JP3795108B2 - Treatment method for organic acid-containing wastewater - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating organic acid-containing wastewater that removes an organic acid from wastewater containing an organic acid such as acetic acid.
[0002]
[Prior art]
Wastewater containing organic acids such as acetic acid is often discharged in factories that produce acetic acid, other organic acids or amino acids by fermentation or synthesis. Since these cannot be released as they are or by simple neutralization treatment, wastewater treatment for removing the organic acid itself from the wastewater is required.
[0003]
As a wastewater treatment method for removing an organic acid, when the organic acid concentration is relatively high, the organic acid can be recovered by a distillation method or an extraction method, but in the case of a low concentration of about 10,000 ppm or less. The above method cannot be used because it is generally economically disadvantageous. Further, even if these distillation methods and extraction methods can recover a high concentration of organic acid, it is inevitable that a small amount of organic acid remains in the waste water after all.
[0004]
Various methods for removing an organic acid from a low concentration organic acid aqueous solution are also known. For example, an organic acid in waste water is immobilized by contacting with a metal salt that forms a water-insoluble salt, for example, calcium carbonate, or adsorbent such as activated carbon, diatomaceous earth, or ion exchange resin. There are methods such as adsorption and immobilization. However, these methods are not necessarily economical and effective methods because immobilization and adsorption are insufficient, the apparatus becomes large-scale, and the treatment and regeneration of solid waste are complicated.
Biological removal methods such as the activated sludge method are also used, but this method cannot be applied when the concentration of organic matter is high, so it is necessary to dilute the raw material wastewater, and therefore the treatment equipment is large-scale. It becomes.
[0005]
Conventionally, a submerged combustion method has been known as one of means for solving this problem. In this method, for example, as described in Japanese Patent Publication No. 55-11848, the waste water is preliminarily concentrated using the waste heat of the waste water combustion furnace, and then this concentrated waste water is supplied to the combustion furnace and burned. This is a method for oxidative decomposition of organic matter. This method is effective for removing not only organic acids but also all water-soluble or water-dispersible organic substances.
[0006]
[Problems to be solved by the invention]
However, this method also requires a large amount of heat because it is necessary to evaporate a large amount of water and convert it into a high-temperature gas even if preconcentration is performed when the concentration of organic substances in the raw material wastewater is low. Not right. Also, disposal of ash generated during combustion becomes a problem.
[0007]
A reverse osmosis method is known as a method for concentrating and separating an organic acid from a dilute solution without requiring heating of water that consumes a large amount of energy. In this method, when an organic aqueous solution is supplied under pressure to one of the reverse osmosis membranes made of cellulose acetate, polyamide, etc., if this organic matter does not permeate the reverse osmosis membrane, It is permeated to the side and discharged.
[0008]
However, when this method is applied to an aqueous solution of an organic acid, the water-soluble organic acid has a strong affinity for the reverse osmosis membrane and has a low degree of dissociation. Become imperfect. Japanese Patent Publication No. 56-20051 proposes a method of converting an organic acid to an organic acid salt by adding an alkali agent to an organic acid aqueous solution in advance and supplying the organic acid ion to the reverse osmosis membrane device. . Since an organic acid in an ionic state cannot permeate the reverse osmosis membrane, the organic acid can be recovered by concentrating the diluted organic acid by this method and further concentrating the concentrated liquid by evaporation.
[0009]
The reverse osmosis membrane method can concentrate the organic acid with low energy, so it is advantageous when recovering and using the obtained organic acid. However, when the purpose is not to recover the organic acid, the concentrated organic acid salt is used. Post-processing becomes a problem.
The present invention has been made in order to solve the above-mentioned problems. Therefore, the object of the present invention is to remove organic acids from waste water containing organic acids without removing organic substances and ash, and to clean the organic acids efficiently. Another object of the present invention is to provide a method for treating organic acid-containing wastewater that can discharge wastewater.
[0010]
[Means for Solving the Problems]
The above problems include a neutralization step in which an organic acid is converted to an organic acid alkali metal salt by adding an alkali metal carbonate to the organic acid-containing wastewater as a neutralizing agent, and an organic acid alkali metal salt obtained in this neutralization step Membrane separation process of supplying aqueous solution to reverse osmosis membrane device and separating into concentrated liquid and waste water rich in organic acid alkali metal salt, and burning this concentrated liquid to convert organic acid alkali metal salt into alkali metal carbonate And a method for treating organic acid-containing wastewater that is circulated to the neutralization step using at least a part of the alkali metal carbonate obtained in the combustion step as a neutralizing agent.
In the present specification, all “carbonates” are names including carbonates and bicarbonates.
[0011]
In the method for treating organic acid-containing wastewater, the organic acid-containing wastewater preferably contains an organic acid within a range of 500 ppm to 20000 ppm.
In addition, in the above-described organic acid-containing wastewater treatment method, the concentrated liquid obtained in the membrane separation process is supplied to an evaporator, and part of the water is removed by evaporation, and the concentrated liquid is further concentrated and then supplied to the combustion process. It is preferable to include a concentration step.
In the method for treating organic acid-containing wastewater, the combustion temperature in the combustion step is preferably in the range of 800 ° C to 1100 ° C.
The method for treating organic acid-containing wastewater of the present invention can be suitably used when the organic acid-containing wastewater is acetic acid synthesis wastewater.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in the order of steps by an example shown in FIG. FIG. 1 shows an example of a wastewater treatment process according to the present invention, but omits liquid feed pumps and intermediate storage tanks.
[0013]
(Neutralization process)
In the wastewater treatment process of FIG. 1, organic acid-containing wastewater containing an organic acid in the range of 500 ppm to 20000 ppm (hereinafter simply referred to as “wastewater”) is sent from a wastewater generation source (not shown) via line 1 to a stirrer. It is supplied to the attached neutralization tank 2. A sodium carbonate (or sodium bicarbonate, hereinafter the same) aqueous solution is supplied to the neutralization tank 2 as a neutralizing agent via a line 29. Further, the return liquid from the second reverse osmosis membrane device 7 described later is also supplied via the line 11.
In the neutralization tank 2, the organic acid in the wastewater is neutralized by sodium carbonate to be converted to sodium organic acid, and dissociated into organic acid ions and sodium ions. This process is a neutralization process.
[0014]
The neutralized waste water (neutralized waste water) is extracted from the neutralization tank 2 and introduced into the filtration tank 4 via the line 3. The filter tank 4 is a sand filter, and the suspended matter in the neutralized waste water is removed here. The filtrate from which the suspended matter has been removed is then sent to the membrane separation process via line 5.
[0015]
(Membrane separation process)
The membrane separation step includes a first reverse osmosis membrane device (hereinafter referred to as “first RO device”) 6 and a second reverse osmosis membrane device (hereinafter referred to as “second RO device”) connected in series. It consists of seven. These RO apparatuses are composed of a reverse osmosis membrane module and a pressure pump.
[0016]
The filtered waste water (5) is first supplied to the first RO device in the membrane separation step and is pressurized by a pressure pump. When the filtered waste water (5) is supplied to the reverse osmosis membrane module of the first RO device in a pressurized state, the water passes through the reverse osmosis membrane and the organic acid sodium dissociated into ions hardly permeates. A concentrated liquid rich in organic acid sodium (hereinafter simply referred to as “concentrated liquid”) and permeated water containing a small amount of organic acid sodium are separated from the osmotic membrane as an interface, and the concentrated liquid is led to line 8.
[0017]
When the permeated water from the first RO device 6 is supplied to the second RO device 7 through the line 9 and is supplied to the reverse osmosis membrane module in a state of being pressurized by the pressure pump, the water is reverse osmosis membrane. , And is discharged out of the system from the line 10 as wastewater substantially free of sodium organic acid.
On the other hand, the impermeable liquid remaining without permeating through the reverse osmosis membrane contains most of the organic acid sodium contained in the permeated water (9). It returns to the neutralization tank 2.
In this membrane separation process, most of, for example, about 90% of the water contained in the wastewater is discharged as clean wastewater.
[0018]
(Concentration process)
The concentrated liquid (8) is then introduced into a concentration step comprising the evaporator 12, the condenser 13, and the decompression device 14. The evaporator 12 is composed of a multi-tube heat exchanger, and the introduced concentrated liquid (8) is produced by the high-temperature combustion waste gas (23) discharged from the gas-liquid separation can 22 of the combustion furnace 20 described later. Heated and separated into water vapor (15) and concentrate (18) enriched with sodium organic acid.
[0019]
Steam (15) is sent from the top of the evaporator 12 to the condenser 13 via the line 15 where it is converted to condensed water and supplied to the gas-liquid separation can 22 of the combustion furnace 20 via the line 16. The surplus is discharged out of the system via the line 31.
The evaporator 12 is kept in a reduced pressure state by the decompression device 14 connected to the line 17 branched from the line 16.
The concentrated liquid (18) of the evaporator 12 is extracted from the bottom of the can and supplied to the combustion unit 21 of the combustion furnace 20 via the line 18.
[0020]
(Combustion process)
The combustion furnace 20 is a submerged combustion furnace, and includes a combustion part 21 and a gas-liquid separation can 22, and the combustion part 21 burns the concentrated liquid (18) together with fuel and oxygen (air) to produce combustion gas. Is blown into the water phase 22 a of the gas-liquid separation can 22.
In the combustion unit 21, the supply temperature of fuel and oxygen is adjusted so that the combustion temperature is kept within the range of 800 ° C to 1100 ° C. If the combustion temperature is less than 800 ° C, the organic acid alkali metal salt may not be decomposed into alkali metal carbonate, and the combustion temperature exceeding 1100 ° C is wasted only by consuming excessive fuel. The organic acid sodium contained in the concentrated liquid (18) is oxidatively decomposed by combustion in the combustion section 21 and converted into sodium carbonate (or sodium bicarbonate) and water.
[0021]
The combustion gas blown into the aqueous phase 22a is separated into a gas phase 22b and an aqueous phase 22a, and sodium carbonate generated by the combustion is collected in the aqueous phase 22a. Then, at least a part of the aqueous phase 22 a containing sodium carbonate is circulated through the line 29 to the neutralization tank 2 as at least a part of the neutralizing agent.
During operation, the water phase 22a in the gas-liquid separation can 22 is maintained at a water level by supplying condensed water (16) in the concentration step. When the water level rises above the limit, excess water phase 22a is discharged out of the system from the line 30.
[0022]
The gas phase 22 b is supplied as a combustion waste gas (23) via the line 23 as a heat source to the evaporator 12, where heat exchange is performed and the combustion waste gas whose temperature has dropped is sent to the separator 25 via the line 24. Water droplets and the like accompanying the combustion waste gas are separated. The aqueous phase discharged from the bottom of the separator 25 is returned to the gas-liquid separation can 22 via the line 26. Since the combustion waste gas from which the water droplets have been removed is mainly composed of water vapor and carbon dioxide, it is discharged from the chimney 28 through the line 27 into the atmosphere.
[0023]
According to the method for treating organic acid-containing wastewater comprising the neutralization step, membrane separation step, concentration step, and combustion step described with reference to FIG. 1, sodium carbonate used as a neutralizing agent for organic acid in wastewater is a system. Since it is circulated in the inside, there is no need to add anything except the initial input amount and the natural weight loss.
Most of the wastewater (eg, about 90%) is drained from the membrane separation process and this process does not require thermal energy.
[0024]
The scale of the combustion furnace 20 used in the combustion process and the amount of fuel consumed depend on the total amount of the concentrate (18) supplied to the combustion furnace 20 and the concentration of organic acid sodium, but the concentrated liquid obtained in the membrane separation process ( 8) is a small proportion compared to the amount of waste water, and is concentrated in advance by the evaporator 12, so that the combustion facility may be relatively small and consume less fuel. Furthermore, since the amount of heat required for the evaporator 12 in the concentration step is covered by the combustion waste gas (23) of the combustion furnace 20, the energy consumption throughout this waste water treatment step is minimized.
Both the waste water (10) and the combustion waste gas (27) discharged by this embodiment are clean, have low environmental pollution, and are of a level that can be reused or released.
[0025]
Hereinafter, each process will be described in detail.
(Neutralization process)
The wastewater introduced into this process contains an organic acid, but the types of organic acids contained in the wastewater are acetic acid, formic acid, propionic acid, butyric acid, valeric acid, (meth) acrylic acid, oxalic acid, It may be any of tartaric acid, citric acid, maleic acid, gluconic acid, various amino acids, and salts thereof, or a mixture thereof. Also, other water-soluble organic substances such as aldehydes, alcohols, esters, ethers, saccharides, proteins, etc. are contained in the waste water as long as they can be separated by a filtration tank or RO device. There is no problem. Furthermore, water-insoluble or hardly soluble contaminants may be included as long as they can be separated by the filtration tank 4.
[0026]
The concentration of the organic acid contained in the wastewater is preferably in the range of 500 ppm to 20000 ppm. If it is less than 500 ppm, the treatment of wastewater by living organisms is generally more economically advantageous. If it exceeds 20000 ppm, the osmotic pressure of the liquid increases, making it difficult to obtain permeated water by reverse osmosis. From this viewpoint, the range of 500 ppm to 20000 ppm is a concentration range particularly suitable for wastewater treatment by membrane separation.
[0027]
If the neutralizing agent added to the neutralization tank 2 is an alkali metal-based basic compound at the start of operation, any of sodium carbonate, sodium hydrogen carbonate, sodium hydroxide, potassium carbonate, potassium hydrogen carbonate, potassium hydroxide, etc. It may be. Whichever is used, these are converted into alkali metal carbonates in the combustion process through the organic acid alkali metal salt in the wastewater treatment process of the present invention. For example, sodium carbonate or potassium carbonate is converted into the neutralization process. It will be circulated.
[0028]
The addition amount of the neutralizing agent (29) depends on the neutralization equivalent of the waste water (1) containing the organic acid. Usually, the amount of addition is preferably adjusted so that the pH of the aqueous phase in the neutralization tank 2 is 7 or more, preferably in the range of 7-8.
[0029]
(Membrane separation process)
The first RO device 6 and the second RO device 7 used in the membrane separation step are usually composed of a reverse osmosis membrane module and a pressure device. This reverse osmosis membrane module was molded in single layer or multilayer from polyamide, aromatic polyamide, polyimide, polyolefin, polysulfone, polyacrylonitrile, polyether, polyethersulfone, polyetherketone, polybenzimidazolone, cellulose acetate, etc. It is formed of a semipermeable membrane, and generally, about 20 to 100 modules are connected in series or in parallel as an RO device.
As the reverse osmosis membrane module, any of a hollow fiber type, a spiral type, and a tube type can be used.
[0030]
In the RO device, it is necessary to pressurize the stock solution supply side. For this purpose, a pressure pump is used. The pump discharge pressure is preferably in the range of 150 mAq to 1000 mAq for both the first RO device 6 and the second RO device 7, preferably about 300 mAq to 700 mAq. This pressurization causes reverse osmosis action, and organic acid ions and alkali metal ions do not permeate the membrane, and water selectively permeates through the membrane.
[0031]
In the embodiment shown in FIG. 1, the RO device is used in two stages in series. However, the connection type and the number of units used are not limited to the above. In short, the amount of membrane leakage of the organic acid alkali metal salt is the design value. It should be determined together with the module material, membrane area, stock solution supply pressure, etc. so as to improve the separation efficiency while maintaining the following. If necessary, an RO device or other separation device for removing BOD substances other than organic acids can be added.
[0032]
(Concentration process)
The concentration step is a step for further increasing the concentration of the organic acid alkali metal salt in the concentrated liquid (8) discharged from the membrane separation step, and can be omitted if the concentration of the concentrated liquid (8) is sufficiently high. It is. However, since the combustion waste gas (23) having a high temperature is generated in the next combustion process, this enrichment process is added and the combustion waste gas is used as the heat source unless the thermal energy is effectively used for other purposes. Using is very advantageous from the viewpoint of energy saving.
[0033]
The apparatus used for the concentration step may be any one conventionally used for evaporation concentration. From the viewpoint of effective use of thermal energy, it is preferable to use a heat exchange system of the evaporator 12 for multiple effects and to use a decompression device 14 as a decompression system. As the type of the heat exchanger 12 and the condenser 13, a multi-tube type is usually used, but other types such as a fin type and a rotary plate type may be used. The decompression device 14 may be any one of a rotary vacuum pump, a Nash suction pump, an ejector, and the like.
[0034]
(Combustion process)
The combustion furnace 20 used in the combustion process includes a combustion unit 21 and a gas-liquid separation can 22. The combustion unit 21 may be of any type as long as the aqueous solution can be combusted, but in general, a jet combustion method is suitable. The organic acid alkali metal salt dissolved in the concentrated liquid (18) undergoes oxidative decomposition due to combustion in the combustion section 21, and is converted into an alkali metal carbonate. This alkali metal carbonate is introduced into the water phase 22a of the gas-liquid separation can 22 together with carbon dioxide gas, water, and other decomposition products generated by the combustion of the fuel. Here, most of the water-soluble components are trapped in the aqueous phase 22a. The water-insoluble component forms a high-temperature gas phase 22 b and is sent from the line 23 to the evaporator 12.
Instead of the gas-liquid separation can 22, a spray tower, a packed tower, a plate tower, or the like can be used.
[0035]
【Example】
By the wastewater treatment process shown in FIG. 1, wastewater containing acetic acid discharged during acetic acid synthesis was treated.
(Neutralization process)
As raw material wastewater, an aqueous solution containing 3000 ppm of acetic acid was supplied via line 1 to a neutralization tank 2 having a capacity of 20 m ³ . The supply flow rate was 50 m ³ / hour. The neutralizing tank 2 was also added with the return liquid (11) from the second RO device 7 at a flow rate of 11 m ³ / hour.
An aqueous sodium carbonate solution (2% by weight of sodium carbonate) was added from the line 29 so that the pH in the neutralization tank 2 was maintained at 7.5.
Neutralization wastewater was extracted from the bottom of the neutralization tank 2 and supplied to the first RO device 6 through the filtration tank 4.
[0036]
(Membrane separation process)
As the first RO device 6, a high rejection rate RO module was used.
The supply amount of the filtrate (5) to the first RO device 6 was 68 m ³ / hour, and the water pressure was 700 mAq. At this time, the permeated water (9) flowed out at a flow rate of 55 m ³ / hour, and this permeated water (9) contained 150 ppm of sodium acetate. Concentrated liquid (8) flowed out at a flow rate of 12 m ³ / hour, and this concentrated liquid (8) contained about 1.7% by weight of sodium acetate.
[0037]
The second RO device 7 has the same configuration as that of the first RO device 6 except that the number of modules is 70% of that of the first RO device 6.
When the permeated water (9) was supplied to the second RO device 7 at a water pressure of 700 mAq and a flow rate of 56 m ³ / hour, the waste water (10) was discharged at a flow rate of 45 m ³ / hour. This waste water (10) contained 10 ppm sodium acetate.
The return liquid (11) flowed out at a flow rate of 11 m ³ / hour and contained 900 ppm sodium acetate. This return liquid (11) was returned to the neutralization tank 2.
[0038]
(Concentration process)
The concentrated liquid (8) was concentrated in the evaporator 12 to obtain 10.5 m ³ / hour condensed water (16) and 1.5 m ³ / hour concentrated liquid (18). This concentrate (18) contained about 14% by weight sodium acetate.
[0039]
(Combustion process)
The concentrated liquid (18) was supplied to the combustion furnace 20 at 1.5 m ³ / hour to burn and decompose sodium acetate. At this time, the concentration of sodium carbonate contained in the aqueous phase 22a of the gas-liquid separation can 22 was 2% by weight. This aqueous phase 22a was extracted from the bottom of the gas-liquid separation can 22 at a flow rate of 5 to 7 m ³ / hour and supplied to the neutralization tank 2 as a neutralizing agent from the line 29 to complete the cycle of the waste water treatment process.
[0040]
According to the operation shown in the above-mentioned Examples, waste water of 50 m ³ / hour containing 3000 ppm of acetic acid was discharged 45 m ³ / hour of waste water containing 10 ppm of sodium acetate. Since this wastewater had a very small acetic acid content, it could be reused or discharged. The neutralizing agent was also circulated and the additional amount was negligible during the long-term operation cycle.
The fuel consumption in the combustion furnace 20 is very small because only 1.5 m ³ / hour of the concentrated liquid (18) is combusted for the treatment of the waste water of 45 m ³ / hour. There was no need to supply heat energy from the outside, and the energy saving effect was great.
[0041]
【The invention's effect】
The organic acid-containing wastewater treatment method according to the present invention is obtained by adding an alkali metal carbonate to the organic acid-containing wastewater as a neutralizing agent to convert the organic acid to an organic acid alkali metal salt, and the neutralization step. The membrane separation step of supplying the organic acid alkali metal salt aqueous solution to the reverse osmosis membrane device and separating it into a concentrated liquid and drainage rich in organic acid alkali metal salt, and burning the concentrated liquid to organic acid alkali metal salt Containing an organic acid since it is circulated to the neutralization step using at least a part of the alkali metal carbonate obtained in this combustion step as a neutralizing agent. Without discharging organic matter and ash from wastewater, it is possible to remove organic acid energy-efficiently and to discharge clean wastewater.
[0042]
If a concentrating step for supplying the concentrated liquid to the evaporator and further concentrating it is inserted between the membrane separation step and the combustion step, the amount of heat required for this concentration can be covered by using the amount of heat released in the combustion step. The scale of the combustion apparatus can be reduced, and the energy saving effect of the entire wastewater treatment process can be further improved.
[Brief description of the drawings]
FIG. 1 is a process chart showing an embodiment of a wastewater treatment method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Organic acid containing wastewater introduction line 2 ... Neutralization tank 6, 7 ... Reverse osmosis membrane apparatus 8 ... Organic acid alkali metal salt concentrate 10 ... Drain line 12 ... Evaporator 20 ... Combustion furnace 29 ... Alkali metal carbonate Circulation line

Claims (5)

In removing the organic acid from the organic acid-containing wastewater, a neutralization step of converting the organic acid to an organic acid alkali metal salt by adding an alkali metal carbonate to the organic acid-containing wastewater as a neutralizing agent, A membrane separation step of supplying the obtained organic acid alkali metal salt aqueous solution to a reverse osmosis membrane device and separating it into a concentrated liquid and drainage rich in organic acid alkali metal salt, and burning this concentrated liquid to produce an organic acid alkali metal An organic acid-containing wastewater characterized by comprising a combustion step of converting a salt into an alkali metal carbonate, and circulating to the neutralization step using at least a part of the alkali metal carbonate obtained in the combustion step as a neutralizing agent Processing method. The organic acid-containing wastewater treatment method according to claim 1, wherein the organic acid-containing wastewater contains an organic acid within a range of 500 ppm to 20000 ppm. Between the membrane separation process and the combustion process, the concentrated liquid obtained by the membrane separation process is supplied to the evaporator, a part of the water is evaporated and removed, and the concentrated liquid is further concentrated before being supplied to the combustion process. The organic acid containing wastewater processing method of Claim 1 or Claim 2 including the concentration process to perform. The method for treating organic acid-containing wastewater according to any one of claims 1 to 3, wherein the combustion temperature in the combustion step is in the range of 800C to 1100C. The organic acid-containing wastewater treatment method according to claim 1, wherein the organic acid-containing wastewater is a wastewater of acetic acid synthesis.

Images