JP2020082036A - Backwashing method for ceramic membrane, and membrane separator - Google Patents

Abstract

To suppress fouling of a ceramic membrane without need for chemicals that may pose an obstacle to the ceramic membrane.SOLUTION: In a membrane separator 1 for bringing about solid-liquid separation of solid matter, a control part 71 of pH controlling equipment 7 controls injection of pH controlling chemicals into backwash water so that a pH of the backwash water supplied to a ceramic membrane of a membrane module 5 can be a pH making a surface charge of the solid matter and a surface charge of the ceramic membrane electrified in an electrically homopolar manner, in a step of backwashing the membrane module 5.SELECTED DRAWING: Figure 1

Description

The present invention relates to a technique for eliminating fouling of a ceramic film.

Ceramic membranes are used for solid-liquid separation of various wastewaters because of their robustness, high physical and chemical durability, and high hydrophilicity. Then, a device for suppressing the deterioration of filtration performance due to fouling (clogging) of the ceramic membrane has been conventionally made. Fouling countermeasures mainly include methods for suppressing fouling and methods for recovering from fouling.

As a method for suppressing fouling, there is a method for adjusting the surface charge of a solid material or a separation film. For example, in the methods of Patent Documents 1 to 3, fouling is performed by adjusting the surface charge of the solid-liquid separation particles in the water to be treated and the surface charge of the ceramic filtration membrane to have the same polarity (potential sign is the same sign). To reduce the power required for membrane cleaning and increase the efficiency of filtration operation. In particular, in the method of Patent Document 3, when the solid matter contained in the liquid phase is separated, the surface charge of the solid matter is electrically charged to the same polarity as the surface charge of the inorganic film for solid-liquid separation of the solid matter. In addition, the pH of the liquid phase is adjusted to suppress the fouling of the inorganic film by the solid matter.

Examples of methods for recovering from fouling include an in-line cleaning method in which a chemical solution is injected into a backwash line for a ceramic membrane and an immersion cleaning method in which the membrane is immersed in the chemical solution.

JP 2002-136969 A JP, 2010-227836, A WO 2013/187378 JP, 2001-334265, A

As a method for suppressing fouling, there is a method in which scrubbing air and backwashing are combined, but there is a limit in the long-term suppressing effect of adhesion and accumulation of pollutants. Therefore, a method for effectively recovering from fouling is required in order to suppress deterioration of filtration performance and maintain stable membrane separation characteristics.

There are cases where insoluble fine crystals or fine metals composed of dispersed particles in water are targeted for separation by utilizing the characteristics of the ceramic membrane. If the fine crystals or fine metals are sparingly soluble in acids and alkalis, the fouling substance trapped by the pores inside the ceramic membrane surface in the in-line cleaning method or the immersion cleaning method using a normal chemical solution. Cannot be removed. Therefore, when such insoluble fine particles clog the ceramic film, it may not be possible to remove it unless hot concentrated sulfuric acid or hydrofluoric acid is required as a chemical for chemical cleaning. Such a situation becomes remarkable when the object to be separated is, for example, a fluorine compound contained in the fluorine-containing wastewater or a factory wastewater containing a fine metal such as tungsten.

Therefore, if the object to be separated is the fine crystals or fine metal and is hardly soluble in acid or alkali, in order to remove the fouling substance, hot concentrated sulfuric acid or hot concentrated sulfuric acid may be used for cleaning the chemical solution as described above. Hydrofluoric acid (hydrofluoric acid) is required.

However, it is difficult to apply these agents to the ceramic membrane for the following reasons.

That is, these agents may reduce the strength and separation performance of the ceramic membrane. Further, since hot concentrated sulfuric acid and hydrofluoric acid are drugs designated as non-medical deleterious substances and external toxic substances, they are difficult to handle during work.

In view of the above circumstances, it is an object of the present invention to suppress fouling of a ceramic film without requiring a chemical liquid that may cause a trouble in the ceramic film.

One aspect of the present invention is a method of backwashing a ceramic membrane used for separating solid matter contained in a liquid phase, wherein the pH of the backwash water supplied to the ceramic membrane is set to the surface charge of the solid matter and the ceramic. It is adjusted so that the surface charge of the film is electrically charged to the same polarity.

One aspect of the present invention is the method of backwashing a ceramic membrane, wherein the pH of the backwash water is lower or higher than both the isoelectric point of the solid and the isoelectric point of the ceramic membrane. Adjust to one of the pH values.

In one aspect of the present invention, in the method for backwashing a ceramic film, the solid matter is insoluble fine crystals or fine metals.

One aspect of the present invention is a membrane separation apparatus, which comprises a membrane module provided with a ceramic membrane for separating solids contained in a liquid phase, and the ceramic membrane supplied in the backwashing step of the membrane module. A control unit that controls the injection of the pH adjusting chemical liquid into the backwash water so that the pH of the backwash water becomes a pH at which the surface charge of the solid matter and the surface charge of the ceramic film are electrically charged to the same polarity. With.

According to the present invention, fouling of a ceramic film can be suppressed without requiring a chemical solution that may cause a trouble in the ceramic film.

The schematic block diagram of one mode of the membrane separation device to which the present invention is applied. The characteristic view which showed the isoelectric point and zeta potential of various inorganic compounds.

Embodiments of the present invention will be described below with reference to the drawings.

[Outline of the present invention]
INDUSTRIAL APPLICABILITY The present invention utilizes the characteristic that the zeta potential of the ceramic membrane changes with pH, and is used for the backwashing step of the ceramic membrane so that the zeta potential has the same polarity as that of the substance to be filtered (repulsive force works). Adjust the pH of the backwash water. Fouling is eliminated by the approach of using the repulsive force of the zeta potential to remove the fouling substance in a state in which the repulsive force works instead of melting it.

That is, the present invention provides a method of backwashing a ceramic membrane used for separating solid matter contained in a liquid phase, wherein the pH of the backwash water supplied to the ceramic membrane is set to the surface charge of the solid matter and the ceramic membrane. It is adjusted so that it is electrically charged to have the same polarity as the surface charge of. For example, the pH of the backwash water is adjusted to either a lower pH value or a higher pH value than both the isoelectric point of the solid matter and the isoelectric point of the ceramic membrane.

The dispersed particles in water, which are the objects of separation of the following ceramic membranes, are insoluble fine crystals and fine metals. Examples of this fine crystal are calcium fluoride and the like, and fouling is eliminated by applying the present invention to a fine crystal or a fine metal.

Examples of the material of the ceramic membrane and the separation target include aluminum oxide described in Patent Document 3, aluminum hydroxide, titanium oxide, titanium hydroxide, zirconium oxide, zirconium hydroxide, zinc oxide, zinc hydroxide, and silicon oxide. , Containing at least one compound from silicon hydroxide.

In the backwashing method, any one of a high-pressure backwashing method, a high-frequency backwashing method, and a long-time backwashing method is arbitrarily applied when the pH-adjusted backwashing water is applied to the ceramic membrane.

[Example of Embodiment of Membrane Separation Device 1 to which the Present Invention is Applied]
The membrane separation device 1 illustrated in FIG. 1 includes a raw water tank 2, a membrane separation tank 3, a filtered water tank 4, a membrane module 5, an air diffusing pipe 6 and a pH adjusting device 7.

The raw water tank 2 temporarily stores a liquid phase containing a solid substance to be removed from the outside of the system, and then supplies the liquid phase to the membrane separation tank 3 by a raw water supply pump P1.

The membrane separation tank 3 filters the liquid phase supplied from the raw water tank 2 with the membrane module 5. A membrane module 5 and an air diffusing tube 6 are provided in the membrane separation tank 3.

The membrane module 5 has, for example, an external pressure type solid-liquid separation type flat membrane-shaped ceramic membrane. The membrane module 5 is arranged so that the membrane surface is vertical when immersed in the liquid phase in the membrane separation tank 3. Then, the filtered water is filtered from the membrane module 5 by setting the negative pressure on the drain side of the membrane module 5 by the suction pump P while monitoring the transmembrane pressure difference by the pressure gauge PI and the flow rate of the filtered water by the flowmeter FI. Transferred to the water tank 4. In addition, in the piping line of the primary side of the suction pump P (drain side of the membrane module 5), a flow path of filtered water is secured in the filtration step of the membrane module 5, while the flow path is provided in the back washing step of the membrane module. A valve V1 for shutting off is provided.

The air diffuser 6 introduces air from the blower B to perform aeration cleaning of the membrane surface of the membrane module 5.

The filtered water tank 4 stores the filtered water discharged from the membrane module 5. This filtered water is used for advanced treatment outside the system. Further, a part of the filtered water is used as backwash water for the membrane module 5 in the backwashing step of the membrane module 5.

A backwash pump P2 that supplies the filtered water from the filtered water tank 4 to the membrane module 5 to periodically backwash the membrane module 5 is provided in the backwash process piping line. Furthermore, a valve V2 that secures a flow path of filtered water supplied to the membrane module 5 in the backwashing process is provided in a pipeline line on the backwashing water supply side of the backwash pump P2.

In the pH adjuster 7, the pH of the filtered water in the filtered water tank 4 used as the backwash water of the membrane module 5 is such that the surface charge of the solid matter and the surface charge of the ceramic membrane of the membrane module 5 are electrically the same polarity. A pH-adjusting chemical liquid is supplied to the filtered water so as to have a charged pH.

The pH adjusting device 7 includes a chemical liquid pump P3 that supplies a pH adjusting chemical liquid from the pH adjusting chemical liquid tank 70 to the backwash water, and a control unit 71 that controls the operation of the chemical liquid pump P3.

As the pH adjusting chemical solution, a well-known acidic or alkaline chemical solution used for pH adjustment in the water treatment field is applied.

The controller 71 controls the operation of the chemical liquid pump P3 so that the pH of the backwash water is a pH at which the surface charge of the solid matter and the surface charge of the ceramic film are electrically charged to the same polarity.

For example, in the control unit 71, based on the surface charge of the ceramic membrane of the membrane module 5 shown in FIG. 2 and the surface charge of the backwash water, the pH adjusting chemical solution for the backwash water becomes the same polarity. The injection rate is preset. Then, in the membrane separation device 1, when the process proceeds to the backwashing step of the membrane module 5, the chemical liquid pump P3 supplies the pH-adjusted chemical liquid to the backwash water by the control signal from the control unit 71 based on the injection rate.

[Operation Example of Membrane Separation Device 1]
In the filtration step of the membrane separation device 1, the liquid phase containing the solid matter is supplied from the raw water tank 2 to the membrane separation tank 3 by the raw water supply pump P1. The liquid phase in the membrane separation tank 3 is supplied to the membrane module 5 by the suction force of the suction pump P and filtered. Air is appropriately supplied to the membrane surface of the membrane module 5 from the air diffuser 6 for aeration cleaning. The filtered water discharged from the membrane module 5 is transferred to the filtered water tank 4. The filtered water stored in the filtered water tank 4 is further transferred to the outside of the system for advanced treatment.

In the membrane separation device 1, at the time of the backwash process, the valve V1 shuts off the filtered water supply path to the filtered water tank 4, while the valve V2 secures the filtered water supply path to the membrane module 5. , The backwash pump P2 is operated. Then, the chemical liquid pump P3 of the pH adjusting device 7 receives the control signal from the control unit 71, sucks the pH adjusting chemical liquid from the pH adjusting chemical liquid tank 70, and injects it into the filtered water supplied to the membrane module 5.

The filtered water into which the pH-adjusting chemical solution has been injected is used as backwash water for the membrane module 5 from the drain side of the membrane module 5 to the ceramic membrane of the membrane module 5. The pH of the backwash water is adjusted to a pH value lower or higher than both the isoelectric point of the solid matter and the isoelectric point of the ceramic membrane of the membrane module 5. There is.

The backwashing process is executed based on an operation mode arbitrarily selected from high-pressure backwashing, high-frequency backwashing, and long-time backwashing transmission modes. The high-pressure cleaning mode is, for example, a mode in which backwash water is supplied under a pressure value higher than at least the filtration step. The high-frequency backwash mode is, for example, a mode in which the backwash process is executed under an arbitrarily set constant cycle. The long-time backwashing mode is, for example, a mode in which the backwashing process is executed under a relatively long backwashing time that is arbitrarily set. The pressure value, cycle, and backwash time of these modes are set based on empirical rules obtained from the characteristics of the solid matter and the ceramic membrane.

[Example of Membrane Separation Device 1]
Specific examples of the membrane separation device 1 will be shown below.

Example 1 Separation of Calcium Fluoride by Ceramic Membrane In the membrane separation apparatus 1 of FIG. 1, in the raw water tank 2, wastewater containing calcium fluoride as a solid substance (adjusted to pH 7, concentration 1000 mg/L, average particle size) Diameter 0.5 μm, having a particle size distribution). Further, the membrane module 5 used a flat membrane-shaped ceramic membrane (pore diameter 0.7 μm) whose membrane material is alumina (aluminum oxide).

In the test, backwash water whose pH was adjusted to 6.0 and 9.0 was used as an example, and backwash water whose pH was adjusted to 7.0 was used as a comparative example.

In each of the tests, in the membrane module 5 to which an unused flat membrane-like ceramic membrane was applied, water was passed through the wastewater containing no solid matter to make it familiar, and then, under the same conditions (filtration speed, water passage time, etc.). The calcium fluoride-containing wastewater was filtered. The recovery of the transmembrane pressure difference was evaluated by the transmembrane pressure ratio (value immediately before backwashing/value immediately after backwashing). The higher the cleaning effect is, the smaller the value immediately after the backwashing is, so that the transmembrane pressure difference ratio is increased. The transmembrane pressure is a pressure necessary for obtaining permeated water by filtration, and is also called a filtration pressure difference or TMP (Trans Membrane Pressure), which is an evaluation index for membrane fouling.

The backwash water pH and transmembrane pressure ratio in Example 1 are shown in Table 1 below.

Further, the results of detailed investigation on the pH of the backwash water are as follows.

When the film material of the ceramic film is alumina (aluminum oxide) and the dispersed particles which are the solid matter to be removed are calcium fluoride, the pH of the backwash water is set to pH 6.3 or lower or pH 9.0 or higher. When the adjusted water was supplied to the ceramic membrane, the recovery of the transmembrane pressure difference was excellent as compared with the backwash water without pH adjustment (pH-unadjusted water).

As described above, the backwashing water having a pH of 9.0 or higher, which is equal to or higher than the isoelectric point of alumina, is supplied to the ceramic membrane, so that the ceramic membrane can be obtained without requiring a chemical solution that may cause trouble to the ceramic membrane. It was shown that the fouling can be suppressed.

(Example 2) Separation of Dispersed Particles (Oxidation Type or Hydroxide Type Ceramics) by Ceramic Membrane The results obtained by performing the same procedure as in (Example 1) using titanium oxide as the solid particles to be removed are shown below. .. Waste water containing titanium oxide particles (adjusted to pH 7.0, concentration 1000 mg/L, average particle size 0.5 μm, and having particle size distribution) was supplied to the raw water tank 2 of Example 2 as raw water.

The backwash water pH and the transmembrane pressure ratio in Example 2 are shown in Table 2 below.

When the film material of the ceramic film is alumina (aluminum oxide) and the dispersed particles that are the solid matter to be removed are titania (titanium oxide), the pH of the backwash water is pH 6.3 which is the isoelectric point of titania. Below, or if pH-adjusted water having a pH of 9.0 or more, which is the isoelectric point of alumina, is supplied to the ceramic membrane, the transmembrane pressure difference will be higher than that of backwash water without pH adjustment (pH-unadjusted water). The recovery was excellent.

As is clear from the above examples, in the backwashing process of the ceramic membrane, the pH of the backwashing water of the ceramic membrane is such that the surface charge of the object to be separated and the surface charge of the ceramic membrane are electrically charged to the same polarity. By being adjusted, fouling of the ceramic film is suppressed.

In particular, when an object to be separated is an insoluble fine crystal or fine metal consisting of dispersed particles in water and the substance to be separated is a substance that is hardly soluble in acid or alkali, fouling due to the substance is suppressed.

DESCRIPTION OF SYMBOLS 1... Membrane separation device 2... Raw water tank 3... Membrane separation tank 5... Membrane module, 6... Diffuser tube 4... Filtration water tank 7... pH adjusting device, P3... Pump, 70... pH adjusting chemical liquid tank, 71... Control unit

Claims (4)

In the backwashing method of the ceramic membrane provided for the separation of solid matter contained in the liquid phase,
The backwashing of the ceramic membrane, wherein the pH of the backwashing water supplied to the ceramic membrane is adjusted so that the surface charge of the solid matter and the surface charge of the ceramic membrane are electrically charged to the same polarity. Method. The pH of the backwash water is adjusted to a pH value lower or higher than both the isoelectric point of the solid and the isoelectric point of the ceramic membrane. The method for backwashing a ceramic membrane according to 1. The method of backwashing a ceramic membrane according to claim 1, wherein the solid matter is insoluble fine crystals or fine metals. A membrane module including a ceramic membrane provided for separation of solid matter contained in a liquid phase,
In the backwashing step of this membrane module, the pH of the backwash water supplied to the ceramic membrane is adjusted to a pH at which the surface charge of the solid matter and the surface charge of the ceramic membrane are electrically charged to the same polarity. A membrane separation device comprising: a control unit that controls the injection of the pH adjusting chemical liquid into the backwash water.

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