Operating method for water treatment device, and water treatment device

Abstract

The present invention provides an operating method for a water treatment device that contributes to energy saving while reducing the operating load on an operator.　Provided is an operating method for a water treatment device comprising a biological treatment tank including an aerobic tank in which at least a membrane separation device is disposed in an immersed state, the operating method being characterized in that a drawing coefficient K (K < 1) is set as a control factor so that a sludge drawing amount Qwas satisfies [sludge drawing amount Qwas] = [raw water inflow amount Qraw × K], the membrane separation device is operated so that a filtration amount Qtrt from the membrane separation device satisfies [filtration amount Qtrt] = [raw water inflow amount Qraw × (1 – K)], and excess sludge is caused to naturally overflow from an overflow path provided to the biological treatment tank.

Description

Method of operating water treatment equipment and water treatment equipment

[0001] The present invention relates to a method for operating a water treatment device and a water treatment device.

[0002] As shown in FIG. 5 , a water treatment device 1 employs a membrane bioreactor to purify raw water, such as organic wastewater. The water treatment device 1 includes a biological treatment tank 8 including at least an anoxic tank 2 and an aerobic tank 3 in which a membrane separation device 4 is immersed, and a sludge circulation path 7 for circulating sludge from the aerobic tank 3 to the anoxic tank 2.

[0003] The raw water that flows into the anoxic tank 2 is mixed with the activated sludge in the tank by the agitation mechanism 2a and then flows down to the aerobic tank 3. In the aerobic tank 3, organic matter is oxidized and decomposed by aerobic microorganisms in an aerobic environment created by the auxiliary air diffuser 6, and ammonia components are nitrified. A portion of the purified water to be treated is withdrawn as treated water via the membrane separation device 4. The water to be treated in the aerobic tank 3 is circulated together with sludge via the sludge pump P1 and sludge circulation line 7 to the anoxic tank 2, where it is denitrified by anaerobic microorganisms in an anaerobic environment.

[0004] In the membrane bioreactor system described above, the biological treatment system must be operated so that the amount of microorganisms present and the viscosity of the circulating activated sludge fall within target control ranges. The control index used in this process is the MLSS (Mixed Liquor Suspended Solids) concentration. Because the MLSS concentration in the system increases depending on the SS contained in the inflowing raw water and the proliferation of microorganisms, an appropriate amount must be continuously or intermittently withdrawn from the system. For this reason, a sludge withdrawal pipe 11 equipped with an withdrawal valve 10 is branched off from the sludge circulation line 7.

[0005] Since it is difficult to accurately predict the increase in MLSS concentration, operators measure and monitor the MLSS concentration in the system, and in order to maintain it within the control value range, they determine and carry out the amount of sludge extraction according to their personal know-how. In general, the amount of sludge extraction is controlled by setting a guideline as a percentage of the raw water inflow.

[0006] Patent Documents 1 and 2 disclose prior art techniques relating to the extraction of such excess sludge.

[0007] JP 2010-194482 A JP 2013-176710 A

[0008] In water treatment systems that employ the membrane bioreactor, in addition to the sludge extraction process described above, an operator monitors the water level in the system and adjusts the amount of filtration by the membrane separator according to the amount of raw water that flows in.

[0009] In other words, the operator was required to monitor the amount of inflowing raw water and independently adjust the amount of treated water withdrawn through the membrane separation device and the amount of excess sludge withdrawn through the sludge withdrawal pipe, which was a complicated operation.

[0010] Furthermore, the pumps used to extract treated water and excess sludge consume a lot of electricity, and an operating method that would be acceptable in a decarbonized society was also needed from the perspective of energy conservation.

[0011] An object of the present invention is to provide a method for operating a water treatment device and a water treatment device that contribute to energy conservation while reducing the operating burden on an operator.

[0012] In order to achieve the above-mentioned object, a first characteristic configuration of the method for operating a water treatment device according to the present invention is a method for operating a water treatment device equipped with a biological treatment tank including at least an aerobic tank in which a membrane separation device is immersed, wherein an extraction coefficient K (K<1) is set as a control factor so that the sludge extraction amount Qwas satisfies the following equation: sludge extraction amount Qwas = raw water inflow amount Qraw × K; the membrane separation device is operated so that the filtration amount Qtrt from the membrane separation device satisfies the following equation: filtration amount Qtrt = raw water inflow amount Qraw × (1-K); and excess sludge is allowed to naturally overflow from an overflow path provided in the biological treatment tank.

[0013] In addition to the sludge extraction pipe, the biological treatment tank is equipped with an overflow path to prevent sludge from overflowing from the tank in the event of an abnormality. By using this overflow path for sludge extraction and allowing excess sludge to naturally overflow, the conventional sludge extraction operation using a sludge extraction pipe is no longer necessary. In a steady state where the water level in the biological treatment tank is approximately constant, the sludge extraction volume Qwas is determined by the raw water inflow volume Qraw minus the filtration volume Qtrt. Therefore, by operating the system with the filtration volume Qtrt set as an operating factor relative to the raw water inflow volume Qraw, the excess sludge can be allowed to naturally overflow from the overflow path.

[0014] Specifically, the sludge extraction amount Qwas is controlled by the equation: sludge extraction amount Qwas = raw water inflow amount Qraw × K (K is the extraction coefficient, K<1), and the membrane separation device is operated so that the filtration amount Qtrt is equal to the raw water inflow amount Qraw × (1-K). If K>0, the water level in the biological treatment tank rises, and excess sludge naturally overflows from the overflow path. If K<0 is set, it is also possible to actively lower the water level in the biological treatment tank. The extraction coefficient K, which is a control factor, can be set appropriately for the raw water inflow amount Qraw captured by a flow sensor or the like, and can be set in advance based on the sludge conversion rate, for example. The sludge conversion rate refers to the amount of sludge generated relative to the amount of organic matter treated by the water treatment facility. For example, the BOD sludge conversion rate is the increase in the amount of sludge (kg) relative to the amount of BOD inflow (kg), and is calculated as follows: {excess sludge concentration (mg / L) × amount of excess sludge extracted (m ３ / day)}÷{Effluent BOD concentration (mg / L) x Drainage volume (m ３ / day)) and the BOD sludge conversion rate for general activated sludge is 0.3 to 0.5.

[0015] The second characteristic configuration is that, in addition to the first characteristic configuration described above, the filtration volume Qtrt is updated and set at predetermined time intervals based on the raw water inflow volume Qraw measured at the predetermined time intervals.

[0016] Since the raw water inflow amount Qraw constantly fluctuates, it is preferable to update and set the filtration amount Qtrt at predetermined time intervals based on the raw water inflow amount Qraw measured at predetermined time intervals.

[0017] The third characteristic configuration is that, in addition to the second characteristic configuration described above, the MLSS of the biological treatment tank is measured regularly or irregularly, and the withdrawal coefficient K is increased or decreased based on the measured MLSS.

[0018] If the MLSS in the biological treatment tank falls below the appropriate range, the extraction coefficient K can be reduced to reduce the amount of sludge extracted (increase the amount of filtration), thereby restoring the MLSS to the appropriate range.If the MLSS rises above the appropriate range, the extraction coefficient K can be increased to increase the amount of sludge extracted (decrease the amount of filtration), thereby restoring the MLSS to the appropriate range.This allows the water treatment device to be operated in a stable state.

[0019] The fourth characteristic configuration is that, in addition to the third characteristic configuration described above, the water level in the biological treatment tank is measured, and if the measured water level deviates from a predetermined allowable range, the withdrawal coefficient K is increased or decreased.

[0020] Furthermore, if the water level in the biological treatment tank deviates from a predetermined allowable range, abnormalities in the water level can be prevented by increasing or decreasing the withdrawal coefficient K. For example, if the filtration rate Qtrt is large relative to the raw water inflow rate Qraw, the withdrawal coefficient K can be set to a large value to prevent a sudden drop in the water level, and if the filtration rate Qtrt is small relative to the raw water inflow rate Qraw, the withdrawal coefficient K can be set to a small value to prevent a sudden rise in the water level.

[0021] The fifth characteristic configuration is that, in addition to the third or fourth characteristic configuration described above, the water level in the biological treatment tank is measured, and if the measured water level deviates from a predetermined allowable range, an abnormal condition is reported via an alarm mechanism.

[0022] If the water level in the biological treatment tank deviates from the specified allowable range, an abnormal condition is reported via an alarm mechanism, alerting the operator and preventing serious accidents from occurring.

[0023] A first characteristic configuration of the water treatment device according to the present invention is that it comprises a biological treatment tank including at least an aerobic tank in which a membrane separation device is immersed, and a control device that sets an extraction coefficient K (K<1) as a control factor so that the sludge extraction amount Qwas satisfies the following equation: sludge extraction amount Qwas = raw water inflow amount Qraw × K, operates the membrane separation device so that the filtration amount Qtrt from the membrane separation device satisfies the following equation: filtration amount Qtrt = raw water inflow amount Qraw × (1-K), and causes excess sludge to naturally overflow from an overflow path provided in the biological treatment tank.

[0024] As described above, according to the present invention, it is possible to provide a method for operating a water treatment device and a water treatment device that contribute to energy conservation while reducing the operating burden on the operator.

[0025] FIG. 1 is an explanatory diagram of a first embodiment of a water treatment device according to the present invention. FIG. 2 is an explanatory diagram of a second embodiment of a water treatment device according to the present invention. FIG. 3A is a partially cutaway perspective view of a third embodiment of a water treatment device according to the present invention. FIG. 3B is an explanatory diagram of the same in plan view. FIG. 4A is an explanatory diagram of a first aspect of the A-A cross section of FIG. 3B. FIG. 4B is an explanatory diagram of a second aspect of the same. FIG. 5 is an explanatory diagram of a conventional water treatment device.

[0026] The present invention relates to a method for operating a water treatment device and a water treatment device, and more particularly to a biological treatment tank 8 including an anoxic tank 2 and an aerobic tank 3 in which a membrane separation device 4 is submerged. The biological treatment tank 8 includes a sludge circulation path 7 for circulating sludge from the aerobic tank 3 to the anoxic tank 2. The biological treatment tank 8 includes a control device C.

[0027] The control device C is composed of a computer equipped with a CPU board, a memory board, an input / output unit, a display unit, etc., and a control program for the water treatment device 1 is stored in the memory mounted on the memory board, and the operation of the water treatment device 1 is controlled by executing the control program by the CPU mounted on the CPU board.

[0028] The input / output unit receives signals from various sensors installed in the biological treatment tank 8, such as a flow rate sensor, MLSS sensor, DO sensor, water level sensor, and a pressure sensor that detects the transmembrane pressure difference of the membrane separation device 4, and outputs control signals for each load from the input / output unit, such as a filtration rate control signal for the membrane separation device 4, an aeration rate control signal for the aeration device 5 and auxiliary aeration device 6, and a sludge circulation rate control signal for the sludge pump P1. The flow rate sensors include a sensor that measures the raw water inflow rate, a sensor that measures the sludge circulation rate, and a sensor that measures the flow rate of treated water. An MLSS sensor, DO sensor, and water level sensor are installed in the aerobic tank 3. The display unit is a touch-panel liquid crystal display device, allowing the operator to set and input various information via the screen.

[0029] The raw water that flows into the anoxic tank 2 is mixed with the activated sludge in the tank by the agitation mechanism 2a and then flows down through the lower opening of the partition wall 8W into the aerobic tank 3. In the aerobic tank 3, organic matter is oxidized and decomposed by aerobic microorganisms in an aerobic environment created by the auxiliary air diffuser 6, and ammonia components are nitrified. A portion of the purified water to be treated is withdrawn as treated water via the membrane separation device 4. The water to be treated in the aerobic tank 3 is circulated together with sludge via the sludge pump P1 and sludge circulation path 7 to the anoxic tank 2. The water to be treated that has been circulated together with sludge to the anoxic tank 2 is denitrified by anaerobic microorganisms in an anaerobic environment.

[0030] The sludge that has grown in the biological treatment tank 8 naturally flows out of the tank along an overflow path 9 consisting of an overflow hole 9h formed in the upper part of the side wall of the biological treatment tank 8, a flange pipe 9f installed in the overflow hole 9h, and a sludge extraction pipe 9t connected to the flange pipe 9f, and is treated in an excess sludge treatment device such as a sludge storage tank not shown in the figure.

[0031] Multiple membrane separation devices 4 are submerged in the aerobic tank 3, and auxiliary air diffusers 6 that promote aerobic treatment are installed on both sides of the membrane separation devices 4. Each membrane separation device 4 has a membrane case that is open at the top and bottom, and multiple plate-shaped membrane elements are arranged at regular intervals so that each membrane surface is in a vertical position, and the membrane surfaces are purified by an upward flow caused by air diffused from an air diffuser 5 installed below the membrane case.

[0032] The air diffuser 5 and auxiliary air diffuser 6 are equipped with air diffuser pipes with multiple air diffuser holes, and are connected to a blower installed outside the tank via an air diffuser header connected to the air diffuser pipes. Each membrane element is connected to a water collection pipe via a tube, and the water collection pipe is connected to a pump P2 as a suction mechanism installed outside the tank. The filtration rate Qtrt of the membrane separation device 4 is controlled by adjusting the suction pressure of the pump P2 so that the transmembrane pressure difference becomes a predetermined value.

[0033] The control device C sets the extraction coefficient K (K<1) as a control factor so that the sludge extraction amount Qwas satisfies the following equation: sludge extraction amount Qwas = raw water inflow amount Qraw × K; and operates the membrane separation device 4 so that the filtration amount Qtrt from the membrane separation device 4 satisfies the following equation: filtration amount Qtrt = raw water inflow amount Qraw × (1-K); and causes excess sludge to naturally overflow from the overflow path 9 provided in the biological treatment tank 8. The sludge rises due to the upward flow caused by aeration by the aeration device 5, and the treated sludge with a high sludge concentration overflows from the overflow path 9.

[0034] In order to prevent sludge from overflowing from the biological treatment tank 8 in the event of an abnormality, the overflow path 9, which is a safety mechanism pre-installed in the biological treatment tank 8, is used for sludge extraction, allowing excess sludge to naturally overflow. This eliminates the need for the operator to operate the sludge pump P1 and open and close the extraction valve 10 as in the conventional sludge extraction operation using the sludge extraction pipe 11 shown in Figure 5.

[0035] If the extraction coefficient K is K > 0, the water level in the biological treatment tank 8 rises and excess sludge naturally overflows from the overflow path 9. If the extraction coefficient K is set to K < 0, it is also possible to actively lower the water level in the biological treatment tank 8.

[0036] The extraction coefficient K may be set by an operator via a touch panel type liquid crystal display device as appropriate. For example, the extraction coefficient K can be set based on the sludge conversion rate relative to the amount of raw water inflow Qraw captured by a flow rate sensor or the like. The sludge conversion rate refers to the amount of sludge generated relative to the amount of organic matter treated by the water treatment facility. For example, the BOD sludge conversion rate is the amount of sludge increase (kg) relative to the amount of BOD inflow (kg), and is expressed as {excess sludge concentration (mg / L) × amount of excess sludge extracted (m ３ / day)}÷{Effluent BOD concentration (mg / L) x Drainage volume (m３ It can be calculated using formulas such as (days / day).

[0037] In order to cope with the constantly fluctuating raw water inflow rate Qraw, the control device C measures the raw water inflow rate Qraw at predetermined time intervals and updates the filtration rate Qtrt based on the measurement results at predetermined time intervals. The predetermined time is not particularly limited and may be set in the range of 10 minutes to several hours.

[0038] Furthermore, the control device C measures the MLSS in the biological treatment tank 8 periodically or irregularly, and adjusts the extraction coefficient K to increase or decrease based on the measured MLSS.

[0039] If the MLSS in the biological treatment tank 8 falls below the appropriate range, the extraction coefficient K is reduced to reduce the amount of sludge extracted (increase the amount of filtration), thereby restoring the MLSS to the appropriate range.If the MLSS rises above the appropriate range, the extraction coefficient K is increased to increase the amount of sludge extracted (decrease the amount of filtration), thereby restoring the MLSS to the appropriate range.This allows the water treatment device 1 to be operated in a stable state.

[0040] Furthermore, it is preferable that the control device C measures the water level in the biological treatment tank 8, and when the measured water level deviates from a predetermined allowable range, it controls the extraction coefficient K to be adjusted to increase or decrease.

[0041] If the water level in the biological treatment tank 8 deviates from a predetermined allowable range, abnormal fluctuations in the water level can be prevented by increasing or decreasing the withdrawal coefficient K. For example, if the filtration rate Qtrt is large relative to the raw water inflow rate Qraw, the withdrawal coefficient K can be set to a large value to prevent a sudden drop in the water level, and if the filtration rate Qtrt is small relative to the raw water inflow rate Qraw, the withdrawal coefficient K can be set to a small value to prevent a sudden rise in the water level.

[0042] The control device C is configured to notify an abnormal condition via an alarm mechanism when the water level in the biological treatment tank 8 deviates from a predetermined allowable range. The alarm mechanism may be a sounding device or a mailer device that notifies an operator of an abnormality. By notifying an abnormal condition via the alarm mechanism, the operator is alerted and a serious accident can be prevented.

[0043] [Second embodiment] In the above-described embodiment, an example was described in which the overflow path 9 is composed of an overflow hole 9h formed in the upper part of the side wall of the biological treatment tank 8, a flange pipe 9f installed in the overflow hole 9h, and a sludge extraction pipe 9t connected to the flange pipe 9f. However, the configuration of the overflow path 9 is not limited to this form and may be, for example, a form as shown in Figure 2.

[0044] In other words, the overflow path 9 is composed of a vertical pipe 9a arranged along the side wall of the biological treatment tank 8, a flange pipe 9f installed in an overflow hole 9h formed in the side wall of the biological treatment tank 8, and a sludge withdrawal pipe 9t, with the upper end of the vertical pipe 9a, which has an open lower end, connected to one end of the flange pipe 9f, and the other end of the flange pipe 9f connected to the sludge withdrawal pipe 9t. By opening part of the upper end of the vertical pipe 9a to the atmosphere or by filling the overflow path 9 with water in advance, the sludge will naturally flow out when the liquid level of the water to be treated in the biological treatment tank 8 rises above the position where the overflow hole 9h is formed.

[0045] 3A, 3B, 4A, and 4B show a water treatment device 1 including a biological treatment tank 8 whose bottom and side walls are formed of steel plates. The biological treatment tank 8, which is rectangular in plan view, is divided longitudinally into three regions R1, R2, and R3 by two partition walls 8W, and a membrane separation device 4, which is rectangular in plan view, is submerged in the central region R2. The membrane separation device 4 is located in the center of the width of the central region R2. The upstream flow caused by aeration from the aeration device 5 provided below the membrane separation device 4 causes the water to rise from the bottom to the top of the membrane separation device 4, forming a circulating flow that descends to both sides from the top of the membrane separation device 4.

[0046] Auxiliary aeration devices 6 are installed on both sides of the membrane separation device 4 at the bottom of the biological treatment tank 8 (see Figures 4A and 4B). When the auxiliary aeration devices 6 are operating, the entire central region R2 of the biological treatment tank 8 functions as the aerobic tank 3, and when the auxiliary aeration devices 6 are stopped, the upper part of the central region R2 of the biological treatment tank 8 functions as the aerobic tank 3, and the lower part functions as the anoxic tank 2. This allows the biological treatment function to be switched.

[0047] The lower parts of the two partition walls 8W are open, and the treated water flows from the central region R2 separated by the partition wall 8W through the lower part of the partition wall 8W into the two side regions R1 and R3, flows from the lower part of the partition wall 8W into the lower part of the membrane separation device 4, and a portion of it is extracted as treated water by the membrane separation device 4.

[0048] A raw water inlet pipe 12 is installed horizontally above the center of the central region R2, and raw water inlet branch pipes 12t are installed on both sides of a partition plate 12W extending from the longitudinal center of the membrane separation device 4, and raw water is supplied to the bottom of the biological treatment tank 8. That is, in the biological treatment tank 8, a flow is formed in which the water to be treated together with sludge flows from the central region R2 to both side regions R1 and R3, and from both side regions R1 and R3 to the membrane separation device 4 below and above the central region R2.

[0049] As shown in FIG. 3A, an overflow path 9 similar to that described in FIG. 1 is provided on the side wall of the central region R2 of the biological treatment tank 8, and as with the control device C in FIG. 1, an extraction coefficient K (K < 1) is set as a control factor so that the sludge extraction amount Qwas is equal to the raw water inflow amount Qraw × K, and the membrane separation device 4 is operated so that the filtration amount Qtrt from the membrane separation device 4 is equal to the raw water inflow amount Qraw × (1 - K), and excess sludge is allowed to naturally overflow from the overflow path 9 provided in the biological treatment tank 8.

[0050] 4A and 4B, the control device C calculates the sludge circulation ratio based on the difference between the values ​​of the MLSS sensor provided in the upper layer of the central region R2 and the MLSS sensor provided in the lower layer (below the membrane separation device 4), and adjusts the amount of air diffused from the air diffuser 5 so that the circulation ratio becomes the target value. The control device C also adjusts the amount of air diffused by the auxiliary air diffuser 6 so that the value of the DO sensor provided in the middle layer of the central region R2 becomes the target value. The control device C then adjusts the extraction coefficient K based on the water level detected by the water level sensor and the value of the MLSS sensor provided in the lower layer of the central region R2 (below the membrane separation device 4).

[0051] [Fourth embodiment] In the above-described embodiment, the water treatment device 1 is described as comprising a biological treatment tank 8 including the anoxic tank 2 and the aerobic tank 3 in which the membrane separation device 4 is submerged, and a sludge circulation path 7 for circulating sludge from the aerobic tank 3 to the anoxic tank 2. However, the biological treatment tank 8 to which the present invention is applied may further comprise an anaerobic tank for removing phosphorus, and the sludge may be circulated and supplied from the aerobic tank 3 to the anaerobic tank via the sludge circulation path 7.

[0052] In the above description, the water treatment device 1 is automatically operated by the control device C, but the operation of the water treatment device 1 may be controlled by an operator instead of the control device C or in cooperation with the control device C.

[0053] In other words, the method for operating a water treatment device according to the present invention is a method for operating a water treatment device equipped with a biological treatment tank including at least an anoxic tank and an aerobic tank in which a membrane separation device is immersed, and a sludge circulation path for circulating sludge from the aerobic tank to the anoxic tank, characterized in that an extraction coefficient K (K<1) is set as a control factor so that the sludge extraction amount Qwas satisfies the following equation: sludge extraction amount Qwas = raw water inflow amount Qraw × K; the membrane separation device is operated so that the filtration amount Qtrt from the membrane separation device satisfies the following equation: filtration amount Qtrt = raw water inflow amount Qraw × (1-K); and excess sludge is allowed to naturally overflow from an overflow path provided in the biological treatment tank.

[0054] It is also preferable to update and set the filtration volume Qtrt at predetermined time intervals based on the raw water inflow volume Qraw measured at predetermined time intervals.

[0055] It is more preferable to measure the MLSS in the biological treatment tank periodically or irregularly and adjust the withdrawal coefficient K to increase or decrease based on the measured MLSS.

[0056] It is preferable to measure the water level in the biological treatment tank, and adjust the extraction coefficient K to increase or decrease if the measured water level deviates from a predetermined allowable range.

[0057] It is preferable to measure the water level in the biological treatment tank, and if the measured water level deviates from a predetermined allowable range, to notify the user of an abnormal state via an alarm mechanism.

[0058] The above-described embodiment is one aspect of the present invention, and the present invention is not limited to this description. It goes without saying that the specific configuration of each part can be appropriately modified and designed within the scope of the effects of the present invention.

[0059] 1: Water treatment device 2: Anoxic tank 3: Aerobic tank 4: Membrane separation device 5: Aeration device 6: Auxiliary aeration device 7: Sludge circulation path 8: Biological treatment tank 9: Overflow path

Claims

1. A method for operating a water treatment device equipped with a biological treatment tank including at least an aerobic tank in which a membrane separation device is submerged, comprising the steps of: setting an extraction coefficient K (K<1) as a control factor so that the sludge extraction amount Qwas is equal to the following: Sludge extraction amount Qwas = raw water inflow amount Qraw x K; operating the membrane separation device so that the filtration amount Qtrt from the membrane separation device is equal to the following: Filtration amount Qtrt = raw water inflow amount Qraw x (1-K); and allowing excess sludge to naturally overflow from an overflow path provided in the biological treatment tank.

2. The method for operating a water treatment apparatus according to claim 1, further comprising updating and setting the filtration volume Qtrt at predetermined time intervals based on a raw water inflow volume Qraw measured at the predetermined time intervals.

3. A method for operating a water treatment system according to claim 2, further comprising the steps of: measuring the MLSS in the biological treatment tank on a regular or irregular basis; and increasing or decreasing the withdrawal coefficient K based on the measured MLSS.

4. A method for operating a water treatment system according to claim 3, further comprising the steps of: measuring the water level in the biological treatment tank; and increasing or decreasing the withdrawal coefficient K when the measured water level deviates from a predetermined allowable range.

5. A method for operating a water treatment system as claimed in claim 3 or 4, characterized in that the water level in the biological treatment tank is measured, and if the measured water level deviates from a predetermined allowable range, an abnormal condition is notified via an alarm mechanism.

6. A water treatment device comprising: a biological treatment tank including at least an aerobic tank in which a membrane separation device is immersed; and a control device which sets an extraction coefficient K (K<1) as a control factor so that a sludge extraction amount Qwas is equal to: Sludge extraction amount Qwas = raw water inflow amount Qraw x K, operates the membrane separation device so that a filtration amount Qtrt from the membrane separation device is equal to: Filtration amount Qtrt = raw water inflow amount Qraw x (1-K), and causes excess sludge to naturally overflow from an overflow path provided in the biological treatment tank.

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