KR20010025926A - Method for quick digestion of garbage and for producing methane therefrom using three-stage system - Google Patents

Abstract

PURPOSE: Provided is a method for producing methane gas from garbage using three-stage methane fermentation system, which can produce a large quantity of methane gas with low HRT and a small reactor. CONSTITUTION: The method comprises the steps of a hydrolysis/acid fermentation step in which crushed garbage by a crusher (A) is mixed with water at the ratio of 1 to 1, and then conveyed to a hydrolysis/acid fermentation tank (B) wherein hydrolysis is carried out by aerobic bacteria and acid fermentation is carried out anaerobic bacteria in the condition of 28 to 32 deg.C of reaction temperature and pH range of 5.0 to 5.5; a primary acid fermentation step in which pretreated admixture of garbage and water is conveyed to the bottom part of a UASB (Upflow Anaerobic Sludge Blanket) type primary acid fermentation tank (C) by a pump (P), and then seeding thereto Clostridium butyricum for increasing yield rate of acetic acid and butyric acid in the condition of 33 to 37 deg.C of reaction temperature and pH range of 5.0 to 5.5; a methane fermentation step in which only liquid obtained during the primary acid fermentation step is conveyed to a methane fermentation tank (D) through a pipe installed up to middle part from upper part of the primary acid fermentation tank so that the conveyed liquid is fermented. The methane fermentation step is performed in the condition of 7.6 to 7.9 of pH range and 39 to 43 deg.C of reaction temperature.

Description

Method for quick digestion of garbage and for producing methane therefrom using three-stage system

The present invention relates to a method of rapidly extinguishing food waste and producing methane therefrom. More specifically, three-stage methane fermentation produces a large amount of methane from food waste in order to process food waste quickly and use it as an alternative energy. The present invention relates to a fast digestion of food waste and a methane production method using a system.

Food waste generated in our country accounted for 31% of urban waste per day in 1994, but accounted for 27.3% in 1997 due to the government's food waste reduction policy, which translates into 8 trillion won. There is a report.

According to 97 recycling of food waste, 59.2% of the recycling was used for livestock feed and 40.8% was recycled for compost. However, foddering is difficult to separate and collects, causing rot / pathogenic microorganisms and foreign matters in feed, which can cause damage to livestock, and can lead to imbalances in the nutritional status of livestock due to unbalanced nutrients in food waste. Composting can also damage crops due to excess moisture and salt in the food itself, and can cause odors in the process of landification of composted food waste.

Therefore, in recent years, research has been actively conducted to recycle food waste into methane gas. As a result, a two-stage digestion process, which is divided into an acid fermentation phase and a methane fermentation phase, is currently in practical use. However, the two-stage fire extinguishing method still has a disadvantage of increasing the size of the fermenter because of the slow processing speed and accordingly, the installation cost is high. In addition, the optimum pH for fermentation in the methane fermentation tank is 7.0 is the best, but the acid generated in the acid fermentation tank directly enters the methane fermentation tank has a disadvantage that the operation of the process itself is stopped.

Therefore, the present application has been continuously conducted to supplement the disadvantages of the two-stage digestive system as described above, using the three-stage methane fermentation system to quickly ferment the waste organic matter to further reduce the residence time in the fermenter and thereby In addition to miniaturizing the size of the fuel economy, a large amount of methane can be produced efficiently from food waste, and a method of rapidly extinguishing food waste that can be used as an alternative energy and obtaining methane therefrom has been devised.

1 is a process flow chart showing a high-speed fire extinguishing and methane production method of food waste using a three-stage methane fermentation system according to the present invention;

Figure 2 is a graph of the production of volatile fatty acids (VFA) during the first hydrolysis and acid fermentation process and the secondary acid fermentation process according to the present invention,

Figure 3 is a change in pH during the methane fermentation process according to the present invention,

Figure 4 is a change in SCOD, TCOD, BOD during the methane fermentation process according to the present invention,

Figure 5 is a change in ammonia nitrogen and total nitrogen during the methane fermentation process according to the present invention.

*** Explanation of symbols for main parts of drawing ***

A: Crusher B: Primary hydrolysis and acid fermenter

C: secondary acid fermenter D: tertiary methane fermenter

E: Sedimentation tank F: Biological media container

G: sodium hydroxide supply vessel H: methane storage tank

P: Pump AP: Air Injection Pump

That is, the present invention is a high-speed digestion of food waste and the production of methane therefrom, the food waste pulverized into small particles in the shredder mixed in a 1: 1 ratio with water and the first semi-anaerobic hydrolysis and acid through a screw A primary hydrolysis and acid fermentation step which is transported to the fermenter and hydrolyzed by aerobic bacteria and acid fermented by anaerobic bacteria; The secondary acid fermentation process in which the first fermented solid-liquid component is injected through the bottom of a secondary acid fermentation tank in the form of an upstream anaerobic sludge blanket (UASB) by a pump, and inoculated with acid clotidium butyricum to ferment acid. and; It consists of a third methane fermentation process of producing methane by the methane-producing bacteria inoculated by transporting by the metering pump to the third methane fermentation tank through the pipe descending from the upper end of the fermentation tank to the middle of the solid-liquid fermentation secondary It is a high-speed fire extinguishing and methane production method of food waste using three-stage methane fermentation system.

In the first hydrolysis and acid fermentation step, the operating temperature and pH in the semi-anaerobic hydrolysis and acid fermentation tank are 28 to 32 ° C. and 5.0 to 5.5, respectively.

In the secondary acid fermentation step, the secondary acid fermentation tank has an operating temperature of 33 to 37 ° C and a pH of 5.0 to 5.5.

In the third methane fermentation process, the methane fermentation tank is designed to circulate the fermentation broth at a temperature of 39-43 ° C., a pH of 7.6 to 7.9 and an upper portion and a lower portion of 55 to 70 cm from the upper portion. .

Microorganisms used to digest food waste in the primary, secondary and tertiary fermenters are shown in Table 1.

Table 1. Microorganisms Used to Digest Food Waste

fair Strain Resolution Semi-anaerobic Hydrolysis / Acid Fermentation Process Cellulose cellulose Cellulose, chitin, pectin Flavobacterium bribes cellulose Bacillus amyloliquecucience carbohydrate Bacillus licheniformis protein Bacillus subtilis Carbohydrates, protein Bacillus Alcalophilus Fat Anaerobic Acid Fermentation Process Clostridium Butyrim Sugars, amino acids, long chain fatty acids Anaerobic Methane Fermentation Process Methane fermentation bacteria Acetate, formate

Referring to Figure 1 attached to the overall process of the high-speed digestion of food waste and methane production method using a three-step methane fermentation system according to the present invention.

The food waste crushed in the crusher (A) is mixed with water in a 1: 1 ratio and transported to the first semi-anaerobic hydrolysis and acid fermentation tank (B). The primary hydrolysis and acid fermentation tank (B) is designed for fast hydrolysis and acid production from food waste. In the first fermentation tank, the agitated food waste is agitated and a large amount of air is injected through the compressor for effective hydrolysis to inject the aerobic bacteria to hydrolyze the macromolecule organic material to low molecular weight in a short time. Causes the substance to be converted to acid. Materials such as cans, rubber, shells, etc., which are not decomposed in this process, are removed through a valve hole provided at the bottom of the fermenter.

The solid solution fermented from the primary fermentation tank (B) is injected into the lower end of the secondary acid fermentation tank (C) by the pump (P). The secondary acid fermenter (C) is inoculated with Clostridium butyricum to increase the production of acids such as acetic acid and butyric acid. The acid effluent produced in the secondary acid fermenter (C) is sent to the tertiary methane fermenter (D) for methane production, and the third methane fermenter (D) is inoculated with methane producing bacteria.

The methane fermentation tank D is equipped with a heat converter, a gas meter, a combustion piping device, and a gas exhaust hole. Several additional processes are employed to remove residual organics, nitrogen and phosphorus compounds in the effluents from the methane fermentation process. The effluent is sent to a settling tank E, of which solids settle. Supernatants are transferred to a 45-liter container containing an aerobic biological filter with immobilized nitrifying bacteria. After 3 days, the waste fluid is transferred to 90 liters of an anoxic biological filter to remove nitrogen by denitrifying bacteria. The biological media used in this study are made from PVC sticks with a wide outer surface and are recovered from waste polyvinyl chloride. The filters are submerged into the wastewater and bacteria are fixed on them.

The wastewater is discharged after 6 days of treatment. In addition, the exhaust gas generated from the primary fermenter is connected to the aerobic biofilm media container so that odor is significantly removed.

Methane is a colorless, toxic, clean energy produced only after complete combustion. In addition, the methane combustion emits a large amount of energy of 12,000 ㎉ / ㎏, and because it is a single-atomic carbon compound, the combustion efficiency is high, so that incomplete combustion rarely occurs. This feature means that the methane fermentation process can be an alternative energy source production process by producing clean biogas, and can be a clean waste treatment process because there is less untreated waste and soot. Have. The gas produced after the methane fermentation contains about 70% of methane and about 30% of carbon dioxide, and thus has sufficient utility value as energy. Since hydrogen sulfide and ammonia gas are less than 1%, respectively, odor is not a problem.

Hereinafter, the present invention will be described in detail with reference to preferred embodiments.

<Example 1>

1. First Hydrolysis and Acid Fermentation Process;

The collected food waste was crushed into small particles of 3 mm or less in a crusher and diluted 1: 1 with water, of which 10 liters were placed in a primary hydrolysis and acid fermenter. The food waste put into the fermenter was stirred at a speed of 100 rpm and kept at 30 ° C. for 2 days to be hydrolyzed by aerobic and anaerobic bacteria and acid fermented.

The first hydrolysis and acid fermentation process is a semi-anaerobic fermentation where both aerobic and anaerobic bacteria coexist and can be active at the same time. The easily decomposable polymers are rapidly hydrolyzed into oligomers or monomers by aerobic bacteria and at the same time in food waste. The low molecular weight hydrolyzed by the anaerobic bacteria present was converted back to acid.

In other words, by injecting aerobic bacteria to accelerate the hydrolysis, acid fermentation by anaerobic bacteria is also accelerated, the hydraulic retention time to digest the waste in the primary fermenter is reduced to two days. In addition, as the fermentation rate in the primary fermenter is faster, the size of the primary fermenter can be reduced.

Bacteria used in the hydrolysis and acid fermentation process are selected as aerobic bacteria that can decompose complex organic substances such as carbohydrates, proteins, and fats in the food (Table 1), and then cultured on a substrate that can grow well. , Was injected into the substrate containing the extract of food waste. After adaptation by continuous cultivation, the bacteria were mixed and injected into the fermenter.

During the fermentation process, small amounts of glucose, cellobiose, rhamnose and various unidentified sugars were detected (no data), while a large amount of lactic acid, acetic acid, propionic acid were observed continuously and the results are shown in FIG.

The initial pH was adjusted to 6.0, but lowered to 4.0 as the acid was produced, which is shown in FIG.

As shown in FIG. 4, the total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD) of the hydrolyzed low molecular weight materials were 45,000 mg / l and 32,000 mg / l, respectively. However, the concentration of biological oxygen demand (BOD) was 51,000 mg / l, which was higher than the total chemical oxygen demand. This is because the mixture still contains a large amount of fine particles, making it difficult to obtain precise values of the total chemical oxygen demand and the biological oxygen demand.

As the food waste was extinguished, heavy materials that did not decompose settle to the bottom and were removed through a valve hole installed in the bottom center of the fermentor.

2. secondary acid fermentation process;

The hydrolyzed low molecular weight material from the primary fermenter was transferred to the secondary acid fermenter by a gear pump. In the secondary acid fermentation process, Clostridium butyridin was inoculated and subjected to continuous culture as described above and injected into the substrate containing the extract of food waste. After adaptation by continuous cultivation, the bacteria were mixed and injected through the lower portion of the secondary acid fermenter in the form of an upstream anaerobic sludge blanket and sufficient acid fermentation occurred during the residence time for two days.

The product still showed high amounts of total and soluble chemical oxygen demand, but the biological oxygen demand was reduced to 60%. This shows that most of the biodegradable material from food waste was effectively digested and converted to volatile fatty acids and carbon dioxide. As shown in Table 2, the main component of the gas produced from the acid-producing fermentor was carbon dioxide. During the fermentation, the pH range was maintained at 5.0-5.5 to prevent interference with acid production.

The low pH and high acidic environment that caused the hydrolysis and acidification reaction to stop were thought to be due to end-product inhibition of acid production by fermentation. In other words, acetic acid decomposed and methane-producing activity was very suppressed under these acidic digestion conditions. It was therefore necessary to mix with alkali effluents from the methane fermenter (pH7.6-8.0) to neutralize the acidic liquids, resulting in more acid being produced continuously.

3. tertiary methane fermentation process;

Only liquid components were conveyed to the bottom of the tertiary methane fermenter in the form of an upstream anaerobic sludge blanket by a pump through a tube descending 90 cm vertically from the top of the fermentation solids component of the secondary fermentation solids.

When the acid effluent was injected directly into the methane digester, the methane fermentation was virtually stopped under acidic conditions and adjusted to optimal conditions using 5N sodium hydroxide (NaOH) to reduce acidic impact on methane producing bacteria.

The operating temperature and pH were 41 ° C and 7.6 to 7.9, respectively, so that the upper part of the fermentation broth and the fermentation broth of the lower part 60 cm from the upper part were allowed to circulate.

The acid liquid remained in the methane digester for 12 days. Under these conditions, the total chemical oxygen demand, soluble chemical oxygen demand and biological oxygen demand of the effluent were reduced by 95%, 96% and 97%, respectively.

This shows that the three-stage digestive system developed by this study is very effective and can save processing time for high total solids-containing food waste. At the same time, the total nitrogen decreased from 4287 mg / l to 2624 mg / l, but ammonia nitrogen was slowly increased in anaerobic acid fermentation and accumulated to 1,200 mg / l in methane fermentation (Fig. 5). Since these nitrogen concentrations are too high to be discharged into the waste water, other additional treatments were required. To solve this problem, aerobic or anoxic biological filtration systems have been adopted. The biological filter was fixed with heterotrophic as well as nitrifying and denitrifying bacteria to reduce residual waste contained in the wastewater. As a result, the concentrations of total nitrogen and ammonia nitrogen were reduced to 166 mg / l and 66 mg / l, respectively, and the biological oxygen demand was also reduced to 287 mg / l. In other words, total nitrogen and biological oxygen requirements were reduced by 96% and 99.9%.

Less than 10 mg / l of total phosphorus was detected in the final effluent and the final effluent was properly treated to discharge.

In the third methane fermentation process, 72% methane and 28% carbon dioxide were generated from volatile fatty acids, and methane was produced at a yield of 0.45 to 0.50 m ³ / kgVS for 12 days of hydraulic retention time.

The total three stage methane process showed a high total chemical oxygen demand reduction of 95%. Total nitrogen also decreased 96% and total phosphorus was detected below 10 mg / l.

As shown in FIG. 2, 4,100 mg / L of total volatile fatty acids were produced in the first digestion process, of which acetic acid was the highest at 77%. Interestingly, the concentration of lactic acid was nine times higher than that of acetic acid under these conditions, but lactic acid was converted back to acetic acid (no data). The concentration of total volatile fatty acids produced in the secondary acid fermentation process was 6,100 mg / l and several types of volatile fatty acids were produced.

The data obtained by performing the secondary acid fermentation process several times also showed that the concentration of total volatile fatty acids produced could be increased from 6,100 mg / L up to 9,100 mg / L.

Of the 6,100 mg / l total volatile fatty acids produced, the main volatile fatty acids were 3,000 mg / l acetic acid, 1,540 mg / l valeric acid and 1,000 mg / l propionic acid. Although a small amount of volatile fatty acids (310 mg / l) was left in the waste liquid from the methane fermenter, most of the organic carbon was converted to methane and carbon dioxide (Table 2).

The most important parameters for the general operation of the digestive system were pH, organic loading rate, and volume fraction between acid and methane fermenters.

The total gas generation amount was 0.65-0.70 m ³ / kg VS. Main gas compositions were methane, carbon dioxide, ammonia, hydrogen sulfide. 72% of the total gas volume generated was methane and methane production was 0.45 to 0.50 m ³ / kg VS (Table 2). Methane conversion and generation yielded similar values compared to other previous studies. The high ammonia nitrogen concentration of 1,200 mg / l generated from the methane fermenter did not interfere with methane production. The data obtained from the overall process are summarized in Table 2.

Compared with the methods using the conventional two-stage system, the three-stage system developed by this study was an effective methane-producing system (Table 3).

Table 2. Operation conditions and results of the three-stage system

parameter Primary process 2nd process 3rd process Biological media Hydraulic stay (days) 2 2 12 9 Load capacity (kgVS / m ³ d) 20 to 22.8 25 to 27.4 12-18.8 1 to 1.2 pH 5.0 to 5.5 5.0 to 5.5 7.6 to 7.9 8.3-8.7 Temperature (℃) 28-32 33-37 39-43 28-32 T-N (mg / L) 4287 3216 2624 166 NH ₃ -N (mg / L) 117 172 1205 66 COD (mg / L) TCODSCOD 4494832223 3058220632 23821071 11451104 BOD (mg / l) 51081 20876 1356 287 Gas generation amount (m ³ / ㎏VS) - - 0.65 to 0.70 - Gas composition CH ₄ (%) CO ₂ (%) - 8.9% 91.1% 72% 28% - Methane generation amount (m ³ / ㎏VS) - - 0.45 to 0.50 - Volatile acid (mg / L) Acetic acid propionate Butyric acid valeric acid caproic acid 3151012382904103 29761013562154306094 3130000313 ------

Table 3. Mesophilic and conventional two-stage anaerobic digestion system

Performance comparison of the three stage system of the present invention

Processing parameters Three-stage system of the present invention (food waste) Conventional two-stage system (1) (food waste) Conventional two-stage system 2 (activated sludge) Hydrolysis mountain methane mountain methane mountain methane Hydraulic stay (days) 2 2 12 5 15 3.1 9.1 Load (kgVS / m ³ d) 20 to 22.8 25 to 27.4 12-18.8 25 to 35 10 to 15 18.9 6.2 pH 5.0 to 5.5 5.0 to 5.5 7.6 to 7.9 5.5 to 6.5 7.4-7.8 5.6 7.7 Total solids at the time of administration 17.53 7.4 6.5 25.8 5 to 6 7.5 4.3 Total Volatile Fatty Acid (mg / l) 4,100 6,100-9,100 313 9,000 to 13,000 4,000-7,000 9,445 172 Methane generation amount (added m ³ / ㎏VS) - - 0.45 to 0.50 - 0.44 - 0.29

As described above, the present invention effectively reduced the hydraulic retention time (HRT) by increasing the hydrolysis of food waste and the production rate of acid, thereby reducing the size of the fermenter. It was able to produce more methane than the system.

Claims (4)

In the method of digesting food waste and producing methane, In the crusher, the food wastes pulverized into small particles are mixed with water in a ratio of 1: 1 and transported to the first semi-anaerobic hydrolysis and acid fermenter through a screw to be hydrolyzed by aerobic bacteria and acid fermented by anaerobic bacteria. Hydrolysis and acid fermentation process, A secondary acid fermentation step of transferring the first fermented solid-liquid component through a lower acid fermentation tank in the form of an upstream anaerobic sludge blanket by a pump and injecting Clostridium butyricum to acid fermentation; It consists of a third methane fermentation process of producing methane by the methane-producing bacteria inoculated by transporting by the metering pump to the third methane fermentation tank through the pipe descending from the upper end of the fermentation tank to the middle of the solid fermentation of the secondary fermentation Fast digestion of food waste and methane production method using a three-stage methane fermentation system. The three-stage methane fermentation system according to claim 1, wherein the temperature of the first semi-anaerobic hydrolysis and acid fermentation tank in the first hydrolysis and acid fermentation process is 28 to 32 ° C and pH is 5.0 to 5.5. Fast digestion of food waste and the methane production method. 2. The fast digestion of food waste using a three-stage methane fermentation system according to claim 1, wherein the operation temperature in the secondary acid fermentation tank of the secondary acid fermentation process is 33 to 37 DEG C and pH is 5.0 to 5.5. Methane production method. The fermentation broth of claim 1, wherein the temperature in the tertiary methane fermentation tank of the tertiary methane fermentation process is 39 to 43 DEG C, pH is 7.6 to 7.9, and the fermentation broth at the upper end and the lower 55 to 70 Cm from the upper end are circulated. Fast digestion of food waste and methane production method using a three-stage methane fermentation system, characterized in that designed to be.