration in the wastewater before and after the addition exceeds the target removal rate is taken as the optimum addition amount for that temperature. [3] The wastewater treatment method according to [1] or [2], further comprising a step of separating heavy metals after the step of adding a chelating agent. [4] The wastewater treatment method according to [3], wherein the step of separating heavy metals is at least one selected from the group consisting of a filtration treatment step using a microfiltration membrane or an ultrafiltration membrane, a coagulation sedimentation treatment step, a pressure flotation treatment step, and a centrifugal separation treatment step. [5] The wastewater treatment method according to any one of [1] to [4], wherein the heavy metal-containing wastewater contains cadmium. [6] The wastewater treatment method according to any one of [1] to [5], wherein the heavy metal-containing wastewater is flue gas desulfurization wastewater from a thermal power plant. [7] The wastewater treatment method according to any one of [1] to [6], wherein the chelating agent is a piperazine-based dithiocarbamate.

Advantages of the Invention

[0010] According to the present invention, in a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater, an appropriate amount of a chelating agent can be added. Thereby, wastewater treatment can be efficiently performed while suppressing the cost of the wastewater treatment method. According to the present invention, by approximating an appropriate amount of the chelating agent as a linear function, an optimal chelating agent can be selected in a wide range of water temperatures, the excess addition amount can be suppressed, and high-temperature wastewater can be efficiently treated.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

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 the present 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) This embodiment includes a step of adding a chelating agent to heavy metal-containing wastewater, and relates to a wastewater treatment method for determining the addition amount of the chelating agent based on a function of the temperature of the heavy metal-containing wastewater and the addition amount of the chelating agent that gives a target removal rate (%) calculated by the following formula. Target removal rate (%) = (cadmium concentration in wastewater (mg / L) - 0.02) / cadmium concentration in wastewater (mg / L) × 100 In this embodiment, the chelating agent is an agent that collects heavy metals contained in wastewater, and purified water can be obtained by appropriately removing the chelating agent that has collected heavy metals.

[0014] In this embodiment, the function of the temperature of the heavy metal-containing wastewater and the addition amount of the chelating agent that gives a target removal rate (%) calculated by the following formula is represented by an approximate straight line. Specifically, the addition amount of the chelating agent when the removal rate calculated by measuring the cadmium concentration in the wastewater before and after the addition while keeping the temperature of the heavy metal-containing wastewater constant exceeds the target removal rate is defined as the optimum addition amount, and the optimum addition amount is calculated by the following method. Then, for each temperature of the heavy metal-containing wastewater, the optimum addition amount calculated by the following procedure is plotted to obtain an approximate straight line. (Calculation of optimum addition amount) While keeping the temperature of the heavy metal-containing wastewater constant, the chelating agent is added in each addition amount, and the addition amount when the removal rate calculated by measuring the cadmium concentration in the wastewater before and after the addition exceeds the target removal rate is defined as the optimum addition amount for that temperature.

[0015] In the wastewater treatment method of this embodiment, since the amount of chelating agent added is determined based on the approximate straight line of the function of the temperature of the heavy metal-containing wastewater and the target removal rate (%) calculated by the above formula, in the wastewater treatment method for removing heavy metals from heavy metal-containing wastewater, an appropriate amount of chelating agent can be added. That is, since the amount of chelating agent added can be optimized, the cost of the wastewater treatment method can be suppressed while increasing the wastewater treatment efficiency.

[0016] Moreover, in the wastewater treatment method of this embodiment, even when wastewater containing heavy metals at a high concentration is used, the impurity concentration of the finally obtained purified water can be made very low. In particular, even when heavy metal-containing wastewater containing cadmium at a high concentration (0.1 mg / L or more) is used, purified water with a cadmium concentration surely less than the discharge regulation value (0.03 mg / L) of the Water Pollution Control Law can be obtained.

[0017] In the wastewater treatment method of this embodiment, for each temperature of the heavy metal-containing wastewater, the slope of the approximate straight line obtained by plotting the optimal addition amount is preferably 0.5 or less, more preferably 0.4 or less, and even more preferably 0.3 or less. The lower limit value of the slope is not particularly limited and is preferably 0.01 or more. Also, the slope may be substantially 0.0. The fact that the value of the slope of the above approximate straight line is below the above upper limit means that the change in the amount of chelating agent required depending on the temperature of the wastewater is small. By appropriately selecting a chelating agent whose slope value is below the above upper limit, the cost of the wastewater treatment method can be suppressed. Also, the control of the amount of chelating agent added in the wastewater treatment process becomes easy, and the wastewater treatment efficiency can be more effectively increased.

[0018] Here, the optimal addition amount can be determined experimentally. Specifically, it is preferably determined experimentally at a plurality of arbitrary constant temperatures, for example, 20°C, 40°C, and 60°C, within a water temperature range of at least 20°C or higher and 60°C or lower. The maximum temperature within this water temperature range is preferably set with reference to the highest water temperature of the discharged water throughout the year. From the perspective of approximating the appropriate amount of the chelating agent as a linear function, the highest annual water temperature of the target heavy metal-containing wastewater is preferably 90°C or lower, more preferably 80°C or lower, and even more preferably 70°C or lower. On the other hand, the highest annual water temperature of the heavy metal-containing wastewater is preferably 40°C or higher, more preferably 50°C or higher. Also, the difference between the highest and lowest annual water temperatures of the heavy metal-containing wastewater is preferably 10°C or higher, more preferably 20°C or higher. The upper limit of the difference between the highest and lowest annual water temperatures of the heavy metal-containing wastewater is not particularly limited, but is preferably 40°C or lower, more preferably 30°C or lower. In this embodiment, even for such heavy metal-containing wastewater, the excess addition amount can be stably suppressed.

[0019] The addition amount of the chelating agent when obtaining the approximate straight line may be appropriately selected from the perspective of suppressing the cost of the wastewater treatment method, and is preferably any value between 1 mg / L and 100 mg / L, more preferably between 3 mg / L and 40 mg / L, and even more preferably between 5 mg / L and 20 mg / L. When calculating the optimal addition amount, the addition amount of the chelating agent is gradually increased, but the interval of the increase is not particularly limited, and is preferably about 10 mg / L, more preferably about 5 mg / L, even more preferably about 2.5 mg / L, and particularly preferably about 1.5 mg / L.

[0020] In this embodiment, in order to determine the addition amount of the chelating agent based on the approximate straight line obtained by plotting the optimal addition amount for each temperature of the heavy metal-containing wastewater, a chemical injection coefficient consisting of the addition amount at the reference temperature and the slope of the approximate straight line is used. A temperature control formula adjusted to the decomposition degree of the chelating agent is preset in advance, and the injection amount can be controlled by inputting the wastewater water temperature. For example, if the highest water temperature of the discharged water throughout the year is 60°C and the reference temperature is 20°C, the temperature control formula for obtaining the chemical addition amount from the wastewater water temperature is represented by the following formula. Drug addition amount (mg / L) = ((Addition amount at 60°C (mg / L) - Addition amount at 20°C (mg / L)) / (60 - 20)) × (Wastewater temperature - 20) + Addition amount at 20°C (mg / L)

[0021] This embodiment may relate to a wastewater treatment apparatus for implementing a wastewater treatment method. The wastewater treatment apparatus has a chelating agent adding means, and in this chelating agent adding means, based on an approximate straight line of a function between the temperature of heavy metal-containing wastewater and the addition amount of a chelating agent that gives a target removal rate (%) calculated by the following formula, the chelating agent is added. Target removal rate (%) = ((Cadmium concentration in wastewater (mg / L) - 0.02) / Cadmium concentration in wastewater (mg / L)) × 100 Regarding the approximate straight line, it is the same as that described in the item of the wastewater treatment method.

[0022] 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 has a heavy metal collection means 31 including a means 13 for adding a chelating agent. Here, the addition amount of the chelating agent in the means 13 for adding the chelating agent is calculated based on the approximate straight line described above. Downstream of the heavy metal collection means 31, a step of separating heavy metals is preferably provided. For example, as shown in FIG. 1, it is preferable that a heavy metal separation means 12 is arranged. In the heavy metal separation means 12, the heavy metals collected by the chelating agent are separated and removed. Upstream of the heavy metal collection means 31 or at the same position, a water temperature measuring means for heavy metal-containing wastewater or treated water 2 is provided. In this embodiment, means for adding a plurality of chelating agents may be provided, and further, switching means that can be switched between these adding means may be provided. Thereby, using a plurality of chelating agents at which the approximate straight lines intersect, comparing the temperature at which the approximate straight lines intersect with the wastewater temperature, and the chelating agent can be selectively selected and used according to the water temperature range. Thereby, it is also possible to selectively collect heavy metals other than cadmium eluted depending on the water temperature range.

[0023] In the wastewater treatment apparatus for implementing the wastewater treatment method of the present embodiment, upstream of the metal collection means 31, for example, there may be a primary treatment means 11 for passing the heavy metal-containing wastewater 1 to obtain treated water. In the primary treatment means 11, preliminary treatment can be performed so that the addition effect of the chelating agent can be maximally obtained. 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 primary treatment means 11, it is preferable to remove components such as protein and oil and fat, for example. In this specification, the wastewater that has passed through the primary treatment means 11 is also referred to as treated water 2. When the primary treatment means 11 is not provided, the water quality of the heavy metal-containing wastewater 1 and the treated water 2 upstream of the metal collection means 31 shown in FIG. 1 is the same.

[0024] Upstream of the metal collection means 31 or at its upstream, it is preferable that means for measuring the water quality of the treated water 2 (for example, the concentration of heavy metals such as cadmium in the treated water) is provided. According to the cadmium concentration measured there, the addition amount of the chelating agent in the means 13 for adding the chelating agent will be calculated.

[0025] The wastewater treatment apparatus may include means (not shown, for example, an arbitrary pump, etc.) for returning at least a part of the treated water 2 as the returned treated water 4 to the primary treatment means 11. In particular, when the cadmium concentration and the concentrations of other components (for example, COD, BOD, SS concentration, turbidity, etc.) of the treated water 2 that has passed through the primary treatment means 11 are not less than a predetermined value, at least a part of the treated water 2 may be returned as the returned treated water 4 to the primary treatment means 11. Also, at least a part of the treated water 3 that has passed through the heavy metal separation means 12 may be returned as the returned treated water 4' to the primary treatment means 11.

[0026] In the wastewater treatment apparatus, as shown in FIG. 1, it is preferable that each of the means of the primary treatment means 11, the metal collection means 31, and the heavy metal separation means 12 can discharge the sludge 21. When discharging the sludge 21 and the returned treated water 4 from the metal collection means 31, the returned treated water 4 may contain the returned sludge.

[0027] <Heavy metal-containing wastewater> Examples of heavy metal-containing wastewater include, for example, industrial wastewater (such as wastewater discharged from seafood processing plants, automobile factories, non-ferrous metal industries, plating factories, semiconductor factories, etc.), thermal power plant wastewater, and sewage. The heavy metal-containing wastewater to be treated in this embodiment is preferably wastewater containing cadmium. However, in addition to cadmium, heavy metals such as copper (Cu), zinc (Zn), tin (Sn), nickel (Ni), and lead (Pb) may also be contained.

[0028] Examples of heavy metal-containing wastewater include, for example, seafood processing wastewater. Seafood processing wastewater is wastewater discharged from seafood processing establishments and mainly contains organic matter 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 seafood processing plants that process these mollusks and crustaceans. Seafood 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.

[0029] In addition, in this embodiment, it is preferable to use the wastewater of 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 COD components, heavy metals, fluorine, etc. 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 to 300 mg / L, a cadmium concentration of 0.1 to 0.5 mg / L, and a fluorine concentration of 10 to 2000 mg / L.

[0030] When using the flue gas desulfurization wastewater of a thermal power plant as heavy metal-containing wastewater, the difference between the maximum annual water temperature and the minimum annual water temperature of the heavy metal-containing wastewater is preferably 10 °C or more, and the maximum annual water temperature is preferably 40 °C or more. In the wastewater treatment method of this embodiment, even when the difference between the maximum annual water temperature and the minimum annual water temperature of the heavy metal-containing wastewater is 10 °C or more and the maximum annual water temperature 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 it is easy to control the addition amount of the chelating agent in the wastewater treatment process. 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 increased.

[0031] In the wastewater treatment method of this embodiment, the amount of heavy metal-containing wastewater that can be treated 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.

[0032] <Primary treatment process> When the heavy metal-containing wastewater contains organic substances (for example, proteins and oils and fats), it may inhibit the reaction between the chelating agent and the heavy metal, 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, the organic substances adhere to the membrane surface and interfere with filtration, increase the viscosity of the heavy metal-containing wastewater, increase the passing resistance of the filtration membrane, increase the frequency of backwashing the membrane, or increase the required membrane area, causing various problems. For this reason, in the wastewater treatment method of this embodiment, primary treatment (pretreatment) may be performed before adding the chelating agent to the heavy metal-containing wastewater. Such a primary treatment process 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 becomes possible.

[0033] <Step of adding chelating agent> In the step of adding a chelating agent, a chelating agent is added to the heavy metal-containing wastewater or the treated water obtained through the primary treatment step.

[0034] Examples of the chelating agent used in the present 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 the 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. By using a piperazine-based dithiocarbamate, the increase in the amount of the chelating agent added with the increase in the temperature of the wastewater can be suppressed, and the cost of the wastewater treatment method can be reduced. In addition, the control of the amount of the chelating agent added in the wastewater treatment step becomes easy, and the wastewater treatment efficiency can be more effectively enhanced.

[0035] For example, when dialkyldithiocarbamic acid (EPO Flock L-1 manufactured by Miyoshi Oil & Fat Co., Ltd.) is used as the chelating agent and when a piperazine-based dithiocarbamate (OJI-FLOCK CH-140 manufactured by Oji Engineering Co., Ltd.) is used, an approximate straight line as shown in FIG. 2 can be obtained. When dialkyldithiocarbamic acid (EPO Flock L-1 manufactured by Miyoshi Oil & Fat Co., Ltd.) is used, the value of a in the approximate straight line (y = ax + b) is 0.60, but when a piperazine-based dithiocarbamate (OJI-FLOCK CH-140 manufactured by Oji Engineering Co., Ltd.) is used, the value of a in the approximate straight line (y = ax + b) is found to be 0.25. The approximate straight line as shown in FIG. 2 is calculated by experimentally measuring the amount of the chelating agent added to achieve the target removal rate (%) after adjusting the temperature of the heavy metal-containing wastewater with each cadmium concentration to each temperature in the range of 20 to 60°C.

[0036] The step of adding a chelating agent preferably includes means for measuring the temperature of the heavy metal-containing wastewater or the treated water obtained through the primary treatment step, and means for measuring the cadmium concentration contained in the heavy metal-containing wastewater or the treated water obtained through the primary treatment step. By providing means for measuring the temperature and means for measuring the cadmium concentration, the amount of chelating agent added can be quickly controlled based on the above approximate straight line. Thereby, it is possible to suppress the addition of an excessive amount of chelating agent, and effectively reduce the heavy metal wastewater treatment cost. Each result obtained by the means for measuring the temperature and the means for measuring the cadmium concentration may be transmitted in real time to the means for adding the chelating agent. Thereby, it becomes possible to determine the amount of chelating agent added in real time.

[0037] In the step of adding a chelating agent, after adding the chelating agent, it is preferable to mix well for an appropriate mixing time.

[0038] In the step of adding a chelating agent, an inorganic flocculant or a polymer flocculant may be added in addition to the chelating agent. The timing of adding the flocculant is preferably before the addition of the chelating agent. Examples of the inorganic flocculant include aluminum-based flocculants such as PAC (polyaluminum chloride) or 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), from the viewpoint of suppressing membrane fouling, it is preferable not to add a polymer flocculant.

[0039] 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 and calcium hydroxide.

[0040] 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.

[0041] <Step of separating heavy metals> In the wastewater treatment method of the present embodiment, it is preferable to further include a step of separating heavy metals after the step of adding a chelating agent. Examples of the step of separating heavy metals include biological treatment, filtration treatment steps using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane), coagulation sedimentation treatment, pressurized flotation treatment, and centrifugal separation treatment. Among them, the step of separating heavy metals is preferably at least one selected from the group consisting of a filtration treatment step using a microfiltration membrane or an ultrafiltration membrane, a coagulation sedimentation treatment step, a pressurized flotation treatment step, and a centrifugal separation treatment step. In biological treatment, a biological treatment membrane may be used. In this case, the biological treatment membrane may be a honeycomb tube, a sponge, or a resin product.

[0042] Among them, the step of separating heavy metals is preferably a filtration treatment step using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane) from the viewpoint of suppressing the outflow of insolubilized heavy metals into the treated water by floating according to the floc formation state. By adopting a filtration treatment step using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane), it is also possible to completely remove bacteria such as Escherichia coli from heavy metal-containing wastewater and obtain sterile water. 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. In the case of the dead-end method, it is desirable to clean 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. In addition, although there is no particular limitation on the cleaning device, since the cleaning waste liquid containing an 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.

[0043] The purified water 3 obtained through the above-described 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 demineralized water can be easily obtained.

[0044] (Method for determining the required addition amount of chelating agent) This embodiment relates to a method for determining the required addition amount of a chelating agent for adding a chelating agent to heavy metal-containing wastewater to remove heavy metals in the heavy metal-containing wastewater, and is based on an approximate straight line of a function between the temperature of the heavy metal-containing wastewater and the addition amount of the chelating agent that gives the target removal rate (%) calculated by the following formula. It may also relate to a method for determining the required addition amount of the chelating agent. Target removal rate (%) = ((cadmium concentration in wastewater (mg / L) - 0.02) / cadmium concentration in wastewater (mg / L)) × 100 Regarding the approximate straight line, it is the same as that described in the item of the wastewater treatment method.

[0045] By adopting a method for determining the required addition amount of such a chelating agent, in a wastewater treatment method for removing heavy metals from heavy metal-containing wastewater, an appropriate amount of the chelating agent can be added. That is, since the addition amount of the chelating agent can be optimized, it is possible to increase the wastewater treatment efficiency while suppressing the cost of the wastewater treatment method.

Examples

[0046] The features of the present invention will be described more specifically below by giving examples and comparative examples. The materials, usage amounts, 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.

[0047] (Example 1) Taking the heavy metal-containing wastewater discharged from a thermal power plant as raw water, it was collected in a 500 ml beaker, and the liquid temperature was adjusted to 20 °C, 40 °C, and 60 °C respectively. The cadmium concentration in the heavy metal-containing wastewater was as shown in Table 1. A chelating agent (Epo Floc L-1 manufactured by Miyoshi Oil & Fat Co., Ltd.) was added to the heavy metal-containing wastewater with the adjusted liquid temperature so as to be the predetermined addition amounts shown in Table 1, and it was stirred at 150 rpm for 10 minutes. Then, filtration was performed using a syringe filter with a pore size of 0.45 μm to obtain treated water. The cadmium concentration in the treated water was as shown in Table 1.

[0048] The target removal rate of cadmium was calculated from the cadmium concentration in the raw heavy metal-containing wastewater by the following formula. Target removal rate (%) = (cadmium concentration in wastewater (mg / L) - 0.02) / cadmium concentration in wastewater (mg / L) × 100 In addition, the actual cadmium removal rate was calculated by the following formula. Removal rate (%) = (cadmium concentration in wastewater (mg / L) - cadmium concentration in treated water (mg / L)) / cadmium concentration in wastewater (mg / L) × 100 When the removal rate first exceeded the target removal rate at a predetermined addition amount, that addition amount was defined as the optimal addition amount for that temperature. Specifically, at 20°C, 6 mg / L was the optimal addition amount of the chelating agent; at 40°C, 22 mg / L was the optimal addition amount of the chelating agent; and at 60°C, 30 mg / L was the optimal addition amount of the chelating agent. The dotted line in Figure 2 is an approximate straight line (with a slope of 0.60) plotting the optimal addition amount against the liquid temperature when adding Epofloc L-1 as the chelating agent.

[0049] (Example 2) The optimal addition amount was determined in the same manner as in Example 1, except that the chelating agent was changed to OJI-FLOCK CH-140 manufactured by Oji Engineering Co., Ltd. OJI-FLOCK CH-140 was added so as to reach the predetermined addition amount shown in Table 2. The solid line in Figure 2 is an approximate straight line (with a slope of 0.25) plotting the optimal addition amount against the liquid temperature when adding OJI-FLOCK CH-140 as the chelating agent.

[0050]

Table 1

[0051]

Table 2

Explanation of Symbols

[0052] 1 Heavy metal-containing wastewater 2 Treated water 3 Purified water 4 Return treated water 11 Primary treatment means 12 Means for separating heavy metals 13 Means for adding a chelating agent 14 Means for adjusting pH 21 Sludge 31 Heavy metal collection means

Claims

1. A wastewater treatment method including a step of adding a chelating agent to heavy metal-containing wastewater, wherein the amount of the chelating agent to be added is determined based on a function of the temperature of the heavy metal-containing wastewater and the addition amount of the chelating agent that gives a target removal rate (%) calculated by the following formula. Target removal rate (%) = (cadmium concentration in wastewater (mg / L) - 0.02) / cadmium concentration in wastewater (mg / L) × 100

2. For each temperature of the heavy metal-containing wastewater, when the optimum addition amount calculated by the following procedure is plotted to obtain an approximate straight line, the slope of the approximate straight line is 0.5 or less. Wastewater treatment method: (Calculation of the optimum addition amount) While keeping the temperature of the heavy metal-containing wastewater constant, the chelating agent is added in each addition amount, and the addition amount when the removal rate calculated by measuring the cadmium concentration in the wastewater before and after the addition exceeds the target removal rate is defined as the optimum addition amount for that temperature.

3. The wastewater treatment method according to claim 1 or 2, further comprising a step of separating heavy metals after the step of adding the chelating agent.

4. The step of separating the heavy metals is at least one selected from the group consisting of a filtration treatment step using a microfiltration membrane or an ultrafiltration membrane, a coagulation sedimentation treatment step, a pressure flotation treatment step, and a centrifugal separation treatment step. The wastewater treatment method according to claim 3.

5. The heavy metal-containing wastewater contains cadmium. The wastewater treatment method according to claim 1 or 2.

6. The heavy metal-containing wastewater is flue gas desulfurization wastewater from a thermal power plant. The wastewater treatment method according to claim 1 or 2.

7. The chelating agent is piperazine-based dithiocarbamate. The wastewater treatment method according to claim 1 or 2.

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