JP7699084B2 - METHOD FOR EVALUATING MEMBRANE CLOUDDING PROPERTY OF COAGULATED WATER, COAGULATION MEMBRANE FILTRATION METHOD, AND COAGULATION MEMBRANE FILTRATION SYSTEM - Google Patents

Description

The present invention relates to a method for evaluating membrane clogging of coagulation-treated water, a coagulation membrane filtration method, and a coagulation membrane filtration system, and in particular to a method for evaluating membrane clogging of coagulation-treated water that occurs during the coagulation membrane filtration process, and a coagulation membrane filtration method and coagulation membrane filtration system that utilize this method.

Traditionally, sand filtration has been the mainstream solid-liquid separation process in water purification, but in recent years, low-pressure membrane filtration methods using microfiltration membranes (MF membranes) and ultrafiltration membranes (UF membranes), which are expected to achieve a higher level of solid-liquid separation, have been increasingly introduced.

For example, membrane filtration is increasingly being applied to equipment upgrades to aging medium- to large-scale water purification plants. In such cases, since surface water such as river water is used as the raw water for the water supply, coagulation treatment is often combined as a membrane pretreatment process to remove soluble substances such as color components.

Coagulation treatment as a membrane pretreatment is also effective in reducing biopolymers, which are a cause of organic membrane fouling, one of the issues with membrane filtration, but it also causes the problem of membrane fouling due to residual coagulants. In particular, aluminum-based coagulants such as polyaluminum chloride (PACl) and aluminum sulfate (Alm) are used for coagulation in water purification treatment, and residual aluminum present in the coagulated water is a cause of membrane fouling.

Residual aluminum in coagulation treated water can be divided into micro-level aluminum particles on the order of μm, soluble aluminum that can be filtered through a 0.45 μm membrane, and nano-aluminum particles on the order of several nm to several tens of nm that have a negative charge due to their positive charge and barely pass through the 0.45 μm membrane. Of these, micro-level aluminum particles are in a form that is incorporated into coarse particles such as microflocs, and these microflocs are not a problem because they can be removed by the physical sieving effect of the membrane, but nano-aluminum particles cause membrane blockage, so knowing the presence of nano-aluminum particles in coagulation treated water is important in understanding the membrane fouling potential of coagulation treated water.

Patent Document 1 is a patent application filed by the applicant of the present application, and discloses that in water purification treatment using coagulation membrane filtration, the hydraulic retention time of the rapid stirring process of the treated water is set to be equal to or longer than the hydraulic retention time of the slow stirring process. The removal of nano-aluminum particles from the coagulation treated water occurs during the flocculation formation process, in which the particles collide with and are taken up by coarse particles such as micro-flocs. According to the invention of Patent Document 1, the incorporation of nano-aluminum particles into sub-micro-flocs and micro-flocs is promoted, making it possible to effectively reduce the nano-aluminum particles remaining in the coagulation treated water.

Patent Document 2 proposes an attempt to suppress membrane fouling by measuring and controlling the zeta potential of nanoaluminum particles (reported to be about 20 to 500 nm). Specifically, the method irradiates electromagnetic waves to membrane-filtered water to be subjected to membrane treatment, mixes the water with a coagulant after the electromagnetic wave irradiation to form flocs, measures the zeta potential of meso particles (i.e., nanoaluminum particles) in the pretreated water containing the flocs, and controls the properties of the irradiated electromagnetic waves so that the measured zeta potential of the meso particles approaches 0 mV. According to the invention of Patent Document 2, pretreated water can be obtained that can sufficiently suppress the occurrence of membrane fouling when filtered through a membrane.

JP 2021-186745 A JP 2022-030987 A

Yoshihide Kaitani et al., "A Study on the Analysis of Nanoparticles in Coagulated Water Treated by Centrifugation," Lectures of the 2018 National Conference (Waterworks Research Presentation), pp. 366-367

According to the invention of Patent Document 1, it is possible to effectively reduce the nano-aluminum particles remaining in the coagulation treatment water, but the nano-aluminum particle concentration was not actually measured.

In Patent Document 2, the zeta potential of meso particles (nano aluminum particles) is measured, but nanoparticle meters such as zeta potential measuring devices are very expensive and require skilled analytical techniques. In addition, they are not basically measuring devices for instrumentation, so it is not practical to apply them to actual water treatment facilities.

In addition, when using nanoparticle counters, centrifugation and membrane filtration are often used as pretreatment to suppress interference from coarse particles larger than the measurement target particles. However, because the nanoaluminum particles to be evaluated are removed by centrifugation and membrane filtration, it has been pointed out that careful selection of pretreatment conditions is necessary depending on the purpose of the analysis and evaluation. This also suggests that the aggregation and settling of nanoaluminum particles and the collision of coarse particles, known as floc formation, have the potential to remove nanoparticles (Non-Patent Document 1).

Furthermore, unlike conventional sand filtration, membrane filtration does not require a settling tank, so the coagulated treated water becomes the membrane supply water as is. In this case, however, the above-mentioned micro-level aluminum particles, nano-aluminum particles, and soluble aluminum are mixed in the membrane supply water, and it is extremely difficult to determine the aluminum concentration of the nano-aluminum particles that cause membrane blockage.

If it is not possible to determine the aluminum concentration of nanoaluminum particles, one can actually perform coagulation membrane filtration and control the membrane filtration conditions while observing the increase in filtration resistance, or one can perform membrane filtration in advance using experimental equipment equipped with a membrane with the same physical properties as the membrane used in the water purification equipment, and evaluate the membrane clogging potential of the coagulation treated water by determining the filtration constant of the coagulation treated water. However, in the former case, it is not possible to evaluate the membrane clogging potential of the coagulation treated water in advance, and in the latter case, separate evaluation equipment is required to evaluate the membrane clogging potential of the coagulation treated water, and the evaluation takes time.

The present invention was made in consideration of the above problems, and its purpose is to provide a coagulation membrane filtration method and a coagulation membrane filtration system that can easily and quickly evaluate the membrane clogging of coagulation treated water on-site without having to evaluate the membrane clogging of the coagulation treated water in a separate facility.

Another object of the present invention is to provide a method for evaluating the membrane clogging of coagulation-treated water, which can easily and quickly evaluate the membrane clogging of the coagulation-treated water during coagulation membrane filtration.

The inventors conducted extensive research to achieve the above-mentioned objective, and noticed that when measuring nano-aluminum particles, which are the cause of membrane blockage, in coagulation-treated water containing a mixture of micro-level aluminum particles, nano-aluminum particles, and soluble aluminum, only nano-aluminum particles and soluble aluminum were present in the supernatant water obtained by leaving a portion of the coagulation-treated water to settle, but no micro-level aluminum particles were present, and that only soluble aluminum was present in the membrane-filtrated water after membrane filtration.

Then, the inventors discovered that by calculating the difference between the aluminum concentration of the supernatant water in the coagulation treated water and the aluminum concentration of the membrane filtered water, it is possible to easily calculate the concentration of nano-aluminum particles in the coagulation treated water, which is difficult to measure directly, and that the calculated concentration of nano-aluminum particles in the coagulation treated water has a significant correlation with the apparent cake filtration constant when the coagulation treated water is filtered through a separation membrane, thereby completing the present invention.

That is, it has been found that the above-mentioned object can be achieved by a coagulation membrane filtration method comprising: a coagulation treatment process in which a coagulant is mixed with the water to be treated to obtain coagulated treated water; a membrane filtration process in which the coagulated treated water obtained in the coagulation treatment process is subjected to membrane filtration using a separation membrane selected from a microfiltration membrane and an ultrafiltration membrane to obtain membrane filtered water; a ΔAl calculation process in which ΔAl is calculated, which is the difference between the aluminum concentration of the supernatant water of the coagulated treated water and the aluminum concentration of the membrane filtered water; and an operating condition change process in which the operating conditions of at least one of the coagulation treatment process and the membrane filtration process are changed from the operating conditions before the change based on the calculated ΔAl value.

A preferred embodiment of the method for treating wastewater generated in a water purification process according to the present invention is as follows.
(1) In the operating condition changing step, if the ΔAl value calculated in the ΔAl calculation step exceeds 0.5 mg/L, the operating conditions of at least one of the coagulation treatment step and the membrane filtration step are changed from the operating conditions before the change so that the ΔAl value becomes 0.5 mg/L or less.
(2) In addition, in the operating condition changing step, if the value of ΔAl calculated in the ΔAl calculation step exceeds 0.3 mg/L, the operating conditions of at least one of the coagulation treatment step and the membrane filtration step are changed from the operating conditions before the change so that the value of ΔAl becomes 0.3 mg/L or less.
(3) The method further includes an apparent cake filtration constant calculation step of calculating an apparent cake filtration constant K2 when the coagulation treated water is subjected to membrane filtration with a separation membrane, and an operating condition changing step of changing the operating conditions of at least one of the coagulation treatment step and the membrane filtration step from the operating conditions before the change, based on the calculated ΔAl value and the calculated K2 value.

The above object is to provide a coagulation membrane filtration system having a coagulation treatment means for performing coagulation treatment on water to be treated to which a coagulant has been added to obtain coagulated treated water, a membrane filtration means for membrane filtering the coagulated treated water obtained by the coagulation treatment means using a separation membrane selected from a microfiltration membrane and an ultrafiltration membrane to obtain membrane filtered water, a first measurement means for measuring the aluminum concentration in the supernatant of the coagulated treated water, and a second measurement means for measuring the aluminum concentration in the membrane filtered water,
This can also be achieved by a coagulation membrane filtration system further comprising: a control unit that calculates a value of ΔAl which is the difference between the aluminum concentration in the supernatant water of the coagulation treated water measured by the first measuring means and the aluminum concentration in the membrane filtered water measured by the second measuring means, and controls a change in the operating conditions of at least one of the coagulation treatment means and the membrane filtration means from the operating conditions before the change based on the calculated value of ΔAl.

Further, the above object is to provide a method for evaluating membrane clogging of coagulation-treated water, which evaluates membrane clogging of the coagulation-treated water obtained by coagulation treatment of water to be treated,
This can also be achieved by a method for evaluating membrane clogging of coagulation-treated water, which comprises a ΔAl calculation step of calculating ΔAl, which is the difference between the aluminum concentration of the supernatant water of the coagulation-treated water and the aluminum concentration of the membrane-filtrated water, and a membrane clogging evaluation step of evaluating the membrane clogging of the coagulation-treated water based on the calculated ΔAl value.

According to the present invention, the nanoaluminum particle concentration in the coagulation treated water can be calculated simply and quickly as ΔAl without the need for separate membrane filtration equipment. In addition, since this ΔAl has a significant correlation with the apparent cake filtration constant K2 when the coagulation treated water is subjected to membrane filtration using a separation membrane, it is possible to easily and quickly evaluate the membrane clogging of the coagulation treated water at the water purification site using ΔAl as an indicator.

1 is a flowchart for explaining the coagulation membrane filtration method of the present invention. FIG. 1A is a model diagram of a blockage of cake filtration, and FIG. 1B is a model diagram explaining an increase in resistance due to accumulated particles p on a filter medium (a bundle of circular tubes) by replacing the increase in resistance with an increase in the length of the circular tubes t. 1 is a flowchart showing a first example of an operating condition changing step of the coagulation membrane filtration method of the present invention. 4 is a flowchart showing a second example of the operating condition changing step of the coagulation membrane filtration method of the present invention. FIG. 1 is a block diagram illustrating a coagulation membrane filtration system 10 of the present invention. 1 is a flowchart for explaining a method for evaluating membrane clogging of coagulation-treated water according to the present invention. 1 is a graph in which K2 and ΔAl values for the coagulated treated water obtained under treatment conditions 1 to 3 in Example 1 are plotted, with K2 on the vertical axis and ΔAl on the horizontal axis.

<Coagulation membrane filtration method>
The coagulation membrane filtration method of the present invention is carried out as a solid-liquid separation process for water purification. The coagulation membrane filtration method of the present invention includes a coagulation treatment step, a membrane filtration step, a ΔAl calculation step, and an operating condition change step. Hereinafter, the method will be described with reference to FIGS. 1 to 4. FIG. 1 is a flow chart for explaining the coagulation membrane filtration method of the present invention, FIG. 2(a) is a model diagram of a cake filtration blockage, and FIG. 2(b) is a model diagram for explaining an increase in resistance due to deposited particles p on a filter medium (a bundle of circular tubes) by replacing it with an increase in the length of the circular tubes t, FIG. 3 is a flow chart showing a first example of an operating condition change step of the coagulation membrane filtration method of the present invention, and FIG. 4 is a flow chart showing a second example of an operating condition change step of the coagulation membrane filtration method of the present invention.
[Aggregation treatment step (S100)]
In this process, a flocculant is mixed with the water to be treated to obtain flocculated treated water. Raw water for water supply is used as the water to be treated. Examples of raw water for water supply include river water, groundwater, reservoir water, lake water, underground water, and groundwater.

In the present invention, the flocculant is an aluminum-based flocculant, such as polyaluminum chloride or aluminum sulfate. In the specific implementation of the present invention, inorganic flocculants other than aluminum-based flocculants or polymer flocculants may be added separately.

The mixing of the flocculant may be carried out in a pipe such as an in-line mixer, or by stirring in a stirring tank. Preferably, the mixing of the flocculant is carried out in a rapid stirring tank upstream of the flocculation treatment section, which includes a combination of a rapid stirring tank upstream and a slow stirring tank downstream. By mixing the aluminum-based flocculant with the water to be treated, flocculated treated water containing a mixture of micro-level aluminum particles, nano-aluminum particles, and soluble aluminum is obtained.

As already mentioned, micron-level aluminum particles refer to aluminum particles on the order of μm, soluble aluminum refers to aluminum that can be filtered through a 0.45 μm membrane, and nano-aluminum particles refer to nano-aluminum particles on the order of several nm to several tens of nm that are negatively charged due to their positive charge and therefore hardly pass through the 0.45 μm membrane (above, the agglomeration process (S100)).

[Membrane filtration step (S110)]
In this step, the coagulation treated water obtained in the coagulation treatment step (S100) is subjected to membrane filtration using a separation membrane selected from a microfiltration membrane and an ultrafiltration membrane to obtain membrane filtered water.

As mentioned above, the separation membrane is selected from microfiltration membranes and ultrafiltration membranes, and the type of separation membrane may be any of polymer membranes, inorganic membranes, MF membranes, and UF membranes, but polymer membranes that can be used with immersion type membrane modules, particularly membranes made of physically and chemically strong PVDF, are preferred, and MF membranes that can also be used with immersion type membrane modules are preferred. In addition, a hollow fiber membrane is preferred for the membrane shape in terms of volume efficiency.

From the viewpoint of energy conservation, a membrane with a pore size of 0.05 μm or more is preferable since it allows the use of the water level difference, and furthermore, from the viewpoint of preventing membrane clogging by organic matter, a membrane with a pore size of 0.05 μm, which is the smallest diameter, is optimal.

The structure of the membrane filtration device can be either a casing type or a tank-immersed type, but the tank-immersed type is preferred as it is more applicable to raw water with high turbidity.

The membrane filtered water obtained by membrane filtration is retained as treated water in a treatment water tank (not shown) for a certain period of time and used as purified water (this concludes the membrane filtration process (S110)).

[ΔAl calculation step (S120)]
In this step, ΔAl, which is the difference between the aluminum concentration in the supernatant water of the coagulation treated water and the aluminum concentration in the membrane filtered water, is calculated.

First, prior to calculating ΔAl, the aluminum concentration of the supernatant water of the coagulation treated water and the aluminum concentration of the membrane filtered water are measured.

The supernatant of the coagulation-treated water is preferably obtained by collecting the coagulation-treated water obtained in the coagulation treatment step (S100) in a container such as a 500 ml beaker, leaving it to stand for 3 to 5 minutes to allow the flocs to settle, and then collecting approximately 20 mL of the water above the water surface with a digital pipette. By collecting the supernatant of the coagulation-treated water using this procedure, it is possible to separate the coarse particles from the nanoparticle group containing nanoaluminum particles.

If the flocculated water is left to stand for less than 3 minutes, the settling of coarse particles will be insufficient, so a setting time of 3 to 5 minutes is preferable, with 5 minutes being more preferable. If it is longer than that, the nanoaluminum particles may flocculate and settle, which is not preferable.

The supernatant water from the coagulation treatment is used to measure the aluminum concentration.

The membrane filtered water obtained in the membrane filtration process (S110) can be used directly for measuring the aluminum concentration.

The aluminum concentration can be measured by colorimetry (absorption spectrophotometry), atomic absorption spectrometry, ICP atomic emission spectrometry, ICP mass spectrometry, etc., but from the viewpoint of simple and rapid measurement of the aluminum concentration, it is preferable to use colorimetry (absorption spectrophotometry).

When measuring the aluminum concentration by colorimetry (spectrophotometric method), commercially available measuring equipment such as a portable aluminum meter (product number: HI 96712, manufactured by Hanna Instruments) or an aluminum meter (product number: MF2PTM-7712H, manufactured by Shiro Sangyo Co., Ltd.) can be used. When using a commercially available spectrophotometer such as an ultraviolet-visible spectrophotometer (product number: UVmini-1240, manufactured by Shimadzu Corporation), the desired aluminum concentration can be obtained by measuring the absorbance of the reaction solution at 535 nm using spectrophotometric method using, for example, Eriochrome Cyanine Red (ECR).

After measuring the aluminum concentration in the supernatant water of the coagulation treated water and the membrane filtered water, ΔAl is obtained by subtracting the aluminum concentration in the membrane filtered water from the aluminum concentration in the supernatant water of the coagulation treated water (this is the ΔAl calculation step (S120)).

[(Optional) Apparent cake filtration constant (K2) calculation step (S130)]
This step is an optional step, and in this step, an apparent cake filtration constant (K2) is calculated when the coagulated treated water is subjected to membrane filtration using a separation membrane.

The apparent cake filtration constant (K2) can be measured using a cake filtration equation based on a cake filtration clogging model (see, for example, Masato Kadoya, edited and published by Marketing and Communications Group, Nippon Pall Co., Ltd., SPRING Pall News, 2013, Vol. 117, pp. 10-15).

The clogging model of cake filtration is a model in which, assuming that the filter medium is a bundle of circular tubes with uniform inner diameter and length, the loaded particles accumulate on the surface of the filter medium (bundle of circular tubes) without clogging the circular tubes. In this case, as shown in Figure 2(a), the thickness of the deposited particles p (cake layer) increases in proportion to the amount of particles p loaded on the bundle of circular tubes t. Here, the increase in resistance due to the deposited particles p can be replaced by the lengthening of the circular tubes t shown in Figure 2(b).

Then, by applying the Hagen-Poiseuille equation to the filter medium and carrying out calculations while taking into account the above-mentioned cake filtration clogging model, the following equation (1) is obtained for constant pressure filtration.

（式（１）中、Ｊは単位ろ過面積あたりの流量（ｍ^３／ｍ^２・ｓ）を、Ｊ_０は単位ろ過面積あたりの初期流量（ｍ^３／ｍ^２・ｓ）を、Ｋ２は見かけのケーキろ過定数（１／ｍ）、閉塞係数ともいう）を、ｖは単位面積あたりのろ液量（ｍ^３／ｍ^２）を、それぞれ示す。）を、
定流量ろ過の場合には、以下の式（２）

（式（２）中、ΔＰは円管両端での差圧（Ｐａ）を、ΔＰ_０は円管両端での初期差圧（Ｐａ）を、Ｋ２は見かけのケーキろ過定数（１／ｍ）、閉塞係数ともいう）を、ｖは単位面積あたりのろ液量（ｍ^３／ｍ^２）を、それぞれ示す。）
を、それぞれ得ることができる。

(In formula (1), J represents the flow rate per unit filtration area ( ^m3 / ^m2 ·s), _J0 represents the initial flow rate per unit filtration area ( ^m3 / ^m2 ·s), K2 represents the apparent cake filtration constant (1/m), also known as the clogging coefficient, and v represents the amount of filtrate per unit area ( ^m3 / ^m2 ).)
In the case of constant flow rate filtration, the following formula (2)

(In formula (2), ΔP is the pressure difference (Pa) between both ends of the circular tube, _ΔP0 is the initial pressure difference (Pa) between both ends of the circular tube, K2 is the apparent cake filtration constant (1/m), also called the clogging coefficient, and v is the amount of filtrate per unit area ( ^m3 / ^m2 ).)
can be obtained, respectively.

The apparent cake filtration constant is known as an index of membrane clogging of the coagulation-treated water. Therefore, when determining whether to change the operating conditions in the operating condition change step (S140) described below, the apparent cake filtration constant K2 of the coagulation-treated water obtained in the coagulation treatment step (S100) is measured in this step, and the membrane clogging of the coagulation-treated water can be determined from a more comprehensive perspective based on ΔAl and K2.

[Operating condition changing step (S140)]
In this step, the operating conditions of at least one of the flocculation treatment step (S100) and the membrane filtration step (S110) are changed from the operating conditions before the change based on the calculated value of ΔAl.

Operating conditions that may be subject to change include coagulation treatment conditions and membrane filtration conditions.

Examples of coagulation treatment conditions include the injection rate of the aluminum-based coagulant, mixing conditions, and pH. For example, the injection rate of the aluminum-based coagulant and the coagulation pH can be adjusted (to optimal values) so that the concentration of nano-aluminum particles in the coagulation treatment water is at its lowest, and extending the mixing time in the mixing tank promotes the incorporation of nano-aluminum particles into micro-flocs and sub-micro-flocs, thereby lowering the concentration of nano-aluminum particles in the coagulation treatment water.

As a membrane filtration condition, for example, it is possible to change the operating conditions to lower the filtration flow rate. This can extend the time until the membrane becomes clogged.

An example of changing the operating conditions of this process is explained with reference to FIG. 3. As shown in FIG. 3, in step S140-1, it is determined whether ΔAl is 0.5 mg/L or less. If ΔAl is greater than 0.5 mg/L (NO judgment), the process proceeds to step S140-2. If ΔAl is 0.5 mg/L or less (YES judgment), the process proceeds again to the flocculation treatment process (S100) without changing the operating conditions.

In step S140-2, the operating conditions are changed to reduce the value of ΔAl. The operating conditions to be changed can be selected from the above-mentioned coagulation treatment conditions or membrane filtration conditions. After the operating conditions are changed, the process proceeds again to the coagulation treatment step (S100).

Furthermore, when changing the operating conditions, not only the value of ΔAl but also the value of the apparent cake filtration constant (K2) may be taken into consideration. Below, the case where the value of K2 is taken into consideration in addition to ΔAl will be described with reference to FIG. 4 as a second example of changing the operating conditions of this process. As shown in FIG. 4, in step S140-3, it is determined whether ΔAl is 0.5 mg/L or less and K2 is 10 (1/m) or less. If ΔAl is more than 0.5 mg/L and K2 is more than 10 (1/m) (NO judgment), if ΔAl is more than 0.5 mg/L and K2 is 10 (1/m) or less (NO judgment), or if ΔAl is 0.5 mg/L or less and K2 is more than 10 (1/m) (NO judgment), the process proceeds to step S140-4. If ΔAl is 0.5 mg/L or less and K2 is 10 (1/m) or less (YES judgment), the process proceeds again to the flocculation treatment step (S100) without changing the operating conditions.

In step S140-4, the operating conditions are changed to reduce the values of ΔAl and K2. The operating conditions to be changed can be selected from the coagulation treatment conditions and membrane filtration conditions already described. After the operating conditions are changed, the process moves back to the coagulation treatment step (S100).

Therefore, according to the coagulation membrane filtration method of the present invention, the concentration of nanoaluminum particles derived from the coagulant that causes membrane clogging in membrane filtration can be easily and quickly calculated as ΔAl, and by using this ΔAl as an indicator of membrane clogging in the coagulation treated water, the operating conditions can be quickly changed at the water purification site and the operating conditions of the coagulation membrane filtration can be optimized.

Furthermore, by using ΔAl and K2 together as indicators of membrane clogging of the coagulation-treated water, it is possible to more accurately evaluate the membrane clogging of the coagulation-treated water.

In the above-mentioned operating condition changing step (S140), the operating conditions of the coagulation membrane filtration are changed when ΔAl exceeds 0.5 mg/L. However, from the viewpoint of maintaining stable operation of the coagulation membrane filtration and economic efficiency, it is preferable to change the operating conditions of the coagulation membrane filtration when ΔAl exceeds 0.3 mg/L.

The steps from the ΔAl calculation step (S120) to the operating condition change step (S140) may be performed under the control of a control unit, as in the coagulation membrane filtration system described below, but the aluminum concentration of the supernatant water and membrane filtered water of the coagulation treatment water, the calculation of ΔAl (and optionally the measurement of K2), and the change of the operating conditions of the coagulation membrane filtration based on the value of ΔAl (and K2) may also be performed manually.

<Coagulation membrane filtration system>
5 is a block diagram illustrating the coagulation membrane filtration system 10 of the present invention. As shown in the figure, the coagulation membrane filtration system 10 of the present invention has a coagulation treatment means 12 that performs coagulation treatment on the treatment water 1 to which a coagulant 2 has been added to obtain coagulated treated water 3, a membrane filtration means 14, a first measurement means 16, a second measurement means 18, and a control unit 20.

The treated water 1, the coagulant 2, and the coagulated treated water 3 are the same as those in the coagulation membrane filtration method described above, so their explanation will be omitted here.

Examples of the flocculation treatment means 12 include an in-line mixer and a stirring tank equipped with a stirring means. It is preferable that the flocculation treatment means 12 is equipped with a rapid stirring tank and a slow stirring tank located downstream of the rapid stirring tank.

The flocculant 2 is injected into the water 1 to be treated by a flocculant injection device (not shown) at a position upstream of the flocculation treatment means 12 or at the flocculation treatment means 12. In FIG. 5, the flocculant 2 is injected into the water 1 to be treated by the flocculation treatment means 12.

When the coagulation treatment means 12 is equipped with a rapid mixing tank and a slow mixing tank located downstream of the rapid mixing tank, the rapid mixing tank may be a single tank, or multiple tanks may be arranged in series. If there are multiple tanks, it is preferable that there are three tanks or less, and it is particularly preferable that there are three rapid mixing tanks. The mixing strength of the rapid mixing is 100 1/s or more, when considered in terms of the mixing strength (speed gradient) G value according to the Waterworks Facility Design Guidelines 2012, and the G value of the slow mixing tank is in the range of 10 to 75 1/s.

In the coagulation treatment means 12, coagulation treatment is performed on the treated water 1 to which the aluminum-based coagulant 2 has been added, to obtain coagulated treated water 3 containing a mixture of micro-level aluminum particles, nano-aluminum particles, and soluble aluminum.

The membrane filtration means 14 is a means for membrane filtering the coagulated treated water 3 obtained by the coagulation treatment means 12 using a separation membrane selected from a microfiltration membrane and an ultrafiltration membrane to obtain membrane filtered water 4. Information about the separation membrane, such as the type and pore size, has already been explained in the coagulation membrane filtration method described above, and will not be explained here.

The first measuring means 16 is a means for measuring the aluminum concentration in the supernatant of the coagulation treated water 3, and the second measuring means 18 is a means for measuring the aluminum concentration in the membrane filtered water 4.

As the first measuring means 16 and the second measuring means 18, the devices described in the above coagulation membrane filtration method can be used as appropriate. The first measuring means 16 and the second measuring means 18 are controlled by the control unit 20 described below.

The control unit 20 is a computer equipped with a CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), etc. The control unit 20 expands a program stored in the ROM onto the RAM and causes the CPU to execute the corresponding process. Note that the above program need not necessarily be stored in the ROM, but may be stored in a NVRAM (Non-Volatile Random Access Memory).

The control unit 20 calculates the value of ΔAl, which is the difference between the aluminum concentration in the supernatant of the coagulation treatment water 3 measured by the first measurement means 16 and the aluminum concentration in the membrane filtered water 4 measured by the second measurement means 18, and controls a change in the operating conditions of at least one of the coagulation treatment means 12 and the membrane filtration means 14 from the operating conditions before the change based on the calculated value of ΔAl.

The method for calculating ΔAl is the same as the ΔAl calculation step (S120) of the coagulation membrane filtration method described above, except that the control unit 20 is limited to calculating ΔAl, so a description of this method will be omitted here.

After calculating ΔAl, the control unit 20 judges whether ΔAl has reached a predetermined value, as described in the first example of the operating condition changing step (S140) of the coagulation membrane filtration method described above, and if it exceeds the predetermined value, sends a signal to at least one of the coagulation treatment means 12 and the membrane filtration means 14 to change the operating conditions, and if it does not exceed the predetermined value, no signal is sent, and therefore the operating conditions are not changed.

The types of operating conditions for at least one of the coagulation treatment means 12 and the membrane filtration means 14 are the same as those in the operating condition change step (S140) of the coagulation membrane filtration method described above, and so a description thereof will be omitted here.

After the operating conditions are changed or after it is decided not to change the operating conditions, the control by the control unit 20 may be terminated, or ΔAl may be calculated again after a predetermined time has elapsed, and the control of changing (or not changing) the operating conditions of at least one of the flocculation treatment means 12 and the membrane filtration means 14 may be repeated based on the result.

As with the coagulation membrane filtration method of the present invention, the coagulation membrane filtration system 10 of the present invention can easily and quickly calculate the concentration of nanoaluminum particles derived from the coagulant that cause membrane clogging in membrane filtration as ΔAl, and by using this ΔAl as an index of membrane clogging in the coagulation treated water, the operating conditions can be quickly changed at the water purification site and the operating conditions of the coagulation membrane filtration can be optimized.

<Method for evaluating membrane clogging of coagulation-treated water>
The method for evaluating membrane clogging of coagulation-treated water of the present invention is a method for evaluating membrane clogging of coagulation-treated water when the coagulation-treated water obtained by coagulation treatment of water to be treated is subjected to membrane filtration. The water to be treated, the coagulation treatment, the coagulation-treated water, and the membrane filtration are the same as those in the above-mentioned coagulation membrane filtration method, and therefore the description thereof will be omitted here.

The membrane clogging evaluation method for coagulation-treated water of the present invention includes a ΔAl calculation step and a membrane clogging evaluation step. The method will be described below with reference to FIG. 6. FIG. 6 is a flow chart for explaining the membrane clogging evaluation method for coagulation-treated water of the present invention.

[ΔAl calculation step (S200)]
In this step, ΔAl, which is the difference between the aluminum concentration of the supernatant water of the coagulation-collected water and the aluminum concentration of the membrane-filtered water, is calculated. Note that this step is no different from the ΔAl calculation step (S120) of the coagulation membrane filtration method described above, and therefore a description thereof will be omitted here (the ΔAl calculation step (S200) is the end).

[Membrane clogging evaluation step (S210)]
In this step, the membrane clogging property of the coagulation treated water is evaluated based on the value of ΔAl calculated in the ΔAl calculation step (S200).

The inventors discovered that the ΔAl obtained when an aluminum-based coagulant is added to perform coagulation treatment in a water purification process using coagulation membrane filtration has a good correlation with the apparent cake filtration constant (K2) obtained when the coagulated water from the same water purification process is filtered through a membrane.

The apparent cake filtration constant (K2) is known as an index of membrane clogging, so by adding an aluminum-based coagulant to the treated water in advance and performing coagulation treatment under various conditions, and then determining the apparent cake filtration constant (K2) and ΔAl when the resulting coagulated treated water (and membrane filtered water for ΔAl) is subjected to membrane filtration, and obtaining a relational equation between the two, it is possible to obtain the value of ΔAl for the coagulated treated water with an unknown apparent cake filtration constant (K2) and the membrane filtered water, and by applying the value of ΔAl to the relational equation, it is possible to determine the value of the apparent cake filtration constant (K2) without performing a long-term membrane filtration test. And since the value of the apparent cake filtration constant (K2) is an index of membrane clogging, it is possible to easily and quickly evaluate the membrane clogging of the coagulated treated water by calculating the value of ΔAl.

Furthermore, the inventors have found that in a water purification process using coagulation membrane filtration, when an aluminum-based coagulant is added to perform coagulation treatment, it is preferable that the apparent cake filtration constant (K2) when the resulting coagulated treated water is subjected to membrane filtration is 10 (1/m) or less. From the above-mentioned relational equation between the apparent cake filtration constant (K2) and ΔAl, the value of ΔAl is 0.5 (mg/L) when the apparent cake filtration constant (K2) is 10 (1/m). Therefore, when the value of ΔAl calculated in the ΔAl calculation step (S200) is 0.5 (mg/L) or less, the membrane clogging property of the coagulated treated water can be evaluated as good.

Furthermore, if the ΔAl value calculated in the ΔAl calculation step (S200) is 0.3 (mg/L) or less, the apparent cake filtration constant (K2) becomes even smaller, and the membrane clogging property of the coagulation treated water can be evaluated as being even better.

In addition, when evaluating the membrane clogging of the coagulation treated water, it is preferable to combine not only the ΔAl value but also the apparent cake filtration constant (K2) value in order to more accurately evaluate the membrane clogging of the coagulation treated water. In this case, if the apparent cake filtration constant (K2) value is 10 (1/m) or less, the membrane clogging of the coagulation treated water can be evaluated as good, and if the apparent cake filtration constant (K2) value is 5 (1/m) or less, the membrane clogging of the coagulation treated water can be evaluated as even better (the above is the membrane clogging evaluation process (S210)).

Therefore, according to the method for evaluating the membrane clogging of coagulation-treated water of the present invention, the nanoaluminum particle concentration in the coagulation-treated water can be calculated simply and quickly as ΔAl without the need for separate membrane filtration equipment. In addition, since this ΔAl has a significant correlation with the apparent cake filtration constant when the coagulation-treated water is filtered through a separation membrane, the membrane clogging of the coagulation-treated water can be evaluated simply and quickly at the water purification site using ΔAl as an indicator.

After evaluating the membrane clogging of the coagulation treated water using ΔAl (and K2) as an index, the operating conditions of the coagulation membrane filtration can be optimized by changing the operating conditions based on the evaluation results.

In addition, in FIG. 6, all steps of the membrane clogging evaluation method of the present invention for coagulation treated water are completed when the membrane clogging evaluation step (S210) is completed, but the ΔAl calculation step (S200) and the membrane clogging evaluation step (S210) may be repeated after a predetermined time has elapsed.

The present invention will be explained in more detail below with reference to the following examples.

Example 1
(1) Water to be treated and coagulation treatment conditions The water to be treated (raw water) was river water (turbidity: 2.7 degrees, color: 4.1 degrees, pH = 7.5, TOC: 0.7 mg/L, _UVA260 : 0.086 (5 cm cell)) collected in winter when the water was relatively clear and collected from a water purification plant equipped with membrane filtration equipment.

Polyaluminum chloride (PACl, basicity 50%) was added to the treated water, and coagulation treatment was carried out using a 500 mL beaker.

The coagulation conditions were pH 7.0 and PACl injection rate 25 mg/L. These are the same as the on-site operating conditions when the treated water was collected. The mixing conditions are as shown in Table 1 below.

(2) Measurement of apparent cake filtration constant (K2) (1) 80 mL each of the water to be treated and the coagulated water obtained under the coagulation treatment conditions was collected, and this was passed through the membrane at constant pressure filtration (suction pressure 90 kPa) using a suction pump to measure the change in the amount of filtrate over time. From the results, the apparent cake filtration constant (K2) was calculated based on the cake filtration theory.

The membrane used was a Merck VVHP membrane (hydrophobic PVDF membrane, pore size 0.1 μm), and a 25 mm diameter glass filter holder for flat membranes was used.

(3) Measurement of ΔAl (1) About 20 mL of the water surface after 5 minutes of settling in the treated water and the coagulation treatment conditions was collected with a digital pipette and used as the supernatant of the coagulation treatment water. The same coagulation treatment water was passed through a membrane in (2) the measurement of the apparent cake filtration constant (K2) to obtain membrane filtrate.

The aluminum concentrations in the supernatant water and membrane filtered water from these coagulation treatments were measured according to JIS K 0102-2008 "58.4 ICP atomic emission spectrometry."

Then, ΔAl, which is the difference between the aluminum concentration in the supernatant water of the coagulation treated water and the aluminum concentration in the membrane filtered water, was calculated. The values of K2 and ΔAl for the coagulation treated water obtained under treatment conditions 1 to 3 in Example 1 are shown in Table 2 below.

Figure 7 is a graph plotting the values of K2 and ΔAl for the coagulated water obtained under treatment conditions 1 to 3 in Example 1, with K2 on the vertical axis and ΔAl on the horizontal axis.

As shown in Table 2 and Figure 7, under all treatment conditions, the values of ΔAl and K2 can be reduced by increasing the mixing time. Furthermore, with treatment condition 1, if rapid mixing is performed only, the reduction in K2 and ΔAl reaches a plateau after 6 minutes of rapid mixing. On the other hand, with treatment condition 2, if rapid mixing is performed for only 2 minutes, slow mixing for 9 minutes or more is required to reduce K2 to 10 (1/m) or less and ΔAl to 0.5 (mg/L) or less, and the mixing time for the coagulation treatment is extended in order to reduce the nanoaluminum particle concentration in the coagulation treatment water.

It was found that rapid stirring for 6 minutes, as in treatment condition 3, followed by slow stirring for just 1 to 2 minutes, can sufficiently reduce the values of K2 and ΔAl.

7, a good correlation was confirmed between ΔAl and K2, and it was therefore found that the value of K2 can be predicted by measuring ΔAl. Similarly, it is also possible to predict ΔAl from the value of K2.
Example 2
ΔAl and K2 were also measured in the membrane feed water of an actual facility.

The treated water was the same as in Example 1.

The water purification process flow of the actual equipment is as follows: rapid mixing section (hydraulic retention time (HRT): 2 min, 130 rpm) → slow mixing section (HRT: 20 min, 30 rpm) → settling tank → membrane filtration equipment. The coagulation conditions were coagulation pH = 7.0, PACl injection rate 25 mg/L.

The measurement conditions for ΔAl and K2 were the same as those in Example 1. The measurement results were ΔAl = 0.29 (mg/L) and K2 = 6.93 (1/m). In the water purification flow of the actual facility, ΔAl was in the range of 0.5 (mg/L) or less and K2 was in the range of 10 (1/m) or less, indicating that the membrane clogging of the coagulated treated water at the site was small and the operating conditions were good.

10 Coagulation membrane filtration system 12 Coagulation treatment means 14 Membrane filtration means 16 First measurement means 18 Second measurement means 20 Control unit

Claims (6)

a flocculation treatment step of mixing a flocculant containing an aluminum-based flocculant with the water to be treated to obtain flocculated treated water;
a membrane filtration step of filtering the flocculated water obtained in the flocculation step through a separation membrane selected from a microfiltration membrane and an ultrafiltration membrane to obtain membrane filtered water;
A ΔAl calculation step of calculating ΔAl, which is the difference between the aluminum concentration of the supernatant water of the coagulation treated water and the aluminum concentration of the membrane filtrate;
An operating condition changing step of changing the operating conditions of at least one of the flocculation treatment step and the membrane filtration step from the operating conditions before the change based on the calculated ΔAl value;
A coagulation membrane filtration method comprising the steps of: The coagulation membrane filtration method according to claim 1, characterized in that, in the operating condition change step, if the value of ΔAl calculated in the ΔAl calculation step exceeds 0.5 mg/L, the operating conditions of at least one of the coagulation treatment step and the membrane filtration step are changed from the operating conditions before the change so that the value of ΔAl becomes 0.5 mg/L or less. The coagulation membrane filtration method according to claim 1, characterized in that, in the operating condition change step, if the value of ΔAl calculated in the ΔAl calculation step exceeds 0.3 mg/L, the operating conditions of at least one of the coagulation treatment step and the membrane filtration step are changed from the operating conditions before the change so that the value of ΔAl becomes 0.3 mg/L or less. Further, an apparent cake filtration constant calculation step of calculating an apparent cake filtration constant K2 when the coagulated treated water is filtered through the separation membrane,
The coagulation membrane filtration method according to any one of claims 1 to 3, characterized in that the operating condition change step changes the operating conditions of at least one of the coagulation treatment step and the membrane filtration step from the operating conditions before the change based on the calculated ΔAl value and the calculated K2 value. a flocculation treatment means for performing flocculation treatment on the water to be treated to which a flocculant containing an aluminum-based flocculant has been added, to obtain flocculated treated water;
a membrane filtration means for filtering the flocculated water obtained by the flocculation treatment means through a separation membrane selected from a microfiltration membrane and an ultrafiltration membrane to obtain membrane filtered water;
A first measuring means for measuring an aluminum concentration in the supernatant water of the coagulation treated water;
A second measuring means for measuring an aluminum concentration in the membrane filtrate,
a control unit that calculates a value of ΔAl which is the difference between the aluminum concentration in the supernatant water of the coagulation treated water measured by the first measuring means and the aluminum concentration in the membrane filtered water measured by the second measuring means, and controls a change of the operating conditions of at least one of the coagulation treatment means and the membrane filtration means from the operating conditions before the change based on the calculated value of ΔAl. A method for evaluating membrane clogging of coagulated treated water, which evaluates membrane clogging of the coagulated treated water obtained by coagulation treatment of water to be treated, comprising the steps of:
The flocculation treatment is a treatment in which a flocculant containing an aluminum-based flocculant is mixed with the water to be treated to obtain the flocculated treated water,
A ΔAl calculation step of calculating ΔAl, which is the difference between the aluminum concentration of the supernatant water of the coagulation treated water and the aluminum concentration of the membrane filtrate;
A method for evaluating membrane clogging of coagulation-treated water, comprising: a membrane clogging evaluation step of evaluating membrane clogging of the coagulation-treated water based on the calculated ΔAl value.

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