JPH0741270B2 - Anaerobic digestion method for organic sludge - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for floating and concentrating organic sludge such as sewage sludge and then subjecting it to anaerobic digestion.

[0002]

2. Description of the Related Art Conventional flotation concentration-anaerobic digestion treatment methods for organic sludge include (a) pressurized flotation concentration-anaerobic digestion treatment method, and (b) foaming agent added flotation concentration-anaerobic digestion treatment method. and so on. In the method (a) shown in Fig. 9, the power for pressurization is large, and since the air is dissolved under pressure, oxygen in the air causes the sludge to become an aerobic atmosphere. The sexual digestion treatment was also problematic in terms of inhibition of digestion by absolutely anaerobic bacteria. See also FIG.
Also in the method (a) shown in (1), the cost of the foaming agent is high, and there is a possibility that the activity of anaerobic bacteria may be inhibited depending on the type and amount of the foaming agent to be dissolved in sludge.

On the other hand, in any of the methods (a) and (b), in order to efficiently carry out the digestion treatment, the optimum temperature of the anaerobic bacterium for medium temperature or high temperature is 35 to 37 ° C. or 55 ° C.
It is necessary to heat up to the vicinity, and heating is also required when separately performing solubilization treatment by heat before digestion treatment, digestion tank heating cost, pressurizing power cost, foaming The cost of the agent was enormous. Further, when the above-mentioned solubilization treatment step is not provided, a long digestion time is required due to the slow digestion reaction rate, which has been a problem in terms of equipment scale and heating (heat retention) cost.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems and does not require a mechanical concentrating means such as pressurization which requires a high power cost in the flotation concentrating process, and a foaming agent or the like. Floating concentrated sludge can increase the sludge concentration stably while maintaining an anaerobic atmosphere during the concentration process, and the deterioration of the properties of the separated liquid during the flotation concentration process can be minimized. In the anaerobic digestion process, the digestion tank heating cost required in the conventional method is hardly required, and the digestion time is significantly shortened to reduce the initial cost and digestion operation required for digestion equipment. It is made to provide a method for anaerobic digestion treatment of organic sludge capable of reducing the running cost required.

[0005]

[Means for Solving the Problems] The first invention made to solve the above problems is to add mixed sludge obtained by mixing organic sludge and returning sludge containing inoculum to a temperature of 30 to 80 ° C. Heat the mixed sludge during heating or after heating
The method is characterized in that air bubbles generated by aerial digestion are used to float and concentrate under atmospheric pressure for a predetermined time, and then the buoyant concentrated sludge is immediately subjected to anaerobic digestion treatment while being maintained at 30 to 60 ° C.

A second aspect of the invention is to convert the separation liquid in the flotation concentration tank and / or the digestion / desorption liquid in the digestion tank into organic sludge and / or mixed sludge in which return sludge containing inoculum is mixed with organic sludge. Heat-exchange the sludge to a temperature of 30 to 80 ° C, and heat the mixed sludge after heating by heating deaeration or anaerobic digestion.
The method is characterized in that the air bubbles generated are used to float and concentrate under a normal pressure for a predetermined time, and then the levitation concentrated sludge is immediately subjected to an anaerobic digestion treatment while being maintained at 30 to 60 ° C.

A third aspect of the present invention is to heat a mixed sludge obtained by mixing organic sludge and return sludge containing inoculum to a temperature of 30 to 80 ° C., and then to a predetermined amount of aggregated sludge after heating. The gas generated by adding and stirring the agent and heating degassing or anaerobic digestion
It is characterized in that it is subjected to levitation concentration under atmospheric pressure for a predetermined time using bubbles, and then anaerobic digestion treatment is performed immediately while maintaining the levitation concentration sludge at 30 to 60 ° C. Furthermore, a fourth invention is characterized in that the above-mentioned steps are carried out by using a plurality of floating concentrating tanks.

In the present invention, in the conventional method, what was heated in the previous stage of the digestion treatment process is transferred to the previous stage of the flotation concentration process, and the medium temperature digestion, high temperature digestion,
This is based on the principle of utilizing the gas generated by heating degassing or a combination thereof to float and concentrate the organic sludge without consuming extra energy or chemicals. Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.

[0009]

EXAMPLE FIG. 1 is a flow sheet showing an example of the first invention, which is the basic flow of the present invention, in which 1 is a sludge heating tank,
2 is a flotation concentration tank, 3 is an anaerobic digestion tank, 4 is a precipitation tank, and 5 is a gas holder. FIG. 2 is a flow sheet showing an embodiment of the second invention, in which heat exchangers 6 and 7 are loaded in the first flow. FIG. 3 is a flow sheet showing an embodiment of the third invention, in which the chemical dissolution tank 8 and the chemical mixing tank 9 are loaded in the first flow.

In FIG. 1, first, the organic sludge is put into a sludge heating tank 1, where it is heated to 30 to 80 ° C, preferably 30 to 60 ° C. The preferable heating temperature is 35 to 37 ° C., which is optimal for mesophilic anaerobes when viewed from the fermentation side in the flotation tank 2.
Alternatively, it is 53 to 55 ° C, which is optimal for thermophilic anaerobes. In addition, when viewed from the heating degassing method that utilizes the difference in gas solubility in water depending on temperature, gas is often generated and floats at 50 to 80 ° C.
As shown in Fig.2, less gas is generated below 30 ° C, so levitation concentration does not occur. If the heating temperature is high, levitation concentration tank 2
Although it takes only a few hours to shorten the concentration time, the advantage of increasing the heating temperature is small considering that the digestion days in the subsequent anaerobic digester require a long time of one month, as shown in Fig. 6. Since the concentration of separated liquid (TS) increases as the heating temperature rises, the heating temperature in the anaerobic digestion process is 40 ~
60 ° C is preferred.

Here, as the heating means, heating by external energy is used as shown in FIG. 1, but as shown in FIG. 2, internal energy can be recovered by using a heat exchanger to improve thermal efficiency. . As a heat exchange method, first, the organic sludge is heated by the heat exchanger 6 using the digestion / desorption liquid separated and generated by the settling tank 4 as a heat source before being fed into the sludge heating tank 1. The separated liquid generated from the flotation tank 2 is heated by a heat exchanger 7 that uses it as a heat source. The sludge thus sent to the sludge heating tank 1 is further heated to a predetermined temperature by external energy when the temperature of the sludge has not reached the predetermined temperature. The desorbed liquid and the separated liquid pass through the heat exchangers 6 and 7, respectively, and then are returned to the water treatment system as return water.

Although heating by electricity or the like is also used as the external energy, a gas boiler using a digestion gas whose main component is methane collected in the gas holder 5 is burned, and steam is preferable from the viewpoint of thermal efficiency. It may be heated to a predetermined temperature by the direct blowing method. The sludge heated to a predetermined temperature in the sludge heating tank 1 in this way is mixed with the sludge returning to the flotation concentration tank, which is a part of the digested sludge flowing out from the settling tank 4. However, at this time, when the amount of sludge returned to the flotation concentration tank is large and the temperature is lower than the set temperatures of the sludge heating tank 1 and the anaerobic digestion tank 3, the temperature of the mixed sludge is expected to decrease, Will it drop 2 ° C or more below the specified temperature, or 30
If the temperature is lower than 0 ° C, the sludge heating tank 1 is mixed with the sludge in the flotation / concentration tank before the sludge is heated, and then the organic sludge is heated to a predetermined temperature.

Next, the mixed sludge containing the sludge returning to the flotation / concentration tank that has reached a predetermined temperature is sent to the flotation / concentration tank 2. However, as for the mixed sludge having poor flotation separation, the flotation / concentration tank 2 is shown in FIG. The flocculant is added before being sent to. Aggregation is performed by a method in which a flocculant previously dissolved in the chemical dissolution tank 8 is added and mixed in the chemical mixing tank 9 to form flocs. As shown in FIG. 7, the addition ratio of the coagulant is preferably 0.05% or more per mixed sludge solids concentration (TS), and if it is less than that, suspended solids may remain in the generated separation liquid.

Next, the mixed sludge is sent to the flotation tank 2.
After being heated in the sludge heating tank 1, it is concentrated while being kept at the same temperature as the final temperature. At this time, the solubilization in the early stage of anaerobic digestion and the acid fermentation process are mainly started within the concentrating temperature of 60 ° C. to generate gas, and at the concentrating temperature of 60 to 80 ° C., the gas solubility is decreased mainly due to the temperature increase. Generation occurs, and a layer of floating concentrated sludge is formed in the upper part, and a layer of separated liquid is formed in the lower part. As shown in FIG. 8, the concentration time varies greatly depending on the heating temperature, and at least 1 hour is required to secure high concentration. As shown in Fig. 8, 20 ℃
Below, the solid concentration of the flotation sludge is low, and at 90 ° C and above, convection occurs and the flotation concentration is significantly reduced.

The flotation sludge obtained in the flotation tank 2 is sent to the anaerobic digestion tank 3 immediately when the heating temperature is 30 to 60 ° C. and when it exceeds 60 ° C., it is cooled to 60 ° C. or lower.
Here, the digestion temperature is the same as the flotation tank 2 (30-60 ℃).
Since the anaerobic digestion tank 3 costs about the same as the heat retention in the anaerobic digestion tank 3, and the mixed sludge has already reached the initial stage of anaerobic digestion in the flotation tank 2, the anaerobic digestion reaction is performed quickly. To be done. As a result, the digestive days of the anaerobic digester 3 will be about 80% of the conventional one.

The digested sludge generated in the anaerobic digestion tank 3 is subjected to solid-liquid separation in the precipitation tank 4 to be divided into digestion-desorption liquid and digestion sludge. In the first invention, the heat exchanger 6 is directly used.
Part of the digested sludge is sent to the flotation / concentration tank return sludge and the rest is sent to the next sludge treatment step. Also, the digestive gas mainly composed of methane generated in the anaerobic digestion tank 3 and the precipitation tank 4 is collected by the gas holder 5, and in some cases, a part thereof is used for heating the sludge heating tank, and the rest is useful. Recovered as a resource.

Next, FIG. 4 is a flow sheet showing an embodiment of the fourth invention, which comprises a plurality of sludge thickening tanks 2 of the flow sheet showing the embodiment of the first invention shown in FIG. The flotation sludge storage tank 10 is provided in. First to
The sludge fed to the flotation / concentration tank heated in the same manner as in the third embodiment of the invention is sequentially charged into a plurality of flotation / concentration tanks 2 and, after being floated and concentrated in the same manner as in the first to third embodiments of the invention, semi-finished. Floating concentrated sludge storage tank 1 with a thermal insulation structure
0 is sequentially sent and stored in the floating concentrated sludge storage tank 10 and then semi-continuously or continuously charged into the anaerobic digestion tank 3, while the separated liquid which is sequentially generated with the floating concentrated sludge in each floating concentrated tank 2. Is also sent to the heat exchanger 7, and the subsequent processing is the first
The invention is carried out in the same manner as in the embodiment of the invention.

Table 1 shows the treatment process of the method of the present invention in which the method for anaerobic digestion of organic sludge of the second invention is incorporated after the gravity concentration method (gravity concentration → biological flotation concentration → anaerobic digestion). And the conventional process consisting of gravity concentration method, floating agent addition floating concentration and anaerobic digestion treatment method (gravity concentration →
Sewage sludge, which is a type of organic sludge, is treated at a rate of 480 m ³ / day (concentration of sludge after gravity concentration is 1.86%) by effluent concentration with blowing agent → anaerobic digestion. The processing conditions in the processing process and the results of the processing are shown.

Comparing the method of the present invention in Table 1 with the conventional method, not only the operating cost of the method of the present invention can be remarkably reduced as compared with the conventional method, but also the digestion time can be shortened by about 6 days at the same digestion temperature of 37 ° C. Even with the driving, the digestion rate is about 50% of the conventional method.
A 10% improvement to 59.3%, which is an improvement in digestibility. In addition, the gas generation rate was 7.3 compared with the conventional method with respect to the amount of input sludge.
m ³ / m ³ , 11.5 m ³ / m ³ with the method of the present invention, which is a dramatic improvement, and this method allows high load operation, reducing the digester tank volume to about 2/3 of the conventional method be able to.

[0020]

[Table 1]

[0021]

As described above, according to the present invention, the heating operation of the organic sludge, which is conventionally performed immediately before flowing into the anaerobic digestion tank, is heated in the sludge heating tank and then floated. Since the concentration is performed in the concentration tank, the acid fermentation process can be promoted even in the sludge concentration stage. As a result, the present invention has the following effects. ADVANTAGE OF THE INVENTION According to this invention, the density | concentration of the organic sludge sent to an anaerobic digestion tank can be raised at low cost and low energy. According to the present invention, the capacity of the anaerobic digestion tank can be reduced. According to the present invention, it is possible to operate the anaerobic digestion tank under high load. According to the present invention, it is possible to mainly reduce floating substances which are hardly decomposable substances in the separated liquid generated in the pretreatment step, to clean the separated liquid, and to improve the digestibility and increase the generation of digestive gas. You can Therefore, the present invention has an extremely great contribution to the industrial development as an anaerobic digestion treatment method for organic sludge such as sewage sludge that eliminates the conventional problems.

[Brief description of drawings]

FIG. 1 is a flow sheet showing an embodiment of the first invention.

FIG. 2 is a flow sheet showing an embodiment of the second invention.

FIG. 3 is a flow sheet showing an embodiment of the third invention.

FIG. 4 is a flow sheet showing an embodiment of the fourth invention.

FIG. 5 is a graph showing the relationship between heating temperature and floating concentrated sludge concentration.

FIG. 6 is a graph showing the relationship between the heating temperature and the concentration of the separated liquid.

FIG. 7 is a graph showing the relationship between the amount of coagulant added and the concentration of the separated liquid.

FIG. 8 is a graph showing the relationship between concentration time and flotation concentrated sludge concentration.

FIG. 9 is a flow sheet of Conventional Method 1 (pressure floating concentration method).

FIG. 10 is a flow sheet of Conventional Method 2 (foaming agent-added flotation concentration method).

[Explanation of symbols]

 1 Sludge heating tank 2 Flotation concentration tank 3 Anaerobic digestion tank 4 Precipitation tank 5 Gas holder 6, 7 Heat exchanger 8 Chemical dissolution tank 9 Chemical mixing tank

Claims (4)

[Claims] 1. A mixed sludge obtained by mixing organic sludge and return sludge containing inoculum is heated to a temperature of 30 to 80 ° C., and the mixed sludge is heated or deaerated during or after heating. By sexual digestion
Anaerobic digestion treatment of organic sludge characterized by performing anaerobic digestion treatment immediately after maintaining the temperature of the suspended flotation sludge at 30 to 60 ° C after air-generated bubbles are used for flotation concentration under normal pressure for a predetermined time. Method. 2. Heat conversion from the separation liquid of the flotation tank and / or the digestion / desorption liquid of the digestion tank to an organic sludge and / or a mixed sludge obtained by mixing the organic sludge with return sludge containing inoculum. Keep the sludge at a temperature of 30 to 80 ° C, and heat the mixed sludge after heating by using the bubbles generated by deaeration and anaerobic digestion.
A method for anaerobic digestion of organic sludge, which comprises subjecting the flocculated sludge to anaerobic digestion immediately after being floated and concentrated under normal pressure for a predetermined time and maintaining the flotation concentrated sludge at 30 to 60 ° C. 3. A mixed sludge obtained by mixing organic sludge and return sludge containing inoculum is heated to a temperature of 30 to 80 ° C.,
Adding a predetermined amount of coagulant to the mixed sludge after heating and stirring, pressurizing
After floating and concentrating under atmospheric pressure for a specified time using air bubbles generated by hot deaeration and anaerobic digestion , the floating concentrated sludge is
An anaerobic digestion treatment method for organic sludge, which comprises immediately performing anaerobic digestion treatment while maintaining the temperature at 60 ° C. 4. The method for anaerobic digestion of organic sludge according to claim 1, 2, or 3, wherein a plurality of flotation tanks are used.