JP6016405B2 - Waste water treatment system and waste water treatment method - Google Patents

Description

  The present invention relates to a wastewater treatment system and a wastewater treatment method.

  An aromatic carboxylic acid such as terephthalic acid is produced, for example, by oxidizing an aromatic hydrocarbon such as paraxylene having an alkyl group in a solvent containing an aliphatic carboxylic acid such as acetic acid in the presence of a catalyst. .

When the aromatic carboxylic acid is produced, wastewater containing the by-product paratoluic acid and the like is discharged in addition to the target aromatic carboxylic acid, so this wastewater is treated, and COD _cr (chemical It is necessary to reduce the required amount of oxygen) to below the regulation value.

For example, the following method has been proposed as a method for treating waste water discharged when producing an aromatic carboxylic acid.
As shown in FIG. 6, in the first aeration tank 101, the step of biologically treating the wastewater by the aerobic microorganisms in the activated sludge, and in the first sedimentation tank 102, the sludge and the supernatant liquid are solid-liquid. In the second aeration tank 103, the step of performing biological treatment of the supernatant liquid by the aerobic microorganisms in the activated sludge, and in the second sedimentation tank 104, solid-liquid separation into the sludge and the supernatant liquid is performed. (See Patent Document 1).

However, in this method, the COD _cr of the supernatant liquid (treatment liquid) separated by the second precipitation tank is insufficiently reduced, and it takes time for the solid-liquid separation in the first and second precipitation tanks. There is a problem that a large area is required for the installation of the first and second sedimentation tanks.

  In addition, as a wastewater treatment system capable of reducing the BOD of the treatment liquid, a wastewater treatment system including a first aeration tank, a precipitation tank, and a second aeration tank provided with a membrane module has been proposed (patent) Reference 2). Since this wastewater treatment system performs solid-liquid separation by means of a membrane module, the time for solid-liquid separation can be shortened, and the number of settling tanks can be reduced compared with that of Patent Document 1, so that the installation area of the system can be reduced.

However, when wastewater discharged when producing aromatic carboxylic acid is treated by this wastewater treatment system, the persistent COD _cr component flowing into the second aeration tank increases, and the second aeration tank There is a problem in that COD _cr of the permeated water (treatment liquid) separated by the membrane module is not sufficiently reduced.

JP 2005-095754 A JP 2008-200277 A

The present invention provides a wastewater treatment system and a wastewater treatment method that can sufficiently reduce the COD _cr of the treatment liquid, shorten the treatment time, and reduce the installation area of the system.

The wastewater treatment system of the present invention comprises an aeration tank that performs biological treatment of wastewater by aerobic microorganisms in activated sludge while maintaining a COD _cr volume load of 1 to 3 [COD _cr -kg / m ³ / day], A sedimentation tank that solid-liquid separates the aeration tank mixed water biologically treated in the aeration tank into sludge and supernatant liquid, and the COD _cr volume load is 0.1 to 0.5 [COD _cr -kg / m ³ / day. A membrane separation activated sludge treatment apparatus that performs biological treatment of the supernatant liquid from the settling tank by aerobic microorganisms in the activated sludge and performs solid-liquid separation treatment on sludge and permeate by a membrane module. It is characterized by comprising.

The wastewater treatment method of the present invention performs biological treatment of wastewater by aerobic microorganisms in activated sludge while maintaining a COD _cr volume load of 1 to 3 [COD _cr -kg / m ³ / day] in an aeration tank. A step of solid-liquid separation of the aeration tank mixed water biologically treated in the aeration tank into a sludge and a supernatant liquid in a settling tank; and a COD _cr volume load in a membrane separation activated sludge treatment apparatus. While maintaining 0.1 to 0.5 [COD _cr -kg / m ³ / day], biological treatment of the supernatant liquid from the settling tank is performed by aerobic microorganisms in the activated sludge, and the membrane module allows the sludge to permeate. And solid-liquid separation into water.

The COD _cr / BOD ₅ of the waste water is preferably 3-10.
It is preferable that the said waste water is waste water discharged | emitted in the manufacturing process of aromatic carboxylic acid.

According to the waste water treatment system and waste water treatment method of the present invention, COD _cr of the treatment liquid can be sufficiently reduced, the treatment time can be shortened, and the installation area of the system can be reduced.

It is a schematic block diagram which shows an example of the waste water treatment system of this invention. It is a perspective view which shows a partially broken example of the membrane unit. It is a perspective view of the diffuser generation device of a membrane unit. It is a schematic block diagram which shows the other example of the waste water treatment system of this invention. It is a perspective view and a partial sectional view showing an example of a reverse osmosis membrane module. It is a schematic block diagram which shows an example of the conventional waste water treatment system.

[Wastewater treatment system]
FIG. 1 is a schematic configuration diagram showing an example of a wastewater treatment system of the present invention. The waste water treatment system includes an aeration tank 10 for biologically treating waste water from a raw water tank (not shown) with aerobic microorganisms in activated sludge; and aeration tank mixed water biologically treated in the aeration tank 10 with sludge and supernatant. And a membrane for solid-liquid separation of sludge and permeated water (treated water) by the membrane module 35 simultaneously with biological treatment of the supernatant from the sedimentation tank 20 by aerobic microorganisms in the activated sludge. Separation activated sludge treatment device 30; drainage flow path 50 for supplying wastewater from the raw water tank to the aeration tank 10; aeration tank mixed water flow for transferring the aeration tank mixed water biologically treated in the aeration tank 10 to the precipitation tank 20 A path 51; a supernatant liquid channel 52 for transferring the supernatant from the sedimentation tank 20 to the membrane separation activated sludge treatment device 30; an excess sludge channel 53 for discharging excess sludge from the sedimentation tank 20; Of excess sludge A return sludge flow path 54 for returning the part to the aeration tank 10; a permeate flow path 55 for discharging the permeated water from the membrane separation activated sludge treatment apparatus 30; and a surplus for discharging excess sludge from the membrane separation activated sludge treatment apparatus 30 A sludge channel 56; and a chemical channel 59 for supplying the chemical to the membrane module 35.

(Aeration tank)
The aeration tank 10 includes: a tank body 11; a diffuser pipe 12 disposed in the vicinity of the bottom of the tank body 11; a blower 13 for supplying air to the diffuser pipe 12; and an air introduction for connecting the diffuser pipe 12 and the blower 13 Tube 14.

(Settling tank)
The sedimentation tank 20 is not particularly limited as long as it can separate the aeration tank mixed water transferred from the aeration tank 10 into sludge and supernatant liquid by gravity sedimentation. The settling tank 20 may be a general settling tank.

(Membrane separation activated sludge treatment equipment)
The membrane-separated activated sludge treatment apparatus 30 includes a tank body 31; an air diffuser 32 disposed near the bottom of the tank body 31; a blower 33 that supplies air to the air diffuser 32; an air diffuser 32 and the blower 33. An air introduction pipe 34 to be connected; a membrane module 35 disposed in the tank body 31 and above the diffuser pipe 32; provided in the middle of the permeate flow path 55, and by allowing the inside of the membrane module 35 to be depressurized to permeate sludge. A suction pump 36 for performing solid-liquid separation from water (treated water) and sending permeate to the reverse osmosis membrane filtration device 40; provided in the middle of the chemical liquid flow path 59; And a backwash pump 37 to be fed to 35.

Examples of the membrane module 35 include a known membrane module including a known filtration membrane.
As a kind of filtration membrane, a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane) is preferable. Examples of the shape of the filtration membrane include a hollow fiber membrane, a flat membrane, a tubular membrane, and a bag-like membrane. Of these, hollow fiber membranes are preferred because they can be highly integrated when compared on a volume basis.

  Examples of the material of the filtration membrane include organic materials (cellulose, polyolefin, polysulfone, polyvinyl alcohol, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, etc.), metals (stainless steel, etc.), inorganic materials (ceramics, etc.). It is done. The material of the filtration membrane is appropriately selected according to the properties of the waste water.

  What is necessary is just to select the pore diameter of a filtration membrane suitably according to the objective of a process. In the membrane separation activated sludge method, the pore size of the filtration membrane is preferably 0.001 to 3 μm. When the pore diameter is less than 0.001 μm, the resistance of the film increases. If the pore diameter exceeds 3 μm, the activated sludge cannot be completely separated, and the water quality of the permeate may be deteriorated. The pore size of the filtration membrane is more preferably 0.04 to 1.0 μm, which is the range of the microfiltration membrane.

In the membrane separation activated sludge treatment apparatus 30, a membrane unit in which the air diffuser 32 and the membrane module 35 are integrated as shown in FIGS. 2 and 3 may be used.
The membrane unit includes a diffuser generating device 60; a membrane module 35 provided on the upper portion of the diffuser generating device 60;

  The air diffuser 60 includes a rectangular casing 61 that is open at the top and bottom; a column 62 that extends downward from the four corners of the casing 61; an outer wall of the casing 61; A connection pipe 63 for supplying air supplied through the casing 61; an air supply header 64 provided along the inner wall of the casing 61 and communicating with the connection pipe 63; an inner wall orthogonal to the air supply header 64 And a plurality of air diffusers 32 connected to 64a.

  The air diffuser 32 discharges air supplied from the blower 33 upward, and is composed of a single tube with a hole or a membrane type one end connected to an air supply header 64 and the other end closed. Yes.

  The membrane module 35 includes a frame 65 extending upward from four corners of the air diffuser 60; a plurality of hollow fiber membrane elements 70 supported by the frame 65 and arranged in parallel; and covers four side surfaces And a casing 66.

  The hollow fiber membrane element 70 has a passage (not shown) formed along the longitudinal direction inside, and has a permeated water outlet 71a formed at one end of the passage and communicating with the passage of the vertical rod 72. A pair of vertical rods 72, 73 that extend upward from both ends of the lower frame 71 and have a passage (not shown) formed along the longitudinal direction therein; and upper ends of the vertical rods 72, 73 An upper frame 74 having a passage (not shown) formed along the longitudinal direction and having a permeate outlet 74a formed at one end of the passage; and connected to the permeate outlet 74a. An L-shaped joint 75 that bends upward; a hollow fiber membrane sheet 76 composed of a number of hollow fiber membranes 76a arranged along the vertical rods 72 and 73 in a direction perpendicular to the water surface; and an upper frame 74 And the end of the hollow fiber membrane 76a is opened in the passage inside the lower frame 71. At state, the upper and lower ends of the hollow fiber membrane 76a, respectively on frame 74, secured in a liquid tight manner to the lower frame 71 comprises a potting material 77 which supports.

  The number of hollow fiber membranes 76 a constituting the hollow fiber membrane sheet 76 is preferably 500 to 3000 per one hollow fiber membrane sheet 76. When the number of the hollow fiber membranes 76a is less than 500, the membrane area of the hollow fiber membranes is reduced, and the amount of water per unit volume is reduced. When the number of the hollow fiber membranes 76a exceeds 3000, the installation area of the membrane unit becomes too large.

  It is preferable that the hollow fiber membranes 76a have a slack and are arranged in a substantially vertical direction with respect to the water surface. By arranging the hollow fiber membranes 76a in a substantially vertical direction with respect to the water surface, it becomes difficult for solids to accumulate on the surface of the hollow fiber membranes 76a, and the air from the diffuser tube 32 causes the surface of the hollow fiber membranes 76a to be deposited. It is cleaned efficiently. Further, since the hollow fiber membrane 76a has slackness, the air from the air diffuser 32 causes the hollow fiber membrane 76a to swing, and the surface of the hollow fiber membrane 76a is efficiently cleaned.

  The water collection header 80 is provided along the arrangement direction of the hollow fiber membrane elements 70. The water collecting header 80 includes a plurality of water collecting ports 80a formed along the longitudinal direction; one end is connected to the water collecting port 80a, and the other end is connected to the L-shaped joint 75 of the hollow fiber membrane element 70. A bent L-shaped joint 81; and a water suction port 80b provided on the upper surface of the water collecting header 80 and connected to the permeate flow passage 55.

(Reverse osmosis membrane filtration device)
As shown in FIG. 4, the wastewater treatment system includes a reverse osmosis membrane filtration device 40 that permeates the membrane module 35 and filters the permeate discharged from the permeate flow channel 55 through a reverse osmosis membrane; A purified water channel 57 that discharges purified water that has permeated through the reverse osmosis membrane; and a concentrated water channel 58 that discharges concentrated water that has not permeated through the reverse osmosis membrane filtration device 40.

The reverse osmosis membrane filtration device 40 includes one or more reverse osmosis membrane modules.
The reverse osmosis membrane module is not particularly limited as long as the purified water that has permeated through the reverse osmosis membrane and the concentrated water that does not permeate the reverse osmosis membrane can be separated.

Examples of the reverse osmosis membrane module include a so-called spiral reverse osmosis membrane module in which a cylindrical reverse osmosis membrane element in which a reverse osmosis membrane is wound around a water collection pipe is housed in a cylindrical casing.
Examples of the material of the reverse osmosis membrane include polyamide, polysulfone, cellulose acetate, and the like, and polyamides including aromatic polyamides or cross-linked aromatic polyamides are preferable.

  FIG. 5 is a perspective view and a partial cross-sectional view showing an example of a spiral type reverse osmosis membrane module. The spiral type reverse osmosis membrane module 41 folds the reverse osmosis membrane 42 in a state where a water collection pipe 43 having a plurality of water collection holes 43 a is placed at the center of the reverse osmosis membrane 42. 3 sides of the reverse osmosis membrane 42 overlapped with the liquid-permeable support fiber 44 sandwiched between them, and a double reverse osmosis membrane 42 with a mesh spacer 45 overlapped, and a double pipe centering on the water collecting pipe 43 A reverse osmosis membrane 42 is wound into a columnar shape to form a reverse osmosis membrane element 46, and the reverse osmosis membrane element 46 is accommodated in a cylindrical casing 47.

[Wastewater treatment method]
The waste water treatment method using the waste water treatment system of FIG. 1 has the following steps (a) to (c), and the waste water treatment method using the waste water treatment system of FIG. 4 includes the following steps (a) to ( d).
(A) In the aeration tank 10, while maintaining the COD _cr volume load at 1 to 3 [COD _cr -kg / m ³ / day], the waste water from the raw water tank (not shown) is discharged by aerobic microorganisms in the activated sludge. Biological treatment step.
(B) A step of solid-liquid separation of the aeration tank mixed water biologically treated in the aeration tank 10 in the precipitation tank 20.
(C) In the membrane-separated activated sludge treatment device 30, while maintaining the COD _cr volume load at 0.1 to 0.5 [COD _cr -kg / m ³ / day], the supernatant liquid from the sedimentation tank 20 is activated sludge. A step of solid-liquid separation into sludge and permeated water (treated water) by the membrane module 35 simultaneously with biological treatment by the aerobic microorganisms therein.
(D) A step of filtering permeated water (treated water) that has permeated through the membrane module 35 with a reverse osmosis membrane in the reverse osmosis membrane filtration device 40 as necessary.

(Drainage)
The waste water to be treated in the present invention preferably has a COD _cr / BOD ₅ of 3 to 10.
If COD _cr / BOD ₅ is less than 3, it is easily (bio) degradable and can be decomposed even in one stage, and when processing in two stages, the target is set as a normal load for the second stage. The quality of treated water is obtained. When COD _cr / BOD ₅ exceeds 10, it is extremely difficult to decompose and is outside the corresponding range of the present invention.
COD _cr is the amount of oxygen consumed by potassium dichromate and is measured according to JIS K0102.
BOD ₅ is a 5-day biochemical oxygen demand and is measured according to JIS K0102.

Examples of wastewater that satisfies the above-mentioned ranges of COD _cr and BOD ₅ include wastewater discharged when producing aromatic carboxylic acids such as terephthalic acid, for example, crude aromatics by oxidizing aromatic hydrocarbons having alkyl groups An oxidation step for making a carboxylic acid slurry, a first solid-liquid separation step for recovering the crude aromatic carboxylic acid by solid-liquid separation of the crude aromatic carboxylic acid slurry, hydrogen after dissolving the crude aromatic carboxylic acid in water or the like Addition treatment, a hydrogenation treatment step to make a hydrogenation treatment solution, a crystallization step to crystallize the hydrogenation treatment solution to make an aromatic carboxylic acid slurry, and separate the aromatic carboxylic acid slurry into a solid and liquid to separate it A process for producing an aromatic carboxylic acid comprising a second solid-liquid separation process for preparing a carboxylic acid cake and a drying process for drying the aromatic carboxylic acid cake to obtain an aromatic carboxylic acid Drainage and the like to be discharged in Oite. In addition, the waste_water | drain in the manufacturing process of aromatic carboxylic acid in this invention is the waste_water | drain discharged | emitted from one of the said each process. The wastewater treatment system and wastewater treatment method of the present invention are suitable for such wastewater treatment.

  Specific examples of the aromatic hydrocarbon having an alkyl group used in the oxidation step include dialkylbenzenes such as m-diisopropylbenzene, m-cymene, p-cymene, paraxylene, metaxylene, orthoxylene; dimethylnaphthalene, diethyl Examples thereof include dialkylnaphthalenes such as naphthalenes and diisopropylnaphthalene; dialkylbiphenyls such as dimethylbiphenyls, preferably dialkylbenzenes, and more preferably paraxylene.

  In the oxidation step, it is preferable to oxidize the aromatic hydrocarbon having the alkyl group in a solvent. Furthermore, the oxidation step is preferably performed in the presence of a catalyst. In general, the oxidation step is preferably performed using a molecular oxygen-containing gas. In the oxidation step, a crude aromatic carboxylic acid slurry is generated. In the oxidation step, it is preferable that after the oxidation reaction is once performed, the oxidation reaction is performed again (sometimes referred to as additional oxidation) in order to eliminate unreacted substances and incomplete oxides.

  In the first solid-liquid separation step, the crude aromatic carboxylic acid slurry and the mother liquor are solid-liquid separated to recover the crude aromatic carboxylic acid cake, and then washed and dried to obtain the crude aromatic carboxylic acid. is there.

  The hydrogenation treatment step is a step in which the above crude aromatic carboxylic acid is dissolved in water or the like to form a hydrogenation treatment solution by hydrogenation.

  The crystallization step is a step of obtaining an aromatic carboxylic acid slurry by crystallization of the hydrogenation treatment liquid.

  The second solid-liquid separation step is a step of solid-liquid separation into the aromatic carboxylic acid slurry and the separation mother liquor to separate into the aromatic carboxylic acid cake and the separation mother liquor.

  A drying process is a process of drying the said aromatic carboxylic acid cake and obtaining aromatic carboxylic acid.

  From the oxidation step, a gas containing an aromatic hydrocarbon containing an alkyl group such as a solvent and unreacted p-xylene is discharged along with the molecular oxygen-containing gas introduced for the oxidation reaction. After the gas is contact cleaned with alkaline water or the like, it is released to the atmosphere, and the oxidized exhaust gas cleaning water is discharged as waste water.

  From the first solid-liquid separation step, a solid-liquid separated mother liquor is generated. A part of the mother liquor is recycled to the oxidation step. The mother liquor not to be reused is preferably discharged as waste water together with the washing waste water after recovering the catalyst.

  From the crystallization step, condensed water generated at the time of crystallization, which is obtained by cooling the vapor generated during crystallization, is generated. The condensed water generated during crystallization is discharged as waste water.

In the second solid-liquid separation step, the aromatic carboxylic acid cake is solid-liquid separated and a separation mother liquor is generated. The separated mother liquor is filtered and separated by a pressure relief cooling or the like and discharged as waste water.
In addition to the second solid-liquid separation step and the drying step, vent gas washing water generated when the vent gas is collectively processed by the scrubber is discharged as waste water.

(Step (a))
Drainage discharged when producing aromatic carboxylic acid such as terephthalic acid and stored in the raw water tank (not shown) is supplied to the aeration tank 10 via the drainage channel 50.
About the waste_water | drain supplied to the aeration tank 10, a coarse suspended solid, earth and sand, etc. may be removed previously, pH may be adjusted, or it may dilute.

  In the aeration tank 10, the blower 13 is operated to introduce air from the air diffuser 12, and oxygen is given to aerobic microorganisms in the activated sludge to perform biological treatment of the waste water. By this biological treatment, aromatic compounds (such as aromatic carboxylic acids) in the wastewater are decomposed into aliphatic carboxylic acids (such as acetic acid).

Examples of the aromatic carboxylic acid include terephthalic acid, p-toluic acid, benzoic acid, 4-carboxybenzaldehyde, salts thereof (alkali metal salts, alkaline earth metal salts, ammonium salts) and the like.
Aerobic microorganisms are bacteria etc. which can decompose | disassemble an aromatic compound etc. using oxygen. Examples of such bacteria include Alkaligenes and Rhodococcus.

  1000-10000 mg / L is preferable and, as for MLSS (mixed water floating substance) in the aeration tank 10, 2000-8000 mg / L is more preferable. When MLSS is less than 1000 mg / L, there is a possibility that aerobic microorganisms are insufficient and biological treatment of waste water cannot be performed sufficiently. When MLSS exceeds 10,000 mg / L, there is a risk that the sludge floats on the water surface and the sludge flows out.

The aeration tank 10 is operated so as to keep the COD _cr volume load at 1 to 3 [COD _cr -kg / m ³ / day]. When the COD _cr volumetric load is less than 1 [COD _cr -kg / m ³ / day], the processing time becomes long. When the COD _cr volumetric load exceeds 3 [COD _cr -kg / m ³ / day], the biological treatment of waste water cannot be performed sufficiently. The COD _cr volumetric load can be adjusted to the above range by adjusting the hydraulic residence time (HRT) according to the COD _cr in the waste water treated per hour.

(Step (b))
The aeration tank mixed water biologically treated in the aeration tank 10 is transferred to the precipitation tank 20 through the aeration tank mixed water flow path 51.
In the settling tank 20, the aeration tank mixed water is solid-liquid separated into sludge and supernatant liquid by gravity settling.

  The separated excess sludge is discharged through the excess sludge channel 53. Further, since the surplus sludge contains aerobic microorganisms, a part of the surplus sludge is returned to the aeration tank 10 via the return sludge flow path 54 and used again for biological treatment of waste water. In addition, since the kind of aerobic microorganisms in the aeration tank 10 is different from the kind of aerobic microorganisms in the membrane separation activated sludge treatment apparatus 30, excess sludge from the aeration tank 10 should not be returned to the membrane separation activated sludge treatment apparatus 30. Is preferred.

The COD _cr of the supernatant is preferably 200 mg / L or more. If the COD _cr of the supernatant is to be less than 200 mg / L, the burden of biological treatment in the aeration tank 10 becomes too large. In addition, there is a risk that the biological treatment becomes insufficient due to a shortage of organic matter that is used as an energy source by the aerobic microorganisms in the membrane separation activated sludge treatment apparatus 30.

After separating the sludge containing aerobic microorganisms in the sedimentation tank 20, the type of aerobic microorganisms in the aeration tank 10 and the membrane separation activated sludge treatment apparatus 30 are transferred by transferring only the supernatant liquid to the membrane separation activated sludge treatment apparatus 30. The type of aerobic microorganisms in this is different. That is, since the kind of organic matter (aromatic compound or the like) in the aeration tank 10 is different from the kind of organic matter (aliphatic carboxylic acid or the like) in the membrane separation activated sludge treatment apparatus 30, the aeration tank 10 and the membrane separation activity naturally occur. In each of the sludge treatment apparatuses 30, different aerobic microorganisms that prefer different organic substances will grow.
Instead of the precipitation tank 20, a membrane separation device may be installed in the aeration tank 10 to perform solid-liquid separation using a membrane.

  If the aeration tank mixed water of the aeration tank 10 is transferred to the membrane separation activated sludge treatment apparatus 30 without passing through the settling tank 20, aerobic microorganisms different from the aeration tank 10 grow in the membrane separation activated sludge treatment apparatus 30. Therefore, the types of aerobic microorganisms in the aeration tank 10 and the membrane separation activated sludge treatment apparatus 30 are almost the same. Therefore, in the membrane separation activated sludge treatment apparatus 30, different aerobic microorganisms that favor aliphatic carboxylic acids and the like are difficult to grow, and biological treatment such as aliphatic carboxylic acids in the membrane separation activated sludge treatment apparatus 30 is insufficient. Become.

(Step (c))
The supernatant liquid of the sedimentation tank 20 is transferred to the membrane separation activated sludge treatment apparatus 30 via the supernatant liquid flow path 52.
The supernatant liquid from the precipitation tank 20 may be mixed with other liquids (such as other wastewater with high COD _cr ) as long as the mixed COD _cr and BOD ₅ do not exceed the above-mentioned drainage range. Good.

In the membrane separation activated sludge treatment apparatus 30, the blower 33 is operated to introduce air from the air diffusion pipe 32, and oxygen is given to aerobic microorganisms in the activated sludge to perform biological treatment of the supernatant liquid. By this biological treatment, the aliphatic carboxylic acid (such as acetic acid) in the wastewater is decomposed into carbon dioxide.
Aerobic microorganisms are bacteria etc. which can decompose aliphatic carboxylic acid etc. using oxygen.

  3000-15000 mg / L is preferable and, as for MLSS (mixed water floating substance) in the membrane separation activated sludge processing apparatus 30, 5000-10000 mg / L is more preferable. When MLSS is less than 3000 mg / L, there is a possibility that aerobic microorganisms are insufficient and biological treatment of the supernatant liquid cannot be performed sufficiently. When MLSS exceeds 15000 mg / L, fluidity is deteriorated as the viscosity of the sludge increases, and the cleaning efficiency of the membrane surface is lowered and may be clogged.

The membrane separation activated sludge treatment apparatus 30 is operated so as to maintain the COD _cr volume load at 0.1 to 0.5 [COD _cr -kg / m ³ / day]. When the COD _cr volumetric load is less than 0.1 [COD _cr -kg / m ³ / day], the processing time becomes long. When the COD _cr volumetric load exceeds 0.5 [COD _cr -kg / m ³ / day], the biological treatment of the supernatant cannot be performed sufficiently. The COD _cr volumetric load can be adjusted to the above range by adjusting the hydraulic residence time (HRT) according to the COD _cr of the supernatant processed per hour.

  Further, in the membrane separation activated sludge treatment apparatus 30, the mixed water is separated into sludge and permeate (treated water) by operating the suction pump 36 to reduce the pressure in the membrane module 35. At this time, by introducing air into the membrane module 35 from the diffuser tube 32, solid-liquid separation can be efficiently performed while cleaning the surface of the hollow fiber membrane 76a.

  The separated excess sludge is discharged through the excess sludge channel 56. Further, since the excess sludge contains aerobic microorganisms, a part of the excess sludge may be returned to the membrane separation activated sludge treatment apparatus 30 and used again for biological treatment of the supernatant liquid. In addition, since the kind of aerobic microorganisms of the membrane separation activated sludge treatment apparatus 30 is different from the kind of aerobic microorganisms of the aeration tank 10, excess sludge from the membrane separation activated sludge treatment apparatus 30 should not be returned to the aeration tank 10. Is preferred.

The smaller the COD _cr of the permeated water (treated water), the more preferable, specifically 80 mg / L or less. If the COD _{cr of the} permeated water is 80 mg / L or less, the influence on the environment is sufficiently small.

(Step (d))
The permeated water (treated water) from the membrane separation activated sludge treatment apparatus 30 may be discharged to the outside as it is. However, the permeated water has high electrical conductivity because it contains metal ions and the like derived from the catalyst used when producing the aromatic carboxylic acid. Therefore, it is preferable that the permeated water from the membrane separation activated sludge treatment device 30 is transferred to the reverse osmosis membrane filtration device 40 through the permeate flow channel 55 and the ions in the permeated water are removed by the reverse osmosis membrane 42.

  In the spiral reverse osmosis membrane module 41 of the reverse osmosis membrane filtration device 40, the permeated water (treated water) is introduced into a permeated water inlet 41a formed on one end surface of the columnar reverse osmosis membrane element 46, and is used as a mesh spacer. It flows through the flow path formed by 45. During this time, a part of the permeated water permeates through the reverse osmosis membrane 42 to become purified water, is led to the water collecting pipe 43 through the flow path formed by the liquid-permeable support fibers 44, and reaches the end of the water collecting pipe 43. It is discharged from the purified water outlet 41b. On the other hand, the permeated water that has not passed through the reverse osmosis membrane 42 becomes concentrated water and is discharged from the concentrated water outlet 41 c formed on the other end face of the reverse osmosis membrane element 46.

The purified water that has passed through the reverse osmosis membrane filtration device 40 is transferred to the aromatic carboxylic acid production facility through the purified water channel 57 and reused as cooling water or the like.
The concentrated water that has not permeated through the reverse osmosis membrane filtration device 40 is discharged to the outside through the concentrated water channel 58. The concentrated water may be discharged after sterilizing with chlorine or the like.

(Function and effect)
In the wastewater treatment system and wastewater treatment method of the present invention described above, the COD _cr volume load is maintained at 1 to 3 [COD _cr -kg / m ³ / day] in the aeration tank 10, Biological treatment of wastewater is performed by aerobic microorganisms in the activated sludge, and the COD _cr volume load is set to 0.1 to 0.5 [COD _cr -kg / m ³ / day] in the membrane separation activated sludge treatment apparatus 30. While maintaining the biological treatment of the supernatant liquid from the sedimentation tank 20 by the aerobic microorganisms in the activated sludge, the COD _cr of the permeate (treatment liquid) is sufficiently reduced compared to the conventional waste water treatment system and waste water treatment method. it can.

In addition, after separating the sludge containing aerobic microorganisms in the sedimentation tank 20, the type of aerobic microorganisms in the aeration tank 10 and the membrane separation activated sludge treatment are transferred by transferring only the supernatant liquid to the membrane separation activated sludge treatment apparatus 30. Since the types of aerobic microorganisms in the apparatus 30 are different, the biological treatment in the aeration tank 10 and the membrane separation activated sludge treatment apparatus 30 can be efficiently performed by different aerobic microorganisms. COD _cr can be sufficiently reduced.
Moreover, since it processes in the membrane separation activated sludge processing apparatus 30, processing time can be shortened compared with the conventional waste water treatment system and waste water treatment method of only an aeration tank and a sedimentation tank.
Further, since the membrane separation activated sludge treatment device 30 is used instead of the aeration tank and the settling tank, the number of settling tanks is reduced as compared with the conventional wastewater treatment system and the wastewater treatment method having two stages of the combination of the aeration tank and the settling tank, System installation area can be reduced.

(Other forms)
In addition, the waste water treatment system and waste water treatment method of this invention should just be a system and a method using a membrane separation activated sludge processing apparatus, and are not limited to the waste water treatment system of the example of illustration, and the waste water treatment method using this.

For example, when the permeated water (treated water) from the membrane separation activated sludge treatment device 30 is not reused as cooling water or the like, the reverse osmosis membrane filtration device 40 may be omitted.
Further, as shown in the illustrated example, the membrane separation activated sludge treatment apparatus may be an integrated type in which biological treatment by aerobic microorganisms in activated sludge and solid-liquid separation treatment by a membrane module are performed in one tank. A separate tank type may be used in which biological treatment with aerobic microorganisms in sludge and solid-liquid separation treatment with a membrane module are performed in separate tanks.

[Method of producing terephthalic acid]
One example of a method for producing terephthalic acid is shown as the following steps (I) to (VI).
(I) A step of obtaining a crude terephthalic acid slurry by liquid phase oxidation of para-xylene in a solvent containing water and acetic acid in the presence of a catalyst and a promoter.
(II) A step of solid-liquid separation of the crude terephthalic acid slurry to recover the crude terephthalic acid.
(III) A step of hydrogenating an aqueous solution in which crude terephthalic acid is dissolved in water.
(IV) A step of crystallizing terephthalic acid from a hydrogenated aqueous solution to obtain a terephthalic acid slurry.
(V) A step of solid-liquid separation of the terephthalic acid slurry to recover terephthalic acid.
(VI) The process of processing the waste_water | drain discharged | emitted from said process (I)-(V).

(Process (I))
Para-xylene is oxidized with oxygen in the air in the presence of a catalyst and a promoter in a solvent containing an aliphatic carboxylic acid such as acetic acid. At this time, water produced by oxidation distills together with the solvent as reaction vapor.
Examples of the catalyst include a catalyst mainly composed of a transition metal compound such as cobalt and manganese and / or a bromine compound such as hydrogen halide.

(Process (II))
A crude terephthalic acid crystal is further precipitated from a mixed slurry containing terephthalic acid produced by oxidation of para-xylene, its intermediate, and impurities to obtain a crude terephthalic acid slurry. The crude terephthalic acid slurry is solid-liquid separated into a separated mother liquor and a crude terephthalic acid cake.
The crude terephthalic acid cake is dried to obtain crude terephthalic acid crystals.

(Process (III))
Since the crude terephthalic acid crystal contains 4-carboxybenzaldehyde which is an oxidation intermediate as an impurity, in order to obtain the terephthalic acid crystal, 4-carboxybenzaldehyde in the crude terephthalic acid crystal is changed to highly soluble p-toluic acid. It is necessary to reduce and separate the paratoluic acid by the difference in solubility.
Crude terephthalic acid crystals are dissolved in water under high temperature and high pressure. The resulting aqueous solution is treated with hydrogen to reduce 4-carboxybenzaldehyde in the aqueous solution to p-toluic acid.

(Process (IV))
The obtained reduction treatment liquid is cooled under pressure to precipitate terephthalic acid crystals to obtain a terephthalic acid slurry. Relief pressure cooling means evaporating and cooling a liquid in a high pressure state by abruptly shifting to a lower pressure state. At this time, crystallization vapor accompanied by impurities such as p-toluic acid is obtained.

(Process (V))
The terephthalic acid slurry is solid-liquid separated into a separation mother liquor and a terephthalic acid cake.
The terephthalic acid cake is dried to obtain terephthalic acid crystals.
From the separated mother liquor, crystals containing impurities such as p-toluic acid are recovered by concentration and cooling, and returned to step (I) for reuse as a raw material. In addition, an aqueous solution containing an aromatic compound that could not be recovered may be partially returned to step (III) and used as a solvent, but a part or all of the aqueous solution is treated as waste water.

(Process (VI))
Some of the reaction steam, water component, crystallization steam, aqueous solution containing aromatic compounds, etc. discharged from the steps (I) to (V) are treated with waste water while containing the organic matter that could not be recovered. . In addition, wastewater containing organic matter may be discharged from processes other than those described above. Furthermore, the washing water used for washing the terephthalic acid production facility is also a wastewater containing organic substances. When the wastewater treatment system and the wastewater treatment method of the present invention are used when treating these wastewaters, COD _cr of wastewater discharged when producing terephthalic acid can be greatly reduced.

  Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

(Drainage)
Cobalt acetate and manganese acetate were used as catalysts, and paraxylene was subjected to liquid phase oxidation in a solvent containing water and acetic acid in the presence of hydrogen bromide as a co-catalyst to obtain a crude terephthalic acid slurry. Next, the crude terephthalic acid slurry was made into a mother liquor and a crude terephthalic acid cake by solid-liquid separation. The crude terephthalic acid cake was washed with water, dried and recovered as crude terephthalic acid. The recovered crude terephthalic acid and water were put into a hydrogenation reactor and subjected to hydrogenation treatment using hydrogen to form a slurry containing terephthalic acid, and then terephthalic acid was recovered. After recovering the catalyst from the mother liquor separated in the solid-liquid separation, waste water discharged together with the washing waste water was prepared. Table 1 shows the properties of this waste water.

[Example 1]
As the waste water treatment system, the waste water treatment system of FIG. 1 was used.
As the membrane unit, the membrane unit shown in FIG. 2 in which the air diffuser 32 and the membrane module 35 were integrated was used.
Table 1 shows water tank capacities of the aeration tank 10 and the membrane separation activated sludge treatment apparatus 30.

Wastewater treatment was performed using this drainage system.
The amount of treated water was the amount of water shown in Table 1.
The MLSS concentration, HRT (hydraulic residence time), and COD _cr volumetric load in the aeration tank 10 were as shown in Table 1.
The MLSS concentration, HRT (hydraulic residence time), and COD _cr volume load of the membrane separation activated sludge treatment apparatus 30 were values shown in Table 1.
The amount of returned sludge from the settling tank 20 was the amount shown in Table 1.
Table 1 shows the properties of the supernatant liquid from the sedimentation tank 20 and the permeated water (treated water) from the membrane module 35.

[Examples 2 and 3]
The amount of treated water is changed to the amount shown in Table 1, the HRT (hydraulic residence time) of the aeration tank 10 and the COD _cr volume load are changed to the values shown in Table 1, and the HRT (water) of the membrane separation activated sludge treatment apparatus 30 is changed. Waste water treatment was carried out in the same manner as in Example 1 except that the physical residence time) and the COD _cr volume load were changed to the values shown in Table 1.
Table 1 shows the properties of the supernatant liquid from the sedimentation tank 20 and the permeated water (treated water) from the membrane module 35.

[Comparative Example 1]
As the waste water treatment system, the conventional waste water treatment system shown in FIG. 6 was used.
Table 1 shows the water tank capacities of the first aeration tank 101 and the second aeration tank 103.

Wastewater treatment was performed using this drainage system.
The amount of treated water was the amount of water shown in Table 1.
The MLSS concentration, HRT (hydraulic residence time), and COD _cr volume load of the first aeration tank 101 were set to values shown in Table 1.
The MLSS concentration, HRT (hydraulic residence time), and COD _cr volume load of the second aeration tank 103 were set to values shown in Table 1.
The amount of returned sludge from the first settling tank 102 and the amount of returned sludge from the second settling tank 104 were the amounts shown in Table 1.
Table 1 shows the properties of the supernatant liquid from the first precipitation tank 102 and the supernatant liquid (treated water) from the second precipitation tank 104.

[Comparative Examples 2 and 3]
The amount of treated water is changed to the amount shown in Table 1, the HRT (hydraulic residence time) of the aeration tank 10 and the COD _cr volume load are changed to the values shown in Table 1, and the HRT (water) of the membrane separation activated sludge treatment apparatus 30 is changed. Waste water treatment was carried out in the same manner as in Example 1 except that the physical residence time) and the COD _cr volume load were changed to the values shown in Table 1.
Table 1 shows the properties of the supernatant liquid from the sedimentation tank 20 and the permeated water (treated water) from the membrane module 35.

(result)
In Examples 1 to 3, the quality of treated water is good and the residence time is small, so the installation area is small. On the other hand, in Comparative Example 1 that does not use membrane separation, the quality of the treated water is somewhat worse than that in Example 1, but the installation area becomes large because the second sedimentation tank is used. Since the comparative example 2 has a high load, the treatment in the aeration tank 10 is not sufficient, and the load on the membrane separation activated sludge treatment apparatus 30 becomes large. As a result, the quality of the treated water cannot be satisfied. In Comparative Example 3, the quality of the treated water is satisfactory, but the residence time is long and the installation area is large.

  The waste water treatment system and waste water treatment method of the present invention are useful for the treatment of waste water discharged when an aromatic carboxylic acid (such as terephthalic acid) is produced.

10 Aeration tank 20 Precipitation tank 30 Membrane separation activated sludge treatment equipment 35 Membrane module

Claims (6)

An aeration tank that performs biological treatment of wastewater by aerobic microorganisms in activated sludge while maintaining a COD _cr volume load of 1 to 3 [COD _cr -kg / m ³ / day];
A settling tank for solid-liquid separation of the aeration tank mixed water biologically treated in the aeration tank into sludge and supernatant;
While maintaining the volume load of COD _cr at 0.1 to 0.5 [COD _cr -kg / m ³ / day], biological treatment of the supernatant from the settling tank is performed by aerobic microorganisms in the activated sludge, and a membrane A wastewater treatment system comprising a membrane separation activated sludge treatment device that performs solid-liquid separation treatment on sludge and permeate using a module. The waste water treatment system according to claim 1, wherein COD _cr / BOD _{5 of the} waste water is 3 to 10.   The wastewater treatment system according to claim 1 or 2, wherein the wastewater is wastewater discharged in an aromatic carboxylic acid production process. A biological treatment of wastewater by aerobic microorganisms in activated sludge while maintaining the COD _cr volume load at 1-3 [COD _cr -kg / m ³ / day] in an aeration tank;
Separating the aerated tank mixed water biologically treated in the aerated tank into a sludge and a supernatant liquid in a settling tank;
Using a membrane-separated activated sludge treatment apparatus, the supernatant from the sedimentation tank by aerobic microorganisms in the activated sludge while maintaining the COD _cr volume load at 0.1 to 0.5 [COD _cr -kg / m ³ / day] A wastewater treatment method comprising: biological treatment of the liquid, and solid-liquid separation into sludge and permeate by a membrane module. The waste water treatment method according to claim 4, wherein COD _cr / BOD _{5 of the} waste water is 3 to 10.   The wastewater treatment method according to claim 4 or 5, wherein the wastewater is wastewater discharged in an aromatic carboxylic acid production process.

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