JP2000102779A - Method for accelerating generation of methane gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the concn. of soluble nutrient salts which can be biologically used in an anaerobic fermentation tank and to increase the production amt. of methane gas by reducing the ion concn. of sulfides in digested sludge when org. waste is introduced into the fermentation tank and decomposed by anaerobic digestion to produce methane gas. SOLUTION: Garbage which is an easily decomposable org. substance as the object 1 of digestion is fed to a fermentation tank 2 and subjected to anaerobic digestion at a specified temp. for specified days, and the digested sludge 3 is drained. In this process, the digested gas 4 produced in the fermentation tank 2 is introduced into a desulfurization tower 5 and desulfurized. The desulfurized gas 6 is separated and introduced into a gas holder 7 while a part of the gas is returned to the fermentation tank 2 and blown into the digested sludge 3. By this method, for example, the amt. of propionic acid accumulated in the digested sludge 3 in the fermentation tank 2 is about 2000 mg/L (pH 7.9) amt. and the decomposition rate of the substrate reaches 85%. Namely, the concn. of hydrogen sulfide in the digested gas 4 is decreased and the emission of hydrogen sulfide in the digested sludge 3 is accelerated, while the ion concn. of sulfides is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for accelerating methane gas generation when organic waste is introduced into a fermenter and decomposed by anaerobic digestion to generate methane gas.

[0002]

2. Description of the Related Art Conventionally, organic waste has been recycled. For example, Japanese Patent Application Laid-Open No. Hei 9-201699 discloses that human waste, septic tank sludge, sewage sludge, agricultural sludge, livestock manure, garbage and foods are disclosed. A method is disclosed in which organic wastes having different properties and concentrations, such as wastes, are treated with the same system to recover useful substances such as methane gas.

[0003]

It is well known that when organic waste is treated in an anaerobic fermenter, nutrients at a certain concentration or higher are required for the decomposition of the organic matter.
However, since hydrogen sulfide contained in digestive gas generated in anaerobic digestion has the property of forming sulfide precipitates by binding to metals, soluble metals essential for biological growth precipitate as sulfides, As a result, this is a factor that causes a shortage of nutrients. In particular, in the high-temperature methane fermentation system, since the solubility of sulfide is lower than that in the medium-temperature methane fermentation system, it is necessary to pay particular attention to the concentration of soluble nutrients.

[0004] The present invention solves the above-mentioned problems, and provides a methane gas generation promoting method capable of increasing the concentration of bioavailable soluble nutrients in an anaerobic fermenter and increasing the amount of methane gas generated. The purpose is to do so.

[0005]

In order to solve the above-mentioned problems, a method for promoting methane gas generation according to the present invention is directed to a method for introducing organic waste into a fermenter and decomposing it by anaerobic digestion to generate methane gas. This is to reduce the sulfide ion concentration in the digested sludge inside.

[0006] The sulfide ion concentration can be reduced by removing hydrogen sulfide in digested gas in the tank and releasing hydrogen sulfide from digested sludge. Further, the reduction of the sulfide ion concentration can be performed by mixing a desulfurizing agent with the organic waste in a stage preceding the fermenter or in the fermenter.

[0007] The sulfide ion concentration can be reduced by lowering the pH of digested sludge in the tank. The amount of soluble metal in the fermenter is represented by the following equation.

[0008]

(Equation 1)

That is, the soluble metal concentration {Me ²⁺ } is inversely proportional to the sulfide ion concentration {S ^2- }, and therefore inversely proportional to the hydrogen sulfide concentration {H ₂ S}, and the hydrogen ion concentration {H ⁺ } Is proportional to the square of

For this reason, the amount of soluble metal can be controlled by adjusting the sulfide ion concentration {S ^2- }, the hydrogen sulfide concentration {H ₂ S}, and the hydrogen ion concentration {H ⁺ }.

In other words, the sulfide ion concentration {S ^2- } and the hydrogen sulfide concentration {H
By adjusting ₂ S｝ and hydrogen ion concentration {H ⁺ ｝
Prevention of precipitation of metals essential for anaerobic digestion as sulfides can prevent shortage of soluble nutrients, which tends to be insufficient in fermenters. As a result, anaerobic digestion activity can be increased, and the amount of methane gas generated can be increased.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. (Example 1) As shown in Fig. 1, kitchen garbage, which is an easily decomposable organic substance, is put into a fermentation tank 2 as a digestion target 1, and is heated to about 55 ° C and retained in a hydrological state. Anaerobic digestion with a time (HRT) of 16 days, digested sludge 3
Was pulled out.

At this time, the digested gas 4 generated in the fermenter 2
Is introduced into a desulfurization tower 5 for desulfurization, whereby the desulfurization gas 6 from which hydrogen sulfide has been removed is diverted and guided to a gas holder 7, and a part of the gas is returned to the fermenter 2 and blown into the digested sludge 3. It is.

As a result, the amount of propionic acid accumulated in the digested sludge 3 in the fermenter 2 is about 2000 mg / L (p
H7.9), and the substrate decomposition rate was 85%. Desulfurization gas 6
In the conventional method similar to the above, except that the blowing of methane gas is not performed, the anaerobic digestion activity is increased and the generation of methane gas is increased as compared with the case where propionic acid is accumulated at about 5000 mg / L and the substrate decomposition rate is also about 60%. It can be seen that the amount (substrate decomposition rate) increased.

This is because the concentration of hydrogen sulfide in the digested gas 4 in the tank is reduced by the desulfurization of the digested gas 4 and the blowing of the desulfurized gas 6, and the release of hydrogen sulfide in the digested sludge 3 is promoted. This is because the concentration was reduced, and the precipitation of metal essential for anaerobic digestion as sulfide was suppressed, and the metal was maintained in a dissolved state.

In this method, since only a return line for the desulfurization gas 6 is added to the conventional digestion gas desulfurization line, new capital investment is minimized, and no special control is required.
Instead of the above-described method, the digestion gas 4 is provided with a desulfurization means in which the desulfurization compound is put in the fermenter 2 or is desulfurized by using a desulfurization compound existing in another treatment step of the same treatment system. Alternatively, the generation of methane gas can be promoted by blowing a separate stirring gas containing no hydrogen sulfide. (Example 2) As shown in FIG. 2, kitchen garbage is digested anaerobically as digestion target 1 in the same manner as in Example 1,
The pH of digested sludge 3 was reduced from 7.9 to 7.2 by adding hydrochloric acid 8 to fermenter 2.

As a result, the substrate decomposition rate was improved to 90%. This is because the release of hydrogen sulfide from the digested sludge 3 was promoted by the increase in the hydrogen ion concentration by the hydrochloric acid 8. In this case, the consumption of the desulfurizing agent in the desulfurization tower 5 increased about twice by the increase in the amount of hydrogen sulfide released, but this increase in running cost was sufficiently compensated by the increase in the amount of methane gas recovered. (Embodiment 3) As shown in FIG. 3, human waste and septic tank sludge carried into a human waste processing plant are introduced into a concentration apparatus 10 such as a gravity concentration tank or a dehydrator as a treatment object 9, and
As a coagulant, 8% of iron chloride per solid was added to coagulate. Then, the aggregate is used as the digestion target 12.
The fermenter 2 was charged into the same fermenter 2 as in Example 1, heated to about 55 ° C., and agitated with the stirring gas 13 to perform anaerobic digestion with a hydrological residence time (HRT) of 16 days.

As a result, the digested gas 4 generated in the fermenter 2
The concentration of hydrogen sulfide therein became 50 ppm. When using polyaluminum chloride (PAC), the concentration of hydrogen sulfide in the digested gas reached 1000 ppm,
Hydrogen sulfide concentration has dropped significantly, and methane gas has also been reduced to 1.
It was recovered three times. This is because the iron chloride 11 acts as a desulfurizing agent, and the sulfide ion concentration in the digested sludge 3 decreases. (Embodiment 4) As shown in FIG. 4, the same human waste and septic tank sludge as in Embodiment 3 were introduced into a concentrating apparatus 10 as a treatment object 9, and 8% polyaluminum chloride per solid as a coagulant (PAC) 14 was added for aggregation. Then, the aggregate is used as the digestion target 12 and the fermenter 2
, Heated to about 55 ° C., and stirred with stirring gas 13, and iron chloride 11 as a desulfurizing agent
It was introduced so as to be about 00 ppm.

The results were the same as in Example 3. further,
By the addition of the iron chloride 11, the pH of the digested sludge 3 was lowered from 8.0 to 6.8. In other words, iron chloride 11 is desulfurized and pH
Both of these effects were effective in suppressing the precipitation of metal sulfide. (Example 5) As shown in Fig. 5, anaerobic digestion was carried out in the same manner as in Example 4 except that iron powder 15 was introduced so as to be about 10000 ppm instead of iron chloride 11. The results were the same as in Example 4.

The introduction of iron chloride as in Example 4 is effective because the pH of the digested sludge 3 is lowered when the pH is high, but may be rancid when the pH is already low to some extent. In such a case, it is effective to add iron powder or iron scrap as in the fifth embodiment.

[0021]

As described above, according to the present invention, it is possible to suppress the precipitation of metals essential for anaerobic digestion by reducing the sulfide ion concentration in digested sludge in a fermenter. Thus, anaerobic digestion activity can be enhanced, and the amount of methane gas generated can be increased.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a first embodiment of a methane gas generation promoting method of the present invention.

FIG. 2 is an explanatory view illustrating a methane gas generation promoting method according to a second embodiment of the present invention.

FIG. 3 is an explanatory diagram for explaining a methane gas generation promoting method according to a third embodiment of the present invention.

FIG. 4 is an explanatory diagram for explaining a methane gas generation promoting method according to a fourth embodiment of the present invention.

FIG. 5 is an explanatory view illustrating a fifth embodiment of the methane gas generation promoting method of the present invention.

[Explanation of symbols]

 Reference Signs List 1 digestion target 2 fermenter 3 digestion sludge 4 digestion gas 5 desulfurization tower 6 desulfurization gas 8 hydrochloric acid 9 treatment target 11 iron chloride 12 digestion target 15 iron powder

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideki Iwabe 2-47, Shishitsuhigashi, Namiwa-ku, Osaka-shi, Osaka F-term (reference) 4D059 AA01 AA02 AA03 AA07 BA15 BA17 BE55 BF12 DA16 DA22 DA24 DA32

Claims (4)

[Claims] 1. A method for promoting methane gas generation, comprising: introducing organic waste into a fermenter, decomposing it by anaerobic digestion to generate methane gas, and reducing sulfide ion concentration in digested sludge in the tank. Method. 2. The methane gas generation promotion according to claim 1, wherein the reduction of the sulfide ion concentration is performed by removing hydrogen sulfide in the digested gas in the tank and releasing hydrogen sulfide from the digested sludge. Method. 3. The method for promoting methane gas generation according to claim 1, wherein the reduction of the sulfide ion concentration is carried out by mixing the organic waste with a desulfurizing agent before the fermenter or in the fermenter. 4. The method according to claim 1, wherein the sulfide ion concentration is lowered by lowering the pH of the digested sludge in the tank.