JP2005254054A - Method and system for anaerobic digestion of organic sludge - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anaerobic digestion method for organic sludge which enables efficiency improvement of operations from a digestion process to a dehydration process, such as improvement of a digestion rate and reduction of the amount of sludge subjected to final treatment in comparison with conventional anaerobic digestion methods, without increasing water treatment load mainly in a sewage treatment plant. <P>SOLUTION: In addition to a process for discharging a part of digested sludge in a digestion tank 2 and dehydrating it with a centrifugal dehydrator 4 to discard it, a series of processes for pulling out a part of the digested sludge from the digestion tank 2 to concentrate the sludge with a centrifugal thickener 5 and heating the concentrated sludge in a heating tower 6 to circulate the sludge to the digestion tank 2 is installed, and the sludge is anaerobically digested at high temperature and high concentration in the digestion tank 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention provides anaerobic digestion by anaerobic bacteria such as raw sludge and biological sludge discharged from sewage treatment plants and organic high-concentration sludge discharged from food factories and chemical factories (hereinafter referred to as organic sludge). The present invention relates to a method and the same digestion system, and more particularly, to a method for digesting at a high temperature and high concentration and the same digestion system.

  In this type of prior art, for example, heated sludge is brought into contact with digested sludge and concentrated sludge in a heating tower to heat the sludge, and these heated sludge is supplied to a high-temperature digester and subjected to anaerobic digestion. In this method, the air discharged from the heating tower is supplied to at least one of a gas heat exchanger or a digestion gas combustion furnace, the heat is recovered as the heated air, and the organic sludge is supplied to the heating tower. A method has been proposed (see, for example, Patent Document 1).

In addition, after separating and separating waste water containing organic solids in a sedimentation tank, the supernatant liquid with a small amount of organic solids is anaerobically treated with an anaerobic treatment device, An anaerobic digestion method for high-concentration organic wastewater has been proposed in which a precipitated solid concentrate containing solids is solubilized at a high temperature, and then the solubilized solubilization treatment liquid is introduced into an anaerobic treatment apparatus ( For example, see Patent Document 2).
JP-A-11-319895 (paragraph number 0005, FIGS. 1 to 3) Japanese Patent Laid-Open No. 9-1179 (paragraph numbers 0014 and 0021, FIG. 1)

However, the above prior art digestion methods still have problems to be solved in the following points. That is,
In the former case, since heated air is brought into direct contact with the sludge, it is necessary to perform a deodorizing process when exhausting. In the heating process, the digested sludge from the digester is heated without being concentrated, and therefore requires a considerable amount of heat.

  In the latter case, the supernatant is separated into the supernatant liquid and the precipitated solid liquid in the precipitation tank, the supernatant liquid is subjected to anaerobic digestion, and the precipitated solid liquid is subjected to anaerobic digestion after being solubilized under high temperature conditions. Is complicated and takes many days to digest.

  The present invention has been made in view of the above-mentioned points, and the load of water treatment mainly in a sewage treatment plant, such as improving the digestibility compared with conventional anaerobic digestion methods and reducing the amount of sludge to be finally treated. The purpose is to provide a high-temperature, high-concentration anaerobic digestion method and an anaerobic digestion system for organic sludge that can improve the efficiency of the work from the digestion process to the dehydration process without increasing the amount.

  In order to achieve the above object, the anaerobic digestion method according to the present invention is a method in which organic sludge is introduced into a digestion tank and anaerobic digestion is performed by methane fermentation. Separately from the step of processing and discarding, a part of the digested sludge is extracted from the digestion tank and concentrated, and the digestion tank is provided with a series of steps for heating and circulating the concentrated sludge to the digestion tank. It is characterized by anaerobic digestion at high temperature and high concentration.

  Conventionally, sludge is heated in advance and put into the digestion tank. However, according to the organic sludge anaerobic digestion method of the present invention, the sludge is put into the digestion tank at room temperature and then digested in the digestion tank. A part of the sludge is extracted, concentrated, that is, water is removed and then heated, and the heated / high-concentration sludge is circulated to the digester again, so the temperature of the sludge in the digester is anaerobic due to high-temperature methane-producing bacteria It can be easily heated up to around 53 ° C. required for digestion, and the concentration of sludge becomes considerably high. Therefore, the digestion rate is improved to about twice that of conventional anaerobic digestion with medium temperature (around 40 ° C.) methanogen, and the working efficiency is greatly improved.

  As described in claim 2, concentrated digested sludge having a non-heated solid concentration (for example, 3%) is introduced into the digestion tank, a part of the digested sludge in the digestion tank is extracted, and the solids are extracted by a concentrator. After concentrating to a solid concentration of, for example, 5% so as to increase by several percent from the concentration, the sludge concentrated by a heating device is heated to near 100 ° C. (eg, 90 ° C.) and circulated through the digester, and the digester The amount of sludge circulated inside can be determined, for example, at around 53 ° C. so that the temperature of the sludge is maintained at a temperature at which high-temperature methane bacteria can be anaerobically digested.

  In this way, conventionally, if the solid concentration of the sludge to be introduced is about 3%, for example, the organic matter is digested by methane fermentation, and the organic matter concentration of the sludge drawn out from the digester is as low as about 2%. By extracting a part of the digested sludge, concentrating it, heating it to around 90 ° C and circulating it, the sludge in the digestion tank is concentrated to a high concentration and becomes high temperature, so it is digested at a high concentration and high temperature. The digestion rate is greatly improved, and the solid concentration of the digested sludge to be extracted is increased to nearly 4%, so that the dehydration efficiency becomes very high and the working efficiency is improved.

  As described in claim 3, the amount of heat necessary for the heating device is generated by the combustion heat of digestion gas generated in the digester.

  In this way, auxiliary fuel is unnecessary and economical.

  The organic sludge anaerobic digestion system according to claim 4 is an anaerobic digestion of organic sludge by methane fermentation in a digestion tank, and the digested sludge is pulled out and the sludge cake dehydrated by a centrifugal dehydrator is incinerated. In the digestion system to be discarded, a heating tower for heating a part of the digested sludge is provided on the downstream side of the digestion tank, and the digested sludge is concentrated between the heating tower and the digestion tank. A concentrator is interposed, and a circulation path for circulating the sludge heated in the heating tower to the digester is provided.

  According to the digestion system of this invention which has the said structure, the digestion method of Claim 1 can be implemented smoothly.

  The digested sludge concentrated in the heating tower by providing a digestion gas combustion furnace generated in the digestion tank and a heat exchanger that generates superheated steam by the combustion heat as described in claim 5 Can be heated by direct contact with superheated steam and returned to the heat exchanger in the form of steam without condensation.

  In this way, superheated steam is used as a heat medium, and sensible heat is given to the concentrated digested sludge by direct contact and moisture is not given. Therefore, highly efficient heating can be achieved and the sludge concentration is not lowered. Moreover, since the water vapor | steam which heats sludge circulates through the closed circuit of a heat exchanger and a heating tower, it is not necessary to deodorize.

  As described in claim 6, the centrifugal dehydrator for extracting and dewatering part of the digested sludge in the digester can also be used as the concentrator.

  According to the anaerobic digestion system of the sixth aspect, since the centrifugal dehydrator that has been used conventionally can be used as a concentrator, the equipment cost is reduced. Specifically, for example, the flocculant is drawn out during the day, and about 1% of the solid content of the sludge is thrown in. Centrifugal dehydration is performed to dispose of the digested sludge. Concentrate with, warm in a heating tower and return to digester. In addition, a flocculant having a solid content of about 0.5% is added to the separation liquid generated after concentration, and water treatment is performed.

  As described in claim 7, by concentrating a part of the digested sludge with the concentrator and removing the separation liquid, the digester can have a function as a temporary storage tank for sludge.

  If it does in this way, the said digestion tank can be used as a temporary storage tank of sludge.

Since the digestion method and digestion system according to the present invention are configured as described above, they have the following excellent effects. That is,
1) Because sludge is anaerobically digested at high temperature and high concentration, digestion rate is improved to about twice that of conventional anaerobic digestion with medium temperature (around 40 ° C) methanogen, and digestibility is around 40% Therefore, the amount of sludge to be finally disposed of is greatly reduced.

  2) By supplying the sludge extracted from the digester directly to the concentrator while maintaining a high temperature, concentration without chemical injection becomes possible.

  3) Although some SS flows out in the concentrated filtrate of 2) above, the amount of solid matter returned to the water treatment can be reduced by attaching an SS recovery device. Moreover, SS can be efficiently recovered by adding a small amount of an aggregating agent to the SS recovery device.

  4) Since sludge is concentrated (reduced) in the sludge circulation line and then heated, it requires less heat and is economical.

  5) In the sludge circulation line, a part of the sludge is concentrated by a concentrator and the separated liquid is removed, so that the digester can function as a temporary sludge storage tank. Even if no machine is installed, stable operation of the plant becomes easy.

  6) Since the digestion tank can be maintained at a high concentration as well as high temperature, the sludge treatment capacity per tank can be increased, and it is possible to cope with the increase in the amount of sludge generation year by year without the addition of facilities.

  BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the best embodiment of an organic sludge anaerobic digestion method and digestion system according to the present invention will be described with reference to the drawings.

  FIG. 1 is a flow sheet showing a first embodiment of an organic sludge anaerobic digestion system according to the present invention. As shown in the figure, the digestion system 1 of this example includes a digestion tank 2, and on the upstream side of the digestion tank 2, a sludge receiving line A for receiving sludge for digestion treatment is provided on the downstream side. The sludge extraction line B for extracting and treating the digested sludge from the digester 2 is connected.

The sludge receiving line A is provided with a sludge storage tank 3 for storing the sludge separated in the water treatment process. Here, sludge having a concentration of 3% and room temperature (for example, 10 ° C.) is pumped to the digestion tank 2. It is thrown. In the sludge extraction line B, a centrifugal dehydrator 4 is provided, and the digested sludge periodically extracted from the digestion tank 2 is added by 1% by weight of the flocculant and dehydrated. The separated liquid is returned to the water treatment step, and the dewatered sludge cake (dehydrated cake) is incinerated in this example and discarded.

  Apart from the lines A and B, a sludge circulation line C for maintaining the high temperature and high concentration by concentrating and heating the sludge in the digestion tank 2, which is a characteristic part of the present invention, is provided. This sludge circulation line C includes a digester 2 and is configured by connecting a centrifugal concentrator 5 and a heating tower 6 in series in this order through a circulation path 7. In addition, a heat supply line D is connected to the heating tower 6 in order to supply heat necessary for heating sludge. In this example, the digestion gas (mainly methane gas) generated in the digestion tank 2 is burned to indirectly generate water vapor, and this water vapor is introduced into the heating tower 6 as a heat medium and directly brought into contact with the digested sludge. Heat exchange is used, and digested sludge is heated to a high temperature (90 ° C.) without using auxiliary fuel.

  The heat quantity supply line D includes a heat exchanger 8, and the superheated steam generated from the heat exchanger 8 is supplied to the heating tower 6 and the superheated steam from the heating tower 6 is returned to the heat exchanger 8. A closed circuit composed of the steam return pipe 10b connects the two alternately. The heat exchanger 8 is provided with a digestion gas combustion furnace 9, and digestion gas discharged from the digestion tank 2 is supplied to the digestion gas combustion furnace 9 through a digestion gas supply pipe 11, and at the same time, air is supplied from a forced blower (not shown). Combustion air is supplied through the supply pipe 14. Further, combustion exhaust gas is discharged from the heat exchanger 8, and the discharge path 12 is connected to the chimney 13.

  The organic sludge digestion system 1 of the present embodiment is configured as described above, and the use mode thereof will be described below together with the digestion method.

As shown in FIG. 1, in the digestion system 1 of this example, the concentrated sludge (solid concentration 3%, organic matter concentration 80%, 10 ° C.) from the sludge storage tank 3 in the sludge receiving line A is 43.5 m ³ / h. Is continuously fed into the digester 2. On the other hand, the digestion tank 2 is anaerobically digested by high-temperature methane bacteria at a high temperature of around 53 ° C. and a solid concentration of 3.7%. And in the sludge extraction line B, the digested sludge in the digestion tank 2 (solid concentration 3.7%, organic matter concentration 61.5%, 53 ° C.) is withdrawn by 18 m ³ / h, and the flocculant is added 1% of the solid amount. Then, it is dehydrated by the centrifugal dehydrator 4. The separated liquid is returned to the water treatment process, and the dehydrated cake is incinerated and disposed of. In this example, the above numerical value is displayed on the assumption that the digestibility is 60%, but it is estimated that it is actually 60% or more.

In the sludge circulation line C, the digested sludge in the digestion tank 2 (solid concentration 3.7%, organic matter concentration 61.5%, 53 ° C.) is withdrawn 98 m ³ / h at a time and conveyed to the centrifugal concentrator 5 by a pump. To do. In the centrifugal concentrator 5, the flocculant is not added (that is, without chemical injection) and concentrated. By this work, a separation liquid (53 ° C.) is generated at a rate of 25.5 m ³ / h. Since this separation liquid contains SS (solid content), a flocculant of about 0.1% of the solid amount is added to the separation liquid here, and after removing SS, the separation liquid is returned to the water treatment step. .

The circulating sludge concentrated in the centrifugal concentrator 5 (solid concentration 5.0%, organic matter concentration 61.5%, 53 ° C.) is pumped to the heating tower 6 by 72.5 m ³ / h. In the heating tower 6, the circulating sludge is sprayed and sprayed, and is heated to 90 ° C. by direct contact with superheated steam (550 ° C.) supplied from the heat exchanger 8 through the supply pipe 10 a. The amount of heat required at this time is 2682500 kcal / h. This amount of heat corresponds to a temperature difference caused by the temperature of the superheated steam returned to the heat exchanger 8 through the return pipe 10b being reduced to 120 ° C. The biogas combustion furnace 9, digestion gas is 652.5m ³ N / h supplied at the same time the combustion air and burned by being 5640m ³ N / h supplied, the flue gas passes through the heat exchanger 8 6292 By discharging 5 m ³ N / h, a heat amount of 3192000 kcal / h is imparted to the superheated steam (120 ° C.) of the return pipe 10b, and the temperature of the superheated steam rises to 550 ° C.

Circulating sludge (solid concentration 5.0%, organic matter concentration 61.5%, 90 ° C.) heated in the heating tower 6 is conveyed to the digester 2 in the circulation path 7 by a pump at a rate of 72.5 m ³ / h. Then, the concentrated sludge from the sludge storage tank 3 (solid concentration 3%, organic matter concentration 80%, 10 ° C., 43.5 m ³ / h) is put into the digestion tank 2. At this time, the apparent input sludge combined with both sludges has a solid concentration of 4.25%, an organic matter concentration of 66.4%, 60 ° C., and an input amount of 116 m ³ / h.

The digestion system 1 and digestion method according to the above embodiment have the following merits. That is,
The centrifugal concentrator 5 has an action of a separation tank for extracting a separation liquid (detachment liquid) in addition to the action of concentrating the digested sludge. Digestion efficiency is almost doubled and digestion time is shortened because digestion in the digestion tank 2 is digested at a high temperature of around 53 ° C and at a solid concentration of 3.7%, maintaining a high concentration of 3.7%. In addition, the digestibility is improved from the conventional 40-50% to 60% or more. Further, by concentrating a part of the digested sludge in the sludge circulation line C and removing the separation liquid in the centrifugal concentrator 5, the digester tank 2 can have a function as a temporary sludge storage tank.

  FIG. 2 is a flow sheet showing an anaerobic digestion system for organic sludge according to another embodiment of the present invention. As shown in FIG. 2, the digestion system 1 ′ of the present embodiment is different from the above-described embodiment in that the dehydrator 4 and the concentrator 5 are used as a single concentration / dehydration combined machine 4 ′ and sludge is concentrated. That is, the dehydration is performed by one apparatus. A centrifugal dehydrator is used for this combined machine 4 '. Since other configurations are common, common members are denoted by the same reference numerals in FIG. 2, and detailed description thereof is omitted.

  In the digestion system 1 ′ of this example, in order to use it as a dehydrator in the daytime, 1% of the solid matter amount of the sludge extracted from the digester 2 is added to the flocculant. Concentrate without adding a flocculant for use as a concentrator at night. Since other usage modes are the same as those in the above embodiment, description thereof is omitted. In particular, in a sewage treatment plant equipped with an existing centrifugal dehydrator 4, the high-temperature / high-concentration digestion system 1 ′ of the present invention can be implemented without newly installing the concentrator 5.

  Two examples of the organic sludge digestion system of the present invention have been described above, but the present invention can also be implemented as follows, for example.

  It replaces with the heat exchanger 8 and the digestion gas combustion furnace 9, a boiler is provided, water vapor | steam is generated with this boiler, it introduce | transduces into the heating tower 6, and water vapor | steam can be sprayed and heated with respect to concentrated sludge. In this case, since water vapor is condensed and water is mixed into the concentrated sludge, the solids concentration is slightly reduced, but is not greatly affected.

It is a flow sheet which shows the 1st example of the anaerobic digestion system of organic sludge concerning the present invention. It is a flow sheet which shows the 2nd example of an anaerobic digestion system of organic sludge concerning the present invention.

Explanation of symbols

1. 1 'Digestion System 2 Digestion Tank 3 Sludge Storage Tank 4 Centrifugal Dehydrator 4' Concentration / Dehydration Combined Machine 5 Centrifugal Concentrator 6 Heating Tower 7 Circulation Path 8 Heat Exchanger 9 Digestion Gas Combustion Furnace 10a Steam Supply Pipe 10b Steam Return pipe 11 Digestion gas supply pipe 12 Discharge passage 13 Chimney 14 Air supply pipe

Claims (7)

In a method of anaerobic digestion by introducing organic sludge into the digester and methane fermentation,
Apart from the process of drawing out part of the digested sludge in the digester and dewatering it,
Extracting and concentrating a portion of the digested sludge from the digestion tank, providing a series of steps for heating the concentrated sludge and circulating it to the digestion tank, and performing anaerobic digestion at a high temperature and high concentration in the digestion tank An anaerobic digestion method for organic sludge.   The digestion tank is filled with unheated solid sludge having a solid concentration, a part of the digested sludge in the digestion tank is extracted and concentrated by a concentrator to increase the solid concentration by several percent, and then heated. The sludge concentrated by the apparatus is heated to nearly 100 ° C. and circulated in the digester, and the amount of sludge circulated in the digester is the temperature at which the sludge can be anaerobically digested by high-temperature methane bacteria. The method for anaerobic digestion of organic sludge according to claim 1, wherein the method is determined so as to be maintained at a low temperature.   The method for anaerobic digestion of organic sludge according to claim 1 or 2, wherein a heat amount necessary for the heating device is generated by combustion heat of digestion gas generated in the digester. In the digestion system where organic sludge is anaerobically digested by methane fermentation in the digestion tank, the digested sludge is pulled out and discarded by incineration of the sludge cake dehydrated with a centrifugal dehydrator,
A heating tower for heating a part of the digested sludge is provided on the downstream side of the digestion tank, and a concentrator for concentrating the digested sludge is interposed between the heating tower and the digester. In addition, an anaerobic digestion system for organic sludge characterized in that a circulation path for circulating the sludge heated in the heating tower to the digester is provided.   A digestion gas combustion furnace generated in the digestion tank and a heat exchanger that generates superheated steam by this combustion heat are provided, and the digested sludge concentrated in the heating tower is heated by direct contact with the superheated steam. 5. The organic sludge anaerobic digestion system according to claim 4, wherein the system is returned to the heat exchanger in a steam state without being condensed.   The anaerobic digestion system for organic sludge according to claim 4 or 5, wherein the centrifugal dehydrator for extracting and dewatering a part of the digested sludge in the digestion tank is also used as the concentrator.   The organic sludge anaerobic digestion system according to claim 4, wherein the digester is provided with a function as a temporary sludge storage tank by concentrating a part of the digested sludge with the concentrator and removing the separation liquid. .

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