JP4863545B2 - Method and apparatus for digesting organic sludge - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for digesting organic sludge using hot alkali pretreatment.
[0002]
[Prior art]
An anaerobic digestion treatment method is known as one of the treatment methods for organic sludge generated from sewage treatment plants. This method has the advantage that organic sludge can be decomposed in an anaerobic digester in an anaerobic digester to simultaneously decompose organic matter and at the same time recover methane gas, which is a valuable resource.
[0003]
The present inventor has improved the treatment capacity in the anaerobic digestion tank by promoting the solubilization of organic matter by installing a hot alkali pretreatment tank in the previous stage of the anaerobic digestion tank and treating the organic sludge with the heat alkali treatment. A number of patent applications have already been filed, such as Japanese Patent Application Laid-Open No. 06-099199, Japanese Patent Application Laid-Open No. 04-326998, Japanese Patent Application Laid-Open No. 06-071297, and the like.
[0004]
However, even if solubilization of organic substances is promoted by this method, anaerobic digestion cannot be completely performed if the concentration of the organic sludge to be treated becomes too high. For this reason, in the past, the organic sludge was operated with the VS (total organic matter concentration in the sludge) suppressed to less than 2%, and it was difficult to increase the organic matter load of the anaerobic digester to 2 kg / m ³ · day or more. Met.
[0005]
[Problems to be solved by the invention]
The present invention solves the above-mentioned conventional problems, can further increase the organic load of the anaerobic digester, and can further promote resource recovery from the organic sludge. It is made in order to provide the processing method and apparatus.
[0006]
As a result of repeated studies to solve this problem, the present inventors have focused on ammonia in the liquid, which is one of the factors that inhibit methane fermentation in an anaerobic digester. As for ammonia, it is said that free ammonia in particular inhibits methane fermentation. And, by increasing the VS of organic sludge from the conventional level and setting the pH of the hot alkali pretreatment tank to 10 or more, ammonia gas can be generated in the hot alkali pretreatment tank or its subsequent stage, and ammonia can be separated from the liquid. discovered. In addition, it was discovered that the solubilization rate of organic substances in sludge is improved by setting the pH to 10 or more.
[0007]
[Means for Solving the Problems]
The organic sludge digestion treatment method of the present invention has been completed based on the above findings, and the step of centrifugally concentrating sludge such that the total organic matter concentration (VS) in the sludge exceeds 2%, The organic sludge having a high concentration exceeding VS of 2% is introduced into a hot alkali pretreatment tank having a pH of 10 or more and a temperature of 35 to 100 ° C., and the organic matter contained in the organic sludge is allowed. The process of promoting solubilization, the process of promoting the solubilization of the organic matter, the ammonia nitrogen produced at the same time is guided to an ammonia removal device, and separated and removed from the pretreated sludge as ammonia gas, and the ammonia gas is separated and removed the pretreatment sludge after, pH is Ri Do and a step of methane fermentation is introduced into the anaerobic digestion tank is controlled to 7-8.5, between the ammonia removal apparatus and heat the alkali pretreatment reservoir By pump Is characterized in that the circulating part of the sludge.
Ammonia nitrogen produced simultaneously in the process of promoting solubilization of the organic matter is guided to an ammonia removal device, and as a process of separating and removing ammonia gas from the pretreated sludge, gas purging with anaerobic atmosphere gas, suction, gas-liquid contact It can be performed by any one of the following means.
[0008]
In addition, the organic sludge digestion treatment apparatus of the present invention comprises a hot alkali pretreatment tank for treating high-concentration organic sludge at a pH of 10 or more and a temperature of 35 to 100 ° C. , and this hot alkali pretreatment. Ammonia removal device for separating and removing generated ammonia gas from pretreatment sludge, a circulation path for circulating a part of sludge by a pump between the ammonia removal device and the hot alkali pretreatment tank, and separation and removal of ammonia gas It consists of an anaerobic digester which carries out methane fermentation of the pre-treatment sludge after having been pH 7-8.5 .
[0009]
According to the organic sludge digestion treatment method and apparatus of the present invention described above, ammonia, which is an inhibitory factor of methane fermentation in an anaerobic digester, can be removed from the liquid in the previous stage, so the activity of methane fermentation bacteria is increased. The organic load in the anaerobic digester can be significantly increased as compared with the conventional case. Moreover, since ammonia gas can be recovered from organic sludge as well as methane gas, there is a great advantage from the viewpoint of resource recovery.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described below. First, a reference embodiment will be described with reference to FIG.
In FIG. 1, 1 is a hot alkali pretreatment tank, and 2 is an anaerobic digestion tank. Organic sludge such as primary sludge generated from a sewage treatment plant, excess sludge, and protein-containing sludge generated from a food factory or the like first flows into the hot alkali pretreatment tank 1 by a pump or the like. As shown below, the preferred conditions for each tank differ depending on the type of sludge. This is because the primary sludge is mainly composed of carbohydrates, and surplus sludge (as well as protein-containing sludge) is mainly composed of proteins. It is.
[0011]
As described above, VS of inflowing organic sludge (inflow sludge) has been conventionally suppressed to less than 2%. However, in the present invention, high-concentration organic sludge having a VS of over 2% is used before hot alkali. It flows into the processing tank 1. In general, when the higher concentration sludge is digested, the ammonia concentration becomes higher and the inhibition to the methane fermentation reaction becomes remarkable, but in the present invention, in order to provide an ammonia removal device, the more concentrated organic sludge is targeted, Compared with a normal high-concentration digestion method, the effect in the methane fermentation reaction is increased. Therefore, it is preferable to set the VS to 3% or more in the case of an easily settled initial settling sludge and to set the VS to 2% or more in the case of excess sludge.
[0012]
Moreover, in this invention, in connection with having raised VS of inflow sludge from the past, pH of the hot alkali pretreatment tank 1 shall be 10 or more higher than before. In the case of primary sludge, the pH is preferably about 10 to 12, and in the case of excess sludge, the pH is preferably about 10 to 11. An alkali such as NaOH is added to the hot alkali pretreatment tank 1 so that such a pH is maintained. Moreover, the hot alkali pretreatment tank 1 is heated to 35 to 100 ° C. by an appropriate heating means. Specifically, about 35 ° C. is preferable in the case of initial settling sludge, and about 70 ° C. is preferable in the case of excess sludge.
[0013]
When the organic sludge is subjected to hot alkali treatment under such conditions, the organic matter in the organic sludge is solubilized as in the conventional case, but at the same time, ammoniacal nitrogen is generated. Therefore, the ammonia nitrogen is separated and removed from the liquid as ammonia gas by the ammonia removing device 5. This ammonia gas can be reused as a valuable resource, for example, for denitration of a sludge treatment plant.
[0014]
Various methods can be considered as a method for separating ammonia gas. For example, as shown in FIG. 2, there is a method in which ammonia gas accumulated in the hot alkali pretreatment tank 1 or ammonia nitrogen in sludge is taken out by insoluble anaerobic, for example, nitrogen gas purge. Here, anaerobic nitrogen gas was used instead of air in order to prevent the anaerobic digestion tank 2 in the subsequent stage from becoming an aerobic atmosphere and causing the activity of the anaerobic bacteria to decrease. Therefore, generally, even if an insoluble aerobic atmosphere gas such as air is initially sealed, an anaerobic atmosphere is formed by a long-term operation. Therefore, these gases can be used in the long term.
[0015]
Moreover, as shown in FIG. 3, the method of attracting | sucking the ammonia gas collected in the inside of the hot alkali pretreatment tank 1 with the blower 3 is also employable. According to this method, ammonia gas can be taken out alone, but if the contact area at the gas-liquid interface is small, ammonia removal may not be sufficiently performed and the rate may be limited.
[0016]
Furthermore, as shown in FIG. 4, ammonia gas can also be taken out by gas-liquid contact. In the method of FIG. 4, the pretreated sludge that has flowed out of the hot alkali treatment tank 1 is brought into contact with an anaerobic atmosphere gas or the like blown from below while flowing down along a multistage flow path, and gas (ammonia gas + anaerobic) Atmosphere gas) and pretreatment sludge can be separated. However, according to the method shown in FIGS. 2 to 4, there is a disadvantage that the pH of the hot alkali pretreatment tank 1 is lowered by the separation of ammonia from the liquid, and the solubilization rate of organic substances in the sludge, which is the original purpose, is lowered.
[0017]
Therefore, as a more preferable separation method, there is a method of providing a U-shaped gas seal 4 before and after the ammonia removing device 5 as shown in FIGS. In the example of FIG. 5, ammonia gas is purged by the blower 3 from the ammonia removing device 5 provided in the intermediate portion. In the example of FIG. 6, ammonia gas is sucked by the blower 3. Further, in the example of FIG. 7, a gas-liquid contact device is used as the ammonia removing device 5, and ammonia gas is separated by gas-liquid contact. According to these methods, there is an advantage that the influence of the pH decrease accompanying ammonia removal does not easily reach the hot alkali pretreatment tank 1. In FIG. 1, these various separation means are shown as an ammonia removing device 5. Naturally, when the ammonia removing device is provided in this way, the ammonia removal proceeds more efficiently at a temperature equal to or higher than that of the hot alkali pretreatment tank. Therefore, heating with jacket water or the like is considered. Further, the pH of the ammonia removing device is lower than the pH of the hot alkali pretreatment tank due to the influence of ammonia removal.
[0018]
In the examples of FIGS. 2, 4, 5, and 7, since the gas purge is performed using an anaerobic atmosphere gas such as nitrogen, the gas flow is a closed circulation system as shown in FIG. It is desirable to provide an absorption tank for absorbing ammonia.
[0019]
The sludge after the pretreatment from which the ammonia gas has been separated and removed in this manner is sent to the anaerobic digestion tank 2 and subjected to methane fermentation in the same manner as before. However, unlike the conventional case, ammonia is previously removed from the liquid, so the methane fermentation is not inhibited in the anaerobic digester 2, and the efficiency of the influent sludge is increased despite the fact that the VS of the influent sludge is increased compared to the conventional case. Methane fermentation is often performed. Methane gas generated at this time is recovered as a valuable resource.
[0020]
In addition, in order to advance methane fermentation efficiently, it is preferable to make pH% of the anaerobic digester 2 7-8.5. More specifically, it is desirable that the initial sludge is about 7.0 to 7.8, and the excess sludge is about 7.8 to 8.1. The temperature is about 35 ° C. for medium temperature fermentation and about 55 ° C. for high temperature fermentation. Thus, the pH of the anaerobic digester 2 is naturally lowered without any special operation. The reason is that carbon dioxide gas generated by methane fermentation dissolves in the liquid and shifts the digested sludge to the acidic side.
[0021]
As described above, according to the present invention, methane fermentation is efficiently performed despite the fact that the VS of the influent sludge is increased as compared with the prior art, so the organic load in the anaerobic digester 2 is reduced to the conventional 2 kg / m ^3. from-days to more than ^3-days 3kg / ^m can be greatly increased. In addition, if this method is applied, when there is a two-stage anaerobic digestion tank, it is possible to provide an ammonia removing device 5 between them to activate the anaerobic digestion at the latter stage. Alternatively, it is possible to circulate a part of the digested sludge flowing out from the anaerobic digestion tank 2 to the ammonia removing device 5 to promote the removal of ammonia and activate the anaerobic digestion.
[0022]
In the reference form described above, as shown in FIG. 1, the pretreatment sludge after removing ammonia is introduced into the anaerobic digestion tank 2 by being naturally flowed from the hot alkali pretreatment tank 1 to the downstream ammonia removal device 5. I let it flow down naturally. However, in the present invention, as shown in FIGS. 9 and 10, a part of the sludge is circulated between the ammonia removing device 5 and the hot alkali pretreatment tank 1 by a pump or the like. By removing ammonia from the pretreatment sludge in this way, the equilibrium of ammonium salt, ammonium ions, and non-dissociated ammonia in the hot alkali pretreatment tank 1 is shifted to the non-dissociated ammonia side, and a larger amount of ammonia is removed in the ammonia removing device. Gas separation becomes possible. The ammonia removing device 5 may be provided in parallel with the hot alkali pretreatment tank 1 as shown in FIG. 10 even at the rear stage of the hot alkali pretreatment tank 1 as shown in FIG.
[0023]
The experimental results regarding ammonia removal are shown below. The experimental apparatus used is a combination of a 1 L capacity hot alkali pretreatment tank and a 10 L capacity anaerobic digestion tank combined with an ammonia removal apparatus. First, the excess sludge from the sewage treatment plant is centrifugally concentrated to give organic sludge with a VS concentration of 3%, and placed in a hot alkali pretreatment tank controlled at pH 10.5 and temperature 70 ° C at a rate of 1 liter per day. And processed. At this time, 400 mg-N ammonia gas was generated per day in the ammonia removing device, and this was separated and recovered.
[0024]
When the pretreated sludge thus obtained was placed in an anaerobic digester controlled at pH 8.0 and temperature of 37 ° C. and treated for 10 days, digestion of methane progressed efficiently and digestion of organic matter was achieved. The rate was 56%. Further, 10 L / day of methane gas and 1 L / day of carbon dioxide gas were recovered. The organic load in an anaerobic digester with a capacity of 10 L is 30 g per day, which is 3 kg / m ³ · day when converted to a unit normally used.
[0025]
Next, the test by the conventional method which does not isolate | separate ammonia by the same load using the same experiment apparatus was done. Excess sludge from the sewage treatment plant was adjusted to 3% VS and placed in a hot alkali pretreatment tank controlled at pH 10.5 and temperature 70 ° C at a rate of 1 L per day. There was no gas evolution during the treatment. This pretreated sludge was put in an anaerobic digester controlled at pH 8.0 and temperature 37 ° C. and subjected to methane fermentation treatment for 10 days. In this case, the digestibility of the organic matter was 40%. Further, 7.0 L / day of methane gas and 0.7 L / day of carbon dioxide gas were recovered. It can be judged that the fact that there was little generation of methane gas due to the absence of an ammonia removal device was the result of ammonia inhibition.
[0026]
Furthermore, tests were conducted by the conventional method which does not separate the ammonia in a low load of excess sludge VS2% 2kg / m 3 ^· day (low density) using the same experimental apparatus. In the hot alkali pretreatment tank controlled at pH 9.0 and temperature of 70 ° C., it was processed at a rate of 1 L per day, but no gas was generated in the hot alkali pretreatment. This pretreated sludge was subjected to methane fermentation treatment for 10 days in an anaerobic digester controlled at pH 8.0 and temperature of 37 ° C. In this case, the organic matter digestibility was 50%. Further, 5.8 L / day of methane gas and 0.58 L of carbon dioxide gas were recovered.
[0027]
【Effect of the invention】
As described above, according to the organic sludge digestion treatment method and apparatus of the present invention, the organic matter load of the anaerobic digester that has been limited to about 2 kg / m ³ · day is reduced to 3 kg / m ³ · It can be greatly increased to about a day. At the same time, since a large amount of methane gas and ammonia gas can be recovered from the organic sludge, there are excellent effects from the viewpoint of resource recovery and the prevention of global warming. In particular, if an ammonia removal device equipped with a gas seal is used in advance, the generation of ammonia gas from the hot alkali pretreatment tank is suppressed, so that the pH does not decrease in the hot alkali pretreatment tank, and organic substances in sludge are allowed. There is an advantage that a decrease in the dissolution rate can be prevented. In particular, if an ammonia removal device equipped with a gas seal is used later, the ammonia gas generated in the ammonia removal device can be prevented from moving to an anaerobic digester and re-dissolved, and the ammonia of the digested gas generated in the anaerobic digester It is possible to more completely recover the digestion gas by suppressing the movement to the removal device.
[Brief description of the drawings]
FIG. 1 is a block diagram showing steps of a reference example .
FIG. 2 is an explanatory view showing an ammonia separation method by gas purging.
FIG. 3 is an explanatory diagram showing an ammonia separation method by suction.
FIG. 4 is an explanatory diagram showing an ammonia separation method by gas-liquid contact.
FIG. 5 is an explanatory diagram showing an ammonia separation method combining a gas seal and a gas purge.
FIG. 6 is an explanatory view showing an ammonia separation method combining gas sealing and suction.
FIG. 7 is an explanatory diagram showing an ammonia separation method combining a gas seal and gas-liquid contact.
FIG. 8 is an explanatory diagram showing an ammonia separation / recovery method using an ammonia removal device and an ammonia absorption device.
FIG. 9 is a block diagram showing a process of the present invention .
FIG. 10 is a block diagram showing another arrangement example of the ammonia removing device.
[Explanation of symbols]
1 thermal alkali pretreatment tank, 2 anaerobic digestion tank, 3 blower, 4 gas seal, 5 ammonia removal device

Claims (4)

Centrifugal concentration of the sludge so that the total organic matter concentration (VS) in the sludge exceeds 2%;
The organic sludge having a high concentration exceeding VS of 2% is introduced into a hot alkali pretreatment tank having a pH of 10 or more and a temperature of 35 to 100 ° C., and the organic sludge contained in the organic sludge A step of promoting solubilization;
A step of introducing ammonia nitrogen simultaneously generated in the step of promoting solubilization of the organic matter to an ammonia removing device, and separating and removing it as pretreatment sludge as ammonia gas;
The pretreatment sludge after ammonia gas separated off, Ri Do and a step of methane fermentation pH is introduced into the anaerobic digestion tank is controlled to 7-8.5,
A method for digesting organic sludge, wherein a part of the sludge is circulated by a pump between the ammonia removing device and the hot alkali pretreatment tank . The step of introducing ammonia nitrogen simultaneously generated in the step of promoting solubilization of the organic substance to an ammonia removing device and separating and removing it from the pretreated sludge as ammonia gas,
2. The method for digesting organic sludge according to claim 1 , wherein any one of gas purging, suction, and gas-liquid contact with an anaerobic atmosphere gas is used. A hot alkali pretreatment tank for treating high-concentration organic sludge under conditions where the pH is 10 or more and the temperature is 35 to 100 ° C . ;
An ammonia removal device for separating and removing ammonia gas generated by this hot alkali pretreatment from the pretreatment sludge;
A circulation path for circulating a part of sludge by a pump between the ammonia removing device and the hot alkali pretreatment tank;
An apparatus for digesting organic sludge, comprising an anaerobic digester for methane fermentation of pretreated sludge after separation and removal of ammonia gas under conditions of pH 7 to 8.5 .   The organic sludge digestion treatment apparatus according to claim 3, wherein the ammonia removal apparatus includes a gas seal before and after the ammonia removal apparatus.

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