JPH0796298A - Operation of digestion tank - Google Patents

Abstract

PURPOSE:To remove scum during operation by spraying and stirring digesting sludge in downward flow in the case of normal and, in the case of scum removal, spraying and stirring the sludge in upward flow and then returning the upward flow to downward flow and at the same time keeping the liquid surface level of the digesting sludge at the time of spraying and stirring in downward flow be under the normal time standard level. CONSTITUTION:At normal time, digesting sludge is sprayed in downward flow 31 out of a draft tube 6 of a stirring apparatus 2 and stirred. Meanwhile, at the time of scum removal, digesting sludge is sprayed in upward flow 30 out of a screw casing 5 of the stirring apparatus 2 and stirred and a scum layer 21 is pulverized. Then, the spraying and stirring in upward flow are turned back to the spraying and stirring in downward flow. At that time, a valve for a sludge taking out pipe 11 is opened and the liquid surface level of the digesting sludge is lowered to be between the lower limit LWL of the standard liquid surface level at the normal time and the height of the upper edge of the screw casing 5. After the scum layer 21 is removed, sludge to be treated is supplied through a sludge supplying pipe 10 and the liquid surface level is returned to the standard level at the normal time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating a digestion tank equipped with a stirrer having a draft tube and for anaerobic fermentation of organic sludge such as sewage sludge.

[0002]

2. Description of the Related Art FIG. 2 is a view showing a digestion tank equipped with an agitator having a draft tube. Reference numeral 1 is a digestion tank, and a stirrer 2 is vertically provided at the center thereof. The stirrer 2 includes a rotary shaft 3 provided on the upper portion of the digestion tank 1,
The screw 4 attached to the rotating shaft, and the screw casing 5 surrounding the screw 4
And a draft tube 6 connected to the lower end of the screw casing 5. Reference numeral 9 is a motor for rotating the rotary shaft 3, 20 is digested sludge during fermentation processing which is temporarily stored in the digestion tank 1, and 21 is a scum layer floating on the upper surface of the digested sludge.

When the sludge is fermented by the digestion tank 1, the treated sludge is supplied from the upper sludge supply pipe 10 and the digested sludge 20 corresponding to the supply amount of the treated sludge is supplied from the lower sludge extraction pipe 11. Extract as treated sludge. In this way, the liquid level of sludge is maintained within the specified range,
Operation continues.

During the fermentation process of sludge, it is necessary to thoroughly mix the newly supplied sludge to be treated with the digested sludge in the digestion tank 1, and therefore the agitator 2 is used for the digestion tank 1
Stir the digested sludge inside. The stirrer 2 can change the rotating direction of the screw 4, and two stirring patterns can be created by changing the rotating direction. One of them is a stirring pattern in which digested sludge is discharged from the lower end of the draft tube 6 to generate a downward flow jet, and the jet flow forms an upward flow 30 in the digestion tank 1, and the other one is a stirring pattern. This is a stirring pattern in which digested sludge is discharged from the upper end of the screw casing 5 to generate an upward flow jet, and the downward flow 31 is formed in the digestion tank 1 by this jet flow. In normal operation, the stirrer 2 performs an operation of generating a downward flow jet from the lower end of the draft tube 6.

By the way, the digested sludge contains suspended matter, which floats up during operation and collects on the liquid surface of the digested sludge to form a scum layer 21. Then, if the operation is continued for a long time, the thickness of the scum layer 21 increases, the effective volume of the digestion tank 1 decreases, or there is a problem in that the generated gas is hindered from being discharged. Therefore, the operation is stopped when the thickness of the scum layer 21 reaches the limit,
The operation of removing the scum layer 21 by opening the digestion tank 1 is performed.

[0006]

However, there is a problem that the operation must be stopped in order to carry out the scum layer removing operation. As a solution to this problem,
A method of removing the scum layer by performing the following operation is considered.

First, the screw 4 of the stirrer is reversely rotated to blow up an upward jet from the upper end of the screw casing 5 to crush the scum layer 21. Next, the rotation direction of the screw 4 is returned to the original state, the digested sludge is discharged from the lower end of the draft tube 5 (the digested sludge is sucked from the screw casing 5), and the scum crushed together with the digested sludge is sucked from the screw casing 5. , Disperse in digested sludge in digestion tank 1.

According to such a method, it is possible to remove the scum layer to some extent without stopping the operation. However, there is still a problem that the scum layer cannot be efficiently removed due to the reasons described below.

FIG. 3 is a diagram showing the agitated state of the digested sludge in normal times. In FIG. 3, the parts already described in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. Reference numeral 7 is an upper screw, and 8 is a disc for scattering sludge discharged in an upward flow on the scum layer in the periphery. HW
L and LWL indicate the standard of the liquid level of digested sludge in normal times, and represent the upper limit level and the lower limit level, respectively.

When the digested sludge is sucked from the screw casing 5, the digested sludge flows in the direction of the rotary shaft 3 of the screw and is sucked into the screw casing 5, as shown by the arrow in the figure. However, the digested sludge directly sucked into the screw casing 5 according to the streamline as shown by the arrow is much lower than the liquid level, and the clear flow as described above is observed at the lower limit of the reference liquid level. It is an area further below. Then, the digested sludge near the liquid surface seems to be still at a glance. Therefore, when operating within the range of the standard liquid level,
Even if the operation for sucking the digested sludge into the screw casing 5 is performed, the scum floating on the liquid surface of the digested sludge is not sucked.

Therefore, according to the above-mentioned method, the scum layer 21 can be crushed by rotating the screw 4 of the stirrer in the reverse direction and blowing up the jet of the upward flow from the upper end of the screw casing 5. Even if an attempt is made to return the rotation direction of the above to suck the crushed scum from the screw casing 5, only the digested sludge in the region below the lower limit of the reference liquid surface level is sucked,
The scum floating near the liquid surface remains floating.

Therefore, even if the above-mentioned method is carried out, the accumulation of scum cannot be prevented, and after long-term operation,
It is necessary to open the digestion tank 1 and directly perform the work of removing scum. It is an object of the present invention to provide a method for operating a digestion tank that can solve the above-mentioned problems and efficiently remove the scum layer during operation.

[0013]

In order to achieve the above object, in the present invention, the digested sludge is discharged in a downward flow from a draft tube of the stirrer in a normal state to stir, and the screw of the stirrer is removed when scum is removed. After stirring the digested sludge from the casing to discharge it in the upward flow, return it to stirring to discharge it in the downward flow, and during the stirring to discharge it in the downward flow, the liquid level of the digested sludge is changed to the normal reference liquid level. After removing the scum, the digestive sludge liquid level is returned to the normal standard liquid level.

[0014]

When the scum is removed, first, the screw of the stirrer is rotated in the reverse direction to blow up the upward jet from the upper end of the screw casing, so that the scum layer 21 is crushed by the jet. Next, when the digested sludge is sucked into the screw casing (discharged from the draft tube in a downward flow) and returned to the stirring pattern, the liquid level of the digested sludge falls below the lower limit of the reference liquid level, so Even in the vicinity, a strong flow of digested sludge sucked into the screw casing occurs. Therefore, the scum crushed and floating on the liquid surface is sucked into the strong flow of digested sludge, and the scum layer is removed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. In FIG. 1, the parts already described with reference to FIGS. The liquid level in normal operation is usually maintained at a height of 300 mm to 1500 mm from the upper end of the screw casing 5. And, the lower limit level of the liquid level is 300 mm ~
It is set to a height of 500 mm and the upper limit level is 1300 mm ~
It is set to a height of 1500 mm. The lower limit level is a value set to maintain the suction flow rate of the agitator 2 at a predetermined amount or more, and the upper limit level is to prevent the digested sludge from being scattered to the upper part of the tank in order to prevent the bearing of the agitator 2 from being damaged. This is the setting value to be set.

According to the operating method of the present invention, the scum layer 21
In order to remove the above, first, the operation of the stirrer 2 is changed from a normal stirring state to stirring in which the digested sludge is discharged from the screw casing 5 in an upward flow. The stirring time for discharging in the upward flow may be short, for example, about 1 hour. The scum layer 21 is crushed by blowing up the digested sludge. Next, the valve of the sludge extraction pipe 11 is opened to lower the liquid level. The liquid level to be lowered is set to a height between the lower limit LWL of the reference liquid level and the upper end of the screw casing 5.

While the liquid level is maintained within the above range, the operation of the agitator 2 is changed to the agitation in which the draft tube 6 discharges in a downward flow, and the agitation is continued for about several hours.
At this time, the stirrer 2 is operated in the state shown in FIG. In FIG. 1, h indicates the range of the liquid level when scum is removed. By lowering the liquid level to the range of h, the crushed scum is sucked into the screw casing 5 together with the digested sludge and discharged from the draft tube 6. Then, it is dispersed in the digested sludge in the digestion tank 1. The sludge to be treated is supplied from the sludge supply pipe 10, the liquid level is returned to the normal liquid level, and the normal operation is restored.

In the above operation, a particularly preferable range of the liquid level at the time of removing scum is 100 mm to 200 mm from the upper end of the screw casing 5. When the stirrer 2 is operated within this liquid level range, a strong flow of digested sludge sucked into the screw casing 5 is generated and swirls are formed. The scum is then easily sucked into the vortex.

When the liquid surface level exceeds 200 mm from the upper end of the screw casing 5, the flow of sucking the digested sludge near the liquid surface into the screw casing 5 becomes gradually weaker and the scum becomes difficult to be sucked. Further, when the liquid level is set lower than the position 100 mm above the upper end of the screw casing 5, the amount of digested sludge sucked gradually decreases and scum sucking also deteriorates.

[0020]

Industrial Applicability According to the present invention, when scum is removed, digested sludge is discharged from the screw casing of the stirrer in an upward flow to agitate the scum layer to crush the scum layer. This is a method of lowering the liquid surface level of the above than the normal standard liquid surface level, and sucking the crushed scum into the digested sludge to disperse it. Therefore, according to the present invention, the scum layer can be removed during the operation, so that the loss due to the suspension of the operation does not occur and it is not necessary to directly remove the scum layer.

[Brief description of drawings]

FIG. 1 is a diagram showing a stirring state when removing a scum layer according to the present invention.

FIG. 2 is a view showing a digestion tank equipped with a stirrer having a draft tube.

FIG. 3 is a diagram showing a stirring state of digested sludge in normal times.

[Explanation of symbols]

 1 Digestion Tank 2 Stirrer 4 Screw 5 Screw Casing 6 Draft Tube 10 Sludge Supply Pipe 20 Digestion Sludge 21 Scum Layer 30 Upflow 31 Downflow

Claims (1)

[Claims] 1. A screw casing is provided so as to surround a vertically extending screw, and a stirrer having a draft tube connected to the lower end of the screw casing is provided. By this stirrer, digested sludge is directed downward from the draft tube. In the operation method of the digestion tank capable of both stirring by discharging the digested sludge from the screw casing or stirring by discharging the digested sludge from the screw casing in the upward flow, the digested sludge is discharged downward from the draft tube of the stirrer in normal times. When the scum is removed, the digestion sludge is discharged from the screw casing of the stirrer in an upward flow, and then returned to a downward flow discharge agitation mode. Lower the sludge liquid level below the normal standard liquid level, and after removing scum, remove the digested sludge liquid level. A method for operating a digestion tank, which is characterized by returning the bell to a normal standard liquid level.