JP2002239538A - Treatment method for anaerobically digested liquid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a treatment method for an anaerobically digested liquid capable of removing ammonia inexpensively without adding a large amount of heat energy in the anaerobically digested liquid from the outside nor adding alkali. SOLUTION: A liquid to be treated containing an organic substance is anaerobically digested to be separated into digestion gas and the digested liquid and ammonia contained in the digested liquid is subsequently removed. In this case, the digested liquid is introduced into a diffusion column and gas containing ammonia is volatilized under reduced pressure and subsequently introduced into a catalytic reactor to oxidize ammonia and diffused into the atmosphere as nitrogen gas and the digested liquid in the diffusion column is heated by hot wastewater used in the cooling of the high temperature gas of a dynamo utilizing the digestion gas. The volatilized gas containing ammonia is heated by the high temperature gas of the dynamo utilizing the digestion gas and subsequently diluted with air to be supplied to the catalytic reactor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating an anaerobic digested liquor, and more particularly, to removing ammonia from a digested liquor containing a high concentration of ammonia after anaerobic digestion of organic sludge. About technology.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for improving the living environment, and proper treatment of municipal waste, industrial waste, organic waste liquid, organic sludge, and the like has become an important issue. Of these, the treatment of organic waste liquid and sludge is often performed using an anaerobic digestion method (also called a methane fermentation method). The anaerobic digestion method refers to, for example, as shown in FIG.
40 ° C) and adding methanogens,
This is a technique of decomposing organic substances in the liquid to be treated into methane gas and carbon dioxide gas (these gases are called digestion gases) to make them harmless. Treated digested fluid (also called digestive fluid)
Is continuously recovered through various processes as a fertilizer or the like, or subjected to biological treatment for detoxification and reused as industrial water. On the other hand, the digestion gas is usually used as a heat source by providing a power generation facility. In this power generation, a large amount of cooling water is used to cool the power generation equipment,
Inevitably, 80-90 ° C. hot water is obtained. This hot water is
Although partly used for heating the liquid to be treated, it is only a small part and most of the hot water remains unused. Regardless of the case where a business that supplies heat to the area around the digestion treatment plant and the hot water can be used effectively, in reality there is no way to use it effectively, and the temperature is forcibly reduced to about 70 ° C or less by a cooling tower or the like. After cooling, the water is usually circulated and used as cooling water for the power generation equipment.

[0003] By the way, the digested liquid is usually 100
0 to 2000 mg / liter (0.1 to 0.2% by mass)
Since it contains a relatively high concentration of ammonia, ammonia is radiated to the surrounding air as it is, which is not preferable in terms of environmental protection. Therefore, it is desired to remove ammonia even when performing the biological treatment.

Generally, in the treatment of exhaust gas from a coke oven,
The liquid with a high ammonia concentration is called stripping, and the contained ammonia is volatilized using a “dissipation tower” described later, and then the volatilized ammonia is gasified in a “catalytic reactor” using an ammonia oxidation catalyst or the like. It is oxidized and then removed. In this case, JP-A-60-1
As disclosed in Japanese Patent No. 14389, steam is blown into an ammonia-containing liquid, and “steam stripping” is performed by volatilizing the steam with entraining ammonia. There is "air stripping" in which ammonia is stripped by the air.

[0005] However, the former “steam stripping” requires not only large-scale facilities for producing steam and pure water, but also the necessity of supplying a large amount of thermal energy from outside the process. There is an economic problem of not becoming. On the other hand, the latter "air stripping"
However, there is a problem that an alkali needs to be added in order to make ammonia easy to volatilize by free ammonia conversion, which increases the processing cost. Also, if inexpensive lime is used as an alkali source, scale will adhere to equipment and piping,
There is also a problem that the operation cannot be performed smoothly. Therefore, it is difficult to apply such a “stripping” technique to anaerobic digested liquid such as organic sludge as it is.

[0006]

SUMMARY OF THE INVENTION In view of the foregoing, the present invention provides a method for converting ammonia into a conventional anaerobic digested solution without adding a large amount of thermal energy from outside the process and without adding an alkali agent. It is an object of the present invention to provide a method for treating an anaerobic digested solution that can be removed at a lower cost.

[0007]

Means for Solving the Problems The inventor has conducted intensive studies to achieve the above object and has embodied the results in the present invention.

That is, according to the present invention, the anaerobic digestion of a liquid to be treated containing an organic substance is performed to separate the digested gas and the digested liquid from each other, and then the ammonia contained in the digested liquid is removed. The spent liquid is introduced into the stripping tower, and the gas containing ammonia is volatilized under reduced pressure. Then, the gas is introduced into the catalytic reactor to oxidize the ammonia, and diffused into the atmosphere as nitrogen gas. The digested liquid in the
A method for treating an anaerobic digested liquid characterized by heating with hot wastewater used for cooling high-temperature exhaust gas of a generator using the digestion gas.

Further, the present invention is characterized in that the gas containing ammonia volatilized is heated with a high-temperature exhaust gas of a generator using the digestion gas, and then diluted with air and supplied to the catalytic reactor. This is a method for treating an anaerobic digested solution. In this case, it is preferable to use, instead of the air, odor gas or environmental air generated in all the steps of anaerobic digestion by heating with the high-temperature effluent gas from the catalytic reactor.

[0010] According to the present invention, not only a separate boiler and a pure water apparatus are not provided, and steam is not generated, but also an excess hot gas generated by anaerobic digestion is efficiently used without using an alkali agent. This makes it possible to strip ammonia from the anaerobic digested solution at low cost. Also, by heating the gas introduced into the catalytic reactor with the excess high-temperature gas, ammonia can be oxidized to nitrogen gas by the oxidation catalyst without condensing water vapor, thereby rendering the gas harmless. Furthermore, since the gas introduced into the catalytic reactor was diluted with the odorous gas or the environmental air sucked from the odor generating point in all the steps of anaerobic digestion,
Deodorizing equipment becomes unnecessary in all processes.

[0011]

Embodiments of the present invention will be described below.

In order to detoxify and remove ammonia from the anaerobic digested solution, the inventor volatilizes ammonia in a stripping tower, oxidizes it in a catalytic reactor, and releases it to the atmosphere as nitrogen gas. We decided to use the "stripping" method. However, simply diverting the conventional “steam stripping” method or “air stripping” method is not economically favorable, and therefore needs to be improved. Therefore,
The present inventors have completed the present invention by devising the following measures for volatilizing the gas containing ammonia in the stripping tower and heating the digested liquid and the gas containing the volatilized ammonia.

First, regarding the volatilization of ammonia, FIG.
The inside of the stripping tower 1 shown in FIG.
The pressure was reduced to about 35,000 Pa or more. By doing so, the digested liquid (also referred to as undiluted liquid) 2 introduced into the stripping tower 1 can be boiled at 60 to 70 ° C., and ammonia can be easily generated only by generated steam without separately introducing steam. It was thought that it could be volatilized. The reason why the reduced pressure state is set to 20000 to 35000 Pa is that if it exceeds 35000 Pa, the volatilization of ammonia is insufficient, and if it is less than 20,000 Pa, the volatilization degree is saturated, and it is not necessary to further increase the reduced pressure state. .

Next, the digested liquid 2 is usually kept at a temperature of about 20 to 30 ° C. in a digestive liquid tank (see FIG. 2). In order to introduce this into the stripping tower 1 and volatilize the ammonia component, it is necessary to raise the temperature to 60 to 70 ° C. as described above. For this purpose, it is conceivable to separately introduce thermal energy from outside the process. However,
In the case of performing anaerobic digestion (methane fermentation) of a slurry containing a high concentration of organic substances, as shown in FIG. 2, a power generation facility using the generated methane gas is provided, so that the cooling of the power generation facility is inevitable. Hot water of less than 100 ° C. (80 to 90 ° C.) is obtained as waste water used in the above. Although only a small part of this warm water is used to heat the liquid to be treated during anaerobic digestion, most of the warm water remains unused. Therefore, the inventor has determined that the hot water is effectively used, and performs heat exchange as described below, which is the present invention.

That is, the atmosphere in the stripping tower 1 is reduced to a level at which water boils below 100 ° C. (for example, 20,000 Pa.).
Then, the pressure is reduced at about 60 ° C.), and ammonia is diffused by water vapor evaporating from water in the anaerobic digested liquid 2. At that time, the digested liquid 2 flowing from the upper part of the stripping tower 1 passes through the filler layer 3 provided inside the tower 1, and then forms a liquid layer 4 at the lower part of the tower. The liquid is extracted from the liquid layer 4 to the outside of the tower by the circulation pump 5, and a part of the liquid is
By heating with a heat exchanger A using hot water of less than 100 ° C. (indicated by reference numeral 6 in FIG. 1) and returning to the liquid layer 4 again, the temperature of the digested liquid 2 is maintained at the boiling point under reduced pressure. did. In the present invention, as the hot water 7, hot effluent for cooling a generator using methane gas or biogas (digestion gas of organic sludge) generated during anaerobic digestion is used. In addition, the heat exchanger A
The portion not guided to 6 is discharged as a processed liquid 8. If necessary, since the treated liquid 8 has heat of about 60 ° C., the heat exchanger B (FIG.
In this case, the anaerobic digested liquid 2 may be used as a heat medium for preheating the anaerobic digested liquid 2.

The gas 10 containing ammonia volatilized in the stripping tower 1 (also referred to as a volatilized gas) is 1/1 of the stock solution 2.
It consists of about 10 water vapor and ammonia, but if this is returned to normal pressure, the water vapor condenses to water. Since ammonia has water solubility, the ammonia volatilized with great care may be dissolved in water again. Although it depends on the concentration of ammonia in the undiluted solution, according to the inventor's calculation, almost all of the ammonia is redissolved when water vapor condenses into water. It would be good if we could find a use to reuse this condensed ammonia water, but in reality it is difficult. Therefore, before redissolving, it is necessary to oxidize ammonia to nitrogen gas to make it harmless.

Therefore, before returning to normal pressure (specifically, on the upstream side of a vacuum pump installed to put the stripping tower in a reduced pressure state), water does not condense even if the volatile gas 10 is returned to normal pressure. We considered heating to temperature, returning it to normal pressure and then oxidizing ammonia. As an oxidation method, there is direct combustion. However, in this case, a high concentration of nitrogen oxides is generated, which causes a problem of air pollution. Therefore, the treatment is performed by catalytic oxidation at 300 to 400 ° C.

That is, the heat exchanger C shown by reference numeral 11 in FIG.
After heating the volatile gas 10 mixed with ammonia to a temperature of 100 ° C. or more (to a level at which water vapor does not condense even when the pressure is returned to normal pressure) under reduced pressure extracted from the upper part of the stripping tower 1 by using Dilute the ammonia concentration to an acceptable level for the catalyst. The allowable level varies depending on the type of the catalyst, but for example, the ammonia concentration is set to 1% or less. Thereafter, the temperature is further increased (usually 3
Around 00 ° C.), the ammonia is oxidized into nitrogen gas through the catalytic reactor 13 and released to the atmosphere.

In the present invention, it is preferable to use the exhaust gas 14 of the generator using the digestion gas for the above heating to 100 ° C. or more. In addition, heating to about 300 ° C. performed before introduction into the catalyst reactor 13
As shown in FIG. 1, part of the high-temperature effluent gas 15 from FIG. 3 is led to a separately provided hot blast stove (burner) 16, where it is heated to, for example, about 600 ° C. It is preferable to mix with the introduced gas.
This is because the amount of fuel used in the hot blast stove 16 can be very small.

Further, according to the present invention, the air 12 for diluting the gas introduced into the catalytic reactor 13 is supplied with an odorous gas or environmental air sucked from an odor generating point in all the processes of anaerobic digestion. It is preferable to use the effluent gas 15 by heating it with a heat exchanger D18 using a heat medium. This is because a good working environment can be maintained without providing a deodorizing device in all the processes of anaerobic digestion.

[0021]

EXAMPLE After digestion of livestock manure (methane fermentation) using the anaerobic digestion equipment shown in FIG.
Was subjected to solid-liquid separation, and the method for treating a digested solution according to the present invention was applied to the remaining solution (referred to as desorbed solution). The applied processing steps are as shown in FIG. 1, and the amount of the desorbed liquid is 100
Tons / day. Table 1 shows the main operating conditions.

On the other hand, in order to confirm the effect of the present invention, the same desorbed liquid was also subjected to a conventional "steam stripping" or "air stripping" treatment.

The results of these operations are shown in Table 2 based on the amounts of steam, chemicals and the like introduced from outside the system for the operation. Table 2 shows that the present invention is very excellent.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

As described above, according to the present invention, not only a separate boiler and a pure water apparatus are provided, but also steam is not generated, and excess hot water generated by anaerobic digestion can be obtained without using an alkali agent. Efficient use allows stripping of ammonia from anaerobic digested liquids at low cost. As a result, the subsequent processing (biological processing) of the digested liquid can be simplified. As a side effect,
By using the air having a bad odor as the dilution air for the catalytic reactor, the deodorizing device of the entire anaerobic digestion equipment can be omitted.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing a method for treating an anaerobic digested solution according to the present invention.

FIG. 2 is a diagram showing an example of the entire anaerobic digestion process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Stripping tower 2 Digested liquid (digested liquid) 3 Filler layer 4 Liquid layer 5 Circulation pump 6 Heat exchanger A 7 Hot water 8 Treated liquid 9 Heat exchanger B 10 Gas containing volatilized ammonia (volatilized gas) 11 Heat Exchanger C 12 Air 13 Catalytic reactor 14 Exhaust gas from generator 15 Outflow gas from catalytic reactor 16 Hot blast stove (burner) 17 Vacuum pump 18 Heat exchanger D

Claims (3)

[Claims] 1. A liquid to be treated containing an organic substance is anaerobically digested and separated into a digested gas and a digested liquid, and then the digested liquid is stripped off when removing the ammonia contained in the digested liquid. And evaporates the gas containing ammonia under reduced pressure, and then introduces the gas into the catalytic reactor to oxidize the ammonia and emit it into the atmosphere as nitrogen gas, and the digested liquid in the stripping tower. Is heated with hot waste water used for cooling high-temperature exhaust gas of a generator using the digestion gas. 2. A gas containing the volatilized ammonia,
2. The method for treating an anaerobic digested liquid according to claim 1, wherein the digested gas is heated with a high-temperature exhaust gas from a generator, diluted with air, and supplied to the catalytic reactor. 3. The method according to claim 2, wherein, instead of the air, odor gas or environmental air generated in all the steps of anaerobic digestion is heated and used with a high-temperature effluent gas from the catalytic reactor. How to treat anaerobic digested liquid.