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
1
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
from a methane fermentation tank to solid-liquid separation includes a gravity sedimentation
method.
[0004] Further, in order to enhance the methane fermentation treatment in efficiency, there
2
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
fatty acid, the dispersibility of the fat is degraded remarkably, and hence, the fat is likely to be
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
3
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
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 the lower layer side of the sludge sedimentation liquid to the methane fermentation tank;
4
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 ratio and a high melting point is charged so that the charging load of a
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
5
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
fermentation treatment of the present invention is described with reference to FIG. 1.
[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.
6
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
methanogens, and decomposes the waste to biogas such as methane gas. In the methane
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
7
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
L2 connected to the pretreatment tank 1 extends from a lower portion (bottom portion in this
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
8
[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%,
in which the fat has the saturated fatty acid ratio of preferably 30% or more, and more
preferably 40% or more, is preferably used in the methane fermentation treatment of the
present invention.
9
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.
[0032] In contrast, in the present invention, the following is considered: the liquid
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
10
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 is melted and dispersed completely, and after the temperature of 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.
11
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
methane fermentation tank 2 in a day. The amount of the saturated fatty acid contained in
the fat can be calculated by multiplying the total fatty acid amount by the ratio of the
saturated fatty acid.
12
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
organic waste can be enhanced. Therefore, un-decomposed fat is hardly contained in the
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
13
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%.
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
by dividing the mass of a solid obtained by evaporating and drying the fermentation liquid
(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
14
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
A methane fermentation treatment was performed in the same way as in 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
15
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%.
From these results, the following is understood. When the sludge concentrated
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
16
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
load (g/L/day) 1 0.8 0.69 0.57 0.5 0.4 0.35 0.29 0.25 0.22 0.2
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
17 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
18
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
