JP6441712B2 - Method for evaluating membrane clogging of treated water - Google Patents

Description

  The present invention relates to a method for evaluating the degree of blockage of water to be treated.

  Membrane filtration devices using ultrafiltration membranes or microfiltration membranes are provided with a separation membrane in the pressure vessel, and the separation membrane separates the vessel into the raw water side and the permeate side (filtrated water side), and the raw water side The raw water is pressurized and introduced with a pump, and permeate is obtained from the permeate side by membrane filtration.

In such a membrane filtration apparatus, the components contained in the raw water adhere to and deposit on the raw water side membrane surface and membrane pores of the separation membrane, the separation membrane is contaminated, and the filtration performance gradually decreases.
Organic substances are the most important causative substances for film contamination as causative substances for film contamination. Total organic carbon (TOC) is used as the water quality indicator, but even when the raw water has the same TOC value, the rate of increase in the filtration resistance of the membrane is often different, and the actual operation and design expectations differ greatly. Therefore, there are not a few troubles such as frequent chemical cleaning.
The same applies to the operation management, and there is a case where the film contamination rapidly progresses even though the TOC concentration does not change. This is because the organic matter that causes film contamination is an extremely small part of the TOC component, and its concentration is low, so that the change cannot be detected even if the TOC is measured.

  As a method for evaluating the membrane blockage of the separation membrane supply water in the overall rather than the individual water quality such as TOC, there is a method using the fouling index (FI value) defined in JIS K3802, etc. This index is basically an index that assumes the evaluation of particles having a particle size of 0.45 μm or more, and the soluble organic substance that is a causative substance for film contamination cannot be evaluated.

In addition, as a related conventional method, for example, Patent Document 1 discloses a membrane permeation flow rate, a physical washing interval, a chemical washing from a function of a turbid mass of membrane feed water, a measured value of dissolved organic carbon (DOC), and a membrane permeation flow rate. A method for optimizing the interval, preprocessing conditions and the like is described. However, in this method, it is necessary to analyze DOC, ultraviolet absorbance (E260), and turbidity, and since it uses a relatively difficult theoretical formula, it is complicated and not versatile.
In addition, this method limits the cause of contamination from organic components to humic substances, and simply calculates the rate of progress of membrane contamination from the ratio of DOC and E260, so that organic components other than humic substances are involved in membrane contamination. Cannot be evaluated correctly.
In recent studies, it has been clarified that an important organic substance involved in membrane contamination is a polysaccharide rather than a humic substance that expresses E260. In that sense, the method described in Patent Document 1 is It lacks validity.

  Further, Patent Document 2 and Patent Document 3 also describe that the coagulation treatment and the like are controlled based on humic substances such as raw water, membrane supply water, and membrane filtered water, and ultraviolet absorbance (E260). This is a driving method that lacks rationality.

This inventor repeated earnest research and developed "Fouling Potential (FP)" which is a new parameter | index regarding the film | membrane obstruction | occlusion organic substance in film | membrane supply waters, such as raw water supply of a nonpatent literature 1-4.
This index is an index that gives useful information regarding the abundance and molecular weight of the membrane clogging organic matter in the raw water supply water or the membrane supply water that has been pretreated therefor, that is, polysaccharides.

JP 2001-327967 A JP 2001-170458 A JP 2008-126223 A

Koji Kashimada and one other, “Proposal of Fouling Potential in Water Treatment and Its Characteristic Evaluation (I)”, Proceedings of the 60th National Waterworks Research Conference, Japan Water Works Association, May 2009, p. 134-135 Yoshihide Kaitani and one other, “Proposal of Fouling Potential in Water Treatment and Its Characteristic Evaluation (II)”, Proceedings of the 60th National Waterworks Research Conference, Japan Waterworks Association, May 2009, p. 136-137 Yoshihide Kaitani and one other, “Proposal of Fouling Potential in Water Purification and Its Characteristic Evaluation (III)”, 61st National Waterworks Research Conference Lecture, Japan Waterworks Association, May 2010, p. 252-253 Yoshihide Kaitani and one other, “Proposal of Fouling Potential in Water Treatment and Its Characteristic Evaluation (IV)”, 62nd National Waterworks Research Presentation Lecture, Japan Waterworks Association, May 2011, p. 352-353

  However, the present inventor has found that detection may be difficult when the concentration of the membrane occluding substance is extremely low or depending on its characteristics.

  That is, an object of the present invention is to provide a method capable of evaluating the membrane occluding property with high sensitivity even when the concentration of the membrane occluding material contained in the water to be treated such as raw water for tap water is low.

  The present inventor has conducted extensive research, and that the polysaccharide-like substance is a substance group that has a much more significant effect on membrane occlusion than the E260-expressing substance, the fouling potential (FP), etc., that is, raw water and membrane It has been found that there is a good correlation with the blockage of the feed water, not the abundance of the E260-expressing substance but the abundance of the polysaccharide-like substance. Further, the present invention has been completed by finding a method capable of satisfactorily measuring even when the concentration of a membrane occluding substance is extremely low, or for a substance group that is difficult to detect with FP or the like.

The present invention is a method for evaluating the degree of clogging of water to be treated, comprising the step of further filtering a liquid having a lower salt concentration and / or pH than the water to be treated after filtering the water to be treated using a separation membrane. is there.
Hereinafter, such a method for evaluating the degree of membrane occlusion is also referred to as “the evaluation method of the present invention”.

  In the evaluation method of the present invention, the liquid having a low salt concentration and / or pH is preferably pure water.

  In the evaluation method of this invention, it is preferable that the said to-be-processed water is seawater, a tap raw water, or biological treatment water.

In the evaluation method of the present invention, the separation membrane is a hydrophobic PVDF membrane having a nominal pore size of 0.22 μm, and this separation membrane is attached to a stirring type pressure cell, and the rotational speed of the stirring bar of the cell is 1,450 rpm, Under the conditions of constant volume filtration (membrane permeation flux 20 m / day), pressure filtration is performed on the water to be treated, and after the membrane differential pressure has increased, the separation membrane is removed from the cell and washed with 1% oxalic acid. (Washing time 60 minutes, washing temperature 20 ° C.), and then again mounting the separation membrane after the oxalic acid washing on the cell, the salt concentration and / or pH is lower than the water to be treated The liquid (preferably pure water, more preferably ultrapure water) is further filtered, and the membrane differential pressure at that time is measured. The difference between this membrane differential pressure and the membrane differential pressure at the start of filtration (m-Aq, at 25 ° C. ) total water filtration (m ³ / m ² - film) by dividing the object to be processed It is preferred to obtain a second fouling potential is a membrane closure evaluation index (FP _S).

The present invention also includes a step of obtaining the first membrane blockage degree (ID _F ) of the treated water by further filtering the obtained filtered water after filtering the treated water using the separation membrane; After filtering the water to be treated using a separation membrane, a liquid having a lower salt concentration and / or pH than the water to be treated is further filtered to obtain the second membrane blockage (ID _S ) of the water to be treated. A water to be treated, which determines the mode of the membrane pollutant contained in the water to be treated from the relationship between the first membrane occlusion degree (ID _F ) and the second membrane occlusion degree (ID _S ). This is a method for determining the state of a film contaminant.
Hereinafter, such a method for evaluating the degree of membrane occlusion is also referred to as a “determination method of the present invention”.

  ADVANTAGE OF THE INVENTION According to this invention, even if it is a case where the density | concentration of the film | membrane obstruction | occlusion substance contained in to-be-processed water, such as raw water for taps, is low, the method which can evaluate the film | membrane obstruction | occlusion property with sufficient sensitivity can be provided.

It is the graph obtained in the Example. It is another graph obtained in the Example. It is another graph obtained in the Example.

<Evaluation method of the present invention>
The evaluation method of the present invention will be described.
The evaluation method of the present invention is based on a conventional method for evaluating the degree of clogging of water to be treated using a separation membrane. A part of this method is a liquid having a salt concentration and / or pH lower than that of the water to be treated. Further, the operation may be changed to an operation of filtering, or an operation of further filtering a liquid having a salt concentration and / or pH lower than that of the water to be treated may be added to this method.

  As an example, the evaluation method of the present invention will be described with reference to fouling potential (FP), which is one of the methods for evaluating the degree of blockage of water to be treated using a conventional separation membrane.

The method for measuring the fouling potential (FP) is as follows.
First, a hydrophobic PVDF membrane having a nominal pore size of 0.22 μm is prepared as a separation membrane, and this separation membrane is attached to a stirring type pressure cell. The rotating speed of the stirring bar of the cell is 1,450 rpm, and the whole volume is constant rate filtration. Under the condition of (membrane permeation flux 20 m / day), the water to be treated was subjected to pressure filtration, and after the membrane differential pressure increased, the separation membrane was removed from the cell and washed with 1% -oxalic acid (washing time 60). (Preferably further cleaning of the membrane surface using a sponge), and then again mounting the separation membrane after the oxalic acid cleaning on the cell and then applying the pressure Filtration is performed using the filtrate produced by filtration, and the membrane pressure difference is measured again. The difference between the membrane pressure difference and the membrane pressure difference at the start of filtration (m-Aq, at 25 ° C.) divided by the total amount of filtered water (m ³ / m ² -membrane) is the fouling potential of treated water ( FP).

In the evaluation method of the present invention, the filtration operation using the filtrate generated by pressure filtration in the conventional fouling potential (FP) measurement method as described above is carried out by using a salt concentration and / or more than the water to be treated. Change to an operation that filters liquids with low pH. Similarly, the difference between the membrane differential pressure and the membrane differential pressure at the start of filtration (m-Aq, at 25 ° C.) is divided by the total amount of filtered water (m ³ / m ² −membrane), thereby treating the water to be treated. Obtain a second fouling potential (FP _S ).

Such a second fouling potential (FP _S ) has a larger value than the conventional fouling potential (FP). Therefore, even when the concentration of the membrane occluding substance contained in the water to be treated is low, the membrane occluding property can be found and evaluated with high sensitivity.

In addition to the above-mentioned fouling potential (FP), for example, MFI (MFI _0.45 ), MFI-UF, MFI-NF, CFS-MFI _UF , and UMFI can be used as a method for evaluating the degree of film blocking of treated water. Furthermore, other methods for evaluating the degree of blockage of water to be treated may be used.

As described above, in the evaluation method of the present invention, a liquid having a low salt concentration and / or pH is used. This liquid is preferably pure water, and more preferably ultrapure water.
Here, pure water means deionized water. It means what is obtained by conventionally known deionization treatment (for example, treatment using a combination of a cation exchange resin and an anion exchange resin).
Ultrapure water refers to high-purity water that is as close as possible to H ₂ O as obtained by combining elemental technologies related to purification that have been known so far. For example, specific resistance is 18 MΩcm, and the number of particles is 20 / cm ³ ( Examples thereof include those having the following characteristics: diameter 0.1 μm) or less, viable count 0.01 or less / cm ³ , and TOC (total organic carbon) 0.1 mg / dm ³ .

  Pure water (preferably ultrapure water) has a lower salt concentration and pH than the water to be treated regardless of the type of water to be treated. It is preferable to use pure water (preferably ultrapure water) because even if the concentration of the membrane occluding substance contained in the water to be treated is low, the membrane occluding property can be evaluated with higher sensitivity. Moreover, it is preferable also from a point with easy acquisition.

  The salt in the liquid having a low salt concentration and / or pH is not particularly limited, and examples thereof include Ca, Mg, Na, and Cl.

  In the evaluation method of the present invention, the water to be treated specifically includes raw water (river water, groundwater, etc.), seawater, and biological treated water.

  In the evaluation method of the present invention, an ultrafiltration membrane (UF membrane), a microfiltration membrane (MF membrane), a reverse osmosis membrane (RO membrane), a nanofiltration membrane (NF membrane) or the like can be used as a separation membrane.

<Determination method of the present invention>
Next, the determination method of the present invention will be described.
The determination method of the present invention is a first method obtained from the former by combining a conventional method for evaluating the degree of blockage of water to be treated using a separation membrane and the evaluation method of the present invention in which a part of the method is changed. This is a method for judging the mode of membrane contaminants contained in the water to be treated from the relationship between the degree of blockage of one membrane (ID _F ) and the degree of blockage of the second membrane (ID _S ) obtained from the latter.

For example, a fouling potential (FP) (corresponding to the first membrane clogging degree (ID _F )), which is a method for evaluating the degree of clogging of water to be treated using a conventional separation membrane, is obtained, and further the evaluation method of the present invention When the second fouling potential (FP _S ) (corresponding to the second membrane blockage degree (ID _S )) is obtained by, the mode of the membrane contaminant contained in the treated water is determined from the relationship between FP and FP _S. can do.
Specifically, when the average value of the molecular weight of the membrane contaminant contained in the water to be treated is small (for example, the molecular weight is 100 kDa or less), the value of FP _S with respect to FP tends to increase (for example, the value of FP _S / FP). Is about 2 to 6, more specifically about 4). Conversely, when the average value of the molecular weight of the membrane pollutant contained in the water to be treated is large (for example, the molecular weight exceeds 100 kDa), the value of FP _S for FP tends to be small (for example, the value of FP _S / FP). Is about 1.5 to 5, more specifically about 2).
Therefore, from this tendency, it is possible to determine the mode (specifically, molecular weight) of the membrane contaminant contained in the water to be treated.

  Examples of the present invention will be described below. The present invention is not limited to the following examples.

1. Supply water (treated water)
Raw seawater (hereinafter also referred to as “raw water”) is prepared, and a portion thereof is filtered using a UF membrane (PVDF, nominal molecular weight cut off 150 KDa, manufactured by Toray Industries, Inc.) and filtered water (hereinafter also referred to as “UF filtered water”). Got.
Below, it experimented using raw | natural water and UF filtered water as to-be-processed water.

2. Measurement of fouling potential The fouling potential (FP) was measured for each of the raw water and the UF filtrate.
A method for measuring the fouling potential (FP) will be described.
First, a hydrophobic PVDF membrane (Millipore, GVHP, diameter 25 mm) having a nominal pore size of 0.22 μm was used, and this was attached to a stirring type pressure cell, and pressure filtration was performed with a liquid feed pump for HPLC. . Filtration is performed by constant volume filtration (membrane permeation flux 20 m / day) while rotating the stirring bar of the cell at 1,450 rpm. After the membrane differential pressure has risen to some extent, the membrane is removed from the cell and 1%- Oxalic acid cleaning (cleaning time 60 minutes, cleaning temperature about 20 ° C.) and sponge cleaning of the membrane surface were performed. After washing, the membrane was attached to the cell, filtered with GVHP membrane filtered water of the test water, and the membrane differential pressure was measured again. The difference between the membrane pressure difference and the membrane pressure difference at the start of filtration (m-Aq, at 25 ° C.) divided by the total amount of filtered water (m ³ / m ² -membrane) is the fouling potential (FP) of the test water. It was.

Next, after calculating | requiring FP in the said procedure, it filtered again using the ultrapure water instead of GVHP membrane filtration water, and measured the membrane differential pressure similarly. And the value which remove | divided the difference (m-Aq, at25 degreeC) of this membrane differential pressure and the membrane differential pressure at the time of filtration start by the total amount of filtrate water (m < ³ > / m <2> -membrane) is ^{2nd of} test water. The fouling potential (FP _S ) was used.

FIG. 1 shows the relationship between FP and FP _S in raw water and UF membrane filtered water. Moreover, in FIG. 2, the figure which shows only the relationship between FP and FP _S in UF filtered water is shown.
As shown in FIG. 1 and FIG. 2, FP and FP _S have a high correlation coefficient, indicating a substantially proportional relationship.
Further, since the differential pressure on the basis of actual measurement is increased, the value of FP _S is larger than that of FP, and apparently FP is amplified to FP _S. The amplification ratio, that is, FP _S / FP is different between raw water and UF filtered water, and is 1.80 to 4.55 (average 2.56) in raw water, but 2.03 to 5.5 in UF filtered water. It was 69 (average 4.03), and the polymer substance having a molecular weight of about 100 kDa or more, that is, a state influenced by the abundance and characteristics of the biopolymer was shown.

Next, the vertical axis in FIG. 1 is replaced in FP _S / FP, Fig. 3 shows the relationship between FP and FP _S / FP.
Although there is variation, the overall trend is that FP _S / FP tends to increase as FP decreases, and conversely, FP _S / FP converges to about 2 when FP increases. It was.
Although the detailed mechanism of this phenomenon is unknown, by using the FP _S of the present invention, it is possible to improve the measurement dynamic range in a relatively low FP region (less than about 1) and to use it together with the FP. Thus, the characteristics (mainly molecular weight distribution and prediction) of the membrane contaminant can be roughly determined.

Thus, it was found that the measurement sensitivity can be improved by about 2 to 4 times the FP by using the FP _S of the present invention.
Further, it was found that the characteristics (mainly molecular weight distribution and prediction) of the membrane contaminant can be roughly judged by using together with the conventional FP.

Claims (2)

After filtering the treated water using the separation membrane, further filtering the obtained filtered water to obtain a first membrane blockage degree (ID _F ) of the treated water;
After filtering the water to be treated using the separation membrane, a second membrane blockage degree (ID _S ) of the water to be treated is further filtered by filtering a liquid having a lower salt concentration and / or pH than the water to be treated. Obtaining
And determining the form of the membrane contaminant contained in the water to be treated from the relationship between the first membrane occlusion degree (ID _F ) and the second membrane occlusion degree (ID _S ). Water membrane blockage evaluation method. The relationship between the first membrane occlusion degree (ID _F ) and the second membrane occlusion degree (ID _S ) is:
When the average value of the molecular weight of the membrane pollutant contained in the treated water is not more than a predetermined value, the second fouling potential (FP _s ), which is an evaluation index of the degree of membrane clogging of the treated water relative to the fouling potential (FP). When the average value of the molecular weight of the membrane contaminant contained in the water to be treated is larger than the predetermined value, the value of the second fouling potential (FP _s ) with respect to the fouling potential (FP) is increased. The method for evaluating the degree of blockage of the water to be treated according to claim 1 , wherein the film has a tendency to become smaller.