JP7103577B2 - Mixed methane fermentation method of sewage sludge and swill - Google Patents

Description

The present invention relates to a methane fermentation method in which sewage sludge and swill are mixed and supplied into a methane fermentation tank, and methane fermentation is performed while separating the liquid in the tank by membrane separation.

In recent years, studies have been conducted to separate food waste from general waste under the jurisdiction of the Ministry of the Environment into a sewage sludge methane fermentation facility under the jurisdiction of the Ministry of Land, Infrastructure, Transport and Tourism, and introduce the separated food waste into the above-mentioned methane fermentation facility.

As a technique for treating sludge and food waste in septic tanks, urine and septic tank sludge are dehydrated and separated into dehydrated sludge and dehydration separation liquid, and the dehydration separation liquid is biologically treated in a biological treatment process, and surplus generated in the biological treatment process. Sewage sludge is solubilized in an ozone reaction tank to produce easily decomposable organic matter, and this ozone-capable sludge is returned to the biological treatment process for biological treatment to eliminate excess sludge, resulting in dehydrated sludge and food waste. A method has been developed in which organic waste is methane-fermented in a methane fermentation step and fermented sludge generated in the methane fermentation step is composted in a composting step (Patent Document 1). This method adjusts the production of compost by leading part or all of the fermented sludge to the biological treatment process, and adjusts the production of compost by leading part or all of the surplus sludge to the methane fermentation process. Is what you do.

In addition, a methane fermentation tank that performs methane fermentation treatment of organic substances in digestion sludge, and a raw material supply that supplies organic substances that are fluid and have a higher temperature than the digestion sludge in the methane fermentation tank to the methane fermentation tank. It is a methane fermentation treatment device that has means and membrane separation means and performs methane fermentation treatment while concentrating the solid content in digestive sludge in the methane fermentation tank by the membrane separation means, and permeates the separation membrane of the membrane separation means. Heat exchange is performed between the membrane permeation liquid outlet pipe that takes out the membrane permeate liquid to the outside, the membrane permeate liquid that flows in the membrane permeate liquid outlet pipe, and the organic substance that is supplied to the methane fermentation tank by the raw material supply means. A methane fermentation treatment apparatus (Patent Document 2), which is characterized by being provided with a heat exchange means for heating the membrane permeate, has also been developed.

This technology is magnesium ammonium phosphate deposited on pipes, etc. during methane fermentation treatment of organic substances containing a large amount of phosphorus, magnesium, and nitrogen, such as distilled liquor, distilled waste liquid from alcohol manufacturing processes, and food factory waste. Can be suppressed at low cost.

The swill is subdivided into a paste with a crusher, and the swill is supplied to the swill treatment facility in a sealed state through a pipe by a pumping pump, and the organic matter in the swill is decomposed by methane fermentation at the methane treatment facility. A swill treatment method (Patent Document 3), which is characterized by gasifying into methane gas and carbon dioxide gas, is also known.

JP-A-2002-273491 JP-A-2010-207700 Japanese Unexamined Patent Publication No. 2002-119937

The present inventors considered and examined the treatment of sewage sludge in a facility where methane fermentation treatment is performed on food waste by the membrane separation methane fermentation method. , Found that it is difficult to mix sewage sludge.

Regarding this point, in Patent Document 1, it is recognized that excess sludge generated in the biological treatment step is basically solubilized in the ozone reaction tank and returned to the biological treatment step, which causes a problem in the methane fermentation step. Not.

Further, Patent Document 2 shows that organic waste is solubilized, but this organic waste does not contain sewage sludge.

Patent Document 3 also solubilizes swill, but it is not shown to solubilize sewage sludge.

An object of the present invention is to provide a method capable of efficient methane fermentation treatment in a facility for mixed fermentation of sewage sludge and swill.

The present inventors have made diligent studies to solve such a problem, and the digested sludge of food waste has high biodegradability and good membrane filterability, but excess sludge has already been generated by decomposing sewage. It was found that the sludge has low biodegradability, and the sludge concentration is as low as about 4% by volume, so that there is no problem in handling, but the membrane filterability is low. Therefore, the present inventor has come to pay attention to a method of solubilizing sewage sludge in advance and then adding it to a methane fermenter, and has found that this can solve the problem of membrane clogging, and can complete the present invention. rice field.

That is, the present invention is characterized in that when sewage sludge and food waste are methane-fermented by a membrane-separated methane fermentation method, excess sludge of sewage sludge is solubilized in advance and then put into a methane fermentation tank. It provides a mixed methane fermentation method of sludge and food waste.

According to the present invention, sewage sludge can be treated inexpensively and efficiently at an existing facility where food waste is treated by the membrane separation methane fermentation method, and biogas generated by methane fermentation can be used to generate electricity. Available.

It is a flow sheet which shows one Embodiment of this invention. It is a figure which shows the schematic structure of an example of the apparatus which carries out this embodiment.

The food waste to which the methane fermentation of the present invention is applied is not particularly limited, but is discharged from households, supermarkets, convenience stores, restaurants, etc., as well as from food factories, fish processing plants, livestock product processing plants, etc. And so on. If necessary, they are methane-fermented after pretreatment such as crushing or separating wastes other than food waste. Generally, for handleability (pumping into the fermenter, etc.), it is better to add water to dilute this swill and then put it into the methane fermenter, and the degree of dilution is 7 to 10 in terms of total solid content concentration. It is appropriate to set the volume to about%.

Sewage sludge is sludge discharged from a sewage treatment plant, and is divided into initial sludge and surplus sludge. The initial sludge is sludge that is separated by first settling the received sewage in a settling basin, and excess sludge is generated in a biological treatment tank such as an aeration tank or an oxygen-free tank.

Excess sludge discharged from the aeration tank or the like is concentrated from about 1 volume% to 4 volume% by mechanical concentration or the like. After that, it is put into a solubilization tank, solubilized continuously or batchwise, and put into a methane fermentation tank.

The solubilization of excess sludge is to destroy the cell membrane by thermal energy, physical energy, or chemical treatment, extract the cytoplasm, or break the bond of high molecular weight organic matter to reduce the molecular weight.

As a method for measuring the degree of solubilization, there are a TS decomposition rate (TS: Total Solid) and a solubility CODcr increase rate (hereinafter, the solubility CODcr will be referred to as S-CODcr).
The TS decomposition rate is obtained by measuring the TS of raw sludge and the TS of treated sludge, and using the respective analytical values as TS1 and TS2 by the following calculation. TS corresponds to JIS K0102 of total evaporation residue. The outline is that a predetermined volume of sample is taken and placed in an evaporating dish. After heating at 105 ° C. for 2 hours and allowing to cool in a desiccator, the weight of the dried product is measured and converted in mg / L.
(TS1-TS2) / TS1 × 100
The CODcr increase rate is expressed by the following formulas as CODcr1, S-CODcr1 and S-CODcr2 for raw sludge and S-CODcr for treated sludge, respectively.
(S-CODcr2-S-CODcr1) / CODcr1 × 100

In the present invention, it is desirable to solubilize the excess sludge with a solubility COD _cr increase rate of about 10 to 70%, preferably about 15 to 40%.

Examples of the solubilization method include heat treatment, bead mill treatment, hydrothermal treatment, ultrasonic treatment, alkaline ultrasonic treatment, and alkaline treatment.

The heat treatment is a method in which excess sludge is placed in a heat treatment tank and heat-treated at 60 to 90 ° C. with stirring, and the heating time is usually about 0.5 to 2 hours at 60 to 90 ° C. If the temperature is lower than 60 ° C., the solubilization degree of 10 is not reached, and if the temperature is 100 ° C. or higher, the protein tends to harden and the solubilization rate decreases as if the egg becomes a boiled egg. The heat treatment may be performed in a tank equipped with a normal stirrer.

In the bead mill treatment, sludge is put into a reaction tank filled with about 30 to 60% of alumina or ceramic beads having a particle size of about 0.2 to 1.0 mm, and mixed at about room temperature for about 10 minutes to 1 hour. good. Excess sludge can be left as it is, and no special water needs to be added.

The hydrothermal treatment may be carried out by inflowing steam into the excess sludge and treating it in a high-temperature and high-pressure container having a temperature of 160 to 180 ° C. and a pressure of 0.6 to 0.8 MPa for about 20 to 30 minutes. When the pressure is 0.5 MPa or less, the solubilization rate decreases. On the other hand, 1 MPa or more may be used, but it is not preferable in terms of increasing the container cost.

In the ultrasonic treatment, excess sludge may be put into a water tank having an ultrasonic irradiation device and irradiated with ultrasonic waves having a wavelength of about 5 to 20 kHz at 0.01 to 0.5 kwh for about 10 to 60 minutes.

Alkaline sonication is a method of adding alkali to excess sludge to adjust the pH to 10 to 12 and irradiating it with ultrasonic waves. By making it alkaline in advance, the degree of solubilization is increased or the ultrasonic irradiation time is shortened. be able to.

Alkali treatment is a method in which alkali is added to excess sludge to adjust the pH to 10 to 12, and this is stirred for about 30 minutes to 2 hours.

Among these, heat treatment, bead mill treatment, hydrothermal treatment and ultrasonic treatment are preferable because the running cost is relatively low and the solubilization effect is high, and the heat treatment and hydrothermal treatment can apply a part of the waste heat of the gas engine. Is particularly preferable.

The methane fermentation method of the present invention is carried out by mixing this solubilized excess sludge with swill. The mixing ratio is not particularly limited, but is usually about 0.25 to 1.0 of excess sludge solubilized with respect to 1 food waste in terms of the weight ratio of solid matter. However, the amount of food waste and excess sludge to be treated varies greatly depending on the season, etc., and may be either food waste alone or excess sludge alone. Further, not only food waste and excess sludge but also other organic wastes and the like can be subjected to methane fermentation treatment. Other organic wastes can also be methane-fermented. For example, sewage sludge initial sedimentation sludge, human waste sludge, livestock manure and the like can be methane-fermented together.

Methane fermentation may be carried out according to a known method. The methane fermentation tank has an anaerobic fermentation for methane fermentation, and has a closed structure in order to effectively utilize the methane produced by the fermentation. The shape may be box-shaped or cylindrical. A stirrer may not be provided inside, but a draft tube or the like may be provided.

Since the fermented liquid that flows out after methane fermentation is performed in the methane fermentation tank contains a large amount of methane bacteria, in order to maintain a high concentration of methane bacteria and allow fermentation to proceed efficiently, use a membrane separation tank. The treated water is separated and the remaining fermented sludge is collected and returned to the methane fermentation tank. As the membrane separation tank attached to the methane fermentation tank, those disclosed in Japanese Patent Application Laid-Open No. 2001-170631 can be used, and the membrane separation tank may be provided inside or outside the methane fermentation tank. As the membrane to be used, a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane) and the like can be used, and the shape of the membrane includes a flat membrane, a hollow yarn membrane and the like. An air diffuser is provided under the membrane to increase the flow velocity on the membrane surface and prevent fermentation sludge from adhering to the membrane surface. The gas used in the air diffuser may be nitrogen gas, carbon dioxide gas, or the like, but it is preferable to use biogas containing methane as a main component generated by methane fermentation.

One embodiment of the present invention is shown in FIGS. 1 and 2. This device consists of a solubilization tank (heat treatment tank) for excess sludge, a methane fermentation tank, a membrane separation device, a biogas desulfurization tower, a gas engine and a heat exchanger. In this apparatus, excess sludge whose concentration has been adjusted to about TS4 volume% by mechanical concentration or the like is put into a solubilization tank, heated at 70 ° C. for about 1 hour, and solubilized by continuous treatment at, for example, HRT for 1 hour. It is put into a methane fermenter. The initial sludge concentrated to TS4 volume% by gravity sedimentation is directly put into the methane fermenter, and the swill is crushed as necessary, diluted with water to TS7-10 volume%, and then placed in the methane fermenter. It is charged and methane fermentation is performed. The input amounts of surplus sludge, initial sludge, and swill are each carried in and depend on the amount generated, and the input amount ratio fluctuates. During methane fermentation, the methane fermentation broth is filtered by a membrane separation device to take out the membrane-filtered water, and the fermentation sludge containing methane bacteria is returned to the methane fermentation tank. The membrane-filtered water contains ammoniacal nitrogen and organic substances, which are separately biologically treated. During methane fermentation, biogas containing methane as the main component is generated, which is desulfurized in a desulfurization tower and then sent to a gas engine to generate electricity. Since the steam discharged from the gas engine has a high temperature, it is sent to a heat exchanger to recover the waste heat contained in the steam, and the remaining heat is sent to a solubilization tank for more effective use. On the other hand, the hot water discharged as a heating source in the solubilization tank can be effectively used by recovering the heat of steam with the above heat exchanger.

Assuming that the heat balance image in the case of heat treatment is 400,000 people in a city, the amount of sewage generated is 200 L / person / day, and the amount of garbage generated is 200 g / day *.
* Regarding food waste, household food waste is 10.5 million tons / year (2010, Consumer Affairs Agency estimate)
Population 120 million people, from 240 g / person / day on a 365-day basis, about 200 g / person / day

[Amount of sludge generated in sewage treatment]
Assuming that the total treatment amount of the city is 80,000 m ³ / day, the initial sedimentation running water SS is 200 mg / L, the treated water SS is about 0 mg / L, the SS generation rate from the initial sedimentation is 0.9, and the excess sludge (concentration) is 4.0%. , Excess sludge amount 180m ³ / day Assuming that the SS removal rate at the initial sedimentation rate is 50%, the initial sedimentation sludge amount is estimated to be equivalent to the excess sludge amount. (4.0%, 180m ³ / day)

[Amount of garbage generated]
In the case of 200 g / person / day and 400,000 people, if the amount of garbage is 8 tons / day and the moisture content is 10%, the dry weight is 7.2 tons.

[Estimation of biogas amount]

SS (suspended solid content):% by weight of residue obtained by evaporating water at 100 ° C.
VTS: SS was burned at 600 ° C and the organic matter contained therein was burned to determine the weight of the remaining ash.

ＶＳ：ＶＳ＝ＳＳ×ＶＴＳ

VS: VS = SS x VTS

[Estimation of waste heat from power generation]
Assuming that the amount of heat of biogas is 22MJ / Nm ³ and the waste heat recovery rate of the generator is 40%, the amount of heat that can be recovered from biogas is 82GJ / day.

[Estimation of required heat]
When the excess sludge at 15 ° C is heated to 90 ° C and 4.18 kJ / kcal,
56GJ / day Power generation waste heat> It may be possible to cover the required heat amount with power generation waste heat rather than the required heat amount.

For food waste, initial sludge, and surplus sludge, the membrane filterability after methane fermentation was examined.

As the swill, a slurry diluted to TS7 to 10% by volume was used. As the initial sludge, the gravity-concentrated TS2-4% by volume sludge of the sewage treatment plant was used. As the excess sludge, sludge of TS3 to 5% by volume of the mechanically concentrated sewage treatment plant was used. Regarding excess sludge, the untreated sludge and the heat-treated sludge were compared. The heat treatment conditions were continuous treatment at 70 ° C. for 1 hour of HRT. Each of these was subjected to membrane separation methane fermentation treatment. The methane fermentation was medium temperature fermentation (38 ° C.) and HRT 15 days. Membrane filtration was performed using a hollow fiber membrane (effective area 0.1 m ² , pore diameter 0.2 μm, made by PTFE) manufactured by Sumitomo Electric Co., Ltd. at a membrane permeation rate of 0.1 m ³ / m ² / day. The experiment period is 3 weeks.

The results are shown in Table 2.

○ in the table indicates the differential pressure between membranes when intermittent suction is performed for 9 minutes suction-1 minute stop with a flux of 0.1 m / day (permeation water volume m ³ per 1 m ² of membrane) for 3 weeks. It shows that the degree of increase of was less than 10 kPa.

Membrane separation In methane fermentation, it is essential to filter the sludge generated by methane fermentation into filtered water and sludge, so the membrane filterability of methane fermentation sludge was evaluated. Throughout the experimental period, the concentration of methane fermentation sludge generated from methane fermentation of food waste was TS1.5 to 2% by volume, and the concentration of methane fermentation sludge in the initial sedimentation sludge was TS1 to 2% by volume. There was no tendency for the pressure to rise, and there was no tendency for sludge to adhere to the membrane and block the membrane. However, the excess sludge methane-fermented sludge has a TS of 2 to 4% by volume, the differential pressure between membranes increases by 10 KPa or more, and the membrane surface tends to be blocked by sludge, and stable operation for a long period of time is considered difficult. Was done. However, by heat-treating the excess sludge, a solubilization degree of 25 to 40% was obtained. The TS concentration of the methane-fermented sludge of the excess sludge that had been heat-treated decreased to 1.5 to 2% by volume, and stable membrane filtration became possible. Therefore, it is considered that the sludge property to which membrane separation methane fermentation can be applied can be obtained by heat treatment.

As a result of examining the membrane filterability of food waste and sewage sludge (initial sedimentation sludge, surplus sludge), the surplus sludge has low membrane filterability, and the surplus sludge is solubilized by heat treatment to improve the membrane filterability. It turned out to be done.

In this study, the amount of garbage collected is affected by collection days such as collecting every month and Friday, so the effect of mixing various types of garbage is not taken into consideration, and the film filterability of each sludge is good. I thought it needed to be.

According to the present invention, existing food waste can be treated inexpensively and efficiently in a methane fermentation treatment facility for sewage sludge, so that the present invention can be widely used.

Claims (4)

When sewage sludge consisting of initial sludge and excess sludge and food waste are methane-fermented by the membrane-separated methane fermentation method, the initial sedimentation sludge of sewage sludge is put into the methane fermentation tank together with the food waste, and the excess sludge is solubilized. It is characterized in that it is solubilized in advance to 15 to 40% and then put into a methane fermenter at a mixing ratio of 0.25 to 1.0 of excess sludge solubilized with respect to 1 garbage by weight of solid matter. , Mixed methane fermentation method of sewage sludge and food waste. The mixed methane fermentation method according to claim 1, wherein the solubilization is performed by heat treatment of excess sludge or hydrothermal treatment. The mixed methane fermentation method according to claim 1, wherein the solubilization is performed by heat-treating the excess sludge at 60 to 90 ° C. for 0.5 to 2 hours. The mixed methane fermentation method according to any one of claims 1 to 3, wherein power is generated using biogas generated during methane fermentation, and the waste heat is used for solubilizing excess sludge.

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