JP5131005B2 - Water treatment method and water treatment apparatus - Google Patents

Description

  The present invention relates to a water treatment method in which a coagulation treatment is followed by a filtration treatment with a membrane.

  Filtration with porous filtration membranes has features such as energy saving, space saving, labor saving and quality improvement of treated water, so it can be used in various fields such as food industry, medical field, water production field, water purification treatment field, wastewater treatment field, etc. Applications in various fields are expanding.

  However, if raw water is filtered through a membrane as it is, removal objects such as turbidity, organic matter, and inorganic matter contained in the raw water accumulate on the membrane surface, resulting in membrane clogging (membrane fouling). As a result, the filtration resistance of the membrane increases, and it becomes impossible to continue the filtration over time. Therefore, in order to maintain the filtration performance of the membrane, it is necessary to periodically wash the membrane. As a membrane washing method, reverse flow washing (hereinafter referred to as “back washing”) in which the membrane filtrate is caused to flow backward from the secondary side (filtrated water side) of the membrane to the primary side (raw water side), or gas is used as the primary membrane. There is physical cleaning such as air cleaning (hereinafter referred to as “air cleaning”) that is supplied to the side to remove the dirt on the membrane. When the rising speed of the filtration resistance of the membrane is fast, it is necessary to increase the frequency of backwashing, but there is a problem that an increase in the frequency of backwashing leads to a decrease in the recovery rate. Further, when the film fouling adheres firmly to the film surface, it is necessary to increase the amount of washing, but there is a problem that the abrasion and deterioration of the film are promoted when the amount of washing is increased.

  On the other hand, for dirt that cannot be removed only by physical cleaning, chemical cleaning is performed in which a chemical solution containing acid, alkali, surfactant, oxidant such as chlorine is brought into contact with the film for a certain period of time. However, when chemical cleaning is performed, there is a problem that the deterioration of the film is accelerated and the film life is shortened. Disposal of waste liquid after chemical cleaning is also a problem.

  In order to increase the efficiency of cleaning by these methods and reduce the frequency, it is effective to perform pretreatment before membrane filtration to reduce membrane fouling, and it is effective to perform agglomeration treatment as this pretreatment. It is.

  When aggregating is performed as a pretreatment for membrane filtration, it is important to appropriately determine the aggregating conditions such as the type of aggregating agent and the amount of aggregating agent injected. Therefore, Patent Document 1 uses the fact that the ultraviolet absorbance at a specific wavelength is an indicator of the concentration of dissolved organic matter, and measures the ultraviolet absorbance of the membrane filtrate extracted from the agglomerated treated water by membrane filtration. It is described that the amount of the flocculant injected is controlled based on the value. Patent Document 2 measures the organic substance concentration in the raw water and controls the amount of the flocculant injected based on the measured value. Patent Document 3 measures the organic substance concentration in the agglomerated treated water and measures this. It is described that the flocculant injection amount is controlled based on the value. However, since the causative substance of the film fouling is not only the organic substance, the progress of the film fouling cannot be sufficiently suppressed only by paying attention to the organic substance.

  Patent Document 4 describes that the amount of flocculant injected is controlled from the time required to filter a predetermined amount of sample water obtained by solid-liquid separation of flocculated water using a filter. However, when filtration is continued for a long period of time, it is difficult to control the accumulation of membrane fouling only by the index and to operate stably for a long period of time.

  Further, in the conventional flocculation sedimentation sand filtration method, since the zeta potential of the flocculation flocs indicates suitability of flocculation, this is measured and the flocculation treatment conditions are controlled. In order to carry out the coagulation sedimentation process well in the conventional coagulation sedimentation sand filtration method, it is preferable to control the zeta potential of the coagulation floc to be in the vicinity of zero mV, that is, between −10 mV to +10 mV. Has been. However, when agglomeration treatment is applied as a pretreatment for membrane filtration, the accumulation of membrane fouling cannot be suppressed by controlling the zeta potential of the agglomeration floc within the above range, and stable operation for a long period of time. It was difficult.

Japanese Patent No. 3312507 JP 2007-203249 A JP 2007-245078 A JP 2006-055804 A

  The present invention is a water treatment method in which raw water is agglomerated and then filtered with a porous filtration membrane, and when filtration is performed with a porous filtration membrane having a negative membrane surface zeta potential at agglomeration pH, the water is stably operated for a long time. Therefore, an object of the present invention is to provide an effective control method for agglomeration treatment conditions.

In order to achieve the above object, the water treatment method of the present invention has the following characteristics.
(1) In a water treatment method in which raw water is agglomerated and then filtered with a porous filtration membrane, when filtration is performed with a porous filtration membrane having a negative membrane surface zeta potential at agglomeration pH, agglomerated flocs in the agglomerated water The water treatment method is characterized in that the agglomeration treatment conditions are controlled such that the zeta potential is determined and the zeta potential becomes negatively charged.
(2) The water treatment method according to (1) above, wherein the coagulation treatment conditions are controlled so that the zeta potential of the coagulation floc in the coagulation treatment water is within a range of −10 mV to less than 0 mV.
(3) The water treatment method according to (1) or (2) above, wherein the aggregating treatment condition controlled based on the zeta potential of the agglomerating floc is an aggregating agent injection amount and / or agglomerating pH.

(4) The water treatment method according to any one of (1) to (3), wherein the zeta potential of the aggregated floc is calculated from the flow potential of the aggregated treated water.
(5) The water treatment method according to any one of (1) to (4) above, wherein filtration is performed with a porous filtration membrane having a membrane surface zeta potential at flocculation pH of -15 mV or more and less than 0 mV. .
(6) The water treatment method according to any one of (1) to (5), wherein a porous filtration membrane made of a polyvinylidene fluoride-based resin is used.
(7) An aggregating apparatus for adding an aggregating agent to the raw water for an aggregating treatment , and a porous filtration having a negative membrane surface zeta potential at an agglomerating pH for filtering the agglomerated water in the aggregating apparatus comprising a multi Anashitsumaku module consisting of film, and a potential measuring device for measuring the streaming potential of the zeta potential or aggregation treated water floc aggregation process water, furthermore, floc zeta by measurement of said potential measuring device water treatment apparatus characterized by comprising a coagulation treatment condition control device for controlling the agglomeration process conditions in agglutination processing apparatus as the zeta potential of the floc is negatively charged on the basis of the potential value.

  If the agglomeration treatment conditions are controlled by the method of the present invention, fouling of the porous filtration membrane can be suppressed, and the membrane module can be stably operated for a long time.

  The best mode for carrying out the method of the present invention will be described below with reference to FIG. 1, taking as an example the case of membrane filtration using a pressurized hollow fiber membrane module filtration device. However, the present invention is not limited to the embodiment described below.

  FIG. 1 is a schematic flow diagram showing a membrane filtration apparatus to which the method of the present invention is applied. In the process of this figure, membrane filtration by the pressure type hollow fiber membrane module 10 is performed. In this figure, the raw water stored in the raw water tank 1 is introduced into the rapid stirring tank 3 by the intake pump 2. In the rapid stirring tank 3, the raw water is stirred and agglomerated by the agitator 6 together with the aggregating agent injected from the aggregating agent solution storage tank 4 by the aggregating agent injection pump 5. In order to control the aggregation pH, it is preferable to inject the pH adjusting solution from the pH adjusting solution storage tank 7 by the pH adjusting solution injection pump 8.

  The agglomerated water that has been agglomerated in the rapid stirring tank 3 is supplied to the hollow fiber membrane module 10 by the supply pump 9. The agglomerated water is filtered from the hollow fiber membrane primary side (raw water side) to the hollow fiber membrane secondary side (permeate water side) by the hollow fiber membrane housed in the hollow fiber membrane module 10 and stored in the filtered water tank 11. It is done.

  In order to measure the characteristics of the agglomerated treated water, the membrane feed water sampling valve 12 is opened as appropriate, the agglomerated treated water is sampled from the membrane feed water sampling pipe 13, and the zeta potential of the agglomerated floc is measured. Based on the zeta potential of the aggregation floc, the aggregation processing conditions are controlled.

  Zeta potential refers to the electrical potential that exists across the solid-liquid interface and refers to the surface charge for colloidal particles in water. Usually, since colloidal particles contained in natural water are negatively charged, the particles repel each other electrically and are dispersed in water. The aggregating agent weakens the repulsive force by neutralizing this charge, and then agglomerates, that is, agglomerates. The zeta potential also changes depending on the aggregation pH. For this reason, controlling the aggregation pH with an acid or alkali is also effective for the aggregation treatment. The agglomeration pH is the pH of the agglomerated water after injecting the aggregating agent.

  The zeta potential of the aggregated floc can be measured from the moving speed of the aggregated floc by electrophoresis using a surface potential measuring device such as an electrophoretic light scattering device (ELS-8000: manufactured by Otsuka Electronics Co., Ltd.). Also, the zeta potential of the floc floc is calculated using the Helmholtz-Smoluchowski equation (the following equation) from the flow potential obtained by measuring the potential generated between the electrodes when the agglomerated treated water is swept away at a constant pressure. It can also be determined by the method of

  The streaming potential can be measured by a streaming potential measuring device such as a colloid particle charge meter (manufactured by Nippon Luft). For zeta potential measurement and streaming potential measurement, it is preferable to use a potential measurement device capable of continuous measurement online. Furthermore, it is more preferable to use an electric potential measuring device capable of continuously measuring raw water without dilution.

  The position at which the zeta potential of the floc floc is measured, that is, the place where the measurement sample is sampled may be collected anywhere after the addition of the flocculant and before the membrane filter module. Since the properties of the flocs flocs may change before the supply to the membrane module 10, it is preferable to collect water at a position as close as possible (immediately before) to the filtration membrane module 10. In order to collect water, it is preferable to branch the membrane supply water sampling pipe 13 and provide the membrane supply water sampling valve 12.

  It is preferable that the agglomeration treatment conditions to be adjusted according to the zeta potential so that the zeta potential of the agglomeration floc is negatively charged is the aggregating agent injection amount and / or the agglomeration pH.

  Examples of the inorganic flocculant injected when the raw water is agglomerated include aluminum salts such as sulfate bands and polyaluminum chloride (PAC), and iron salts such as ferric chloride and polyferric sulfate. Can be used. In addition to these inorganic flocculants, an organic polymer flocculant can be used in combination as a flocculant auxiliary agent, and by using this combination in combination, extremely good flocculation treatment may be performed. In order to adjust the pH, an inorganic acid solution such as sulfuric acid, hydrochloric acid, or nitric acid, an inorganic alkaline solution such as sodium hydroxide, sodium carbonate, or sodium bicarbonate may be added.

  As the tank for coagulation and stirring, only the rapid stirring tank 3 may be used as shown in the figure, but a slow stirring tank may be installed at the subsequent stage of the rapid stirring tank. As the agitation means, a general agitation means such as an agitation means using an agitation blade or an agitation means bypassing the water channel may be used. Moreover, you may stir raw | natural water and a coagulant | flocculant using a pump or a static mixer in piping, without providing a stirring tank.

  Further, the agglomerated water may be supplied to the filtration membrane module after removing suspended substances by precipitation treatment, pressure levitation treatment and filtration using sand or other filter media.

  The raw water to be treated in the method of the present invention includes natural water such as river water, lake water, ground water, underground water, various factory waste water, agricultural waste water, sewage or treated water, sea water, and the like. In the case where it is difficult to increase the electric field strength and it is difficult to measure the zeta potential as in the case of processing highly conductive water such as seawater, the measurement may be performed after dilution.

  The porous membrane used in the filtration membrane module in the present invention is not particularly limited in pore diameter on the membrane surface, and may be a microfiltration membrane or an ultrafiltration membrane, but 0.001 μm to 10 μm The pore diameter in the range may be selected for convenience. Further, the membrane form may be a flat membrane or a hollow fiber membrane. As long as the film has the removal performance, it may be either a homogenous film in which similar structures are continuous in the thickness direction of the film or an asymmetric film in which two or more kinds of structures are laminated. Examples of the material for the porous film include organic materials such as polyacrylonitrile, polysulfone, polyether sulfone, polyphenylene sulfone, polyphenylene sulfide sulfone, polyvinylidene fluoride, and polypropylene, and inorganic materials such as ceramic.

  Although not particularly limited in view of the gist of the present invention, the effect of the present invention is easily exhibited in the case of a porous film made of a hydrophobic polyvinylidene fluoride resin, which is generally said to cause film fouling. . The polyvinylidene fluoride resin is a polymer having a vinylidene fluoride residue structure as a main unit, that is, a resin containing a vinylidene fluoride homopolymer and / or a vinylidene fluoride copolymer. A plurality of types of vinylidene fluoride copolymers may be contained. Examples of this copolymer include copolymers of vinylidene fluoride monomer and other fluorine-based monomers.

  Laser doppler electrophoresis is preferably used for measuring the zeta potential of the porous membrane used in the present invention. The measuring means of the laser Doppler electrophoresis method itself is not particularly limited, but it is preferable to use, for example, an electrophoretic light scattering apparatus (ELS-8000: manufactured by Otsuka Electronics Co., Ltd.) as the measuring apparatus. The membrane surface zeta potential in the present invention is a membrane surface zeta potential obtained by measuring after holding an unused filtration membrane in an aqueous solution having a predetermined pH for 10 minutes before measurement.

  Usually, membrane fouling components contained in natural water are often negatively charged. Therefore, in order to suppress membrane fouling by electrostatic adsorption, the membrane surface zeta potential at the aggregation pH may be negative. It is effective. On the other hand, when the membrane surface potential is smaller than −15 mV (larger on the negative side), electrostatic adsorption with the positively charged membrane fouling component is facilitated. Therefore, in order to suppress membrane fouling, the membrane surface zeta potential at the aggregation pH is more preferably −15 mV or more and less than 0 mV.

  In the present invention, from the viewpoint of electrostatic action with the negatively charged porous filtration membrane, it is important to control the aggregation treatment conditions so that the zeta potential of the aggregation floc becomes negative. It is preferable that the zeta potential of the aggregated floc is −10 mV or more and less than 0 mV. When the zeta potential of the aggregation floc is less than −10 mV, charge neutralization does not occur sufficiently, and the colloidal particles float while maintaining the repulsive force, so that aggregation is likely to be insufficient, and membrane fouling can be sufficiently suppressed. Have difficulty. Further, when the zeta potential of the aggregated floc becomes 0 mV or more, the adhesion force to the membrane having a negative membrane surface zeta potential is rapidly increased, and fouling is promoted. More preferably, the zeta potential of the aggregated floc is in the range of −5 ± 2.5 mV. This is because if the zeta potential of the aggregated floc is controlled too close to 0 mV, there is a risk that the control of the zeta potential of the aggregated floc will erroneously become 0 mV or more due to changes in the raw water quality, and the zeta potential of the aggregated floc will be 0 mV or more. This is because membrane fouling is likely to occur rapidly.

  The water treatment method of the present invention is preferably performed using the water treatment apparatus of the present invention described below. An example of the water treatment apparatus of the present invention is shown in FIG. In FIG. 2, the solid line arrows indicate the flow of the water to be treated, the injection liquid, and the treated water, and the dotted line indicates the flow of information transmission for feedback control.

  The water treatment apparatus of the present invention includes a porous membrane module 10, and includes an aggregation treatment device 14 on the upstream side of the porous membrane module 10. The coagulation treatment apparatus 14 may be provided with at least a coagulant adding means for adding the coagulant to the raw water and an agitation means for mixing the raw water and the coagulant after the addition. In the case of FIG. 2, a flocculant solution storage tank 4, a flocculant injection pump 5 and an injection pipe extending therefrom are provided as a flocculant addition means, and a rapid agitation tank 3 and a stirrer 6 are provided as an agglomeration treatment tank. Yes.

  Further, a zeta potential measuring device 15 is provided for measuring the zeta potential of the coagulation floc in the coagulation treated water from before the coagulation treatment device to the filtration membrane module 10. A part of the agglomerated treated water is collected by the zeta potential measuring device bypass 16 and supplied to the zeta potential measuring device 15 to automatically measure the zeta potential of the agglomerated floc.

  This water treatment device includes a coagulation treatment condition control device 17 that controls the coagulation treatment conditions of the coagulation treatment device based on the zeta potential of the coagulation floc. The zeta potential of the coagulation floc can be adjusted by changing the injection amount of the coagulant, the type of the coagulant, the pH of the coagulation water, and the like. Therefore, when the measured value of the zeta potential of the flocs is outside the range of the present invention, for example, an operation for changing the amount of flocculant added or the amount of pH adjustment solution injected may be performed by the flocculant treatment condition controller 17. Good.

  In order to change the injection amount of the flocculant, feedback control is performed so that the discharge amount of the flocculant injection pump 5 falls within the zeta potential range of the present invention. When the zeta potential value is smaller than the range of the present invention, the flocculant injection amount may be increased, and when the zeta potential value is large, the flocculant injection amount may be decreased. Further, in order to change the injection amount of the pH adjustment solution, feedback control is performed so that the discharge amount of the pH adjustment solution injection pump 8 is within the zeta potential range of the present invention. In this case, when the value of the zeta potential is smaller than within the scope of the present invention, the pH of the pH adjustment solution is adjusted so that the pH of the condensed water is shifted to the acidic side, and when the value of the zeta potential is large, the pH is adjusted to the alkaline side. Control is sufficient.

  As control of the coagulation treatment condition, feedback control of only one of the change of the coagulant injection amount and the change of the injection amount of the pH adjusting solution described above may be performed, or both may be feedback controlled. Further, the flow potential of the flocculated water may be continuously measured online and feedback controlled.

  The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples.

  In order to investigate the generation degree of membrane fouling due to the difference in the coagulation treatment conditions, the coagulation treatment water was filtered through a membrane, the membrane was then washed, and the filtration resistance after washing was examined. It shows that the smaller the filtration resistance after washing, the easier it is for the fouling to peel off from the membrane and membrane fouling is less likely to occur. Details of the test are shown below.

Using a filter holder for vacuum filtration (ADVANTEC), the agglomerated water was filtered at a constant pressure (60 kPa) with a polyvinylidene fluoride resin MF flat membrane (pore size 0.08 μm, diameter 47 mm). After 50 ml of flocculated water was filtered, the flat membrane was washed, and then 100 ml of control water (pure water) was filtered at 60 kPa. The time t ₁ required for filtering the control water and the water temperature T _{1 of the} control water were measured.

In addition, before conducting a filtration test in advance, using the flat membrane to be used, 100 ml of control water was filtered at 60 kPa, and time t ₂ and water temperature T ₂ required for filtration were measured. As control water, pure water (reverse osmosis membrane-treated water: electric conductivity of 15 μS / cm or less) was used. The flat membrane was washed using a method of spraying pure water (RO-treated water: electric conductivity of 15 μS / cm or less) on the flat membrane for about 10 seconds using a washing bottle.

The filtration resistance after washing was calculated using the following formula (1).

In the above formula (1), R _m is a value calculated from the following formula (2).

Here, t ₁ : time required to filter 100 ml of control water after washing,
t ₂ : Time required to filter 100 ml of control water in an unused membrane.
Further, the membrane area in the formula is the area of the flat membrane used for filtration, and the viscosity and density are the viscosity and density of water at each water temperature (T ₁ , T ₂ ).

<Experimental example 1>
Aggregated treated water is 1 mg-Fe / L of ferric chloride (FeCl ₃ ) as a flocculant added to 1 L of sewage secondary treated water, and rapidly stirred at 150 rpm using a jar tester without adjusting pH. Prepared by running for minutes. The coagulation pH of this coagulation-treated water was 6.5, and the zeta potential of the coagulation floc in the coagulation-treated water was −5.4 mV. The membrane surface zeta potential at pH 6.5 was -9.7 mV.

This agglomerated water was put in a filter holder, filtered and washed in the above procedure, and the filtration resistance was measured. In this case, since the relationship of the zeta potential was within the range of the present invention, the filtration resistance after washing was as small as 1.6 × 10 ¹⁰ (m ⁻¹ ), and membrane fouling was suppressed.

<Experimental example 2>
Aggregated treated water is 10 mg-Fe / L of ferric chloride (FeCl ₃ ) as a flocculant added to 1 L of sewage secondary treated water, and rapidly stirred at 150 rpm using a jar tester without adjusting pH. Prepared by running for minutes. The coagulation pH of the coagulation-treated water was 5.5, and the zeta potential of the coagulation floc in the coagulation-treated water was 3.5 mV. The membrane surface zeta potential at pH 5.5 was -5.5 mV.

This agglomerated water was put in a filter holder, filtered and washed in the above procedure, and the filtration resistance was measured. In this case, since the relationship of the zeta potential was outside the range of the present invention, the filtration resistance after washing was 1.3 × 10 ¹¹ (m ⁻¹ ), which is 8.2 times that in the case of Experimental Example 1. The membrane fouling was great.

<Experimental example 3>
Agglomerated treated water is added with 0.5 mg-Fe / L of ferric chloride (FeCl ₃ ) as a coagulant to 1 L of sewage secondary treated water, and rapidly stirred at 150 rpm using a jar tester without adjusting pH. For 5 minutes. The coagulation pH of the coagulation-treated water was 6.9, and the zeta potential of the coagulation floc in the coagulation-treated water was −12.9 mV. The membrane surface zeta potential at pH 6.9 was -11.4 mV.

This agglomerated water was put in a filter holder, filtered and washed in the above procedure, and the filtration resistance was measured. The zeta potential in this case was negatively charged but was less than −10 mV, so the filtration resistance after washing was 4.4 × 10 ¹⁰ (m ⁻¹ ), which was 2.8 times that in the case of Experimental Example 1. It was.

  From the above experimental examples, when filtration is performed with a porous filtration membrane having a negative membrane surface zeta potential, in particular, the zeta potential of the aggregated floc in the agglomerated treated water becomes negatively charged so that it is −10 mV or more and less than 0 mV. It can be seen that controlling the agglomeration treatment conditions so as to be within the range is effective in suppressing the generation of membrane fouling and the increase in filtration resistance.

  The method of the present invention is suitable for membrane filtration of natural water such as river water, lake water, groundwater, underground water, various factory wastewater, agricultural wastewater, sewage or treated water, seawater, and the like with a membrane module. Available.

It is a schematic flowchart which shows the membrane filtration process to which this invention method is applied. It is the schematic which shows an example of the water treatment apparatus for implementing this invention.

Explanation of symbols

1: Raw water tank 2: Intake pump 3: Rapid stirring tank 4: Flocculant solution storage tank 5: Flocculant injection pump 6: Stirrer 7: pH adjustment solution storage tank 8: pH adjustment solution injection pump 9: Supply pump 10: Filtration membrane module 11: Filtration water tank 12: Membrane supply water sampling valve 13: Membrane supply water sampling tube 14: Coagulation treatment device 15: Zeta potential measurement device 16: Bypass for zeta potential measurement device 17: Coagulation treatment condition control device

Claims (7)

In the water treatment method in which the raw water is agglomerated and then filtered with a porous filtration membrane, the zeta potential of the agglomerated floc in the agglomerated treated water is used for filtration with a porous filtration membrane having a negative membrane surface zeta potential at the agglomeration pH. And the coagulation treatment conditions are controlled so that the zeta potential becomes negatively charged. The water treatment method according to claim 1, wherein the coagulation treatment conditions are controlled so that the zeta potential of the coagulation floc in the coagulation treatment water is within a range of -10 mV or more and less than 0 mV. The water treatment method according to claim 1 or 2, wherein the coagulation treatment condition controlled based on the zeta potential of the coagulation floc is the coagulant injection amount and / or the coagulation pH. The water treatment method according to claim 1, wherein the zeta potential of the aggregated floc is calculated from the flow potential of the aggregated treated water. The water treatment method according to any one of claims 1 to 4, wherein filtration is performed with a porous filtration membrane having a membrane surface zeta potential of -15 mV or more and less than 0 mV at the aggregation pH. The water treatment method according to claim 1, wherein a porous filtration membrane made of a polyvinylidene fluoride resin is used. A coagulation treatment device for adding an aggregating agent to raw water for coagulation treatment and a porous filtration membrane having a negative membrane surface zeta potential at the coagulation pH for filtering water coagulated by the coagulation treatment device and a multi Anashitsumaku module, and a potential measuring device for measuring the streaming potential of the zeta potential or aggregation treated water floc aggregation process water, furthermore, the flocs zeta potential levels of the measurement of said potential measuring device water treatment apparatus characterized by zeta potential of the floc is provided with a coagulation treatment condition control device for controlling the agglomeration process conditions in agglutination processing apparatus so that the negative charge on the basis of.

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