JPS6410771B2 - - Google Patents

Description

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

In water treatment equipment that purifies wastewater containing organic pollutants such as sewage using activated sludge, which is a collection of microorganisms, the wastewater that flows into the aeration tank or its primary treated water is treated with activated sludge returned from the settling tank. The mixed liquid is aerated to absorb most of the contaminants into activated sludge, and the sludge is precipitated in a settling tank for purification.

In this way, water treatment using activated sludge is a method of purifying organic wastewater that uses biological effects in the aeration tank and solid-liquid separation in the sedimentation tank as the main processes. The condition of the sludge has a major impact on the purification effect and operational stability.

Therefore, when wastewater containing a large amount of organic pollutants, such as sewage, is purified using activated sludge (hereinafter simply referred to as sludge), the amount of organic matter F in the inflowing sewage and the amount of microorganisms M in the aeration tank, That is, it has been confirmed that it is preferable to maintain the ratio of the mixed liquid to the sludge concentration, the so-called F/M ratio, at a constant value in order to enhance the treatment effect and to perform stable purification.

The amount of inflowing organic matter described above is often highly correlated with the flow rate of inflowing sewage, so it is easy to predict.

However, since the sludge concentration in the aeration tank is controlled by the mixing state inside the tank, it is not easy to predict it. The mixing state in the aeration tank differs depending on the structure of the tank and the aeration pipe, and therefore the ratio of inflowing wastewater to activated sludge, that is, the F/M ratio, also changes as it flows down the tank. If the disturbance of these changes becomes large, the biochemical reaction of sludge organic matter by microorganisms in activated sludge will not work effectively, which will reduce the treatment effect. That is, in order to reduce disturbances in the F/M ratio to a preferable target value, it is first necessary to accurately know the mixing state in the aeration tank.

Conventionally, many methods have been proposed for determining the mixing state based on mathematical models representing mixing states such as single-stage or multi-stage complete mixing flow, extrusion flow, combined flow thereof, or diffusion flow. .

However, mathematical models such as perfect mixed flow, extrusion flow, and diffusion flow represent ideal mixing conditions, so in many cases they do not match the actual situation.
The errors between the values calculated from these numerical models and the actual values are extremely high.

The reason is that in many cases the aeration tank of the sewage treatment plant is
It is a long and narrow rectangular tank, and usually has partition walls in several places or is arranged in several folds, and the whole aeration tank is incomplete due to partitions such as partitions and folds. This is because short-circuit flow and reverse flow occur through the gaps between the divided parts.

If the number of divisions mentioned above is large enough and the flow of the mixed flow is transient, the accuracy of a mathematical model based on a complete mixing flow, an extrusion flow, or a flow combining them should be high. . Furthermore, if the aeration tank is an elongated rectangular tank with no divisions such as partition walls or folds, a mathematical model based on diffusion flow should fit well.

Therefore, in addition to the model representing the ideal mixing state described above, a mathematical model in which the ratio of the aforementioned short-circuit flow, reverse flow, or a combination of these is added is used to more accurately represent the mixing state of the tank. By doing so, it becomes possible to determine the sludge concentration of the mixture to match the actual value.

The present invention has been made in view of the above circumstances, and is made by sending sewage or its primary treated water and activated sludge to an aeration tank divided into multiple stages of fractionation tanks, and sequentially flowing them into subsequent stages of fractionation tanks to liquefy the mixture. In water treatment equipment where sludge is sent to a settling tank for purification, the main stream of each fractionation tank (i.e. complete mixed flow, diffusion flow, extrusion flow, Mathematics that calculates the relationship between each element of the flow (or a combination thereof) and the side flow (short-circuit flow, counterflow, or combination thereof) and the sludge concentration of the mixed liquid in the fractionation tank, using each of the main flow and side flow as parameters. Based on a time series of numerical values obtained by means of monitoring the flow rate of sewage or primary treated water flowing into the aeration tank and its suspended solids concentration, and the flow rate of activated sludge and its concentration. , the value of the parameter is determined by comparing the flow rate and suspended solids concentration of the wastewater or its primary treatment water flowing into the aeration tank with the actual measured value of the flow rate and concentration of the activated sludge, and then the formula An object of the present invention is to provide a method for determining an aeration tank sludge concentration in which an accurate mixed liquid sludge concentration can be determined by calculating the mixed liquid sludge concentration in each fractionation tank.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a fractionating tank in an aeration tank divided by dividing parts such as partition walls and folds. In the figure, 1 is an aeration tank, and 2 is a fractionating section arranged at appropriate intervals within this aeration tank 1. This dividing part 2 has the shape of a partition, and has a shape that blocks the flow of the mixed liquid 3 in the aeration tank 1, and its lower and upper parts are formed and provided as flow paths for the mixed liquid 3. There is. Reference numeral 4 designates a fractionating tank formed by being divided by the dividing portion 2, and 5 designates an aeration pipe for discharging air arranged near the inner bottom of the aeration tank 1.

In such an aeration tank 1, the mixed liquid 3 flows down from the upstream side A of the aeration tank 1 toward the downstream side B, becomes a swirling flow due to the air ejected from the aeration pipe 5, and reaches the downstream side B. The fractionation tank 4 is separated from other fractionation tanks by an adjacent division part 2. Here, fractionation tank 4
When paying attention to the flow of the mixed liquid 3 flowing down, in addition to the main stream Fm that mixes due to the swirling flow,
A part of the mixed liquid 3 flowing into the fractionation tank 4 from a passes through the bottom of the dividing part 2 and directly flows into the subsequent fractionation tank 4b.
The flow Fa short-circuits to the flow Fa and the reverse flow flowing back from the downstream fractionation tank 4b to the fractionation tank 4 through the gap in the dividing part 2.
It can be divided into side streams such as Fb.

The present invention uses a complete mixing flow, a diffusion flow, an extrusion flow, or a combined flow thereof as a mathematical model of the main flow Fm, and a short-circuit flow as the side flow.
A mathematical model is used in which the ratio of Fa or counterflow Fb to the total flow rate of the aeration tank is used as a parameter.

FIG. 2 is a flow diagram showing the relationship between the main stream and the substream, which is the basis of the mathematical model used in the present invention. If we assume that the fractionation tank 4 we are currently focusing on is the jth stage counting from the inlet side of the aeration tank 1, then the fractionation tank 4a in the j-1st stage before it passes through the gap 2a of the dividing part 2. A part of the liquid mixture that has flowed into the fractionation tank 4 of the j-th stage becomes a short-circuit flow Fa and is branched off.
Reverse flow from the subsequent fractionation tank 4b, i.e., the j+1st stage
Together with Fb′, it becomes the mainstream Fm and undergoes mixing. After that, the main flow Fm merges with the short circuit flow Fa mentioned above, but a part of it passes through the gap 2a from the fractionation tank 4 of the j-th stage,
It becomes a counterflow Fb and returns to the fractionation tank 4a of the j-1st stage, and then flows down to the fractionation tank 4b of the j+1st stage through the gap 2a of the dividing part 2 at the boundary between the fractionation tanks 4 and 4b. .

In Fig. 2, the short-circuit flow Fa and counterflow Fb in the fractionation tank 4 of the jth stage and the fractionation tank 4b of the j+1st stage
Let the ratios of the reverse flow Fb′ to the flow rate v of the aeration tank 1 be a _j , b _j and b _j+1 , respectively, then j−1
The flow rate v _j of the liquid mixture flowing from the fractionation tank 4a at the th stage to the fractionation tank 4 at the jth stage is expressed by the following equation.

v _j =(1+b _j )v (1) Moreover, the flow rate v _j ' of the main stream Fm is given by the second equation.

v _j ′=(1+b _j −a _j +b _j+1 )v …(2) Therefore, the sludge concentration x _j ′ of the mixed liquid flowing into the fractionation tank 4 of the j-th stage as the main stream Fm is If the sludge concentrations in the divided tank 4a of the second stage and the fractionating tank 4b of the j+1 stage are x _j-1 and x _j+1 , respectively, then x _j ′=x _j-1・(1+b _j −a _j )v+x _j+1・b _j+1・v/v _j ′ …(3) It is given by.

Further, the flow rate v j ₊₁ of the mixed liquid flowing out from the fractionation tank 4 of the j-th stage and flowing into the fractionation tank 4b of the j+1 stage is expressed as v _j+1 = (1+b _j+1 ) v …(4) is given.

Here, the sludge concentration v _j of the mixed liquid flowing out from this j-th stage fractionation tank 4 to the j+1st stage fractionation tank 4b is x _j =x _j ″・v _j ′+x _j-1・a _j・v/(1+b _j+1 )v…(5
) can be expressed as Here, x _j ″ is the sludge of the mainstream mixed liquid of the fractionation tank from the j-th stage calculated by the mathematical model of the mainstream Fm using a complete mixing flow, diffusion flow, extrusion flow, or a combined flow of these. It is concentration.

If the ratios a _j , b _j and b _j+1 of the short-circuit flow Fa and counterflows Fb, Fb′ to the aeration tank flow rate v are given in this way, the sludge concentration of the mixed liquid in each fractionation tank will be calculated as follows: Calculated from a total of four variables: the flow rate of inflowing sewage, the concentration of suspended solids contained in this inflowing sewage, and the flow rate and concentration of return sludge, that is, activated sludge returned from a settling tank or sludge well and mixed with the inflowing sewage. It can be determined by

Next, one embodiment of the present invention will be described using FIGS. 3 and 4.

FIG. 3 shows four partition walls (i.e., dividing portions) 2-1,
2-2, 2-3, 2-4, five fractionation tanks 4
It is a cross-sectional view of the elongated rectangular aeration tank 1 consisting of a, 4b, 4c, 4d, and 4e. The aeration tank 1 is divided into fractionation tanks 4a to 4e by partition walls 2-1 to 2-4. The depth of the aeration tank 1 is 5.5 [m], and the partition walls 2-1 to 2-4 are installed between 0.5 and 4.5 [m] below the water surface WL, with 0.5 [m] at the top and 1.0 [m] at the bottom. Has a high opening. Diffusion pipes 5a and 5 for dispersing air to the side wall near the bottom of each fractionation tank 4a to 4e
b, 5c, 5d, and 5e are installed. The primary treated sewage water is passed from the initial settling tank side A through a valve 31 and a flow meter 32 to the first stage fractionation tank 4a of the aeration tank 1.
The return sludge returned from the final settling tank or sludge well side B passes through a valve 33 and a flow meter 34 to the first stage fractionation tank 4a of the aeration tank 1 for the primary treatment. It flows in with water. Further, the mixed liquid mixed and aerated in the final stage fractionation tank 4e is sent from C to the settling tank.

In the aeration tank 1 having such a configuration, the sludge concentration in each of the fractionation tanks 4a to 4e is expressed by equations 1 to 5.
It can be determined by calculation based on the concept shown in the formula.

Here, the sludge concentration in the first stage fractionation tank 4a is the third.
It is calculated based on the concept of the formula, but since it is the first stage, it is necessary to know the values of the suspended solids concentration of the primary treated water and the returned sludge concentration.

All of these use detection data from a densitometer that detects each concentration, but it is also possible to collect each sample and manually analyze it and use that data, or to set up a mathematical model for each. It is also possible to calculate the value using a formula based on the calculated value.

For example, a regression model based on weekly data on suspended solids concentration can be used as the data on the suspended solids concentration in the primary treated water, and the returned sludge concentration can be used in other mathematical models to calculate the amount of settled sludge in the settling tank. You can set a basic formula and calculate from this. still,
To find the sludge concentration in the second and subsequent fractionation tanks,
Since the above explanation of the present invention was explained using a general solution as a formula for determining the sludge concentration in the jth stage, if j = 1, the formula for the first stage, and if j = 2, the formula for the second stage. Similarly, if j=3 is set, the sludge concentration of the third stage can be determined, and if j=N is set, the sludge concentration of the Nth stage can be determined.

Figure 4 compares the sludge concentration in the aeration tank calculated by the present invention with the measured value and the value calculated by the conventional method (combined model based on the optimal ratio of complete mixed flow and extrusion flow) for the aeration tank in Figure 3. This is a graph showing the results of an implementation test conducted for this purpose. In the experiment, the flow rate of the primary treated water and the flow rate of the returned sludge were kept constant and the operation was carried out for about a day and night, and then the flow rate of the returned sludge was reduced to 10%.
Figure 3 shows the changes in the sludge concentration in the third-stage fractionation tank 4c of the aeration tank when the system was stopped for two hours from 12:00 to 12:00. In the figure, A is an actual measured value, B is a calculated value using the method of the present invention, and C is a calculated value using the conventional method, and it can be seen that the present method shows a concentration close to the actual state. As a result, the ratio of each mixed flow to the aeration tank flow rate was 69% for the complete mixed flow, 5% for the extrusion flow, 28% for the short circuit flow, and 9% for the reverse flow. The calculation method used to obtain B (value calculated by the method of the present invention) in Fig. 4 is ◎ The main flow Fm is a completely mixed flow, and the main flow of the fractionating tank in the j-th stage is determined by the mathematical model. The sludge mixed concentration x _j ″ is used as shown below.

◎ In Equations (1) and (4) above, the inflow amount from the previous stage v _j , the flow rate of the mainstream Fm taking into account the retrograde flow Fb and short circuit flow Fa
v _j ′, sludge concentration of the mixed liquid flowing into the j-stage fractionation tank
x _j ′, determine the inflow amount v _j ′ from the j−1st stage (second
(see figure), then calculate x _j ″ as follows.

However, x _j is the sludge concentration of the mainstream Fm in the j-stage fractionation tank before calculation, and V _a is the volume of the fractionation tank.

x _j ″=x _j・v _a +x _j ′・v _j ′/v _a +v _j ′ (4a) This equation (4a) is a perfect mixture equation.Using this x _j ″, the above (5 ) can be used to determine the sludge concentration x _j flowing from the j-th stage fractionating tank to the j+1-th stage fractionating tank.

Also, how to obtain C (calculated value in the conventional method) in Fig. 4 is as follows: ◎ Use an integrated model based on the optimal ratio of the complete mixing tank row model and the extrusion model.

◎ Calculation method is B Extrusion model The outlet sludge concentration of the j-th stage is given by the following formula.

x _pj = x _i (t-τ _j ) where t is the current time, τ _j is the average residence time of water up to the jth stage, and x _i is the concentration of sludge flowing into the aeration tank (i, j = 1, 2, 3...) (b) Complete mixing tank row model Let the aeration tank volume be Va and the number of fractionation tanks be N.

Then, the sludge concentration x ₁ ′ in the first fractionation tank is x ₁ ′ = x _i · v + x ₁ · (V _a /N) / v + (V _a /N) The sludge concentration x in the second fractionation tank is x ₂ ′=x ₁ ′・v+x ₂・(V _a /N)/V+(V _a /N) The sludge concentration x in the jth fractionation tank is x _j ′=x _j-1 ′・v+x _j (V _a /N )/V+(V _a /N) where v is the inflow flow rate.

C Model combination Then, from the formula x _j = (1-α)・x _pj + α・x _j ′, x _j
Calculate and compare this with the actual measurement value A (Figure 4),
Ratio α and number of fractionation tanks N that minimizes the error
I explored and decided. The result of calculation using this optimal ratio α and fractionation tank N is shown in C in FIG.

In this way, from the time series of the flow rate of sewage or its primary treated water flowing into the aeration tank, its suspended solids concentration, return sludge flow rate, and its sludge concentration, it is possible to create an ideal model of mixing such as complete mixing, extrusion flow, or diffusion flow. By using a mathematical model of the aeration tank mixture that takes into account short-circuit flow, counterflow, or a combined flow thereof, the sludge concentration of the mixture can be determined with high accuracy.

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 mixed liquid sludge concentration M corresponding to the amount of contaminated organic matter F, which is the inflow characteristic of sewage, can be determined. It becomes easy to operate to maintain the F/M ratio.

As described in detail above, sewage or its primary treated water and activated sludge are sent to an aeration tank divided into multiple stages of fractionation tanks, sequentially flowed into the subsequent division tanks, mixed, aerated, mixed and liquefied, and then settled in a sedimentation tank. In a water treatment system in which the mixed liquid sludge concentration in each fractionation tank in the aeration tank is determined, the main stream of each fractionation tank, that is, the complete mixed flow, diffusion flow, extrusion flow, or their combination and substream, is used. In other words, short-circuit flow (components flowing toward the lower fractionation tank without contributing to mixing in the fractionation tank), reverse flow (components flowing from the lower side), or their combination and sludge concentration in the mixed liquid in the fractionation tank. In addition, a mathematical model formula is set for the relationship between the main stream and the side stream as parameters, and the relationship between the flow rate of the sewage or its primary treated water flowing into the aeration tank, the suspended solids concentration, and the activated sludge Actual measurement of the flow rate and suspended matter concentration of the wastewater or its primary treatment water flowing into the aeration tank and the flow rate of the activated sludge and its concentration, based on a time series of numerical values obtained by a means for monitoring the flow rate and its concentration. The value of the parameter is determined by comparing the values, and then the concentration of each fractionation tank is known by calculating the above formula. Therefore, the concentration can be known with high accuracy, and therefore the F/ It is possible to maintain the M ratio at a constant value during operation, which improves the treatment effect and performs stable purification.In addition, the concentration of the mixed liquid in each fractionation tank can be determined by calculation, making maintenance management easier. It is possible to provide an aeration tank sludge concentration determination method having excellent features such as eliminating the need to provide a concentration meter or the like in each fractionation tank.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of an aeration tank showing the swirling flow, short-circuit flow, and reverse flow; Figure 2 is a flow diagram showing the interrelationship of each flow in Figure 1; Figure 3 is the implementation of the method of the present invention. Fig. 4 is a cross-sectional view showing the input-output relationship of the aeration tank used in the test, and Fig. 4 is a graph showing changes in sludge concentration showing the results of the implementation test. 1... Aeration tank, 2-1 to 2-4... Partition wall, 4a
~4e... Fractionation tank, 32, 34... Flow meter.

Claims (1)

[Claims] 1 Wastewater or its primary treated water and activated sludge are sent to an aeration tank divided into multiple stages of fractionation tanks, sequentially flowed into the latter stages of the division tanks, mixed and aerated to liquefy the mixture, and this is sent to a settling tank. As a method for determining the concentration of mixed liquid sludge in each fractionation tank in water treatment equipment that sends water for purification, a mathematical model of the concentration of sludge in the fractionation tank is set up using the ratio of the main flow and side stream of each fractionation tank as parameters, and then The value of the parameter is determined by comparing the flow rate and suspended solids concentration of the wastewater or its primary treatment water flowing into the aeration tank with the actual measured value of the flow rate and concentration of the activated sludge, and then the mathematical model A method for determining sludge concentration in an aeration tank, characterized in that the sludge concentration of the mixed liquid is determined by calculating an equation according to the following.