JPH034859B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for determining sludge concentration in an aeration tank in a sewage treatment plant that uses activated sludge to purify sewage.

Generally, when wastewater containing a large amount of organic sludge such as sewage is purified using activated sludge, the ratio of the amount of organic matter F in the inflowing sewage to the amount M of microorganisms in the aeration tank, that is, the F/M ratio, is set to a constant value. It is said that it is preferable to maintain and operate the sewage system at a constant temperature in order to increase the treatment effect and stably purify the sewage. The amount of inflowing organic matter described above is often highly correlated with the amount of inflowing sewage, so it can be easily predicted. However, the sludge concentration in the aeration tank is greatly affected by the mixing state within the tank, and it is not easy to maintain a constant F/M ratio. The mixing condition of the aeration tank differs depending on the structure of the tank and the aeration pipe. Furthermore, the ratio between the amount of organic matter in the inflowing sewage and the amount of microorganisms in the activated sludge, ie, the F/M ratio, changes as it flows down the tank. If the disturbance in the F/M ratio becomes large, the biochemical reaction of organic matter caused by microorganisms in the activated sludge will not work effectively, which will reduce the treatment effect. That is, in order to maintain the F/M ratio in a good state and to reduce disturbances from the target value, it is necessary to accurately know the mixing state in the aeration tank.

Conventionally, methods for determining the mixing state include complete mixing flow, extrusion flow (flow flowing down without being mixed), combined flow and diffusion flow, etc.
Many methods based on mixture models have been proposed.
However, these models are so-called ideal mixing conditions, and in many cases these models do not match the reality, and the value at the aeration tank outlet calculated from these models has a large error from the actual value. It was hot. This aeration tank is usually a long and narrow rectangular tank, with partition walls installed in several places,
In addition, the aeration tank is installed in several stages in a zigzag manner, and the entire aeration tank is incompletely divided by partitions such as partition walls and folds, which results in short-circuit flows passing through the gaps between the partitions. This is because a reverse flow occurs. If the fractionating sections in the aeration tank are uniformly installed and the flow of the mixed liquid is uniform, the accuracy of the complete mixing flow, extrusion flow, or their combined model should be high.

An object of the present invention is to use, in addition to the model representing the ideal mixing state, an arithmetic circuit that adds the ratio of the short-circuit flow, reverse flow, or a combination thereof, to more accurately determine the mixing state of the aeration tank. It is an object of the present invention to provide a method for determining sludge concentration in an aeration tank that can determine the sludge concentration of a mixture without much difference from the actual sludge concentration by expressing it as follows.

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. FIG. 1 is a cross-sectional view of an aeration tank divided by partition walls, folds, etc. The mixed liquid flows down from the upstream side A of the aeration tank 1, becomes a swirling flow by the air jetted out from the aeration pipe 2, and flows to the downstream side B.
leading to. Fractionation tank 3 is separated from other fractionation tanks by fractionation units 4 and 5. Here, if we pay attention to the flow of the mixed liquid flowing down the fractionating tank 3 separated by partition walls, in addition to the main stream 6 mixed by the swirling flow, there is also a flow from the previous stage fractionating tank 7-1 to the current fractionating tank A part of the liquid mixture that has flowed into Flow 8 is directly short-circuited to the downstream fractionation tank 7-2, and a part of the liquid mixture that has flowed into the downstream fractionation tank 7-2 flows through the gap in the fractionation unit 5 and is then returned to the current fractionation tank 3. There are a countercurrent 9 flowing therein and a side stream such as a countercurrent 13 counterflowing from the current fractionation tank 3 through the gap in the fractionation section 4 to the preceding fractionation tank 7-1.

The present invention expresses the mixing state of the mainstream by a completely mixed flow, an extrusion flow, or a diffusion flow.
Or, using a model consisting of a mixed flow combined by taking into account the weights of these flow rates, the volume ratio of the complete mixing flow part and the extrusion flow part, which are the main stream in the fractionation tank, and the short-circuit flow or reverse flow, which is the side flow, is calculated. It has an arithmetic circuit that uses as a parameter the ratio of the flow to the inflow flow rate of the fractionating tank.

FIG. 2 is a flow diagram showing the relationship between the main flow and the sub flow, which is the basis of the arithmetic circuit used in the present invention. Assuming that the fractionation tank 3 is the j-th one counting from the aeration tank inlet, a part of the mixed liquid that has flowed into the J tank from the previous stage J-1 tank 7-1 through the gap 10 in the fractionation part passes through the short-circuit flow 8. After branching, it merges with the counterflow 14 and becomes the main flow 12. After that, a part of the main stream 12 flows in the opposite direction 1
7 and is returned to tank j-1 through gap 10. It also merges with the counterflow 9 from the subsequent tank j+1 to form one main stream 12'. Thereafter, a part of the flow in the complete mixed flow section 12-1, which is a part of the main flow, becomes a counterflow 13 and is returned to the j-1 tank together with the above-mentioned counterflow 17. In addition, another part of the main flow 12 is the extrusion flow 1
After passing through the extrusion section 12-2 as 2'', it merges with the flow of the complete mixing flow section 12-1.Then, the main stream 12 merges with the short-circuit flow 8, and also with the reverse flow 15 from the j+1 tank in the subsequent stage. Then, a part of it branches off into a flow 14 which flows in the opposite direction to the inlet of the fractionation tank 3 and then flows down through the gap 11.

In Fig. 2, the ratios of the short-circuit flow 8 and reverse flows 9, 13, 14, 15 and 17 to the aeration tank inflow flow rate v in the j-stage tank are expressed as a _j , b _j , c _j , d _j , e _j and
When g _j is assumed, the flow rate v _j of the mixed liquid flowing from tank j-1 to tank j is given by the following equation.

v _j = (1 + c _j + g _j ) v ... (1) Here, v is the load water amount flowing into the aeration tank, which is the sum of the primary treatment water amount Q _s and the return sludge flow rate Q _r .

v= _Qs + _Qr ...(1)' Also, the sludge concentration Xj ₌₀ flowing into the first fractionation tank is expressed by the following equation.

X _j=p =X _s・Q _s +X _r・Q _r /v (1)″ The flow rate v _j ′ of the main stream 12 is given by the following equation.

v _j ′=(1+c _j +g _j −a _j +d _j )v...(2) Furthermore, if the ratio of the extrusion flow 12'' to the aeration tank inflow flow rate v is h _j , then the main flow 12 to the complete mixing section 12-1 is ′ flow rate v _j ″ is as follows.

v _j ″=(1+g _j +c _j −a _j −h _j ×d _j +b _j )v ……(2)′ The sludge concentration X _j ′ of the mixed liquid flowing into the j-stage tank as the main stream 12 is j−1 The sludge concentration in the stage tank and the sludge concentration flowing out from the j stage tank to the j+1 stage tank are respectively
Assuming X _j-1 and X _j , it is determined by the following formula.
For X _j-1 , use the value calculated for tank j-1 (for the first stage, use the inflow concentration to the aeration tank), and for X _j , use the value calculated in the previous cycle (for the first time, use the value calculated for the first cycle) (using records).

X _j ′=X _j-1 (1+c _j +g _j −a _j )v+X _j・d _j・v/v _j ′
...(3) Furthermore, the sludge concentration X _j '' of the mixed liquid flowing into the complete mixing section 12-1 can be calculated using the following equation.

X _j ″＝X _j ′・v _j +X _j+1・b _j /v _j ″ ……(3)′ Also, the flow rate v _j+1 of the mixed liquid flowing from the j-stage tank to the j+1-stage tank is It is obtained in the same way as equation (1).

v _j+1 = (1+b _j +e _j )v (4) Here, the sludge concentration X _j of the mixed liquid 18 flowing out from the j-stage aeration tank to the j+1-stage aeration tank is given by the following equation.

X _j =X _ja・(v _j ″−c _j・v)+X _jg・h _j v+X _j-1・a _j・
v+X _j+1・e _j・v/v _j ″−c _j v+h _j・v+a _j v+e _j v…
...(5) Here, X _ja is the concentration of sludge flowing out from the complete mixing section 12-1, and X _jg is the concentration of sludge flowing out from the extrusion section 12-2, which is calculated using the same calculation as the conventional one. It is something that

X _ja = X _ja (t-1)・V _c +X _j ″・V _j ″/V _c +V _j ″……
(5)' Here, X _ja (t-1) is the concentration of sludge in the complete mixing part of the previous calculation cycle, and V _c is the volume of the complete mixing part.

_{In addition ,} X _{jg =} It is a value.

Thus, the parameters in equations (1) to (5)
a _j , b _j , c _j , d _j , e _j , g _j , and by setting c _j , g _j to 0 for the calculation of the first fractionation tank (j = 1). Therefore, the inflow amount and sludge concentration can be calculated.

Furthermore, the concentration of sludge flowing out can be calculated from the flow rate of water flowing into each fractionation tank and the concentration of sludge.

The above parameters include actual measurement data for at least 24 hours of the flow rate of inflow water to the fractionation tank, its sludge concentration, and the sludge concentration at the outlet of the fractionation tank, and the above formula ( The sludge concentration of the fractionating tank effluent calculated in 5) can be easily determined by a calculation method such as a grid search method so that the error with the above-mentioned actual measurement data is minimized.

Next, details of an embodiment of the present invention will be explained with reference to FIGS. 3 and 4. FIG. 3 is a cross-sectional view of an elongated rectangular aeration tank having four partition walls and consisting of five fractionation tanks. Aeration tank 1 has partition walls 19-1 to 1
It is divided into fractionation tanks 18-1 to 18-5 by 9-4. The depth of the aeration tank is 5.5 m, the partition wall 19 is installed between 0 and 4 m below the water surface, and air is sent from a blower and mixed with water. Sewage or its primary treated water flows from the inlet A through the valve 23 and the flow meter 24 into the fractionation tank 18-1 of the aeration tank. The returned sludge flows into the fractionation tank 18-1 from the sludge well side B through the valve 21 and the flow meter 22.

Figure 4 shows an experiment conducted to compare the sludge concentration in the aeration tank calculated by the present invention with the measured value and the value calculated by the conventional method using the complete mixing and short-circuit flow models and the simplest back-mixing flow. It is a figure showing the result of the response of the aeration tank sludge concentration. Aeration tank 3 when the flow rate of primary treated water and the flow rate of return sludge are kept constant for a certain period of time and then the flow rate of return sludge is stopped for 2 hours.
This is an actual measurement of fluctuations in sludge concentration at the outlet of the fractionation tank.

As a result of the above experiment, the ratio (cm ³ /H)/(cm ³ /H) of each mixed flow to the inflow flow rate to the aeration tank was determined by the grid search method.
50%, short-circuit flow is 19% (the total value of 129% is the flow rate including 29% of reverse flow), and 10% is reverse flow returning to the second fraction as reverse flow, 4th fraction tank 10% of the flow is in the reverse direction returning to the third fractionation tank, and 19% is in the reverse flow within the tank. Ratio to aeration tank volume (cm ³ /
^cm3 ), the volume ratio of complete mixing is 70%, and the volume ratio of extrusion is 70%.
It was 30%.

Here, the volume ratio of complete mixing is the ratio of the complete mixing portion to the total aeration volume.

In this way, short-circuit flow or It has an aeration tank mixing calculation circuit that takes into account several counterflows or their combined flows.

The simulation model allows the sludge concentration of the mixed liquid to be determined with high accuracy.

FIG. 5 is a schematic flowchart of a simulation program for obtaining calculated values as shown in FIG. 4, and calculations are performed according to this flowchart.

That is, first, each parameter a _j , b _j , c _j , d _j ,
e _j , f _j , g _j , h _j , aeration tank volume, and number of fractionation tanks are input (step 101). Next, the flow rate and concentration of the primary treated water flowing into the aeration tank and the flow rate and concentration of the returned sludge are input as time series data for 7 hours every 10 minutes (step 102).

In step 103, the period t is updated from 1 to 42 so that 42 calculations are performed every 10 minutes. Further, in step 104, the fractionation tank number J is updated from 1 to 5 in order to sequentially execute calculations for the five fractionation tanks.

The calculation procedure is to calculate the mixed liquid flow rate using equation (1).
Find V _j (step 105). Next, the flow rate V _j ' of the main stream 12 is determined using equation (2) (step 106). Furthermore, the flow rate V _j of the flow rate 12' to the complete mixing section 12-1 is determined by (2)' (step 107). Next, calculate the sludge concentration X _j ' in the main stream 12 using equation (3) (step
108). Furthermore, the sludge concentration X _j '' flowing into the complete mixing section 12-1 is determined using equation (3)' (step 109).
Next, calculate the effluent sludge concentration X _j from the complete mixing section 12-1 using equation (5)' (step 110), and
The concentration of sludge flowing out from the extrusion portion 12-2, X _jg, is determined by (5)'' and (5) (step 111).Then, from these results, the concentration of sludge flowing out from tank J, X _j, is determined by equation (5). (Step 112).

The above calculations are performed sequentially for the five fractionation tanks (step 113), and are performed 42 times every 10 minutes (step 114) to create and output data for 7 hours (step 115).

As described above, according to the present invention, it is possible to accurately know the sludge concentration of the mixed liquid in each fractionation tank of the aeration tank, and the sludge concentration M of the mixed liquid corresponding to the organic capacity F, which is the inflow characteristic of sewage. It becomes easy to operate the engine to the desired F/M ratio.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of an aeration tank in an embodiment of the present invention, FIG. 2 is a flow diagram of an arithmetic circuit in the present invention, and FIG. 3 is a cross-sectional view for explaining an embodiment of the present invention. FIG. 4 is a graph showing the experimental results of the method of the present invention, and FIG. 5 is a flowchart showing an example of calculation by the method of the present invention. 1... Aeration tank wall, 2... Aeration pipe, 5... Partition wall, 10,
11...Gap, 12-1...Complete mixing part, 12-2
...extrusion part.

Claims (1)

[Scope of Claims] 1. Means for monitoring the flow rate and suspended matter concentration of sewage or its primary treated water, and the flow rate and sludge concentration of returned sludge, and a plurality of said sewage or primary treated water and returned sludge. In a method for determining the sludge concentration in an aeration tank in sewage treatment in which the sludge is mixed and aerated by flowing into a stage fractionation tank, the volume ratio of the complete mixing flow and its volume to the volume of the fractionation tank, and the volume ratio of the extrusion flow and its volume to the volume of the fractionation tank are determined. ratio, the ratio of the short-circuit flow to the total amount of water flowing into the aeration tank, the ratio of the reverse flow from the outlet of the fractionation tank to the outlet of the previous stage fractionation tank, and the ratio of the flow from the inlet of this fractionation tank to the previous stage fractionation. Using an arithmetic circuit whose parameters are the ratio of the flow flowing backward to the inlet of the tank and the ratio of the flow flowing backward from the exit to the inlet of the fractionation tank,
The above parameters are determined in advance as values that minimize the error between the measured data of the flow rate of water flowing into the fractionating tank, its sludge concentration, and the sludge concentration at the outlet of the fractionating tank, and the calculation result by the arithmetic circuit. The arithmetic circuit uses the time-series numerical data from each of the means to calculate the flow rate from the target fractionation tank for each fractionation tank at regular intervals based on the flow rate flowing into the aeration tank. Calculate the flow rate into the target fractionation tank by considering the ratio of reverse flow, and calculate the flow rate of the mainstream in this fractionation tank and the complete mixing part of this mainstream based on the flow rate into the fractionation tank. Using these calculated values, each parameter, the already determined sludge concentration of the front-stage fractionation tank, and the sludge concentration of the target fractionation tank, calculate the flow rate in the complete mixing section of the target fractionation tank. The sludge concentration from the sludge concentration and the sludge concentration from the extrusion section are calculated, and the effluent sludge concentration from the target fractionation tank is sequentially determined from these calculated values, each of the above-mentioned parameters, and the already determined next-stage division tank concentration. Characteristic method for determining sludge concentration in an aeration tank.