JPS588312B2 - Sludge concentration control method in sewage treatment process - Google Patents

Description

Detailed Description of the Invention The present invention is an activated sludge treatment process that aims to purify sewage by mixing activated sludge, which has the ability to absorb organic matter in sewage, into sewage in an aeration tank. The present invention relates to a sludge concentration control method for maintaining sludge concentration (MLSS) at a target value.

FIG. 1 is a block diagram showing a conventional example of this type of device.
2 is the aeration tank into which sewage flows, 2 is the final settling tank, 20 is the sludge that has settled there, 3 is the sludge return pump, 4 is the sludge extraction pump, 5 and 6 are the flow rate adjustment valves, and 7 is the aeration tank inside the aeration tank. An adder for comparing the measured value S of the sludge concentration meter 12 with its target value 100; 8 is, for example, a proportional integrator for determining the sludge return rate or amount according to the output ΔS of the adder 7; 11 is a proportional integrator A flow meter 10 that measures the total output of 8 and the amount of flow into the aeration tank 1
A multiplier 9 calculates the return sludge amount QR or the opening degree of the flow rate adjustment valve 5 from the output of the multiplier 9, and a flow rate control device that controls the adjustment valve 5 according to the output of the multiplier.

In such a device, sewage is purified by activated sludge in the aeration tank 1, but at this time, the sludge concentration (MLSS, or mixed liquor suspended solids concentration) in the aeration tank has a large effect on sewage purification, so its stability is important. It is hoped that this will become a reality.

The aeration tank sludge concentration is controlled and adjusted by the amount of returned sludge QR and the amount of sludge drawn out QW.

Generally, as shown in FIG. 1, according to the difference ΔS between the detected value S of MLSS by the sludge concentration meter 12 and its target value S, the amount of returned sludge QR is determined by also referring to the amount of sewage inflow QO at that time.
By determining and controlling the flow rate adjustment valve 5, MLS
A method is adopted in which S matches a target value.

However, such a control device has the following drawbacks.

The MLSS at the inlet of the aeration tank 1 is expressed by equation (1) from the balance of sludge in the aeration tank 1.

S=Dun-topped paddy tllR (1) QO+QB, SO is sludge concentration of inflow sewage (mg/l) SR is return sludge concentration (mg/l) QO is inflow sewage volume (m3/h) QR is return sludge volume ( m3/h) P is the amount of sludge growth in the aeration tank (mg/l) Generally, as the amount of inflowing sewage QO increases, the sludge concentration S decreases.

Therefore, in order to keep the MLSS constant, it is necessary to increase the amount of returned sludge QR, and the smaller the returned sludge concentration SR, the larger the amount needs to be.

Therefore, depending on the value of SR, when the amount of inflowing sewage QO increases or decreases, the range of change in QR necessary to compensate for it may exceed the upper and lower limits, and the MLSS may not be stabilized after all.

The present invention provides a sludge concentration control method that can always maintain the sludge concentration in an aeration tank at a target value despite changes in the amount of inflowing sewage.

When considering the average value of sludge concentration over one day, aeration tank 1
The balance of the amount of sludge in the process in the settling tank 2 is as shown in Figure 2 a and b.

That is, the sum of the sludge amount SOQO flowing into the aeration tank 1, the sludge amount SRQR that is returned, and the sludge amount PQ that grows in the aeration tank 1 is the increase in sludge in the aeration tank 1, and on the other hand, The amount of sludge flowing out of the aeration tank and flowing into the settling tank 2 is expressed as S(QO+QR).

Furthermore, if the settling constant in the settling tank 2 is α, the amount of sludge that accumulates in the settling tank 2 is αS (QO + QR), and on the other hand, the amount of sludge flowing out from the settling tank 2 is (1 - α) S (QO + QR).
).

Furthermore, the amount of sludge drawn from the settling tank 2 is SR (QR+
QW).

From the above results, the sludge concentration values S and SR are calculated using formula (2)
and (3).

SR=αS(QO+QR)/(QR+QW)(3)QW
: Amount of sludge drawn (m3/h) α: Constant (0.9≦α≦1.0) When the above equations are solved simultaneously, the relationship between the amount of returned sludge QR, which is a manipulated variable related to sludge concentration, and the amount of sludge drawn QW is obtained. Equation (4) is obtained.

When P and SO are given, the relationship between Qw and QR is shown as shown in FIG. 3 using sludge concentration S as a parameter.

From now on, the amount of returned sludge Q to keep the sludge concentration S constant
It can be seen that R is greatly influenced by the amount of sludge drawn QW.

In other words, when QW is large (which means that SR is small), QR must also be made large in order to keep the sludge concentration constant; when QW is small, QR may be made small. It turns out.

In the present invention, the values of QR and QW are determined using the relationship shown in FIG. 3 in order to set the sludge concentration to the target value.

It is well known that in order to keep the sludge concentration constant despite daily fluctuations in the amount of inflowing sewage QO, QR should be changed in proportion to QO.

QR=β・QO (5)β=
Constant (0≦β≦1) If this relationship is satisfied, the relationship between the daily average value QR of QR and the daily maximum value QRmax of QR satisfies equation (6).

QR/QRmax=QO/QOmax (6) QO is the average daily inflow (m3/h) QOmax is the maximum daily inflow (m3/h) QRmax
is the maximum value of QR. Among these, QRmax is determined from the capacity of the return pump 3, and QO and QOmax can be estimated from past data.

In order to keep QR within the capacity of the pump when the amount of inflowing sewage QO reaches its maximum in a day, the daily average value QR of QR can be determined from equation (7).

QR = QRmax・QO/QOmax (7) Once the daily average value QR of the amount of returned sludge is determined, using the relationship shown in Figure 3, the average daily sludge concentration can be set as the target as shown in Figure 4. The amount of drawn sludge QW that can be made into a value of 100 can be determined.

Specifically, QW is calculated using equation (8).

Figure 4 shows the method of controlling the average values QR and QW, and the QO
For changes within a day, the QR operation variable amount (QRm
The sludge concentration can be made constant using the difference between ax and QR).

In this way, the method for determining QR and QW in Fig. 4 and the conventional Q
By combining the MLSS control method for operating R,
The sludge concentration can be maintained at the target value while keeping the QR within the operable range.

An example of a device suitable for carrying out the invention is shown in FIG.

In the figures, the same reference numerals as in FIG. 1 represent the same or equivalent parts.

13 is a sludge concentration meter for inflowing sewage, 14 is a flow rate control device, 1
5 is a calculation device, 16 is a multiplier, and 17 is an operation console for inputting the target value S of sludge concentration, the maximum return sludge amount QRmax, and the sludge growth amount P in the dusting tank into the calculation device 15.

The calculation device 15 performs the process shown in FIG. 6 using the inflow sewage amount QO, the sludge concentration SO measured by the measuring devices 10 and 13, and the return sludge pump capacity QR and the sludge growth amount P set by the operator. Sludge amount QW drawn to the flow rate control device 14
It also outputs the return rate β to the multiplier 16.

The calculation of QW and β is performed according to the procedure shown in steps 01 to 06 of FIG.

In step 01, QO, SO, QRmax, P and S are input.

In step 02, hourly values of QO are stored for the past week.

Using those stored data, calculate the average flow rate QO and daily QOmax/QO for one week at step 0.
Find it in 3.

The reason why one week's worth of data is used in this way is to remove unusual data such as in the case of rainy weather.

In step 04, the average return sludge amount QR to be set is determined from QRmax, QOmax, and QO using equation (7).

In step 05, QR, QO, sludge concentration target values S, P
, SO, and is calculated using the above-mentioned equation (8) also shown in FIG.

In step 06, the return rate β (=QR/QO) that allows the sludge concentration to reach the target value is determined from QR and QO.

In step 07, the QW obtained in step 05 is sent to the flow rate control device 14, and the return rate β obtained in step 06 is
is output to the multiplier 16.

The flow rate control device 14 adjusts the opening degree of the flow rate adjustment valve 6 to make the amount of sludge drawn match QW, and when the daily average value of the returned sludge amount QR reaches QR, the daily average value of the sludge concentration matches the target. I'll do what I do.

In addition, the multiplier 16 adjusts the amount of returned sludge QR so that the return rate becomes β, thereby making the daily average value of the amount of returned sludge coincide with QR, and adjusting the sludge concentration against daily fluctuations in QO. Aim for constantization.

In this way, by controlling the drawn sludge amount QW and the return rate β, the daily average value of sludge concentration can be controlled to the target value, and the MLSS can be kept constant within the allowable operating range of the returned sludge amount. can always be maintained at the target value,
This helps stabilize water quality.

Another embodiment of the invention is shown in FIG.

In the figures, the same reference numerals as in FIGS. 1 and 5 represent the same or equivalent parts.

The difference from Fig. 5 is that the calculation device 15 calculates the amount of sludge Q
Only W is output, and the operation control of QR is left to the conventional control device shown in FIG.

Although this device is more complex than the embodiment shown in FIG. 5, it has the advantage that even if there are errors in the 15 mathematical models, the sludge concentration can be brought closer to the target value.

According to the present invention, even when the concentration of returned sludge is small, the amount of returned sludge QR necessary to keep the sludge concentration constant exceeds the pump capacity, which makes it impossible to achieve a constant sludge concentration. defects can be removed,
The QR required for constant sludge concentration can always be kept within the operable range.

Therefore, the sludge concentration can always be kept at the target value, and the process can be stabilized.

[Brief explanation of the drawing]

Figure 1 is a block diagram explaining a conventional example, Figure 2 is an illustration of sludge balance in the water treatment process, and Figure 3 shows the relationship between the amount of returned sludge, the amount of extracted sludge, and the aeration tank sludge concentration. 4 is an explanatory diagram of the principle of the present invention, FIG. 5 is a block diagram of an example of an apparatus suitable for carrying out the present invention, FIG. 6 is a flow chart showing the method of the present invention, and FIG. FIG. 2 is a block diagram of an example device. 1...Aeration tank, 2...Final sedimentation tank,
3... Sludge return pump, 4... Sludge extraction pump, 5, 6... Flow rate adjustment valve, 9, 14.
...Flow rate control device, 10...Flow rate meter, 1
2, 13...Sludge concentration meter, 15...Calculation device, 16...Multiplier, 17...Operation console.

Claims (1)

[Claims] 1. Return sludge is mixed with sewage flowing into the aeration tank, the resulting mixed sewage is transferred to a settling tank, and a portion of the sludge settled there is returned to the aeration tank again. At the same time, in the sewage treatment process in which another part is extracted and discharged, the amount of returned sludge is such that the ratio between the maximum amount of returned sludge and its average value is equal to the ratio between the maximum amount of inflow sewage and its average value. The average value of the drawn sludge amount for the target sludge concentration in the aeration tank is calculated and set from the relationship between the average value of the returned sludge amount and the drawn sludge amount, and the average value of the drawn sludge amount and the returned sludge amount are calculated and set. A method for controlling sludge concentration in a sewage treatment process, which comprises controlling the amount of sludge drawn and the amount of returned sludge based on the average amount of sludge, thereby maintaining the sludge concentration in an aeration tank at a target value. 2 Mix return sludge with sewage flowing into the aeration tank, transfer the resulting mixed sewage to a settling tank, return part of the sludge settled there, and return the other part to the aeration tank. In the sewage treatment process where sludge is extracted and discharged, the average value of the amount of returned sludge is set so that the ratio between the maximum amount of returned sludge and its average value is equal to the ratio between the maximum amount of inflowing sewage and its average value. , From the relationship between the amount of returned sludge and the amount of drawn sludge, calculate and set the average value of the amount of drawn sludge for the target sludge concentration in the aeration tank, control the amount of drawn sludge based on this, and calculate the average amount of returned sludge and the average amount of drawn sludge. A method for controlling sludge concentration in a sewage treatment process, characterized by setting a return rate from a ratio of the amount of inflowing sewage and controlling the amount of returned sludge based on this.