JPS5814837B2 - Predictive control method for mixed liquid suspended solids concentration in activated sludge method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for predicting and controlling the concentration of suspended solids in a mixed liquid in an aeration tank in an extrusion flow type sewage treatment process using an activated sludge method.

In sewage treatment using the activated sludge method, microorganisms in the sludge take in organic matter in the sewage in an aeration tank, the microorganisms multiply, and the concentration of organic matter in the sewage decreases.

In order to obtain good treated water quality, it is necessary to maintain an appropriate amount of sludge in the aeration tank depending on the amount and quality of inflowing sewage.

One possible method is to know the mixed liquid suspended solids concentration (MLSS concentration) of sewage and sludge in the aeration tank and set it to an appropriate value, and attempts are being made to control the amount of returned sludge and inflow. It is being

The formula used at this time is often expressed as the following formula.

MLSS=QoSo+QnSR/Qo+QR
(1) Qo: Inflow sewage volume So: Inflow sewage suspended solids concentration (SS concentration) QR: Returned sludge volume SR: Returned sludge concentration However, this formula only represents the MLSS concentration at the aeration tank inlet, and the sludge due to aeration is The effect of growth rate was not taken into account.

That is, the MLSS concentration at the middle and outlet side of the aeration tank is not shown.

In this way, in the past, it was difficult to know the amount of sludge growth, so it was necessary to use equation (1) to control the concentration on the outlet side, which is considered to have a large effect on the quality of treated water. was there.

On the other hand, as a return sludge control method that does not use equation (1),
There is a feedback control that detects the MLSS in the tank and controls the amount of sludge to be returned according to the deviation from the target value, and a control that returns the sludge in proportion to the amount of inflowing sewage.

However, since the aeration tank has a delay time of 4 to 8 hours called residence time, the response to control is slow, and neither method can be said to be reliable.

An object of the present invention is to provide a predictive control method for MLSS concentration that takes into account the amount of proliferation.

This eliminates the drawback of ignoring the amount of proliferation.

Furthermore, since this is a method of predicting and controlling the MLSS concentration, it can also cover the effects of residence time.

Furthermore, the present invention also aims to reduce errors in the MLSS concentration caused by ignoring the step ratio and residence time of each circuit when the aeration tank is operated by the step aeration method. .

To simplify the explanation of the present invention, consider an aeration tank consisting of four circuits in which sewage flows in from two places and a complete extrusion flow can be assumed, as shown in FIG. 1.

Now, considering the sewage Qo(t) that has flowed into the aeration tank at time t, the concentration at the inlet is expressed by equation (1) as shown above.

MLSS(x=o,t) =QA(t)・SO(t)+QR,(t)・SR(t)
/QA(t)+QR(t) (1) However, QA(t)=β・Qo(t) (2
) β: Step ratio (distribution ratio of inflow sewage to A-B circuit) In addition, the distance from the aeration tank inlet is represented by x.

The inflowing sewage QA(t) is mixed with the return sludge QR(t), and is pushed by the sewage that flows later and flows from A to B circuit. During this time, the microorganisms in the sludge are They take in organic matter and multiply.

Therefore, if the sludge growth rate during this period is αA, the MLSS concentration when it reaches the A circuit exit is expressed by the following equation.

MLSS (x=xA, t+τA) = (QA(t)・So(t)+QR(t)・Sn(t)
)(I+αA(X-XA,t)/QA(t)+QR(t
) (3) However, τA is the residence time until reaching the exit of circuit A, f j+'rA (QA(,t+QB(t'))at
'=VA. (4) t ■ A: Capacity of aeration tank A circuit Next, at the inlet of circuit B, new sewage is added and further pushed out to reach the outlet of the aeration tank.

If the sludge growth rate during this period is αBCD, the MLSS concentration at the aeration tank outlet is as follows.

MLSS(x=xD, t+r) = {(QA(t)・So(t)+QR(t)・SR(t
)) (.1+αA(x-=xA, t))+[QR(t+
τA)・So(t+τA)]*}×(1+αBCD(x
=xD,t))/{QA(t)+QR(t)+(QB(
t+τA))*} (5) Here τ
is the residence time from entering the aeration tank to exiting it, f1 + Toshi A(t') + (QB(t'
-V (6)t ■: Total capacity of the aeration tank Newly mixed sewage from circuit B is (QR(t+TA))*=(1-B)(Qo(t+τA)
)]*(7) In equation (7), the [ ]* is added at time t.
The sewage that has flowed into the aeration tank at
This is to show that when mixed with the inflow amount from the B circuit of +τA), the amount is not exactly (1-B)×Qo(t+τA), but is slightly shifted back and forth.

In other words, if the inflow measurement interval is 5 minutes, and the residence time in circuit A is 123 minutes, new sewage will begin to be mixed at the entrance of circuit B between 120 and 125 minutes. It represents that.

As mentioned above, if the sludge growth rate αA and αBCD are known in advance, the MLS at the outlet of the aeration tank
It becomes possible to more accurately predict and control the S concentration.

Therefore, let's consider the growth rate of this sludge.

Activated sludge growth depends on the quality and quantity of organic nutrients in the influent sewage,
The quality and quantity of microorganisms in sludge, water temperature, aeration amount, aeration time,
It changes depending on etc.

Although the quality of inflowing sewage and activated sludge varies greatly between treatment plants, it can be considered that there is a fairly constant pattern when viewed at a single treatment plant.

In addition, the inflow amount changes in the same pattern on a daily basis, and the aeration time (residence time) also changes in a certain pattern accordingly.

Therefore, there is some relationship between the growth rate of sludge and the amount of returned sludge, or the variation pattern of the amount of organic matter in inflowing sewage over a day.

Furthermore, each treatment plant has a unique variation pattern in its growth rate.

Therefore, it is possible to find the relationship between the sludge growth rate and the amount of returned sludge or the amount of organic matter in inflowing sewage based on past data, or to calculate the daily fluctuation pattern of the growth rate. Using this method, it becomes possible to predict the MLSS concentration more accurately.

Furthermore, using this predicted value, it becomes possible to control the MLSS more reliably.

Hereinafter, the present invention will be explained by examples.

FIG. 2 shows a flowchart using this calculation method when controlling the MLSS concentration at a constant level at the aeration tank outlet.

First, input the actual measured values of past inflow sewage volume, inflow sewage SS concentration, aeration tank outlet MLSS concentration, return sludge, and return sludge concentration, and use them to calculate the sludge growth amount and growth rate for that period. .

This can be calculated from a balance equation for the amount of sludge at the inlet and outlet of the aeration tank, taking into account the residence time.

The balance equation is expressed by the following equation, where P is the amount of proliferation.

P(t)-{QA<t)+(QB(t+τA)]
*}×{SD(t+τ)−So(t)}−QR,(t)
-SR(t) (8) Next, predict the proliferation rate for that day from the past proliferation rate.

To do this, use the following method. One method is to perform a regression analysis of the inflow amount, the amount of returned sludge, and the growth rate to determine the relationship between the growth rate and these.

The second method is to extract a pattern of daily growth rate changes from past data.

By these methods, the multiplication rate for that day is known, and using the equation (5) mentioned above, the MLSS at the aeration tank outlet is calculated.
Calculate concentration.

At this time, the residence time is calculated using equation (6) using the predicted value of the inflow amount and the value of the amount of returned sludge scheduled for future operation.

Once the predicted MLSS concentration of the currently inflowing sewage after the residence time at the aeration tank outlet is calculated, it is compared with the separately input MLSS setting value.

If the set value cannot be satisfied, change the amount of returned sludge and calculate again.

In this way, the amount of returned sludge that brings the MLSS concentration to the target value is determined.

FIG. 3 shows an example of an MLSS control device that determines the amount of returned sludge using the above procedure.

1 is an aeration tank, 11 and 12 are detection devices for the amount of inflow sewage and its SS concentration, the aeration tank outlet flow rate and its MLSS concentration,
2 is a final settling tank, and 13 is a device for detecting the amount and concentration of returned sludge.

Using the past measurement values of 11, 12, and 13, the relationship between the sludge growth rate in the aeration tank and the amount of inflowing sewage is calculated using the sludge growth rate calculation device in 3.

Using this result and the measured values of 11 and 13, and assuming the amount of returned sludge, the MLSS prediction calculation is performed with the apparatus of 5.

Compare this with the MLSS setting value input in step 4, and if satisfied, adjust the amount of returned sludge in step 6.

If it deviates from the set value, change the returned sludge ■ and recalculate.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of an aeration tank. FIG. 2 is a flowchart of a method for determining the amount of returned sludge using this method. FIG. 3 shows an MLSS control device using the above method for determining the amount of returned sludge. 1 1 is the aeration tank, 2 is the final settling tank, 3 is the sludge growth rate calculation device, 4 is the MLSS setting value input device, 5 is the aeration tank ML
SS prediction calculation device, 6 is return sludge amount control device, 11,1
2 and 13 are detection devices for inflowing sewage volume and SS concentration, aeration tank outlet flow rate and MLSS concentration, and return sludge volume and concentration, respectively.

Claims (1)

[Claims] 1. In the sewage treatment process using the activated sludge method, the scheduled values from now until after a predetermined period of time have elapsed for each of the inflow sewage volume, inflow sewage suspended solids concentration, return sludge volume, and return sludge concentration, and in the case of step aeration. , using the step ratio and the previously determined activated sludge growth rate, to perform a predictive calculation of the mixed liquid suspended solids concentration at a predetermined point in the aeration tank where the predetermined time becomes the aeration tank residence time; The planned value of the returned sludge amount is determined so that the predicted mixed liquid suspended solids concentration becomes a predetermined value, and based on the determined planned value of the returned sludge amount, the returned sludge from the aeration tank is A predictive control method for the concentration of suspended solids in a mixed liquid, characterized by controlling the amount.