Wastewater treatment method

Abstract

To provide a wastewater treatment method capable of treating a heavy metal-containing waste water including fluorine and a heavy metal in a single treatment tank (by a single step).SOLUTION: A wastewater treatment method includes a step, to heavy metal-containing waste water containing heavy metals and fluorine for insolubilization as hydroxides in the presence of an alkali agent, of adding an alkali agent, an aluminum-based inorganic coagulant, and a metal scavenger in this order, for insolubilization of the heavy metal and the fluorine respectively, wherein an aluminum concentration in the heavy metal-containing waste water is an aluminum concentration or over of those calculated by the following formula. An aluminum concentration in heavy metal-containing waste water (mg / L)=objective fluorine decrease amount before and after treatments (mg / L)×8-an aluminum amount included in an aluminum-based inorganic coagulant (mg / L).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wastewater treatment method, and more particularly to a wastewater treatment method for removing heavy metals and fluorine from heavy metal-containing wastewater containing heavy metals and fluorine. [Background technology]

[0002] Heavy metal components contained in wastewater from the metallurgical industry, non-ferrous metal primary smelting and refining industry, non-ferrous metal secondary smelting and refining industry, hot-dip plating industry, and seafood food manufacturing industry, such as cadmium, the causative agent of Itai-itai disease, can have an impact on human health. For this reason, UN agencies and various countries, including Japan, have established environmental standards and emission standards for heavy metals.

[0003] Heavy metals contained in wastewater containing heavy metals include copper (Cu), zinc (Zn), tin (Sn), nickel (Ni), lead (Pb), and cadmium (Cd). Known methods for treating wastewater containing heavy metals include precipitation removal, electrolytic removal, and chelating resin adsorption removal. Among these, precipitation removal is preferred for treating large amounts of wastewater containing heavy metals. Known precipitation removal methods include the hydroxide method and sulfide method, and various methods are selected depending on the wastewater standards and wastewater facilities. The hydroxide method removes heavy metals by precipitating metal hydroxides, while the sulfide method removes heavy metals by precipitating metal sulfides.

[0004] Incidentally, heavy metal-containing wastewater may contain fluorine. A known method for removing fluorine from wastewater is the aluminum hydroxide method, in which aluminum hydroxide is precipitated and fluorine is adsorbed onto the precipitate to effect coprecipitate. In wastewater containing fluorine and heavy metals, the optimal pH ranges for insolubilizing fluorine and heavy metals are different, resulting in a problem of reduced removal efficiency. For this reason, for example, Patent Document 1 proposes a wastewater treatment method for removing fluorine by adding an aluminum source to fluorine-containing wastewater to coprecipitate fluorine together with an aluminum hydroxide precipitate, the method comprising: introducing the fluorine-containing wastewater into a reaction tank to form the precipitate; separating the precipitate slurry from the wastewater; introducing a part or all of the precipitate into a conditions tank; adding an aluminum source and an alkaline agent to the precipitate slurry in the conditions tank; returning the precipitate slurry to the reaction tank to adjust the pH; and repeatedly circulating the precipitate slurry to reduce the fluorine concentration in the wastewater. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-320865 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, methods for coprecipitating heavy metal-containing wastewater containing fluorine and heavy metals have been investigated, but in conventional techniques, wastewater containing heavy metals as well as fluorine has had to be treated in two stages: a fluoride removal step in which an aluminum source and a coagulant are added to remove fluorine in a pH range of 6.5 to 7.5, and a heavy metal removal step in which an alkaline agent is added to insolubilize the heavy metals in the wastewater in a pH range of 9 to 10. This makes the treatment process complicated, and further poses the problem of the treatment equipment becoming large because multiple treatment tanks are required.

[0007] Therefore, in order to solve the problems of the conventional technology, the present inventors have conducted studies with the aim of providing a wastewater treatment method capable of treating wastewater containing fluorine and heavy metals in a single treatment tank (in one stage). [Means for solving the problem]

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

[0009] [1] A process for insolubilizing heavy metals and fluorine in heavy metal-containing wastewater, which contains heavy metals and fluorine that are insolubilized as hydroxides in the presence of an alkaline agent, by adding an alkaline agent, an aluminum-based inorganic coagulant, and a metal scavenger in this order, to insolubilize the heavy metals and fluorine, respectively; A wastewater treatment method, wherein the aluminum concentration in the heavy metal-containing wastewater is equal to or greater than the aluminum concentration calculated by the following formula: Aluminum concentration in heavy metal-containing wastewater (mg / L) = Target fluorine reduction amount before and after treatment (mg / L) x 8 - Amount of aluminum contained in aluminum-based inorganic coagulant (mg / L) [2] The wastewater treatment method according to [1], wherein the pH of the heavy metal-containing wastewater after adding the alkaline agent is 8.5 to 10. [3] The wastewater treatment method according to [1] or [2], wherein in the step of adding an aluminum-based inorganic coagulant, an amount of aluminum-based inorganic coagulant is added such that the pH of the heavy metal-containing wastewater does not fall below 7.0. [4] The wastewater treatment method according to any one of [1] to [3], wherein the aluminum-based inorganic flocculant is polyaluminum chloride. [5] The wastewater treatment method according to any one of [1] to [4], comprising the steps of adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to a single tank, and recovering the precipitate after adding the metal scavenger. [Effects of the Invention]

[0010] According to the wastewater treatment method of the present invention, wastewater containing fluorine and heavy metals can be treated in a single treatment tank (in one stage), thereby simplifying the treatment process and the treatment device. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram illustrating the processing steps in this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail below. The following description may be based on representative embodiments and specific examples, but the present invention is not limited to such embodiments. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] (Wastewater treatment method) This embodiment relates to a wastewater treatment method including a step of adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to heavy metal-containing wastewater containing heavy metals and fluorine that are insolubilized as hydroxides in the presence of an alkaline agent, thereby insolubilizing the heavy metals and fluorine, respectively. Here, when the aluminum concentration of the heavy metal-containing wastewater calculated by the following formula is a positive value, the heavy metal-containing wastewater must contain aluminum at a concentration equal to or greater than that value. Note that when the aluminum concentration calculated by the following formula is a negative value, the heavy metal-containing wastewater may not contain aluminum. Aluminum concentration in heavy metal-containing wastewater (mg / L) = Target fluorine reduction amount before and after treatment (mg / L) x 8 - Amount of aluminum contained in aluminum-based inorganic coagulant (mg / L)

[0014] In this embodiment, by adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to heavy metal-containing wastewater containing heavy metals and fluorine, it is possible to treat heavy metal-containing wastewater containing fluorine in a single treatment tank (in one stage). This makes it possible to simplify the treatment process and treatment device. In other words, the wastewater treatment method of this embodiment is a wastewater treatment method that includes adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to heavy metal-containing wastewater containing heavy metals and fluorine in a single tank.

[0015] For example, in this embodiment, the heavy metal and fluorine contents can be kept below the emission standards. The emission standards are set by each factory in accordance with general wastewater standards and local ordinances. Examples of the emission standards include fluorine 8 mg / L or less, zinc 0.6 mg / L or less, and copper 1.0 mg / L or less.

[0016] In this embodiment, the heavy metal-containing wastewater contains, in addition to fluorine, heavy metals that become insoluble as hydroxides in the presence of an alkaline agent. Examples of such heavy metals include copper (Cu), zinc (Zn), tin (Sn), nickel (Ni), lead (Pb), and cadmium (Cd). In particular, the heavy metal-containing wastewater to be treated in this embodiment preferably contains copper (Cu) and zinc (Zn).

[0017] In this embodiment, the heavy metal-containing wastewater may further contain aluminum. When the aluminum concentration calculated by the following formula is a positive value, the heavy metal-containing wastewater must contain aluminum at a concentration equal to or greater than that value. When the value is a negative value, the wastewater may not contain aluminum. Aluminum concentration in heavy metal-containing wastewater (mg / L) = Target fluorine reduction amount before and after treatment (mg / L) x 8 - Amount of aluminum contained in aluminum-based inorganic coagulant (mg / L) In the wastewater treatment method of this embodiment, it is preferable to provide a step of measuring the amount of aluminum contained in the heavy metal-containing wastewater before the step of adding the alkaline agent.

[0018] In addition, when the target fluorine amount after treatment is not so low (i.e., when the target fluorine reduction amount before and after treatment is small) or when the aluminum content of the aluminum-based inorganic flocculant is relatively high, the amount of aluminum contained in the heavy metal-containing wastewater may be small, and in some cases the heavy metal-containing wastewater may not substantially contain aluminum. Thus, the amount of aluminum contained in the heavy metal-containing wastewater is determined by the target fluorine amount after treatment and the amount of aluminum contained in the aluminum-based inorganic flocculant. In addition, when the heavy metal-containing wastewater to be subjected to wastewater treatment does not contain the amount of aluminum calculated by the above formula, it is preferable to add aluminum to achieve the predetermined concentration.

[0019] In the wastewater treatment method of this embodiment, an alkaline agent is first added to heavy metal-containing wastewater containing heavy metals and fluorine that are insolubilized as hydroxides in the presence of an alkaline agent. After the addition of the alkaline agent, the pH of the heavy metal-containing wastewater is preferably 8.5 to 10, more preferably 9.0 to 10. In the step of adding the alkaline agent, the alkaline agent is preferably added so that the pH of the wastewater falls within the above range. By adjusting the pH of the heavy metal-containing wastewater to fall within the above range, the hydroxides of the heavy metals in the heavy metal-containing wastewater can be insolubilized.

[0020] Examples of alkaline agents used in this embodiment include sodium hydroxide, potassium hydroxide, barium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, and calcium carbonate. These may be used alone or in combination of two or more. Among these, the alkaline agent is preferably at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide, and more preferably sodium hydroxide.

[0021] The temperature conditions when adding the alkaline agent are not particularly limited, and may be room temperature. After adding the alkaline agent, stirring is preferably carried out at 10 to 1000 rpm. The stirring time is not particularly limited, but should be 30 seconds or more.

[0022] In the wastewater treatment method of this embodiment, after the alkaline agent is added, an aluminum-based inorganic flocculant is added. Examples of aluminum-based inorganic flocculants include aluminum sulfate (Al2(SO4)3) and polyaluminum chloride (Al2(OH) n Cl 6-n Among them, the aluminum-based inorganic flocculant is preferably polyaluminum chloride (PAC).

[0023] In the step of adding the aluminum-based inorganic coagulant, it is preferable to add the aluminum-based inorganic coagulant in an amount that does not cause the pH of the heavy metal-containing wastewater to fall below 7.0. Specifically, it is preferable to add the aluminum-based inorganic coagulant while measuring the pH of the wastewater containing an alkaline agent. By adding an aluminum-based inorganic coagulant in an amount that does not cause the pH of the heavy metal-containing wastewater to fall below 7.0, it is possible to effectively prevent the re-elution of the heavy metal hydroxides precipitated in the previous step. Furthermore, by adding an aluminum-based inorganic coagulant in an amount that does not cause the pH of the heavy metal-containing wastewater to fall below 7.0, it is possible to precipitate fluorine together with the aluminum hydroxide precipitate. This makes it possible to simultaneously insolubilize fluorine and heavy metals from heavy metal-containing wastewater containing fluorine and heavy metals.

[0024] The temperature conditions when adding the inorganic flocculant are not particularly limited, and may be room temperature. After adding the inorganic flocculant, it is preferable to stir at 10 to 1000 rpm. The stirring time is not particularly limited, but it should be 30 seconds or more.

[0025] In the wastewater treatment method of this embodiment, after the addition of the aluminum-based inorganic flocculant, a metal scavenger is added. A chelating agent is preferably used as the metal scavenger. Examples of chelating agents include EDTA (ethylenediaminetetraacetic acid) and PDTK (dipotassium piperazine-1,4-bis(carbodithioate)).

[0026] The amount of metal scavenger added is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and even more preferably 0.03% by mass or more, based on the total mass of the heavy metal-containing wastewater. Furthermore, the amount of metal scavenger added is preferably 1% by mass or less, based on the total mass of the heavy metal-containing wastewater. By setting the amount of metal scavenger added within the above range, it becomes possible to sufficiently remove heavy metals.

[0027] The temperature conditions when adding the metal scavenger are not particularly limited and may be room temperature. After adding the inorganic flocculant, stirring is preferably carried out at 10 to 1000 rpm. After adding the metal scavenger, stirring is preferably carried out for 1 to 60 minutes, more preferably 2 to 30 minutes, and even more preferably 3 to 20 minutes, and then the mixture is allowed to stand for preferably 1 minute, more preferably 3 minutes, and even more preferably 5 minutes or more.

[0028] This embodiment preferably includes a step of recovering a precipitate after the step of insolubilizing heavy metals and fluorine, respectively. This precipitate contains at least heavy metals and fluorine. That is, this embodiment preferably includes a step of adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to a single tank, and recovering the precipitate after adding the metal scavenger. The precipitate is recovered and disposed of as sludge containing fluorine and heavy metals. In this embodiment, the sludge extraction step can be performed at a single location, thereby reducing the amount of sludge discharged and further simplifying the wastewater treatment process.

[0029] The treated water (supernatant water) obtained through the above steps may be discharged into rivers, the ocean, etc. Alternatively, the treated water (supernatant water) may be recovered and reused as industrial water.

[0030] (Wastewater treatment equipment) This embodiment may relate to an apparatus for treating heavy metal-containing wastewater containing heavy metals and fluorine that are insolubilized as hydroxides in the presence of an alkaline agent, the apparatus having a coagulation tank equipped with means for adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order. In the treatment apparatus of this embodiment, the aluminum concentration in the heavy metal-containing wastewater is equal to or greater than the aluminum concentration calculated by the following formula: Aluminum concentration in heavy metal-containing wastewater (mg / L) = Target fluorine reduction amount before and after treatment (mg / L) x 8 - Amount of aluminum contained in aluminum-based inorganic coagulant (mg / L)

[0031] As shown in Fig. 1, the treatment device of this embodiment may include a raw water tank in addition to the coagulation tank. Wastewater containing heavy metals stored in the raw water tank is appropriately sent to the coagulation tank to perform wastewater treatment.

[0032] As shown in Figure 1, the coagulation tank is preferably equipped with a stirring means. The stirring means is not particularly limited, and stirring blades, stirring shafts, etc. can be used. The coagulation tank is also preferably equipped with a pH meter. This makes it easy to appropriately control the amount of aluminum-based inorganic flocculant added.

[0033] The coagulation tank may be provided with a sludge withdrawal pipe and a sludge withdrawal valve, and may also be provided with a flow path for discharging treated water (supernatant water).

[0034] In this embodiment, it is preferable to provide only one coagulation tank, that is, the insolubilization of fluorine and heavy metals is preferably carried out in a single tank. [Example]

[0035] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0036] Example 1 Wastewater containing the following components was used as the heavy metal-containing wastewater. The standard values ​​for fluorine and heavy metals (zinc and copper) after wastewater treatment were as follows: Fluoride 8mg / L or less, zinc 0.6mg / L or less, copper 1.0mg / L or less. [Table 1]

[0037] While stirring 300 mL of the heavy metal-containing wastewater at 150 rpm using a jar tester, 5.6 mL of 1 mol / L aqueous sodium hydroxide solution was added to adjust the pH to 8.5 to 10. Next, 0.11 mL of 10% by weight polyaluminum chloride (PAC) was added to the maximum amount necessary to maintain the pH above 7. Two minutes after the addition of polyaluminum chloride (PAC), 0.75 mL of a 10x diluted solution of a piperazine-based chelating agent (containing 40 to 42% by weight of dipotassium piperazine-1,4-bis(carbodithioate) (hereinafter referred to as PDTK)) was added. After 5 minutes, stirring was stopped and the solution was allowed to stand for 10 minutes. The pH, water temperature, and component amounts of the treated water (supernatant water) at each step were as follows:

[0038] [Table 2]

[0039] (Comparative Example 1) 300 mL of the same heavy metal-containing wastewater as in Example 1 was stirred at 150 rpm using a jar tester, while 0.189 mL of 10% by mass polyaluminum chloride (PAC) was added. Next, 5.67 mL of a 1 mol / L aqueous sodium hydroxide solution was added to adjust the pH to 6.5 to 7.5. After 2 minutes, 0.75 mL of a 10-fold diluted solution of a piperazine-based chelating agent (containing 40 to 42% PDTK) was added. After 5 minutes, stirring was stopped and the solution was allowed to stand for 10 minutes. The pH, water temperature, and component amounts of the treated water (supernatant water) in each step were as follows:

[0040] [Table 3]

[0041] In the examples in which the alkaline agent, aluminum-based inorganic coagulant, and metal scavenger were added in this order, the fluorine, zinc, and copper contents were all below the standard values. On the other hand, in the comparative examples, the zinc content after treatment did not meet the standard. Thus, the present invention makes it possible to remove both fluorine and heavy metals in a single treatment tank (in one stage).

Claims

1. The method comprises the step of adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to heavy metal-containing wastewater containing heavy metals and fluorine that are insolubilized as hydroxides in the presence of an alkaline agent, thereby insolubilizing the heavy metals and the fluorine, respectively; The wastewater treatment method, wherein the aluminum concentration in the heavy metal-containing wastewater is equal to or greater than the aluminum concentration calculated by the following formula: Aluminum concentration in heavy metal-containing wastewater (mg / L) = target fluorine reduction amount before and after treatment (mg / L) × 8 - amount of aluminum contained in aluminum-based inorganic coagulant (mg / L)

2. 2. The wastewater treatment method according to claim 1, wherein the pH of the heavy metal-containing wastewater after adding the alkaline agent is 8.5 to 10.

3. 2. The wastewater treatment method according to claim 1, wherein in the step of adding the aluminum-based inorganic flocculant, the aluminum-based inorganic flocculant is added in an amount such that the pH of the heavy metal-containing wastewater does not fall below 7.

0.

4. 2. The wastewater treatment method according to claim 1, wherein the aluminum-based inorganic flocculant is polyaluminum chloride.

5. 2. The wastewater treatment method according to claim 1, further comprising the steps of adding an alkaline agent, an aluminum-based inorganic flocculant, and a metal scavenger in this order to a single tank, and recovering the precipitate after adding the metal scavenger.

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