JPH0699199A - Anaerobic digestion treatment of organic sludge - Google Patents

Abstract

PURPOSE:To efficiently decrease sludge volume and recover gaseous methane by alkalinizing primary sludge and excess sludge and also performing heat alkali treatment wherein organic substance contained in sludge is made soluble, thereafter mixing both the sludges and performing anaerobic digestion treatment for the mixed sludge. CONSTITUTION:Primary sludge produced in a sewage disposal plant is firstly introduced into a heat alkali treatment tank 2 therefor. Excess sludge is introduced into a heat alkali treatment tank 3 therefor. The primary sludge is made alkaline and also maintained at ordinary temperature to the temperature of anaerobic digestion treatment. Thereby heat alkali treatment is performed wherein organic substance contained in sludge is made soluble. Further excess sludge is made alkaline and also maintained at 50-100 deg.C. Thereby heat alkali treatment is performed wherein organic substance contained in excess sludge is made soluble. Thereafter both the organic sludges is mixed and pH regulation is performed in a pH regulation tank 4 and then anaerobic digestion treatment is performed in an anaerobic digestion tank 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anaerobic digestion treatment method for organic sludge, and more particularly to a method for anaerobic digestion treatment after solubilizing organic sludge such as sewage sludge with an alkali. .

[0002]

2. Description of the Related Art The method for anaerobic digestion of organic sludge generated from a sewage treatment plant is to stabilize and detoxify landfill,
In addition to the advantages of volume reduction and improvement of dehydration, there is an advantage that methane can be recovered as a valuable resource. This anaerobic digestion treatment method is an important treatment method especially in sewage treatment plants due to the increase of methane gas due to the change of organic components in sludge caused by the change of consumer life in modern society and the development of gas power generation technology. Most of them are operated in a one-phase reaction tank at the medium temperature of digestion temperature (37 ℃).

However, at present, the digestibility is low, the volume of sludge is reduced, and the recovery rate of methane gas is insufficient.
At the research level, it promotes solubilization before anaerobic digestion,
Methods for improving the efficiency of anaerobic digestion have been investigated. Specifically, a method of adding alkali to promote solubilization,
A method of applying heat to promote solubilization, a method of adding heat and alkali to promote solubilization (referred to as hot alkali treatment), and the like have been studied.

However, the method (1) has a drawback that solubilization cannot be expected sufficiently and anaerobic digestion treatment cannot be performed. A process is also considered in which these solubilizations are applied only to excess sludge and not solubilization to the first settling sludge, but the digestibility does not rise sufficiently and the volume of sludge is reduced and the recovery rate of methane gas is reduced. Was insufficient in terms of. on the other hand,
Is a method of applying heat and adding alkali to promote solubilization, and the solubilization rate is high, and as a result, the digestibility can be improved, and the volume of sludge can be reduced and the recovery rate of methane gas can be improved. Expected to improve efficiency. However, in order to perform a solubilizing operation on a large amount of organic sludge such as sewage sludge, there is a drawback that the cost required for adding heat or alkali increases depending on the setting of conditions.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and treats first settled sludge and surplus sludge having different degradability in anaerobic digestion with hot alkali treatment suitable for each suitability. To provide an anaerobic digestion treatment method for organic sludge, which can improve the digestibility and the recovery rate of methane gas while suppressing the cost required for adding heat and alkali to the minimum by applying It was completed in.

[0006]

Means for Solving the Problems The present invention made to solve the above-mentioned problems is to make organic matter in the initial sludge by keeping the initial sludge sludge alkaline and maintaining at room temperature to anaerobic digestion treatment temperature. Perform hot alkali treatment to solubilize and make excess sludge alkaline and
The method is characterized by performing a hot alkali treatment for solubilizing the organic matter in the excess sludge by maintaining it at 0 ° C, and then performing an anaerobic digestion treatment by combining both organic sludges.

FIG. 1 is a basic flow sheet of the present invention, which is an alkali storage tank 1 and a first sludge heat alkali treatment tank 2.
It has an excess sludge hot alkali treatment tank 3, a pH adjusting tank 4, an anaerobic digestion tank 5, a precipitation tank 6, a gas holder 7, a boiler or gas generator 8, and an acid storage tank 9. In this flow, the first settling sludge generated in a sewage treatment plant or the like is first put into the first settling sludge hot alkali treatment tank 2, and the surplus sludge is put into the surplus sludge heat alkali treatment tank 3.

In the first sludge heat alkali treatment tank 2, the alkali is introduced from the alkali storage tank 1 by controlling the pH and treatment temperature. At this time, as shown in Fig. 2, the solubilization rate of the initial sludge and the control pH have a positive relationship, but when the control pH is less than 7.5, the solubilization rate is low, and conversely, when the control pH is 10.5 or more, the anaerobic digestion pH is decreased. When uncontrolled, the pH of anaerobic digestion becomes higher than 9.2, and the cost of acid addition to the pH adjusting tank 4 for efficient anaerobic digestion treatment increases, so the pH should be adjusted to 7.5 to 10.5. It is preferable to control.

On the other hand, since the treatment temperature has no significant relationship with the solubilization rate of the initial sludge as shown in FIG.
It may be at room temperature as it is with the initial sludge. However, the digestion temperature of the anaerobic digester 5 in the latter stage is medium (35 to 40).
(° C) or high temperature (50-55 ° C) is desirable from the viewpoint of anaerobic digestion treatment, and the anaerobic digestion tank 5 or the first settling sludge heat alkaline treatment tank 2 in the preceding stage should be set to those temperatures.
It is necessary to heat at. Considering this, the temperature of the first sludge hot alkaline treatment tank 2 is in the range of normal temperature to anaerobic digestion treatment temperature. Also, the processing time is 1Hr and 22Hr.
Since there is almost no difference between and, a processing time of about 1 Hr is sufficient.

The above is a study of the hot alkaline treatment conditions assuming that only the first settled sludge enters the anaerobic digestion tank 5. However, actually, excess sludge treated separately with hot alkali is also mixed. Therefore, it is needless to say that it is necessary to determine the hot alkaline treatment condition of the first settling sludge in consideration of the state (temperature, pH) of the anaerobic digester 5 in combination with it.

In the excess sludge heat alkali treatment tank 3, alkali is introduced from the alkali storage tank 1 by controlling pH and treatment temperature. At this time, as shown in FIG. 3, the solubilization rate of the excess sludge and the control pH have a positive relationship, but when the control pH is less than 7.5, the solubilization rate is low, and conversely, when the control pH is 12.5 or more, the anaerobic digestion pH does not exist. When controlled, the pH of anaerobic digestion becomes higher than 9.2, and the cost of acid addition to the pH adjusting tank 4 for efficient anaerobic digestion treatment increases, so control to pH 7.5-12.5. It is desirable to do.

On the other hand, the treatment temperature also has a positive relationship with the solubilization rate of the excess sludge as shown in FIG. However, setting the treatment temperature to 100 ° C or higher has equipment limitations, and the degree of solubilization is unsuitable for applying heat energy, and as a result, the biodegradability at the anaerobic digestion treatment temperature is low. The treatment temperature is
50 to 100 ℃ is recommended. Further, when the treatment temperature is 70 ° C. or higher, the carbon dioxide solubility of the excess sludge hot alkali treatment tank 3 becomes remarkably low, and the feature that carbon dioxide dissolves in the hot alkali treatment tank cannot be fully utilized, and the excess sludge heat alkali treatment is performed. Since a degassing facility will be required in the tank 3, it is preferable to set the temperature to 50 to 70 ° C. if this becomes a problem. Further, the treatment temperature of 22 Hr is preferably about 22 Hr because the solubilization rate is increased by about 20% compared to 1 Hr.

The above is a study of the hot alkaline treatment conditions assuming that only excess sludge enters the anaerobic digestion tank 5. However, actually, the first settled sludge separately treated with hot alkali is also mixed. Therefore, it is needless to say that it is necessary to determine the hot alkali treatment condition of the excess sludge in consideration of the state (temperature, pH) of the anaerobic digester 5 in combination with it.

The organic sludge treated in the first set sludge hot alkali treatment tank 2 and the surplus sludge heat alkali treatment tank 3 is as it is, or is solid-liquid separated in the solid-liquid separation tank 10 and is sent to the pH adjusting tank 4 as a separated liquid. After being supplied and adjusted to pH 7.3 to 9.2 with the acid from the acid storage tank 9 in the pH adjusting tank 4, it is sent to the anaerobic digestion tank 5 together with the returned sludge. If the pH is 7.3 to 9.2 when the hot alkali-treated sludge and the returned sludge are mixed and put into the anaerobic digestion tank 5, the pH adjustment in the pH adjustment tank 4 is not necessary. In anaerobic digester 5, pH
Anaerobic digestion is performed by stirring at a digestion temperature of 20 to 60 ° C with an alkaline pH of 7.3 to 9.2, and digestive gas containing methane gas is stored in the gas holder 7 and digested from the anaerobic digestion tank 5. The sludge is sent to the settling tank 6.

[0015]

EXAMPLES Examples of the present invention will be shown below together with conventional examples. FIG. 4 shows the flow of the embodiment, and FIG. 5 shows the flow of the conventional example. In the conventional example, the hot alkali treatment conditions for solubilizing the mixed raw sludge were 70 ° C., 22Hr, and pH 9.0. 37 for first sludge
A hot alkali treatment was carried out at a temperature of 1 hour and a pH of 9.0. This is because, as shown in FIG. 3, with respect to the first settling sludge, the difference due to temperature and treatment time is not significant. The results are shown in Table 1.

[0016]

[Table 1]

As shown in Table 1, regarding the solubilization by the hot alkali treatment, the solubilization rate of 38% in the conventional example is intermediate between the solubilization rate of excess sludge of 47% and the solubilization rate of initial sludge of 30%. However, the organic acid concentration in the dissolved components reached 2760 mg / L due to the solubilization of the excess sludge in the examples. This indicates that the excess sludge was pretreated to the level of acid fermentation with further solubilization in the hot alkaline treatment time of 22 Hr. In the conventional example, the organic acid concentration was 17 even when the mixed raw sludge in which the initial sludge and the excess sludge were mixed was treated with hot alkali under the same conditions.
The effect of the method of the present invention in which sludge having different properties was treated under different conditions was observed, which increased only up to 50 mg / L. As described above, in terms of properties, it has the effect of increasing the concentration of organic acids that are assimilated to anaerobic digestion in the next step, in excess of the solubilization rate based on the mere VSS solubilization.

Regarding the amount of alkali added, in the conventional example, TS was added at a rate of 12% (gNaOH / gTS) relative to the TS of the mixed raw sludge, whereas in the example, the first settled sludge was added. The addition rate can be lowered to 5%, and the cost can be reduced. This is because the initial alkaline sludge has the same thermal alkaline treatment conditions of pH 9.0 as the excess sludge, but the treatment time can be shortened to about 1 hour. As a result of the anaerobic digestion treatment, the digestibility was 55% in the conventional example, but was improved to 62% in the example, and the methane gas generation rate was 10.3 (m ³ / sludge m ³ ) in the conventional example. On the other hand, in the example, the efficiency was confirmed to be 11.8 (m ³ / sludge m ³ ).

[0019]

As described above, according to the present invention, the optimum solubilization treatment is performed on the primary sludge and the surplus sludge having different properties, so that the cost can be kept low and the effective solubilization can be performed. Processing can be performed. as a result,
By improving the digestibility, which is a feature of anaerobic digestion treatment, it becomes possible to reduce sludge volume and recover methane gas more efficiently than before.

[Brief description of drawings]

FIG. 1 is a flow sheet showing a processing method of the present invention.

FIG. 2 is a graph showing the relationship between the pH of the hot alkali treatment and the solubilization rate of the initial sludge.

FIG. 3 is a graph showing the relationship between the pH of hot alkali treatment and the solubilization rate of excess sludge.

FIG. 4 is a flow sheet showing a processing method of an example.

FIG. 5 is a flow sheet showing a processing method of a conventional example.

[Explanation of symbols]

 1 Alkali storage tank 2 Initial sludge heat alkali treatment tank 3 Excess sludge heat alkali treatment tank 4 pH adjustment tank 5 Anaerobic digestion tank 6 Precipitation tank 7 Gas holder 8 Boiler or gas generator 9 Acid storage tank 10 Solid-liquid separation tank

Claims (1)

[Claims] 1. A hot alkali treatment is carried out to make the initial sludge sludge alkaline and maintain the normal temperature to anaerobic digestion treatment temperature to solubilize organic matter in the initial sludge sludge. Anaerobic digestion treatment of organic sludge, characterized in that both organic sludge are combined and subjected to anaerobic digestion treatment after thermal alkali treatment to solubilize organic matter in excess sludge by maintaining at ~ 100 ℃ Method.