JP2003103287A - Treatment apparatus for oils and fats-containing wastewater and treatment method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide not only a concentration and separation method for oils and fats- containing wastewater capable of achieving maintenance properties, economical efficiency and space saving with respect to a pressure floating method or a centrifugal separator but also a treatment apparatus for oils and fats-containing wastewater capable of reducing industrial waste treatment cost in pretreatment and not damaging biological treatment capacity in post-treatment and a treatment method using the same. SOLUTION: The treatment apparatus for oils and fats-containing wastewater is constituted of a specific gravity separation means 3 for concentrating and separating oils and fats of oils and fats-containing wastewater by utilizing specific gravity difference, a biological treatment means 4 for high concentration oil-containing wastewater for treating the concentrated and separated high concentration oil-containing wastewater in a biological tank to which oil decomposing bacteria are added, a biological treatment means 5 for low concentration oil-containing wastewater adding oil decomposing bacteria to the low concentration oil- containing wastewater from which a part of oils and fats is removed by the specific gravity separation means 3 to treat the wastewater under an aerobic condition, a sedimentation means 6 for subjecting the wastewater treated by the biological treatment means 5 for the low concentration oil-containing wastewater to solid-liquid separation treatment and a sludge return means 9 for returning the sediment of the sedimentation means 6 to the biological treatment means 5 for the low concentration oil-containing wastewater.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oil-containing wastewater treatment apparatus and method for biologically removing wastewater containing animal and vegetable oils and mineral oils.

[0002]

2. Description of the Related Art At a vegetable oil manufacturing plant, waste water containing fats and oils as high as several thousand ppm to tens of thousands ppm is discharged from the manufacturing process of soybean oil, rapeseed oil, corn oil, etc., especially from the deoxidizing process. Also,
Water-soluble cutting oil (mineral oil) is used to cool and lubricate the cutting surface in manufacturing plants that use many machine tools such as automobile factories. This water-soluble mineral oil is recycled and reused. However, the waste oil that has deteriorated and cannot be reused must be industrially disposed of, or treated to comply with the discharge standards and then discarded. In addition, relatively high concentrations of oil-containing wastewater derived from animal and vegetable oils are discharged from food factories, hotels and restaurants.

Conventional techniques such as a device and method for treating oil-and-fat-containing wastewater are known techniques described in, for example, water treatment techniques learned from the beginning (industrial research group; published on September 6, 1999). , Chemical treatment using chemicals such as coagulant, static separation, pressure floating separation, physical treatment such as grease trap, biological treatment such as activated sludge method and biofilm method, and these treatment methods A processing device which is a combination of the above is generally used.

In the case of treating oil-containing wastewater, it is often difficult to stably remove fats and oils by chemical methods and physical methods due to load fluctuations and changes with time of various treatments. It is impossible to remove the fats and oils in advance. Further, normally, in the biological method which is the post-treatment of the above-mentioned treatment method, the oil and fat content in the waste water can be decomposed and removed to some extent, but as described above, the pre-treatment makes the removal of the oil and fat unsatisfactory and the load of the oil and fat content. When the amount increases, oil adheres to the flocs of the activated sludge and the surface of the biofilm, impairing the permeability of oxygen, substrates, nutrients, etc., and gradually lowering the treatment efficiency, and treating organic substances other than fats and oils. It was pointed out that there was a problem that it could not be performed sufficiently.

On the other hand, in terms of operability and economy,
In chemical treatment, the working environment such as sludge handling and odor is bad, and the amount of sludge is large. In the physical method, a device that is left standing for a long time to spontaneously float and separates requires a vast site area, and a grease trap or the like requires time and effort for maintenance, resulting in high cost. In addition, a common problem of chemical treatment and physical treatment is that labor and cost are required for disposing sludge. Generally, sludge is incinerated or treated as industrial waste, but large-scale equipment and enormous cost are required for incineration, and in recent years, the dioxin problem caused by chlorine in the incinerator is a problem. Due to environmental issues such as the above, it is becoming difficult to incinerate. On the other hand, landfill disposal as industrial waste has the drawbacks that the processing cost increases due to the recent tightness of the final disposal site and that it is difficult to secure the disposal site.

[0006]

Therefore, for example, Japanese Patent Application Laid-Open No. 5-245439 relates to the treatment of oil-containing wastewater in which waste oil concentrated and separated (pressure floating or centrifuge) is contacted with a lipase enzyme to decompose fats and oils. Invention and JP-A-5-24
Japanese Patent No. 5489 discloses an invention relating to a method for treating oil-containing wastewater in which waste oil concentrated by a centrifuge is used as a fuel for heating cells in a biological treatment tank, improving performance of pretreatment means and reducing extra sludge. The invention has been disclosed which leads to reduction of industrial waste processing cost.

However, even in such a technique, in the former case, in order to prevent the performance from being deteriorated due to the outflow of the added enzyme, there is a problem that the operating cost increases due to the replenishment of an expensive substance such as a lacking enzyme. In the latter case, it is considered that concentrated waste oil can be used as a bacterial cell warming fuel in the winter season, but in the summer season, the temperature of the biological treatment tank is the outside temperature, the aeration air rises due to the compression insulation of the blower, and the waste oil is decomposed. It rises from the fermentation heat generated from time to time, and in our experience, there are cases where the cooling system is warped and required. Therefore, in this case, it is expected that not only the concentrated waste oil cannot be used as a fuel for heating bacterial cells, but also there are various problems such as handling and storage of sludge, and deterioration of working environment such as odor due to decay of the concentrated waste oil.

In the pressure flotation method and the concentration separation by the centrifuge described in the above-mentioned known example, the former secures a site area and, depending on the separation conditions, operating costs such as power cost for pressurization, The handling of the coagulant to improve the separation performance is a problem. The latter has a lot of mechanical drive parts and has some problems in maintainability, and since the entire container rotates, operating costs such as power costs and workability become problems.

An object of the present invention is to propose a method for concentrating and separating oil-and-fat-containing wastewater, which can achieve maintenance, operability, economy and space saving, in addition to the above-mentioned pressure flotation method and centrifuge. It is an object of the present invention to propose an apparatus and method for treating oil-and-fat-containing wastewater which can reduce industrial waste treatment cost and which does not impair the biological treatment performance of post-treatment.

[0010]

[Means for Solving the Problems] Specific gravity separation means for concentrating and separating oil and fat-containing wastewater by utilizing specific gravity difference, and treating the concentrated and separated high-concentration oil-containing wastewater in a biological tank to which oil-degrading bacteria are added. Biological treatment means for high-concentration oil impregnation, and low-concentration oil-impregnated biological treatment means for treating a low-concentration oil-containing wastewater from which a part of fats and oils have been removed by the specific gravity separation means under aerobic conditions by adding oil-degrading bacteria, This can be achieved by sedimentation means for solid-liquid separation of the wastewater treated by the oil-containing biological treatment means, and sludge returning means for returning the precipitate of the sedimentation means to the low-concentration oil-containing biological treatment means.

Further, in the invention of claim 2, the low-concentration oil-containing biological treatment means of claim 1 is constituted by a plurality of stages of means, at least one of which means is a filter bed having a carrier fixed thereon and organisms under aeration. This can be achieved by using an aeration tank for treatment and a sprinkling filter means composed of a circulation / sprinking means for sprinkling a part of the treated water in the aeration tank on the filter bed.

According to the third aspect of the invention, the specific gravity separating means of the first aspect can be achieved by a liquid specific gravity separator for separating by utilizing the centrifugal force of the means for sending the oil and fat-containing wastewater.

The invention of claim 4 can be achieved by controlling, in the liquid specific gravity separator of claim 3, the amount of liquid fed by the means for feeding the oil and fat-containing wastewater according to the frequency.

The invention according to claim 5 is achieved in the liquid specific gravity separator according to claim 3 by varying the diameter of the lower discharge pipe at the bottom of the liquid specific gravity separator to control the distribution ratio of the drainage discharged vertically. it can.

According to the invention of claim 6, the high-concentration oil-containing wastewater concentrated and separated by the specific gravity separation means is treated by a biological treatment means to which oil-decomposing bacteria are added and which is equipped with a means for stirring the inside of the tank. After that, it can be achieved by providing a returning means for returning to the upstream of the specific gravity separating means.

According to a seventh aspect of the present invention, the returning means of the sixth aspect is a precipitation means for solid-liquid separating the treated wastewater of the biological treatment means for treating the high-concentration oil-containing wastewater, and the supernatant of the precipitation means is separated by the specific gravity means. Alternatively, it can be achieved by returning the precipitate to the biological treatment means for low-concentration oil impregnation and returning the precipitate to the biological treatment means.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS.

Raw water 1 containing animal and vegetable oils and mineral oils is sent to a liquid gravity separator 3 by a raw water supply pump 2. The liquid specific gravity separator 3 is composed of an inlet pipe 30, a separator body (cylindrical portion + cone portion), an upper discharge pipe 31, a lower discharge pipe 32, and a seal component, and the upper discharge pipe 31 is a biological treatment tank 4 for high-concentration oil impregnation.
In addition, the lower discharge pipe 32 is connected to the biological treatment tank 5 for low-concentration oil impregnation.

The high-concentration oil-containing wastewater concentrated and separated by the liquid gravity separator 3 is oil-decomposed in the high-concentration oil-containing biological treatment tank 4 equipped with a stirrer 40, and then the treated wastewater is sucked into the raw water supply pump 2. Or, return it to the raw water storage tank. On the other hand, the low-concentration oil-containing wastewater discharged from the lower discharge pipe 32 of the liquid gravity separator 3 flows into the settling tank 6 via the low-concentration oil-containing biological treatment means 5. In the settling tank 6, the liquid is not agitated and mixed and is in a stationary state, so that solid-liquid separation is performed and the supernatant liquid overflows as the discharged water 7. On the other hand, a part or all of the excess sludge 8 that has been settled and concentrated in the settling tank 6 is returned to the low-concentration oil-containing biological treatment means 5 by the return pump 9, and the work of removing organic matter from the low-concentration oil-containing wastewater is repeated again.

The flow of raw water 1 inside the liquid gravity separator 3 is as follows. Inlet tube 30 of liquid gravity separator 3
Is attached tangentially to the separator body (cylindrical part), and the raw water 1 becomes a spiral in the separator and centrifugal force causes an oil and fat component in the raw water 1 and a component / suspension having a lighter specific gravity than water. (Hereinafter referred to as high-concentration oil-containing wastewater) gathers in the center of the separator. The high-concentration oil-containing wastewater is transferred from the upper discharge pipe 31 attached to the center of the separator to the high-concentration oil-containing biological treatment tank 4 together with a small amount of oil and fat and a part of the emulsified liquid (hereinafter referred to as low-concentration oil-containing wastewater). Is discharged. On the other hand, the remaining low-concentration oil-containing wastewater flows down from the wall side of the separator to the lower discharge pipe 32,
It is discharged to the low-concentration oil-containing biological treatment tank 5.

The amount of raw water 1 to be treated in the liquid gravity separator 3 is controlled by the operating frequency of an inverter 20 connected to the power source of the raw water supply pump 2, for example, a cylinder diameter φ.
In the liquid specific gravity separator 3 having a height of 270 mm and an overall height of about 1,000 mm, as shown in FIG. 2, 10 to 30 m ³ / h (operating frequency: 2
A throughput of 0-60 Hz) is obtained. In addition, the upper drainage rate (defined as the upper drainage rate / total raw water rate) at this time varies by 0 to 90% depending on the operating frequency, but the upper drainage rate is 0 to 75%.
During (operating frequency: 20 to 30 Hz), the change is rapid and it is difficult to finely adjust. In this case, as shown in FIG. 3, the throughput is maintained constant by changing the diameter of the lower pipe diameter (the upper pipe diameter is constant) (in this example, the operating frequency is 40 Hz, the throughput is 25 m).
³ / h), the upper drainage rate, that is, the distribution ratio of drainage to the upper side and the lower side, can be accurately controlled. The distribution ratio of the liquid gravity separator varies depending on the separation status (ratio) between the oil layer and the liquid layer of the raw water to be treated, but in the experiments conducted by the authors using actual wastewater from an oil refinery, the upper drainage rate was 20% ( The oil layer and the liquid layer could be completely separated at a distribution ratio of about 20:80. Further, the amount of n-hexane extracted substance at this time was 4,600 mg / L on the high-concentration oil-containing drainage side and 200 mg / L on the low-concentration oil-containing drainage side.

Next, the operation of treating the high-concentration oil-containing wastewater (including part of the low-concentration oil-containing wastewater) concentrated and separated by the liquid specific gravity separator 3 in the high-concentration oil-containing biological treatment tank 4 will be described below.

The high-concentration oil-containing wastewater supplied to the high-concentration oil-containing biological treatment tank 4 to which the oil-degrading bacteria have been added is agitated by the agitating device 40, whereby the oil layer and the liquid layer are suspended, and the oil layer and the liquid layer are entrained by agitation. The highly concentrated oil-containing wastewater is decomposed under aerobic conditions by the air in the atmosphere.

The oil-decomposition treatment in the high-concentration oil-containing biological treatment tank 4 does not have to be a complete treatment, and decomposition that does not separate the oil layer and the liquid layer is sufficient. By reducing the amount, the separation of the oil layer and the liquid layer was completely eliminated.

In this way, the treated wastewater which has been subjected to the oil decomposition treatment in the high-concentration oil-containing biological treatment tank 4 is returned to the suction port of the raw water supply pump 2 or the raw water storage tank and concentrated again in the liquid gravity separator 3. To be separated.

The stirring device 40 is preferably operated at a low speed such that an oil layer and a liquid layer are suspended, and an operation method in which forward and reverse operations are alternately repeated at a low speed is preferable. As another means of the stirring device 40, the whole tank is shaken. The method of allowing is also suitable. However, aeration and agitation by stirring with air like an aeration tank is not preferable because it promotes separation of the oil layer and the liquid layer as in the pressure floating method. In addition, the origin of the oil-decomposing bacteria added to the high-concentration oil-containing biological treatment tank 4 is not limited. For example, Pseudomonas spp., Bacillus spp. Hypolka O (manufactured by Shikoku Kasei) as a degrading bacterium,
SYBRON (Biochemical Co., Ltd.), Oppenheimer Formula (Oppenheimer Formula-
・ Japan) can also be used.

As described above, according to the embodiment of the present invention, it is very compact as compared with the pressure levitation method and the centrifuge, and the number of mechanical operating parts is small, so that the maintainability is high, the operating frequency and the liquid specific gravity separation are performed. By simply changing the structure of the vessel, it is possible to concentrate the wastewater containing oil and fat with excellent operability. Also,
By using the high-concentration oil-containing wastewater treatment together, it is possible to treat the scum (oil layer) that is usually treated as industrial waste in pretreatment, so the industrial waste treatment cost can be reduced and the biological treatment performance of posttreatment is not impaired.

On the other hand, an example of the operation of treating the low-concentration oil-containing wastewater separated by the liquid gravity separator 3 by the biological treatment means 5 will be described below with reference to FIG.

The biological treatment means 5 is composed of, for example, an oil decomposing tank 10, a sprinkling filter means 11, a post-aeration tank 12 and the like, to which oil decomposing bacteria and sludge bacteria are added and which treats organic substances under aerobic conditions. In addition, air diffusers 100, 112, and
120 is installed, and air is sent to the air diffuser from the aeration blower 13 through the aeration flow rate adjusting valves 130 to 132. The origins of the oil-degrading bacteria and the sludge bacteria are not limited, and for example, the same bacteria as the oil-degrading bacteria added to the high-concentration oil-containing biological treatment tank 4 can be used.

The oil decomposing tank 10 has a nutritional balance of raw water such as BOD (biochemical oxygen demand): N (nitrogen): P.
(Phosphorus) = 100: 5: 1 In addition to being adjusted, the oil and fat components are actively hydrolyzed (glycerin and fatty acids) and oxidatively decomposed (carbon dioxide, water). The number of oil-decomposing bacteria and the aeration flow rate are adjusted so that 50% or more, preferably 70% or more, of the raw water can be decomposed. In addition to fats and oils, for example, organic substances such as carbohydrates and proteins are similarly hydrolyzed and oxidatively decomposed, so that BOD, COD (chemical oxygen demand), SS (suspended suspended matter), T -N (total nitrogen), T
-P (total phosphorus) will also decrease.

In the sprinkling filter bed means 11, for example, the filter bed 110 is located above the aeration tank 111, and the filter bed 1 is installed from the oil decomposition tank 10.
10 or aeration tank 111 is supplied with wastewater, and aeration tank 11
In No. 1, the wastewater is oxidized and decomposed under aerobic conditions, while the aeration tank 11
The treated wastewater in 1 is circulated and sprinkled by a pump 113 to filter bed 1
By using the water sprinkling pipe 114 to 10 to spray water evenly on the front surface,
Oil-degrading bacteria and sludge bacteria are carried (activated) on the carrier loaded on the filter bed 110, and the activated bacteria oxidize and decompose the waste water, and the natural falling water 115 is returned to the aeration tank 111.
Further, a part of the air aerated in the aeration tank 111 is dissolved in the waste water, and the other passes through the filter bed 110 from the aeration tank 111, and the carrier surface loaded on the filter bed 110 is forced to be aerobic. Normalize and discharge out of the system. The amount of water sprinkled from the circulation / sprinkling pump 113 through the air diffuser 114.
(m ³ / h) and actual capacity of the aeration tank 111 (reservoir capacity: m ³ )
The ratio is preferably 0.4 or more per unit time. Further, as the carrier loaded on the filter bed 110, crushed stone, plastic-based packing, etc. can be used, but preferably SiO ₂ is 80 to 85% and AL ₂ O ₃ is 5 to 10.
%, K ₂ O is 4 to 6%, other Fe ₂ O ₃ , CaO, Ti
Quartz trachyte with a composition such as O ₂ and MnO is effective. When the carrier crushed in the filter bed 110 is, for example, the above-described quartz trachyte, the quartz trachyte has a porous interior, so that the substantial surface area is large and anaerobic bacteria are mainly present in the interior with poor ventilation. Breeding and aerobic bacteria are mainly propagated on the surface leaning under aerobic conditions. Therefore, in addition to oxygen respiration, nitrification,
Respiratory metabolism such as denitrification is also actively performed, which is excellent in catabolic metabolism,
The amount of sludge generated is small and odor can be suppressed. Moreover, since the bacterial cell concentration in the filter bed is high, stable operation is possible due to fluctuations in the water quality of the raw water 1. In addition to the major elements of C, O, N, H, P, K, S, Na,
Since it is composed of trace elements such as Ca, Mg, Cl and Fe, the components lacking in the raw water 1 can be obtained from quartz trachyte. In addition, the filter bed 110 and the aeration tank 111 have a dephosphorizing effect due to the phenomenon that phosphorus is released under anaerobic conditions and that phosphorus is excessively ingested under aerobic conditions.

The post-aeration means 12 mainly assists solid-liquid separation in the settling tank 6 which is the next means, and promotes floc formation of the bacterial cells.

FIG. 5 shows an example of the flow of the oil-containing wastewater treatment equipment which is another embodiment of the present invention. This embodiment also has a configuration,
The operation is the same as that of the embodiment of FIGS. 1 to 4, and the feature of this embodiment is that the treated wastewater treated in the high-concentration oil-containing biological treatment tank 4 is settling tank 14 for the high-concentration oil-containing treatment wastewater (hereinafter referred to as settling tank 14). Note)
The point is that a means for solid-liquid separation is provided. The effect peculiar to this embodiment is that the oil-decomposing bacteria in the high-concentration oil-containing biological treatment tank 4 are subjected to solid-liquid separation in the settling tank 14 to increase the bacterial cell concentration in the high-concentration oil-containing biological treatment tank 4 to a high concentration. It can be maintained without degrading the oil and fat decomposition treatment performance. In addition, since the outflow of the bacterial cells is suppressed, the high-concentration oil-containing biological treatment tank 4 and the low-concentration oil-containing biological treatment tank 5 have different fat and oil decomposing capacities (use of fat and oil decomposing bacteria suitable for each concentration). In addition to the use of bacteria, when the supernatant has a small amount of oil film, the return line 140 is used to return the supernatant water to the suction port of the raw water supply pump 2 or the raw water storage tank without using the return line 141. The supernatant water can be returned to the low-concentration oil-containing biological treatment tank 5 to reduce the load on the liquid gravity separator 3.

[0034]

According to the embodiments of the present invention, it is possible to concentrate oil-and-fat-containing wastewater which can be maintained, economically and space-saving in a pressure floating method and a centrifuge. Moreover, the industrial waste treatment cost in the pretreatment can be reduced, and the biological treatment performance of the posttreatment is not impaired.

[Brief description of drawings]

FIG. 1 is a flow chart showing an embodiment of an oil / fat-containing wastewater treatment apparatus according to the present invention.

FIG. 2 is a characteristic diagram showing an example of operating characteristics of a liquid gravity separator.

FIG. 3 is a special drawing showing an example of the relationship between the ratio of the discharge pipe diameter of the liquid gravity separator and the upward drainage rate.

FIG. 4 is a flow chart showing another embodiment of the oil and fat-containing wastewater treatment apparatus according to the present invention.

FIG. 5 is a flow chart showing another embodiment of the oil / fat-containing wastewater treatment apparatus according to the present invention.

[Explanation of symbols]

1 ... Raw water, 2 ... Raw water supply pump, 3 ... Liquid gravity separator,
4 ... High-concentration oil-containing biological treatment tank, 5 ... Low-concentration oil-containing biological treatment means, 6 ... Sedimentation tank, 7 ... Discharge water, 8 ... Excess sludge, 9 ...
Return pump, 10 ... Oil decomposition tank, 11 ... Sprinkling filter bed, 12 ...
After aeration tank.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Seiji Nomura             Yamaguchi Prefecture Kudamatsu City Oita Toyoi 794 Stock Association             Inside Hitachi Kasado Works F-term (reference) 4D003 AA02 AB02 BA02 BA03 CA03                       CA07 EA14 EA23 EA30 FA05                       FA06                 4D028 AB00 BB02 BB06 BC01 BC12                       BC18 BD11 BD16                 4D037 AA13 AB06 BA28 CA07                 4D040 DD03 DD12

Claims (8)

[Claims] 1. A specific gravity separation means for concentrating and separating oil and fat-containing wastewater by utilizing a difference in specific gravity, and a high-concentration oil-containing wastewater in which the concentrated and separated high-concentration oil-containing wastewater is treated in a biological tank to which oil-degrading bacteria are added. Biological treatment means, low-concentration oil-containing wastewater from which a part of oils and fats has been removed by the specific gravity separation means, and treated under aerobic conditions by adding oil-degrading bacteria, and the low-concentration oil-containing biological treatment A treatment device for oil-and-fat-containing wastewater, comprising a sedimentation means for solid-liquid separation of the wastewater treated by the means, and a sludge return means for returning the precipitate of the precipitation means to the low-concentration oil-containing biological treatment means. 2. The apparatus for treating oil / fat-containing wastewater according to claim 1, wherein the low-concentration oil-containing biological treatment means comprises a plurality of stages, at least one of which is under aeration with a filter bed having a carrier fixed thereon. A treatment apparatus for oil-and-fat-containing wastewater, which comprises an aeration tank for biological treatment and a sprinkling filter means composed of a circulation / sprinking means for sprinkling a part of the treated water in the aeration tank on a filter bed. 3. The specific gravity separating means for concentrating and separating the oil-and-fat-containing wastewater by utilizing the difference in specific gravity of the oil-and-fat-containing wastewater of the apparatus for treating oil-and-fat-containing wastewater according to claim 1, uses the centrifugal force of the means for sending the oil-and-fat-containing wastewater. A device for treating oil-containing wastewater, characterized in that it is a liquid specific gravity separator for separation. 4. The liquid gravity separator according to claim 3, wherein
A treatment device for oil-and-fat-containing wastewater, characterized in that the amount of liquid sent by means for sending oil-and-fat-containing wastewater is controlled by the frequency. 5. The liquid gravity separator according to claim 3,
A treatment device for oil-and-fat-containing wastewater, characterized in that the diameter of the lower discharge pipe at the bottom of the liquid gravity separator is varied to control the distribution ratio of the wastewater discharged vertically. 6. The apparatus for treating oil-and-fat-containing wastewater according to claim 1, wherein the high-concentration oil-containing wastewater concentrated and separated by the specific gravity separation means is provided with a means for stirring oil in the tank and stirring the inside of the tank. A treatment device for oil-and-fat-containing wastewater, comprising return means for returning the wastewater after the treatment by means to the specific gravity separation means. 7. The returning means according to claim 6 is a settling means for solid-liquid separation of the treated wastewater of the biological treatment means for treating high-concentration oil-containing wastewater, and the supernatant of the precipitating means is provided upstream of the specific gravity separation means or at a low level. An apparatus for treating oil-and-fat-containing wastewater, which is a means for returning the precipitate to the biological treatment means for oil-containing concentration and for returning the precipitate to the biological treatment means. 8. A specific gravity separation step for concentrating and separating fats and oils containing wastewater containing fats and oils by using a difference in specific gravity, and a high-concentration oils-containing wastewater that has been concentrated and separated is treated in a biological tank to which oil-degrading bacteria have been added. Biological treatment step and biological treatment step for low-concentration oil-containing oil treatment, in which low-concentration oil-containing wastewater from which a part of oils and fats has been removed in the specific gravity separation step is treated under aerobic conditions by adding oil-degrading bacteria, and the low-concentration oil-containing biological treatment Means for solid-liquid separation of wastewater treated by means, and a sludge returning step for returning the precipitate obtained in the precipitation step to the low-concentration oil-containing biological treatment step .