JP2000070908A - Method for anaerobically digesting organic waste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the treatment of the org. waste contg. protein or lipid by controlling the water content of the digested sludge to a value at which a fermentation inhibitor does not exceed the upper-limit concn. SOLUTION: A garbage 1 as an easily decomposable org. matter is introduced into a fermentation tank 2, heated to about 55 deg.C and anaerobically digested, and the generation of a biogas 3 is measured. Since the garbage 1 contg. 90% water is anaerobically digested without any dilution, the water content of the digested sludge 4 in the fermentation tank 2 is increased to about 99%. Similarly, the garbage 1 is charged into the fermentation tank 2, heated to about 55 deg.C and anaerobically digested and the generation of the biogas 3 is measured. However, the digested sludge 4 in the fermentation tank 2 is sent to a membrane separator 5, soid is separated from liq. by an submerged membrane separator 6, the filtrate 7 is discharged, the digested sludge 4' concentrated thereby is returned into the fermentation tank 2, and the water content of the digested sludge 4 in the tank 2 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 anaerobic digestion of organic waste such as septic tank sludge and sewage sludge, high-concentration organic waste liquid, kitchen waste and food waste.

[0002]

2. Description of the Related Art Organic sludge such as septic tank sludge and sewage sludge,
2. Description of the Related Art Processing methods for anaerobic digestion of high-concentration organic waste such as high-concentration organic waste liquid, kitchen waste, and food waste to recover methane gas have been performed.

[0003] In the conventional fermenter, the sludge concentration was gradually increased. However, an example in which microorganisms involved in anaerobic digestion were maintained at a high concentration by extracting a part of the sludge, concentrating the sludge and returning the sludge was returned. (Japanese Patent Laid-Open No. 6-6359)
No. 8), it was considered that the higher the concentration, the more advantageous in terms of the decomposition rate and decomposition rate.

[0004] In addition, as means for improving the decomposition rate,
A method in which the input substrate is solubilized in advance by an alkali agent or heating, and a method in which CO ₂ is separated from digested gas and then blown into an acid fermentation tank to promote solubilization of cellulose are adopted.

[0005]

When anaerobically digesting organic waste, it is necessary to always pay attention to rancidity. In the fermentation process of easily decomposable organic matter, the rate of acid generation is higher than the rate of acid consumption (methane generation rate). If the concentration of accumulated organic acids such as acetic acid and propionic acid increases due to the difference, the substrate concentration is inhibited. It is known to affect the methanation stage.

In addition, in the anaerobic digestion of organic waste containing a high concentration of protein or lipid, the anaerobic digestion activity is reduced or stopped due to inhibition of adsorption by generated ammonia, hydrogen sulfide, or lipid. I will.

[0007] These problems become more conspicuous as the solids concentration in the fermenter is higher, that is, as the water content is lower. In other words, high-concentration anaerobic digestion, which was considered to be advantageous in terms of decomposition rate and decomposition rate, is compared to low-concentration anaerobic digestion,
It can be said that it is susceptible to concentration inhibition by the input substrate or degradation product.

[0008] However, to cope with such fermentation inhibition, there is no other way but to control the pH or to refrain from introducing the substrate into the fermenter, and it is easy to treat the organic waste containing a large amount of protein or lipid. Was not.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to reduce fermentation inhibition by input substrates or products. As a result, in the anaerobic digestion of organic waste, the TS concentration in the fermenter, In other words, they have found that by controlling the water content of sludge, the influence of inhibitors can be reduced, and the decomposition rate and decomposition rate of organic waste can be increased, and the present invention has been completed.

That is, according to the method for anaerobic digestion of organic waste of the present invention, an organic waste is charged into a fermenter and digested under anaerobic conditions. The water content of the sludge is controlled to a value that does not exceed the upper limit concentration of the fermentation inhibitor in the tank.

[0011] Fermentation inhibitors include input substrate components such as lipids, ammonia, organic acids, hydrogen sulfide, and heavy metals.
And its decomposition products. Referring to FIGS. 3 to 5, 1) Although the methane production rate per fermenter volume increased with an increase in the TS concentration (solid matter concentration) of the sludge in the fermenter,
When the TS concentration exceeded 10%, the increase in the methane production rate became slow (see FIG. 3). 2) With the increase in the TS concentration, the decomposition rate of the input COD and the methane production rate decreased (see FIG. 4). From this, it can be seen that in this case, it is optimal to control the TS concentration to about 10%. That is, the methane generation rate can be controlled by managing the TS concentration.

3) When anaerobic fermentation was performed using kitchen waste as a substrate, the number of days required for 80% decomposition of the input COD decreased with an increase in the TS concentration, but started to increase after a TS concentration of 6%. (See FIG. 5) From this, it can be seen that an optimum TS concentration exists. That is, the decomposition rate can also be controlled by managing the TS concentration.

From the above, it can be said that the decomposition rate and decomposition rate of organic waste in anaerobic digestion can be controlled by controlling the water content. Therefore, based on the above experimental results, by controlling the water content of the digested sludge to an appropriate value that can ensure a high decomposition rate and decomposition rate, the effect of the fermentation inhibitor in the tank is as low as possible, The fermentation inhibitor can be controlled so as not to exceed the upper limit concentration.

If the above experiment is not performed, the concentration of the fermentation inhibitor in the tank is determined as an empirical rule from the composition of the organic waste to be charged into the tank, or the digested sludge in the tank is measured. Ask for it. Then, the water content of the digested sludge is controlled based on a comparison between the obtained concentration value and a predetermined upper limit concentration value.

The upper limit concentration may be determined in a previous experiment, or may be a value known in the literature. The concentration of the fermentation inhibitor is conveniently determined on the basis of the content of protein or lipid in the organic waste.

However, the control value of the water content must satisfy the condition that the stirring operation of the digested sludge can be easily performed (TS concentration about 10%). In addition, since the fermentation conditions gradually change due to the acclimation of microorganisms, it is necessary to appropriately reset the control values. Further, it is necessary to consider conditions such as operating costs.

The relationship between the inhibitory substance concentration range in the fermenter and the TS concentration in the tank generally has a relationship as shown in Table 1 below.

[0018]

[Table 1]

The increase in the water content of digested sludge can be achieved by concentrating the organic waste or feeding it in undiluted form, separating a portion of the digested sludge by solid-liquid separation means, and removing the filtrate from the system. It can be performed by at least one of withdrawing.

The decrease in the water content of digested sludge can be caused by solid-liquid separation of a part of the digested sludge by solid-liquid separation means, withdrawing the filtrate to the outside of the system, and supplying diluting water not containing the fermentation inhibitor. Can be done by

As the solid-liquid separation means, a centrifugal separator or the like may be used, a membrane separation device may be installed in the fermentation tank, or a membrane separation tank provided with the membrane separation device may be used in the fermentation tank. It may be attached. In the case of solid-liquid separation in a membrane separation tank, a part or the whole amount of digested sludge concentrated by extracting a filtrate is returned to a fermentation tank.

As the diluting water, one obtained by subjecting the discharged filtrate to biological treatment, an ammonia stripping method, an ammonia adsorbent, or the like, process water generated in another treatment step, or fresh water can be used.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 In the apparatus configuration as shown in FIG. 1, kitchen waste 1 (low protein: 5% based on the total weight of organic matter, low lipid: 1% based on the total weight of organic matter), which is an easily decomposable organic substance, was added to the fermenter 2. The amount of biogas 3 generated was measured while anaerobic digestion was performed in a state where the mixture was charged into the inside and heated to about 55 ° C. Kitchen waste 1
Of the digested sludge 4 in the fermenter 2 was 99% because it was subjected to anaerobic digestion without dilution.
Converged to a degree.

Similarly, the garbage 1 was put into the fermenter 2 and the amount of biogas 3 generated was measured while being anaerobically digested at a temperature of about 55 ° C. However, the digested sludge 4 in the fermentation tank 2 is sent to the membrane separation tank 5, and the filtrate 7 is separated by solid-liquid separation by the immersion type membrane separation device 6. , The moisture content of the digested sludge 4 in the fermenter 2 was reduced. In reducing the water content, the filtrate 7 was withdrawn so that the NH ₄ —N concentration of the digested sludge 4 in the fermenter 2 was 3000 mg / L or less.

The results are as shown in Table 1. At the time of controlling the water content, the maximum load and the amount of gas generated per volume increased four times as compared with the time of non-control. (Residence time in tank: HRT
(10 days, SRT 30 days)

[0026]

[Table 2]

From these results, it is possible to reduce the size of the fermentation tank and eliminate the need to heat excess water by implementing the method of the present invention as described above, as compared with the conventional method in which the water content is not controlled. It can be seen that the amount of methane gas used for heating the fermenter can be reduced, and the energy that can be recovered outside the system increases dramatically. (Example 2) In the same manner as in Example 1, garbage 1 having a high protein content (protein: 30% of the total organic matter weight) was anaerobically digested in a controlled state and an uncontrolled state, respectively. However, in the control state, the non-NH
By adding _4- N water and extracting the filtrate 7, digested sludge 4
The NH ₄ -N concentration of reduced below 3000 mg / L.

The results are as shown in Table 2. The decomposition rate and the amount of gas generated during control were higher than those without control, and high-protein garbage could be decomposed efficiently.
(In the control state, the residence time in the tank: HRT 6.5 days, SR
T30)

[0029]

[Table 3]

From these results, by using the membrane separation tank 5 (that is, the membrane separation device 6) together, it is possible to keep the load of organic substances high while lowering the concentration of inhibitory substances, and to reduce the load of high-protein garbage. It can be seen that the processing can be performed without dropping. Such a high-protein garbage can be processed by the conventional method of merely diluting the substrate, throwing it in and pulling it out, but the amount of processing is greatly reduced.

In each of the above embodiments, NH ₄ —N
Is controlled so as to be equal to or lower than a predetermined upper limit concentration, but the water content is controlled using an inhibitor such as NH ₄ —N, lipid, organic acid as an index according to the composition of the fermentation target. However, in the case of an organic acid, for example, it is necessary to control the water content in the fermenter so that the instantaneous VFA concentration due to the immediate decomposition of the easily decomposable substance does not inhibit the inhibition in addition to the accumulation.

As shown in FIG. 2, solid-liquid separation of the digested sludge 4 and extraction of the filtrate 7 may be performed by a membrane separation device 6 installed inside the fermenter 2.

[0033]

As described above, according to the present invention, by controlling the moisture content of the digested sludge in the fermentation tank to a value that does not exceed the upper limit concentration of the fermentation inhibitor in the tank, the decomposition of the input substrate is performed. Efficiency can be improved, and stable operation can be performed. It is possible to select either operation that emphasizes energy recovery or operation that emphasizes decomposition rate (stability).

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an apparatus for carrying out the method for anaerobic digestion of organic waste of the present invention, wherein a fermentation tank is provided with a membrane separation tank provided with a membrane separation apparatus.

FIG. 2 is an explanatory view showing an apparatus for performing the anaerobic digestion method of organic waste of the present invention, in which a membrane separation device is installed inside a fermenter.

FIG. 3 is a graph showing the effect of TS concentration on the methane production rate per fermenter volume as a basis for controlling the water content of digested sludge.

FIG. 4 is a graph showing the effect of TS concentration on methane production rate and COD decomposition rate as a basis for controlling the water content of digested sludge.

FIG. 5 is a graph showing the relationship between the number of days required for 80% decomposition of organic waste and TS concentration as a basis for controlling the water content of digested sludge.

[Explanation of symbols]

 Reference Signs List 1 kitchen waste (organic waste) 2 fermenter 3 biogas (methane gas) 4,4 'digested sludge 6 membrane separation device 7 filtrate 8 dilution water

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hideki Iwabe 2-47, Shizitsuhigashi, Namiwa-ku, Osaka-shi, Osaka F-term in Kubota Corporation (Reference) 4D059 AA02 AA03 AA07 AA08 BA12 BA25 BA56 BE38 BE42 BF15 CA21 CA22 CA23 CA30 CC03 EA20 EB01 EB20

Claims (5)

[Claims] Claims 1. An organic waste is charged into a fermenter and digested under anaerobic conditions to recover methane gas.
An anaerobic digestion method for organic waste, comprising controlling the water content of digested sludge in a tank to a value that does not exceed an upper limit concentration of a fermentation inhibitor in the tank. 2. The method according to claim 1, wherein the concentration of the fermentation-inhibiting substance in the tank is determined as an empirical rule from the composition of the organic waste put into the tank, or is determined by actually measuring digested sludge in the tank. Item 4. The method for anaerobic digestion of organic waste according to Item 1. 3. Increasing the water content of the digested sludge can be achieved by concentrating the organic waste or adding it without dilution, by solid-liquid separation of a part of the digested sludge by solid-liquid separation means, The method for anaerobic digestion of organic waste according to any one of claims 1 and 2, wherein the method is performed by at least one of withdrawing the organic waste to the outside. 4. The decrease in the water content of the digested sludge is caused by solid-liquid separation of a part of the digested sludge by a solid-liquid separating means, withdrawing the filtrate out of the system, and supplying diluting water free of the fermentation inhibitor. The method for anaerobic digestion of organic waste according to any one of claims 1 and 2, wherein the method is performed. 5. The method for anaerobic digestion of organic waste according to claim 2, wherein the concentration of the fermentation inhibitor in the tank is determined based on the content of protein or lipid in the organic waste.