Operational method of water treatment apparatus and water treatment apparatus

Abstract

To provide an operational method for a water treatment apparatus that contributes to energy saving while reducing an operational load on an operator.SOLUTION: In an operational method for a water treatment apparatus comprising a biological treatment tank including an aerobic tank in which at least a membrane separator is immersed and installed, a withdrawal factor K(K<1) is set as a control factor so that a sludge withdrawal volume Qwas is equal to a raw water inflow volume Qraw*K, the membrane separator is operated so that a filtration volume Qtrt from the membrane separator becomes such that the filtration volume Qtrt is equal to the raw water inflow Qraw*(1-K), and excess sludge is naturally overflowed from an overflow path in the biological treatment tank.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an operation method of a water treatment apparatus and a water treatment apparatus.

Background Art

[0002] As shown in FIG. 5, a water treatment apparatus 1 adopting a membrane separation activated sludge method (Membrane Bio Reactor) to purify raw water such as organic wastewater as treated water includes at least an anoxic tank 2 and an aerobic tank 3 in which a membrane separation device 4 is immersed, a biological treatment tank 8, and a sludge circulation path 7 for circulating sludge from the aerobic tank 3 to the anoxic tank 2.

[0003] The raw water flowing into the anoxic tank 2 is mixed with the activated sludge in the tank by a stirring mechanism 2a and then flows down to the aerobic tank 3. In the aerobic tank 3, organic substances are oxidized and decomposed by aerobic microorganisms in an aerobic environment by an auxiliary aeration device 6, and the ammonia component is nitrified. A part of the purified treated water is withdrawn as treated water through the membrane separation device 4. The treated water in the aerobic tank 3 is circulated and conveyed to the anoxic tank 2 together with the sludge through a sludge pump P1 and a sludge circulation path 7, and in the anoxic tank 2, denitrification treatment is performed by anaerobic microorganisms in an anaerobic environment.

[0004] In the above-described membrane separation activated sludge method, it is necessary to perform an operation so that the amount of microorganisms present in the biological treatment system and the viscosity of the circulating activated sludge are within the range of target management values. At that time, the MLSS (Mixed Liquor Suspended Solids) concentration is used as a management index. Since the MLSS concentration in the system increases depending on the SS contained in the inflowing raw water and the growth of microorganisms, it is necessary to continuously or intermittently draw out an appropriate amount outside the system. Therefore, a sludge extraction pipe 11 provided with an extraction valve 10 is branched and connected to the sludge circulation path 7.

[0005] Since it is difficult to accurately predict the increase in MLSS concentration, the operator measures and monitors the MLSS concentration in the system and determines the sludge withdrawal amount according to personal know-how and executes the withdrawal in order to continuously maintain it within the range of the control value. Generally, the sludge withdrawal amount is often managed by setting a standard of what percentage of the raw water inflow.

[0006] Patent Document 1 and Patent Document 2 disclose prior arts related to the withdrawal of such excess sludge.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, in a water treatment apparatus adopting the membrane separation activated sludge method, apart from the sludge withdrawal treatment described above, the operator monitors the water level in the system and performs operation management to adjust the filtration amount by the membrane separation apparatus according to the inflow amount of raw water.

[0009] That is, the operator is required to perform complicated operation operations of independently performing both the adjustment operation of the withdrawal amount of treated water through the membrane separation apparatus and the adjustment operation of the withdrawal amount of excess sludge through the sludge withdrawal pipe while monitoring the inflow amount of raw water.

[0010] In addition, the power consumption required for the pumps driven for the withdrawal of treated water and the withdrawal of excess sludge increases, and a driving method that is acceptable in a decarbonized society is also required from the viewpoint of energy saving.

[0011] An object of the present invention is to provide an operation method and a water treatment apparatus for a water treatment apparatus that contribute to energy saving while reducing the operation load on an operator.

Means for Solving the Problems

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

[0013] In the biological treatment tank, an overflow path for preventing sludge from overflowing from the water tank in the event of an abnormality is installed separately from the sludge withdrawal pipe. By using this overflow path for sludge withdrawal to naturally overflow the excess sludge, the sludge withdrawal operation using the conventional sludge withdrawal pipe can be made unnecessary. In a steady state where the water level of the biological treatment tank is substantially constant, the sludge withdrawal amount Qwas is determined by the raw water inflow amount Qraw - the filtration amount Qtrt. Therefore, if the filtration amount Qtrt is determined as an operation factor with respect to the raw water inflow amount Qraw and the operation is performed, the excess sludge can be naturally overflowed from the overflow path.

[0014] Specifically, the sludge extraction amount Qwas is managed by Qwas = raw water inflow amount Qraw × K (where K is the extraction coefficient and K < 1), and the membrane separation device is operated such that the filtration amount Qtrt is Qtrt = raw water inflow amount Qraw × (1 - K). If K > 0, the water level in the biological treatment tank rises, and excess sludge overflows naturally from the overflow path. If K is set to < 0, it is also possible to actively lower the water level in the biological treatment tank. For the raw water inflow amount Qraw captured by a flow sensor or the like, the extraction coefficient K as a control factor can be set as appropriate, for example, it can be set in advance based on the sludge conversion rate. The sludge conversion rate refers to the amount of sludge generated with respect to the amount of organic matter treated by the water treatment facility. For example, the BOD sludge conversion rate is the increase in sludge amount (kg) with respect to the BOD inflow amount (kg), and is obtained by calculation formulas such as {excess sludge concentration (mg / L) × excess sludge extraction amount (m 3 / day)} ÷ {drainage BOD concentration (mg / L) × drainage amount (m 3 / day)}, etc. The BOD sludge conversion rate for general activated sludge is 0.3 to 0.5.

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

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

[0017] The third characteristic configuration, in addition to the second characteristic configuration described above, is that the MLSS in the biological treatment tank is measured regularly or irregularly, and the extraction coefficient K is adjusted to increase or decrease based on the measured MLSS.

[0018] When the MLSS in the biological treatment tank decreases from the appropriate range, the draw coefficient K is decreased to reduce the sludge draw amount (increase the filtration amount) to return the MLSS to the appropriate range. When the MLSS increases from the appropriate range, the draw coefficient K is increased to increase the sludge draw amount (decrease the filtration amount) to return the MLSS to the appropriate range, so that the water treatment device can be operated while maintaining a stable state.

[0019] In addition to the third characteristic configuration described above, the fourth characteristic configuration is that the water level of the biological treatment tank is measured, and when the measured water level deviates from a predetermined allowable range, the draw coefficient K is adjusted to increase or decrease.

[0020] Also, when the water level of the biological treatment tank deviates from the predetermined allowable range, by adjusting the draw coefficient K to increase or decrease, the abnormality of the water level can be avoided. For example, when the filtration amount Qtrt is large with respect to the raw water inflow amount Qraw, the draw coefficient K is set to a large value to avoid a rapid decrease in the water level. When the filtration amount Qtrt is small with respect to the raw water inflow amount Qraw, the draw coefficient K is set to a small value to avoid a rapid increase in the water level.

[0021] In addition to the third or fourth characteristic configuration described above, the fifth characteristic configuration is that the water level of the biological treatment tank is measured, and when the measured water level deviates from a predetermined allowable range, an abnormal state is notified via a notification mechanism.

[0022] When the water level of the biological treatment tank deviates from the predetermined allowable range, by notifying the abnormal state via the notification mechanism, the operator can be alerted, and the occurrence of a major accident can be avoided.

[0023] The first characteristic configuration of the water treatment apparatus according to the present invention includes a biological treatment tank including an aerobic tank in which at least a membrane separation device is immersed, and a sludge withdrawal amount Qwas is set with a withdrawal coefficient K (K < 1) as a control factor so that the sludge withdrawal amount Qwas = raw water inflow amount Qraw × K, and the membrane separation device is operated so that the filtration amount Qtrt from the membrane separation device is Qtrt = raw water inflow amount Qraw × (1 - K), and a control device that naturally overflows excess sludge from an overflow path provided in the biological treatment tank.

Effect of the Invention

[0024] As described above, according to the present invention, it has become possible to provide an operation method and a water treatment apparatus for a water treatment apparatus that contribute to energy saving while reducing the operation load by an operator.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0026] Hereinafter, an operation method and a water treatment apparatus for a water treatment apparatus according to the present invention will be described with reference to the drawings. [First Embodiment] As shown in FIG. 1, the water treatment apparatus 1 includes a biological treatment tank 8 including an anoxic tank 2 and an aerobic tank 3 in which a membrane separation device 4 is immersed, a sludge circulation path 7 for circulating sludge from the aerobic tank 3 to the anoxic tank 2, and 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. A control program for the water treatment device 1 is stored in the memory mounted on the memory board, and the control program is executed by the CPU mounted on the CPU board, whereby the water treatment device 1 is operationally controlled.

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

[0029] The raw water flowing into the anoxic tank 2 is mixed with the activated sludge in the tank by the stirring mechanism 2a and then flows down from the lower opening of the partition wall 8W into the aerobic tank 3. In the aerobic tank 3, the organic matter is oxidized and decomposed by aerobic microorganisms in an aerobic environment by the auxiliary aeration device 6, and the ammonia component is nitrified, and a part of the purified water to be treated is drawn out as treated water through the membrane separation device 4. The water to be treated in the aerobic tank 3 is circulated and conveyed to the anoxic tank 2 through the sludge pump P1 and the sludge circulation path 7 together with the sludge. The water to be treated circulated and conveyed to the anoxic tank 2 together with the sludge is denitrified by anaerobic microorganisms in an anaerobic environment.

[0030] The sludge grown in the biological treatment tank 8 naturally flows out of the tank along the overflow path 9 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, and is treated by an excess sludge treatment device such as a sludge storage tank (not shown).

[0031] A plurality of membrane separation devices 4 are immersed and installed in the aerobic tank 3, and auxiliary air diffusers 6 for promoting aerobic treatment are installed on both sides of the membrane separation device 4. Each membrane separation device 4 has a large number of plate-shaped membrane elements arranged at regular intervals inside a membrane case with openings at the top and bottom, such that each membrane surface is in a vertical posture, and the membrane surface is purified by the upward flow generated by the air diffusion from the air diffuser 5 provided below the membrane case.

[0032] The air diffuser 5 and the auxiliary air diffuser 6 are equipped with air diffuser pipes formed with a plurality of air diffuser holes, and are connected to a blower installed outside the tank via an air diffuser header connected to the air diffuser pipe. Each membrane element is connected to a water collecting pipe via a tube, and the water collecting 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 by the pump P2 so that the membrane differential pressure reaches a predetermined value.

[0033] The control device C sets the extraction coefficient K (K < 1) as a control factor such that the sludge extraction amount Qwas is Qwas = raw water inflow amount Qraw × K, operates the membrane separation device 4 such that the filtration rate Qtrt is Qtrt = raw water inflow amount Qraw × (1 - K), and allows excess sludge to overflow naturally from the overflow path 9 provided in the biological treatment tank 8. The sludge rises according to the upward flow generated by the air diffusion by the air diffuser 5, and the treated water 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 when an abnormality occurs, by using the overflow path 9, which is a safety mechanism installed in advance in the biological treatment tank 8, for sludge extraction to allow excess sludge to overflow naturally, the sludge extraction operation using the conventional sludge extraction pipe 11 shown in FIG. 5, that is, the operation of the sludge pump P1 and the opening and closing operation of the extraction valve 10 by the operator, become unnecessary.

[0035] If the extraction coefficient K is K > 0, the water level of the biological treatment tank 8 rises, and excess sludge overflows naturally 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 of the biological treatment tank 8.

[0036] The extraction coefficient K may be appropriately set by the operator via a touch panel type liquid crystal display device. For example, the extraction coefficient K can be set based on the sludge conversion rate with respect to the raw water inflow rate Qraw captured by a flow sensor or the like. The sludge conversion rate refers to the amount of sludge generated with respect to the amount of organic matter treated by the water treatment facility. For example, the BOD sludge conversion rate is the increase amount (kg) of sludge with respect to the BOD inflow amount (kg), and is obtained by a calculation formula such as {surplus sludge concentration (mg / L) × surplus sludge extraction amount (m 3 / day)} ÷ {drainage BOD concentration (mg / L) × drainage amount (m 3 / day)}.

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

[0038] In addition, the control device C measures the MLSS of the biological treatment tank 8 regularly or irregularly, and increases or decreases the extraction coefficient K based on the measured MLSS.

[0039] When the MLSS of the biological treatment tank 8 drops from the appropriate range, the extraction coefficient K is decreased to reduce the sludge extraction amount (increase the filtration amount) to return the MLSS to the appropriate range. When the MLSS rises from the appropriate range, the extraction coefficient K is increased to increase the sludge extraction amount (decrease the filtration amount) to return the MLSS to the appropriate range, so that the water treatment device 1 can be operated while maintaining a stable state.

[0040] Furthermore, the control device C preferably measures the water level of the biological treatment tank 8, and controls to increase or decrease the extraction coefficient K when the measured water level deviates from a predetermined allowable range.

[0041] When the water level in the biological treatment tank 8 deviates from a predetermined allowable range, abnormal fluctuations in the water level can be avoided by increasing or decreasing the extraction coefficient K. For example, when the filtration amount Qtrt is large relative to the raw water inflow amount Qraw, the extraction coefficient K is set to a large value to avoid a sharp drop in the water level, and when the filtration amount Qtrt is small relative to the raw water inflow amount Qraw, the extraction coefficient K is set to a small value to avoid a sharp rise in the water level.

[0042] When the water level in the biological treatment tank 8 deviates from a predetermined allowable range, the control device C is configured to notify an abnormal state via a notification mechanism. As the notification mechanism, a sounding device, an e-mail device that notifies an operator of the occurrence of an abnormality, or the like is used. By notifying an abnormal state via the notification mechanism, the operator is alerted and the occurrence of a major accident is avoided.

[0043] [Second Embodiment] In the above-described embodiment, an example 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 has been described. However, the configuration of the overflow path 9 is not limited to such a mode, and for example, a mode as shown in FIG. 2 may be used.

[0044] That is, 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 extraction pipe 9t. The upper end side of the vertical pipe 9a with an open lower end is connected to one end of the flange pipe 9f, and the other end of the flange pipe 9f is connected to the sludge extraction pipe 9t. By opening a part of the upper end side of the vertical pipe 9a to the atmosphere or filling the overflow path 9 with water in advance, when the liquid level of the water to be treated in the biological treatment tank 8 rises above the formation position of the overflow hole 9h, the sludge naturally flows out.

[0045] [Third Embodiment] Figures 3(a), (b) and 4(a), (b) show a water treatment apparatus 1 including a biological treatment tank 8 whose bottom and side walls are formed of iron plates. The biological treatment tank 8, which is rectangular in plan view, is divided into three regions R1, R2, and R3 by two partition walls 8W along the longitudinal direction, and a membrane separation device 4 that is rectangular in plan view is immersed and disposed in the central region R2. The membrane separation device 4 is disposed at the center in the width direction of the central region R2, and an upward flow due to aeration from an aeration device 5 provided below the membrane separation device 4 forms a circulation flow in which the water to be treated rising from the bottom to the top of the membrane separation device 4 descends from the top of the membrane separation device 4 to both sides.

[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 4(a) and (b)). In a state where the auxiliary aeration devices 6 are operating, the entire central region R2 of the biological treatment tank 8 functions as an aerobic tank 3, and in a state where the auxiliary aeration devices 6 are stopped, the upper layer of the central region R2 of the biological treatment tank 8 functions as an aerobic tank 3 and the lower layer functions as an anoxic tank 2, so that the biological treatment function is configured to be switchable.

[0047] The lower parts of the two partition walls 8W are open, and the water to be treated flowing into the both side regions R1 and R3 from the central region R2 partitioned by the partition walls 8W via the lower parts of the partition walls 8W flows into the lower part of the membrane separation device 4 from the lower parts of the partition walls 8W, and a part of the water is taken out as treated water by the membrane separation device 4.

[0048] A raw water inflow pipe 12 is horizontally installed above the center of the central region R2, and raw water inflow branch pipes 12t are installed from both sides with a partition plate 12W extending from the central part in the longitudinal direction of the membrane separation device 4 interposed therebetween, and the 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 the both side regions R1 and R3, and from the both side regions R1 and R3 to above the central region R2 via the lower part of the membrane separation device 4.

[0049] As shown in Fig. 3(a), 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. Similar to the control device C in Fig. 1, the sludge withdrawal amount Qwas is set with the withdrawal coefficient K (K < 1) as a control factor so that Qwas = raw water inflow amount Qraw × K, and the membrane separation device 4 is operated so that the filtration amount Qtrt = raw water inflow amount Qraw × (1 - K), and excess sludge is naturally overflowed from the overflow path 9 provided in the biological treatment tank 8.

[0050] As shown in Figs. 4(a) and (b), the control device C calculates the sludge circulation ratio based on the difference value 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 aeration amount from the aeration device 5 so that the circulation ratio becomes the target value. Also, the control device C adjusts the aeration amount by the auxiliary aeration device 6 so that the value of the DO sensor provided in the middle layer of the central region R2 becomes the target value. Then, the withdrawal coefficient K is adjusted based on the detected water level by the water level sensor and the value of the MLSS sensor provided in the lower layer (below the membrane separation device 4) of the central region R2.

[0051] [Fourth Embodiment] In the above-described embodiment, the water treatment apparatus 1 including the biological treatment tank 8 including the anoxic tank 2 and the aerobic tank 3 in which the membrane separation device 4 is immersed, and the sludge circulation path 7 for circulating sludge from the aerobic tank 3 to the anoxic tank 2 has been described. However, the biological treatment tank 8 to which the present invention is applied may further include an anaerobic tank for removing phosphorus, and may be configured such that sludge is circulated and supplied from the aerobic tank 3 to the anaerobic tank via the sludge circulation path 7.

[0052] In the above description, the mode in which the water treatment apparatus 1 is automatically operated by the control device C has been described. However, the operator may operate and control the water treatment apparatus 1 instead of the control device C or in cooperation with the control device C.

[0053] That is, the operation method of the water treatment apparatus according to the present invention is an operation method of a water treatment apparatus including at least an anaerobic 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 anaerobic tank, wherein the sludge withdrawal amount Qwas is set with a withdrawal coefficient K (K < 1) as a control factor so that the sludge withdrawal amount Qwas = raw water inflow amount Qraw × K, and the membrane separation device is operated so that the filtration amount Qtrt from the membrane separation device is Qtrt = raw water inflow amount Qraw × (1 - K), and surplus sludge is naturally overflowed from an overflow path provided in the biological treatment tank.

[0054] Further, 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.

[0055] More preferably, the MLSS of the biological treatment tank is measured regularly or irregularly, and the withdrawal coefficient K is adjusted to increase or decrease based on the measured MLSS.

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

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

[0058] The above-described embodiments are one aspect of the present invention, and the present invention is not limited by the description. Needless to say, the specific configuration of each part can be appropriately changed and designed within the range in which the effects of the present invention are achieved.

Explanation of Reference Numerals

[0059] 1: Water treatment apparatus 2: Anaerobic 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. An operation method of a water treatment apparatus including a biological treatment tank having an aerobic tank in which at least a membrane separation device is immersed, wherein the sludge withdrawal amount Qwas is sludge withdrawal amount Qwas = raw water inflow amount Qraw × K and the withdrawal coefficient K (K < 1) is set as a control factor so that the filtration amount Qtrt from the membrane separation device is filtration amount Qtrt = raw water inflow amount Qraw × (1 - K) and the membrane separation device is operated so that excess sludge is naturally overflowed from an overflow path provided in the biological treatment tank, characterized in that it is an operation method of a water treatment apparatus.

2. The operation method of the water treatment apparatus according to claim 1, characterized in that the filtration amount Qtrt is updated and set at the predetermined time interval based on the raw water inflow amount Qraw measured at the predetermined time interval.

3. The operation method of the water treatment apparatus according to claim 2, characterized in that the MLSS of the biological treatment tank is measured regularly or irregularly, and the withdrawal coefficient K is adjusted to increase or decrease based on the measured MLSS.

4. The operation method of the water treatment apparatus according to claim 3, characterized in that the water level of the biological treatment tank is measured, and when the measured water level deviates from a predetermined allowable range, the withdrawal coefficient K is adjusted to increase or decrease.

5. The operation method of the water treatment apparatus according to claim 3 or 4, characterized in that the water level of the biological treatment tank is measured, and when the measured water level deviates from a predetermined allowable range, an abnormal state is notified via a notification mechanism.

6. A biological treatment tank including an aerobic tank in which at least a membrane separation device is immersed, wherein the sludge withdrawal amount Qwas is sludge withdrawal amount Qwas = raw water inflow amount Qraw × K and the withdrawal coefficient K (K < 1) is set as a control factor so that the filtration amount Qtrt from the membrane separation device is filtration amount Qtrt = raw water inflow amount Qraw × (1 - K) and the membrane separation device is operated so that a control device for naturally overflowing excess sludge from an overflow path provided in the biological treatment tank, characterized in that it is a water treatment apparatus provided with.

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