JP2022114586A - Organic waste methane fermentation method and device - Google Patents

Abstract

To provide inexpensive and simple means that can prevent calcium scale deposition.SOLUTION: There are provided organic waste methane fermentation method and device. In the organic waste membrane separation methane fermentation method, a relay tank is provided between a fermentation tank and a membrane separation tank, and a CO2 partial pressure of a gas phase part in the relay tank is adjusted so as to be smaller than a CO2 partial pressure of any gas phase part in the fermentation tank and the membrane separation tank.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method and apparatus for treating organic waste by membrane separation methane fermentation.

Methane fermentation of organic waste such as raw garbage discarded from households and restaurants, raw garbage discharged from factories for preparing side dishes and cooking, processing plants for agricultural products and marine products, and sewage sludge among municipal solid waste. is being done. Fermentation sludge increases during methane fermentation, so it is pulled out of the fermentation tank, but this also pulls out methane bacteria. Therefore, a membrane separation fermentation method has been developed in which the fermentation liquid is sent to a membrane separation tank for membrane separation, and the sludge is returned to the fermentation tank.

An outline of this membrane separation fermentation method is shown in FIG. Organic waste is put into a fermenter and methane fermentation takes place. During this time, the fermentation liquid is circulated between the membrane separation tanks, the permeated liquid separated by the membrane is discharged outside the system, the fermentation sludge is concentrated and returned to the fermentation tank, and the surplus sludge that settles in the fermentation tank is removed. It is pulled out, dehydrated in a dehydrator, and taken out as a dehydrated cake.

By the way, waste such as kitchen waste contains calcium components such as egg shells, shells, bones, etc., and this precipitates as calcium scale in tanks and pipes.

As a countermeasure, Patent Document 1 regards the precipitation of calcium carbonate on the membrane surface of the membrane separation tank as a problem, and adds sodium carbonate to the membrane permeated liquid of the membrane separation tank to precipitate and separate calcium as calcium carbonate. Then, the calcium-removed permeate is returned to the fermenter to prevent precipitation of calcium on the membrane surface.

JP 2004-148269 A

In the membrane separation methane fermentation method, if calcium scale is generated in the system, the operation efficiency will decrease, and in particular, if the deposition on the membrane surface of the membrane separation tank becomes severe, membrane separation will not be possible, so stop the operation and remove the scale. conduct. If all of this scale is removed using acid, the cost of the acid will be high, so pickling should be limited to the membrane module, the scale inside the tank should be scraped off with a scraper, or the piping itself should be replaced. It will be a lot of work. Since the method of Patent Document 1 is costly to use sodium carbonate, it is necessary to develop a more inexpensive and practical method.

An object of the present invention is to provide an inexpensive and simple means for preventing calcium scale deposition.

The inventor of the present invention made intensive studies to solve the above problems, and noticed that calcium scale is likely to occur in fermentation tanks, membrane separation tanks, and peripheral piping. Then, the mechanism of precipitation of calcium carbonate scale was considered as follows.

In other words, in the membrane separation tank, bubbling is performed to prevent sludge clumps from adhering to the surface of the membrane, but since methane fermentation is anaerobic, air cannot be used. is used for bubbling. Carbon dioxide, which is more soluble in water than methane, easily dissolves in the sludge of the membrane separation tank, and the sludge in the membrane separation tank is supersaturated with _CO2 . When the sludge in the membrane separator is returned to the fermenter, the supersaturated CO ₂ is released into the gas phase in the fermenter, and the concentration of dissolved CO ₂ in the sludge decreases. As the CO ₂ concentration in the liquid decreases, the reaction 2HCO ³⁻ →CO ₂ ↑+CO ₃ ²⁻ +H ₂ O proceeds due to equilibrium of carbon ions, and the pH rises. Calcium carbonate (CaCo ₃ ) precipitates in the fermentor due to the increase in pH.

Therefore, as a countermeasure, the present inventors installed a relay tank in the line that returns the sludge from the membrane separation tank to the fermenter, and changed the _CO2 partial pressure in the gas phase of the relay tank to If the partial pressure is less than ₂ partial pressures, CO ₂ is removed from the sludge returned in the relay tank by moving to the gas phase, and precipitation of calcium carbonate in the fermenter can be prevented. Then, in order to lower the CO ₂ partial pressure in the gas phase of the relay tank, it can be achieved by providing a suction mechanism for the gas phase in the relay tank, and this relay tank returns the sludge from the membrane separation tank to the fermentation tank. The present inventors have found that the deposition of calcium carbonate can be prevented even if it is installed in the line that sends the fermentation liquid from the fermenter to the membrane separation tank instead of the line, and that the effect of preventing the deposition of calcium carbonate is exerted throughout the system.

The present invention was made based on such knowledge,
In the membrane separation methane fermentation method for organic waste, a relay tank is provided between the fermenter and the membrane separation tank, and the CO ₂ partial pressure in the gas phase part of the relay tank is adjusted to the gas phase of either the fermentation tank or the membrane separation tank. A method for methane fermentation of organic waste, characterized by adjusting the CO ₂ partial pressure of the
A membrane separation methane fermentation apparatus for organic waste, characterized in that a relay tank having a gas phase suction mechanism is provided between the fermentation tank and the membrane separation tank. When,
The relay tank provides the methane fermentation apparatus described above, which is installed in the line from the membrane separation tank to the fermentation tank.

ADVANTAGE OF THE INVENTION According to the present invention, when organic waste is subjected to methane fermentation by membrane separation, precipitation of calcium carbonate scale in the fermentation apparatus can be prevented, and methane fermentation can be stably operated for a long period of time.

FIG. 4 is a block diagram showing an example of attachment of the relay tank of the present invention; 1 is a block diagram illustrating one embodiment of the present invention; FIG. FIG. 4 is a block diagram illustrating another embodiment of the invention; FIG. 4 is a block diagram illustrating yet another embodiment of the present invention; It is a block diagram which shows the basic composition of the conventional membrane-separation methane fermentation method.

Organic waste is produced by methane fermentation, and includes food waste from homes and restaurants, food waste from factories that produce side dishes and dishes, processing plants for agricultural products and marine products, and waste from sewage treatment plants. This includes discharged sludge.

Garbage discharged from these households and restaurants is generally put in plastic bags, and raw garbage discharged from manufacturing plants for delicatessens and dishes is often put in cardboard boxes. Combustible waste is crushed with a crusher, bag breaker, etc., and the contents are taken out.

Conventionally, the organic waste received in this way is first sorted in order to remove fragments of plastic bags generated during shredding and stones mixed in the organic waste.

The organic waste from which these unsuitable substances for fermentation have been removed as residue is put into the fermentation tank.

The fermenter has a closed structure, with a gas discharge port at the top and a sludge extraction port at the bottom. An inlet is provided for the return sludge from the fermenter and is usually also fitted with an agitator to agitate the fermentation liquor. In addition, instruments necessary for fermentation management such as liquid level gauges and thermometers are also installed.

The sludge withdrawal port is connected to a dehydrator where the sludge is dewatered and discharged as a dehydrated cake.

A plurality of membrane modules are provided in the membrane separation tank, and an air diffuser for cleaning the membranes is installed below them. It is preferable that the membrane has an average pore diameter of about 0.05 to 0.4 μm which does not allow methane bacteria to pass through. Since the methane fermentation is anaerobic, air is not preferable for the air diffuser, and biogas or the like, which is mainly composed of methane and carbon dioxide discharged from the fermentation tank, is used.

The organic waste is usually added with water before being put into the fermenter, and a mixing tank for that purpose can be provided if necessary. Addition of water brings the TS concentration to about 4 to 10% by weight. The concentration of organic waste in the fermentation tank is about 2 to 5% by weight of TS, about 10,000 to 60,000 mg/L of COD-Cr, about 1,000 to 7,000 mg/L of Ca, usually 2,000 to 6,000 mg/L. The pH is about 6 to 8, usually about 7.0 to 7.9. Methane fermentation is performed by inoculating methane bacteria to this. Methane fermentation may be carried out according to a conventional method, and high-temperature fermentation may be carried out using thermophilic bacteria, but it may be carried out at a usual temperature of about 35 to 40°C. During fermentation, it is necessary to maintain the concentration of methane bacteria so that the fermentation proceeds smoothly, and the TS concentration is maintained at about 2 to 5% by weight. Therefore, the fermented liquid is circulated between the fermenter and the membrane separation tank, the water separated by the membrane is drawn out, the sludge is returned to the fermenter, and the excess sludge precipitated in the fermenter is taken out from the sludge withdrawal port. .

The fermented liquid in the fermentation tank is stirred with a stirrer, but if the stirring is too strong, the sludge that settles in the layer will rise and the amount of transfer to the membrane separation tank will increase. Better to

Fermentation may be performed continuously or batchwise.

The present invention is characterized in that a relay tank is provided between the fermenter and the membrane separation tank in such a membrane separation methane fermentation method.

As an example, as shown in Fig. 1, organic waste is put into a fermentation tank and methane fermentation is performed. is installed. In the membrane separation tank, part of the biogas discharged from the fermentation tank is supplied and bubbling is performed, thereby dissolving the carbon dioxide gas in the biogas, thereby changing the pH of the fermentation liquid from pH 7.8 to pH 7, for example. .6. Although not shown in the figure, the biogas bubbled in the membrane separation tank is extracted from the top of the tank and connected to the gas phase of the fermenter. The fermented liquid that has left the membrane separation tank is put into a relay tank, and the dissolved carbon dioxide gas is released into the gas phase, causing the pH to rise to 7.8. A calcium component precipitates as calcium carbonate. The gas phase in the membrane separation tank is not connected to the relay tank, and a bypass that does not pass through the relay tank from the membrane separation tank is provided to remove calcium carbonate accumulated in the relay tank without stopping the fermentation operation. I am making it possible.

The relay tank has a sealed structure to prevent contact with oxygen in the air, and in order to facilitate the deposition and precipitation of calcium carbonate scale, a stirrer is installed, which is controlled by an inverter to enable gentle stirring. is preferred. Precipitated calcium carbonate particles are present in the relay tank, and act as seed crystals to precipitate carbon dioxide present in the sludge in a supersaturated state as calcium carbonate. If the CO ₂ partial pressure in the gas phase of the relay tank is lowered, the reaction of 2HCO ³⁻ →CO ₂ ↑+CO ₃ ²⁻ +H ₂ O due to the equilibrium of carbonate ions proceeds more easily, and the pressure in the relay tank increases. The pH of the fermented liquid rises and the amount of precipitated calcium carbonate increases.

_Lowering the CO ₂ partial pressure in the gas phase of the relay tank can be done by sucking the gas phase and lowering its pressure. It is preferable to lower the lower CO ₂ partial pressure by about 0.1 to 2 kPa, preferably about 0.5 to 1.5 kPa, from the lower CO ₂ partial pressure. The CO ₂ partial pressure is obtained by measuring the total pressure of the gas phase and the CO ₂ concentration in the gas phase, and calculating the gas phase pressure×CO ₂ concentration (vol %)=CO ₂ partial pressure. The total pressure of the gas phase can be measured, for example, with a pressure gauge, and the CO ₂ concentration can be obtained, for example, by collecting gas from the gas phase and measuring it by gas chromatography.

Means for lowering the CO ₂ partial pressure in the gas phase can be, for example, an intake pump, a blower, an aspirator, etc., but it is preferable to use a suction device in the fermentation facility, for example, as shown in FIG. Thus, the gas phase part of the relay tank can be connected to the deodorizing equipment. The deodorizing equipment sucks and deodorizes the gas phase of the garbage receiving equipment and the mixing tank before putting it into the fermentation tank so that the odor does not leak to the outside. is used.

Further, if the relay tank is provided with a mechanism for supplying a pH adjuster to raise the pH of the fermentation liquid in the relay tank, the precipitation amount of calcium carbonate can be further increased. Alkali such as sodium hydroxide can be used as the pH adjuster. The pH to be adjusted is pH 6 or more and 8 or less, and a pH of about 7 to 7.9 is particularly preferable. Since the pH adjuster is added to the fermented liquid in the relay tank, the addition position may be outside the relay tank and slightly upstream.

Most of the calcium carbonate deposited in the relay tank is solid and can be extracted from the bottom of the relay tank. In addition, since it is deposited on the tank wall, stirring blades, etc., these are scraped off. Since the sludge withdrawn from the relay tank contains a large amount of precipitated calcium scale, the proportion of inorganic substances in the sludge increases and the moisture content of the dewatered cake decreases. The sludge pulled out from this relay tank may be dewatered as it is by a dehydrator, but as shown in FIG. can. Fermentation liquor may be withdrawn from the relay tank with the sludge of the relay tank rather than from the fermenter.

As shown in FIG. 4, the relay tank may be installed on the line from the fermenter to the membrane separation tank, thereby suppressing the deposition of calcium scale in the membrane separation tank. The relay tank may be installed in both the line from the membrane separation tank to the fermenter and the line from the fermenter to the membrane separation tank.

The apparatus shown in Figure 2 was used. As the organic waste, raw garbage collected from households in a plastic bag was put into a hopper, and the bag was broken using a crusher. Then, after removing unsuitable materials for fermentation such as plastic bag fragments and stones using a separator, water was added, stirred and mixed in a mixing tank, and pumped into the fermentation tank. The amount of dilution water is controlled so that the concentration of TS in the mixing tank is about 4 to 10% by weight. The concentration of organic waste in the fermenter was 3 wt% TS, 23,000 mg/L COD-Cr, 4,000 mg/L Ca, and pH 7.8.

This organic waste is inoculated with methane bacteria to start methane fermentation. and let it bubble. The gas phase portion of the relay tank was continuously sucked at a constant speed by a deodorizer, and sludge was drawn out exclusively from the bottom of the relay tank at 100 L/h (not shown). The organic waste was continuously fed into the fermenter, thus measuring the gas phase pressure and _CO2 partial pressure in the fermenter, relay tank, and membrane separation tank when the methane fermentation reached a steady state. The _CO2 partial pressures were 1.2 kPa, 0 kPa, 1.2 kPa and the pressures in the gas phase were 3 kPa, 0 kPa, 3 kPa. The above CO ₂ partial pressure was obtained by measuring the CO ₂ concentration by gas chromatography.

Then, when the calcium concentration of the extracted sludge was measured, the results shown in Table 1 were obtained.

In Table 1, the method without a relay tank is the conventional method, and among the cases with a relay tank, pH 7.8 is without suction by the deodorizing device, and pH 7.9 is an example of the present invention in which suction was performed. Thus, it can be seen that the provision of the relay tank removes calcium, and the suction of the gas phase portion of the relay tank further increases the calcium removal rate.

In addition, when the extracted sludge was dehydrated under the same conditions using the same dehydrator, the water content of the dehydrated cake was 77% by weight in the conventional method without a relay tank. In Example 1, it was shown that the dehydration effect was also good at 72% by weight.

Then, when the operation is continued and the flow rate of the sludge circulation pump from the fermentation tank to the membrane separation tank drops by 20% or more, it becomes necessary to remove the scale from the piping from the fermentation tank to the membrane separation tank. In the conventional method without a tank, once a month was necessary, but in the example of the present invention, once every four months, the operation period could be greatly extended.

INDUSTRIAL APPLICABILITY According to the present invention, precipitation of calcium in a fermenter or a membrane separation tank can be prevented, and the methane fermentation operation can be continued smoothly. Therefore, it can be widely used for methane fermentation of organic waste containing calcium.

In other words, in the membrane separation tank, bubbling is performed to prevent sludge clumps from adhering to the surface of the membrane, but since methane fermentation is anaerobic, air cannot be used. is used for bubbling. Carbon dioxide, which is more soluble in water than methane, easily dissolves in the sludge of the membrane separation tank, and the sludge in the membrane separation tank is supersaturated with _CO2 . When the sludge in the membrane separator is returned to the fermenter, the supersaturated CO ₂ is released into the gas phase in the fermenter, and the concentration of dissolved CO ₂ in the sludge decreases. As the CO ₂ concentration in the liquid decreases, the reaction 2HCO ³⁻ →CO ₂ ↑+CO ₃ ²⁻ +H ₂ O proceeds due to equilibrium of carbon ions, and the pH rises. Calcium carbonate ( CaCO ₃ ) precipitates in the fermentor due to the increase in pH.

The present invention was made based on such knowledge,
In the membrane separation methane fermentation method for organic waste, a relay tank equipped with a gas phase suction mechanism is provided between the fermenter and the membrane separation tank, and the CO ₂ partial pressure in the gas phase of the relay tank is transferred to the fermenter. A method for methane fermentation of organic waste, characterized in that calcium carbonate is precipitated in the relay tank by adjusting it to be lower than the CO ₂ partial pressure of the gas phase of any of the and membrane separation tanks;
An organic waste membrane separation methane fermentation apparatus for organic waste, characterized in that a relay tank is provided between the fermentation tank and the membrane separation tank to precipitate calcium carbonate with a suction mechanism for the gas phase part. a methane fermentation device for a substance;
The relay tank provides the methane fermentation apparatus described above, which is installed in the line from the membrane separation tank to the fermentation tank.

Claims (10)

In the membrane separation methane fermentation method for organic waste, a relay tank is provided between the fermenter and the membrane separation tank, and the CO ₂ partial pressure in the gas phase part of the relay tank is adjusted to the gas phase of either the fermentation tank or the membrane separation tank. A method for methane fermentation of organic waste, characterized by adjusting the partial pressure of CO ₂ to be lower than the partial pressure of CO 2 in the organic waste. A membrane separation methane fermentation apparatus for organic waste, characterized in that a relay tank having a gas phase suction mechanism is provided between the fermentation tank and the membrane separation tank. . 3. The methane fermentation apparatus according to claim 2, wherein an agitator having an inverter control mechanism for controlling agitation is attached to the relay tank. 4. The methane fermentation apparatus according to claim 2 or 3, wherein the suction mechanism for the gas phase is deodorizing equipment attached to the methane fermentation apparatus. 5. The methane fermentation apparatus according to any one of claims 2 to 4, further comprising a mechanism for supplying a pH adjuster to the relay tank. 6. The methane fermentation apparatus according to any one of claims 2 to 5, wherein the relay tank is provided with a sludge extraction section. 7. The methane fermentation apparatus according to any one of claims 2 to 6, wherein the sludge drawn from the fermenter is combined with the sludge drawn from the relay tank. The methane fermentation apparatus according to any one of claims 2 to 7, wherein the relay tank is installed in a line from the fermentation tank to the membrane separation tank. The methane fermentation apparatus according to any one of claims 2 to 7, wherein the relay tank is installed on a line from the membrane separation tank to the fermentation tank. 8. The methane fermentation apparatus according to any one of claims 2 to 7, wherein the relay tank is installed in both the line from the fermenter to the membrane separation tank and the line from the membrane separation tank to the fermenter.

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