JP7635581B2 - Methane fermentation treatment method - Google Patents

Description

The present invention relates to a method for treating organic waste such as food waste and sludge by methane fermentation, and in particular to a methane fermentation treatment method that can reduce the amount and cost of disposal of dewatered sludge generated by methane fermentation treatment.

Methane fermentation, which uses methane fermentation bacteria to anaerobicly decompose organic waste such as sewage sludge, food waste, and livestock manure, has the excellent advantage of significantly reducing the volume of waste and producing biogas that contains methane gas. This biogas is currently being used to generate electricity.

In order to improve profitability in biogas power generation businesses, it is important to not only increase electricity sales by increasing the methane recovery rate from methane fermentation raw materials, but also to reduce digestate processing costs.

The following methods are described in Patent Documents 1 and 2 as methods for reducing digestate processing costs.

Patent Document 1 describes a method for treating organic sludge that involves aeration treatment of difficult-to-dewater digested sludge, which is produced after anaerobically treating organic sludge, by passing an oxygen-containing gas through the sludge to reduce the sludge viscosity, and then injecting steam into the sludge for heating and dehydration. The method described in Patent Document 1 requires high equipment and electricity costs for the aeration treatment, resulting in high costs.

Patent Document 2 describes a method of treating waste liquid by adding acid and fiber to the methane fermentation digestive liquid remaining after subjecting organic residue to methane fermentation, and then carrying out a solid separation process, a phosphorus recovery process, and a nitrogen control process.

JP 2020-157261 A JP 2009-66499 A

When fiber is added to the digestate as in Patent Document 2, 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 raw material for methane fermentation, and methane is not recovered.

The objective of the present invention is to provide a methane fermentation treatment method that can reduce the cost of reducing the amount of dewatered sludge generated.

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).

A wet methane fermentation tank is suitable as the methane fermentation tank.

In one aspect of the present invention, the fiber content, which is the residual dry solids contained in the digested liquid from the methane fermentation tank and which are collected using 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 solids 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.

In one aspect of the present invention, the digestive fluid is subjected to coagulation treatment using a cationic polymer, and the coagulated sludge is dewatered using a dehydrator.

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

In the present invention, organic matter containing insoluble dietary fiber is used as part of the methane fermentation raw material. This allows fiber to remain in the digestive fluid, and when the digestive fluid is dehydrated, the remaining fiber acts as a conduit for water, reducing the moisture content of the dehydrated cake.

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

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, in an amount of 10 wt% or more (wet weight excluding dilution water) of the total organic waste, which is the raw material for methane fermentation, the fiber content (% SS concentration) in the digestive liquid in the methane fermentation tank is adjusted to 10% or more. As a result, the fiber serves as a conduit for water in the dehydration process after coagulation of the digestive liquid, significantly reducing the moisture content of the dehydrated cake.

FIG. 1 is a block diagram showing a methane fermentation treatment method according to an embodiment. 1 is a graph showing the results of an example.

The embodiment will be described below 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.

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

Although not shown, the methane fermentation tank 7 is provided with a chemical feeder for adding nutrient inorganic salts and a pH adjuster, as well as measuring devices for measuring indicator values such as pH, alkalinity, temperature, COD _Cr , TOC, and ORP (oxidation-reduction potential) in the tank. The measurement data from these measuring devices is input to a controller. The controller receives data on the properties of each of the organic wastes 1a, 1b... (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 waste received. The controller controls the amount of organic waste supplied by the feeders 3a, 3b..., 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 feeder, based on the input data and a preset appropriate range of indicator values.

The sludge produced in the methane fermentation tank 7 is sent to the dehydrator 9 for dehydration, and the dehydrated sludge is extracted as dehydrated cake. The dehydrated filtrate is sent to the wastewater treatment tank 10 for treatment.

In the present invention, organic waste with a high fiber content is preferably 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 suitable for use as a raw material for normal methane fermentation because most of the components decomposed by methane fermentation bacteria have been removed, but they are suitable for use in the present invention because they contain a lot of fiber. The content of insoluble dietary fiber is measured using the modified Prosky method and the method specified in the AOAC 2011.25 method.

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

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 from dehydrated 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).

As described above, in the present invention, it is preferable to add organic waste such as coffee grounds, tea grounds, and wheat bran that contains 5% or more insoluble dietary fiber (wet weight excluding dilution water added to the organic waste before methane fermentation treatment) at a ratio of 10 wt% or more, for example 10 to 30 wt% (wet weight excluding dilution water added to the organic waste before methane fermentation treatment) of the total methane fermentation raw material. It is also preferable to adjust the amount of raw material containing insoluble dietary fiber added to 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% (relative to SS concentration) of fiber (residual solids recovered through a sieve with 150 μm openings).

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

The dehydrator 9 is preferably a belt press, a screw press, or a centrifugal dehydrator, but is not limited to these.

Various types of wastewater treatment tanks, such as aeration tanks, nitrogen treatment tanks, and solid-liquid separation tanks, can be used as the wastewater treatment tank 10.

[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 (content of insoluble dietary fiber: 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 in 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.

<Wet methane fermentation experiment conditions>
The input amount of each raw material is shown in Table 1. 8 L of seed sludge was placed in a methane fermentation tank with a volume of 10 L, and each raw material was input once a day under the conditions in Table 1 (but not input 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 in the methane fermentation tank was kept at about 35°C using a heater. The amount of biogas generated was constantly measured using a gas counter (manufactured by Bioprocess Control, Sweden).

The discharged digestive fluid was periodically (approximately once a week) coagulated with a cationic polymer according to the dehydration test conditions below, then squeezed in a press to check the moisture content of the cake.

<Dehydration test conditions>
150 mL of a 0.3 wt% DAA (dimethylaminoacrylate)-based high cationic polymer aqueous solution was added to 200 mL of digestive fluid as a flocculant, and the mixture was stirred at 4000 rpm for about 6 seconds using a hand mixer (Nihon Seiki Seisakusho, Japan). After stirring, the flocculation state of the digestive fluid was confirmed, and the sludge in which flocs of about 3 mm or larger were formed was gravity filtered using a Buchner funnel with a filter cloth set. If the formation of flocs 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×17.5 mmH) using a spatula, and squeezed with a squeezer for 5 minutes at a surface pressure of 2 kg/ ^cm2 . 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, and 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 FIG. 2.

[Comparative Example 1]
A methane fermentation treatment was carried out in the same manner as in Example 1, except that sewage sludge (content of insoluble dietary fiber: 2 wt % (based on wet weight)) was used instead of coffee grounds. 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 the digestive fluid. The change over time in the moisture content of the dehydrated cake of the digestive fluid is shown in FIG.

[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.

<Results and 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 the moisture content reached 69% on the 130th day of the test.
In contrast, in Comparative Example 1, the water content of the dehydrated cake of the digestive fluid on the 130th day of the test was about 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.

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

Claims (4)

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. 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. The method for methane fermentation treatment according to claim 1 or 2 , wherein the fibrous organic waste is coffee grounds. The methane fermentation method according to any one of claims 1 to 3 , wherein the methane fermenter is a wet methane fermenter.

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