JPH0364198B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a control device for returning sludge from a digestion tank in which organic sludge in a sewage treatment plant is digested by anaerobic bacteria.

[Technical background of the invention and its problems]

Organic sludge is extracted from the initial settling tank and final settling tank at a sewage treatment plant, and after being concentrated in a thickening tank, it is put into a digestion tank where it is digested. The organic sludge put into the digester is decomposed into carbon dioxide and methane gas by anaerobic bacteria through the process of liquefaction and gasification, and the methane gas is led to a gas storage tank and used as a heat source to keep the digester warm.
On the other hand, the digested sludge is extracted from the digestion tank and sent to the next process of washing and dewatering equipment.

Normally, a high-efficiency digestion method is used for digestion, and two-stage digestion is performed using two digestion vessels. In this case, it is necessary to return the sufficiently digested seed sludge from the secondary digestion tank to the primary digestion tank.

Conventionally, for returning this type of sludge, a method has often been adopted in which a value obtained by multiplying the raw sludge flow rate input from the thickening tank by a certain ratio is used as a target value for the amount of return type sludge, and a return type sludge pump is controlled.

In FIG. 1, raw sludge drawn from a thickening tank (not shown) is sent to a primary digestion tank 2 by a raw sludge input pump 1. The flow rate is measured by a flow meter 6. In the primary digestion tank 2, it is decomposed into methane gas and carbon dioxide gas through liquefaction and gasification processes by anaerobic bacteria under stirring and heating. Further, the digested sludge in the lower part of the primary digestion tank 2 is pulled out and sent to the secondary digestion tank 4 by the sludge transfer pump 3. Secondary digestion tank 4
After post-digestion and concentration, part of the digested sludge is returned to the primary digestion tank 2 by a return sludge pump 5. The flow rate of the returned sludge is measured by a flow meter 7. Other digested sludge is sent to the next process, such as cleaning equipment. In this case, the input raw sludge flow rate is measured by the flowmeter 6 so that the return seed sludge flow rate is approximately 1 to 3 times the input raw sludge flow rate, and the ratio is amplified by the ratio device 8 to multiply this signal by a certain ratio. The output of this ratio device 8 is inputted to the flow rate regulator 9 as a target flow rate of return type sludge. The flow rate regulator 9 receives feedback of the flow rate of return sludge from the flow meter 7, and controls the return sludge pump 5 to a predetermined flow rate of return sludge by PID operation.

By the way, such conventional methods simply measure the flow rates of input raw sludge and returned seed sludge, and therefore have the following problems.

Firstly, the seed sludge is returned regardless of the properties of the input raw sludge, making it impossible to follow changes in the properties of the input raw sludge (suspended solids concentration), and the excess or deficiency of the seed sludge (more specifically, the anaerobic bacteria This disturbs the acidic and slightly alkaline environments during the liquefaction and gasification processes, respectively), making it impossible to maintain optimal digestion efficiency. On the other hand, the digestion efficiency cannot be maintained at an optimum level even when the properties of the sludge in the secondary digestion tank change.

[Purpose of the invention]

The purpose of the present invention is to efficiently perform digestion in the primary and secondary digestion tanks even when the properties of the raw sludge input to the primary digestion tank change, or even when the properties of the sludge in the secondary digestion tank change. An object of the present invention is to provide a return sludge control device for a digestion tank that can be operated efficiently and stably.

[Summary of the invention]

The feature of the present invention is that the control device is configured not only based on the flow rate of both input raw sludge and returned sludge, but also based on the suspended solids concentration in relation to the flow rate.

[Embodiments of the invention]

The present invention will be explained in detail below with reference to an embodiment shown in FIG. In FIG. 2, reference numeral 1 denotes a raw sludge input pump, which inputs raw sludge drawn from a thickening tank (not shown) into a primary digestion tank 2. 3 is a sludge transfer pump that transfers the lower sludge in the primary digestion tank 2 to the secondary digestion tank 4.
Transfer to. 5 is a return type sludge pump, and 6 and 7 are sludge flowmeters, which are installed in pipes 20 and 21, respectively.

The configuration up to this point is the same as that shown in Figure 1, and the digestion gas (mainly composed of carbon dioxide gas and methane gas) generated by the digestion process in the first and second digestion tanks 2 and 4. is sent to a gas tank (not shown) through a pipe 22. In addition, the supernatant water generated by the digestion process in the secondary digestion tank 4 is transferred to the pipe 23.
Also, the thickened sludge in the secondary digestion tank 4 is transferred to the pipe 2.
4, each is sent to the next processing step.

10 and 11 are suspended solids concentration meters, and pipes 21 and 2
0 to measure the concentration of suspended solids in the sludge flowing through them. 12 is a multiplier which inputs the flow rate of input raw sludge and the suspended solids concentration signal from the corresponding measuring device, calculates and outputs the amount of suspended solids by multiplication. 13
is a similar multiplier for return seed sludge. 8
is a ratio device that multiplies a constant constant, 9 is a flow rate controller, the signal from ratio device 8 is set as a target value, and multiplier 1
The return species sludge pump 5 is controlled by PID operation using the signal from 3 as feedback.

In the above configuration, the flow rate and suspended solids concentration of raw sludge introduced into the primary digestion tank 2 by the raw sludge input pump 1 are measured by the flow meter 6 and the concentration meter 10, and these are input to the multiplier 12. Ru.
The multiplier 12 calculates (raw sludge flow rate) x (raw sludge suspended solids concentration) = raw sludge suspended solids amount, and inputs it to the ratio machine 8 . Ratio device 8 is determined in advance (determined from the digestion tank volume, return type sludge pump capacity, etc.)
The raw sludge suspended matter amount inputted from the multiplier 12 is multiplied by a constant (1 to 3 times), and the result is inputted to the flow rate regulator 9.

On the other hand, the flow rate and suspended solids concentration of the seed sludge drawn out from the secondary digestion tank 4 by the return seed sludge pump 5 are measured by the flow meter 7 and the concentration meter 11, respectively.
It is returned to the primary digestion tank 2. The flow rate and suspended solids concentration of the returned seed sludge are inputted to the multiplier 13 in order to obtain the suspended solids of the returned seed sludge in the same way as raw sludge, and the output of the multiplier 13 (the amount of suspended solids in the returned seed sludge) is input to the flow rate regulator 9. is input to.

The raw sludge and returned seed sludge introduced into the primary digestion tank 2 are stirred and heated in the primary digestion tank 2, and are digested by anaerobic digestion bacteria. After the digestion is completed, the sludge is transferred to the secondary digestion tank 4 by the sludge transfer pump 3, and after post-digestion and concentration are performed in the secondary digestion tank 4, some of it is used as the above-mentioned return seed sludge, and the rest is sent to the cleaning equipment. The slurry is sent to the next process.

The flow rate regulator 9 uses the input from the ratio device 8 as a target value and the input from the multiplier 13 as feedback to control the return species sludge pump by PID operation. As a result, the amount of returned seed sludge necessary for digesting the amount of suspended solids in the input raw sludge can be qualitatively and optimally controlled.

Another embodiment of the present invention will be described with reference to FIG. The embodiment shown in Fig. 2 is a closed-loop control device such as a PID controller, but the embodiment shown in Fig. 3 is an open-loop control device by introducing integration and comparison means. . Description of the same parts as in the embodiment of FIG. 2 will be omitted.

The output of the ratio device 8 (the amount of suspended solids in the input raw sludge) is input to the integrator 14 , and similarly the output of the multiplier 13 (the amount of suspended solids in the returned raw sludge) is input to the integrator 15 . Normally raw sludge is added to the digestion tank 6 to 10 times/
This is done in batch operation on a daily basis, and since it is unlikely that continuous injection is carried out, the integration should be carried out every time this batch operation is performed. The outputs of the integrators 14 and 15 (the integrated value KΣSS _I of the input raw sludge suspended solids and the integrated value ΣSS _R of the returned raw sludge suspended solids) are output by the comparators 1, respectively.
6, and the comparator 16 outputs logic 1 to the sequence controller 17 when KΣSS _I ≦ΣSS _R. The sequence controller 17 sends a start signal to the return type sludge pump 5 based on the operation signal of the raw sludge input pump 1, and a stop signal based on the logic 1 signal from the comparator 16. Furthermore, return species sludge pump 5
At the same time as transmitting a stop signal to integrator 14, 15
Outputs the integration reset signal to

Under the above configuration, the return species sludge pump 5 is started at the same time as the raw sludge input pump 1 is started, and the target return species sludge is proportional to (K times) the total suspended solids amount of the input raw sludge in one batch operation. When the amount of suspended solids has been returned (KΣSS _I ≦ΣSS _R ), the return type sludge pump stops, and the optimal return type sludge can be achieved. The embodiment shown in FIG. 3 has the advantage that the return sludge pump can be a fixed speed pump.

In addition, in the above description, a multiplier, a ratio machine, etc. were illustrated, but it may be performed by the calculation means of these functional electronic calculators.

〔Effect of the invention〕

As explained above, according to the present invention, even if the properties of the raw sludge input to the digestion tank (suspended solids concentration...SS) change, or the properties of the digested sludge in the secondary digestion tank change, the species The return of sludge can be maintained optimally, and the efficiency and stability of digestion can be maintained. Furthermore, depending on the method of implementation as in the embodiment shown in FIG. 3, the speed of the return sludge pump can be fixed and the apparatus can be simplified.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of a conventional device, Fig. 2 is a block diagram showing an embodiment of a return sludge control device for a digester tank according to the present invention, and Fig. 3 is a block diagram showing another embodiment of the present invention. It is a diagram. 1... Raw sludge input pump, 2... Primary digestion tank,
4... Secondary digestion tank, 5... Return species sludge pump,
6, 7...Flowmeter, 10,11...Concentration meter, 1
2, 13... Suspended solid amount calculation means, 8... Ratio calculation means, 9, 16, 17... Flow rate adjustment means.

Claims (1)

[Claims] 1 A flowmeter and a concentration meter are installed to measure the flow rate and suspended solids concentration of the raw sludge input into the primary digestion tank where organic raw sludge is digested by anaerobic bacteria, and the input raw sludge is measured based on the flow rate and concentration of the input raw sludge. Calculating means for outputting the amount of suspended solids in sludge, and measuring the flow rate and suspended solids concentration of seed sludge returned to the primary digestion tank from the secondary digestion tank that thickens the sludge transferred from the primary digestion tank. A flow meter and a concentration meter are used to calculate the ratio between the amount of suspended solids in the input raw sludge and the amount of suspended solids in the returned sludge. A return sludge control device for a digester tank, comprising a flow rate adjustment means for controlling a return seed sludge pump so that the ratio is given in advance.