JPH0811240B2 - Anaerobic digestion method of sewage sludge - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for anaerobic digestion of sewage sludge.

BACKGROUND ART Sludge generated at a sewage treatment plant is usually discharged in a state of a solid content concentration of about 2%. As one of the measures to reduce the amount of sludge to be discharged, a method of introducing sludge into a digestion tank and performing methane fermentation has been conventionally performed.

However, such a conventional method requires a large-capacity digestion tank because the fermentation rate is slow and the sludge has a dilute concentration, and the digestibility is at most 40-50%, so A considerable part of the digestion gas generated by the above was consumed for heating this sludge, and the economical efficiency was extremely low.

The first point to be improved in the sludge digestion technology is the downsizing of the digestion tank, and the second point is the improvement of the heat balance and the digestion gas generation rate in the process. Recently, attempts have been made to concentrate sludge having a solid content concentration of about 2% to 4 to 6%. By such concentration, the volume of the digestion tank is reduced and the heat absorption is improved, enabling energy recovery by digestion gas power generation. However, even with such a concentration method, the economical efficiency is still insufficient, and further improvement in performance is desired.

As a method for improving the efficiency of methane fermentation, a method of performing fermentation at a high temperature of 50 to 55 ° C and a method of preliminarily heat treating sludge before methane fermentation are known. In the former method, a fermentation rate of 2 to 2.5 times that in the normal methane fermentation at 35 ° C can be obtained. In the latter case, raw sludge in advance
It is said that heat treatment at a temperature of 120 to 180 ° C. can improve the digestibility to 50 to 60% more than usual. These methods are extremely effective methods for improving the efficiency of sludge digestion. However, when the sludge concentration is 4 to 6%, a large amount of heat is required for heating, so heating to about 35 ° C was the limit. Therefore, methane fermentation at a temperature of 50 to 55 ° C or preheating of raw sludge to 120 to 180 ° C can be performed in a normal sewage treatment plant because a large amount of heat is required. There wasn't.

SUMMARY OF THE INVENTION An object of the present invention is to provide an anaerobic digestion method capable of improving the digestion rate, that is, the digestion gas generation rate and reducing the volume of the digestion tank.

The anaerobic digestion method of the present invention is characterized by using dehydrated sludge in place of conventional dilute sludge, dehydrating raw sludge to a solid content concentration of at least 10%,
The dehydrated raw sludge is added to the digested sludge and uniformly kneaded, and the kneaded sludge is subjected to methane fermentation at a predetermined temperature.

In the present invention, dehydration of raw sludge is preferably carried out at a solid content concentration of 15% or more, more preferably 20% or more.

In this invention, the dehydrated raw sludge is preferably heat-treated at a temperature of 120 to 180 ° C. before being added to the digested sludge.

In this case, it is particularly preferable that the methane fermentation is carried out in a high temperature fermentation region of 50 to 55 ° C where the fermentation rate is high.

Further, the present invention can be applied to a so-called two-phase anaerobic digestion method in which methane fermentation is separated into an acid production step and a methane production step. In this case, the raw sludge is dehydrated to a solid content concentration of 10% or more, the dehydrated raw sludge is added to the digested sludge after the acid-producing fermentation and uniformly kneaded, and the kneaded sludge is acid-generated and fermented, and then acid-fermented. The product of productive fermentation is introduced into a methanator to produce methane. That is, the product of acid production fermentation is
The digested sludge after the acid-producing fermentation is separated with a dehydrator as a desorbed liquid. The production of acid-producing fermentation contained in the obtained desorbed liquid is brought into contact with the immobilized biocatalyst in the methane fermenter to cause methane-producing fermentation.

Here, the acid-producing fermentation is a process of converting organic matter from sludge into organic acid,
Fermentation that produces alcohol and carbon dioxide. Even in this two-phase anaerobic digestion method, the performance is further improved if the raw sludge is preheated and fermented at a high temperature.

By adopting the above two-layer digestion method, the kneading conditions of viscous raw sludge and seed sludge can be relaxed, and the efficiency of methane fermentation can be further improved.

When the solid content concentration of sludge becomes 4 to 6% or more, the viscosity sharply increases, and when it exceeds 10%, the consumption of stirring power becomes excessive in a normal methane fermentation tank equipped with a stirrer, and the economical efficiency is lost. Further, if it exceeds 15%, it becomes impossible to stir with a general stirrer. Therefore, it has been conventionally considered that the practical concentration is 4 to 6% or less. Therefore, conventionally, methane fermentation has not been performed using living mud having a solid content concentration of 10% or more, and it has been considered that such a thing is impossible.

The present inventor has made it possible to efficiently perform methane fermentation by sufficiently increasing the microbial cell concentration even in a raw sludge having a solid content of 20% or more, and by uniformly mixing the microbial cells in the raw sludge. Found out. That is, the present invention is a high-concentration seed sludge, that is, a small amount of high-concentration raw sludge is added to the digested sludge, the mixture is uniformly mixed, methane fermentation by maintaining a constant temperature in the digestion tank I am doing it.

According to the present invention, since raw sludge dehydrated to a solid content concentration of at least 10% or more is used, a digester tank having a smaller capacity than before can be obtained. Further, when the digestion layer having the same capacity as the conventional one is used, a larger amount of raw sludge can be digested than the conventional one.

Further, according to the present invention, since the solid content concentration of sludge is high, the amount of water to be heated becomes smaller than in the conventional case. Therefore, in terms of heat balance, it is possible to adopt a heat treatment method of sludge or a high temperature fermentation method which has not been practically used in the past.

The sewage treatment plant may be equipped with a digestion gas power generation facility. According to the present invention, the sludge can be heated by using the exhaust heat generated from this power generation system, so that the present invention is particularly useful in such a treatment plant.

Description of the Preferred Embodiment Experiment of Fill and Draw Method The present invention was examined by the Fill and Draw method.

The seed sludge used was digested at 50 to 50 ° C and dehydrated to a solid concentration of 15%. The raw sludge used was dehydrated to a solid concentration of 21%. As heat treatment sludge,
The raw sludge used was heat-treated at about 170 ° C for 30 minutes.

Example 1 200 g of seed sludge and 2.6 g of raw sludge were placed in a completely sealed experimental kneader having a gas outlet with an internal volume of about 1000 ml, and the inside of the kneader pot was replaced with nitrogen gas. Then, after kneading for 5 minutes, the kneader was placed in a constant temperature water bath at 53 ° C. and allowed to stand. The generated digested gas was collected in a collection bottle, and the amount of gas generated and the methane content were measured the next day. Next, 2.6 g of sludge was extracted from the kneader pot, 2.6 g of fresh sludge was newly added, and the mixture was kneaded again for 5 minutes. This operation was repeated once a day. The experiment was continued for 16 days. The gas generation amount is shown in FIG. 1, and the methane content of the generated gas is shown in FIG.

Comparative Example 1 200 g of seed sludge and 2.6 g of raw sludge were placed in a 300 ml Erlenmeyer flask, and the sludge was stirred and mixed with a glass rod while displacing the inside of the flask with nitrogen gas. The Erlenmeyer flask was covered with a rubber stopper having a digestion gas outlet, placed in a constant temperature water bath at 53 ° C., and allowed to stand. The generated digestive gas was collected in a collection bottle, and the methane content was measured the next day. Next, 2.6 g of sludge was extracted from the flask, 2.6 g of fresh sludge was newly added, and the mixture was again stirred with a glass rod to mix. This operation was repeated once a day. The experiment was conducted for 16 days. The obtained results are shown in FIGS. 1 and 2.

Example 2 Instead of the raw sludge used in Example 1, heat-treated sludge was used, and the gas generation amount and the methane content of the generated gas were measured in the same manner as in Example 1. The results are shown in FIGS. 1 and 2.

Comparative Example 2 Instead of the raw sludge used in Comparative Example 1, heat-treated sludge was used, and the heat-treated sludge was mixed with seed sludge and digested in the same manner as in Comparative Example 1 to measure the gas generation amount and the methane content. . The obtained results are shown in FIGS. 1 and 2.

As shown in FIG. 1, Example 1 in which the sludge was sufficiently mixed using the experimental kneader exhibited a much higher gas generation amount than Comparative Example 1 in which the stirring was insufficient. This gas generation amount is the gas generation amount per 1 g of organic matter in sludge.
The gas generation amount of Example 1 was almost the same level as the gas generation amount in the case of digestion which was performed at a low concentration in the related art.

In Example 2, raw sludge was heat-treated in advance. First
As shown in the figure, the gas generation amount of Example 2 is higher than that of Example 1, and it can be seen that the heat treatment improves the digestibility and generates a large amount of gas. From comparison with Comparative Example 2 using heat-treated sludge, it can be seen that even in this case, it is necessary to sufficiently mix the sludge.

Bench Scale Experiment FIG. 3 is a process chart for the first specific explanation according to the present invention. Referring to FIG. 3, the raw sludge dehydrated to 10% or more is added to the kneader 1. In Knead 1,
The digested sludge discharged from the digestion tank 2 is circulated as seed sludge, and the raw sludge is mixed in this digested sludge. As the kneader 1, a kneader capable of kneading a high viscosity is used, and for example, a kneader, a ribbon mixer, a guillotine mixer or the like can be used. The sludge obtained by kneading the raw sludge into the digested sludge in the kneader 1 is then sent to the digestion tank 2. In the digestion tank 2, digestion is performed and the generated digestion gas is extracted. Sludge that has been digested for a predetermined time,
It is added to the kneader 1 again, and a part is discarded. Knead 1
The larger the mixing ratio of digested sludge / raw sludge in the above, the more stable results can be obtained. Depending on the efficiency of the kneader, 1 to 20 is actually used.

It is said that methane fermentation produces methane through two processes, a process of producing organic acids and alcohols from organic substances of substrates in sludge and a process of producing methane from organic acids and alcohols. Is the bacterial concentration in the raw sludge low?
Alternatively, when the mixed state of raw sludge and seed sludge is non-uniform and the bacterial cell concentration is locally low, the production of organic acids and alcohols becomes dominant, and the activity of methanogenic bacteria is suppressed. Regarding the rate of methane fermentation, it is said that the methane production process is rate-determining, and methanogens are very sensitive to pH and organic acid concentration. Therefore, it is necessary to keep the concentration of methanogens in the raw sludge high and knead the raw sludge and the seed sludge so that the concentration of organic acids or alcohols around the methanogen does not exceed a certain limit. Is. When mixing the raw sludge and the seed sludge, it is preferable to increase the amount of seed sludge to be circulated to increase the initial concentration of the bacterial cells so that the organic acid and alcohol concentrations do not become excessive. This balances the symbiotic relationship between acid-producing and methanogenic bacteria.

FIG. 4 is a process chart for a second specific explanation according to the present invention, showing a specific example of heat treatment of raw sludge before kneading with seed sludge. With reference to FIG. 4, the dehydrated raw sludge is supplied to the sludge preheater 3 and preheated. The raw sludge is then fed into the heater 4 and heated to a temperature of 120-180 ° C. The heater 4 can be heated by steam, for example. For such steam, steam recovered from waste heat of digestion gas power generation installed in a sewage treatment plant can be used. The heated sludge is then put in the first flash tank 5. Here the raw sludge is cooled to 100 ° C. The saturated steam generated in the first flash tank is guided to the sludge preheater 1 and used for preheating the raw sludge. Next, the raw sludge is put into the second flash tank 6 and cooled to, for example, 55 ° C.

The cooled raw sludge is supplied into the kneader 1 and kneaded with the digested sludge circulating as seed sludge. The kneaded sludge is then sent to the digestion tank 2 at a predetermined temperature, for example 50 to 55.
Methane fermentation is carried out at ° C. The generated digestion gas is taken out from the outlet, the sludge that has been digested for a predetermined time is partially discarded, and the rest is circulated to the kneader 1.

This bench-scale experiment was conducted based on the process shown in FIG. A tube type digester as shown in FIG. 5 was used as the digester. Referring to FIG. 5, a tube 12 is provided inside the jacket 11. tube
A sludge supply nozzle 13 and a digestion gas discharge port 15 are provided at the upper end of 12. At the bottom of the jacket 11, the sludge discharge port 14
Is provided. The sludge kneaded by the kneader 1 with a heating jacket is supplied from the mud supply nozzle 13 into the tube 12. The inside of the tube 12 is kept at a predetermined temperature by the jacket 11. In the tube 12, sludge moves downward while digesting. Tube within a certain period
The sludge that has moved inside 12 is then discharged from the sludge discharge port 14. The gas generated by digestion is discharged from the digestion gas discharge port 15 provided at the top of the tube 12.

As the tube 12, a tube having a diameter of 160 m / m and a length of 3500 m / m was used. The jacket 11 was kept warm by flowing warm water so that the temperature inside the tube 12 became 50 to 55 ° C. The sludge supply rate from the sludge supply nozzle 12 was adjusted so that the sludge supplied from the sludge supply nozzle 13 could pass through the tube 12 in one day.

In Kneader 1 heated to 50-55 ℃, 0.84kg of seed sludge, that is, 12.0kg of digested sludge, was preliminarily heat-treated with raw sludge with a solid content of 21% at 160 ℃ for 30 minutes. Then, the sludge was supplied into the tube 12 once every 6 hours from the sludge supply nozzle 13.

As described above, this bench scale experiment was performed for 15 days, and the amount of gas generated by digestion and the methane content in the generated gas were measured. As a result, according to this system, about 750 gases were generated per kg of organic matter in the raw sludge. With conventional normal anaerobic digestion, organic matter 1k
Since the gas generation amount is about 500 per gram, the gas generation amount obtained by this system is a considerably high value. In addition,
The methane content was about 60%, which was almost the same as the conventional anaerobic digestion method. Compared with the conventional digestion method, the amount of gas generated per unit organic matter was significantly improved, and it is clear that the digestion method of the present invention is an excellent method even from bench scale experiments, which are more realistic conditions. . FIG. 6 is a partially cutaway sectional view showing an example of a digestion tank when the present invention is applied to a large-scale plant.
In this invention, since the sludge has a high viscosity, digestion in a conventional stirring tank cannot be performed. Therefore, one of the effective methods is to knead well in advance with a kneader and then to digest while moving in a sludge tube as shown in FIG. Therefore, in a large-scale plant, a device having a large number of tubes as shown in Fig. 6 can be used. Referring to FIG. 6,
A plurality of tubes 22 are contained in the jacket 21. The tube 22 has a diameter of 0.2 to 1.0 m and a length of 10 to 30 m, for example. A sludge supply port 23 is provided at the bottom of the jacket 21, and the sludge supply nozzle 23 is provided with a sludge distribution nozzle 24 corresponding to each tube 22. A warm water inlet 27 and a sludge discharge port 25 are provided at the bottom of the jacket 21. A digestion gas outlet 26 and a hot water inlet 28 for extracting digestion gas are provided in the upper portion of the jacket 21.

Sludge supplied from the sludge supply port 23 is a sludge distribution nozzle.
By 24, they are extruded into their respective tubes 22. In the tube 22, the sludge is gradually extruded and rises, and at the upper end of the tube 22, the sludge overflows, passes through the outer wall of the tube 22, and then moves downward. The sludge that has moved downward through the outer wall of the tube 22 is discharged through the sludge discharge port 25, part of which is discarded, and the rest is circulated to the kneader.

The sludge digests while rising in the tube 22 and down the outer wall of the tube 22. By adopting such a method, a long retention time can be secured even with a short tube. In addition, in the outer wall of the jacket 21,
The hot water supplied from the hot water inlet 27 is passed through, and the hot water exits from the hot water outlet 28. With this warm water, the jacket 21
The inside of is kept at a predetermined temperature.

By using a digestion tank having a plurality of tubes as shown in FIG. 6, it is possible to make the movement speed of sludge in the tubes uniform and prevent uneven movement of sludge such as short-circuit flow. be able to. In addition, by using a digestion tank with multiple tubes and moving sludge along the walls of the tubes, methane fermentation can be performed effectively even when stirring cannot be performed, and the amount of stirring power consumed It is also possible to avoid losing the economy due to excessive cost.

However, the digester shown in FIG. 6 is an example of the digester that can be used in the present invention, and the present invention is not limited to the use of the digester shown in FIG.

The present invention can also be applied to a two-phase anaerobic digestion method in which methane fermentation is separated into an acid production step and a methane production step as described above. FIG. 7 is a process chart for specific description of the case where the present invention is applied to such a two-layer anaerobic digestion method. With reference to FIG. 7, the dehydrated raw sludge is supplied to the kneader 31. Here, the dehydrated raw sludge is kneaded with the circulated digested sludge. The kneaded sludge is sent to the acid fermentation digestion tank 32, and only the acid production step in the methane fermentation process is performed. A part of the digested sludge after fermentation is dehydrated by the dehydrator 33 and then discarded. The rest is circulated to Kneader 31. By the acid production fermentation, the organic matter in the raw sludge is converted into organic acid, alcohol, carbon dioxide gas or the like and solubilized. For this reason, organic substances and the like generated in the acid generation step are dissolved in the desorbed liquid. The effluent is then fed into a methanogenic fermentor 44 with immobilized biocatalyst to carry out the methanogenic fermentation. Methane production fermentation is performed here, the generated methane gas is taken out, and the desorbed liquid processed in the methanogenic fermentation tank 34 is discarded. As the structure of the acid fermentation digestion tank 32, a tube type as shown in FIGS. 5 and 6 is used.

Furthermore, as another specific example of the two-phase anaerobic digestion method, there is one in which the raw sludge is preheated to a temperature of 120 to 180 ° C. by the method as shown in FIG. 4 and then the two-phase anaerobic digestion is performed. . In this case, fermentation is preferably carried out in a high temperature range of 50 to 55 ° C. This will further improve the efficiency of the two-phase anaerobic digestion.

[Brief description of drawings]

FIG. 1 is a diagram showing a gas generation amount in an experiment by the Fill and Draw method. FIG. 2 is a diagram showing the methane content of the gas generated in the experiment by the Fill and Draw method. FIG. 3 is a process chart for the first specific explanation according to the present invention. FIG. 4 is a process chart for the second specific description according to the present invention. FIG. 5 is a perspective view showing a digester used in a bench-scale experiment according to the process shown in FIG. FIG. 6 is a partially cutaway perspective view of a digestion tank when the present invention is applied to a large-scale plant. FIG. 7 is a process chart for the third specific explanation according to the present invention.

Claims (4)

[Claims] 1. A method for anaerobic digestion of sewage sludge, which comprises discarding a part of digested sludge after methane fermentation, circulating the rest, adding raw sludge to the circulating digested sludge, and performing methane fermentation. The sludge is dehydrated to a solid content concentration of at least 10% or more, the dehydrated raw sludge is added to the circulating digested sludge and uniformly kneaded, and the sludge obtained by the kneading is distributed to a plurality of tube-type digester tanks. Supply and move sludge in each tube-type digester along the wall of the tube while performing methane fermentation at a predetermined temperature, and discarding part of the sludge discharged from the tube-type digester by the transfer. A method for anaerobic digestion of sewage sludge, characterized in that the remainder is kneaded as the circulating digested sludge with the drained raw sludge. 2. The sludge obtained by kneading is extruded into each tube-type digestion tank by a mixing / distributing nozzle, and the sludge is gradually raised in each tube-type digestion tank while causing the sludge to overflow at the upper end thereof. Characterized in that the overflowed sludge is caused to descend along the outer wall of the tube-type digestion tank, a part of the descended sludge is discarded, and the rest is kneaded with the dehydrated raw sludge as the circulation digested sludge. An anaerobic digestion method for sewage sludge according to claim 1. 3. The sewage according to claim 1, wherein the dehydrated raw sludge is heat-treated at a temperature of 120 to 180 ° C., and the heat-treated raw sludge is added to the digested sludge. Anaerobic digestion method of sludge. 4. The methane fermentation is carried out separately into an acid producing step of producing mainly an acid and a methane producing step of producing methane from the components produced in the acid producing step, and the digestion produced in the acid producing step. It is a two-phase anaerobic digestion method for sewage sludge in which a part of sludge is discharged to be sent to the methane production step, the remaining digested sludge is circulated, and raw sludge is added to the circulated digested sludge to perform the acid production step. The raw sludge is dehydrated to a solid content concentration of at least 10% or more, the dehydrated raw sludge is added to the circulating digested sludge and uniformly mixed, and the sludge obtained by the kneading is formed into a plurality of tube types. It is distributed and supplied to the digester, and sludge is moved along the wall of the tube while acid-producing and fermenting the sludge in each tube-type digester at a predetermined temperature. A two-phase anaerobic digestion method for sewage sludge, comprising sending a part of mud to the methane production step and kneading the rest as the circulating digested sludge with the dehydrated raw sludge.