JP7547296B2 - Method for treating oil-containing wastewater and water treatment device for oil-containing wastewater - Google Patents

Description

The present invention relates to a method for treating oil-containing wastewater and a water treatment device for oil-containing wastewater, and in particular to a method for treating oil-containing wastewater and a water treatment device for oil-containing wastewater that appropriately removes oil from the oil-containing wastewater and then performs biological treatment.

Traditionally, biological treatment using microorganisms has been used for wastewater treatment, but wastewater containing oil (hereinafter also referred to as oil-containing wastewater) takes a long time to be decomposed by microorganisms, and when the surface of the microorganisms is covered with oil, the respiration and fermentation of the microorganisms are inhibited, causing the microbial activity to be lost, making biological treatment difficult, and oil-containing sludge floats to the surface during solid-liquid separation after biological treatment, causing problems.

For example, when using a membrane-separated activated sludge process (MBR process) in biological treatment to improve solid-liquid separation, there is a problem that oil will adhere to the separation membrane surface, making membrane separation difficult. Therefore, when using biological treatment, it is necessary to properly remove oil beforehand.

Methods for removing oil from oil-containing wastewater include oil traps, coagulation sedimentation, and pressure flotation. For example, coagulation sedimentation is a method in which the oil and SS (suspended or floating matter) in the wastewater are turned into coagulated flocs using inorganic or polymer coagulants, and then the flocs that have absorbed the oil and SS are removed by solid-liquid separation (gravity separation) to obtain separated water.

However, there was concern that small oil and SS particles would not coagulate even with the addition of inorganic or polymeric coagulants, or that the resulting coagulated flocs would be small or brittle, resulting in some of the oil and SS remaining in the liquid portion (separated water) during solid-liquid separation.

Patent Document 1 is an invention of the present applicant, and discloses an invention that addresses the problem of some of the oil and SS remaining in the liquid portion during solid-liquid separation. Figure 6 is a flow sheet of a conventional wastewater treatment process from oil-containing wastewater in which coagulation treatment is performed before biological treatment. Figure 6 is composed of a coagulation process, a mechanical solid-liquid separation process, and a biological treatment process.

Specifically, as shown in Figure 6, in Patent Document 1, a polymer flocculant 201 is added to water to be treated 200 containing oil and SS, and after flocculation treatment, the separated liquid 202 after mechanical solid-liquid separation is biologically treated. A portion of the separated sludge generated and separated in the biological treatment is returned to the flocculation step or before the flocculation step, mixed with the water to be treated, and flocculated in the flocculation step to form flocs.

The flocs are separated into oil-containing sludge 203 and separated liquid 202 in a mechanical solid-liquid separation process. The oil-containing sludge 203 is dehydrated or discharged directly to the outside. The separated liquid 202 contains residual oil and organic matter from the water to be treated 200, so the oil and organic matter are decomposed and removed by biological treatment.

According to the invention of Patent Document 1, in the coagulation process, the flocs contained in the returned separated sludge act as nuclei for the growth of coagulated flocs, so larger and stronger flocs can be obtained compared to conventional coagulation and sedimentation processes, and the amount of oil and SS remaining in the separated liquid 202 after solid-liquid separation can be reduced.

Patent Document 2 discloses an invention in which wastewater is biologically treated in an activated sludge treatment system consisting of a biological treatment process with an activated sludge return line and a subsequent solid-liquid separation process, in which the activated sludge mixed liquid in the activated sludge treatment system is passed through a porous permeable body such as a screen to selectively remove coarse inactive SS originating from toilet paper contained in sewage and the like. Figure 5 and claim 5 disclose that an organic polymer coagulant is added to excess sludge to perform coagulation treatment to strengthen the activated flocs, and then the flocs are separated using a porous permeable body.

In this way, the excess sludge is subjected to a flocculation treatment using an organic polymer flocculant to increase the amount of activated flocs, and then separated using a porous permeable medium, resulting in sufficient sludge concentration. This allows the excess sludge to be easily dewatered directly using a mechanical dehydrator without the need for a large-volume concentration process as in the past.

Patent No. 6797089 Japanese Patent Application Publication No. 59-6984

However, although Patent Document 1 shows that the amount of oil and SS remaining in the separated liquid after solid-liquid separation can be reduced, there is a demand for further improvement in the oil removal efficiency at the site of oil-containing wastewater treatment. This demand is particularly great when treating wastewater containing high concentrations of oil.

According to Patent Document 2, an organic polymer flocculant is added to the excess sludge, but the purpose of this is to efficiently concentrate and dehydrate the excess sludge and remove it from the system, and there is no disclosure of the purpose of reducing the amount of oil or SS remaining in the separated liquid after solid-liquid separation, or of adding an organic polymer flocculant to sludge sent upstream of biological treatment (specifically, to the flocculation treatment).

The objective of the present invention, which has been made in consideration of the above problems, is to provide a method and apparatus for treating oil-containing wastewater that can remove oil from oil-containing wastewater more efficiently than conventional methods.

The inventors conducted extensive research to achieve the above-mentioned objective, and discovered that, rather than adding a polymer coagulant to the water to be treated that contains small particles of oil and SS to form and grow coagulated flocs, adding a polymer coagulant to separated sludge that already contains particles of a certain size, and then allowing the small particles of oil and SS to adhere to and adsorb onto the resulting complex of polymer coagulant and separated sludge, results in a higher efficiency of oil and SS uptake into the coagulated flocs, and therefore less oil remaining in the separated liquid after solid-liquid separation. This led to the completion of the present invention.

That is, it has been found that the above-mentioned object can be achieved by a water treatment method for oil-containing wastewater, which comprises a coagulation step of forming coagulated flocs from oil-containing wastewater containing oil, a solid-liquid separation step of separating the coagulated water containing the coagulated flocs into solids and a separated liquid, a biological treatment step of biologically treating the separated liquid to obtain treated water and separated sludge, and a polymer coagulant addition/coagulated sludge return step of adding a polymer coagulant to at least a portion of the separated sludge obtained in the biological treatment step and returning the obtained coagulated sludge to the coagulation step, characterized in that the formation of the coagulated flocs in the coagulation step is achieved by mixing the returned coagulated sludge with the oil-containing wastewater.

A preferred embodiment of the method for treating oil-containing wastewater according to the present invention is as follows.
(1) An oil-removing agent addition step is included in which an oil-removing agent is added to the oil-containing wastewater prior to the coagulation step. This allows the oil in the oil-containing wastewater to be adsorbed or adhered to the oil-removing agent, making it possible to more efficiently remove the oil and SS from the oil-containing wastewater.
(2) In the polymer flocculant addition/flocculated sludge return step, the addition rate of the polymer flocculant to the separated sludge is 0.05 wt/wt % to 5.0 wt/wt % to SS.

The above object can also be achieved by an oil-containing wastewater treatment apparatus comprising: a flocculation means for forming flocs from oil-containing wastewater containing oil; a solid-liquid separation means for separating the flocculated water containing the flocs into solids and a separated liquid; a biological treatment means for biologically treating the separated liquid to obtain treated water and separated sludge; a polymer flocculant addition means for adding a polymer flocculant to at least a portion of the separated sludge obtained by the biological treatment means to obtain flocculated sludge; and a return means for returning the flocculated sludge to the flocculation means, wherein the flocculation flocs are formed in the flocculation means by mixing the flocculated sludge returned from the return means with the oil-containing wastewater.

According to the present invention, the formation of flocs from oil-containing wastewater containing oil is achieved by adding a polymer flocculant to at least a portion of the separated sludge obtained after biological treatment, and mixing the flocculated sludge formed with the oil-containing wastewater. This improves the efficiency of the uptake of oil and SS in the oil-containing wastewater into the flocs compared to the case where a polymer flocculant is added directly to the oil-containing wastewater, and therefore makes it possible to significantly reduce the concentrations of oil and SS in the separated liquid obtained by solid-liquid separation. This makes it possible to treat oil-containing wastewater containing higher concentrations of oil and SS than before.

1 is a flowchart for explaining a method for treating oil-containing wastewater of the present invention. FIG. 1 is a schematic diagram showing an example of a treatment flow of the method for treating oil-containing wastewater of the present invention. 1 is a schematic diagram showing the concentration ratio of oil and SS in the treated water and the concentration ratio of oil and SS in the coagulation tank during the coagulation process. FIG. FIG. 2 is a schematic diagram showing another example of a treatment flow according to the method for treating oil-containing wastewater of the present invention. FIG. 1 is a block diagram showing an example of a water treatment device for oil-containing wastewater according to the present invention. This is a flow sheet of a conventional wastewater treatment method for oil-containing wastewater, which involves coagulation and sedimentation treatment before biological treatment.

<Method for treating oil-containing wastewater>
Fig. 1 is a flow chart for explaining the method for treating oil-containing wastewater according to the present invention. As shown in Fig. 1, the present invention is a method for treating oil-containing wastewater, comprising a coagulation step (S110) for forming coagulated flocs from oil-containing wastewater containing oil, a solid-liquid separation step (S120) for separating the coagulated water containing the coagulated flocs into solids and a separated liquid, a biological treatment step (S130) for biologically treating the separated liquid to obtain treated water and separated sludge, and a polymer coagulant addition/coagulated sludge return step (S140) for adding a polymer coagulant to at least a part of the separated sludge obtained in the biological treatment step and returning the obtained coagulated sludge to the coagulation step.

The oil remover addition step (S100) in Figure 1 is an optional step and will be described later.

The present invention is characterized in that the formation of flocs in the flocculation step (S110) is achieved by mixing the returned flocculated sludge with the oil-containing wastewater. This will be explained in more detail below.

(Water to be treated (oil-containing wastewater))
In the present invention, the treated water to be treated is oil-containing wastewater containing oil, SS, soluble organic matter, and insoluble organic matter. Examples include wastewater generated at various business establishments such as beverage production plants, sewage, human waste, and kitchen wastewater from restaurants, etc.

Oil refers not only to oils that are liquid at room temperature, but also to fats that are solid at room temperature, i.e., all fats and oils, and in the present invention, all fats and oils are referred to as oils. Examples of oils contained in the water to be treated include vegetable oils, animal oils, mineral oils, and oils emulsified with surfactants, and one or more of these oils are contained. Generally, the concentration of oil in wastewater is measured as hexane extracts according to the industrial wastewater test method JIS K0102:2019. The concentration of oil is referred to as the hexane extract concentration. In the present invention, all substances measured as hexane extracts are referred to as oil. The lower limit of the oil content of oil-containing wastewater in the present invention is 30 mg/L as the hexane extract concentration. From the viewpoint of the needs of the field where the present invention, which can remove oil more efficiently, is required, the lower limit of the oil content of oil-containing wastewater is preferably 100 mg/L, and particularly preferably 300 mg/L.

Organic matter is a concept that includes the above-mentioned oils and all organic matter other than the oils (soluble organic matter and insoluble organic matter), and is subject to biological treatment. Organic wastewater may contain only oils or may contain oils as well as organic matter other than oils. Examples of organic matter include one or more types of organic matter such as carbohydrates, proteins, lipids, nucleic acids, vegetable oils, animal oils, mineral oils, alcohols, fatty acids, surfactants, and paints. Organic matter may also be substances derived from plants or animals, or chemically synthesized substances.

The present invention does not limit the oil to SS concentration ratio (oil/SS) of the treated water, but it is preferable that the oil to SS concentration ratio (oil/SS) of the treated water is 100 or less, and it is more preferable that the oil to SS concentration ratio (oil/SS) of the treated water is 10 or less.

When the concentration ratio of oil to SS in the water to be treated (oil/SS) is 100 or less, the oil in the water to be treated can be captured by the coagulated flocs by returning the coagulated sludge 8 (described later) mixed with the polymer coagulant 7 to the coagulation step (S110) (see Figures 1 and 2) into which the water to be treated flows, enabling stable solid-liquid separation in the solid-liquid separation step (S120) (see Figures 1 and 2). Furthermore, when the concentration ratio of oil to SS in the water to be treated (oil/SS) is 10 or less, the oil concentration relative to the SS in the water to be treated is low, so even more stable oil removal performance can be achieved.

(Polymer flocculant)
As the polymer flocculant, a cationic polymer flocculant or an amphoteric polymer flocculant can be used, and it may be a commercially available product or a synthetic product obtained by polymerization of commercially available monomers or oligomers, and there is no particular limitation.

The cationic polymer flocculant contains a cationic monomer as an essential component, and one or more of the following may be selected for use: homopolymers or copolymers of cationic monomers, copolymers of cationic monomers and nonionic monomers, copolymers of cationic monomers and anionic monomers, etc. Examples of cationic monomers include dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate, diethylaminoethyl methacrylate, or neutralized salts or quaternary salts of these.

Amphoteric polymer flocculants are copolymerized with cationic monomers, anionic monomers, and nonionic monomers, and have cationic units, anionic units, and nonionic units in the molecule. Cationic polymer flocculants and amphoteric polymer flocculants can be in the form of powder, liquid (dispersion, emulsion), etc.

There are no particular limitations on the molecular weight, viscosity, or cationicity of the cationic polymer flocculant or amphoteric polymer flocculant, and cationic polymer flocculants or amphoteric polymer flocculants that are generally used in sludge treatment, etc. can be used.

Cationic polymer flocculants and amphoteric polymer flocculants are dissolved in tap water, industrial water, groundwater, and wastewater treatment water from various wastewater treatment plants, etc., to prepare the respective polymer flocculant solutions, which are then used in the flocculation process.

(Oil remover)
The oil remover is added to and dispersed in the water to be treated, and has the function of adsorbing or adhering oil. Practical examples include inorganic powder products such as powdered activated carbon, natural short fibers such as cotton, chemically synthesized short fibers, and recycled short fibers. Short fibers recycled from plastic waste and short fibers made of viscose rayon are preferred. A fibrous auxiliary that is a short fiber made of viscose rayon (for example, Evergloss U-700 series, manufactured by Suing Co., Ltd.) has a fiber length of 5 to 10 mm and a moisture content of 30 to 80 wt/wt%.

The method for treating oil-containing wastewater of the present invention will be described below with reference to Figures 1 and 2. Figure 2 is a schematic diagram showing an example of a treatment flow for the method for treating oil-containing wastewater of the present invention.

[Agglomeration step (S110)]
In this step, flocculated flocs are formed from the oil-containing wastewater 1 containing oil. The flocculated flocs are formed by mixing the returned flocculated sludge 8 with the oil-containing wastewater 1, as described below. The efficiency of taking in the oil and SS contained in the sludge 8 into the flocculated sludge 8 is improved, and flocs can be formed more efficiently than before.

The coagulation process is preferably carried out in a coagulation tank into which the water to be treated (oil-containing wastewater 1) and coagulated sludge 8 flow, and when SS is used as the index value, the sludge concentration in the coagulation tank is preferably 100 mg/L or more and 10,000 mg/L or less, and particularly preferably 500 mg/L or more and 2,000 mg/L or less (above, coagulation process (S110)).

[Solid-liquid separation step (S120)]
In this step, the flocculated water 2 containing the flocs is subjected to solid-liquid separation into solids 4 (sludge containing oil and fats) and separated liquid 3.

In the present invention, the following two methods can be applied for solid-liquid separation.
(1) Solid-liquid separation by pressure flotation (2) Mechanical solid-liquid separation using screens or discs Considering the characteristics of oil, which has a lower specific gravity than water and tends to adhere to hydrophobic substances, it is preferable to use a solid-liquid separation device selected from a pressure flotation treatment device after coagulation treatment and a mechanical solid-liquid separation device using a screen or the like after coagulation treatment.

For solid-liquid separation, it is necessary to generate strong flocs that can be separated by flocculation. When the oil to SS concentration ratio (oil/SS) in the treated water is as high as 3.0 or more, the strength of the flocs weakens due to the high oil content of the flocs. However, pressurized flotation is an appropriate solid-liquid separation method that can still maintain the flocs.

On the other hand, when the oil to SS concentration ratio (oil/SS) of the treated water is less than 3.0, the flocs have sufficient strength and can be separated by mechanical solid-liquid separation on a screen or multiple rotating disks, so a mechanical solid-liquid separator is suitable.

In addition, in the present invention, by adding polymer flocculant 7 and returning the formed flocculated sludge 8 to the flocculation step (S110), the concentration ratio of oil to SS (oil/SS) in the flocculation step (S110) is made less than 3.0, which is preferable from the viewpoint of obtaining flocculated flocs with higher strength than when flocculated sludge 8 is not added, allowing the application of a pressurized flotation treatment device or a mechanical solid-liquid separator, and enabling stable solid-liquid separation. Figure 3 is a schematic diagram showing the concentration ratio of oil to SS in the water to be treated and the concentration ratio of oil to SS in the flocculation tank in the flocculation step. As shown in the figure, even if the concentration ratio of oil to SS in the water to be treated is as high as 4.0, for example, the concentration ratio of oil to SS in the flocculation tank can be reduced to 1.0 by returning flocculated sludge 8 to the flocculation step (S110), thereby improving the strength of the flocculated flocs and enabling stable solid-liquid separation.

The solid-liquid separation device is not particularly limited, and a flocculation flotation separation device such as a pressurized flotation device can be used, but commercially available devices such as an inclined screen, drum screen, wedge wire screen, bar screen, vibrating screen, belt thickener, elliptical plate thickener, multiple disk type dehydrator, and multiple plate type screw press dehydrator can be preferably used.

When using any of the above solid-liquid separation devices, it is essential that the oil and SS in the treated water are flocculated into flocculated sludge 8 formed by adding a polymer flocculant to separated sludge 6.

In addition, when any of the above solid-liquid separation devices is used, the separated liquid 3 after solid-liquid separation contains residual oil and organic matter derived from the treated water, and is sent to a biological treatment process, where the oil and organic matter are decomposed and removed. In addition, the solid matter 4 (sludge containing oil and fats) separated in the solid-liquid separation device is transported to the outside as industrial waste, or dehydrated and disposed of as industrial waste (the above is the solid-liquid separation process (S120)).

[Biological treatment step (S130)]
In this process, the separated liquid 3 is biologically treated to obtain treated water 5 and separated sludge 6. The biological treatment process includes a biological treatment operation in which oil and organic matter in the separated liquid 3 are decomposed and removed using aerobic microorganisms, and a solid-liquid separation operation subsequent to the biological treatment operation in which the activated sludge mixed liquid containing the microorganisms is separated into solid-liquid separation 6 and treated water 5.

The biological treatment process is aerobic biological treatment, and examples include standard activated sludge process, carrier flow treatment method in which carriers with attached microorganisms are suspended, and membrane separation activated sludge process (MBR method).

In the solid-liquid separation operation at the latter stage of the biological treatment operation, the activated sludge mixture containing microorganisms is separated into separated sludge 6 and treated water 5 by gravity settling, pressure flotation or membrane separation. The separated sludge 6 is withdrawn from the biological treatment tank having a settling tank or a membrane separation device in the biological treatment process. It is then separated into returned sludge (not shown) which is returned to the biological treatment process to maintain the activated sludge concentration in the biological treatment operation, and the remaining separated sludge. In the present invention, as described below, a polymer flocculant is added to a part of the separated sludge to become flocculated sludge 8 which is returned to the flocculation process. The remaining separated sludge is subjected to sludge treatment such as dehydration, and the dehydrated cake is incinerated or transported to the outside. The treated water 5 is discharged as it is depending on the water quality, or is subjected to advanced treatment such as flocculation and sedimentation treatment as necessary (the above is the biological treatment process (S130)).

[Polymer flocculant addition/flocculated sludge return step (S140)]
In this step, a polymer flocculant 7 is added to at least a portion of the separated sludge 6 obtained in the biological treatment step (S130), and the obtained flocculated sludge 8 is returned to the flocculation step (S110).

The polymer flocculant 7 may be added to the suction or discharge part of the return pump that returns a portion of the separated sludge to the flocculation step (S110), or to a line mixer or mixing tank installed in the return piping. The mixing tank installed in the return piping may be mechanically mixed or mixed by a water flow caused by the excess sludge.

The residence time of the separated sludge in the mixing tank installed in the return pipe varies depending on the mixing method, mixing strength, return flow rate and its concentration, but can be, for example, 0.2 to 3 minutes. If the residence time is 0.2 minutes or more, the polymer flocculant 7 and the separated sludge are mixed sufficiently. On the other hand, if the residence time is 3 minutes or less, the possibility of sludge flocs accumulating in the mixing tank installed in the return pipe and the possibility of the flocculation properties decreasing can be reduced.

As described above, the polymer flocculant 7 added to the separated sludge 6 may be cationic or amphoteric, and since the SS in the flocculation step (S110) is mainly activated sludge, a polymer flocculant 7 of any cationicity or molecular weight can be used.

Conventionally, when a polymer flocculant is added to the water to be treated, the polymer flocculant injection rate is calculated based on the weight of the polymer flocculant (mg/L) relative to the amount of water to be treated. However, when the polymer flocculant 7 is added to the separated sludge 6 as in the present invention, it is preferable to use the "wt/wt% vs. SS" used in dewatering in sludge treatment as an indicator of the polymer flocculant addition rate.

Indicators of sludge concentration include SS (suspended solids) and TS (total solids). Therefore, TS may be used as well as SS as an indicator of sludge concentration. For sludge with a low dissolved salt concentration (concentration of dissolved salts such as salt) such as sewage, the measured values of SS and TS are the same, but in sludge with a high dissolved salt concentration such as sewage treatment, when the SS and TS values of the sludge differ, it is preferable to use SS as the sludge concentration.

In the present invention, SS and TS are defined as follows.
SS:K 0102:2019 Industrial wastewater test methods 14.1 Suspended solids TS:K 0102:2019 Industrial wastewater test methods 14.2 Total evaporation residues When SS is used as an indicator of sludge concentration, in this process, it is preferable to add polymer flocculant 7 to separated sludge 6 at a concentration of 0.05 wt/wt% to 5.0 wt/wt% to SS, and it is even more preferable to add it at a concentration of 0.1 wt/wt% to SS and 4.0 wt/wt% to SS.

Separated sludge 6 drawn out from a biological treatment tank having a settling tank or a separation membrane device is provided with a dedicated extraction pump and dedicated return piping for the return sludge (not shown) that is returned to the biological treatment process and the separated sludge that is subjected to sludge treatment such as dehydration. It is preferable to also provide a dedicated extraction pump and dedicated return piping for the flocculated sludge 8, which is returned to the flocculation process after a polymer flocculant 7 is added to a part of the separated sludge. By having two transfer systems for the separated sludge in this way, one system can be used for sending sludge to sludge treatment such as dehydration, and the other system can be used for returning the sludge to the flocculation process, making it possible to transport and return a stable amount of sludge. However, as shown in Figure 2, there is no problem in the present invention if one transfer line for the separated sludge is branched, one is transferred to sludge treatment, and the other (part of the separated sludge) is returned to the flocculation process.

Since the physical properties of the returned sludge and the separated sludge withdrawn from the settling tank are the same, if there is a surplus in the amount of returned sludge beyond the amount necessary to maintain the activated sludge concentration in the biological treatment tank, a polymer flocculant may be added to at least a portion of the surplus returned sludge, and the resulting flocculated sludge 8 may be returned to the flocculation step (S110). Also, a portion of the returned sludge may be taken from the return sludge piping (not shown) that is returned to the biological treatment tank, and a polymer flocculant 7 may be added to the returned sludge to form flocculated sludge 8, which is returned to the flocculation step (S110). Furthermore, if the flow rate of the separated sludge 6 returned to the flocculation step (S110) is 2 parts or less compared to the flow rate of the returned sludge 10 parts returned to the biological treatment tank, the impact on the activated sludge concentration in the biological treatment tank is small even when considering fluctuations in the amount of water to be treated, so equipment costs can be reduced by separating a portion of the returned sludge from the return sludge piping that is returned to the biological treatment tank, adding a polymer flocculant 7 to the separated sludge to make flocculated sludge 8, and returning it to the flocculation step (S110).

In addition, it is preferable from the viewpoint of stable solid-liquid separation to install a sludge flow meter and a sludge concentration meter in the return piping for returning the sludge to the flocculation step (S110) to monitor the return flow rate and sludge concentration, since this allows the amount of sludge returned to the flocculation step (S110) to be controlled.

To detect the sludge concentration, commercially available sludge concentration meters such as near-infrared sludge concentration meters, laser light sludge concentration meters, and microwave sludge concentration meters can be used. To detect the sludge flow rate, commercially available electromagnetic flow meters and ultrasonic flow meters can be used. The pump that returns the sludge can be a commercially available product, and the sludge can be returned by adjusting the rotation speed to a set flow rate (this concludes the polymer flocculant addition/flocculated sludge return process (S140)).

According to the present invention, the polymer flocculant 7 flocculates the oil and SS in the water to be treated and the sludge particles in the separated sludge 6, and the amount of polymer flocculant 7 to be added should be determined based on the weight of solids such as SS. The amount of polymer flocculant 7 to be added (mg/L) based on the amount of water to be treated does not take into account the amount of solids in the SS and sludge on which the polymer flocculant 7 acts.

In the coagulation process (S110), the SS concentration of the treated water, which contains the SS of the coagulated sludge 8, changes depending on the return conditions of the coagulated sludge 8, so the amount of polymer coagulant added (unit: mg/L), which is the weight of polymer coagulant relative to the amount of treated water or the amount of polymer coagulant to be added, cannot be properly managed or controlled.

As in the present invention, by returning the flocculated sludge 8 obtained by adding polymer flocculant 7 based on the sludge weight to the flocculation step (S110) and mixing the water to be treated and the flocculated sludge 8 in the flocculation step (S110), it is possible to always add the optimal amount of polymer flocculant 7 regardless of the properties of the water to be treated and the wastewater treatment status of the water to be treated, compared to the conventional method of setting the injection rate of the polymer flocculant based on the flow rate of the water to be treated. This makes it possible to optimize the amount of polymer flocculant 7 used for efficient treatment while consistently achieving a high flocculation effect.

If an optimal amount of polymer flocculant 7 is added to the separated sludge 6 and returned to the flocculation step (S110), there is no need to add additional polymer flocculant 7 in the flocculation step (S110). Even if additional polymer flocculant 7 is added in the flocculation step (S110), it may actually result in a decrease in treatment performance, since it takes time to disperse and homogenize the solution in the flocculation step due to its viscosity.

Although an example of the method for treating oil-containing wastewater according to the present invention has been described above with reference to Figures 1 and 2, the present invention is not limited to this example. For example, in the biological treatment process (S130), the water to be treated is subjected to the coagulation process (S110), the obtained coagulated water 2 is subjected to the solid-liquid separation process (S120), and the obtained separated water 3 is subjected to the biological treatment process (S130). However, it is also possible to subject another water to be treated to pretreatment such as coagulation and sedimentation treatment together with the separated water 3, and to supply the obtained treated water to the biological treatment process (S130).

Furthermore, together with the separated water 3, another water to be treated may be subjected to the biological treatment process (S130) without pretreatment.

In this case, the separated sludge 6 that is the source of the flocculated sludge 8 is sludge generated by biologically treating the separated water 3 in the biological treatment process (S130), is sludge obtained by subjecting another treated water to pretreatment such as flocculation and sedimentation treatment together with the separated water 3 and supplying the resulting treated water to the biological treatment process (S130), and is sludge obtained by supplying another treated water to the biological treatment process (S130) together with the separated water 3 without pretreatment.

The flocculated sludge 8 may be flocculated sludge obtained by subjecting the separated liquid obtained by flocculation treatment and solid-liquid separation at another site or another business to the above-mentioned biological treatment step (S130) and adding a polymer flocculant 7 to the sludge obtained, or may be flocculated sludge obtained by performing the biological treatment itself in a separate wastewater treatment system and adding a polymer flocculant 7 to the sludge obtained there.

Furthermore, the flocculated sludge 8 may be obtained by returning a portion of the aeration tank mixed liquid in the biological treatment operation preceding the biological treatment step (S130) and adding a polymer flocculant 7, or by returning a portion of the concentrated sludge obtained by concentrating the separated sludge 6 and adding a polymer flocculant 7.

In addition, an inorganic coagulant may be added to the water to be treated during or before the coagulation step (S110). The inorganic coagulant may be commercially available aluminum sulfate, polyaluminum chloride (PAC), polyferric sulfate (polyferric iron), ferric chloride, or a mixture of these. In addition, the use of these inorganic coagulants reduces the pH of the water to be treated, so commercially available caustic soda or the like is used as an alkaline agent to adjust the pH to an appropriate coagulation pH.

Furthermore, as shown in FIG. 1, the treated water may be subjected to an oil remover addition process (S100) before being subjected to the coagulation process (S110).

Figure 4 is a schematic diagram showing another example of the treatment flow of the oil-containing wastewater treatment method of the present invention. The example of Figure 4 differs from the example of Figure 2 in that, in addition to the example of Figure 2, it includes an oil removing agent addition step (S100) in which an oil removing agent 9 is added to the water to be treated (oil-containing wastewater 1) prior to the coagulation step (S110). Therefore, only the oil removing agent addition step will be described here.

[Oil remover addition step (S100)]
In this process, an oil removing agent 9 is added to the water to be treated (oil-containing wastewater 1) prior to the coagulation process. As described above, the oil removing agent has the function of being added to and dispersed in the water to be treated to adsorb or adhere oil.

In the present invention, short fibers made of recycled plastic waste or short fibers made of viscose rayon are suitable. Among them, fibrous auxiliaries made of viscose rayon (e.g., Evergloss U-700 series, manufactured by Suing Co., Ltd.) have good dispersibility in the water to be treated, and have excellent lipophilicity to the oil in the water to be treated and affinity with SS. A fibrous auxiliary having a fiber length of 5 to 10 mm and a water content of 30 to 80 wt/wt% is suitable. The addition rate of the oil remover 9 is preferably 50 to 1000 mg/L, more preferably 100 to 500 mg/L, per water to be treated.

If the rate of oil remover 9 added is 50 mg/L or more per water to be treated, the oil removal effect from the water to be treated is high, and in the subsequent coagulation step (S110), strong coagulated flocs are formed due to the fibers, improving the solid-liquid separation property in the solid-liquid separation step (S120). In other words, in solid-liquid separation, the moisture content of the solids 4 (oil-containing sludge) is reduced, the solid-liquid separation speed is improved, and the oil, SS concentration, and BOD concentration of the separated liquid 3 are reduced. Since the separated liquid 3 is biologically treated in the subsequent biological treatment step (S130), the load on the biological treatment is reduced, allowing for stable treatment, and the power cost for aeration in the biological treatment step (S130) can be reduced.

Furthermore, if the addition rate is less than 1000 mg/L, the oil removing agent 9 can be uniformly mixed in the water to be treated, making it possible to effectively capture the oil and SS in the water to be treated and also being economically advantageous. Furthermore, by using the oil removing agent 9 in combination, the oil removing agent 9 itself has a high flocculating effect on the sludge generated in the biological treatment in addition to its function of capturing oil and SS, so that it is possible to reduce the amount of flocculated sludge 8 returned to the flocculation step (S110) and to reduce the addition rate of polymer flocculant 7 to the separated sludge 6 (above, the oil removing agent addition step (S100)).

In addition, although not shown in Figures 1 and 2, a mixing tank may be provided before the coagulation step to add a coagulation step. In the coagulation step, a pH adjuster (acid, alkali, etc.) and an inorganic coagulant are added, and the oil and SS in the water to be treated are neutralized by the hydrolysis product of the inorganic coagulant to form fine particles. If the pH of the water to be treated decreases due to the addition of the inorganic coagulant, the pH is adjusted with an alkali such as caustic soda so that the pH is within the optimal coagulation range for the inorganic coagulant. If the pH of the water to be treated is too high to fall within the optimal coagulation pH range even when the inorganic coagulant is added and mixed, the pH is adjusted by adding an acid such as sulfuric acid.
<Water treatment equipment for oil-containing wastewater>
Fig. 5 is a block diagram showing an example of a water treatment apparatus for oil-containing wastewater of the present invention. As shown in the figure, the water treatment apparatus 10 for oil-containing wastewater of the present invention has a flocculation means 14, a solid-liquid separation means 16, a biological treatment means 18, a polymer flocculant addition means 20, and a return means 22.

The flocculation means 14 has a flocculation tank and is a means for forming flocs from oil-containing wastewater 1 (water to be treated) that contains oil.

The water to be treated and flocculated sludge 8 flow into the flocculation tank and are mixed to form flocs. The flocculated water 2 containing the flocs is sent to solid-liquid separation means 16 located downstream of the flocculation means 14.

As described above, the solid-liquid separation means 16 includes two solid-liquid separation devices: (1) solid-liquid separation by pressurized flotation, and (2) mechanical solid-liquid separation using a screen or disk. The flocculated water 2 containing the flocs is separated into solids 4 (oil-containing sludge) and separated liquid 3.

The oil and SS in the treated water are coagulated by the returned coagulated sludge 8 to form strong coagulated flocs, which are then separated into solid and liquid. However, since the coagulated flocs contain oil, they are separated by floating or separated into solid and liquid using a screen that is easy to clean.

The differences between pressurized flotation processing equipment and mechanical solid-liquid separation equipment using screens, etc. are as follows, but are not limited to these.

Empirically, when the amount of treated water is 100 ^m3 /day or more, a pressurized flotation treatment device is suitable for treating large amounts of water as it requires less equipment than a mechanical solid-liquid separation device, while when the amount of treated water is less than 100 ^m3 /day, a mechanical solid-liquid separation device using a screen or the like which is easy to maintain and can be used as a wide range of standard models available on the market are available.

In addition, when the oil to SS concentration ratio (oil/SS) of the water to be treated is 3.0 or more, a pressurized flotation treatment device is suitable, while when the oil to SS concentration ratio (oil/SS) is less than 3.0, a mechanical solid-liquid separation device is suitable.

The solid-liquid separation device is not particularly limited, but may be a flocculation flotation separation device such as a pressurized flotation device, or a commercially available device such as an inclined screen, drum screen, wedge wire screen, bar screen, vibrating screen, belt thickener, elliptical plate thickener, multiple disk type dehydrator, or multiple disk type screw press dehydrator. The solids 4 are either transported to the outside as industrial waste or dehydrated and disposed of as industrial waste, while the separated liquid 3 is sent to the biological treatment means 18.

The biological treatment means 18 has a biological treatment tank in the first stage where oil and organic matter in the water to be treated are decomposed and removed by aerobic microorganisms, and a second solid-liquid separation device in the second stage of the biological treatment tank where the activated sludge mixed liquid containing the microorganisms is separated into sludge and treated water 5. The type of the biological treatment tank and the second solid-liquid separation device is as described in the section <Water treatment method for oil-containing wastewater>, and the description is omitted here. The sludge separated from the second solid-liquid separation device (separated sludge 6) is divided into returned sludge (not shown) that is returned to the biological treatment means 18 and the remaining separated sludge, and a part of the separated sludge is returned to the flocculation means 14 by the return means 22 described below. The remaining separated sludge is sent to a sludge treatment means such as a dehydrator.

The return means 22 is a return pipe that connects the biological treatment means 18 and the flocculation means 14, and returns a portion of the separated sludge to the flocculation means 14 using the power of a return pump or the like.

The polymer flocculant adding means 20 is, for example, a line mixer or a mixing tank provided at the suction section or discharge section of the return pump or in the middle of the return piping. In the polymer flocculant adding means 20, the polymer flocculant 7 is added to and mixed with the sludge, so that the sludge becomes flocculated sludge 8, which is returned to the flocculation means 14. In other words, the return means 22 is a return means that returns the flocculated sludge 8 to the flocculation means 14.

In FIG. 5, the water treatment device 10 for oil-containing wastewater has nothing provided upstream of the coagulation means 14, but an oil remover adding means may be provided as desired. The oil remover adding means is a place where the oil remover 9 is added and mixed, and may be in the form of, for example, an in-line mixer provided in the piping upstream of the coagulation means 14, or a buffer tank for the water to be treated may be used.

The present invention will be explained in more detail below with reference to the following examples.

1. Example 1
1-1. Test conditions Treated water (oil-containing wastewater): Restaurant kitchen wastewater (pH 6.0, SS 450 mg/L, hexane extracts 750 mg/L, BOD 1300 mg/L, all hexane extracts were animal and vegetable oils. The BOD was measured according to the description in the "21. Biochemical oxygen consumption (BOD)" section of the JIS K0102:2019 Industrial Wastewater Testing Method.)

Water treatment equipment: A water treatment equipment having, from the upstream side where the treated water flows in, a mixing tank (oil remover adding means), a coagulation tank (coagulation means), a screen separator (solid-liquid separation means), and a biological treatment means including an aeration tank and a gravity settling device. The equipment specifications and test conditions are shown in Table 1.

Treatment flow: The treated water was treated according to the flow sheet in Figure 2.

1-2. Oil removal test An oil removal test was conducted on the treated water of "1-1. Test conditions" above using the water treatment device of "1-1. Test conditions" above under the conditions of Table 1, with a treated water volume of 4.5 ^m3 /day, a liquid temperature of 20 to 25°C, and a screen with a wire slit width of 1 mm.

In Example 1, no oil remover was added or mixed in the mixing tank, but instead, a cationic polymer flocculant (EvaGrose CS402, manufactured by Suing Co., Ltd.) was added to the treated water in the coagulation tank at a rate of 0.05 to 5.0 wt/wt% to SS, with the added concentration conditions gradually changed. The coagulated sludge obtained by adding this to the separated sludge was then added and mixed to form coagulated flocs.

The flocculated flocs were subjected to solid-liquid separation using a screen separator to obtain oil-containing sludge and separated liquid. The hexane extract concentration of the separated liquid was measured to evaluate the oil removal performance. The BOD of the separated liquid after the solid-liquid separation process was 500-600 mg/L. 4.0 ^m3 /day of this separated liquid was used to conduct an activated sludge treatment test (biological treatment) with a BOD sludge load of 0.2 kg/kg/day and a MLSS of 4,000 mg/L, obtaining separated sludge with a sludge concentration of 8,000-10,000 mg/L. The above-mentioned cationic polymer flocculant was added to a portion of the separated sludge to obtain flocculated sludge, which was then returned to the flocculation tank.

The test results are shown in Table 2.

As shown in Table 2, with a coagulation tank SS of 500 mg/L, coagulation tank SS/treated water SS of 1.1, and a polymer coagulant addition rate of 1.0 wt/wt% vs. SS, the SS of the separated liquid was 410 mg/L and the hexane extractable substances were 650 mg/L (Test No. 1).

At a coagulation tank SS of 1,000-1,500 mg/L, coagulation tank SS/treated water SS of 2.2-3.3, and polymer coagulant addition rate of 1.0 wt/wt% vs. SS, the SS of the separated liquid was 120-270 mg/L, and hexane extracts were 80-330 mg/L, improving the oil removal performance from the separated liquid (Test No. 2-3).

At a coagulation tank SS of 2,000-3,000 mg/L, coagulation tank SS/treated water SS of 4.4-6.6, and a polymer coagulant addition rate of 1.0 wt/wt% vs. SS, the SS of the separated liquid was 50-80 mg/L, and hexane extracts were 35-60 mg/L, further improving the oil removal performance from the separated liquid (Test Nos. 4-6).

2. Example 2
2-1. Test conditions The water to be treated and the water treatment device were the same as those in Example 1.
Treatment flow: The water to be treated was treated according to the flow sheet shown in FIG.
Oil removal test was carried out under the same conditions as in Example 1, except that, under the condition of coagulation tank SS/treated water SS 3.3, Evergloss U-700 (fiber length 5 mm, manufactured by Suing Co., Ltd.) was added as an oil remover to the treated water in the mixing tank at 50 to 1,000 mg/L. The results are shown in Table 3.

When the amount of oil removing agent added was 50-100 mg/L and the polymer flocculant addition rate was 1.0 wt/wt% vs. SS, the SS in the separated liquid was 75-100 mg/L and the hexane extractables were 62-70 mg/L (Test Nos. 15-16). By using an oil removing agent in combination, the SS and oil removal performance was improved compared to the SS of the separated liquid of 120 mg/L and hexane extractables of 80 mg/L when no oil removing agent was added (Test No. 3 in Table 2).

When the amount of oil remover added was 200-1,000 mg/L and the polymer flocculant addition rate was 1.0 wt/wt% versus SS, the SS in the separated liquid was 38-50 mg/L and the hexane extractable substances were 33-43 mg/L, further improving the SS and oil removal performance (Test Nos. 17-20).

When the amount of oil remover added was 200 mg/L and the polymer flocculant addition rate was 0.5-0.75 wt/wt% versus SS, the SS in the separated liquid was 42-56 mg/L and the hexane extractable substances were 35-48 mg/L, showing good SS and oil removal performance even when the polymer flocculant addition rate was lowered. (Test No. 22-23)

3. Example 3
3-1. Test conditions The water to be treated and the water treatment device were the same as those in Example 1.
Treatment flow: The water to be treated was treated according to the flow sheet shown in FIG.

Oil removal test was carried out under the same conditions as in Example 1, except that Evergrose U-700 (fiber length 5 mm, manufactured by Suing Co., Ltd.) was added as an oil remover to the water to be treated in the mixing tank at 50 to 1,000 mg/L under the conditions of coagulation tank SS/water to be treated SS 1.1 to 2.2 and polymer coagulant addition rate 1.0 wt/wt% to SS. The results are shown in Table 4.

At a coagulation tank SS/treated water SS ratio of 1.1, the SS in the separated liquid was 290 mg/L and the hexane extractables were 310 mg/L (Test No. 24). By lowering the ratio of coagulation tank SS/treated water SS and reducing the amount of separated sludge returned, the use of an oil remover in combination improved the SS and oil removal performance of the separated liquid compared to the results of Test No. 1 in Table 2 of Example 1.

When the coagulation tank SS/treated water SS was set to 1.7-2.2, the SS of the separated liquid was 75-110 mg/L and the hexane extractable substances were 65-75, further improving the removal of SS and oil from the separated liquid (Test No. 25-26).

4. Comparative Example 4-1. Test Conditions The water to be treated and the water treatment device were the same as those in Example 1.

Treatment flow: The treated water was treated according to the flow sheet in Figure 6.

4-2. Oil removal test An oil removal test was conducted under the same conditions as in Example 1, except that the polymer flocculant of Example 1 was added to the treated water of Example 1 in a coagulation tank at an amount of 5.0 to 50 mg/L, and the separated sludge to which no polymer flocculant was added was returned to the coagulation tank so that the coagulation tank SS/treated water SS was 3.3. The results are shown in Table 5. In Table 5, the value in parentheses for the amount of polymer flocculant added (mg/L) is the addition rate of the polymer flocculant to the separated sludge added to the coagulation tank (unit: wt/wt% to SS), calculated from the amount of polymer flocculant added and the coagulation tank SS/treated water SS.

When the amount of polymer flocculant added was 5.0-10 mg/L (polymer flocculant addition rate equivalent to 0.5-1.0 wt/wt% vs. SS), the SS in the separated liquid was 160-200 mg/L, and the hexane extracts were 120-130 mg/L (Test No. C1-C2). In contrast, when the polymer flocculant addition rate in Table 2 was 0.5-1.0 wt/wt% vs. SS, the SS in the separated liquid was 120-140 mg/L, and the hexane extracts were 80-90 mg/L, which was a smaller concentration of SS and hexane extracts in the separated liquid compared to Test No. C1-C2 (Test No. 3, 10). From these results, it was found that the removal performance of SS and oil was much improved by returning the flocculated sludge obtained by adding polymer flocculant to the flocculation tank and flocculating it, rather than returning the separated sludge to the treated liquid and then adding polymer flocculant to the flocculation tank.

In addition, even when the amount of polymer flocculant added was 20 mg/L or more (polymer flocculant addition rate equivalent to 2.0 wt/wt% of SS or more), no reduction in SS or hexane extractable substances in the separated liquid was observed (Test Nos. C3 to C5).

10 Water treatment device for oil-containing wastewater 14 Coagulation means 16 Solid-liquid separation means 18 Biological treatment means 20 Polymer flocculant addition means 22 Return means

Claims (4)

a flocculation step of forming flocs from oil-containing wastewater containing oil;
a solid-liquid separation step of separating the flocculated water containing the flocs into a solid and a separated liquid;
A biological treatment step of biologically treating the separated liquid to obtain treated water and separated sludge;
A polymer flocculant addition/flocculated sludge return process for adding a polymer flocculant to at least a portion of the separated sludge obtained in the biological treatment process and returning the obtained flocculated sludge to the flocculation process;
A method for treating oil-containing wastewater, characterized in that the formation of the coagulated flocs in the coagulation step is achieved by mixing the returned coagulated sludge with the oil-containing wastewater. The method for treating oil-containing wastewater according to claim 1, further comprising an oil removing agent addition step of adding an oil removing agent to the oil-containing wastewater prior to the coagulation step. The method for treating oil-containing wastewater according to claim 1 or 2, characterized in that in the polymer flocculant addition/flocculated sludge return process, the addition rate of the polymer flocculant to the separated sludge is 0.05 wt/wt% to SS or more and 5.0 wt/wt% to SS or less. A flocculation means for forming flocs from oil-containing wastewater containing oil;
a solid-liquid separation means for separating the flocculated water containing the flocs into solids and a separated liquid;
a biological treatment means for biologically treating the separated liquid to obtain treated water and separated sludge;
a polymer flocculant adding means for adding a polymer flocculant to at least a portion of the separated sludge obtained by the biological treatment means to obtain flocculated sludge;
A return means for returning the flocculated sludge to the flocculation means,
13. A water treatment device for oil-containing wastewater, wherein the formation of the coagulated flocs in the coagulation means is achieved by mixing the coagulated sludge returned from the return means with the oil-containing wastewater.

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