Woody biomass washing waste liquid saccharification method and methane fermentation method

Abstract

The present disclosure addresses the problem of providing a woody biomass washing waste liquid saccharification method that eliminates the need for a temperature adjustment when a woody biomass washing waste liquid is saccharified and can substantially increase saccharide concentration post-saccharification. A woody biomass washing waste liquid saccharification method according to the present invention is a saccharification method for a washing waste liquid obtained when woody biomass is washed. The woody biomass washing waste liquid saccharification method includes a washing step and a saccharification step. At the washing step, woody biomass is washed with water. At the saccharification step, an oxidizing agent is added to a washing waste liquid obtained at the washing step, and the washing waste liquid is saccharified to obtain a saccharified waste liquid.

Description

Saccharification method and methane fermentation method for woody biomass washing wastewater

[0001] The present disclosure generally relates to a method for saccharifying and methane fermenting woody biomass washing wastewater, and more particularly to a method for saccharifying and methane fermenting woody biomass washing wastewater that can be directly used for methane fermentation, and a method for methane fermentation using sugars.

[0002] Patent Document 1 discloses a method for gas combustion of cellulosic biomass, in which the pomace from which sap has been separated by a juicing process is subjected to a saccharification process, and the saccharified solution obtained by the saccharification process, or the saccharified solution and sap, are subjected to a methane fermentation process.

[0003] However, the saccharification treatment in Patent Document 1 is a saccharification method using enzymes or microorganisms, and it is necessary to adjust the temperature to a level suitable for the reaction of the enzymes or microorganisms used. Furthermore, when saccharification methods using enzymes or microorganisms are used on woody biomass washing wastewater, there is a problem in that the sugar concentration of the washing wastewater cannot be sufficiently increased, making it difficult to directly use the washing wastewater for methane fermentation.

[0004] International Publication No. 2016 / 056353

[0005] An object of the present disclosure is to provide a method for saccharifying woody biomass washing wastewater that does not require temperature adjustment during saccharification of the woody biomass washing wastewater and that can sufficiently increase the sugar concentration after saccharification. Another object of the present disclosure is to provide a methane fermentation method that allows the saccharified woody biomass washing wastewater to be used in methane fermentation.

[0006] A method for saccharifying a washing waste liquid of woody biomass according to one aspect of the present disclosure is a method for saccharifying a washing waste liquid obtained when woody biomass is washed. The method for saccharifying a washing waste liquid of woody biomass includes a washing step and a saccharification step. In the washing step, the woody biomass is washed with water. In the saccharification step, an oxidizing agent is added to the washing waste liquid obtained in the washing step, and the washing waste liquid is saccharified to obtain a saccharified waste liquid.

[0007] A methane fermentation method according to one aspect of the present disclosure includes a fermentation step of performing methane fermentation using the saccharification waste liquid obtained by the method for saccharifying woody biomass washing waste liquid.

[0008] The following describes a method for saccharifying woody biomass washing wastewater according to an embodiment. Note that the following embodiment is merely a portion of the various embodiments of the present disclosure. Furthermore, the following embodiment can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved. Furthermore, although the mechanism of action in the embodiment may be described, this description of the mechanism of action includes an explanation based on speculation, and the present disclosure is not bound by the description of the mechanism of action.

[0009] (Embodiments) (1) Overview The method for saccharifying woody biomass washing wastewater is a method for saccharifying the washing wastewater obtained when washing woody biomass. The method for saccharifying woody biomass washing wastewater includes a washing process and a saccharification process. In the washing process, woody biomass is washed with water. In the saccharification process, an oxidizing agent is added to the washing wastewater obtained in the washing process, and the washing wastewater is saccharified to obtain a saccharified wastewater. This sufficiently increases the sugar concentration in the washing wastewater generated when washing woody biomass, allowing the saccharified wastewater obtained in the saccharification process to be directly used for methane fermentation. Furthermore, adding an oxidizing agent to the washing wastewater allows efficient saccharification without heating, making saccharification easy without the need for heat or other means.

[0010] In the present disclosure, "saccharification" refers to converting sugars in the washing waste liquid into lower molecular weight compounds, i.e., converting polysaccharides into free sugars including monosaccharides and disaccharides (hereinafter also referred to as "sugars"). The converted free sugars may also be further converted into lower molecular weight compounds.

[0011] (2) Details <Saccharification method> A saccharification method for woody biomass washing wastewater according to this embodiment will be described. The saccharification method for woody biomass washing wastewater according to this embodiment includes a washing step and a saccharification step. These steps will be described below in order.

[0012] <Washing Step> The washing step is a step of washing woody biomass with water. More specifically, washing is carried out by extracting sugars from a pulverized woody biomass product (hereinafter referred to as "pulverized product") into water, and then separating the pulverized product from the water containing the sugars.

[0013] Sugars include free sugars, including monosaccharides and disaccharides, and polysaccharides (including oligosaccharides). Disaccharides and polysaccharides are formed by glycosidic bonds between multiple monosaccharides.

[0014] Examples of monosaccharides include glucose, fructose, ribose, arabinose, rhamnose, xylulose, and deoxyribose.

[0015] Examples of disaccharides include sucrose, maltose, trehalose, turanose, lactulose, maltulose, palatinose, gentiobiulose, melibiulose, galactosucrose, rutinulose, and planteobiose.

[0016] Examples of polysaccharides include starch, agarose, alginic acid, glucomannan, inulin, chitin, chitosan, hyaluronic acid, glycogen, cellulose, etc. Examples of oligosaccharides include fructooligosaccharides, galactooligosaccharides, mannanoligosaccharides, stachyose, etc.

[0017] In the method for saccharifying woody biomass washing wastewater according to this embodiment, among the above sugars, starch and polysaccharides formed by decomposition of starch are particularly targeted for saccharification.

[0018] Woody biomass is a plant that contains sugars. Woody biomass is broadly divided into woody plants (so-called trees) and herbaceous plants (so-called grasses), but it may be either woody or herbaceous. Woody biomass is not particularly limited, but examples of woody biomass include palm plants such as oil palm, botrytis, coconut palm, date palm, sago palm, acai, and palm oil palm, grass plants such as sugarcane, and mallow plants such as kenaf. Particularly, oil palm parts include the trunk (OPT: Oil Palm Trunk), empty fruit bunch (EFB), leaves (OPF: Oil Palm Fronds), and stems. Furthermore, oil palm and sugarcane are more preferred because they contain relatively more sugar than other woody biomass and are easily utilized for methane fermentation.

[0019] Here, woody biomass contains parenchyma tissue. Parenchyma tissue is tissue composed of parenchyma cells. Parenchyma tissue includes assimilation tissue, secretory tissue, storage tissue, etc., and has physiological functions such as synthesis, decomposition, and storage. In other words, in woody biomass, parenchyma tissue contains a large amount of sugar.

[0020] The palm oil industry is thriving in Southeast Asia, but because oil palm trees lose fruit production after 20 to 30 years, large quantities of old trees are discarded. These trees emit large amounts of methane, a greenhouse gas, when left abandoned, raising the issue of how to dispose of them. For this reason, effective utilization of old harvested oil palm trees is desired, and their use as biomass molding materials and fuel pellets is being considered. However, freshly harvested oil palm contains approximately 10% free sugar and 25% starch, making it necessary to remove the sugars for effective utilization. Therefore, sugars can be removed from oil palm by washing it with water.

[0021] In the washing process, as described above, the woody biomass is first pulverized to form a pulverized material. Pulverization is a process in which the woody biomass is finely crushed to obtain a pulverized material. Pulverizing the woody biomass into a pulverized material facilitates contact with water during washing and destroys the parenchymal tissue of the woody biomass, allowing for efficient extraction of sugars. The pulverization method is not particularly limited, but examples include a hammer mill, cutter mill, chipper, ball mill, grinder, roller mill, and jet mill. The size of the pulverized material obtained by pulverization is not particularly limited, but preferably has an average length of 0.8 mm to 50.0 mm and an average diameter of 0.10 mm to 2.00 mm. Pulverized material of such sizes facilitates contact between the pulverized material and water, improving sugar extraction efficiency. Furthermore, the pulverized material after extraction can be effectively utilized.

[0022] Washing is a treatment in which the pulverized material obtained by pulverization is washed with water. In washing, extraction is first carried out, and then solid-liquid separation is carried out.

[0023] Extraction is a process in which sugars in the pulverized material are extracted into water. The water from which the sugars in the pulverized material have been extracted is the washing waste liquid. Extraction can be performed by any method that transfers the sugars in the pulverized material to the water used. That is, extraction can be performed by simply contacting the pulverized material with water, or by adding water to the pulverized material. Adding water to the pulverized material increases the moisture content of the pulverized material, making it easier to extract sugars. Furthermore, by squeezing the pulverized material after adding water during the solid-liquid separation process described below, washing waste liquid can be obtained more efficiently. Here, when adding water, bringing the pulverized material into a water-saturated state allows for more efficient extraction of sugars from the pulverized material. A water-saturated state refers to a state in which the cell cavities and cell walls of woody biomass are fully filled with free water and bound water. Free water is water present in the cell cavities in a liquid state, while bound water is water chemically bound to fibers and other components in the cell walls as water molecules. The extraction method is not particularly limited, but examples include spraying water using a sprayer, sprayer, etc., sprinkling water using a hose, nozzle, etc., and immersion in a water tank, etc. When extraction is performed by immersion, the water in which the pulverized material is immersed may be stirred to mix it.

[0024] Solid-liquid separation is a process for separating the pulverized material that has been subjected to extraction treatment from the water (washing wastewater) from which the sugars in the pulverized material have been extracted. The solid-liquid separation method is not particularly limited, as long as it does not contaminate the pulverized material with the washing wastewater and does not separate the sugars from the washing wastewater. That is, it may be performed by filtration using a filter medium such as a mesh, screen, filter, or filter paper, or by centrifugal filtration using a centrifuge or centrifugation including centrifugal sedimentation. Furthermore, when separating the pulverized material into solid and liquid form after adding water, it is preferable to compress the pulverized material. By breaking down the parenchymal tissue by compressing, sugars can be obtained more efficiently. For compressing, a known compressing device can be used, for example, a belt press, a screw press, a plate press, a roll press, a shredder press, etc.

[0025] Furthermore, a second crushing treatment may be carried out between the extraction treatment and the solid-liquid separation treatment, which makes it easier to destroy the parenchymal tissue and allows sugars to be obtained more efficiently.

[0026] The sugar content of the pulverized material after sugar extraction is reduced. Therefore, woody biomass with less mold can be obtained. Therefore, the pulverized material after sugar extraction can be effectively utilized by fiberizing it.

[0027] <Saccharification Step> The saccharification step is a step in which an oxidizing agent is added to the washing waste liquid obtained in the washing step to saccharify the washing waste liquid, thereby obtaining a saccharified waste liquid.

[0028] The temperature of the washing waste liquid during the saccharification step is preferably above 0°C, more preferably 25°C or higher, and even more preferably 35°C or higher. Furthermore, the temperature of the washing waste liquid during the saccharification step is preferably below 100°C, more preferably 85°C or lower, and even more preferably 80°C or lower. When the temperature of the washing waste liquid during the saccharification step is above 0°C, the water in the washing waste liquid does not solidify, and the saccharification of the washing waste liquid proceeds. Furthermore, the higher the lower limit of the temperature of the washing waste liquid during the preferred saccharification step, the higher the solubility of sugars in water, allowing for greater dissolution of sugars. Furthermore, when the temperature of the washing waste liquid during the saccharification step is below 100°C, the water in the washing waste liquid does not evaporate, preventing changes in the oxidant concentration and sugar concentration, making it easier to control the progress and quality of the saccharification of the washing waste liquid. Furthermore, the lower the upper limit of the temperature of the washing waste liquid during the preferred saccharification step, the more effectively starch gelatinization can be suppressed, thereby preventing inhibition of saccharification. The saccharification step may be performed at room temperature without heating the washing waste liquid. In the present disclosure, "room temperature" refers to the range defined by JIS Z 8703, specifically, 5°C or higher and 35°C or lower.

[0029] An oxidizing agent is an additive that hydrolyzes polysaccharides such as starch to lower molecular weights, thereby saccharifying the sugars in the cleaning waste liquid. In other words, adding an oxidizing agent to the cleaning waste liquid converts the sugars into sugars, thereby increasing the sugar concentration in the cleaning waste liquid. The oxidizing agent preferably contains at least one selected from the group consisting of hydrogen peroxide, calcium peroxide, ammonium persulfate, and peracetic acid. In this case, saccharification of the cleaning waste liquid progresses, potentially increasing the sugar concentration. The oxidizing agent is more preferably hydrogen peroxide. In this case, since the by-products are water and oxygen, a process for removing impurities from the cleaning waste liquid is unnecessary, thereby improving treatment efficiency. Furthermore, hydrogen peroxide is more preferable because it is less expensive than other oxidizing agents. After adding the oxidizing agent, the cleaning waste liquid may be stirred to mix.

[0030] The amount of oxidizing agent added relative to the total amount of the washing waste liquid and the oxidizing agent is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more. The amount of oxidizing agent added is preferably 6% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less. When the amount of oxidizing agent added is 1% by mass or more, saccharification of the washing waste liquid proceeds efficiently. When the amount of oxidizing agent added is 6% by mass or less, the risk of using the oxidizing agent is reduced.

[0031] When hydrogen peroxide is used, manganese dioxide may be further added after saccharification. Manganese dioxide acts as a catalyst for the decomposition of hydrogen peroxide, thereby accelerating the decomposition rate of hydrogen peroxide. Furthermore, manganese dioxide is a heterogeneous catalyst and can be separated from the saccharification waste liquid by a simple procedure such as filtration. That is, when hydrogen peroxide is used, the addition of manganese dioxide increases the decomposition rate of hydrogen peroxide, thereby accelerating the removal of oxygen, a by-product, from the saccharification waste liquid. This allows the saccharification waste liquid to be immediately used in the methane fermentation (described below) carried out under anaerobic conditions, thereby improving treatment efficiency.

[0032] The sugar concentration of the saccharification waste liquid is preferably 2% Brix or more. In this case, the sugar concentration of the saccharification waste liquid is sufficiently high so that it can be directly used for methane fermentation.

[0033] <Methane Fermentation> Methane fermentation is a known reaction system in which organic matter is decomposed by the action of multiple anaerobic microorganisms, including methanogens, that function in the absence of oxygen, generating biogas primarily composed of methane gas and carbon dioxide. The organic matter to be decomposed can be the saccharification wastewater obtained by the method for saccharifying woody biomass washing wastewater according to this embodiment. That is, the saccharified woody biomass washing wastewater can be used for methane fermentation. As described above, this saccharification wastewater has an increased sugar concentration due to the saccharification treatment, and therefore can be directly used for methane fermentation. Methane fermentation can be carried out by known methods, such as the activated sludge method, the flotation method, the fixed-bed method, the fluidized-bed method, and the UASB (Upflow Anaerobic Sludge Bed) method. Furthermore, the generated biogas can be used for heat, power generation, fuel, etc. after undergoing processes such as desulfurization.

[0034] (3) Aspects As is clear from the above embodiment, the present disclosure includes the following aspects.

[0035] A method for saccharifying woody biomass washing wastewater according to a first aspect is a method for saccharifying washing wastewater obtained when woody biomass is washed. The method for saccharifying woody biomass washing wastewater includes a washing step and a saccharification step. In the washing step, woody biomass is washed with water. In the saccharification step, an oxidizing agent is added to the washing wastewater obtained in the washing step, and the washing wastewater is saccharified to obtain a saccharified wastewater.

[0036] According to this embodiment, there is no need to adjust the temperature when saccharifying the woody biomass washing waste liquid, and the sugar concentration after saccharification can be increased.

[0037] The method for saccharifying woody biomass washing wastewater according to the second aspect is the first aspect, wherein the temperature of the washing wastewater during the saccharification step is higher than 0°C and lower than 100°C.

[0038] According to this embodiment, the oxidizing agent concentration and the sugar concentration in the washing waste liquid do not change, and the progress and quality of saccharification can be easily controlled.

[0039] A third aspect of the method for saccharifying woody biomass washing wastewater is the method of the first or second aspect, wherein the oxidizing agent includes at least one selected from the group consisting of hydrogen peroxide, calcium peroxide, ammonium persulfate, and peracetic acid.

[0040] According to this embodiment, the washing waste liquid can be saccharified.

[0041] A method for saccharifying woody biomass washing wastewater according to a fourth aspect is any one of the first to third aspects, wherein the content of the oxidizing agent relative to the total amount of the washing wastewater and the oxidizing agent is 1% by mass or more and 6% by mass or less.

[0042] According to this embodiment, the washing waste liquid can be saccharified efficiently and safely.

[0043] A fifth aspect of the method for saccharifying woody biomass washing wastewater is any one of the first to fourth aspects, wherein the woody biomass includes at least one species selected from the group consisting of oil palm and sugarcane.

[0044] According to this aspect, oil palm and sugarcane can be effectively utilized as resources.

[0045] A methane fermentation method according to a sixth aspect includes a fermentation step of performing methane fermentation using saccharification waste liquid. The saccharification waste liquid is obtained by the method for saccharifying woody biomass washing waste liquid according to any one of the first to fifth aspects.

[0046] According to this embodiment, methane fermentation can be carried out using the wastewater from washing of woody biomass, and energy such as heat and electricity can be obtained.

[0047] The present disclosure will be specifically described below with reference to examples, but the present disclosure is not limited to the following examples.

[0048] Example 1 First, 2 g of dry oil palm trunk (OPT) fiber as woody biomass and 20 g of water were placed in a beaker at 20°C and stirred for 1 hour to extract sugars such as starch from the oil palm. Next, solid-liquid separation was performed using a tea strainer to separate the oil palm fiber from the water containing the extracted sugars (hereinafter referred to as "extracted water"). After that, 10 g of the extracted water was measured, and 1 g of 30% hydrogen peroxide solution was added to the extracted water as an additive. The extract was then allowed to stand for 5 minutes to saccharify it. After standing for 5 minutes, the sugar concentration (% Brix) of the extracted water was measured using a digital refractometer (HANNA Instruments, Model No. HI 96811). The measurement results are shown in Table 1.

[0049] (Example 2) The extract was separated in the same manner as in Example 1, and then hydrogen peroxide solution was added as an additive. After the additive was added, the extract was heated to 80°C using a heater and then kept at 80°C for 30 minutes. After the warming was completed, the extract was cooled to 20°C, and the sugar concentration of the extract was measured in the same manner as in Example 1.

[0050] Example 3 The same procedure as in Example 1 was carried out, except that the extraction water was cooled in a refrigerator before the addition of the additives, and the temperature of the extraction water was adjusted to 5°C.

[0051] Comparative Example 1 The same procedure as in Example 1 was repeated except that 0.1 g of α-amylase (manufactured by Amano Enzyme Inc., product name: Claistase T10S) was added as an additive.

[0052] Comparative Example 2 The same procedure as in Example 2 was repeated except that 0.1 g of α-amylase (manufactured by Amano Enzyme Inc., product name: Claistase T10S) was added as an additive.

[0053] (Reference Example) The same procedure as in Example 1 was carried out, except that no additive was added.

[0054]

[0055] As shown in Table 1, compared to Comparative Examples 1 and 2, Examples 1 to 3 had sugar concentrations of 2% Brix or more, and it was confirmed that the sugar concentrations were higher than those of the Reference Example, regardless of the temperature of the extraction water.

Claims

1. A method for saccharifying washing waste liquid obtained when woody biomass is washed, comprising: a washing step of washing the woody biomass with water; and a saccharification step of adding an oxidizing agent to the washing waste liquid obtained in the washing step and saccharifying the washing waste liquid to obtain a saccharified waste liquid.

2. The method for saccharifying woody biomass washing wastewater according to claim 1, wherein the temperature of the washing wastewater during the saccharification step is higher than 0°C and lower than 100°C.

3. The method for saccharifying woody biomass washing wastewater according to claim 1 or 2, wherein the oxidizing agent comprises at least one selected from the group consisting of hydrogen peroxide, calcium peroxide, ammonium persulfate, and peracetic acid.

4. A method for saccharifying woody biomass washing waste liquid according to any one of claims 1 to 3, wherein the content of the oxidizing agent relative to the total amount of the washing waste liquid and the oxidizing agent is 1 mass% or more and 6 mass% or less.

5. The method for saccharifying woody biomass washing wastewater according to any one of claims 1 to 4, wherein the woody biomass includes at least one species selected from the group consisting of oil palm and sugarcane.

6. A methane fermentation method comprising obtaining saccharification waste liquid by the method for saccharifying woody biomass washing waste liquid described in any one of claims 1 to 5, and a fermentation step of carrying out methane fermentation using the saccharification waste liquid.

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