Methane fermentation treatment method

Abstract

To provide a methane fermentation method capable of reducing cost for reducing a dewatering sludge generation amount.SOLUTION: In a methane fermentation method for fermentation treatment of an organic waste in a methane fermentation tank, the methane fermentation method is characterized by that a fibrous organic waste containing an insoluble dietary fiber of 5 wt.% (per wet weight; excluding dilution water to be added to the organic waste before the methane fermentation treatment) is made to be contained at a rate of 10% or more (per wet weight, excluding the dilution water) of the whole organic waste.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for treating organic waste such as food waste and sludge by methane fermentation, and more particularly to a methane fermentation treatment method capable of reducing the amount and cost of disposal of dewatered sludge generated by the methane fermentation treatment. [Background technology]

[0002] Methane fermentation, which uses methane fermentation bacteria to anaerobicly decompose organic waste such as sewage sludge, food waste, and livestock manure, has the excellent advantages of significantly reducing the volume of waste and generating biogas containing methane gas. This biogas is currently being used to generate electricity.

[0003] In order to improve the profitability of biogas power generation businesses, it is important to increase the electricity sales price by increasing the methane recovery rate from methane fermentation raw materials, as well as to reduce digestate processing costs.

[0004] As a method for reducing digestate processing costs, the following methods are described in Patent Documents 1 and 2.

[0005] Patent Document 1 describes a method for treating organic sludge, which involves aeration treatment of difficult-to-dewater digested sludge resulting from anaerobically treating the organic sludge with an oxygen-containing gas to reduce the sludge viscosity, and then injecting steam into the sludge for thermal dehydration treatment. The method described in Patent Document 1 requires high costs due to the equipment and electricity costs for the aeration treatment.

[0006] Patent Document 2 describes a method in which an acid and fiber are added to the methane fermentation digestive liquid remaining after methane fermentation of organic residues, and the waste liquid is then treated through a solid separation process, a phosphorus recovery process, and a nitrogen control process. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-157261 A [Patent Document 2] JP 2009-66499 A Summary of the Invention [Problem to be solved by the invention]

[0008] As in Patent Document 2, when fiber is added to the digestate liquid after the digestion, fiber costs are often incurred. Fiber chemicals are sold as dehydration aids, but they are expensive. Even if the fiber is available free of charge, the added fiber is not used as a methane fermentation raw material, and methane is not recovered.

[0009] An object of the present invention is to provide a methane fermentation treatment method capable of reducing the cost for reducing the amount of dewatered sludge generated. [Means for solving the problem]

[0010] The methane fermentation treatment method of the present invention is a methane fermentation treatment method in which organic waste is fermented in a methane fermentation tank, and is characterized in that fibrous organic waste containing 5 wt% or more of insoluble dietary fiber (wet weight excluding dilution water added to the organic waste before methane fermentation treatment) is treated so that it contains 10% wt% or more of the total organic waste (wet weight excluding dilution water).

[0011] The methane fermenter is preferably a wet methane fermenter.

[0012] In one embodiment of the present invention, the fiber content, which is the residual dry solid matter contained in the digested liquid from the methane fermentation tank and collected through a sieve with a mesh size of 150 μm, is set to 10% or more (% relative to SS concentration). The SS concentration is the residual dry solid matter when filtered through a 1 μm filter. The fiber content and SS concentration are calculated based on the weight of the residue filtered through the sieve or filter after drying at 105° C. for 2 hours or more.

[0013] In one embodiment of the present invention, the digestive liquid is subjected to a flocculation treatment using a cationic polymer, and the flocculated sludge is dewatered using a dewatering machine.

[0014] In one embodiment of the present invention, the fibrous organic waste is at least one of coffee grounds, tea grounds, and wheat bran. Effect of the Invention

[0015] In the present invention, organic matter containing insoluble dietary fiber is used as part of the methane fermentation raw material, which allows fiber to remain in the digestive liquid and function as a water channel during dehydration of the digestive liquid, thereby reducing the moisture content of the dehydrated cake.

[0016] The present invention is very economical because it does not require the purchase of fiber as a dehydration aid, and does not require additional equipment such as a digestive liquid aeration tank. In addition, since insoluble dietary fiber is accepted as a methane fermentation raw material, a gate fee can be obtained, and methane gas can be recovered, ensuring profits.

[0017] For example, by adding fibrous organic waste such as coffee grounds, tea grounds, and wheat bran that contains 5 wt% or more, for example 5 to 50 wt% (wet weight excluding dilution water added to the organic waste before methane fermentation treatment) of insoluble dietary fiber, which is 10 wt% or more (wet weight excluding dilution water) of the entire organic waste, which is the raw material for methane fermentation, the fiber content (% SS concentration) in the digested liquid in the methane fermentation tank is adjusted to 10% or more. As a result, the fiber serves as a path for water in the dehydration process after coagulation of the digested liquid, and the moisture content of the dehydrated cake is significantly reduced. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram showing a methane fermentation treatment method according to an embodiment. [Diagram 2] 1 is a graph showing the results of an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment will be described with reference to the drawings. As shown in Fig. 1, multiple types of organic waste 1a, 1b... are dumped into respective pits (receiving tanks) 2a, 2b.... Of these, organic waste 1a is fibrous organic waste with a high content of insoluble dietary fiber.

[0020] The organic waste in each pit is supplied to a crusher 4 by feeders 3a, 3b, ... and crushed, and then supplied to a mixing tank (or acid generation tank) 5 (or directly without passing through the crusher 4) and mixed. If necessary, dilution water is supplied to the mixing tank 5 from a dilution water supply means 6 to make a slurry with an appropriate moisture content, and then the slurry is supplied to a methane fermentation tank (wet methane fermentation tank in this embodiment) 7 and subjected to methane fermentation treatment. In addition to the dilution water, digested liquid from the methane fermentation tank 7 (circulating or excess digested liquid in the methane fermentation tank 7) may be mixed. The generated methane-containing biogas is taken out from an extraction line 8.

[0021] Although not shown in the figure, the methane fermentation tank 7 is equipped with a chemical feeder for adding nutrient inorganic salts and a pH adjuster, as well as a device for controlling the pH, alkalinity, temperature, and COD in the tank. Cr The waste is provided with measuring devices for measuring index values ​​such as TOC, ORP (oxidation-reduction potential), etc. The data measured by these measuring devices is input to a controller. The controller stores data on the properties of each of the organic wastes 1a, 1b, etc. (moisture content, fiber content, TS (total solids) concentration, VS (volatile solids) concentration, BVS (biodegradable volatile solids) concentration, COD Cr , C / N (carbon / nitrogen) ratio, etc.) and the amount of received waste are input. Based on the input data and a preset appropriate range of index values, the controller controls the amount of organic waste supplied by the feeders 3a, 3b, etc., the amount of dilution water injected by the dilution water supply device 6, and the amount of inorganic salts and pH adjuster supplied by the chemical injection device.

[0022] The sludge produced in the methane fermentation tank 7 is sent to a dehydrator 9 for dehydration, and the dehydrated sludge is taken out as a dehydrated cake. The dehydrated filtrate is sent to a wastewater treatment tank 10 for treatment.

[0023] In the present invention, organic waste with a high fiber content is preferably a fibrous organic waste containing 5 wt% or more, for example 5 to 50 wt% (wet weight excluding dilution water added to the organic waste before methane fermentation treatment) of insoluble dietary fiber, such as coffee grounds, tea grounds, and wheat bran. Coffee grounds are not desirable as a raw material for normal methane fermentation because most of the components decomposed by methane fermentation bacteria have been removed, but are suitable for use in the present invention because they contain a lot of fiber. The content of insoluble dietary fiber is measured by the modified Prosky method and the method specified in the AOAC 2011.25 method.

[0024] Examples of organic waste to be treated include, but are not limited to, biological treatment sludge, food waste, livestock manure, agricultural waste, etc. The content of insoluble dietary fiber in these organic wastes is usually 5 wt% or less, and particularly 3 wt% or less (wet weight, excluding dilution water added to the organic waste before methane fermentation treatment).

[0025] The organic waste is preferably finely crushed using a crusher (such as a hammer mill) and made into a slurry using dilution water (in addition to industrial water, wastewater treatment water or dehydrated filtrate obtained by dehydrating sludge from the methane fermentation tank 7 can also be used) or digestion liquid (circulating or excess digestion liquid in the methane fermentation tank 7).

[0026] In the present invention, as described above, it is preferable to charge organic waste such as coffee grounds, tea grounds, and wheat bran that contains 5% or more of insoluble dietary fiber (based on wet weight, excluding dilution water added to the organic waste before methane fermentation treatment) at 10 wt% or more, for example, 10 to 30 wt% (based on wet weight, excluding dilution water added to the organic waste before methane fermentation treatment) of the entire methane fermentation raw material. It is also preferable to adjust the amount of raw material containing insoluble dietary fiber charged into the raw material mixing tank 5 so that the digestive liquid in the methane fermentation tank contains 10 wt% or more, for example, 10 to 30 wt% (based on SS concentration) of fiber (residual solids collected through a sieve with a mesh size of 150 μm).

[0027] It is preferable that the HRT of the methane fermenter 7 is set to 20 to 65 days, the temperature is set to 30 to 60° C., and the TS concentration of the digested liquid is set to 1 to 10%.

[0028] The dehydrator 9 is preferably a belt press, a screw press, a centrifugal dehydrator, or the like, but is not limited thereto.

[0029] As the wastewater treatment tank 10, various types of tanks such as an aeration tank, a nitrogen treatment tank, or a solid-liquid separation tank can be used. EXAMPLES

[0030] [Example 1] <Organic waste> Organic waste to be treated: Mock food waste made using dumplings, sewage sludge, salad oil, rice, dog food, and orange juice (insoluble dietary fiber content 2 wt% (wet weight)) Organic waste with high fiber content: Coffee grounds (25 wt% insoluble dietary fiber (wet weight)) The simulated food waste and coffee grounds were mixed at a ratio of 8:2 on a wet weight basis, and the insoluble dietary fiber was adjusted to 20 wt% of the total organic waste, and the mixture was subjected to a wet methane fermentation experiment.

[0031] <Wet methane fermentation experiment conditions> The amount of each raw material added is shown in Table 1. 8 L of seed sludge was placed in a 10 L methane fermentation tank, and each raw material was added once a day under the conditions in Table 1 (but not added on two days a week), and the tank was operated so that the HRT was 42 days. The methane fermentation tank was sealed to prevent air from entering the tank, and an anaerobic state was maintained. The sludge in the methane fermentation tank was constantly stirred and mixed at 100 rpm using a stirrer. The temperature inside the methane fermentation tank was kept at approximately 35°C using a heater. The amount of biogas generated was constantly measured using a gas counter (Bioprocess control, Sweden).

[0032] The discharged digestive fluid was periodically (approximately once a week) coagulated with a cationic polymer according to the dehydration test conditions described below, and then squeezed in a squeezer to examine the moisture content of the cake.

[0033] [Table 1]

[0034] <Dehydration test conditions> 150 mL of a 0.3 wt% DAA (dimethylaminoacrylate)-based high cationic polymer aqueous solution was added as a flocculant to 200 mL of digestive fluid, and the mixture was stirred at 4000 rpm for approximately 6 seconds using a hand mixer (Nihon Seiki Seisakusho, Japan). After stirring, the flocculation state of the digestive fluid was confirmed, and sludge in which flocs of about 3 mm or larger had formed was gravity filtered using a Buchner funnel equipped with a filter cloth. If floc formation was insufficient, the polymer flocculant was added again, and the stirring operation was repeated under the same conditions. The flocculated sludge after filtration was filled into a cake mold (30 mmφ x 17.5 mmH) using a spatula, and the surface pressure was 2 kg / cm. 2 The cake was then peeled off from the filter cloth using a metal spatula, weighed, and dried at 105°C for 12 hours or more. The weight after drying was measured to determine the moisture content. The change in moisture content of the dehydrated cake over time is shown in Figure 2.

[0035] [Comparative Example 1] Except for using sewage sludge (content of insoluble dietary fiber: 2 wt % (based on wet weight)) instead of coffee grounds, methane fermentation treatment was carried out in the same manner as in Example 1. The raw material feeding conditions are shown in Table 2. The dehydration test was carried out in the same manner as in Example 1, except that 60 mL of a 0.3 wt% DAA (dimethylaminoacrylate)-based high cationic polymer aqueous solution was added as a flocculant to 200 mL of digestive fluid. The change in the moisture content of the dehydrated cake of the digestive fluid over time is shown in FIG.

[0036] [Comparative Example 2] A wet methane fermentation experiment and a dehydration test were carried out in the same manner as in Example 1, using only the simulated food waste without adding coffee grounds.

[0037] [Table 2]

[0038] <Results / Discussion> The change over time in the moisture content of the dehydrated cake of the digestive fluid in Example 1 and Comparative Example 1 is shown in FIG. As shown in FIG. 2, in Example 1, the moisture content of the dehydrated cake was reduced by about 9% compared to Comparative Example 1, and reached 69% on the 130th day of the test. In contrast, in Comparative Example 1, the moisture content of the dehydrated cake of the digestive fluid on the 130th day of the test was approximately 78%. In Comparative Example 2, the moisture content of the dehydrated cake of the digestive fluid on the 130th day of the test was 78%, which was about 9% higher than the results of the Examples. [Explanation of symbols]

[0039] 1a, 1b Organic waste 2a, 2b Pit 3a, 3b Feeder 7. Methane fermentation tank 9 Dehydrator 10 Wastewater treatment tank

Claims

1. A methane fermentation treatment method for fermenting organic waste in a methane fermentation tank, comprising: A methane fermentation treatment method for treating fibrous organic waste containing 5 wt % or more of insoluble dietary fiber (based on wet weight, excluding dilution water added to the organic waste before methane fermentation treatment) in an amount of 10% or more of the total organic waste (based on wet weight, excluding dilution water), The organic waste to be subjected to the methane fermentation treatment is food waste having an insoluble dietary fiber content of 3 wt % or less (wet weight, excluding dilution water added to the organic waste before the methane fermentation treatment), The fibrous organic waste is at least one of coffee grounds, tea grounds, and wheat bran; A methane fermentation treatment method, characterized in that the fiber content, which is the residual dry solid matter contained in the digestion liquid from the methane fermentation tank and recovered through a sieve with an opening of 150 μm, is reduced to 10% or more (% relative to SS concentration).

2. 2. The method for methane fermentation treatment according to claim 1, further comprising the steps of flocculating the digested liquid with a cationic polymer and dewatering the flocculated sludge in a dehydrator.

3. 3. The method for methane fermentation treatment according to claim 1 or 2, wherein the fibrous organic waste is coffee grounds.

4. The methane fermentation method according to any one of claims 1 to 3, wherein the methane fermenter is a wet methane fermenter.

Images