JPS603878B2 - Sludge treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to remove excess sludge, which is produced in the process of aerobically decomposing organic sewage with oxygen-containing gas in the presence of aerobic bacteria, by blowing oxygen-containing gas into The present invention relates to a method for reducing equipment costs and operating costs of a sludge treatment method that includes a step of methane fermentation after heat treatment.

FIG. 1 is a schematic process diagram showing one example of a sludge treatment method that has been used.

The sewage is first led to the settling tank 1, where raw sludge is separated, and then mixed with the sludge (feedback sludge) separated in the final settling tank 3, which will be described later, and flows into the aeration tank 2, where the aerobic bacteria in the sludge are The organic matter contained therein is decomposed and reaches the final settling tank 3 mentioned above.

In the final settling tank 3, the treated water is separated into sludge, which is mostly made up of bacterial cells, and part of it is fed back to the sedimentation tank 2, and the rest is used as surplus sludge.
The sludge is put into the sludge thickening tank 4 and decanted. The sludge from which the top water has been removed in the sludge thickening tank 4 and has been somewhat thickened is pressurized by the sludge pump 5 and sent to the M. 1 double pipe type indirect heat exchanger (hereinafter referred to as HXM.1. The same applies to HXM. ．． 2 and enters H, which will be described later.
XNo. It is indirectly heated by the hot wire heated in step 3 and enters reactor 6. In the reactor 6, steam generated in the waste heat boiler 1 or steam from other equipment is further blown in and heated to, for example, 175°C and a gauge pressure of 8.1k9/ground, and undergoes heat treatment for about 30 minutes. The heat-treated sludge has been modified to be more susceptible to methane fermentation. It is cooled by applying heat to the hot soot in HXNo. In step 4, it is water-cooled and put into a digestion tank 7. In the digestion tank 7, anaerobic digestion (methane fermentation) occurs by methane bacteria and acid-producing bacteria, and the generated mutane-containing gas is sent to the gas tank 8a, and the sludge after digestion (digested sludge) is sent to the sludge storage tank 8. In this storage tank, the sludge is first mixed with raw sludge from the settling tank 1, and a polymer flocculant is added thereto, and then filtered in a pressure furnace 9 to form a dehydrated cake, which is then charged into a vertical multi-stage incinerator 10 and incinerated. be done. Combustion gas from the incinerator 10 is sent to a waste heat boiler 11, and the generated steam is used to heat the reactor 6. On the other hand, gas tank 8
The methane-containing gas in a is used as fuel for the power generation gas engine 12, the exhaust gas of the gas engine 12 is sent to the waste heat hot water boiler 3, and the overflow of the hot water boiler 3 is used as fuel for the power generation gas engine 12. 1
Used for heating. In addition, in this example of the conventional method, the raw sludge separated in the settling tank 1 is not fermented with methane,
Although raw sludge is charged into the sludge storage tank 8, if there are few impurities such as earth and sand in the raw sludge, it is advantageous to overflow the raw sludge and charge it into the digestion tank 7 because the amount of methane gas generated will further increase.
The above conventional method is characterized by heat-treating excess sludge, which is difficult to cause methane fermentation, to convert it into a substrate that is easy to ferment.
The amount of vertebrae generated is 1.5 to 2 compared to the case without heat treatment.
．． It is considered to be an extremely excellent method from the standpoint of energy recovery, as it increases the sound of the air.

However, on the other hand, since an indirect heat exchanger is used, it is not limited to the double pipe type of the above embodiment.
Even if a shell-tube type is used, the equipment cost will be significantly higher.

Furthermore, as is well known, the optimum temperature for methane fermentation is 35±200℃ for medium-temperature digestion and 53±0.5℃ for high-temperature digestion. In the case of an indirect heat exchanger, as the operating time passes, the heat transfer surface inevitably gets dirty and the heat transfer deteriorates, thus making it difficult to adjust the temperature of the heat-treated sludge at the inlet of the digestion tank 7 and, ultimately, of the digestion tank 7 itself. becomes difficult.

The present inventors have conducted various studies in order to solve the above problems, and in particular, by performing heat treatment of sludge in the presence of oxygen (economically speaking, air) in the reactor 6, , the heat treatment temperature is 100-120℃ (vapor pressure 0-lk
It was found that it was possible to lower the value to 9/earth gauge).

This will be specifically explained using experimental results.

First, the experimental method will be explained. As samples, raw sludge and excess sludge from a separate sewage treatment plant were sedimented and separated, and the total solids concentration was 3.8 to 4.0% by weight, respectively (organic concentration 2.7 to 4.0% by weight).
3.0% by weight), 2.8-3.4% by weight (organic concentration 2
．． 0 to 2.5% by weight).

Regarding heat treatment conditions, raw sludge has a sufficiently high decomposition rate through methane fermentation even without heat treatment. Heat treatment) is called anaerobic heat treatment, and heat treatment in the presence of oxygen is called aerobic heat treatment.

) 20oo (substantially no heat treatment), 6000, 100
Excess sludge was subjected to anaerobic and aerobic heat treatment at 30 pm, 60 q, 80 q, 100 ℃, 120 ℃, respectively.
It was held at 140q0, 17500 for 30 minutes.
For aerobic heat treatment, air was added to a total pressure of 3.0 kg/flow. As a digestion method, in a glass bottle with an internal volume of 4, the input sludge, whether raw sludge or surplus sludge, is 12 sludges that have undergone anaerobic or aerobic heat treatment, 1 sludge that has not been heat treated, and 3 seeds sludge. A batch method was used in which the mixture was thoroughly mixed and kept at 1°C for 13 days.

The experimental results were expressed as the decomposition rate (%) by methane fermentation, based on the amount of gas generated during the digestion process and analysis of sludge. A part of the experimental results is shown in Figure 5, with the heat treatment temperature plotted on the axis and the decomposition rate due to methane fermentation plotted on the vertical axis. As is clear from the figure, raw sludge is easily digested, so
Even without heat treatment, the decomposition rate (vertical axis value) is sufficiently high, and heat treatment is not required regardless of whether it is anaerobic or aerobic.

{o' In the case of surplus sludge, when aerobic heat treatment is performed, the effect of heat treatment appears at a lower temperature than anaerobic heat treatment,
It almost reaches its maximum value at 100 to 120 qo. On the other hand,
In the case of anaerobic heat treatment, the decomposition rate reaches approximately 60% starting at 12,000 ℃, and as the temperature is further increased, the decomposition rate increases to about 64% at 17 pu0. From the above experimental results, it has been recognized that energy saving and economic benefits are achieved by carrying out the heat treatment in the reactor 6 in the presence of air (more generally, oxygen-containing gas).
It has been filed under No. 6-144311.

When the heat treatment temperature is lowered to 100o~120q0, the HXM. 2, HXM. 3,
HXNo. 4, etc., the heat recovery efficiency will be significantly reduced.

Also, HXNo. 4, the amount of heat transferred decreases over time due to "dirt" which is the fate of indirect heat exchangers, but if the principle of flash evaporation (vacuum evaporation) is used, cooling is performed by reducing the pressure, which is extremely effective compared to heat transfer. Pressure adjustment with a long response allows extremely stable temperature adjustment with good followability.

Furthermore, if the high temperature heat-treated sludge that has exited the reactor 6 is flushed to atmospheric pressure, it will generate steam, but this steam can be recovered using a steam ejector, and the steam for driving the ejector is Although the waste heat generated in the waste heat boiler 1 is used, a part of the waste steam from the vacuum generation ejector during the vacuum evaporation can also be used.

Of course, a compressor could be used instead of a steam ejector. The purpose of the present invention is to improve the known activated sludge sludge heat treatment method to aerobic heat treatment based on the above findings and considerations, and further improve the sludge treatment method by improving the heat recovery method around the heat treatment reactor. It is to provide. The configuration of the present invention is as follows.

That is, first, raw sludge is separated and removed from the wastewater in the initial settling tank, then organic matter contained in the wastewater is decomposed by aerobic bacteria under aeration in the aeration tank, and then processed in the final settling tank. Water and surplus sludge are separated, and this surplus sludge is heated in a reactor for 1,000 hours while aeration of oxygen-containing gas is carried out.
After heat treatment at a temperature exceeding 0 and a pressure exceeding atmospheric pressure, the heat-treated sludge is subjected to methane fermentation in a digestion tank with or without addition of raw sludge separated from the first settling tank, and methane-containing gas is recovered. In a sludge treatment method including;
The steam generated when the heat-treated sludge exiting the reactor is flushed to approximately atmospheric pressure is compressed and mixed with the surplus sludge before heat treatment to recover heat, and the flushing reduces the pressure to approximately atmospheric pressure. By adding or not adding the raw sludge separated in the first settling tank to the heat-treated sludge, the sludge is flushed into a vacuum cooler with a controlled vacuum degree as a regulating element to keep the temperature of the sulfur sulfur at an appropriate temperature. A sludge treatment method characterized by temperature control.

According to Samurai Seki No. 50-133168, sludge is introduced semi-continuously into two or more containers connected in series, heated to the required temperature by steam recovered from the succeeding containers and newly replenished steam, After being held for the required time, it is transferred to a subsequent container, where it is depressurized and cooled by generating steam;
A method is disclosed in which the generated steam is utilized for heating as described above.

However, this method not only has the limitation of being semi-continuous, but also requires a high temperature of 200 qo, which is the same as before, since no oxygen-containing gas is blown into the heat treatment. Instead of using a heat exchanger, we decided to use a flush method instead.
It is understood that the device was simplified at the expense of thermal efficiency, and its purpose is to improve the dewatering properties of heat-treated sludge, not for digestion (production of methane-containing gas). In contrast, the present invention was made based on the knowledge that the treatment temperature can be lowered to around 100 qC by blowing in oxygen-containing gas during heat treatment, and the treatment temperature can be lowered to around 100 qC.
The steam generated when flushing to near atmospheric pressure is recovered to reduce the power required to create the vacuum necessary for flushing in a vacuum state. Therefore, it was possible to simplify and stabilize the device while maintaining high thermal efficiency. Therefore, when heat treatment is performed while blowing in oxygen gas, the advantage of performing the heat treatment at a temperature of 100 qo or higher is that the treatment time is shortened, which is the same as in the case of general chemical reactions.

Next, an example of an embodiment of the present invention will be specifically described with reference to FIG. 2.

The area other than the area surrounded by the dashed line in the figure is the same as the example of the conventional method shown in FIG. Pressurized by the sludge pump 5, H
XNo. Hot water (9
The sludge, which has been preheated at a temperature of 0°C to 95°C, is heated by the steam ejector 23 by mixing the steam generated in the reduced pressure evaporator 21 (described later) with the steam recovered under pressure, and is further heated at the inlet of the reactor or the reactor. It is heated by steam blown into the inside (or both) and reaches a temperature of over 100℃,
Heat treatment is performed.

As mentioned above, when this heat treatment is carried out under an ant atmosphere of oxygen-containing gas, a temperature of 100 to 120 qo is sufficient and is a desirable working condition. The effect of the present invention can be fully recognized when the temperature is around 0. The sludge that has been heat-treated (heat-treated sludge) reaches the reduced pressure evaporator 21, where it is released to almost atmospheric pressure and generates steam due to the flushing phenomenon, but the generated water vapor is recovered under pressure by the ejector 23 as described above and converted into sludge. mixed with

The heat-treated sludge discharged from the reduced pressure generator 21 is flushed to a vacuum cooler 22 whose degree of vacuum is automatically adjusted by a vacuum ejector 24 in order to maintain the digestion tank at an appropriate temperature. .07k9/earth).

Examples of electric and pneumatic vacuum degree adjustment mechanisms are shown in FIG. 3 for the electric type and FIG. 4 for the pneumatic type. Vacuum cooler 22
The heat-treated sludge that has exited the sludge reaches the digestion tank 7, where it undergoes the same treatment as in the conventional method shown in FIG.

As mentioned in the explanation of the conventional method shown in FIG. 1, in the above embodiment, instead of charging the raw sludge separated in the settling tank 1 to the sludge storage tank 8, the ± sand content of the raw sludge is reduced. In this case, it is advantageous to input the raw material into the digester 7 and use it as a raw material for methane fermentation, but in this case, the present invention does not determine whether to input it into the vacuum cooler 22 and use other temperature control equipment. I don't care.

Note that high-pressure steam used for heating the steam ejector 23, vacuum ejector 24, and reactor 6 is supplied to the waste heat boiler 11.
However, if there is a shortage of steam, it will be supplied from the pond equipment, for example, another boiler (not shown), and if there is an excess, it will be supplied to other equipment or used for heating hot water in the waste heat boiler 13. Some are discarded.

Also, the location of the sludge pump is HXM. Even if it is located upstream of HO rudder. 1 and the steam injector inlet, or between the steam injector inlet and the reactor 6, there is no big difference, and there is no particular limitation on these.

Furthermore, the technical meaning of the present invention is not impaired even if a combustor is used in place of the steam ejector 23, or a vacuum pump is used in place of the vacuum ejector and the cooler that follows it. Don't worry about it. As is clear from the above description, the sludge treatment method of the present invention utilizes mass transfer and phase change, rather than transferring only heat through a heat transfer surface, as a heat recovery method during heat treatment of surplus sludge. Therefore, the change in heat transfer over time based on dirt on the heat transfer surface is HXNo. Except for 1, there is no post-heat exchanger at all, so the equipment cost is low, and by using a vacuum cooler, the temperature of the heat-treated sludge can be accurately controlled.

[Brief explanation of the drawing]

Figure 1 is a process diagram showing an example of a conventional sludge treatment method;
The figure is a process diagram showing one example of the sludge treatment method of the present invention, and FIGS. 3 and 4 are diagrams showing an electric type and an air type regulating method of a vacuum cooler, respectively. Figure 5 also shows the raw sludge and excess sludge, respectively.
It is a graph showing the relationship between heat treatment temperature and decomposition rate by methane fermentation when aerobic heat treatment and anaerobic heat treatment are performed. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. Raw sludge is separated and removed from wastewater in an initial sedimentation tank, then organic matter contained in the wastewater is decomposed by aerobic bacteria under aeration in an aeration tank, and organic matter contained in the wastewater is decomposed by aerobic bacteria in a final sedimentation tank. , the process of separating treated water and sludge, feeding back part of the sludge to the settling tank, heat-treating the remaining surplus sludge in a reactor, fermenting methane in a digestion tank, and recovering methane-containing gas. In a continuous sludge treatment method comprising: (a) carrying out the heat treatment at 100°C or higher in the presence of an oxygen-containing gas;
(b) Heat is recovered by compressing the steam generated when the heat-treated sludge is flushed to almost atmospheric pressure and mixed with the surplus sludge before heat treatment; A method for treating sludge, comprising the steps of: flanging heat-treated sludge in a vacuum cooler to cool it; and (d) adjusting the temperature of a digestion tank using the degree of vacuum of the vacuum cooler as a control means. 2 After adding the raw sludge separated in the initial settling tank to the sludge that has been flushed to almost atmospheric pressure after the heat treatment in step (b) of Section 1, carry out the steps of Section 1 (c) and (d). Features: A sludge treatment method according to claim 1.