2. University of Sumatera Utara - Methane Fermentation Method

Japan  Patent  Office  

  Title   : Methane  Fermentation  

  Method     Number  

  : 5079834     Patentee  

  : 1.  

  

Metawater  Co.,  Ltd  

• -­‐  Yoshimasa  Tomiuchi  

  2.   University  of  Sumatera  Utara  

• -­‐  Dr.  Eng.  Ir.  Irvan,  M.Si  

• ­‐  Ir.  Bambang  Trisakti,  M.Si  
• ­‐  Prof.  Dr.  Urip  Harahap,  Apt  
• ­‐  Prof.  Darwin  Dalimunthe  
• ­‐  Prof.  Dr.  Erman  Munir,  MSc  
• ­‐  Dr.  Mahyuddin  K.M  Nasution,        MIT.,  Ph.D  
• ­‐  Prof.  Dr.  Runtung,  SH,  M.Hum  

  TITLE OF THE INVENTION METHANE FERMENTATION METHOD BACKGROUND OF THE INVENTION

  1. Field of the Invention [0001] The present invention relates to a methane fermentation treatment of an organic waste, and more specifically, to a methane fermentation treatment capable of being performed stably over a long period of time even when a charging load of fat is high.

  2. Description of the Related Art [0002] In the methane fermentation treatment, an organic waste is fermented with methanogens under an anaerobic atmosphere and converted into methane gas. With this treatment, the organic waste is decomposed to biogas and water, and hence, the organic waste can be reduced remarkably. Further, the methane fermentation treatment has an advantage of recovering methane gas to be generated as a by-product as energy. [0003] If a great amount of methanogens can be stored in the methane fermentation tank, the treatment can be performed at higher speed. As one of the method for storing a great amount of methanogens in the methane fermentation tank, there is a method in which the method includes subjecting fermentation liquid extracted from a methane fermentation tank to solid-liquid separation and returning separated sludge obtained by the solid-liquid separation to the methane fermentation tank or the like, to thereby perform methane fermentation. A typical example of the method is the method includes subjecting fermentation liquid extracted method.

  [0004] Further, in order to enhance the methane fermentation treatment in efficiency, there has been conventionally employed a method in which the method includes subjecting an organic waste to a solubilization treatment to perform methane fermentation treatment. [0005] For example, Japanese Patent Application Laid-open No. 2002-66507 discloses a method of treating an organic solid, the method including a slurrying treatment step of slurrying an organic solid which is insoluble in water; a solubilization treatment step of transforming the slurry of organic solid into an organic substance which is soluble in water; an anaerobic treatment step of subjecting the solubilized resultant to methane fermentation in the presence of sludge containing anaerobes; a solubilization treatment step of transforming the sludge generated in the anaerobic treatment into an organic substance which is soluble in water; and a returning step of returning the solubilized resultant to the anaerobic treatment.

  [0006] Patent Application Publication: Japanese Patent Application Laid-open No. 2002-66507 [0007] However, fat containing saturated fatty acids such as palmitic acid having a melting point of 63ºC and stearic acid having a melting point of 69.9ºC contained in palm oil mill effluent, and the like is not melted even in high-temperature methane fermentation at a fermentation temperature of about 55ºC and retains in a solid state. Therefore, the fat cannot come into contact with methanogens or the like in a tank efficiently, and thus, the fat is likely to be discharged without being decomposed.

  [0008] Therefore, when an organic waste containing a great amount of fat having a high saturated fatty acid ratio is subjected to a methane fermentation treatment with an increased charging load, the dispersibility of the fat in a methane fermentation tank is degraded. In particular, when the amount of the fat is set to 0.2 g/L/day or more in terms of a saturated pulled out of the methane fermentation tank without being decomposed. Then, when fermentation liquid containing a great amount of the un-decomposed fat is subjected to solid-liquid separation by a gravity sedimentation method, the un-decomposed fat floats on a liquid surface and is likely to be discharged in a form of scum. This degrades the liquid properties of wastewater, causing time and trouble in the subsequent treatment of wastewater.

  Further, the un-decomposed fat adheres to sludge to impair the sedimentation rate of the sludge remarkably, which makes it necessary to enlarge a gravity sedimentation tank and prolong a retention time in the tank. [0009] According to Japanese Patent Application Laid-open No. 2002-66507, in order to enhance in efficiency in the methane fermentation treatment of a solid waste containing an organic solid which is insoluble in water, such as a beer slop, a food waste, or a vegetable or animal waste, these solid wastes are subjected to a solubilization treatment. However, the above-mentioned problem caused in a case of subjecting an organic waste containing fat having a high saturated fatty acid ratio to a methane fermentation treatment with an increased charging load has not been considered.

  SUMMARY OF THE INVENTION [0010] It is an object of the present invention to provide a methane fermentation method capable of decomposing an organic waste containing fat having a high saturated fatty acid ratio in a high concentration efficiently within a short period of time. [0011] In order to solve the above-mentioned problem, a methane fermentation method of the present invention includes: charging an organic waste containing fat to a methane fermentation tank to subject the organic waste to a methane fermentation treatment; extracting fermentation liquid from the methane fermentation tank by a predetermined amount to form gravity sedimentation; returning at least part of liquid containing sludge in high concentration on the lower layer side of the sludge sedimentation liquid to the methane fermentation tank; and discharging at least part of liquid containing sludge in low concentration on the upper layer side of the sludge sedimentation liquid out of a system, in which the liquid containing sludge in high concentration extracted from the lower layer side of the sludge sedimentation liquid and the organic waste are mixed with each other, heated to a temperature equal to or more than the melting point of the fat contained in the mixture to disperse the fat, and then charged to the methane fermentation tank so that a charging load of a saturated fatty acid in the fat is 0.2 g/L/day or more to perform the methane fermentation treatment.

  [0012] In the methane fermentation method of the present invention, it is preferable that the organic waste contains the fat in an amount of 20 to 35% by mass in a solid, and the fat has a saturated fatty acid ratio of 30% or more. [0013] In the methane fermentation method of the present invention, the mixture of the liquid containing sludge in high concentration and the organic waste is preferably heated to 70ºC or more and then charged to the methane fermentation tank. [0014] According to the methane fermentation method of the present invention, liquid containing sludge in high concentration extracted from the lower layer side of sludge sedimentation liquid and an organic waste are mixed with each other, and heated to a temperature equal to or more than the melting point of fat contained in the mixture. Thus, the fat is dispersed to adhere to the sludge in the liquid containing sludge in high concentration extracted from a gravity sedimentation unit, which can enhance the dispersibility of the fat in the methane fermentation tank. Consequently, the fat is likely to come into contact with methanogens in the tank to enhance the decomposition efficiency of the organic waste. Therefore, even when the organic waste containing fat having a high saturated fatty acid in the fat becomes 0.2 g/L/day or more to perform the methane fermentation treatment, the fat comes into contact with methanogens in the tank, and is decomposed efficiently. Therefore, the decomposition efficiency of the organic waste is enhanced, and a fermentation liquid containing un-decomposed fat in a small amount is discharged from the methane fermentation tank. As a result, when the fermentation liquid is subjected to solid-liquid separation by gravity sedimentation, scum is unlikely to be formed on a liquid surface, and the liquid properties of supernatant after the solid-liquid separation becomes satisfactory, which can reduce the time and labor required in the subsequent treatment of the supernatant as wastewater. Further, the amount of fat in the fermentation liquid pulled out of the methane fermentation tank is small, and hence, the sedimentation rate of sludge in the gravity sedimentation unit is high and the sludge can be sedimented within a short period of time even without enlarging the gravity sedimentation unit, which can simplify the time and labor required in the solid-liquid separation of the fermentation liquid.

  BRIEF DESCRIPTION OF THE DRAWINGS [0015] In the accompanying drawings:

  FIG. 1 is a schematic structural view of a methane fermentation apparatus used in the methane fermentation treatment of the present invention; FIG. 2 is a graph showing the sedimentation characteristics of fermentation liquid; and FIG. 3 is a graph showing the relationship between charging load of saturated fatty acid in fat and VS decomposition ratio.

  DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0016] One embodiment of a methane fermentation apparatus used in a methane [0017] As illustrated in FIG. 1, the methane fermentation apparatus mainly includes a pretreatment tank 1, a methane fermentation tank 2, and a gravity sedimentation tank 3.

  [0018] The pretreatment tank 1 includes a heating apparatus (not shown) and a stirring apparatus (not shown). In the pretreatment tank 1, an organic waste sent from a supply source of the organic waste is mixed with a sludge concentrated liquid returned from the gravity sedimentation tank 3 described later, and the mixture is heated to a temperature equal to or more than the melting point of fat contained in the mixture (hereinafter, referred to as "organic waste"), and thus, the fat is dispersed. Further, the pretreatment tank 1 also has a buffer function for stabilizing an inflow of the organic waste into the methane fermentation tank 2 described later. The heating apparatus is not particularly limited, and conventionally known apparatuses such as a heater, steam under pressure, and an apparatus for heating by heat-exchange of a heat amount of the organic waste to be treated each can be used. Further, there is no particular limit to the above-mentioned stirring apparatus as long as the apparatus can stir the organic waste in the tank, and conventionally known apparatuses such as a stirrer equipped with a stirring blade each can be used.

  [0019] A pipe L1 extending from the supply source of the organic waste and a pipe L2 extending from a bottom portion of the gravity sedimentation tank 3 (pipe L2 may be connected to a lower portion of a side surface of the sedimentation tank 3) are connected to the pretreatment tank 1.

  [0020] In the latter stage of the pretreatment tank 1, the methane fermentation tank 2 is placed. The pretreatment tank 1 and the methane fermentation tank 2 are connected to each other via a pipe L3. [0021] In the methane fermentation tank 2, an organic liquid waste supplied to the tank is subjected to an anaerobic treatment through the function of anaerobic bacteria such as fermentation tank 2, a stirring device (not shown) for stirring fermentation liquid in the tank is placed. Further, a pipe L4 for extracting the biogas extends from an upper portion of the methane fermentation tank 2, and is connected to a gas holder, a gas-using facility, etc.

  [0022] There is no particular limit to the stirring apparatus as long as the apparatus is capable of stirring the fermentation liquid in the tank. Examples of the stirring apparatus include a stirrer equipped with a stirring blade, etc. Further, a mechanism in which a passage for circulating the fermentation liquid in the tank is formed to generate an upward flow or a downward flow in the fermentation liquid in the tank may be equipped. A gas stirring apparatus for circulating the generated biogas to perform blow bubbling also may be provided. [0023] In the latter stage of the methane fermentation tank 2, the gravity sedimentation tank 3 is placed. The methane fermentation tank 2 and the gravity sedimentation tank 3 are connected to each other via a pipe L5. [0024] The gravity sedimentation tank 3 is a treatment tank in which sludge in the fermentation liquid extracted from the methane fermentation tank 2 is subjected to gravity sedimentation to form sludge sedimentation liquid whose sludge concentration increases towards a lower portion. Examples of such tank include a gravity sedimentation pond.

  Further, by placing a water flow gradient plate in the gravity sedimentation tank 3, the sedimentation rate of sludge can be enhanced more. Examples of the gravity sedimentation tank equipped with the water flow gradient plate include the tank described in Japanese Patent Application Laid-open No. Hei 6-63321.

  [0025] A pipe L6 for discharging liquid containing a low concentration of sludge (hereinafter, also referred to as "sludge separated liquid") on an upper layer side outside the system is connected to the side portion of the gravity sedimentation tank 3. Further, the pipe example) of the gravity sedimentation tank 3 so that at least part of liquid containing sludge in high concentration (hereinafter, also referred to as "sludge concentrated liquid") on a lower layer side may be returned to the pretreatment tank 1.

  [0026] Next, the methane fermentation method of the present invention is described using the case using the above-mentioned methane fermentation apparatus as an example.

  [0027] The organic waste used in the methane fermentation treatment of the present invention has only to contain fat in a high concentration. In particular, fat having a high saturated fatty acid ratio tends to have a high melting point so as to form a solid shape even at room temperature. Therefore, the organic waste containing fat having a high saturated fatty acid ratio is preferably used in the methane fermentation treatment of the present invention. Further, a saturated fatty acid having 16 or more carbon atoms has a melting point of 55ºC or more, and fat containing such saturated fatty acid is likely to maintain a solid state without being dissolved even at a time of a high-temperature methane fermentation treatment. Therefore, the organic waste containing fat that includes a saturated fatty acid having 16 or more carbon atoms is particularly preferably used in the methane fermentation treatment of the present invention. Examples of the saturated fatty acid having 16 or more carbon atoms contained in the organic waste include palmitic acid (number of carbon atoms: 16, melting point: 63ºC) and stearic acid (number of carbon atoms: 18, melting point: 69.9ºC). Examples of the organic waste containing fat that includes a saturated fatty acid having 16 or more carbon atoms include oils and fats liquid wastes discharged from an oils and fats factory, etc. and food residues of, for example, ice cream and milk-based drinks. [0028] Further, fat having a higher saturated fatty acid ratio has a higher melting point, and is likely to keep a solid state even at room temperature. Therefore, the organic waste which contains fat in a solid in an amount of preferably 20 to 35%, and more preferably 25 to 30%, preferably 40% or more, is preferably used in the methane fermentation treatment of the present invention. [0029] It should be noted that the saturated fatty acid ratio of fat refers to a value of a ratio expressed in percentage of a saturated fatty acid in fatty acids that are fat constituent components.

  [0030] In the present invention, an organic waste and a sludge concentrated liquid are supplied to the pretreatment tank 1 through the pipes L1 and L2, respectively. Then, while the organic waste and the sludge concentrated liquid supplied to the pretreatment tank 1 are being stirred, the mixture (organic waste liquid) is heated to a temperature equal to or more than the melting point of fat contained in the mixture. Thus, the fat is melted and dispersed to adhere to the surface of sludge, and hence, the dispersibility of the fat in the methane fermentation tank 2 is enhanced and the fat is likely to come into contact with methanogens in the tank. Consequently, the decomposition efficiency of the entire organic waste is enhanced.

  Although the detail of the reason is unknown, it may be assumed as follows. [0031] That is, when an organic waste is heated to solubilize fat contained therein before the organic waste is charged into the methane fermentation tank 2, the fat is solidified and precipitated at a time of methane fermentation, because the fermentation temperature in the methane fermentation tank 2 is lower than the melting point of the fat. When the charging load of the fat is high, fat that has a high saturated fatty acid ratio and is likely to be solidified is present in a great amount in the methane fermentation tank. Therefore, the precipitated fat bumps against and adheres repeatedly to each other to grow gradually to form a large fat particle, which decreases the dispersibility of the fat. Therefore, the fat is unlikely to be decomposed by methanogens and is discharged as fermentation liquid without being decomposed. containing sludge in high concentration and the organic waste are mixed, and the mixture is heated to a temperature equal to or more than the melting point of fat contained in the mixture; thus, the fat having a high melting point is melted to form a liquid state and is dispersed and adheres to sludge of methanogens or the like contained in the liquid containing sludge in high concentration. Even when the methanogens dies out during heating, the dispersion state of the fat having a high melting point is kept. Therefore, the following is considered: even when the fermentation temperature of the methane fermentation tank 2 is lower than the melting point of the fat, the fat adheres to sludge of methanogens or the like to be dispersed in the tank; as a result, even if a large amount of fat having a high saturated fatty acid ratio with a high melting point is present in the methane fermentation tank, the fat is likely to come into contact with methanogens in the tank. Consequently, the decomposition efficiency of the entire organic waste is enhanced, and fermentation liquid containing a small amount of un-decomposed fat is likely to be discharged from the methane fermentation tank.

  [0033] The heating temperature of the organic liquid waste (mixture of the organic waste and the sludge concentrated liquid) has only to be equal to or more than the melting point of the fat contained in the organic liquid waste. The heating temperature can be adjusted appropriately depending upon the kind or saturated fatty acid ratio of the fat contained in the organic liquid waste. For example, in the case of an organic waste containing fat including a saturated fatty acid having 16 or more carbon atoms, the organic liquid waste is heated to preferably 60ºC or more, and more preferably 60 to 80ºC. By heating the organic liquid waste to 60ºC or more, the fat contained in the organic liquid waste can be dissolved sufficiently. Further, when heating is performed at a high temperature, energy loss is caused, and hence, the upper limit is preferably 80ºC.

  [0034] The heating time of the organic liquid waste is set so that the fat in the organic liquid waste reaches a set temperature, the heating is performed for preferably 0.1 to 24 hours, and more preferably 0.5 to 2.0 hours. [0035] The mixing ratio between the organic waste and the sludge concentrated liquid is set so that the solid content of the sludge concentrated liquid becomes preferably 500 to 1000 parts by mass, and more preferably 600 to 800 parts by mass, with respect to 100 parts by mass of the fat in the organic waste. When the solid content of the sludge concentrated liquid is less than 500 parts by mass with respect to 100 parts by mass of the fat in the organic waste, the fat is low in dispersibility. When the solid content exceeds 1000 parts by mass, the solid concentration in the fermentation liquid tends to increase and the methane fermentation efficiency tends to be reduced. [0036] In the pretreatment tank 1, pulverization, crushing, etc. may be performed further, if required.

  [0037] The organic liquid waste subjected to the dispersion treatment of the fat as described above is supplied to the methane fermentation tank 2 via the pipe L3 so that the charging load of the saturated fatty acid in the fat is 0.2 g/L/day or more, and preferably 0.4 to 0.8 g/L/day. Although the methane fermentation treatment can be performed without problems even with a charging load of less than 0.2 g/L/day of the saturated fatty acid in the fat, the charging load of the saturated fatty acid in the fat is set to 0.2 g/L/day or more in the present invention, because an object of the present invention is to treat an organic waste having a high fat content within a short period of time. When the charging load of the saturated fatty acid in the fat is 4 to 0.8 g/L/day, methane fermentation can be performed with a decomposition efficiency of the organic waste enhanced. [0038] It should be noted that "the charging load of the saturated fatty acid in the fat" refers to the amount of the saturated fatty acid in the fat to be charged per 1 L of liquid in the the fat can be calculated by multiplying the total fatty acid amount by the ratio of the saturated fatty acid.

  [0039] In the methane fermentation tank 2, the supplied organic liquid waste is retained for a predetermined period of time while the waste is being stirred continuously or intermittently with a stirrer (not shown) so that the sludge concentration and temperature of the fermentation liquid in the tank become substantially uniform, and the organic liquid waste is subjected to methane fermentation through the function of anaerobic bacteria such as methanogens.

  Then, the fermentation liquid in the same amount as that of the organic liquid waste supplied to the methane fermentation tank 2 is pulled out of the pipe L5 and supplied to the gravity sedimentation tank 3. Further, biogas such as methane gas generated when the organic waste is subjected to methane fermentation is extracted from the tank through the pipe L4, and stored in a biogas holder (not shown), etc. [0040] In the gravity sedimentation tank 3, sludge mainly containing methanogens in the fermentation liquid extracted from the methane fermentation tank 2 is subjected to gravity sedimentation, and thus, sludge sedimentation liquid having a higher concentration of sludge in a lower portion is formed. Then, at least part of liquid containing sludge in low concentration on an upper layer side of the sludge sedimentation liquid is discharged through the pipe L6, and at least part of liquid containing sludge in high concentration on a lower layer side of the sludge sedimentation liquid is returned to the pretreatment tank 1 through the pipe L2.

  [0041] As described above, the fat is present in the fermentation liquid in the methane fermentation tank 2 with dispersibility satisfactory. Therefore, even when the charging load of the saturated fatty acid in the fat is set to 0.2 g/L/day or more, the fat can be decomposed efficiently with methanogens in the tank, and the decomposition efficiency of the entire fermentation liquid pulled out of the methane fermentation tank 2, and a sludge component can be sedimented rapidly and the sludge concentrated liquid can be recovered efficiently in the gravity sedimentation tank 3. Further, as un-decomposed fat is hardly contained, a scum is unlikely to be generated on the liquid surface of the gravity sedimentation tank 3, and the properties of the sludge separated liquid to be discharged from the pipe L6 are satisfactory, which can reduce the time and labor required for the later effluent treatment. Examples [0042] Text Example 1

  Example 1 Methane fermentation was performed using a methane fermentation apparatus illustrated in FIG. 1. As a methane fermentation tank 2, a tank with a capacity of 5 L was used. Further, as a gravity sedimentation tank 3, a tank with a capacity of 0.5 L was used. As an organic waste, palm oil mill effluent with a solid concentration of about 35,000 mg/L and a non-volatile organic substance (VS) concentration of about 30,000 mg/L was used.

  The fat concentration (extracted with chloroform-methanol) amount of the palm oil mill effluent was 10,000 mg/L, and the ratio of palmitic acid and stearic acid occupying in the total fatty acid was 40%.

  It should be noted that a TS concentration (evaporation residue) was measured in accordance with a sewage test method-2.2.9. More specifically, the TS concentration was obtained by evaporating and drying sample liquid at 110ºC. That is, the amount of the remaining dried solid was divided by the volume of the sample liquid. Further, an ash concentration was obtained by dividing the mass of a solid remaining after heating the fermentation liquid at 600ºC±25ºC by the volume of the sample. Then, the VS concentration was obtained by subtracting the ash concentration from the TS concentration (value obtained (mg/l) at 110ºC by the volume of the sample liquid).

  To a pretreatment tank 1 , 833 mL of palm oil mill effluent was charged per day, and 208 mL of a sludge concentrated liquid was returned from the gravity sedimentation tank 3 to be mixed with the palm oil mill effluent. The mixture was heated to 70ºC. After being kept at 70ºC for one day, the mixture was charged to the methane fermentation tank 2 four times per day in a total amount of 1042 mL. In this case, the charging load of saturated fatty acid in fat was 0.67 g/L/day. Further, the same amount of fermentation liquid was pulled out of the methane fermentation tank 2 to be introduced into the gravity sedimentation tank 3.

  Here, the charging load of the saturated fatty acid was calculated by the following expression.

  Charging load of saturated fatty acid=fatty acid concentration (10 g/L)×saturated fatty acid ratio (0.40)×charging amount of organic waste per 1 L of fermentation liquid (0.167L/day=1 L/6 days)

  The VS concentration of the fermentation liquid in the methane fermentation tank 2 after an elapse of 20 days from the 6th day of residence time of the organic waste charged to the methane fermentation tank 2 was measured to be 6,000 mg/L, and the decomposition ratio in the VS was 80.0%.

  Further, the interfacial surface between the sludge separated liquid and the sludge

  

concentrated liquid of the fermentation liquid charged to the gravity sedimentation tank 3 was

  observed visually and the sedimentation characteristics of the fermentation liquid were evaluated. FIG. 2 illustrates the results. As illustrated in FIG. 2, the fermentation liquid was separated almost completely into the sludge separated liquid and the sludge concentrated liquid within about 6 hours.

  [0043] Comparative Example 1 except that the mixture of the palm oil mill effluent and the sludge concentrated liquid was supplied to the methane fermentation tank 2 without heating after the mixture was kept for one day at room temperature (20ºC).

  The VS concentration of the fermentation liquid in the methane fermentation tank 2 after an elapse of 20 days from the 6th day of residence time of the organic waste charged to the methane fermentation tank 2 was measured to be 77,000 mg/L , and the decomposition ratio in the VS was 73%.

  Further, the interfacial surface between the sludge separated liquid and the sludge

  

concentrated liquid of the fermentation liquid charged to the gravity sedimentation tank 3 was

  observed visually and the sedimentation characteristics of the fermentation liquid were evaluated. FIG. 2 illustrates the results. As illustrated in FIG. 2, the sedimentation rate of sludge was low compared with that in Example 1, and the separation between the sludge separated liquid and the sludge concentrated liquid was insufficient even after an elapse of 8 hours from the charging to the gravity sedimentation tank 3. [0044] Comparative Example 2

  To the pretreatment tank 1 , 833 mL of the palm oil mill effluent was charged and heated to 70ºC. The palm oil mill effluent was kept at 70ºC for one day and charged to the methane fermentation tank 2 four times per day in a total amount of 833 mL. The same amount of fermentation liquid was pulled out of the methane fermentation tank 2 and introduced into the gravity sedimentation tank 3.

  The VS concentration of the fermentation liquid in the methane fermentation tank 2 after an elapse of 20 days from the 6th day of residence time of the organic waste charged to the methane fermentation tank 2 was measured to be 10,200 mg/L, and the decomposition ratio in the VS was 64.0%. liquid and the organic waste containing fat having a high saturated fatty acid ratio are mixed while being kept at 70ºC or more, and then, the mixture is charged to the methane fermentation tank to be subjected to a methane fermentation treatment, the decomposition efficiency of the fat is enhanced, the amount of the fat in the fermentation liquid pulled out of the methane fermentation tank is reduced, and sludge can be sedimented within a short period of time in the gravity sedimentation tank. [0045] Test Example 2

  Methane fermentation was performed with the supply amount of an organic waste to the methane fermentation tank adjusted so that the charging load of saturated fatty acid in fat was 0.2 to 1.0 g/L/day in Example 1. Table 1 and FIG. 3 show or illustrate the results.

  [0046] [Table 1] Fatty acid charging

  1

  0.8

  0.69

  0.57

  0.5

  0.4

  0.35

  0.29

  0.25

  0.22

  0.2 load (g/L/day) HRT (day)

  4

  5

  5.8

  7

  8

  10

  11.4

  14

  15.8

  18

  20 VS decomposition ratio

  51

  59

  64

  67

  69

  72

  73

  77

  80

  81

  82 (%)

  [0047] As shown or illustrated in Table 1 and FIG. 3, as the charging load of the saturated fatty acid in the fat increased, the residence time of the organic waste charged to the methane fermentation tank 2 tended to become shorter, and the decomposition ratio in the VS tended to decrease.

  WHAT IS CLAIMED IS:

  1. A methane fermentation method, comprising: charging an organic waste containing fat to a methane fermentation tank to subject the organic waste to a methane fermentation treatment; extracting fermentation liquid from the methane fermentation tank by a predetermined amount to form sludge sedimentation liquid having a higher concentration of sludge in a lower portion by gravity sedimentation; returning at least part of liquid containing sludge in high concentration on a lower layer side of the sludge sedimentation liquid to the methane fermentation tank; and discharging at least part of liquid containing sludge in low concentration on an upper layer side of the sludge sedimentation liquid out of a system, wherein the liquid containing sludge in high concentration extracted from the lower layer side of the sludge sedimentation liquid and the organic waste are mixed with each other, heated to a temperature equal to or more than a melting point of the fat contained in the mixture to disperse the fat, and then charged to the methane fermentation tank so that a charging load of a saturated fatty acid in the fat is 0.2 g/L/day or more to perform the methane fermentation treatment.

  2. A methane fermentation method according to claim 1, wherein the organic waste contains the fat in an amount of 20 to 35% by mass in a solid, the fat having a saturated fatty acid ratio of 30% or more.

  3. A methane fermentation method according to claim 1 or 2, wherein the mixture of the liquid containing sludge in high concentration and the organic waste is heated to 70ºC or more and then charged to the methane fermentation tank.

  ABSTRACT OF THE DISCLOSURE Provided is a methane fermentation method capable of decomposing an organic waste containing fat having a high saturated fatty acid ratio in a high concentration efficiently within a short period of time. The methane fermentation method includes: charging an organic waste containing fat to a methane fermentation tank to subject the organic waste to a methane fermentation treatment; extracting fermentation liquid from the methane fermentation tank by a predetermined amount to form sludge sedimentation liquid having a higher concentration of sludge in a lower portion by gravity sedimentation; returning at least part of liquid containing sludge in high concentration to the methane fermentation tank; and discharging at least part of liquid containing sludge in low concentration out of a system, in which the liquid containing sludge in high concentration and the organic waste are mixed with each other, heated to a temperature equal to or more than the melting point of the fat contained in the mixture to disperse the fat, and then charged to the methane fermentation tank so that the charging load of a saturated fatty acid in the fat is 0.2 g/L/day or more to perform the methane fermentation treatment.

  FIG. 1

  FIG. 2

  FIG. 3