JPS5820320B2 - Anaerobic digestion method for organic waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating organic waste, and in particular, a method for treating organic waste in which organic waste is treated by anaerobic digestion to efficiently reduce the amount of organic waste and at the same time recover methane gas. Regarding.

Conventionally, organic wastes such as sewage sludge and frozen ground have been treated using anaerobic digestion in a single tank.

This method has the advantage that methane gas, a by-product, can be used as power for the digestion equipment at the same time as the volume is reduced, and the digested sludge can be made into a good organic fertilizer.

However, the conventional tank-type method requires a long digestion period of 30 to 50 days, and has the disadvantage that the methane purity in the recovered gas is low.

However, recently, the above advantages have begun to be reevaluated, and improvements in anaerobic digestion have made it possible to improve the energy recovery rate and to shorten the number of days required for digestion through the development of two-phase anaerobic digestion.
The effectiveness of anaerobic digestion has been confirmed, and its practical application is gaining momentum.

In particular, the two-phase anaerobic digestion method separates facultative anaerobic bacteria and obligate anaerobic bacteria, which have different physiological and growth characteristics, into acid production and gasification processes that preferentially retain each type of bacteria. The aim was to maximize the substrate metabolic ability of the molecule and increase the overall rate of digestion.

According to the two-phase anaerobic digestion method, the substrate metabolic ability can be maximized by separating each bacterial cell, and the digestion speed can be particularly increased in the gasification process, which is considered to be the rate-limiting factor in anaerobic digestion. Therefore, the number of days required for digestion can be shortened.

Furthermore, according to the two-phase anaerobic digestion method, the methane content in the gas obtained in the gasification process is reduced to 50 to 65% in the tank type.
However, it can be increased to 60-75%.

This is considered to be because carbon dioxide gas is generated in the acid generation step, and therefore the amount of carbon dioxide gas generated in the gasification step is reduced accordingly.

Conventionally, methane gas generated by anaerobic digestion methods, including two-phase methods, has been consumed as fuel for heat retention and stirring power in digestion equipment, and the surplus has been used as fuel for power generation.

In the future, it is expected that the amount of recovered methane will increase as the amount of organic waste processed such as sewage sludge, frozen ground, household manure, and municipal waste increases. This value is 75%, which is lower than that of synthetic gas derived from high-calorie natural gas, petroleum, etc., and the uses of the recovered gas are extremely limited.

Therefore, it is desirable to further improve methane purity.

On the other hand, the amount of gas generated in the acid generation process is smaller than that in the gasification process, and the hydrogen and methane contents in the gas are low, so it is considered difficult to recover energy.

Furthermore, since the generated gas contains malodorous components such as hydrogen sulfide, it is not preferable to discharge the generated gas directly to the outside of the system.

SUMMARY OF THE INVENTION An object of the present invention is to provide an anaerobic digestion method for organic waste that overcomes the drawbacks of the prior art and can efficiently recover gas with high methane purity in a two-phase treatment method.

According to the present invention, this purpose is to remove carbon dioxide gas from a gas mainly consisting of hydrogen, methane and carbon dioxide generated in the acid generation process, and to use the generated gas from which carbon dioxide has been removed as a gasification slurry in the gasification process. This is achieved by blowing into the mixture.

Removal of carbon dioxide gas can be carried out by a dry method or a wet method, and can be carried out using, for example, a basic substance capable of absorbing carbon dioxide gas, such as slaked lime, calcium oxide, caustic soda, or monoethanolamine.

When the generated gas from which carbon dioxide gas has been removed is blown into the slurry of the gasification process, the hydrogen gas reacts with the carbon dioxide gas generated in the gasification tank by the action of gasification bacteria, and is converted into methane.

Therefore, the gas generated in the gasifier will be richer in methane than with conventional two-phase digestion methods.

As mentioned above, the method of the present invention removes carbon dioxide contained in the gas generated in the acid generation process in the two-phase anaerobic digestion method of organic waste and sprays it onto the gasification slurry in the gasification process. By mixing, extremely beneficial effects can be obtained, such as increasing the amount of gas generated in the gasification process, improving methane purity, and efficiently recovering energy.

Next, the present invention will be explained in detail based on the drawings.

The drawing is a flow sheet showing an embodiment of the method of the present invention, in which organic waste as the raw material to be treated, such as sewage sludge, frozen ground, alcohol distillation waste liquid, and urban dirt, is input into an acid generation tank 1 to generate acid. contact with bacteria.

It may be crushed if necessary before being added.

It is then kept under anaerobic conditions at a constant temperature for several days with stirring.

In the acid-producing tank 1, the polymer substances are reduced in molecular weight by the action of acid-producing bacteria and decomposed into organic acids and alcohols.

The temperature in the acid production tank 1 is preferably 30 to 60°C, and the pH is preferably 4 to 7, and is appropriately selected depending on the type of waste and the acid production bacteria used.

The gas generated in the acid generation tank 1 contains 40 to 90% carbon dioxide gas, and also contains methane, hydrogen, nitrogen, hydrogen sulfide, and the like.

The acid generation treated slurry after acid generation is put into the gasification tank 3, and the organic acid is converted into methane and carbon dioxide gas by the action of gasification bacteria.

In order to perform this gasification efficiently, the temperature must be heated to 30 to 60°C with sufficient stirring in an anaerobic atmosphere, and the pH must be 60°C.
, 5 to 8.0.

In the present invention, the gas generated in the acid generation tank 1 is sent to the carbon dioxide absorption tower 2 and brought into contact with an absorbent to absorb and remove carbon dioxide, and then this gas is converted into a gasified slurry in the gasified species 3. Spray and mix.

In addition to methane gas and carbon dioxide gas, small amounts of nitrogen and hydrogen sulfide are also generated in the gasification tank.

Further, hydrogen in the gas blown onto the gasification slurry is converted to methane together with carbon dioxide gas by the action of gasification bacteria, and the mixed methane and methane generated in the gasification tank 3 are combined to produce highly pure methane gas.

The gas generated from the gasification tank 3 passes through a gas meter 4 and is stored in a gas holder 5.

In the carbon dioxide absorption tower 2, a powder, solution or slurry of slaked lime, calcium oxide or caustic soda, or a solution or slurry of monoethanolamine is used.

The temperature within the iron tower may range from 5 to 100°C.

These absorbents can also absorb hydrogen sulfide.

The used absorbent can be regenerated as needed after use.

Next, the present invention will be explained based on examples.

Example 1 Table 1 shows the results of treating sewage concentrated sludge according to the method of the present invention using a 1M-scale experimental apparatus.

As is clear from the results in Table 1, highly pure methane can be obtained, and the method of the present invention is far more effective than the conventional two-phase anaerobic digestion method.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a flow sheet for treating organic waste, showing an embodiment of the method of the present invention. Explanation of symbols, 1... Acid generation tank, 2...
Carbon dioxide absorption tower, 3...Gasification tank, 5...
...Gas holder.

Claims (1)

[Scope of Claims] 1. In a two-phase anaerobic digestion method of organic waste consisting of an acid production process using acid-producing bacteria and a gasification process using gasification bacteria, hydrogen, methane and carbonic acid generated in the acid production process are An anaerobic digestion method for organic waste, which is characterized by removing carbon dioxide gas from a gas mainly consisting of gas, and blowing and mixing the generated gas from which carbon dioxide gas has been removed into a gasification slurry in a gasification step. 2. The anaerobic digestion method according to claim 1, wherein carbon dioxide is removed by a wet or dry method using a carbon dioxide absorbent such as slaked lime, calcium oxide, caustic soda, or monoethanolamine.