JP7615591B2 - Membrane separation device, its operation method, and biological treatment device - Google Patents

Description

The present invention relates to a membrane separation type biological treatment device that biologically treats wastewater containing organic matter and obtains treated water by membrane separation. The present invention also relates to a membrane separation device suitable for this biological treatment device and its operating method.

Patent Document 1 describes a biological treatment device that obtains treated water by filtering from the primary side of the membrane to the secondary side while circulating sludge from a biological reaction tank to an outer membrane of the tank. When this biological treatment device is in operation, solids gradually adhere to the primary side of the membrane, so a backwash operation is performed at regular intervals by passing treated water from the secondary side to the primary side to peel off and remove the solids adhering to the primary side of the membrane. This prevents a decrease in filtering ability due to sludge adhering to the membrane surface on the primary side, and makes it possible to obtain treated water stably. The backwash wastewater is returned to the biological treatment tank.

JP 2004-249235 A

Backwashing of the membrane is best done for a short time with a higher flux (permeation flux) than during filtration, in terms of the cleaning effect on the membrane surface and the recovery rate of treated water. However, when the backwash pump is started at a specified time, it takes some time (hereinafter referred to as the time lag) from when the backwash pump starts until its discharge flow rate reaches a specified value. During this time lag, the backwash water (treated water is used) has a small flow rate and is returned to the biological reactor as backwash wastewater without having a sufficient cleaning effect. This creates a problem in that the membrane filtration efficiency (recovery rate of treated water) decreases to the extent that treated water that does not contribute to backwashing is returned.

In addition, if the amount of sludge adhering to the primary side of the membrane increases during the filtration cycle, or if the adhering sludge is poorly permeable, the membrane permeation differential pressure increases significantly. Therefore, if the backwash pump output is set to a fixed value, the discharge pressure required for backwashing will be higher than the actual discharge pressure of the backwash pump, and it may not be possible to obtain the desired backwash flux.

The objective of the present invention is to provide a membrane separation device, an operating method thereof, and a biological treatment device that can effectively backwash the membrane separation device, reduce the amount of backwash water used, and stably maintain membrane filtration performance for a long period of time.

The membrane separation device of the present invention is a membrane separation device in which the water to be treated is passed through the primary side, and the treated water that has permeated the membrane is taken out from the secondary side to a treated water tank. The membrane separation device is equipped with a backwash pump for supplying the treated water in the treated water tank to the secondary side, and is characterized by having a switching means for switching the supply destination of the backwash pump's discharge water between the secondary side and the treated water tank.

In one aspect of the present invention, the switching means operates to return discharged water to the treatment tank after the backwash pump starts until the output of the backwash pump reaches a reference value or until a predetermined time has elapsed, and thereafter to introduce the discharged water into the secondary side.

The method of operating the membrane separation apparatus of the present invention is a method of operating the membrane separation apparatus of the present invention, which includes a step of backwashing the membrane by supplying treated water in the treatment water tank to the secondary side by the backwash pump, and is characterized in that after the backwash pump is started, the discharge water of the backwash pump is returned to the treatment water tank until the output of the backwash pump reaches a reference value or a predetermined time has elapsed, and then the discharge water of the backwash pump is supplied to the secondary side to perform backwashing.

The biological treatment device of the present invention has a reaction tank for biologically treating wastewater containing organic matter, a membrane separation device installed outside the reaction tank for obtaining treated water by membrane filtration, a treated water tank for storing the treated water, and a backwash pump for intermittently backwashing the membrane separation device with the treated water, and is characterized by having a switching means for switching the supply destination of the backwash pump's discharge water between the secondary side of the membrane separation device and the treated water tank.

In the present invention, the backwash pump is started during the filtration cycle to circulate the treated water to the treatment tank, and after the output of the backwash pump becomes sufficiently high, the valve is switched to perform membrane backwash. This allows the membrane to be backwashed sufficiently efficiently. In addition, the amount of backwash water used is reduced, improving the recovery rate of treated water.

Furthermore, this effect can be further improved by setting the specified output of the backwash pump to a value that results in a specified backwash flux relative to the pressure inside the tubular membrane during the filtration cycle, as determined from the pump performance curve.

FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment. FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment. FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment. FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment. FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment. FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment. FIG. 1 is a configuration diagram of a biological treatment device according to an embodiment.

The following describes the embodiment with reference to the drawings.

Figure 1 shows a membrane separation device according to an embodiment, where (a) shows the filtration process, (b) shows the state immediately after starting the backwash pump, and (c) shows the backwash process.

The raw water is introduced into the biological treatment reactor 1 and undergoes biological treatment. The reactor 1 is equipped with an agitator 1a.

As shown in FIG. 1(a), in the filtration process, valve 8 is opened and water supply pump 3 is operated. The biological treatment liquid in reaction tank 1 is supplied to the primary side of membrane separation device 5 through pipe 2, water supply pump 3, and pipe 4, and is subjected to membrane separation treatment by membrane 5a. Concentrated water that has not permeated membrane 5a flows out from the primary side of membrane separation device 5 and is returned to reaction tank 1 through pipe 6. Treated water that has permeated membrane 5a flows from the secondary side of membrane separation device 5 through pipe 7, valve 8, and pipe 9 into treated water tank 10, and is taken out through pipe 11. In this filtration process, backwash pump 13, which will be described later, is stopped.

In this embodiment, the treated water tank 10 and piping 7 are connected by piping 12, backwash pump 13, piping 14, 18, valve 19, and piping 20 so that the treated water in the treated water tank 10 backwashes the membrane 5a of the membrane separation device 5. In addition, piping 15, valve 16, and piping 17 are provided so that water from piping 14 is returned to the treated water tank 10 when the backwash pump 13 starts. During the filtration process, both valves 16 and 19 are closed.

To start backwashing the membrane separation device 5, as shown in FIG. 1(b), valve 8 is open, the water supply pump 3 is operating, valve 19 is closed, valve 16 is opened, and the backwash pump 13 is started. The discharge water from the backwash pump 13 is returned to the treatment water tank 10 through pipes 14 and 15, valve 16, and pipe 17.

After a predetermined time has passed and the output of the backwash pump 13 has become sufficiently large, as shown in FIG. 1(c), valve 8 is closed, valve 16 is closed, and valve 19 is opened while the water supply pump 3 is operating. This causes the treated water in the treated water tank 10 to pass through pipes 12, 14, 18, valve 19, and pipes 20 and 7 and be supplied to the secondary side of the membrane separation device 5, where it permeates the membrane 5a from the secondary side to the primary side, peeling off any deposits that have adhered to the primary side of the membrane 5a.

Even during this backwash process, the water supply pump 3 is operating, and the biological treatment liquid from the reaction tank 1 is passed through the primary side of the membrane separation device 5. Therefore, the material that has peeled off from the primary side of the membrane 5a flows into the reaction tank 1 through the pipe 6 together with the biological treatment liquid.

After performing this backwash for a predetermined period of time, as shown in Figure 1(a), the backwash pump 13 is stopped, valve 8 is opened, and valve 19 is closed, and the filtration process is resumed.

In this embodiment, as shown in FIG. 1(b), when the backwash pump 13 is started, valve 19 is closed and valve 16 is opened, and during the start-up period until the output of the pump 13 becomes sufficiently high, the discharge water of the pump 13 is returned to the treated water tank 10, and backwash water is not supplied to the membrane separation device 5. Therefore, during the start-up period of the backwash pump 13, low flow rate backwash water is not supplied to the membrane separation device 5, and treated water with poor backwash effect is not returned. As a result, the wasteful use of treated water for backwashing is avoided, and the treatment efficiency (membrane filtration efficiency) is improved.

Another embodiment will be described with reference to Figures 2 to 7. This embodiment relates to a biological treatment device in which two membrane separation devices 38, 44 are arranged.

Figure 2 shows the state in which the biological treatment liquid is passed through the first membrane separation device 38, and the concentrated water is passed through the second membrane separation device 44 for filtration. In Figure 2, the liquid to be treated is passed through the membrane separation devices 38 and 44 in the forward direction.

In FIG. 2, the backwash pump 62 is stopped, and only the water supply pump 33 is operating. Valves 35, 41, 47, 53, and 58 are open, and the other valves are closed.

The biological treatment liquid in the reaction tank 31 having the agitator 31a is supplied to the primary side of the membrane separation device 38 through the pipe 32, the water supply pump 33, the pipe 34, the valve 35, and the pipes 36 and 37. The concentrated water that did not permeate the membrane 38a is supplied from the primary side of the membrane separation device 38 through the pipes 39 and 40, the valve 41, and the pipes 42 and 43 to the primary side of the membrane separation device 44. The concentrated water that did not permeate the membrane 44a is returned from the primary side of the membrane separation device 44 to the reaction tank 31 through the pipes 45 and 46, the valve 47, and the pipes 48 and 49.

The treated water that has permeated the membrane 38a of the membrane separation device 38 is introduced from the secondary side of the membrane separation device 38 through pipes 51 and 52, valve 53, and pipe 54 into the treated water tank 55. The treated water that has permeated the membrane 44a of the membrane separation device 44 is introduced from the secondary side of the membrane separation device 44 through pipes 56 and 57, valve 58, and pipe 59 into the treated water tank 55.

In this embodiment, in order to backwash the membrane separation device 38 with the treated water in the treated water tank 55, a pipe 61, a backwash pump 62, a valve 90, pipes 63 and 64, a valve 65, and a pipe 66 are provided, and the pipe 66 is connected to the connection point of the pipes 51 and 52.

In addition, in order to backwash the membrane separation device 44, a pipe 67 branched off from the connection point of pipes 63 and 64 is connected to the connection point of pipes 56 and 57 via a valve 68 and a pipe 69.

In this embodiment, valves 90 and 91 and piping 92 are also provided to return the discharge water from the backwash pump 62 to the treatment water tank 55 when the backwash pump 62 is started.

Figure 3 shows the state in which membrane separation device 38 is backwashed and filtration is performed by membrane separation device 44. In this state, pumps 33 and 62 are operating, valves 35, 41, 47, 58, 65, and 90 are open, and the other valves are closed.

The biological treatment liquid in the reaction tank 31 is supplied to the primary side of the membrane separation device 38 through pipe 32, water supply pump 33, pipe 34, valve 35, and pipes 36 and 37. Backwash water is supplied to the secondary side of the membrane separation device 38 as described below. The biological treatment liquid supplied to the primary side and the backwash wastewater that has permeated the membrane 38a from the secondary side to the primary side are mixed and supplied from the primary side of the membrane separation device 38 to the primary side of the membrane separation device 44 through pipes 39 and 40, valve 41, and pipes 42 and 43. The concentrated water that has not permeated the membrane 44a is returned from the primary side of the membrane separation device 44 to the reaction tank 31 through pipes 45 and 46, valve 47, and pipes 48 and 49.

The treated water that has permeated the membrane 44a of the membrane separation device 44 is introduced into the treated water tank 55 from the secondary side of the membrane separation device 44 through pipes 56 and 57, valve 58, and pipe 59.

A portion of the treated water in the treated water tank 55 is supplied to the secondary side of the membrane separation device 38 through pipe 61, backwash pump 62, pipes 63, 64, 66, 51, and valves 90 and 65, and the membrane 38a is backwashed.

Figure 4 shows the state in which filtration is performed in membrane separation device 38 and membrane separation device 44 is backwashed. In this state, pumps 33 and 62 are operating, valves 35, 41, 47, 53, 68, and 90 are open, and the other valves are closed.

The biological treatment liquid in the reaction tank 31 is supplied to the primary side of the membrane separation device 38 through pipe 32, water supply pump 33, pipe 34, valve 35, pipes 36 and 37. The concentrated water that has not permeated the membrane 38a is supplied from the primary side of the membrane separation device 38 through pipes 39 and 40, valve 41, pipes 42 and 43 to the primary side of the membrane separation device 44. Backwash water is supplied to the secondary side of the membrane separation device 44 through pipe 61, backwash pump 62, pipes 63, 67, 69, 56, and valves 90 and 68. The liquid from the membrane separation device 38 supplied to the primary side and the backwash wastewater that has permeated the membrane 44a from the secondary side to the primary side are mixed together and returned from the primary side of the membrane separation device 44 to the reaction tank 31 through pipes 45 and 46, valve 47, pipes 48 and 49.

The treated water that has permeated the membrane 38a of the membrane separation device 38 is introduced into the treated water tank 55 from the secondary side of the membrane separation device 38 through pipes 51 and 52, valve 53, and pipe 54.

When starting the backwash pump 62 to perform the backwash in Figures 3 and 4, the same start-up operation is performed as in Figure 1(a). That is, when starting the backwash pump 62, valve 90 is closed, valve 91 is opened, the backwash pump 62 is started, and the pump discharge water is returned to the treatment water tank 55. Then, once the output of the pump 62 has become sufficiently large, valve 90 is opened and valve 91 is closed as shown in Figures 3 and 4, and backwash is performed. The same applies to the cases of Figures 6 and 7 described below.

In Figures 2 to 4, the biological treatment liquid is passed through the primary side of the membrane separation devices 38, 44 in the above-mentioned direction (hereinafter sometimes referred to as the forward direction), but in Figures 5 to 7, which will be explained next, the biological treatment liquid is passed through the primary side of the membrane separation devices 38, 44 in the opposite direction to the forward direction (hereinafter sometimes referred to as the reverse direction).

Figure 5 shows the membrane separation devices 38 and 44 in a state where they are performing filtration operation with reverse water flow. In this state, the water supply pump 33 is operating and the backwash pump 62 is stopped. In addition, valves 53, 58, 72, 76, and 82 are open, and the other valves are closed.

The biological treatment liquid in the reaction tank 31 is supplied to the primary side of the membrane separation device 38 through pipe 32, water supply pump 33, pipes 34 and 71, valve 72, and pipes 73 and 39. The concentrated water that did not permeate the membrane 38a is supplied from the primary side of the membrane separation device 38 through pipes 37 and 75, valve 76, and pipes 77 and 45 to the primary side of the membrane separation device 44. The concentrated water that did not permeate the membrane 44a is returned from the primary side of the membrane separation device 44 to the reaction tank 31 through pipes 43 and 81, valve 82, and pipes 83 and 49.

The treated water that has permeated the membrane 38a of the membrane separation device 38 is introduced from the secondary side of the membrane separation device 38 through pipes 51 and 52, valve 53, and pipe 54 into the treated water tank 55. The treated water that has permeated the membrane 44a of the membrane separation device 44 is introduced from the secondary side of the membrane separation device 44 through pipes 56 and 57, valve 58, and pipe 59 into the treated water tank 55.

Figure 6 shows the state in which the membrane separation device 38 is backwashed and filtration is performed by reverse water flow in the membrane separation device 44. In this state, the pumps 33 and 62 are operating, the valves 58, 65, 72, 76, 82, and 90 are open, and the other valves are closed.

The biological treatment liquid in the reaction tank 31 is supplied to the primary side of the membrane separation device 38 through the pipe 32, the water supply pump 33, the pipes 34 and 71, the valve 72, and the pipes 73 and 39. Backwash water is supplied to the secondary side of the membrane separation device 38 through the pipe 61, the backwash pump 62, the pipes 63, 64, 66 and 51, and the valves 90 and 65. The biological treatment liquid supplied to the primary side and the backwash wastewater that has permeated the membrane 38a from the secondary side to the primary side are mixed and supplied from the primary side of the membrane separation device 38 to the primary side of the membrane separation device 44 through the pipes 37 and 75, the valve 76, and the pipes 77 and 45. The concentrated water that has not permeated the membrane 44a is returned from the primary side of the membrane separation device 44 to the reaction tank 31 through the pipes 43 and 81, the valve 82, and the pipes 83 and 49.

The treated water that has permeated the membrane 44a of the membrane separation device 44 is introduced into the treated water tank 55 from the secondary side of the membrane separation device 44 through pipes 56 and 57, valve 58, and pipe 59.

Figure 7 shows the state in which filtration is performed in the membrane separation device 38 and the membrane separation device 44 is backwashed. In this state, the pumps 33 and 62 are operating, the valves 53, 68, 72, 76, 82, and 90 are open, and the other valves are closed.

The biological treatment liquid in the reaction tank 31 is supplied to the primary side of the membrane separation device 38 through the pipe 32, the water supply pump 33, the pipes 34, 71, the valve 72, and the pipes 73, 39. The concentrated water that has not permeated the membrane 38a is supplied from the primary side of the membrane separation device 38 through the pipes 37, 75, the valve 76, and the pipes 77, 45 to the primary side of the membrane separation device 44. Backwash water is supplied to the secondary side of the membrane separation device 44 through the pipe 61, the backwash pump 62, the pipes 63, 67, 69, 56, and the valves 90, 68. The liquid from the membrane separation device 38 supplied to the primary side and the backwash wastewater that has permeated the membrane 44a from the secondary side to the primary side are mixed together and returned from the primary side of the membrane separation device 44 to the reaction tank 31 through the pipes 43, 81, the valve 82, and the pipes 83, 49.

The treated water that has permeated the membrane 38a of the membrane separation device 38 is introduced into the treated water tank 55 from the secondary side of the membrane separation device 38 through pipes 51 and 52, valve 53, and pipe 54.

By alternately passing water in the forward direction as shown in Figures 2 to 4 and passing water in the reverse direction as shown in Figures 5 to 7, deposits that have adhered near the inlets of the membrane separation devices 38 and 44 can be removed by switching the water flow direction.

That is, when water is passed in the forward direction, sludge, precipitated inorganic matter, membrane peelings, etc. adhere to the vicinity of the inlet on the pipe 37 side of the membrane separation device 38 and the vicinity of the inlet on the pipe 43 side of the membrane separation device 44. When the forward water flow is switched to the reverse water flow, the deposits that adhered to the vicinity of the inlet on the pipe 37 side of the membrane separation device 38 during the forward water flow flow out from the pipe 37 to the pipe 75 side, and the deposits that adhered to the vicinity of the inlet on the pipe 43 side of the membrane separation device 44 flow out from the pipe 43 to the pipe 81 side.

When water is flowing in the reverse direction, deposits that have adhered near the inlet of membrane separation device 38 on the pipe 39 side and near the inlet of membrane separation device 44 on the pipe 45 side also flow out to pipe 40 and pipe 46, respectively, when water is switched to flowing in the forward direction.

By reversing the direction of sludge flow through the membrane module every 1 to 8 hours, it is possible to prevent blockage of the flow path due to deposits and maintain stable filtration performance.

The following describes the biological treatment tank, membrane separation device, and suitable operating conditions for use in the present invention.

<Biological treatment tank>
The biological treatment tank may be either aerobic or anaerobic, but when an external tubular membrane is used, anaerobic treatment is preferred. In this case, the sludge concentration in the tank is 3,000 to 50,000 mg/L, preferably 8,000 to 40,000 mg/L, and particularly 20,000 to 30,000 mg/L, which is preferred in terms of enabling high-load treatment and preventing deterioration of membrane filtration performance. The temperature is 25 to 40°C or 45 to 60°C, preferably 30 to 38°C, or 50 to 58°C. The pH in the tank is 6.5 to 8.0, preferably 6.8 to 7.4, and pH adjustment is performed by adding an alkaline agent such as NaOH or _NaHCO3 or an acid agent such as HCl to the sludge circulation line to the external membrane (return pipe 49 from the membrane to the anaerobic reaction tank) as necessary.

<Membrane separation device>
As the membrane separation device, an internal pressure type tubular membrane is preferred.

The internal pressure tubular membrane is preferably a membrane module with multiple tubulars (e.g. 100 to 1000 tubulars) with an inner diameter of 3 mm to 10 mm packed inside, with sludge flowing inside each tubular. The permeate seeps out of the tubulars, and the permeate that seeps out of each tubular is collected and discharged from the membrane module as treated water. The pore size of the membrane is preferably 0.01 to 1 μm, and more preferably 0.03 to 0.1 μm.

Pressure sensors are attached to the sludge inlet and outlet pipes of the tubular membrane, and the intermediate value is set as the primary pressure of the tubular membrane to measure the pressure inside the membrane, while a pressure sensor attached to the permeate side pipe measures the secondary pressure.

<Water flow patterns of membrane filtration and backwash>
In the membrane filtration step, it is preferable to allow the sludge from the biological treatment tank to flow through the tubular filter at a flow rate of preferably 0.5 to 2 m/sec, and more preferably 0.8 to 1.5 m/sec. The flux during filtration is preferably 0.1 to 0.8 m/d, and more preferably 0.2 to 0.5 m/d.

In the backwashing process, it is preferable to perform backwashing with treated water for 5 to 30 seconds at a flux of 4 to 10 m/d for 10 to 60 minutes of filtration. It is preferable to operate in a form in which such membrane filtration process and backwashing process are repeated alternately.

<Backwash pump startup operation>
When starting the backwash pump, it is preferable to start the backwash pump in a form that circulates the treated water in the treated water tank during the filtration process 10 seconds to 5 minutes, preferably 1 to 5 minutes, before moving to the backwash process, and after the output of the backwash pump has increased to a specified value, to start backwashing by switching the valve.

The specified output of the backwash pump is preferably determined by measuring the pressure inside the tubular membrane during membrane filtration during trial operation of the biological treatment device, taking into account the membrane's water flow pressure, and then determining the output that results in a specified backwash flux from the pump's performance curve (the calibration relationship between the discharge pressure and discharge flow rate at each pump output). For example, if the pressure inside the tubular membrane during membrane filtration is P1 and the membrane's water flow pressure is P2, the specified output is set by finding from the pump's performance curve the output that results in a discharge flow rate equivalent to the specified backwash flux at discharge pressure P1 + P2. The specified output can also be set from the discharge volume during trial operation or actual backwashing.

In the above embodiment, the backwash pumps 13, 62 are controlled to be ON/OFF, but the water supply pumps 3, 33 are always ON, so even when the membrane is in one stage and the process transitions from the membrane filtration process to the backwash process, the circulating sludge continues, and internal pressure is applied to the primary side of the membrane, just as during membrane filtration.

When multiple membrane modules are used, the modules may be arranged in series, parallel, or both. However, when arranged in series, it is preferable to configure the modules so that when the sludge flow direction is reversed, the sludge in the connecting piping between the modules is returned to the anaerobic reaction tank without passing through the modules, as in the embodiment of Figures 2 to 7 described above, in order to prevent the accumulation of coarse solid particles between the modules.

The retention time in the treatment tank is preferably 0.5 to 8 hours, and more preferably 1 to 4 hours, to prevent contamination of the backwash water due to the proliferation of microorganisms in the treatment tank.

By performing backwashing under these conditions, backwashing can be performed more effectively in a membrane separation type biological treatment device that uses an internal pressure type tubular membrane installed outside the tank, and the membrane filtration performance can be maintained stably for a long period of time.

1, 31 Biological treatment tank 3, 33 Feed pump 5, 38, 44 Membrane separation device 13, 62 Backwash pump

Claims (3)

A membrane separation device in which water to be treated is passed through a primary side, and treated water that has permeated the membrane is taken out to a treated water tank from a secondary side,
A membrane separation apparatus equipped with a backwash pump for supplying treated water in a treatment tank to the secondary side,
A switching means is provided for switching the supply destination of the discharge water of the backwash pump from a supply mode in which the discharge water is supplied to the treatment water tank without being supplied to the secondary side to a supply mode in which the discharge water is supplied to the secondary side ,
A membrane separation apparatus characterized in that the switching means operates to return discharged water to the treatment water tank after the backwash pump is started until the output of the backwash pump reaches a reference value, and thereafter to introduce the discharged water into the secondary side . 2. The method for operating a membrane separation apparatus according to claim 1, further comprising the step of supplying treated water in the treated water tank to the secondary side by the backwash pump to backwash the membrane,
A method for operating a membrane separation apparatus, characterized in that after starting the backwash pump, the discharge water of the backwash pump is returned to the treatment water tank until the output of the backwash pump reaches a reference value, and then the discharge water of the backwash pump is supplied to the secondary side to perform backwashing. A biological treatment device having a reaction tank for biologically treating wastewater containing organic matter, a membrane separation device installed outside the reaction tank for obtaining treated water by membrane filtration, a treated water tank for storing the treated water, and a backwash pump for intermittently backwashing the membrane separation device with the treated water,
A switching means is provided for switching the supply destination of the discharge water of the backwash pump from a supply mode in which the discharge water is supplied to the treated water tank without being supplied to the secondary side of the membrane separation device to a supply mode in which the discharge water is supplied to the secondary side ,
This biological treatment device is characterized in that the switching means operates to return the discharged water to the treatment tank after the backwash pump is started until the output of the backwash pump reaches a standard value, and thereafter to introduce the discharged water into the secondary side .

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