JPS59109296A - Anaerobic digestion treatment - Google Patents

Abstract

PURPOSE:To enable operation under high load and to reduce the size of a digestion tank by removing the digested sludge from the digestion tank during a digestion period, introducing the same into a floatation thickening stage or centrifugal thickening stage and returning the separated and thickened sludge to the digestion tank. CONSTITUTION:Excess activated sludge is introduced through a line 1 into a mixing zone 32. Air bubble is introduced through a line 38 in the zone 32, and the air bubble-sludge mixture is fed through a line 34 into a floatation zone 35. The thickened sludge is deaerated and is supplied through a line 6 into a digestion tank 7. The formed gas is stored in a gas holder 38 and the digested sludge 39 is supplied intermittently through a line 7 into the zone 32, where the sludge is similarly thickened as mentioned above. The thickened and digested sludge is deaerated and returned through the line 6 into the tank 7. The separated water is discharged through a line 31 to the outside of the system. A valve VI is adequately opened or closed to avoid overload in the case of subjecting the sludge 39 to floatation thickening under the atm. pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for anaerobically digesting organic sludge such as raw sludge, excess activated sludge, mixed sludge, human waste, livestock manure, and valve waste generated from sewage treatment plants.

A pneumatic digestion method is known as one of the methods for treating organic sludge. Anaerobic digestion reactions mainly consist of solubilization reactions and gas reactions. In the solubilization reaction, carbohydrates and fats are separated by the action of facultative.

It reduces the molecular weight of organic substances such as proteins to produce lower fatty acids such as formic acid, acetic acid, and gropionic acid. In the gasification reaction, these lower fatty acids are decomposed into methane, carbon dioxide, etc. by obligate anaerobic bacteria. The generated gas becomes an energy source for heating the digester, and excess gas is recovered as secondary energy. As described above, anaerobic digestion methods are energy-saving and are becoming widespread.

(2) However, this method requires a long period of time for digestion, 20 to 40 days, and yields a digestibility of only 0 to 40.

There is a strong desire to reduce the size of the equipment by reducing the number of days required for digestion, and to improve the gas yield by increasing the digestibility.

As a result of intensive investigation of the gaseous digestive reaction, the present inventors found that the solubilization reaction is rate-determining and that the gasification reaction is rapid and lower fatty acids are decomposed into gases such as methane. Since the solubilization reaction is carried out by facultative anaerobes, the solubilization reaction can be promoted by increasing the bacterial cell concentration.

That is, the present invention introduces organic sludge into a first shrinking step by flotation concentration or centrifugal concentration to separate it into first concentrated sludge and first separated water, and introduces the first compacted sludge into a digestion tank. A method of performing anaerobic digestion 2; during the digestion period, the digested sludge is pulled out from the digestion tank and the
Introduced into the concentration process.

The second thickened sludge separated in the second thickening step is returned to the digestion tank. Anaerobic (3) Digestion treatment method for organic sludge.

Does it affect the digestive response? There are many children to give (there are F/
The state of the digestive reaction can be approximately explained by the M ratio. F is the amount of organic matter in the raw material input per day, but here it is expressed as a solid content meter instead of the amount of organic matter, and M is the amount of bacterial cells in the digester (θ), but here, instead of the amount of bacterial cells, There are many two-type digesters that perform solid-liquid separation based on the amount of MLSS in the digester, but here, in order to explain the digestion reaction, we will discuss the first tank of the two-type digester. Furthermore, although there are continuous methods, batch methods, etc. for introducing sludge, here, for the sake of simplicity, it is assumed that sludge is introduced once a day using the single-time method V. Therefore, the extraction of digested sludge (transferred to the second tank in the case of type 2 type) before sludge input is also 1.
Once a day. When such a model is set, F is the amount of solids in the input sludge, M is the amount of solids in the digester before sludge is introduced, and M is the amount of solids in the digester after the digested sludge is extracted.
LSS(4) is the amount. Considering the daily income and expenditure, continuous feeding is also 1
This model can also be used to explain batch-type injections that are divided into several doses per day.

Assuming that a 100A digester is being operated for a digestion period of 10 days, the conventional method and the method of the present invention will be compared.

FIG. 1 is a flow sheet diagram showing an example of the present invention. Conventionally, in the digestion tank, the solid content concentration of the inflowing sludge is 3 to 4%, and the sludge reduction rate, which is the processing rate of the input sludge, is often 30 to 50%, and the MLSS in the digestion tank is 6 to 4%, F/M. The operation is managed so that the ratio is 0.1t1.

Here (engineering, as a conventional method.

The solid content concentration of input sludge is 6%, the MLSS in the digestion tank is 3rd unit, and in the present invention, the inflow sludge with a solid content concentration of 6% is
In the normal pressure flotation concentrator 2, the solid content is reduced to a solid content of 5 cm, for example, and then the M1. Assuming that the operation is performed at a concentration ratio of 8 so that S S becomes 5%, and the sludge reduction rate is 50, the state of the digester from the start of operation to the 61st is shown in Table 1.
It becomes as shown in .

(5) From Table 1, in the conventional method, F/M
It can be seen that the ratio increases. This is because while F remains constant, 5M decreases, resulting in a decrease in the amount of gas generated and the efficiency of digestion. To improve this situation, lower the F7 by reducing the amount of input.

The F/M ratio will be improved. On the other hand, in the present invention, compared to the conventional method, the solid content of the input sludge can be increased by about 1.7 times, and at the same time, the F/M ratio can be kept constant, so the gas generator and digestion efficiency can be improved. It can always be maintained constant and stable operation is possible. In addition, if the amount of sludge flowing into the digestion tank cannot be increased to 9 or more as a solid content, the amount of sludge input to the digestion tank will be 64 (88 concentration 5 parts), and the number of days for digestion will be 10 to 17 days. It can be increased daily.

Is there a relationship between the number of days for digestion and digestion efficiency? Shown in Figure 2. In this way, the amount of gas generated is approximately 1.5 in the case of 10 days and 17 days for 7 days of digestion.
Since the amount is doubled, the digestion efficiency such as the sludge reduction rate can also be increased to almost the same extent.

As described above, two advantages of the present invention are that it is possible to maintain a constant operation with a low F/M ratio (6), and at the same time, it is possible to increase the solid content of sludge fed into the digestion tank. Also, if it is not possible to increase the solid content of input sludge due to sludge treatment in the previous process, the number of days required for digestion is 5? Since it is possible to increase the amount of water produced by a gas generator,
Since it is possible to operate at a higher load than before, the digester can be made smaller.

The present invention will be explained in detail below.

As organic sludge, use raw sludge, surplus activated sludge, mixed sludge generated from sewage treatment plants, human waste, livestock excrement, pulp waste liquid, or various sludge extracted from agricultural and fishery processing processes.

This organic sludge is concentrated in the first concentration step and the first concentrated sludge is concentrated. In the step of obtaining, conventional flotation concentration method or centrifugal concentration method can be applied, but atmospheric flotation concentration method is particularly preferred. The reason for this is that the solid content concentration of the thickened sludge must be stably maintained at a constant value, and as will be explained later, it is preferable to use the normal pressure flotation concentration method in the second concentration step. This is because the process can be simplified by configuring it with equipment. The atmospheric flotation concentration method is to generate bubbles by mixing a foaming agent, liquid, and gas. It is a cationic surfactant that flows in (mixes with filtered sludge to form a bubble-sludge mixture and forms a bubble-sludge mixture, and air (9) can be used as the gas. It is preferable to use a polymer flocculant together in order to improve the bond between the sludge and the sludge.

In the normal pressure flotation concentration method, the flotation concentration conditions are set so that the solid content concentration of the thickened sludge becomes 3 to 10 times. Thickened sludge is lacking. If the solid content concentration of the thickened sludge is 10% or more, the fluidity of the sludge in the digestion tank becomes poor and complete mixing becomes impossible, resulting in a decrease in the number of gases generated, which is not preferable.

Although it depends on the lj, 'i form concentration of the inflowing sludge and the capacity of the digestion tank, it is preferable to make it into a thickened sludge with a solids concentration of 4 to 8%.The thickened sludge is fed into the digestion tank and subjected to anaerobic digestion. The anaerobic digestion method is not particularly limited, and any known technique can be used. The temperature of the digestion tank may be a medium temperature of 60-67C or a high temperature of 5O-57tZ', and the temperature in the tank is kept uniform by mechanical stirring, pumper stirring, gas stirring, etc. to bring the bacterial cells into contact with the sludge. The rate of organic acid production is fast <p
When a decrease in H is detected, add slaked lime or other alkaline material to the tank to keep the pH of the digested sludge neutral. This can prevent a decrease in the gasification reaction rate.

During the digestion period, the digested sludge is continuously or intermittently drawn out of the tank and concentrated in a second concentration step to obtain concentrated digested sludge and separated water. The concentrated digested sludge is returned to the digestion tank, and the separated water is discharged or led to a secondary treatment process. Excess digested sludge may be partially drawn out of the thickened sludge, or may be drawn out of the thread by another method. In this second concentration step, the above-mentioned normal pressure flotation concentration method using air as the gas can be applied. The method of the present invention, which uses this normal pressure flotation-compression method in the second IA compression step, is something that could not even be expected based on conventional knowledge. According to conventional knowledge, the gasification reaction is said to be rate-limiting in the anaerobic digestion reaction, and the gas-killed bacteria that perform the gasification reaction are absolutely anaerobic, so if air gets into the digestion tank, the gas-killed bacteria will die. is inactivated and the digestive reaction &l5
It was said to cause 14 harm. According to the method of the present invention, the second
Does the digestion reaction (11) still occur even if the normal pressure flotation concentration method using air at °C in the concentration process is applied? Not hinder. This is because in the atmospheric pressure floating double concentration method, the action of surfactants prevents the air used for flotation from dissolving into the free water on the surface of the solid, and the area around the solid remains anaerobic at all times. be. Therefore, there is no need to use a gas other than air as the gas used for flotation.Also, in the present invention, it is necessary to stably concentrate to an arbitrary concentration of 6 to 10% in the second concentration step, but the normal pressure flotation concentration method Other floating concentration methods are not preferred because they cannot stably perform such 5-A concentration. In addition, the centrifugal concentration method can also be applied, although it is not as good as the Tsuneta Uki double concentration method in terms of initial cost and running cost.

The present invention will be explained in more detail with reference to the accompanying drawings.

FIG. 6 is a flow sheet diagram showing a preferred anaerobic digestion method of the present invention.

Excess activated sludge is introduced into mixing zone 62 via line 1. Air bubbles are introduced into this mixing zone 52 from line 68, and the air bubble-sludge mixture is passed through line 4 or flotation zone 3.
Get sent to 5. Here, the concentrated sludge (12) is deaerated and then supplied to the digestion tank 7 from the line 6.

The generated gas is intermittently passed through the mixing zone 3 from the line 8 to the gas holder 6B (fko*I"(r'lsaha7-0)
2, the concentrated digested sludge is concentrated in the same manner as described above, and after being deaerated, it is returned to the digestion tank 7 through line 6.The separated water is discharged outside through line 61.

When the digested sludge 69 is concentrated by flotation at normal pressure, the valve v1 is opened and closed as appropriate to prevent the flotation process from becoming overloaded.

A foaming agent 6 and air 66 are supplied to the foaming zone 67, and the generated bubbles are sent to the mixing zone 2 from line Otsu 8.

Example 1 Excess activated sludge from a certain sewage treatment plant (total solid content: 29,560 m!)
j/l, shoulder ridge 24151m? /l) is concentrated using the normal pressure flotation concentration method shown in Figure 6 to obtain thickened sludge (total solids content: 49
285η#-, *aI/1j404o4Iuy#,)v
Obtained. In normal pressure flotation concentration (zero for the solid content of sludge)
．． A flocculant of 1% disodium carbonate and a surfactant of 1% wt. 1 ton of this thickened sludge & 10011 and 7 days
Digested sewage from a certain sewage treatment plant (13), which was put into the digestion tank of 15.
596m? /l) was used as seed sludge, and this seed sludge was cultured in the surplus activated sludge for 2 weeks, and then the sludge obtained by flotation and concentration at atmospheric pressure as described above (solid content: 50.010 m9/l).

900 r# was loaded into the digester 1st tank prior to the experiment. The sludge is removed from the digestion tank once every 6 days, and the above-mentioned atmospheric pressure floating J: 711
The sludge density in the digestion tank was adjusted to 5% by 4 M'x, and the F/M value was maintained at 0111. The digester was kept at 7'C by immersing it in a constant temperature bath. The number of days for digestion is 10 days.

Disposal@thingsadago was ll, Of-vSs/l -El.

On the other hand, in the comparative experiment, the surplus activated sludge was directly charged into the digestion tank without being concentrated, the sludge loaded into the digestion tank prior to the experiment was not concentrated by atmospheric flotation, and the amount of sludge extracted was 10%. [Same as above except that the dose was 1 mg/day. In the comparative experiment, the sentry load was 2. It was pm f-vss/l day.

The results are shown in Figure 4. The average amount of gas generated was 0.0 in the comparative experiment. ,? In the present invention, compared to t/H: 1J(
14) t/day. In addition, the amount of gas generated per input organic matter is
In the comparative experiment, it was 335 me/f-vss, whereas in the present invention, it was 410 me/ff-vss.
It can be seen that the digestibility is high. In addition, when looking at the change over time in the amount of gas generated, in the present invention there is little variation (σ), that is, with the method of the present invention, the processing is stable (・).This means that the F/M value is always constant. by.

Example 2 The same procedure as in Example 1 was carried out except that the concentration of digested sludge in the digestion tank was changed by 3.7% + 2.84Y.

The results are shown in Figure 5. It can be seen that as the F1M value increases, the amount of gas generated decreases.

[Brief explanation of the drawing]

1 and 6 are the methods of the present invention? This is a flow sheet showing the relationship between the number of days for digestion and the amount of gas generated. FIG. 4 is a diagram showing the results of Example 1,
FIG. 5 is a diagram showing the results of Example 2. 2.9...Normal pressure flotation dark blue device 7...Digestion tank (15
) Filter 2... Mixing zone 66... Foaming agent 65...
・Floating zone Filter 6... Air 37... Foaming zone Patent applicant Shinryo Corporation (16) Figure 4 01 23456789 Figure 5

Claims (1)

[Claims] 1. Organic sludge is introduced into a first concentration step by flotation concentration or centrifugal concentration to separate it into first concentrated sludge and first separated water;
A method for performing pneumatic digestion by introducing the 1a reduced sludge into the digestion tank; during the digestion period, the digested sludge is pulled out from the digestion tank and introduced into a second concentration step by flotation concentration or centrifugal concentration; A method for anaerobic digestion of organic sludge, which comprises returning a total of 2 concentrated sludge separated in 2 concentration steps to the digestion tank. 2. The method according to claim 1, wherein the first and second concentration steps are comprised of the same device. 6. The first and second concentration steps are: generating a bubble, mixing the bubbles with incoming sludge to form a bubble-sludge mixture, and directing the bubble-sludge mixture to a flotation zone to form a thickened sludge. Obtained from each flotation concentration process (
1) The method according to claim 1 or 2. 4. σ according to claim 6, wherein the gas is air.
)Method.