JPH08299767A - Method for washing backward pressurized hollow-fiber membrane module - Google Patents

Abstract

PURPOSE: To provide a backward washing method capable of restoring the flux of a pressurized hollow-fiber membrane module to the original state with a simple operation. CONSTITUTION: The pressurized hollow-fiber membrane module 1 is washed backward by injecting backward washing water from a permeated water inlet 1C and discharging it from the end of the module 1. As a first stage, one end 1A of the module 1 and 5 are opened, the other end and 4, 8 and 9 are closed, backward washing water is discharged into the one end 1A of the module 1 and 5 from the permeated water outlet, 11, 12, 14, 15 and 1C, and the module is washed backward. As a second stage, the other end and 9 are opened, the one end and 5 are closed, the backward washing water is discharged into the other end of the module and 9 from the permeated water outlet, 11, 12, 14, 15 and 1C, and the module is washed backward.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal pressure type hollow fiber membrane module in which backwashing water is injected from the permeated water outlet side and the backwashing water is discharged from the end of the membrane module. The present invention relates to a backwash method for hollow fiber membrane modules.

[0002]

2. Description of the Related Art Internal pressure type hollow fiber membrane modules are made of, for example, polyethylene, polysulfone, polyacrylonitrile,
Approximately 1.0 inner diameter formed using cellulose acetate as a raw material
Thousands of porous hollow fibers (0.01 to 0.5 μm) having a length of 1 mm and a length of 1 mm are bundled and housed in a container. The container is equipped with a raw water supply port, a permeated water outlet and a concentrated water outlet.In the case of the internal pressure type, the raw water supply port is at one end of the hollow fiber,
The permeated water outlet communicates with the outside of the hollow fiber, and the concentrated water outlet communicates with the other end of the hollow fiber. When the internal pressure type hollow fiber membrane module is used for water purification by cross-flow filtration, raw water such as river water is pumped from the raw water supply port of the hollow fiber membrane module at a constant flow rate using a raw water supply pump, and further concentrated. The raw water coming out of the water outlet is returned and circulated using a circulation pump, and this process is repeated to collect permeated water that has exuded from the hollow fiber wall and send it to the distribution reservoir. In many cases, suspended solids and colloidal substances float in the treated water, so that suspended solids and colloidal substances accumulate on the inner wall of the hollow fiber during filtration. As the amount of accumulation gradually increases, the permeation resistance of the hollow fiber wall increases, and the pressure of the raw water supply pump also increases to secure a constant amount of water. When the pump pressure is high, the load applied to the hollow fiber membrane is also large. Therefore, after a certain period of time, backwash is usually performed to restore the original flux. For backwashing, the permeated water that was temporarily stored in the backwash tank was pumped with a backwash pump from the permeate outlet of the hollow fiber membrane module to pump a certain amount of water to penetrate the hollow fiber wall and remove the deposits that were peeled at that time. The water is discharged from the concentrated water outlet or the raw water supply outlet together with the backwash water to the outside of the system. In addition to the above, there is a dead end filtration method, but this filtration method has the advantage of low running cost because it does not circulate the treated water unlike the cross-flow filtration method. Since it does not have the effect of peeling off the deposits attached to, there is a demerit that it will be clogged immediately if the water quality is poor.
In any case, when the flux drops, for example, when a certain period of time has passed, backwashing is performed in the same manner as in the crossflow filtration method. Furthermore, if sticky cells or its metabolites adhere to the hollow fiber wall, or if colloidal iron hydroxide is clogged in the pores, it can be removed in a short time with only backwash water. Since this is not possible, backwashing may be performed by adding a few ppm of a chlorine-based oxidizing agent, such as sodium hypochlorite, to the backwashing water. The backwashing treatment is generally performed at a rate of 15 seconds to 1 minute of backwashing for 10 minutes to 1 hour of passing water. If the flux does not recover even with this backwash, chemical cleaning is performed.
Detergent and citric acid are usually used as chemicals.

[0003]

The use of a large amount of backwash water is likely to increase the flux recovery rate, but the conventional backwash method has a problem in that the flux is not recovered as expected. It was Then, the objective of this invention is providing the backwashing method of the internal pressure type hollow fiber membrane module which can anticipate sufficient recovery of a flux.

[0004]

[Means for Solving the Problems] The above problems are solved by injecting backwash water from the permeated water outlet side of an internal pressure type hollow fiber membrane module and discharging the backwash water from the end of the membrane module. In the backwashing method of an internal pressure type hollow fiber membrane module to be performed, first, one end of the membrane module is opened as a first step,
The other end is closed and the backwash water is discharged from the permeate outlet side to one end side of the membrane module for backwashing, and then the other end is opened as the second step and one end is closed and the backwash water is the permeated water. A backwashing method for an internal pressure type hollow fiber membrane module is characterized in that the backwashing is performed by discharging from the outlet side to the other end side of the membrane module. When used in the cross-flow filtration system, one end of the membrane module may be on the raw water inlet side and the other end may be on the concentrated water outlet side. When using the dead end filtration method, one end of the membrane module serves as a backwash water discharge port. In each step, a certain amount of backwash water may be sent to carry out backwashing, or the backwashing may be carried out for a predetermined period of time.

[0005]

In backwashing, it is easy to think that pressure may be applied from the outside of the hollow fiber to remove the deposits adhering to the inner wall of the hollow fiber by the pressure difference between the inside and outside of the hollow fiber. However, the removal of the deposited deposit depends not only on the differential pressure but also on the flow velocity of the backwash water flowing in the hollow fiber. That is, when the backwash water is fed from the permeated water outlet side in the first step, the backwash water permeates the inside of the hollow fiber over the entire length of the hollow fiber. All the backwash water that has soaked flows toward one end of the membrane module, so that the flow velocity is considerable near the end. If the flow velocity is high, the shearing force exerted by the fluid on the inner wall of the hollow fiber is also large, so that the effect of separating the deposit becomes large. On the other hand, near the other end of the membrane module, the amount of backwash water that has soaked into the membrane module is small and it is unlikely to increase. Therefore, in the first step, as seen in the conventional backwash method, the flux near one end recovers to the initial flux, but the flux near the other end does not recover at all. Since the backwash water flows toward the side with the smaller flow resistance, when the flux near the outlet of the backwash water is restored, the flow rate of the backwash water passing through that part increases, but the backwash water flowing near the other end The flow rate of water is reduced. Therefore, even if the backwashing in the first step is further continued, the amount of water near the other end does not increase, so the effect of backwashing cannot be expected so much. In the present invention, backwashing in the second step is performed after the first step. In the backwashing in the second step, the backwashing water sent from the permeated water outlet side flows through the hollow fiber to the other end side of the membrane module, contrary to the first step. The flux on one end side of the hollow fiber has already been restored by the backwashing in the first step, and when backwashing water is passed in this state, the backwashing water permeates from one end side of the hollow fiber with low resistance and the other end Flowing to the side. Even if there is flow resistance, a considerable flow rate enters from one end side, so the flow velocity of the backwash water near the other end becomes considerably faster than in the first step. In addition, since the deposit adheres to the inner wall of the hollow fiber near the other end and the inside of the pipe is narrow, and the flow velocity increases in inverse proportion to the radius, the shear force of the fluid on the deposit is large. This shear force separates the deposit near the other end, so that the recovery of the hollow fiber flux near the other end can be expected as compared with the conventional method.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the accompanying drawings. First, referring to FIG. 1, an outline of the cross-flow filtration device used in the present invention will be described. Reference numeral 1 is a membrane module in which a large number of inner pressure type hollow fibers made of cellulose acetate are bundled. On the right side of this membrane module 1 is a circulation connecting a raw water supply port 1A which is the upper part of the membrane module 1 and a concentrated water outlet side 1B which is the lower part. A circulation passage 3 is formed, and a circulation pump 3 that circulates raw water counterclockwise in the circulation passage 2 is formed.
A circulation valve 4 is connected to the downstream side thereof, and a first backwash discharge valve 5 that branches from the circulation path 2 and discharges the backwash water to the outside of the system is connected to the downstream side thereof. A raw water supply pipe 6 for supplying raw water to the membrane module 1 is connected to the upstream side of the circulation pump 3, and a raw water supply pump 7 and a raw water supply valve 8 are connected to the pipe 6.
Is connected, and a second backwash discharge valve 9 that branches off to the downstream side and discharges the backwash water to the outside of the system is connected. Further, a water supply pipe 10 for guiding the permeated water to the distribution reservoir is arranged on the permeated water outlet side 1C of the membrane module 1, and the permeated water may be temporarily stored in the middle of the pipe 10 for use as backwash water. A tank 11 that can be used is arranged. And
The cleaning pipe 12 for sending the backwash water from the tank 11 to the membrane module 1 side is the water supply pipe 10 on the upstream side of the tank 11.
And the backwash water is transmitted through the water pipe 10 to the tank 1
A permeate valve 13 is connected to the downstream side of the connection point so as not to flow into the inside 1. Backwash pump 1 for backwash pipe 12
4 and a backwash valve 15 are connected downstream thereof.

The backwashing method according to the present invention using this filtration device will be described below. First, the filtration process will be described. For filtration, the raw water supply valve 8, the circulation valve 4 and the permeate valve 13 are opened, and the first backwash drain valve 5, the second backwash drain valve 9 and the backwash valve 15 are closed. The backwash pump 14 is stopped, and the raw water supply pump 7 and the circulation pump 3 are operated. Since the circulation pump 3 of the circulation path 2 is operated, the raw water supplied by the raw water supply pump 7 flows counterclockwise along the circulation path 2 and flows from the raw water supply port 1A of the membrane module 1 to the concentrated water outlet 1B. After exiting, it is further circulated by the circulation pump 3. Raw water is filtered as it passes through the hollow fibers constituting the membrane module 1, and the permeated water that has passed through the hollow fiber wall flows into the tank 11 through the permeated water outlet 1C of the membrane module 1. The tank 11 is for securing water for backwashing,
When the amount of water exceeds the required amount, the tank 11 overflows and the permeate flows into the reservoir. When the permeated water flows out from the permeated water outlet 1C, the amount of water in the circulation path 2 decreases, but the amount is continuously replenished by the raw water supply pump 7. During filtration, clean water passes through the hollow fiber wall, so suspended substances and microorganisms contained in raw water are blocked and attached to the inner wall of the hollow fiber. Since a large amount of raw water is circulated by the circulation pump 3 in the cross flow filtration method,
The water flow separates the deposits attached to the inner wall of the hollow fiber. Therefore, in the initial stage of filtration, the phenomenon in which suspended substances and microorganisms adhere and peel off is repeated alternately. When the filtration proceeds to some extent, the concentration of impurities in the raw water flowing in the circulation path 2 increases, and suspended substances and microorganisms gradually begin to deposit on the inner wall of the hollow fiber. Corresponding to this, the filtration resistance gradually increases and the amount of permeated water decreases, so the pump pressure of the raw water supply pump 7 increases to compensate for it. If the pressure rises above a certain pressure, the efficiency is poor and the membrane module 1 is adversely affected. Therefore, backwash is performed periodically or when the pressure above the certain pressure is detected to restore the flux of the membrane module 1. .

The present invention relates to this backwashing method and is carried out as follows. First, in the backwash of the first step, the raw water supply pump 7 and the circulation pump 3 are stopped, the circulation valve 4 and the permeated water valve 13 are closed, the backwash valve 15 and the first backwash drain valve 5 are opened, and the backwash pump 1
This is done by putting 4 into the operating state. The backwash water stored in the tank 11 is pressurized by the backwash pump 14 provided in the backwash pipe 12 and pressure-fed into the membrane module 1 from the permeated water outlet 1C side of the membrane module 1. FIG. 2 is an enlarged view of the hollow fiber contained in the membrane module 1, and the jelly-like substance deposited by filtration adheres to the inner wall of the hollow fiber over the entire length of the hollow fiber. Since the conditions of backwashing do not change much regardless of the conditions, the backwashing water that has flowed in through the permeate outlet 1C initially permeates the hollow fiber substantially uniformly from every location. The amount of water gradually increases while the permeated backwash water flows toward the raw water supply port 1A. The flow rate is considerably large near the raw water supply port 1A, and since the deposit adheres to the inner wall of the hollow fiber and the effective diameter is also narrow, the flow velocity is considerable. Since the shear force exerted on the sediment by the backwash water is proportional to the average flow velocity of the backwash water and inversely proportional to the effective diameter, the sediment near the raw water supply port 1A, which mainly has a large shear force, peels off in the backwash of the first step. To be done. The backwashing in the first step may be performed for a certain period of time with a fixed time, or may be performed for a certain amount of water with a fixed amount of water. However, even if the backwashing in the first step is continued for a long time, the peeling effect cannot be expected so much. The permeation resistance of the hollow fiber wall near the raw water supply port 1A is small due to the separation of the deposit, and the backwash water that has flowed in from the permeate outlet 1C flows through the part with low resistance, and the sediment on the concentrated water outlet 1B side This is because it does not contribute to peeling.

Therefore, in the present invention, backwashing in the second step is performed. The backwash in the second step is performed by closing the first backwash discharge valve 5 and opening the second backwash discharge valve 9 after the backwash in the first step. When this operation is performed, the direction of the backwash water flowing in the hollow fibers is reversed as shown in FIG. As can be seen from the figure, since the permeation resistance near the raw water supply port 1A is small, a large amount of backwash water flows into the hollow fiber from near the raw water supply port 1A. On the other hand, since the effective diameter is smaller on the concentrated water outlet 1B side, the flow velocity of the backwash water entering from near the raw water supply port 1A is the highest on the concentrated water outlet 1B side. Therefore, the shearing force that the backwash water exerts on the deposit becomes the largest on the concentrated water outlet 1B side, so that the deposit in this part is peeled off intensively. The backwashing in the second step may also be performed for a certain period of time with a fixed time, or may be performed for a certain amount of water with a fixed amount of water.

Although the description of the embodiment is for a cross-flow filtration system, the present invention can also be applied to a dead end filtration method. In addition, add a few p of sodium hypochlorite to the backwash water.
Back washing may be performed by adding pm. In some cases, air may be blown into the hollow fiber. Furthermore, it can be applied not only to water purification treatment, but also to other treatments.

[0011]

As described above, according to the present invention, the flux of the membrane module can be restored to an almost initial state by a simple operation.

[Brief description of drawings]

FIG. 1 is a schematic view of a filtration device used for carrying out the present invention.

FIG. 2 is an explanatory diagram showing a process of backwashing in a first step.

FIG. 3 is an explanatory view showing a process of backwashing in the same second step.

[Explanation of symbols]

 1 Internal pressure type hollow fiber membrane module 1A Raw water supply port 1B Concentrated water outlet 1C Permeate water outlet 2 Circulation path 3 Circulation pump 4 Circulation valve 5 First backwash discharge valve 6 Raw water supply pipe 7 Raw water supply pump 8 Raw water supply valve 9 Second Backwash discharge valve 10 Water pipe 11 Tank 12 Washing pipe 13 Permeate water valve 14 Backwash pump 15 Backwash valve

Claims (3)

[Claims] 1. An internal pressure type hollow fiber membrane module for backwashing by injecting backwash water from the permeate outlet side of the internal pressure type hollow fiber membrane module and discharging the backwash water from the end of the membrane module. In the backwashing method, first, as one step, one end of the membrane module is opened and the other end is closed to discharge backwash water from the permeate outlet side to one end side of the membrane module for backwashing, and then the second step. As a step, the other end is opened and the other end is closed, and the backwash water is discharged from the permeate outlet side to the other end side of the membrane module to perform backwashing. . 2. The backwash of an internal pressure type hollow fiber membrane module according to claim 1, wherein one end of the membrane module is on a raw water inlet side and the other end of the membrane module is on a concentrated water outlet side. Method. 3. The backwashing method for an internal pressure type hollow fiber membrane module according to claim 1 or 2, wherein the backwashing in the first and second steps is performed by sending a constant amount of backwashing water.