JPH0147238B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of operating a sewage treatment system using an activated sludge method, in which the concentration of the aeration tank MLSS and the concentration of the returned sludge are operated with the smallest possible deviation range from the target value. The present invention relates to a method for operating a sewage treatment system, which controls the operation by determining the flow rate value of excess sludge and excess sludge.

FIG. 1 shows the basic configuration of a sewage treatment system using the activated sludge method. That is, sewage containing organic matter enters the aeration tank 2 from the sewage inflow pipe 1, and is thoroughly agitated, mixed, and aerated with microorganisms in the aeration tank 2, that is, mixed suspended solids (hereinafter referred to as MLSS). After that, the mixed liquid flows into the final settling tank 3. Here, solid-liquid separation by precipitation is performed, and the supernatant liquid is discharged from the supernatant water discharge pipe 4.

In addition, most of the solid matter (sludge) that has settled at the bottom of the final settling tank 3 is extracted from the bottom of the settling tank 3 and then sent to the aeration tank 2 via the return sludge inflow pipe 6 by the return sludge pump 5. The remaining sludge is sent back and the remaining sludge is discharged out of the system by the surplus sludge pump 7. Reference numeral 8 designates a control valve for adjusting the flow rate of return sludge provided in the return sludge inflow pipe 6, and reference numeral 9 represents a control valve for adjusting the flow rate of surplus sludge provided on the discharge side of the surplus sludge pump 7.

One method for stable operation management in this type of sewage treatment plant is to maintain the MLSS concentration at a certain target value. That is, in a sewage treatment plant, the characteristics of inflowing sewage vary significantly on a daily or weekly basis, and on the other hand, the quality of water to be treated and discharged must be below a predetermined index value. Therefore, in the above-mentioned sewage treatment plant, one way to perform stable operation management to satisfy such conditions is to maintain the MLSS concentration at a certain target value.

However, the following problems arise with operating methods that only keep MLSS constant. That is, since there is a limit to the capacity of the final sedimentation tank, there is a limit to the weight of sludge accumulated in the final sedimentation tank. Fluctuations in the inflow sewage flow rate are large, and when the flow rate is large, the concentration in the aeration tank MLSS is reduced.
To keep this constant, the flow rate of return sludge must be increased, but as a result, the sludge concentration at the bottom of the final settling tank (i.e., the return sludge concentration) is lowered, and a larger return sludge flow rate is required at the next point. This creates a vicious cycle. Therefore, it is necessary to give consideration to the amount of sludge in the final settling tank as well as the aeration tank.

The present invention has been made in view of the above circumstances.
A sewage treatment system that does not have the drawbacks mentioned above, which maintains the MLSS at a constant value and at the same time maintains the concentration at a constant value using the concentration (return sludge concentration) as an indicator when it is pulled out from the bottom to ensure an appropriate amount of sludge in the final settling tank. The aim is to provide a driving method.

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. The target system of the present invention has the configuration shown in FIG. 1, and the manipulated variables are three flow rates: the settling tank withdrawal flow rate, the return sludge flow rate, and the excess sludge flow rate. Of these three flow rates, once the settling tank withdrawal flow rate and the return sludge flow rate are determined, the remaining one flow rate is automatically determined. Flow rate adjustment is performed by pumps 5, 7 or control valves 8, 9. The sludge concentration in each section is calculated to determine the flow rate, using a mathematical model of the aeration tank and final settling tank.

FIG. 2 shows the configuration of an algorithm used in the method of operating a sewage treatment facility according to the present invention. This algorithm is based on the inflow sewage flow rate, substrate concentration, and return sludge flow rate and concentration.
It consists of an aeration tank model that calculates the MLSS concentration, and a settling tank model that calculates the return sludge concentration from the MLSS concentration at the outlet side of the aeration tank, sewage flow rate, and sludge flow rate pulled out of the settling tank.These two models are used to calculate the predicted inflow. Determine the optimal sludge flow rate for each unit time interval based on the sewage characteristics.

The above-mentioned inflow sewage characteristics are daily fluctuation patterns of the flow rate of inflow sewage (Q _I ), SS in inflow sewage (X _I ), and substrate concentration in inflow sewage (C _I ). The prediction uses past performance data. For example, if the weather forecast for the previous day is the same on the same day of the week and the same time, it is considered as one common state, and Q _I , X _I ,
Find the average value of C _I as shown in Figure 3. Then, since the values at each time on each day of the week are determined in the same way depending on whether the weather is sunny or rainy, Q _I , X _I , and C _I at each time on the next day can be predicted as one fluctuation pattern. can.

The aeration tank model used here uses the following equation, for example, assuming complete mixing inside the tank.

∂x/dt=1/T(A-X)+dx/dt...(1) ∂s/∂t=1/T(B-S)+ds/dt...(2) Here ∂x/∂t …Aeration tank sludge concentration increase rate, ∂s/
∂t... Rate of increase in substrate concentration in the atomization tank, T... Residence time in the atomization tank, A... SS concentration at the inlet side of the aeration tank (suspended matter concentration at the inlet side of the aeration tank), B... Substrate concentration at the inlet side of the aeration tank.

In addition, the settling tank model uses, for example, the following statistically constructed formula to calculate the drawn sludge concentration (return sludge concentration X _R ).
Calculate.

X _R =aX+b(Q _I +Q _R )+cX(Q _I +Q _R )+dQw (3) Here, the coefficients a, b, c, and d are determined, for example, by the method of least squares.

Here, the algorithm for optimally determining the surplus sludge flow rate and the return sludge flow rate as operating variables is as follows. First, the sludge flow rate from the settling tank is the sum of the return sludge flow rate Q _R and the excess sludge flow rate Qw.
Determine Qwd. The method is to change the initial value to Qwd
(min), and simulate the case up to the maximum Qwd (max) in steps of ΔQwd. From the n results obtained, select the index value regarding the concentration of sludge drawn from the settling tank (return sludge concentration), for example, the case where the deviation width (absolute deviation value) from the target value is the minimum, and select the sludge drawn from the settling tank at that time. The operating flow rate is

The returned sludge concentration is determined as follows. First, use the aeration tank model to find the MLSS concentration (X _A ) at the aeration tank outlet. Taking the case of a four-tank type aeration tank as shown in FIG. 4 as an example, the following relationship is obtained.

dX _o /dt=Q _I +Q _R /V(X _o-1 −X _o )+R _xo ...(4) (n=1, 2, 3, 4) dC _o /dt=Q _I +Q _R /V( C _o-1 −C _o )−Rc _o ...(5) However, R _xo is the growth rate of sludge, R _co is the removal rate of organic matter, and the formula is as follows.

R _xo = μ^ X _o・_{C o} /K _s + _{C o} −K _d X _{o ……} (6) R _{co =} 1/Yμ^ Y, K _d , K _s , μ^ are process constants, and C _o is the substrate concentration in the aeration tank.

Further, the concentration when flowing into the first tank in FIG. 4 is determined as follows.

X _O =X _I Q _I +X _R Q _R /Q _I +Q _R ...(8) C _O =C _I Q _I /Q _I +Q _R ...(9) However, only X _R and Q _R at the initial time Enter the measured value. Moreover, X _I , Q _I , and C _I are each predicted values.

Next, substitute the MLSS calculation value X _A obtained above into the sedimentation tank model, that is, equation (3) above, and calculate the return sludge concentration (X _R ) using the following calculation formula (statistical model). .

_{X R} = _a・X _A + b ( _{Q I +} Q _R ) + _C・
It is calculated from the following equation so that the MLSS value becomes a constant value at the target value MLSS value X _O and material balance is maintained.

Q _R = Q _I (X _O −X _I ) − (dx/dt)・V/X _R −X _O ……(10) Here, Q _I …Flow rate of inflowing sewage, X _I … Inflowing sewage
SS concentration (suspended matter concentration in inflowing sewage), X _R ...Return sludge concentration, (dx/dt)...Sludge growth rate (in aeration tank), V...Aeration tank volume, Q _R ...Return sludge flow rate, However, If the sludge growth rate (dx/dt) is a small value, it may be omitted, or it may be set as a constant.
Monod formula etc. may also be used. When using the Monod formula, it can be expressed as follows.

dx/dt=μ^ X・S/K _s +S−KdX ……(11) ds/dt=1 _/ Yμ^ Substrate (removal) rate, Y, K _d , K _s , μ^...process constant, S...substrate concentration in the aeration tank, X...sludge concentration in the aeration tank Return sludge flow rate value Q determined by equation (10) For _R , this is the sedimentation tank withdrawal flow rate value Qwd found earlier.
When the value is above, Q _R = Qwd, that is, the upper limit value is taken as the sedimentation tank withdrawal flow rate; otherwise,
Use the value obtained by equation (10) as is. Also,
At that time, the excess sludge flow rate Qw is calculated using the following formula Qw=Qwd−Q _R (13).

The three flow rate values including the sedimentation tank withdrawal flow rate determined as described above are set as the optimum operating flow rate at that time.

As described above, if the algorithm of the present invention is used as an operation method that gives target values to the MLSS and return sludge concentration of the target system and reduces the deviation range from these values, it is possible to By predicting from the data and specifying target values for MLSS and return sludge concentration, both return and surplus flow rates can be obtained as manipulated variables. Moreover, the value of the return sludge flow rate obtained therein is obtained at a value maintained within a range that does not reduce the sludge concentration drawn out from the final settling tank (return sludge concentration). At the same time, since the time series of MLSS and return sludge concentration can be obtained, it is possible to study the operating conditions in advance, and as a result, stable operation of the target system can be ensured.

In addition, in the above description, the present invention uses an equation other than the above-mentioned equation (1), for example, which is proportional to the inflow sewage flow rate, as the (a) return sludge flow rate calculation formula. (b) A sludge growth term is not added to the aeration tank mathematical model. Or one using a propagation formula other than Monod. The flow model is treated as an intermediate between a complete mixing model and an extrusion model. Alternatively, (c) another statistical model is used as the sedimentation tank model. Or one that uses a model that describes physical phenomena based on its structure. The same results as above can also be obtained.

In this way, according to the present invention, stable operation management of the sludge amount in the aeration tank and the final settling tank is performed,
An effective method for operating a sewage treatment system that can discharge treated water of a predetermined quality can be provided.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of an activated sludge method sewage treatment system, Fig. 2 is a flow chart showing the algorithm of the present invention, Fig. 3 is a characteristic diagram for explaining the characteristics of inflow sewage used in the present invention, and Fig. 4 The figure is a diagram for explaining the aeration tank model used in the present invention. 1... Sewage inflow pipe, 2... Aeration tank, 3... Final settling tank, 4... Supernatant water discharge pipe, 5, 7... Pump, 8, 9... Control valve.

Claims (1)

[Claims] 1 Activated sludge method sewage treatment that is equipped with a sludge adjustment device for returning sludge from the final settling tank to the aeration tank, and a device for extracting excess sludge from the final settling tank, and adjusts the amount of sludge contained in the aeration tank and the final settling tank. In the system operation method, the inflow sewage characteristics, which are the fluctuation patterns of the inflow sewage flow rate, the suspended solids concentration in the inflow sewage, and the substrate concentration in the inflow sewage, are predicted based on actual data, and the Using the air tank model, find the MLSS at the outlet of the aeration tank, substitute this MLSS into the final settling tank model to calculate the return sludge concentration value, and perform the simulation by sequentially changing the value of the sum of the flow rates of return sludge and excess sludge. By doing so, the value that minimizes the deviation width between the calculated return sludge concentration value and its set target value is searched, and this value is set as the optimal sedimentation tank withdrawal sludge flow rate value, and the MLSS value of the aeration tank is
The flow rate value of the returned sludge to be set as the set MLSS target value is determined from the inflow sewage flow rate and suspended solids concentration in the inflow sewage based on the predicted inflow characteristics, and the sludge growth rate and return sludge concentration determined by each of the above models. Calculated based on the mass balance equation of the aeration tank,
If this value is equal to or greater than the optimum settling tank drawn sludge flow rate value, the return sludge flow rate value is made equal to this optimum settling value drawn sludge flow rate value, and the excess sludge flow rate value is calculated from the optimum settling tank drawn sludge flow rate value. A method of operating an activated sludge method sewage treatment system in which the flow values of both return sludge and surplus sludge obtained by subtracting the return sludge flow rate value are used as the optimum operation amount at that time.