JPS588316B2 - How to dispose of organic waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for anaerobic digestion of organic waste.

With the spread of secondary treatment facilities for urban sewage and livestock wastewater, large amounts of surplus activated sludge are being generated.

In addition, a considerable amount of household kitchen waste is also disposed of.

Therefore, it will be an extremely important problem to treat these organic wastes efficiently and non-pollutingly in the future.

Currently, these organic wastes are mainly disposed of by incineration, landfill, and ocean dumping, but it is well known that these causes various secondary pollution.

Incidentally, anaerobic digestion has long been known as one of the methods for treating concentrated organic waste liquid.

It is known that this anaerobic digestion mainly involves two types of reactions.

In other words, first, the organic matter in the waste liquid becomes a group of liquefying bacteria (a group of putrefying bacteria).
It consists of a liquefaction reaction in which the fatty acids are reduced in molecular weight and become volatile fatty acids such as acetic acid, propionic acid, and butyric acid, and a reaction in which these fatty acids are converted into methane by gasifying bacteria (methane bacteria). There is.

In the normally carried out anaerobic digestion, it is thought that both of these bacterial groups coexist or the digestive decomposition proceeds through a process of slow exchange of forces.

For this reason, anaerobic digestion typically requires a long treatment period of 30 to 50 days.

Therefore, even though it has features such as the ability to effectively use digested sludge, which is the residue of digestion, as fertilizer, and the ability to use the generated methane gas as power for the digestion equipment, it is still only used for human waste treatment in Japan.

However, recently, the above-mentioned non-polluting and energy-saving features have begun to be highly valued, and research has begun to improve the low processing efficiency, which is a major drawback.

By the way, the main components of organic waste are carbohydrates, fats, and proteins.

Among these, carbohydrates are easily broken down, but fats and proteins are said to be difficult to break down.

It is also well known that the amount of gas generated per unit weight of each component and the methane purity of the generated gas are higher in the order of lipids, proteins, and carbohydrates.

As described above, the decomposition rate varies depending on the component, so even if an attempt is made to shorten the digestion period, the decomposition of lipids and proteins that are difficult to decompose becomes rate-limiting.

From the above, in order to increase the digestion rate of anaerobic digestion, as well as increase the methane yield and methane purity of the recovered gas, it is necessary to stop the decomposition of lipids and proteins that are difficult to decompose. .

Until now, there have been no attempts to improve from the above point of view, and the main countermeasure has been to simply find the optimal range of anaerobic digestion conditions and use this range.

As a result of various studies on the above points, we have discovered that by preheating the raw organic waste under alkaline conditions with a pH of 9.5 or higher, it is possible to promote the digestion of fat and protein in the subsequent anaerobic digestion. .

Furthermore, as a result of further detailed studies, we found that organic wastes with high lipid and protein contents, such as sewage treatment sludge and kitchen waste with high lipid content, hair washing waste liquid, and fish cane processing waste liquid, For those with a lipid content of 20% or more, conditions of pH 13.60°C and 2 hours or more were found to be effective for decomposing lipids and proteins.

However, when processing above the above conditions, although it is convenient for the decomposition of lipids and proteins, many of the B vitamins, which are important growth factors for anaerobic bacteria, are deactivated, and the overall digestion is not so good. It was found that although the time required for digestion was not shortened, it actually became longer depending on the conditions.

For example, slurry made by crushing household kitchen waste has a pH of 14.10.
By heating at 0℃ for 2 hours, lipids, proteins, etc. can be effectively decomposed, but 90-100% of B group vitamins such as thiamine, riboflavin, B6, pantothenic acid, and biotin can be decomposed.
% decomposes and becomes inactive.

When digestion is performed after the heat treatment, digestion proceeds only after a lag period of several days, so there is no significant difference in the number of days required for digestion compared to control plots that were digested without heat treatment.

However, the yield of CH4 at the end of digestion is improved by about 10% compared to the symmetrical method.

An object of the present invention is to improve the above-mentioned drawbacks of the prior art and to provide a method for treating organic waste with high treatment efficiency.

As a result of various studies on the above points, the inventors of the present invention found that 80 to 95% of the raw material organic waste used was heat treated with alkaline water in the presence of water, and the remaining 5 to 20%
By anaerobically digesting the waste by mixing it without treatment, we solved the above drawbacks and discovered scales that promote the digestion of fat and protein.

Therefore, the feature of the present invention is that the amount of raw material organic waste used is 8.
Before digesting the amount equivalent to 0 to 95%, heat treatment is performed under alkaline conditions as described above, and the waste that has been subjected to the heat treatment described above and the remaining waste that has not been heat treated and is equivalent to 5 to 20% of the total amount used. It is to mix things.

This makes it possible to replenish the growth factors of the anaerobic bacteria that have been decomposed and lost as described above.

As the anaerobic digestion method that can be used in the present invention, in addition to the conventional method in which liquefaction and gasification are performed in parallel in the same tank, the above-mentioned liquefaction and gasification separation methods can also be used.

Regarding the latter, it has recently been proven that both the liquefaction and gasification reactions can be separated, and it has been reported that by optimizing each reaction, the processing period can be shortened compared to the conventional parallel multiple fermentation method. has been done.

In the present invention, when the liquefaction gasification separation method is used as the digestion method, there is an advantage that the alkali used in the alkaline heat treatment can be effectively used as a neutralizing agent for volatile fatty acids produced by the liquefaction reaction.

In this way, the present invention eliminates the difference in decomposition rate depending on the components and enables efficient digestion.

FIG. 1 shows an example of the process of the organic waste treatment method of the present invention, and the steps will be explained in detail below in order.

First, organic waste with a relatively high lipid content stored in the waste liquid storage tank 1, such as kitchen waste, 25 to 40% per dry weight.
80 to 90% of the amount of surplus activated sludge, fish cane production waste liquid, etc. containing a relatively high concentration of lipids is put into the heat treatment tank 2 and heat treated under alkaline conditions.

It goes without saying that in the case of items containing coarse solid materials such as kitchen waste, it is preferable to appropriately crush them into a slurry form in terms of preventing equipment failure and improving processing efficiency.

The effective pH range in this heat treatment is preferably 13 or higher.

The alkali that can be used for pH adjustment is not particularly limited as long as its constituent cations are not biologically toxic.

That is, caustic soda, soda carbonate, slaked lime, and quicklime are used.

Caustic soda and carbonated soda are particularly effective. The amount of alkali added varies depending on the type of organic waste, the composition and concentration of organic components, and the type of alkali, but generally the above effective p
To obtain H, the addition amount is in the range of 1 to 30% (wt/wt) of the raw material organic matter.

The effective temperature range is 60 to 100°C, but a range of 60 to 90°C is preferable in consideration of economic efficiency.

Heating time varies greatly depending on pH and temperature, but
13, 60℃, 2 hours or more, pH 14, 100℃
In this case, more than 1 minute is required.

The upper limit of the time is not particularly limited, but from economical considerations, two days or less is preferable.

The heating method is not particularly limited as long as there is no burning on the heating surface, and known methods are sufficient.

That is, the heating is performed using a heater or heat exchanger provided around or inside the heat treatment tank, or by blowing steam.

For stirring, a known method suitable for the water content of the raw materials and the amount of water added may be used.

When the water content is 75 to 85% (wt/wt), stirring in a rotating cylinder is also effective.

Although the above heat treatment effectively dissolves the lipids in the waste and reduces the protein to a low molecular weight, most of the B group vitamins are destroyed.

Next, the above heat-treated organic waste and the remaining 5 to 2
0% of unheated organic waste is put into the liquefaction tank 3 and brought into contact with liquefaction bacteria.

It is then kept under anaerobic conditions and at constant temperature for several days with stirring.

In this liquefaction process, the high molecular weight substance is reduced in molecular weight, and the generated low molecular weight compound is further decomposed into volatile fatty acids.

Due to the heat treatment in the previous step, the molecular weight of the protein has been reduced, and the fat has been decomposed into constituent higher fatty acids, making it easier to dissolve, but in this liquefaction step, the decomposition further progresses to lower fatty acids.

The temperature in the liquefaction process is preferably 30 to 70°C, and the pH is preferably 4 to 6, and can be appropriately selected depending on the type of waste liquid and the liquefaction bacteria used.

Although acid is produced in the liquefaction reaction, the alkali used in the first step is effectively used as a neutralizing agent in the second step.

If the alkali is insufficient, it is necessary to newly add a neutralizing agent and adjust the pH in the liquid to the above-mentioned preferred range.

At this time, in addition to the alkali used in the first step, calcium carbonate is also effective as a neutralizing agent.

Note that the liquefied bacteria, stirring, and heat retention that have been used in conventional anaerobic digestion methods are sufficient.

For example, the liquefied bacteria include Clostridium, Bacillus, Escherichia, and Staphylococcus.

These are usually used as a group rather than as single bacteria.

The composition of the gas generated in the liquefaction tank varies considerably depending on the type of raw material and processing conditions, but it is generally between 70 and 95 CO2.
%, and also contains small amounts of N2, H2, and H2S.

These gases are CH4 generated during gasification in the subsequent process.
It may be used to dilute a high-calorie gas rich in carbon dioxide, or it may be desulfurized with a ferric salt and then vented to the atmosphere.

Next, the liquefied slurry is sent to the gasification tank 4.

A CH4-rich gas is generated here, but in order to efficiently perform this gasification, it is necessary to heat the gas to 30 to 70° C. under anaerobic conditions and adjust the pH to 7 to 8 while stirring.

The heating and stirring methods can be sufficiently accomplished by the same methods used in the liquefaction step described above.

The pH is adjusted by adding mineral acids or organic acids.

As the gasifying bacteria, conventionally used gasifying bacteria such as Methanosarcina, Methanococcus, and Methanobacterium can be used.

The main components of the generated gas are 60-90% CH4 and 10-90% CH4.
It consists of 40% CO2 and also contains small amounts of N2 and H2S.

The gas generated in the gasification process may be used alone or mixed with the gas generated in the liquefaction process, and after passing through the desulfurizer 5, it is transferred to the gasification tank 6.
is stored in

The stored gas may be used as a heat source for the apparatus and for aeration and agitation of the gasification tank, as in conventional digestion methods.

The slurry that has been gasified is separated into solid and liquid in a settling tank 7 into a supernatant desorbed liquid and a precipitated digested sludge.

The desorbed liquid is then post-treated by activated sludge treatment, etc., and then released.

Digested sludge, like digested sludge obtained by conventional methods, can be dehydrated and dried and used as an organic fertilizer.

Next, specific examples of the present invention will be described and explained in more detail.

Example 1 400 g of surplus activated sludge from urban sewage treatment (solid content concentration: 1
．． 1%, organic matter concentration in solid content 65%, lipid content 30%
, crude protein content 32%) was added with 3.5 ml of 30% caustic soda solution with stirring, and the pH was adjusted to 10.4.

This was placed in a 1 liter separable round bottom flask equipped with a reflux condenser, and heated at 90° C. for 30 minutes with stirring.

After the heat treatment, the mixture was placed in a 2-liter glass digestion tank, and 3 ml of concentrated hydrochloric acid was added thereto to neutralize the pH to around 6.0.

Next, raw surplus activated sludge 10 without heat treatment is added to this.
After adding 0 g of sludge and inoculating 2% (wt/wt) of the raw material sludge slurry using anaerobic digested sludge of human waste as seed sludge, the gas phase inside the digester was replaced with N2 gas.

Then, while stirring at 300-350 rpm, the pH was adjusted to
Sludge 500 while automatically adjusting to 7.2 with H-stat
Digestion tests were conducted in the same manner for the case where the pH was adjusted to 7.2 with caustic soda without the above-mentioned pretreatment, and the case where 500 g of sludge was heat-treated under the same conditions as the above-mentioned alkaline heat treatment.

Figure 2 shows the relationship between the amount of gas for methane generation and the number of days for digestion.

When the entire amount of raw sludge is heat treated with alkaline heat treatment, the induction period for B gas generation becomes longer and the number of days required for digestion is not much shorter than in case A without treatment, whereas raw sludge is added to the alkaline heat treated material When C was used, the lag period was shortened and the number of days required for digestion was shortened by 30%.

In addition, the methane content of the digestion gas is A: 69.0 in volume ratio,
The purity is almost the same as B: 70.5% and C: 70.1%.

According to the present invention, by heat-treating organic waste with alkaline to make difficult-to-digest components such as fat and protein easier to digest, the digestion period can be shortened compared to conventional methods. can.

[Brief explanation of drawings]

FIG. 1 is a flow sheet diagram according to the present invention, and FIG. 2 is a diagram showing the relationship between the amount of digestive gas generated and the number of days for digestion. Explanation of symbols, 1...Organic waste storage tank, 2.
... Alkaline heat treatment tank, 3 ... Liquefaction tank, 4 ... Gasification tank, 5 ... Desulfurization tower,
6... Gas storage tank, 7... Sedimentation tank.

Claims (1)

[Claims] 1. A first step in which 80 to 95% of the total amount of organic waste is heat-treated with alkaline water in the presence of water; remaining 5-20
% of untreated organic waste is mixed and anaerobically digested. 2. The organic waste treatment method according to claim 1, characterized in that the anaerobic digestion method carried out in the second step uses a two-stage fermentation method in which liquefaction treatment is performed and then gasification treatment is performed.