JPH0324248B2 - - Google Patents

Description

Backwash operation number designation means 225 for designating the number A liquid filtration device comprising:

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a filtration device for filtering impurities in various waste liquids, and particularly to a liquid filtration device equipped with a backwash control device for removing impurities attached to a filter body. This invention relates to an improvement in backwash control in which the number of backwash operations during one backwash operation is increased in accordance with the progress of clogging of a filter.

<Prior art> In this type of filtration device, impurities accumulate on the filter body as the filtration process progresses, and eventually the filter body becomes clogged, so these impurities are removed by backwashing. It is common to carry out filtration work while regenerating the filter body. The method of controlling the backwashing work is to focus on the differential pressure before and after the filter (hereinafter simply referred to as differential pressure) during steady operation, and detect the increase in differential pressure due to the accumulation of impurities. This is,
It is common practice to initiate a backwashing operation when a certain preset upper limit is reached.

In such backwashing operations, it is difficult to completely remove the impurities deposited on the filter body with only one backwashing operation, so conventionally, a specific predetermined The backwash operation is repeated several times to regenerate the filter as completely as possible.

However, even if the backwashing operation is repeated several times each time the differential pressure reaches the upper limit, it is not possible to completely remove the accumulated impurities. Therefore, the differential pressure immediately after backwashing (hereinafter referred to as lower limit differential pressure) will tend to rise in the long term without recovering to its original state.
As the filtration work progresses, the filter body gradually becomes contaminated and deteriorates in quality. Eventually, when the lower limit differential pressure reaches the maximum allowable value, the life of the filter comes to an end, and the filter needs to be replaced.

<Problems to be Solved by the Invention> In this conventional technology, the clogging of the filter body, which progresses over a long period of time as the filtration work progresses, is solved.
The concept of performing backwashing by changing the number of backwashing operations optimally each time based on the operating history of the equipment has not been introduced, and the increase in differential pressure is limited to a predetermined upper limit. A simple backwashing control was used in which when the amount reached, the backwashing operation was always performed a fixed number of times regardless of the operating history.

Therefore, in such conventional technology, if the number of backwashing operations performed during one backwashing operation is increased unnecessarily, the time required for the backwashing operation becomes long;
Since the ratio of backwashing time to the operating time of the filtration device increases, the operating efficiency of the filtration device decreases.

On the other hand, on the other hand, if the number of backwash operations performed during one backwash operation is kept to a small number, the backwash operation time will be shortened and the operating efficiency of the filtration device will improve at first glance. Since the regeneration of the filter will be incomplete, the life of the filter will be shortened. In particular, if the filter is a hollow fiber type, if clogging progresses to a certain extent due to incomplete backwashing, the recovery function of the filter will be rapidly lost, resulting in a large economic loss. This will invite

Therefore, in a filtration device, maintaining a high operating efficiency and maintaining a long lifespan of the filter are two trade-offs, and therefore, in the above-mentioned conventional technology, an operation that optimally harmonizes the two is contradictory. However, there was a problem in that it was extremely difficult to ensure this through manual optimization of the number of backwash operations during one backwash operation.

<Means for Solving the Problems> Therefore, the present invention solves the problem of the difficulty in optimizing the number of backwashing operations for each backwashing operation in order to achieve a balance between operating efficiency and filter life in the prior art described above. In view of this, for each backwashing operation performed in the filtration device, the progress of clogging of the filter is expressed based on the previous lower limit differential pressure value and the current lower limit differential pressure value immediately after the current backwashing operation. Then, the lower limit differential pressure increase calculating means 222 calculates the lower limit differential pressure increase of the current lower limit differential pressure value with respect to the previous lower limit differential pressure value, and the lower limit differential pressure increase determining means 224 determines the current lower limit differential pressure value. , relative to the past, i.e.,
Determine whether the current lower limit differential pressure increase is the largest, including multiple lower limit differential pressure increases in the past, and increase or decrease the number of backwash operations in the next backwash operation depending on the determination result. In order to increase the number of backwash operations for the next backwash operation, only if it is determined that the lower limit differential pressure increase is the maximum this time, the number of backwash operations will be increased to a greater extent than the number of backwash operations in normal backwash operations. By specifying this, the above problem is attempted to be solved.

<Function> The configuration of the present invention includes a differential pressure detection means 10 for detecting a differential pressure value between the filtrate in the filtrate chamber 2 and the filtrate in the filtrate chamber 3, which are isolated by the filter body 4.
Each time a filter body is backwashed, the lower limit differential pressure value immediately after the backwashing operation is stored as the current lower limit differential pressure value in the lower limit differential pressure storage means 221 so that it can be read out. and the lower limit differential pressure value obtained in the previous backwashing operation prior to the backwashing operation, which is read out as the previous lower limit differential pressure value from the lower limit differential pressure value storage means 221. The lower limit differential pressure increase calculation means 222 calculates the lower limit differential pressure increase for each washing operation, stores the lower limit differential pressure increase for each backwash operation in the lower limit differential pressure increase storage means 223 in a readable manner, and further , During this time, the current differential pressure increase for each backwashing operation and the lower limit differential pressure increase are stored in the storage means 2.
By comparing the multiple lower limit differential pressure rises for multiple backwash operations before the previous time read from 23, it is determined that the current lower limit differential pressure increase is the largest including the past multiple lower limit differential pressure rises. The lower limit differential pressure increase determining means 224 determines whether or not the lower limit differential pressure increase is the maximum, and if it is determined that the lower limit differential pressure increase this time is not the maximum, the backwash operation in the next backwash operation is performed. As the number of times, the number of backwashing operations for normal backwashing work (hereinafter referred to as the number of normal backwashing operations) is specified by the backwashing operation number designating means 225, and on the contrary, the next lower limit differential pressure increase is specified. When it is determined that this is the maximum number of backwash operations in the next backwash operation,
The number of backwashing operations that is larger than the normal number of backwashing operations (hereinafter referred to as the increased number of backwashing operations) is specified by the backwashing operation number designating means 225, thereby preventing clogging of the filter body 4 for each backwashing operation. Depending on the progress, it works to optimize the number of backwash operations in the next backwash operation.

<Example> Next, an example of the present invention will be described below based on the drawings from FIG. 2 onwards.

FIG. 2 is a block diagram showing the hardware configuration. The filtration device 1 includes a filtrate chamber 2 into which a filtrate is introduced, a filtrate chamber 3 from which a filtrate is taken out, and a filtrate chamber 3 into which a filtrate is taken out. It consists of a filter body 4 interposed between the chambers 2 and 3, and the filtrate chamber 2 has a liquid to be treated tank 5.
is an inlet pipe 8 having a supply pump 6 and a flow rate regulating valve 7.
A discharge pipe 9 is connected to the filtrate chamber 3 via an on-off valve 15 to a next process device, etc. (not shown). A differential pressure gauge 10 is connected between the inlet pipe 8 and the outlet pipe 9, and detects the differential pressure between the liquids in the two chambers 2 and 3, that is, the differential pressure before and after the filter body 4. Outputs a differential pressure signal SP representing the pressure value.
Further, a discharge pipe 12 extends from the lower part of the filtered liquid chamber 2 to the backwash receiving tank 11 via an on-off valve 16, and further, a liquid filling introduction pipe 25 branches from the introduction pipe 8 and connects to the on-off valve. It extends to a liquid filling tank (not shown) via 14.

A top vent pipe 27 is connected from the top of the filtrate chamber 3.
is extended to the backwash receiving tank 11 via the on-off valve 28, and along the way, the dome drain pipe 23 from the filtrate chamber is connected via the on-off valve 24, and the vent pipe 26 is joined via the on-off valve 14a. ing.

An air pipe 19 from a backwash air tank (not shown) is also connected to the top of the filtrate chamber 3 via an on-off valve 20. Each of the on-off valves 14, 14a, 1
5, 16, 20, 24, and 28 are each connected to the backwash control device 13, and are driven to open and close individually.

During the filtration work, only the on-off valve 15 is kept open by the command signal from the backwash control unit 13, so an operation start command signal is manually given to the flow rate control valve 7 to put it into operation. When the liquid is transferred, the liquid to be treated is pumped from the liquid to be treated tank 5 by the supply pump 6 and reaches the liquid to be filtered chamber 2 via the introduction pipe 8 at a constant flow rate set by the valve 7, and passes through the filter body 4 in order. The fluid passes through the filtrate chamber 3 and is then discharged via the discharge pipe 9.

On the other hand, during backwashing work, the backwashing control device 13
At the first stage, only the on-off valves 20 and 24 are kept open by the command signal from the
Compressed air from the backwash air tank is forced into the top of the filtrate chamber 3 via the air pipe 19, and the filtrate filling the chamber is expelled to the backwash receiving tank 11 via the drain pipe 23. Then, the time required for the air-water interface to drop to the dome drain pipe 23 position in the filtrate chamber 3 is measured, and the on-off valve 24 is opened in response to a command signal issued by the control device 13. At the same time, the on-off valve 16 shifts to the open state in response to a command signal issued by the control device 13, so that the filtrate in the filtrate chamber 3 passes through the filter body 4 in the opposite direction and becomes the filtrate. Together with the liquid to be filtered in the chamber 2, it is discharged into the backwash receiving tank 11 via the discharge pipe 12. After the discharge is completed, the on-off valves 14, 14 are controlled by a command signal from the backwash control device 13.
Only a and 24 are kept open, and water from the liquid filling tank is introduced into the filtrate chamber 2 via the liquid filling introduction pipe 25, and then flows into the filtrate chamber 3 via the filter body 4. It is also introduced into the room and fills both chambers. At this time, the air in the filtrate chamber 2 and the filtrate chamber 3 is exhausted via the vent pipe 26 and the dome drain pipe 23, whose on-off valves 14a and 24 are kept open, respectively. And filtrate chamber 3
The on-off valve 2 is activated in response to a command signal issued by the backwash control device 13 by measuring the elapsed time required for the air-water interface to rise to the dome drain pipe 23 position.
4, 14, and 14a transition to the closed state, thereby completing the liquid filling operation in preparation for another backwashing operation during one backwashing operation.

On the other hand, the arithmetic processing unit 22 includes a microprocessor and has three input ports and one output port. The input port 1 is connected to the power supply via the resistor 22a, and is linked to the manual on-off valve 7, and is opened when the on-off valve 7 is in the open state, that is, during operation, and outputs an operating state signal that becomes "1". It is grounded via contact SW1 for supplying W1. When the input port 2 is connected to the power supply through the resistor 22b, the on-off valve 7 is opened and the flow rate passing therethrough, that is, the differential pressure value at the differential pressure gauge 10, is delayed. is opened after stabilization, and "1"
A contact point for supplying the stable state signal W2 that becomes
Grounded via SW2. The input port 3 is connected to the output terminal of the differential pressure gauge 10, from which the differential pressure value represented by the differential pressure signal SP is taken in.

The output port 4 is for a backwash operation number designation signal, and is connected to an input terminal of the backwash control section 13.

A functional block diagram (claim correspondence diagram) showing the function realizing means corresponding to each of the plurality of functions performed by the microprocessor in the arithmetic processing unit 22 in FIG. 2 is included in the arithmetic processing unit 22 in FIG. Illustrated.

Next, the operation of the arithmetic processing section 22 in the configuration of the above embodiment will be described as follows, with reference to FIGS. 3 and 4.

When the arithmetic processing unit 22 starts (FIG. 3a), it clears addresses 8100 to 8108 of the memory block (FIG. 3b), and further clears the B register (FIG. 3c).

Next, the processing device 22 takes in the operating state signal W1 which becomes "1" during operation from the input port 1 (FIG. 3 d), and determines whether it is "0" (FIG. 3). e) When the determination result is YES, that is, when the operation is stopped, the process goes through the steps d to e in FIG. 3 and waits for the operation to start.

When the on-off valve 7 (Fig. 2) is opened manually at the start of operation, the contact SW1 is opened in conjunction with this, and "1" as the operating status signal W1 is supplied to the input port 1, so as shown in Fig. 3. The judgment result of e is NO
Then, from the input port 2, the differential pressure gauge 10
The stable state signal W2 which becomes "1" when the differential pressure value stabilizes in (Fig. 2) is taken in (Fig. 3 f), and it is determined whether it is "0" or not, and the determination result is
When YES, that is, during the transient period immediately after the opening/closing valve 7 is opened, the steps e to g in FIG. 3 are repeated to wait for the differential pressure value at the differential pressure gauge 10 to stabilize.

After a specific period of time in conjunction with the opening of contact SW1
When SW2 is opened, "1" as the stable state signal W2 is supplied to the input port 2, so the determination result in FIG. 3g becomes NO, and then the process advances to the step in FIG. 3h, where The stored contents of each address in the memory block (addresses 8100 to 8104) are sequentially shifted to each adjacent address, the oldest stored lower limit differential pressure value is discarded, and the newest lower limit differential pressure value is stored at the address. Thus, at this point, the four lower limit differential pressure values measured in the past are stored in the memory block at addresses 8100 to 8103.

In the subsequent step of FIG. Each lower limit differential pressure increase is stored at addresses 8105 to 8107.

Further, proceed to the subsequent step in Fig. 3j, where the maximum value of the three lower limit differential pressure increases calculated in the past stored in addresses 8105 to 8107 of the memory block is calculated. Select it and store it in memory at address 8109.

Subsequently, the arithmetic processing unit 22 takes in the lower limit differential pressure value represented by the differential pressure signal SP at this point of the differential pressure gauge 10 from the input port 3 (Fig. 3k),
This is stored in the memory block at address 8104 (FIG. 3K), which is prepared to store the latest lower limit differential pressure value (FIG. 3I). Thus, the latest lower limit differential pressure value stored here is obtained only after the start of operation (Fig. 3 d to g), so for example, after replacing the filter body 4 and restarting the operation, This corresponds to the differential pressure value immediately after the start of operation.

That is, as shown in the fourth graph, when operation is restarted for the first time after replacing the filter 4 (X0 in Figure 4), the differential pressure value immediately after the start of operation is Since the temperature is high, the lower limit differential pressure value (Fig. 4a) is reached.

Furthermore, as the operation continues, as the filter becomes increasingly clogged, the differential pressure before and after the filter gradually increases (Fig. 4 p), which increases to the upper limit differential pressure (the fourth The operation is manually stopped when it is detected by normal means that the condition shown in Figure S) has been reached.
Next, backwashing work (X1 in FIG. 4) is started manually, but the work itself is automatically performed by the backwashing control device 13.

As the details will become clear in the subsequent explanation, each function realizing means realized by the arithmetic processing unit 22 performs appropriate operations during the manually started backwashing operation as described above. They work together to calculate the number of backwash operations prior to starting the backwash operation and specify this to the backwash control device 13.

Next, returning to FIG. 3, the processing device moves the most recent one (stored at address 8104) of the memory block that has been shifted in the block memory and stored at address 8103. ), then store the new lower limit differential pressure value in the C register (Fig. 3 m, n), subtract it from the latest measured lower limit differential pressure value (Fig. 3 o, p),
Determine whether the difference is negative (Fig. 3 q),
If it is negative, convert it to 0 (Fig. 3r) to avoid handling negative numbers.

Next, the difference is stored in the memory block at address 8108 (FIG. 3, i), which is prepared to store the latest lower limit differential pressure increase (FIG. 3, s).

Next, the processing device stores the difference, that is, the latest calculated lower limit differential pressure rise (m to p in FIG. 3) from the past three values stored at address 8109 in the memory block. It is subtracted from the maximum of the lower limit differential pressure increases (Fig. 3, t to v), and it is determined whether the subtraction result is positive or not (Fig. 3, w). At this stage, the memory block The latest lower limit differential pressure value is stored at address 8104, and the latest lower limit increase is stored at address 8108 ("0" stored at address 8103).
Since only the difference (difference between the two addresses) is stored and all other addresses are cleared (FIG. 3b), the result of the above subtraction is not meaningful.

Therefore, since the latest lower limit differential pressure increase is larger than the largest one in the past, the above subtraction result (Fig. 3 v) is negative, and the processing device 22 Even if the judgment result is NO, the latest lower limit differential pressure value currently stored in address 8104 of the memory block will be sequentially shifted four times (the memory block will be shifted five times).
Transferred to address 8100, and as a result, address 8100 ~
Until the four lower limit differential pressure values are stored at address 8103 and three meaningful lower limit differential pressure increases are stored at addresses 8105 to 8107, the course will always lead to step x in Figure 3. and specify "2" as the number of normal backwash operations to the backwash control device 13 via the output port 4 (Fig. 3 x, Z1),
Each time, 1 is added to the contents of the B register, and the number of passes is counted (Z2 in FIG. 3). In order to ensure the above operation, the memory block is shifted five times and until the contents of the B register become 4, when the judgment result in FIG. 3 w is NO,
It is determined whether the content of the B register is less than 4, and if the result of the determination is YES, a course is provided to move to the step x in FIG. 3.

Therefore, while the above operations in each pass of the filter replacement work and each subsequent backwashing work are performed in four passes, that is, the first filter replacement work (the first Regarding the lower limit differential pressure values (Fig. 4 a, b, c, d,) immediately after each operation from the third backwashing operation (Fig. 4 X3) from Fig. 4 X0), regardless of the value, The arithmetic processing unit 22 specifies "2" as the number of normal backwash operations from the output port 4 to the backwash control device 13, and in response, the control device performs each of the above-mentioned backwash operations X1 to X3. While working,
As described above, each on-off valve is operated in sequence to perform the backwash operation twice.

Thus, when the fourth pass is completed and the fifth pass is reached, the content of the B register becomes 4, so the determination result in FIG. 3y becomes NO from then on. Therefore, in this case, the latest lower limit differential pressure increase is larger than the maximum value of the past lower limit differential pressure increases,
If the judgment result shown in Fig. 3 w is NO, the process directly proceeds to step Z shown in Fig. 3, and "3" as the number of increased backwash operations is specified to the backwash control device 13 via the output port 4. (Z, Z1 in FIG. 3), thereby, three backwash operations are performed during one backwash operation.

In other words, the lower limit differential pressure value (Fig. 4 e) immediately after the start of operation after the completion of the fourth backwashing operation (Fig. 4 X4)
and the lower limit differential pressure value (Fig. 4 d) immediately after the start of operation after the third backwashing operation (X3 in Fig. 4) has been completed (Fig. 4). The three calculated lower limit differential pressure increases (Fig. 4 △Pd, △Pc,
△Pb) when it is larger than the largest one in
The number of backwash operations (X5 in Figure 4) after the end of the operation starting from the lower limit differential pressure value (Figure 4 e) is one pass when measuring the lower limit differential pressure value (Figure 4 e). The number of increased backwash operations is specified as "3".

On the other hand, on the contrary, the lower limit differential pressure increase (the fourth
If △Pe) is smaller than the maximum of the three lower limit differential pressure increases calculated in the past (△Pd, △Pc, △Pb), the fifth backwashing operation ( The number of backwash operations in FIG. In this way, the number of increased backwash operations is specified only when the latest lower limit differential pressure rise is greater than the maximum of the past lower limit rises, and otherwise, normal backwash operation is performed. If you specify the number of times and look at this in chronological order, you can see that the number of backwash operations increases, the backwash operation (A in Figure 4) is the number of backwash operations, and the number of backwash operations is normal backwash operation. Backwashing work (B in Figure 4)
As these appear alternately, the density of the former increases relatively depending on the strength of the increasing tendency of the lower limit differential pressure increase, and this causes the total number of backwash operations to increase in chronological order. To effectively suppress an increasing tendency of a lower limit differential pressure value without increasing it.

In other words, the lower limit differential pressure value tends to increase in the conventional technology in which the number of backwashing operations during one backwashing operation is always kept unchanged as the number of normal backwashing operations (Figure 4 PB)
It is possible to set the lower limit differential pressure value on the upward trend (FIG. 4 PA), which has a relatively low rate of increase compared to .

Returning to Figure 3 again, set the contents of the B register to 1.
After adding (Z2 in FIG. 3), the arithmetic processing unit 22
takes in the operating status signal W1 from input port 1 again (Z3 in Figure 3) and determines whether it is "1" (Z4 in Figure 3). While the signal is "1", Fig. 3 Z
The process goes through steps 3 to Z4 and waits for the operation to be stopped for the next backwashing operation. As a result, while the differential pressure value that is the only measurement result immediately after backwashing is taken in as the lower limit differential pressure value, the differential pressure value that follows an upward trend (Fig. 4 p) is not continuously taken in thereafter. .

Then, when the operation is stopped for the next backwashing operation, the decision result in Z4 of FIG. 3 becomes NO, the process moves to step d in FIG. 3, and the above-mentioned operations are repeated.

Of the steps for the arithmetic processing described above, the steps h, k, and l in FIG.
Each step is calculated by the lower limit differential pressure increase calculating means 222.
The steps i and s in FIG. 3 use the lower limit differential pressure increase storage means 223, and the steps j, t, u, v, and w in FIG. 3 use the lower limit differential pressure increase determination means 224,
Each of the steps x, z, and z1 in FIG. 3 is realized by using the backwash operation number specifying means 225 as a function realizing means.

In the above embodiment, the number of normal backwash operations is designated as "2", and the number of increased backwash operations is designated as "3", but these numbers of operations are not limited; Various values should be suitably selected in consideration of the performance of the body, the attributes of the liquid to be filtered, etc.

<Effects> As described above, according to the present invention, the current lower limit differential pressure increase, which is the difference between the current lower limit differential pressure immediately after the current backwashing operation and the previous lower limit differential pressure value immediately after the previous backwashing operation, is the largest among the plurality of lower limit differential pressure increases calculated for each lower limit differential pressure value for each of the previous plurality of backwash operations. By specifying an increased number of backwash operations that is greater than the number of washing operations, the number of backwash operations during the next backwash operation will be increased, and on the other hand, on the other hand, the lower limit differential pressure increase this time will be calculated. If it is not the largest among the past multiple lower limit differential pressure increases,
By configuring the backwash control device to specify the number of normal backwash operations and keep the number of backwash operations during the next backwash operation to the normal number, the backwash operation for each backwash operation is reduced. The number of backwashing operations can be automatically optimized according to the lower limit differential pressure value immediately after each backwashing operation, that is, the degree of clogging of the filter. While it does not cause a decrease in operational efficiency as would be the case if the number of backwash operations were uniformly increased, it would not reduce the life of the filter as it would be if the number of backwash operations were uniformly reduced in each backwash operation in the prior art. An excellent effect is achieved in that the above-mentioned harmony of the two contradictory ratios is automatically and reliably achieved without causing any shortening.

[Brief explanation of drawings]

FIG. 1 is a functional block diagram showing the configuration of the present invention. Figures 2 to 4 relate to an embodiment of the present invention, with Figure 2 being a block diagram showing the hardware configuration, Figure 3 being a flowchart of calculation processing, and Figure 4 showing differential pressure values. This is a time chart showing changes over time. DESCRIPTION OF SYMBOLS 1... Filtration device, 2... Filtered liquid chamber, 3... Filtrate chamber, 4... Filter body, 10... Differential pressure detection means (differential pressure gauge), 13... Backwash control device, 22... Arithmetic processing unit, 221...lower limit differential pressure storage means (h,
k, l), 222... lower limit differential pressure increase calculation means (m, n, o, p, q, r), 223... lower limit differential pressure increase storage means (i, s), 224... lower limit difference Pressure increase determination means (j, t, u, v, w), 225
... Backwash operation number designation means (x, z, z1).

Claims (1)

[Scope of Claims] 1. A filtrate chamber 2 into which a filtrate is introduced; a filtrate chamber 3 from which a filtrate is taken out; and a system for isolating the filtrate chamber 2 and the filtrate chamber 3. In a liquid filtration device consisting of a filter body 4 interposed in A differential pressure detection means 10 that outputs a differential pressure signal SP, and each time the filter body 4 is backwashed, the differential pressure value represented by the differential pressure signal SP immediately after the backwashing operation can be read as a lower limit differential pressure value. Based on the lower limit differential pressure value storage means 221 stored in the lower limit differential pressure value storage means 221, the current lower limit differential pressure value for each backwashing operation, and the previous lower limit differential pressure value read from the lower limit differential pressure value storage means 221, the lower limit for each backwashing operation is determined. A lower limit differential pressure increase calculation means 222 that calculates the differential pressure increase; a lower limit differential pressure increase storage means 223 that readably stores the lower limit differential pressure increase for each backwash operation; and a current lower limit for each backwash operation. The differential pressure increase is compared with a plurality of lower limit differential pressure increases for multiple backwash operations before the previous time, which are read from the lower limit differential pressure increase storage means 223, and it is determined that the current lower limit differential pressure increase is the above. The lower limit differential pressure increase determination means 224 determines whether or not the lower limit differential pressure increase is the maximum including multiple lower limit differential pressure increases, and the lower limit differential pressure increase determination means 224 determines that the lower limit differential pressure increase is not the maximum this time. When it is determined,
The normal number of backwash operations is specified as the number of backwash operations for the next backwash operation, and when the determining means 224 determines that the current lower limit differential pressure increase is the maximum, the number of backwash operations for the next backwash operation is specified. The number of backwashing operations is increased more than the normal number of backwashing operations.