Wastewater treatment method

Abstract

The present invention provides a wastewater treatment method that is excellent in removing oil from wastewater containing mud, oil, and a surfactant, and a wastewater treatment apparatus for carrying out the said method. [Solution] A method for treating wastewater containing mud with a sand particle size of 75 μm or less in content of less than 5% by mass, oil, and a surfactant, and having a pH of 8.0 or higher and 12 or lower, comprising the following steps. Step 1: Adding polyaluminum chloride to the wastewater and mixing it to adjust the pH of the wastewater to between 6.0 and 8.0. Step 2: After performing Step 1, the wastewater is subjected to activated carbon treatment by passing it through activated carbon.

Description

[Technical Field]

[0001] The present invention relates to a wastewater treatment method and a wastewater treatment apparatus for wastewater containing mud, oil, and surfactants. [Background technology]

[0002] Securing safe water is one of the most important activities in daily life. In developed countries, infrastructure for water use is well underway. However, there are still many regions around the world where sufficient water cannot be secured, and people living in such areas are forced to expend considerable effort to obtain water. Therefore, there is a strong need for technologies that can easily purify pollutants such as sludge present in river water, lake water, groundwater, and rainwater. In addition, wastewater that requires treatment includes wastewater discharged from washing oil-contaminated objects in daily life and various industrial fields. This wastewater contains oils such as animal and vegetable oils and petroleum hydrocarbons, which are sources of pollution, as well as surfactants contained in cleaning agents. In particular, in recent years, technologies have been developed to wash contaminated soil with soil cleaning agent compositions, and since wastewater containing the above substances is discharged, there is a need for simple treatment methods.

[0003] Patent Document 1 discloses a wastewater treatment method for treating wastewater containing organic matter, oil, and surfactant, comprising a flocculation treatment step of adding salt and an inorganic flocculant to the wastewater, and a flocculation removal step of removing flocs formed in the wastewater by the flocculation treatment. Patent Document 2 discloses a treatment system characterized by comprising: a dilution step of diluting wastewater or sludge containing a high concentration of suspended solids with water to 10 times (volume) or less, and adjusting the concentration of suspended solids in the diluted solution to a range of 5,000 mg / L to 50,000 mg / L; a flocculation step of adding an inorganic powder flocculant (however, not including an acidic flocculant aid) to the diluted solution to flocculate the suspended solids; and a dewatering step of simply dewatering the sludge generated in the flocculation step. Patent Document 3 discloses a method for purifying emulsion-containing oily wastewater that recovers water from emulsion-containing oily wastewater containing an oil-in-water droplet type emulsion and separated oil which is oil that does not constitute the emulsion, the method comprising: a coagulation step of adding a coagulant to the emulsion-containing oily wastewater to coagulate and remove the separated oil; an emulsion destruction step of adding an oil-water separator to the emulsion-containing oily wastewater to destroy the emulsion; an adsorption step of contacting the emulsion-containing oily wastewater that has gone through the coagulation step and the emulsion destruction step with activated carbon to adsorb and remove the oil separated by the emulsion destruction on the activated carbon; and a reverse osmosis step of passing the emulsion-containing oily wastewater that has gone through the adsorption step through a reverse osmosis membrane to remove impurities. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-124317 [Patent Document 2] Japanese Patent Publication No. 2019-217423 [Patent Document 3] Japanese Patent Publication No. 2023-58187 [Overview of the project] [Problems that the invention aims to solve]

[0005] The present invention provides a wastewater treatment method that is excellent in removing oil from wastewater containing mud, oil, and a surfactant, and a wastewater treatment apparatus for carrying out the said method. [Means for solving the problem]

[0006] The present invention relates to a method for treating wastewater containing mud with a sand content of less than 5% by mass of sand with a particle size of 75 μm or less, oil, and a surfactant, and having a pH of 8.0 or higher and 12 or lower, and comprising the following steps. Step 1: Adding polyaluminum chloride to the wastewater and mixing it to adjust the pH of the wastewater to between 6.0 and 8.0. Step 2: After performing Step 1, the wastewater is subjected to activated carbon treatment by passing it through activated carbon.

[0007] The present invention also relates to a wastewater treatment apparatus comprising: a wastewater storage tank for storing wastewater containing mud with a particle size of 75 μm or less having a content of less than 5% by mass of sand, oil, and a surfactant, and having a pH of 8.0 or higher and 12 or lower; a treatment tank for adding and mixing polyaluminum chloride to the wastewater in the wastewater storage tank and adjusting the pH of the wastewater to 6.0 or higher and 8.0 or lower; and an activated carbon tank for passing the wastewater treated in the treatment tank through activated carbon and performing activated carbon treatment. [Effects of the Invention]

[0008] According to the present invention, a wastewater treatment method that is excellent in removing oil from wastewater containing mud, oil, and a surfactant, and a wastewater treatment apparatus for performing the method are provided. [Modes for carrying out the invention]

[0009] The reason why the wastewater treatment method of the present invention is excellent in removing oil from wastewater containing mud, oil, and surfactants is not entirely clear, but it can be inferred as follows. By adding polyaluminum chloride to the wastewater so that the pH of the wastewater falls within a specific range, the mud is coagulated, and oil and surfactant are incorporated into the mud, forming flocs. This reduces the concentration of surfactant and oil in the wastewater. Furthermore, it is believed that trace amounts of emulsified and dissolved oil and surfactant in the wastewater are removed by activated carbon, and it is estimated that excellent oil removal performance was obtained through the above wastewater treatment. Furthermore, the present invention is not limited in any way to the mechanism of action described above.

[0010] [Wastewater treatment method] The wastewater treated by the wastewater treatment method of the present invention contains mud with a sand content of less than 5% by mass, with a particle size of 75 μm or less, oil, and a surfactant, and has a pH of 8.0 to 12. The wastewater may be wastewater discharged when soil contaminated with oil is washed by mixing it with a washing solution containing a surfactant and water. Soil contaminated with oil may be soil contaminated with one or more types of oil selected from, for example, animal and vegetable oils and petroleum hydrocarbons. The animal or vegetable oil may be one or more selected from, for example, linseed oil, soybean oil, olive oil, coconut oil, palm oil, castor oil, rapeseed oil, rice oil, sesame oil, sunflower oil, and corn oil. The petroleum-based compound may be one or more selected from, for example, gasoline, kerosene, diesel fuel, heavy oil, and machine oil.

[0011] The aforementioned wastewater contains mud. The mud is included in the wastewater as a result of washing the soil with the washing solution. From the viewpoint of the sedimentation properties of the wastewater, the content of sand with a particle size of 75 μm or less in the sediment is less than 5% by mass, preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less, and preferably 0% by mass or more, and more preferably 0.1% by mass or more. From the viewpoint of the sedimentation properties of the sediment in the wastewater, the content of sand with a particle size exceeding 75 μm and not exceeding 150 μm is preferably 15% by mass or less, more preferably 10% by mass or less, and preferably 5% by mass or more, and more preferably 8% by mass or more. From the viewpoint of the sedimentation properties of the sediment in the wastewater, the content of sand with a particle size exceeding 150 μm and 2 mm or less is preferably 95% by mass or less, more preferably 92% by mass or less, and preferably 85% by mass or more, and more preferably 88% by mass or more.

[0012] In the drainage water, the content of sludge is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, still more preferably 1% by mass or more, even more preferably 3% by mass or more, and preferably 30% by mass or less, more preferably 20% by mass or less, still more preferably 10% by mass or less, and even more preferably 7% by mass or less, from the viewpoint of the sedimentation property of the sludge in the drainage water.

[0013] The drainage water contains oil components. The oil components will be contained in the drainage water by washing the soil contaminated with oil with a cleaning liquid. The oil components contained in the drainage water may be one or more oil components selected from vegetable oils and fats and petroleum-based hydrocarbons. The vegetable oils and fats may be, for example, one or more selected from linseed oil, soybean oil, olive oil, coconut oil, palm oil, castor oil, rapeseed oil, rice oil, sesame oil, sunflower oil, and corn oil. The petroleum-based compounds may be, for example, one or more selected from gasoline, kerosene, light oil, heavy oil, and machine oil.

[0014] In the drainage water, the content of oil components is preferably 10 ppm or more, more preferably 50 ppm or more, still more preferably 100 ppm or more, even more preferably 500 ppm or more, even more preferably 1,000 ppm or more, even more preferably 1,500 ppm or more, and preferably 10,000 ppm or less, more preferably 5,000 ppm or less, still more preferably 3,500 ppm or less, from the viewpoint of the treatability with activated carbon.

[0015] The drainage water contains surfactants. The surfactants will be contained in the drainage water by washing the soil with a cleaning liquid containing surfactants. Examples of the surfactants contained in the drainage water include one or more selected from nonionic surfactants, anionic surfactants, cationic surfactants, and amphoteric surfactants.

[0016] Examples of nonionic surfactants include alkyl monoglyceryl ethers, polyoxyalkylene monoalkyl or alkenyl ethers, alkyl (poly)glycosides (glycoside-type nonionic surfactants), sorbitan-based nonionic surfactants, aliphatic alkanolamides, fatty acid monoglycerides, sucrose fatty acid esters, and amidates of alkanolamines such as monoethanolamine, diethanolamine, and methylmonoethanolamine with fatty acids such as lauric acid and myristic acid. The alkyl or alkenyl group of the nonionic surfactant has, for example, 8 to 22 carbon atoms. The average number of added moles of the oxyalkylene group, such as the oxyethylene group, of the nonionic surfactant is, for example, 1 to 25.

[0017] Examples of anionic surfactants include one or more selected from alkyl or alkenyl sulfate salts, polyoxyalkylene alkyl or alkenyl ether sulfate salts, alkanesulfonates, alkyl or alkenylbenzenesulfonates, higher fatty acids or their salts, polyoxyethylene alkyl or alkenyl ether carboxylic acids or their salts, N-acyl amino acids or their salts, alkyl or alkenyl phosphate salts, and polyoxyethylene alkyl or alkenyl ether phosphates. The alkyl or alkenyl group of the anionic surfactant has, for example, 8 to 22 carbon atoms. The average number of added moles of the oxyalkylene group of the anionic surfactant, for example, the oxyethylene group, is, for example, 0 to 10. The salt of the anionic surfactant is, for example, an alkali metal salt such as a sodium salt or a potassium salt.

[0018] Examples of cationic surfactants include quaternary ammonium salt type cationic surfactants. Quaternary ammonium salt type cationic surfactants are those in which one or two of the groups bonded to the nitrogen atom are hydrocarbon groups having 8 to 22 carbon atoms, and the remaining group is selected from the group consisting of alkyl groups having 1 to 3 carbon atoms, hydroxyalkyl groups having 1 to 3 carbon atoms, and arylalkyl groups (such as benzyl groups).

[0019] Examples of amphoteric surfactants include one or more selected from N-alkanoylaminopropyl-N,N-dimethylamine oxide, N-alkyl or alkenyl-N,N-dimethylamine oxide, N-alkanoylaminopropyl-N,N-dimethyl-N-carboxymethylammonium betaine, N-alkyl or alkenyl-N,N-dimethyl-N-carboxymethylammonium betaine, N-alkyl or alkenyl-N,N-dimethyl-N-sulfopropylammonium sulfobetaine, N-alkyl or alkenyl-N,N-dimethyl-N-(2-hydroxysulfopropyl)ammonium sulfobetaine, N-alkanoylaminopropyl-N,N-dimethyl-N-sulfopropylammonium sulfobetaine, and N-alkanoylaminopropyl-N,N-dimethyl-N-(2-hydroxysulfopropyl)ammonium sulfobetaine. In these, the number of carbon atoms in the alkanoyl group is between 8 and 22, for example, lauroyl or myristiroyl. Furthermore, in these, the alkylalkenyl group has 8 to 22 carbon atoms, and examples include a lauryl group or a myristyl group.

[0020] From the viewpoint of treatability with activated carbon, the surfactant content in the wastewater is preferably 10 ppm or more, more preferably 50 ppm or more, even more preferably 100 ppm or more, even more preferably 500 ppm or more, and preferably 10,000 ppm or less, more preferably 5,000 ppm or less, even more preferably 3,000 ppm or less, and even more preferably 1,500 ppm or less.

[0021] The pH of the wastewater is 8.0 or higher, preferably 8.5 or higher, more preferably 9.0 or higher, and 12 or lower, preferably 11.0 or lower, from the viewpoint of dispersibility of the sludge. The pH may be the pH at which the wastewater temperature is 20°C.

[0022] <Process 1> Step 1 is a step of adding polyaluminum chloride to the wastewater, mixing it, and adjusting the pH of the wastewater to 6.0 or more and 8.0 or less.

[0023] The polyaluminum chloride includes, for example, one or more selected from the compounds represented by the following general formulas (1) to (3). [Al2(OH) n Cl 6-n m (1) [Al(OH)3] n AlCl3(2) Al n (OH) m Cl (3n-m) (3) [In the formula, m is independently 0 < m < 3n for each.] The polyaluminum chloride is, for example, [Al6(OH) 15 Cl3, [Al8(OH) 20 Cl4, [Al 13 (OH) 34 Cl5, [Al 21 (OH) 60 Cl3. For adding the polyaluminum chloride to the wastewater, a general polyaluminum chloride aqueous solution may be used. The aluminum oxide concentration (Al2O3) in the polyaluminum chloride aqueous solution is preferably 5% by mass or more, more preferably 8% by mass or more, and preferably 40% by mass or less, more preferably 30% by mass or less from the viewpoint of flocculability. Also, the basicity in the polyaluminum chloride aqueous solution is preferably 30% by mass or more, more preferably 40% by mass or more, and preferably 80% by mass or less, more preferably 75% by mass or less from the viewpoint of flocculability.

[0024] In Step 1, polyaluminum chloride is added to the wastewater and mixed, and the mixing amount of polyaluminum chloride is adjusted so that the pH of the wastewater becomes 6.0 or more, preferably 6.2 or more, more preferably 6.5 or more, and 8.0 or less, preferably 7.5 or less, more preferably 7.0 or less from the viewpoints of sludge sedimentation property and oil removal property. ​The pH may be the pH at which the wastewater temperature is 20°C.

[0025] In step 1, the amount of polyaluminum chloride mixed in the wastewater is preferably 1 ppm or more, more preferably 10 ppm or more, even more preferably 30 ppm or more, even more preferably 60 ppm or more, and preferably 5,000 ppm or less, more preferably 1,000 ppm or less, even more preferably 500 ppm or less, even more preferably 300 ppm or less, and even more preferably 150 ppm or less, from the viewpoint of mud settling properties and oil removal properties.

[0026] In step 1, after adding polyaluminum chloride to the wastewater, stirring and mixing may be performed. From the viewpoint of uniform mixing, the stirring and mixing time is preferably 0.1 minutes or more, more preferably 0.5 minutes or more, even more preferably 1 minute or more, and from the viewpoint of preventing floc breakage, preferably 5 minutes or less, more preferably 3 minutes or less, and even more preferably 2 minutes or less.

[0027] <Process 2> Step 2 is a step in which, after performing Step 1, the wastewater is passed through activated carbon to perform activated carbon treatment.

[0028] As activated carbon, materials such as wood-based, coconut shell-based, peat-based, and coal-based materials are preferably used, which have been activated by physical methods using gases such as steam and carbon dioxide, or by chemical methods using zinc chloride, phosphoric acid, etc.

[0029] The activated carbon is preferably granular activated carbon, and the packing density is preferably 0.1 g / ml or more, more preferably 0.12 g / ml or more, even more preferably 0.14 g / ml or more, even more preferably 0.20 g / ml or more, even more preferably 0.30 g / ml or more, and preferably 0.6 g / ml or less, more preferably 0.5 g / ml or less, and even more preferably 0.45 g / ml or less, from the viewpoint of floc removal. The packing density is determined by placing activated carbon into a container such as a cylinder with volume markings, allowing it to fill naturally without pressing or compressing it from above, and then calculating the packing density (= mass of activated carbon / packing volume) from the mass of the filled activated carbon and the filled volume.

[0030] The average particle size of the activated carbon is preferably 0.05 mm or larger, more preferably 0.1 mm or larger, even more preferably 0.5 mm or larger, even more preferably 1.0 mm or larger, even more preferably 2.0 mm or larger, and preferably 5.0 mm or smaller, more preferably 4.5 mm or smaller, and even more preferably 4.0 mm or smaller, from the viewpoint of water permeability and floc removal. Particle size shall be measured using the sieving method.

[0031] In the wastewater treatment method of the present invention, in step 1, polyaluminum chloride is added to the wastewater to adjust the pH, thereby generating flocs (aggregates containing polyaluminum chloride, mud, oil, and surfactants), and in step 2, the wastewater with the generated flocs is passed through a packed activated carbon. Since the flocs are large enough to be trapped between the layers of activated carbon and do not obstruct the flow of water to the activated carbon, after performing step 1, the wastewater can be passed through the activated carbon without removing the flocs formed in the wastewater. In other words, step 2 may be a step in which, after performing step 1, the wastewater is passed through activated carbon without removing the flocs formed in the wastewater.

[0032] If step 2 is performed without removing the flocs formed in the wastewater after step 1, it is preferable, from the viewpoint of workability, to pass the wastewater through the activated carbon within 120 minutes, more preferably within 90 minutes, and even more preferably within 60 minutes, after adding polyaluminum chloride to the wastewater in step 1.

[0033] In step 2, the rate at which the wastewater is passed through the activated carbon is preferably 0.1 ml / min or more, more preferably 0.5 ml / min or more, even more preferably 1 ml / min or more, and preferably 100 ml / min or less, more preferably 10 ml / min or less, even more preferably 5 ml / min or less, and even more preferably 3 ml / min or less, from the viewpoint of water permeability and floc removal efficiency.

[0034] After completing step 2, if it is confirmed that the wastewater meets general wastewater standards, it may be treated as general wastewater, or it can be reused as domestic water, industrial water, etc.

[0035] [Wastewater treatment equipment] The present invention relates to a wastewater treatment apparatus comprising: a wastewater storage tank for storing wastewater containing mud with a particle size of 75 μm or less having a content of less than 5% by mass of sand, oil, and a surfactant, and having a pH of 8.0 or higher and 12 or lower; a treatment tank for adding and mixing polyaluminum chloride to the wastewater in the wastewater storage tank and adjusting the pH of the wastewater to 6.0 or higher and 8.0 or lower; and an activated carbon tank for passing the wastewater treated in the treatment tank through activated carbon and performing activated carbon treatment. By using the wastewater treatment apparatus of the present invention, the wastewater treatment method of the present invention can be easily carried out. The wastewater treatment apparatus of the present invention can be appropriately adapted by applying the embodiments described in the wastewater treatment method of the present invention to one another.

[0036] The aforementioned wastewater storage tank is a tank for storing wastewater containing mud with a sand particle size of 75 μm or less containing less than 5% by mass, oil, and a surfactant, and having a pH of 8.0 to 12. The wastewater storage tank may be equipped with a stirring mechanism to prevent the sediment in the wastewater from settling. Furthermore, the wastewater storage tank may be equipped with a pH measuring means for measuring the pH of the wastewater.

[0037] The aforementioned treatment tank is a tank that transfers the wastewater stored in the wastewater storage tank, adds and mixes polyaluminum chloride to the wastewater, and adjusts the pH of the wastewater to between 6.0 and 8.0. The aforementioned treatment tank is a tank used for carrying out step 1 of the wastewater treatment method of the present invention. The treatment tank may be equipped with stirring means to prevent the settling of flocs formed by adding and mixing polyaluminum chloride into the wastewater. Furthermore, the treatment tank may be equipped with a pH measuring means for confirming that polyaluminum chloride is added to and mixed with the wastewater, and that the pH of the wastewater is adjusted to between 6.0 and 8.0.

[0038] The activated carbon tank is a tank that transfers the wastewater treated in the treatment tank, passes it through the activated carbon, and performs activated carbon treatment. The activated carbon tank is a tank used for carrying out step 2 of the wastewater treatment method of the present invention. The activated carbon tank is filled with activated carbon and includes a means for transferring the wastewater treated in the treatment tank and passing it through the activated carbon. The means for passing the water may include a means for adjusting the rate at which the wastewater flows. Preferably, the activated carbon tank is divided into three sections by two partition plates or the like, with activated carbon filling the first and second sections in the direction through which the wastewater flows, and the third section receiving the treated water after the activated carbon has passed through. The activated carbon is the same as that described in the wastewater treatment method of the present invention. If it is confirmed that the treated water that has passed through the activated carbon tank is within the general wastewater standards, it may be treated as general wastewater, or it may be reused as domestic water, industrial water, etc.

[0039] The wastewater storage tank and the treatment tank, and the treatment tank and the activated carbon tank, may be connected by piping for transferring the wastewater. Each of these pipes may have a valve, and the transfer of wastewater can be adjusted by opening and closing the valve. Therefore, by opening and closing the valve at regular intervals, batch-type (intermittent) wastewater treatment is possible. On the other hand, by keeping the valve basically open, continuous wastewater treatment is also possible. [Examples]

[0040] [Evaluation methods for wastewater treatment tests] (1) Preparation of simulated wastewater A simulated wastewater solution was prepared by mixing water with sand containing 0.5% by mass of sand with a particle size of 75 μm or less, 9.5% by mass of sand with a particle size exceeding 75 μm and 150 μm or less, and 90% by mass of sand with a particle size exceeding 150 μm and 2 mm or less as the mud component; light oil as the oil component; and polyoxyethylene alkyl ether (with an alkyl group having 12 carbon atoms and an average number of added moles of oxyethylene groups of 8) as the surfactant, in the amounts shown in Table 1 (the remainder being water). The pH of the simulated wastewater at 20°C was the value shown in Table 1.

[0041] (2) Wastewater treatment test A 2 cm stirring bar was placed in a 300 mL glass beaker, and 100 mL of the prepared simulated wastewater was added. While stirring with a stirrer (manufactured by AS ONE Corporation), polyaluminum chloride (hereinafter referred to as PAC, manufactured by Nankai Chemical Co., Ltd.) was added until the pH of the wastewater at 20°C reached the values ​​shown in Table 1, and the mixture was stirred and mixed (Step 1). The stirring time after adding PAC was 1 minute. The amount of PAC added to the wastewater is shown in Table 1. In Comparative Example 1, no PAC was added and Step 1 was not performed. Granular activated carbon (CL-H, packing density 0.45 g / ml, average particle size 3.8 mm, manufactured by Ajinomoto Fine Techno Co., Ltd.) was packed into a 50 ml Terumo syringe, and the simulated wastewater treated in Step 1 was passed through the packed granular activated carbon at a flow rate of 1.2 ml / min within 60 minutes of adding PAC to the wastewater in Step 1, without removing the flocs formed in the wastewater (Step 2). In Comparative Example 2, Step 2 was not performed.

[0042] (3) Measurement of oil concentration after wastewater treatment The oil concentration was measured in the wastewater treated in Step 2 (or in the case of Comparative Example 2, the wastewater treated in Step 1). An oil content measuring reagent set (model: WA-OIL) and an oil content analyzer (also manufactured by Kyoritsu Rika Co., Ltd.) were used to measure the oil concentration. This measurement kit measures the oil concentration in a solution using the polynipamp extractant assay method. The results are shown in Table 1.

[0043] Table 1

Claims

1. A method for treating wastewater containing mud with a sand particle size of 75 μm or less in content of less than 5% by mass, oil, and a surfactant, and having a pH of 8.0 or higher and 12 or lower, comprising the following steps. Step 1: Adding polyaluminum chloride to the wastewater and mixing it to adjust the pH of the wastewater to 6.0 or higher and 8.0 or lower. Step 2: After performing Step 1, the wastewater is subjected to activated carbon treatment by passing it through activated carbon.

2. The wastewater treatment method according to claim 1, wherein step 2 is a step of performing activated carbon treatment by passing the wastewater through activated carbon without removing the flocs formed in the wastewater after performing step 1.

3. The wastewater treatment method according to claim 1, wherein step 2 is a step of performing activated carbon treatment by passing the wastewater through activated carbon within 120 minutes after adding polyaluminum chloride to the wastewater in step 1, without removing the flocs formed in the wastewater after step 1 has been performed.

4. The wastewater treatment method according to claim 1 or 2, wherein the activated carbon is granular activated carbon and has a packing density of 0.1 g / ml or more and 0.6 g / ml or less.

5. The wastewater treatment method according to claim 1 or 2, wherein the wastewater contains a surfactant in an amount of 10 ppm or more and 10,000 ppm or less.

6. The wastewater treatment method according to claim 1 or 2, wherein the wastewater contains oil in an amount of 10 ppm or more and 10,000 ppm or less.

7. The wastewater treatment method according to claim 1 or 2, wherein the oil content contained in the wastewater is one or more oils selected from animal and vegetable oils and petroleum hydrocarbons.

8. A wastewater treatment apparatus comprising: a wastewater storage tank for storing wastewater containing mud with a particle size of 75 μm or less containing less than 5% by mass of sand, oil, and a surfactant, and having a pH of 8.0 or higher and 12 or lower; a treatment tank for adding and mixing polyaluminum chloride to the wastewater in the wastewater storage tank and adjusting the pH of the wastewater to 6.0 or higher and 8.0 or lower; and an activated carbon tank for passing the wastewater treated in the treatment tank through activated carbon and performing activated carbon treatment.

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