JPH0698276B2 - Membrane separation method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a membrane separation method, and in particular, in the membrane separation treatment of raw water to which a coagulant is added, a high water permeation rate is maintained to achieve efficient membrane separation. The present invention relates to a membrane separation method that enables treatment.

[Prior Art] Ultrapure water is indispensable in the semiconductor manufacturing process and is used in large amounts as semiconductor cleaning water. Therefore, as a water-making process for producing such ultrapure water, a membrane separation method has been widely adopted, which uses a semipermeable membrane to remove dissolved substances in water.

The performance of the membrane separator used to produce ultrapure water is
Not only the ability to remove dissolved substances in raw water but also its high water permeation rate is required. That is, in producing a predetermined amount of pure water, if the water permeation rate of the membrane separation device is small, a large membrane area will be required, resulting in an increase in water production cost.

By the way, in a membrane separation apparatus generally applied to fresh water, not only suspended substances and colloidal substances that coexist in raw water but also dissolved organic substances and the like accumulate on the separation membrane surface, and the water permeation rate is Will start to decline.

The biggest problem in the conventional membrane separation method is that the gel-like substance accumulates on the membrane surface in this way, and the gel-like substance with a large filtration resistivity gradually accumulates strongly on the membrane surface, which lowers the water permeation rate. It is to be. This phenomenon has been an unavoidable problem to some extent even when raw water with relatively little pollution is used as in the production of ultrapure water.

It is generally said that even a semipermeable membrane such as a UF membrane having relatively large pores has a surface porosity of several percent or less. If a layer with a high filtration resistivity is added on this membrane surface, the distance from the surface of this layer to the pores of the semipermeable membrane through this layer will increase and the water permeation rate will decrease significantly. .

For this reason, in the past, a pretreatment for removing suspended substances and the like from the raw water has been performed, and in many cases, a pretreatment combining treatment unit operations such as coagulation sedimentation sand filtration and microfiltration is performed. There is. However, these pretreatment methods require a lot of site area in addition to requiring complicated operations. Moreover, even when such pretreatment is performed, it is not possible to completely prevent the accumulation of contaminants on the film surface, and it is necessary to periodically perform a chemical cleaning of the film surface. .

As a method for solving such a problem, a method of adding a crystallization medium or the like to raw water is known. Even with such a method, when water is passed for a long period of time, a crystallization product or an agglomeration is generated. There remains a problem that substances adhere to the membrane surface and contaminate the membrane surface, resulting in a decrease in water permeation rate.

In addition, in order to remove colloidal components, Al salt, Fe salt, Mg
A method is known in which raw water obtained by adding an inorganic coagulant such as a salt and performing coagulation is directly passed through a membrane separation device to perform membrane separation.

It is also known that the water permeation rate is recovered by periodically washing the membrane separation device with back pressure to remove the gel layer accumulated in the separation membrane.

[Problems to be Solved by the Invention] The influence of colloidal particles cannot be completely excluded even when the raw water that has been subjected to the coagulation treatment is passed through a membrane separation device. There is also a problem that the water permeation rate decreases with time because the flocs generated by the aggregation adhere to the membrane surface and are consolidated.

That is, Al, Fe, Mg salt, which is an inorganic flocculant widely used in the field of water treatment, has a filtration specific resistance of 1 to 10 in the flocculation block such as aluminum hydroxide or iron hydroxide formed in water.
It is in the range of × 10 ¹² m / kg, and when compared with organic mucilages,
Its filtration resistivity is small. However, these agglomerated flocs have a property of being compressible and being densified under a high pressure condition to rapidly increase the filtration resistivity. Therefore, although the flocculant flocculates the colloidal substance to reduce the filtration resistivity, the flocculation of these flocs on the membrane surface increases the filtration resistivity, which increases the water permeation rate of the flocculant. The effect of increasing the water content is offset, and as a result, the water permeation rate decreases with time as in the case where no coagulant is used.

The method of peeling and removing the gel layer by performing back pressure washing of the separation membrane is an effective method for recovering the water permeation rate, but when membrane separation of raw water that has not been subjected to coagulation treatment is performed, Since the colloidal particles form a gel layer on the membrane surface, it is necessary to perform back pressure washing frequently, for example, once every 15 minutes (that is, water passing for 14 minutes, washing for 1 minute). For this reason,
The conventional back pressure washing method has a drawback that the washing frequency is high, the treatment efficiency is poor, and the separation membrane is worn early.

The present invention solves the above-mentioned conventional problems, uses a semipermeable membrane, and uses pressure as a driving force to perform membrane separation of raw water containing an organic substance or an inorganic substance. An object of the present invention is to provide a membrane separation method capable of maintaining and efficiently performing a membrane separation treatment.

[Means for Solving the Problem] In the membrane separation method of the present invention, raw water added with a flocculant is directly (that is, as it is, without pretreatment) being passed through a membrane separation apparatus for membrane separation treatment. The method is characterized in that the separation membrane is backwashed once every 30 minutes to 120 minutes.

In the method of the present invention, backwashing the separation membrane once every 30 to 120 minutes means that the membrane separation treatment (raw water flow) and backpressure backwashing are performed once for 30 to 120 minutes. The cycle of 30 to 120 minutes includes the back-pressure backwash time.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a system diagram of a membrane separation apparatus suitable for carrying out the membrane separation method of the present invention.

The membrane separation device shown in the figure comprises a flocculation reaction tank 1 equipped with a stirrer 1a for coagulating the raw water by adding a coagulant, a circulating water tank 2 for the coagulated raw water, a membrane module 3 equipped with a separation membrane 3a, and a raw water membrane module 3 Pressure circulation pump P for supplying
The membrane module 3 mainly comprises a back pressure pressurizing means 4 and a coagulant supplying means 5. Reference numerals 11 to 20 denote liquid distribution pipes. Reference numeral 21 is a pipe provided so as to bypass the pump P, and V ₁ and V ₂ are valves.

In the apparatus shown in FIG. 1, the back pressure pressurizing means 4 includes the separation membrane 3a.
On the other hand, it is composed of a header extending vertically so that a hydrostatic head can be applied.

In the illustrated method, first, raw water is supplied to a coagulation reaction tank 1 through a pipe 11, and a coagulant and a stirrer 1a supplied through a pipe 12.
Agitation reaction is carried out by stirring at to colloid the colloid components.

The raw water that has been subjected to coagulation treatment is pipe 13, circulating water tank 2, pipes 14,1
After passing through 5, it is supplied to the membrane module 3 by the power of the pressurizing circulation pump P and subjected to membrane separation treatment. In this case, the valve V ₁ may be open or closed.

The permeated water of the membrane module 3 is discharged out of the system as treated water through the pipe 16, the back pressure pressurizing means 4, and the pipe 17. On the other hand, the concentrated water is circulated to the circulating water tank 2 through the pipes 18 and 20. At this time, by opening / closing the valve V ₂ , a part of the concentrated water is discharged to the outside of the system through the pipe 19 as needed.

After continuously performing the membrane separation process of the raw water in this way, back pressure washing of the separation membrane 3a is regularly performed as follows.

That is, the valve V ₁ is opened and the pump P is stopped. By stopping the pump P, the water in the membrane module 3 instantaneously flows back to the circulating water tank 2 side through the pipe 21. That is, in the case of FIG. 1, since the open end of the concentrated water withdrawing pipe 20 is located at a position higher than the water surface level of the circulating water tank 2, a backflow is instantaneously generated due to the head difference.

Also, when the pump P is stopped, the back pressure can be applied to the separation membrane 3a of the membrane module 3 by the header as the back pressure pressurizing means 4. The starting height of this header is not particularly limited, but it is preferable that a back pressure of about 0.1 to 0.5 kg / cm ² can be applied, for example.

When backwashing is performed under back pressure in this way, the gel layer (aggregated floc layer) compressed and deposited on the membrane surface (raw water side) of the separation membrane 3a stops the raw water pump P. As a result, the pressure is released, the compression degree becomes loose, and a high-speed backflow momentarily flows along the surface of the gel layer having the loose compression degree, and the back pressure is applied to the separation membrane 3a. By being added, the gel layer is effectively removed.

After removing the gel layer in this way, the pump P is operated to restart the membrane separation process of the raw water.

In the method of the present invention, the raw water to which the coagulant is added is directly passed through the membrane separation device and then backwashed, so that the backwashing frequency can be significantly reduced as compared with the conventional case. That is, although it depends on the quality of the raw water, it can be set to a very low rate of once every 30 to 120 minutes.

The above description is an example of the present invention, and the present invention is not limited to the above description unless it exceeds the gist of the present invention.

For example, the coagulant may be added directly to the circulating water tank without providing the coagulation reaction tank, or the coagulant may be added to the raw water supply pipe or the circulating water pipe.

Further, the back pressure pressurizing means may be provided with a pressurizing gas supply pipe, a pressurizing pump, etc. other than the one shown in the drawing.

When the back pressure backwash is performed, the water flow may be stopped or continued, but as shown in FIG. 1, a bypass pipe 21 bypassing the raw water pressure circulation pump is provided, and
An extremely high cleaning effect can be obtained by providing a concentrated water extraction pipe that can be opened above the water level in the circulating water tank, stopping the water flow, and instantly causing a reverse flow of the raw water.

In the method of the present invention, the aggregating agent is not particularly limited, for example, polyaluminum chloride, aluminum sulfate,
Iron-containing, aluminum chloride, ferric chloride, poly-iron sulfate, etc. can be used, and various aggregating agents can also be used in combination with various polymer polymers as an aggregating aid.

The addition amount of such a coagulant may be appropriately determined by a jar test or the like within a range where the filtration resistance is minimized, and it is not always necessary to make the addition amount as large as the amount generally adopted in the conventional coagulating sedimentation filtration method. Absent. In the present invention, it is usually preferable that the amount of the coagulant added is about 10 to 200 ppm with respect to the raw water. However, it goes without saying that it is added depending on the raw water quality.

The type of separation membrane used in the method of the present invention is not particularly limited, but is particularly effective for UF membranes and RO membranes.

[Function] For example, the relationship between the water permeation rate J _V and the raw water-soluble concentration C in the case of performing membrane separation using a UF membrane or RO membrane can be expressed by the following equation.

J _V = KlnC _W / C _b …… where k: mass transfer coefficient, C _w : solute concentration on the film surface, C _b : solute concentration in solution, and solute is a dissolved or colloidal substance. Show.

From the equation, it is clear that if k is increased and C _b is decreased, J _V is increased. Among these, increasing k can be achieved by raising the temperature of the solution or promoting turbulent flow, but all of these methods require a large amount of energy and are not always advantageous. Not a method.

On the other hand, as a method for reducing C _b , a method using a coagulant is known, and an effect is recognized for a material containing a colloidal substance. That is, the flocculation of the colloidal substance by the agglutination reaction reduces the apparent C _b .
Because of this, J _v is often reported to increase significantly.

However, when J _v increases remarkably, the velocity toward the membrane near the membrane surface increases, so that the floc-like components suspended in the solution are rapidly attracted to the membrane surface when J _v is small. . Therefore, not the colloidal component but the floc component is accumulated on the film surface. Therefore, there was a problem that high J _v could not be maintained for a long time. The decrease in J _{v at} this time is due to the consolidation of the floc-like component accumulation layer.

By the way, as a method for reducing the decrease in J _v due to the consolidation of the floc-like component storage layer, there is backwashing in which the permeated water is periodically backflowed from the permeated water side of the membrane surface. This method has many achievements in microfiltration membranes (MF), and is effective as a method for peeling an accumulation layer of coarse particles, that is, a cake layer from the membrane surface. However, those mainly composed of colloidal components, which are fine particles, are accumulation layers having a high concentration, which promotes association between the colloidal particles and forms a strong gel layer having a large binding force between the particles. For this reason, it is often the case that the effect of backwashing cannot be expected with respect to those containing mainly colloidal components. For this reason, conventionally, after long-term use, a means for performing chemical cleaning to remove the gel layer on the film surface has been required.

Currently, commercially available UF or RO membranes are negatively charged in water even if they are polysulfone membranes having no electric charge. Many particles in water are negatively charged, so J _v
When is small, the clonal repulsive force of the same charge can reduce the accumulation of particles on the membrane surface.
However, such J _v is extremely small,
It cannot be put to practical use. On the other hand, when driving force such as pressure is increased to increase J _v , Coulomb repulsive force causes
The colloidal particles come closer to each other at a speed higher than the speed of trying to move away from the vicinity of the film surface, and as a result, a strong gel layer is formed. On the other hand, commonly used flocculants have an electric charge opposite to that of colloidal particles in water. That is, since an inorganic flocculant such as Al or Fe salt produces a metal hydroxide in water and the periphery of this metal hydroxide is positively charged, it is possible to adsorb negatively charged colloidal particles in water. , Can make floc.

When separating water containing positively charged agglomerated flocs using a separation membrane that is negatively charged in water, J _v is large and strong agglomerated flocs on the membrane surface due to Coulomb attraction. Layers are formed. Generally, in the method of directly performing membrane separation of raw water that has been flocculated using a flocculant, it is possible to carry out continuous operation for a certain period of time because of the high J _v initially obtained. However, if water is passed for a longer period of time, the flocculation layer is compacted and concentrated, so that flocs associate with each other to form a strong gel layer. As a result, due to the resistance of this layer,
J _v decreased, and it was necessary to remove this layer by chemical cleaning or the like.

In view of these things, the present inventor examined the method of continuing _Jv obtained by using an aggregating agent.

Since many commercially available separation membranes are negatively charged in water, Al and
It is unavoidable that flocs aggregated with Fe salt adhere to the film surface. The first layer of this floc adhered to the membrane surface by Coulomb attraction cannot be removed by means such as backwashing.

Layers of aggregated flocs that deposit and grow on the membrane surface associate with time, coarsening of the flocs occurs, and eventually aggregates over the entire area of the membrane surface. When backwashing is performed at a high speed, the gel layer associated with the floc particles can be removed. However, in the case of a UF membrane or RO membrane, if back pressure is applied at a rate higher than the filtration rate, the membrane will break and liquid cannot be passed. For this reason, when performing backwashing on dense membranes such as UF membranes and RO membranes, the rate must be set to a value smaller than the filtration rate. In such a UF membrane or RO membrane, which can be backwashed only with a weak force, the gel layer with advanced association cannot be removed.

On the other hand, at the time of the aggregation of the flocs that proceed on the membrane surface,
The action of incorporating unaggregated colloidal particles also occurs. That is, unaggregated colloidal particles are adsorbed in the process of progressing association to form a strong gel-like layer. Therefore,
If the flocculation flocs are moved away from the membrane surface by backwashing before the association progresses to some extent, the unaggregated colloidal particles are left on the membrane surface, and finally the accumulation of colloidal fine particles. Will form a layer.

Therefore, by using a flocculant, C _b can be reduced, and by regular backwashing, C _w can be reduced to reduce J _v.
The backwash interval is an important factor in maintaining a high value of.

As a result of examining many cases of using a flocculant, the present inventors have found that the backwashing interval can maintain a high value for J _v from 30 minutes to 2 hours.

By regularly backwashing at such intervals, the coarse flocculated flocs containing unaggregated colloidal substances that have progressed to a certain degree of association are removed, except for the strong adhesion layer adhering to the membrane surface. Can be moved away from the plane. As a result, C _b can be made small and C _w can be made small, and J _v can be kept high without consuming a large amount of energy.

[Examples] The present invention will be described more specifically with reference to Examples.

Example 1 Polyaluminum chloride (PAC) 1 in raw water (Lake Sagami)
00 mg / l was added, and the suspension in which agglomerated flocs were generated was
Water was passed and back pressure was backwashed under the following conditions. FIG. 2 shows the change with time of the flux.

Processing conditions Separation membrane: UF membrane (Polysulfone UF membrane: molecular weight cut off 20,000) Membrane module: Spiral module with wavy spacer (3mm height) inserted.

Membrane circulation average flow velocity: 0.93 m / sec Normal pressure force: 1.26 kg / cm ² Back pressure backwash pressure: 0.17 kg / cm ² Operating cycle: 27 minutes water flow / 3 minutes intermittent backwash backwash intermittent operation Comparative example 1 Example 1 except that back pressure backwash was not performed and continuous operation was performed
The same treatment as described in (1) and (2) was performed and the change in flux with time is shown in FIG.

Comparative Example 2 The same treatment as in Example 1 was carried out except that PAC was not added, and the change in flux with time is shown in FIG.

As is clear from FIG. 2, according to the method of the present invention, it is possible to obtain a flux which is about three times as high as that of the conventional method.

Example 2 Kaolin (5 mg / l) and PAC (30 mg / l) were added to raw water (Atsugi city water), and the suspension in which agglomerated flocs were generated was subjected to water flow and back pressure backwash under the following conditions. FIG. 3 shows the change with time of the flux.

Treatment conditions Separation membrane: Low pressure RO membrane (polysulfone-Poval composite membrane) Membrane module: The same as that used in Example 1.

Membrane circulation average flow velocity: 0.93 m / sec Normal pressure force: 10 kg / cm ² Back pressure backwash pressure: 0.17 kg / cm ² Operating cycle: 27 minutes water flow / 3 minutes intermittent backwash backwash intermittent operation Comparative Example 3 The same treatment as in Example 2 was carried out except that PAC was not added and that backwashing was not carried out, and changes in flux with time are shown in FIG.

As is apparent from FIG. 3, according to the method of the present invention, it is possible to obtain about twice the flux as compared with the conventional method.

Example 3 Polyaluminum chloride (PA) was added to raw water (river water of the Sagami River).
C) 100 mg / l was added, and the suspension in which coagulated flocs were generated was subjected to water flow and back pressure backwash under the following conditions. The relationship between the backwash cycle and the average flux was investigated, and the results are shown in FIG.

Treatment conditions Separation membrane: UF membrane (polysulfone UF membrane: molecular weight cut off of 20,000) Membrane module: Same as that used in Example 1.

Membrane circulation average flow velocity: 0.93 m / sec Average pressure: 1.28 kg / cm ² Back pressure Backwash pressure: 0.17 kg / cm ² Operating cycle: Comparative Example 4 A treatment was performed in the same manner as in Example 3 except that PAC was not added, and the relationship between the backwash cycle and the average flux was examined, and the results are shown in FIG. Further, the average flux was also examined for those not backwashed (backwash cycle = 0 min), and the results are shown in FIG.

From FIG. 4, it is clear that a good flux can be obtained in the backwash cycle of 30 to 120 minutes.

[Effects of the Invention] As described in detail above, according to the membrane separation method of the present invention, when the raw water that has been subjected to the coagulation treatment is directly passed through the membrane separation device to perform the membrane separation treatment, the membrane surface of the flocculation floc is Can be prevented from consolidating and a dramatically higher water permeation rate can be maintained as compared with the case of the conventional coagulation treatment only or the back pressure backwash treatment only. Moreover, since the backwashing frequency can be kept low, the number of processing operations is reduced, and the processing operation is simplified, so that the processing cost is reduced and the life of the separation membrane is extended by preventing the wear of the separation membrane. Is possible.

[Brief description of drawings]

FIG. 1 is a system diagram of a membrane separation apparatus suitable for carrying out the membrane separation method of the present invention, FIG. 2 is a graph showing the results of Example 1, Comparative Example 1 and Comparative Example 2, and FIG. 3 is Example 2 And a graph showing the results of Comparative Example 3, and FIG. 4 is a graph showing the results of Example 3 and Comparative Example 4. 1 ... Aggregation reaction layer, 2 ... Circulating water tank, 3 ... Membrane module, 3a ... Separation membrane, 4 ... Back pressure applying means.

Claims (1)

[Claims] 1. A method for membrane separation treatment in which raw water containing a coagulant is directly passed through a membrane separator to perform back separation backwashing of the separation membrane once every 30 minutes to 120 minutes. Characteristic membrane separation method.