JP7637581B2 - Water treatment method and water treatment device - Google Patents

Description

The present invention relates to a water treatment method and a water treatment device that use multiple stages of separation membranes.

When operating a separation membrane device that uses a separation membrane such as a reverse osmosis membrane, suppressing fouling, such as so-called biofouling, in which microorganisms adhere to the separation membrane, is a major issue.

In addition, in order to improve the recovery rate of treated water, a common method in separation membrane devices is to temporarily store the concentrated water concentrated by a separation membrane placed upstream in an intermediate water tank, and then treat it further downstream using a separation membrane to obtain treated water (see Patent Document 1).

As a method for suppressing fouling in separation membrane devices, a method has been proposed in which a chemical such as a bactericide is injected into the water being treated as a fouling inhibitor (see Patent Document 2).

In addition, from the standpoint of economy and other factors, these chemicals are often intermittently injected into the water being treated (see Patent Document 3). In this case, the minimum concentration required to be effective varies depending on the type of chemical, so it is common to inject the chemical so that the chemical concentration in the water being treated at the separation membrane remains at or above the minimum required concentration for a specified period of time or more.

In a device in which concentrated water concentrated by a separation membrane placed upstream (upstream separation membrane) as described above is temporarily stored in an intermediate water tank and then processed by a separation membrane placed further downstream (downstream separation membrane), when a chemical is intermittently injected at the inlet of the upstream separation membrane, the concentration of the chemical at the inlet of the downstream separation membrane may become lower than the concentration of the chemical at the inlet of the upstream separation membrane. To solve this problem, it is possible to inject the chemical again at the inlet of the downstream separation membrane, but this would increase the amount of chemical used and is not efficient.

JP 2020-065963 A Patent No. 6401491 JP 2020-142211 A

The object of the present invention is to provide a water treatment method and water treatment device that can efficiently supply a predetermined concentration of chemicals to the downstream separation membrane in a water treatment process in which concentrated water concentrated by an upstream separation membrane is stored in an intermediate water tank and then further treated by a downstream separation membrane.

The present invention is a water treatment method including a chemical injection step of intermittently injecting a chemical into the water to be treated, an upstream separation membrane treatment step of performing separation membrane treatment using a first separation membrane on the water to be treated into which the chemical has been injected to obtain a first concentrated water and a first permeate, and a downstream separation membrane treatment step of storing the first concentrated water in an intermediate water tank and then performing separation membrane treatment using a second separation membrane on the first concentrated water sent from the intermediate water tank to obtain a second concentrated water and a second permeate, and adjusting the chemical injection time (T _d ), the chemical concentration ratio (a) in the upstream separation membrane treatment step, and the residence time ( _Tr ) in the intermediate water tank so that the maximum value of the chemical inflow concentration (C ₂ ) into the downstream separation membrane treatment step is equal to or greater than the chemical inflow concentration (C ₀ ) into the upstream separation membrane treatment step.

In the water treatment method, the first separation membrane and the second separation membrane are preferably reverse osmosis membranes.

In the water treatment method, it is preferable that the drug concentration ratio (a) in the upstream separation membrane treatment process is in the range of 1 to 10 times.

In the water treatment method, the agent is preferably a biofouling inhibitor.

In the water treatment method, the biofouling inhibitor preferably includes at least one of a stabilized hypobromous acid composition, chlorosulfamic acid, and DBNPA.

The present invention is a water treatment device comprising: a chemical injection means for intermittently injecting a chemical into the water to be treated; an upstream separation membrane treatment means for performing separation membrane treatment on the water to be treated into which the chemical has been injected using a first separation membrane to obtain a first concentrated water and a first permeate; an intermediate water tank for storing the first concentrated water; a downstream separation membrane treatment means for performing separation membrane treatment on the first concentrated water sent from the intermediate water tank using a second separation membrane to obtain a second concentrated water and a second permeate; and an adjustment means for adjusting the chemical injection time (T _d ), the chemical concentration ratio (a) in the upstream separation membrane treatment means, and the residence time ( _Tr ) in the intermediate water tank so that the maximum value of the chemical inflow concentration (C ₂ ) to the downstream separation membrane treatment means is equal to or greater than the chemical inflow concentration (C ₀ ) to the upstream separation membrane treatment means.

In the water treatment device, the first separation membrane and the second separation membrane are preferably reverse osmosis membranes.

In the water treatment device, it is preferable that the drug concentration ratio (a) in the upstream separation membrane treatment means is in the range of 1 to 10 times.

In the water treatment device, the agent is preferably a biofouling inhibitor.

In the water treatment device, the biofouling inhibitor preferably includes at least one of a stabilized hypobromous acid composition, chlorosulfamic acid, and DBNPA.

The present invention provides a water treatment method and water treatment device that can efficiently supply a predetermined concentration of chemicals to the downstream separation membrane in a water treatment process in which concentrated water concentrated by an upstream separation membrane is stored in an intermediate water tank and then further treated by a downstream separation membrane.

1 is a schematic diagram illustrating an example of a water treatment device according to an embodiment of the present invention. FIG. 4 is a schematic configuration diagram showing another example of a water treatment device according to an embodiment of the present invention.

The following describes an embodiment of the present invention. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

An example of a water treatment device according to an embodiment of the present invention is outlined in FIG. 1, and its configuration will be described.

The water treatment device 1 in FIG. 1 includes a pipe 28 as a chemical injection means for intermittently injecting a chemical into the water to be treated, an upstream separation membrane treatment device 10 as an upstream separation membrane treatment means for performing separation membrane treatment using a first separation membrane on the water to be treated to which the chemical has been injected to obtain a first concentrated water and a first permeate, an intermediate water tank 12 for storing the first concentrated water, and a downstream separation membrane treatment device 14 as a downstream separation membrane treatment means for performing separation membrane treatment using a second separation membrane on the first concentrated water sent from the intermediate water tank 12 to obtain a second concentrated water and a second permeate.

In the water treatment device 1 of FIG. 1, a pipe 16 is connected to the inlet of the upstream separation membrane treatment device 10. A pipe 18 is connected to the first permeate outlet of the upstream separation membrane treatment device 10. A pipe 20 connects the first concentrated water outlet of the upstream separation membrane treatment device 10 to the inlet of the intermediate water tank 12. A pipe 22 connects the outlet of the intermediate water tank 12 to the inlet of the downstream separation membrane treatment device 14. A pipe 24 is connected to the second permeate outlet of the downstream separation membrane treatment device 14, and a pipe 26 is connected to the second concentrated water outlet. A pipe 28 is connected to the pipe 16, so that a chemical can be intermittently injected into the water to be treated.

The water treatment method and operation of the water treatment device 1 according to this embodiment will be described.

The water to be treated is sent to the upstream separation membrane treatment device 10 through pipe 16. Here, a chemical is intermittently injected into the water to be treated in pipe 16 through pipe 28 (chemical injection process). A tank for treated water may be provided upstream of the upstream separation membrane treatment device 10, and a chemical may be intermittently injected into the water to be treated in the tank for treated water.

In the upstream separation membrane treatment device 10, separation membrane treatment is performed using a first separation membrane on the water to be treated into which the chemical has been injected, and a first concentrated water and a first permeate are obtained (upstream separation membrane treatment process). The first permeate is discharged through piping 18. The first concentrated water is sent to the intermediate water tank 12 through piping 20 and temporarily stored in the intermediate water tank 12.

The first concentrated water is stored in the intermediate water tank 12 and then sent to the downstream separation membrane treatment device 14 through the pipe 22. In the downstream separation membrane treatment device 14, the first concentrated water sent from the intermediate water tank 12 is subjected to separation membrane treatment using a second separation membrane, and a second concentrated water and a second permeate are obtained (downstream separation membrane treatment process). The second permeate is discharged through the pipe 24, and the second concentrated water is discharged through the pipe 26.

In the water treatment method and water treatment device 1 according to this embodiment, the chemical is added to the water to be treated in the upstream separation membrane treatment device 10 (upstream separation membrane treatment process), and is not added to the first concentrated water, which is the water to be treated in the downstream separation membrane treatment device 14. In other words, the chemical is added only to the water to be treated in the upstream separation membrane treatment device 10 (upstream separation membrane treatment process).

In the water treatment method and water treatment device 1 according to this embodiment, the "chemical injection time (T _d ),""chemical concentration ratio (a) in the upstream separation membrane treatment process (upstream separation membrane treatment device 10)," and "residence time (T _r ) in the intermediate water tank 12" are adjusted so that the maximum value of the chemical inflow concentration (C ₂ ) to the downstream separation membrane treatment process (downstream separation membrane treatment device 14) is equal to or greater than the chemical inflow concentration (C ₀ ) to the upstream separation membrane treatment process (upstream separation membrane treatment device 10).

That is,
_Td : drug injection time, _Tr : intermediate water tank residence time, a: drug concentration ratio at the upstream separation membrane, _C0 : drug inflow concentration to the upstream separation membrane, _C1 : drug outflow concentration from the upstream separation membrane, _C2 : drug inflow concentration to the downstream separation membrane,
_C0 ≦ _C2
T _d (drug injection time), T _r (residence time in the intermediate water tank), and a (drug concentration ratio at the upstream separation membrane) are adjusted so that the above holds.
In addition,
_C1 = _C0 x a
It is.

As described above, in the water treatment in which the concentrated water concentrated by the upstream separation membrane is stored in the intermediate water tank and further treated by the downstream separation membrane, when the chemical is intermittently injected at the inlet of the upstream separation membrane, if the injection amount and injection time of the chemical are determined considering only the required concentration to the upstream separation membrane, the chemical concentration at the inlet of the downstream separation membrane may be lower than the chemical concentration at the inlet of the upstream separation membrane, and the predetermined amount of the chemical may not be supplied to the downstream separation membrane. This is considered to be because the injected chemical is diluted in the intermediate water tank, or the chemical is consumed in the upstream separation membrane. Therefore, the "chemical injection time (T _d )", "chemical concentration ratio (a) in the upstream separation membrane treatment process (upstream separation membrane treatment device 10)", and "residence time (T _r )" are adjusted so that the maximum value of the chemical inflow concentration (C ₂ ) to the downstream separation membrane treatment process (downstream separation membrane treatment device 14) is equal to or higher than the chemical inflow concentration (C ₀ ) to the upstream separation membrane treatment process (upstream separation membrane treatment device 10). By making the chemical concentration ( _C2 ) at the downstream separation membrane inlet equal to or higher than the chemical concentration ( _C0 ) at the upstream separation membrane inlet, a predetermined concentration of chemical can be supplied to the downstream separation membrane (the latter separation membrane) disposed downstream of the primary side of the upstream separation membrane (the former separation membrane). If a biofouling inhibitor is used as the chemical, both the upstream separation membrane and the downstream separation membrane can be efficiently sterilized.

The "drug injection time (T _d )" can be adjusted, for example, by increasing or decreasing the operating time of the drug injection pump provided in the piping 28 or by increasing or decreasing the opening and closing time of the valve provided in the piping 28. The drug injection time (T _d ) may be, for example, 1 to 300 minutes.

The "drug concentration ratio (a) in the upstream separation membrane treatment process" can be adjusted, for example, by changing the operating conditions of the upstream separation membrane or by using a drug with a different rejection rate in the upstream separation membrane. The drug concentration ratio (a) in the upstream separation membrane treatment process is, for example, in the range of 1 to 10 times, and preferably in the range of 1 to 5 times. If the drug concentration ratio (a) in the upstream separation membrane treatment process is less than 1 time, the effect of the drug in the downstream separation membrane treatment process may be insufficient, and if it exceeds 10 times, it may cause a decrease in the performance of the separation membrane.

The "residence time in the intermediate tank (T _r )" can be adjusted by changing the capacity of the intermediate tank 12 or the amount of the first concentrated water flowing into the intermediate tank 12. The retention time in the intermediate tank (T _r ) may be, for example, 1 to 300 minutes.

The water treatment apparatus may be provided with an adjustment means for adjusting the injection time (T _d ), the concentration ratio (a) of the chemical in the upstream separation membrane treatment process, and the residence time (T _r ) in the intermediate water tank so that the maximum concentration (C ₂ ) of the chemical in the downstream separation membrane treatment process is equal to or higher than the concentration (C ₀ ) of the chemical in the upstream separation membrane treatment process. An example of a water treatment apparatus having such a configuration is shown in FIG. 2.

The water treatment device 2 in FIG. 2 may further include a control device 30 as the adjustment means, and a drug concentration measuring device 32 as a drug concentration measuring means for measuring the drug concentration at the inlet of the downstream separation membrane treatment process (downstream separation membrane treatment device 14). As a drug concentration measuring means for measuring the drug concentration at the inlet of the upstream separation membrane treatment process (upstream separation membrane treatment device 10), a drug concentration measuring device may be installed downstream of the connection point of the pipe 28 in the pipe 16.

In the water treatment device 2, a drug concentration measuring device 32 is installed in the piping 22. The control device 30 and the drug concentration measuring device 32, and the control device 30 and the drug injection pump and valves provided in the piping 28 are connected by wired or wireless electrical connections, etc.

In the water treatment device 2, the control device 30 adjusts the "chemical injection time (T _d ),""chemical concentration ratio (a) in the upstream separation membrane treatment process (upstream separation membrane treatment device 10)," and "residence time (T _r ) in the intermediate water tank" so that the maximum value of the chemical inflow concentration (C ₂ ) to the downstream separation membrane treatment process (downstream separation membrane treatment device 14) is equal to or greater than the chemical inflow concentration (C ₀ ) to the upstream separation membrane treatment process (upstream separation membrane treatment device ₁₀ ), thereby making the chemical concentration (C ₂ ) at the downstream separation membrane inlet equal to or greater than the chemical concentration (C ₀ ) at the upstream separation membrane inlet.

The control device 30 is composed of, for example, a microcomputer and electronic circuits that include a calculation means such as a CPU that operates a program, and storage means such as ROM and RAM that store the program and calculation results, and functions as an adjustment means that adjusts the drug injection time (T _d ), the drug concentration ratio (a) in the upstream separation membrane treatment process, and the residence time (T _r ) in the intermediate water tank so that the maximum value of the drug inflow concentration (C ₂ ) to the downstream separation membrane treatment process is equal to or greater than the drug inflow concentration (C ₀ ) to the upstream separation membrane treatment process.

The control device 30 can adjust the "chemical injection time (T _d )" by, for example, controlling a chemical injection pump provided in the pipe 28 to increase or decrease the operating time. The control device 30 can adjust the "chemical concentration ratio (a) in the upstream separation membrane treatment process" by, for example, controlling the opening of a valve provided in the pipe 20 of the upstream separation membrane. The control device 30 can adjust the "residence time in the intermediate water tank (T _r )" by, for example, controlling a valve or the like provided in the pipe 20.

There are no particular limitations on the drug concentration measuring device as long as it can measure the drug concentration, and examples include a residual chlorine meter.

Methods for measuring the residual concentration of halogen-based drugs include the DPD method and the polarographic method.

The drug concentration at the inlet of the downstream separation membrane treatment process (downstream separation membrane treatment device 14) can be measured automatically or manually using a drug concentration measuring device 32.

<Separation membrane>
The first separation membrane used in the upstream separation membrane treatment device 10 and the second separation membrane used in the downstream separation membrane treatment device 14 are not particularly limited, and examples thereof include reverse osmosis membranes (RO membranes), nanofiltration membranes (NF membranes), microfiltration membranes (MF membranes), ultrafiltration membranes (UF membranes), etc. Among these, the reverse osmosis membranes and nanofiltration membranes (NF membranes) that can substantially concentrate ion components, i.e., where drug concentration occurs, are the targets, and the water treatment method and water treatment device according to this embodiment are preferably applied to the reverse osmosis membrane.

Polyamide composite membranes are commonly used as reverse osmosis membranes.

In each of the upstream separation membrane processing device 10 and the downstream separation membrane processing device 14, multiple stages of separation membrane processing devices may be used in series or parallel.

<Water to be treated>
The water to be treated is not particularly limited, and examples of the water to be treated include environmental water (groundwater, river water, seawater, etc.), wastewater, etc.

<Drugs>
The chemicals to be injected into the water to be treated are not particularly limited, and examples include biofouling inhibitors (bactericides), dispersants, etc. The water treatment method and water treatment device according to the present embodiment are preferably applied to the chemicals to be injected into the water to be treated, in the case of biofouling inhibitors that are effective even when injected intermittently.

Biofouling inhibitors include stabilized hypobromous acid compositions containing a bromine-based oxidizing agent and a sulfamic acid compound, stabilized hypochlorous acid compositions containing a chlorine-based oxidizing agent and a sulfamic acid compound, halocyanoacetamide compounds such as 2,2-dibromo-3-nitrilopropionamide (DBNPA), hypochlorous acid, isothiazolinone, peracetic acid, and performic acid.

As the chemical, in particular, a chemical that hardly permeates the upstream separation membrane, i.e., the chemical injected into the water to be treated migrates almost entirely to the first concentrated water side (for example, the rejection rate in a separation membrane such as a reverse osmosis membrane is 95% or more) is preferred. Specific examples include a stabilized hypobromous acid composition, chlorosulfamic acid, and DBNPA, and a stabilized hypobromous acid composition that has a high rejection rate in a separation membrane such as a reverse osmosis membrane is preferred.

The "stabilized hypobromous acid composition containing a bromine-based oxidizing agent and a sulfamic acid compound" may be a stabilized hypobromous acid composition containing a mixture of a "bromine-based oxidizing agent" and a "sulfamic acid compound", or may be a stabilized hypobromous acid composition containing a "reaction product of a bromine-based oxidizing agent and a sulfamic acid compound". The "stabilized hypochlorous acid composition containing a chlorine-based oxidizing agent and a sulfamic acid compound" may be a stabilized hypochlorous acid composition containing a mixture of a "chlorine-based oxidizing agent" and a "sulfamic acid compound", or may be a stabilized hypochlorous acid composition containing a "reaction product of a chlorine-based oxidizing agent and a sulfamic acid compound".

Bromine-based oxidizing agents include bromine (liquid bromine), bromine chloride, bromic acid, bromates, hypobromous acid, etc. Hypobromous acid may be produced by reacting a bromine compound such as sodium bromide with a chlorine-based oxidizing agent such as hypochlorous acid. When the bromine-based oxidizing agent is bromine, there is no chlorine-based oxidizing agent present, and therefore the deterioration effect on separation membranes such as reverse osmosis membranes is significantly reduced.

Examples of chlorine-based oxidizing agents include chlorine gas, chlorine dioxide, hypochlorous acid or its salts, chlorous acid or its salts, chloric acid or its salts, perchloric acid or its salts, and chlorinated isocyanuric acid or its salts. Among these, examples of salts include alkali metal hypochlorite salts such as sodium hypochlorite and potassium hypochlorite, alkaline earth metal hypochlorite salts such as calcium hypochlorite and barium hypochlorite, alkali metal chlorite salts such as sodium chlorite and potassium chlorite, alkaline earth metal chlorite salts such as barium chlorite, other metal chlorite salts such as nickel chlorite, alkali metal chlorate salts such as ammonium chlorate, sodium chlorate and potassium chlorate, alkaline earth metal chlorate salts such as calcium chlorate and barium chlorate, etc. These chlorine-based oxidizing agents may be used alone or in combination of two or more. As the chlorine-based oxidizing agent, it is preferable to use sodium hypochlorite from the viewpoint of handling and the like.

Bromine compounds include sodium bromide, potassium bromide, lithium bromide, ammonium bromide, and hydrobromic acid. Of these, sodium bromide is preferred from the standpoint of formulation costs, etc.

The sulfamic acid compound is a compound represented by the following general formula (1).
R ₂ NSO ₃ H (1)
(In the formula, R is independently a hydrogen atom or an alkyl group having 1 to 8 carbon atoms.)

Examples of sulfamic acid compounds include sulfamic acid (amidosulfuric acid) in which both R groups are hydrogen atoms, as well as sulfamic acid compounds in which one of the R groups is a hydrogen atom and the other is an alkyl group having 1 to 8 carbon atoms, such as N-methylsulfamic acid, N-ethylsulfamic acid, N-propylsulfamic acid, N-isopropylsulfamic acid, and N-butylsulfamic acid, sulfamic acid compounds in which both of the R groups are alkyl groups having 1 to 8 carbon atoms, such as N,N-dimethylsulfamic acid, N,N-diethylsulfamic acid, N,N-dipropylsulfamic acid, N,N-dibutylsulfamic acid, N-methyl-N-ethylsulfamic acid, and N-methyl-N-propylsulfamic acid, sulfamic acid compounds in which one of the R groups is a hydrogen atom and the other is an aryl group having 6 to 10 carbon atoms, such as N-phenylsulfamic acid, and salts thereof. Examples of sulfamic acid salts include alkali metal salts such as sodium salts and potassium salts, alkaline earth metal salts such as calcium salts, strontium salts and barium salts, other metal salts such as manganese salts, copper salts, zinc salts, iron salts, cobalt salts and nickel salts, ammonium salts and guanidine salts. Sulfamic acid compounds and their salts may be used alone or in combination of two or more. As the sulfamic acid compound, it is preferable to use sulfamic acid (amidosulfuric acid) from the viewpoint of environmental load, etc.

The stabilized hypobromous acid composition may further contain an alkali. Examples of the alkali include alkali hydroxides such as sodium hydroxide and potassium hydroxide. From the viewpoint of product stability at low temperatures, sodium hydroxide and potassium hydroxide may be used in combination. The alkali may be used as an aqueous solution instead of a solid.

Examples of dispersants include polyacrylic acid, polymaleic acid, and phosphonic acid.

<Method of drug injection>
The chemical is intermittently injected into the water to be treated in the upstream separation membrane treatment device 10 (upstream separation membrane treatment step). Intermittent injection refers to an injection in which an injection period during which the chemical is injected into the water to be treated and a non-injection period during which the chemical is not injected into the water to be treated are provided. The chemical may be automatically injected into the water to be treated by a chemical injection pump or the like. The place where the chemical is injected may be either the piping or the water tank to be treated.

<Other configurations>
At a stage subsequent to the downstream separation membrane treatment, at least one of a reverse osmosis membrane treatment device, a UV treatment device, or an ion exchange treatment device may be provided, and at least one of a reverse osmosis membrane treatment, a UV treatment, or an ion exchange treatment may be performed on the permeate of the separation membrane treatment.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

<Examples 1 and 2, Comparative Example 1>
In the water treatment device 1 of Figure 1, a chemical was injected into the water to be treated in the upstream separation membrane treatment device, and the chemical concentration flowing into the downstream separation membrane treatment device was measured when the device was operated under the following operating conditions. The chemical concentration (mg-Cl/L) was measured by a total chlorine measurement method (DPD (N,N-diethyl-p-phenylenediamine) method) using a HACH multi-item water quality analyzer DR/4000. The results are shown in Table 1.

[Operating conditions]
· Residence time in intermediate tank (T _r ): 20 minutes · Concentration ratio in upstream separation membrane (a): 2 times · Drug inflow concentration to upstream separation membrane (C ₀ ): 10 mg-Cl/L
Drug: Drug containing a stabilized hypobromous acid composition as an active ingredient. Intermittent injection time (T _d ) of drug per 1440 minutes: 20 minutes (Example 1), 15 minutes (Example 2), 10 minutes (Comparative Example 1).

In the examples, by setting the intermittent drug injection time to 15 minutes or more, the drug inflow concentration to the downstream separation membrane could be made equal to or greater than the drug inflow concentration to the upstream separation membrane.

<Reference Examples 1 to 5>
The rejection rate (%) of the active ingredients of various biofouling inhibitors in a reverse osmosis membrane was measured. The drug concentration was measured by the DPD method. The results are shown in Table 2.
Rejection rate (%) = [1 - (chemical concentration in permeate water/chemical concentration in raw water)] x 100

[Test conditions]
Separation membrane: Polyamide polymer reverse osmosis membrane Operating pressure: 0.75 MPa
・Raw water: Sagamihara well water (pH 7.2, conductivity 240μS/cm)
Chemicals: stabilized hypobromous acid composition (Reference Example 1), chlorosulfamic acid (Reference Example 2), DBNPA (Reference Example 3), sodium hypochlorite (Reference Example 4), chloramine (Reference Example 5)

[Medications used]
(Stabilized hypobromous acid composition)
A stabilized hypobromous acid composition was prepared by mixing 16.9% by weight (wt%) liquid bromine, 10.7% by weight sulfamic acid, 12.9% by weight sodium hydroxide, 3.94% by weight potassium hydroxide, and the remainder of water under a nitrogen atmosphere. The stabilized hypobromous acid composition had a pH of 14 and a total chlorine concentration of 7.5% by weight.

(Chlorosulfamic acid)
12% sodium hypochlorite aqueous solution: 50% by weight, sulfamic acid: 10% by weight, sodium hydroxide: 8% by weight, water: remaining amount are mixed to prepare stabilized hypochlorous acid composition (chlorosulfamic acid).The pH of stabilized hypochlorous acid composition is 14, and the total chlorine concentration is 6% by weight.

(DBNPA)
2,2-Dibromo-3-nitrilopropionamide (DBNPA) was used.

(Sodium hypochlorite)
A 12% aqueous solution of sodium hypochlorite was used.

(Chloramine)
Sodium hypochlorite was injected into the water to be treated from the separation membrane at 10 mgCl/L in terms of available chlorine, and ammonium chloride was injected into the water to be treated at 11.3 mg/L in terms of available chlorine, and mixed to generate chloramines at a predetermined concentration in the water to be treated.

It can be seen that the stabilized hypobromous acid composition (Reference Example 1), chlorosulfamic acid (Reference Example 2), and DBNPA (Reference Example 3) have a rejection rate of 95% or more in the reverse osmosis membrane.

As described above, the method of the embodiment allows for efficient supply of a specified concentration of chemicals to the downstream separation membrane in a water treatment process in which concentrated water concentrated by the upstream separation membrane is stored in an intermediate tank and then further treated by the downstream separation membrane.

1, 2 Water treatment device, 10 Upstream separation membrane treatment device, 12 Intermediate water tank, 14 Downstream separation membrane treatment device, 16, 18, 20, 22, 24, 26, 28 Piping, 30 Control device, 32 Drug concentration measuring device.

Claims (10)

An agent injection step of intermittently injecting an agent into the water to be treated;
an upstream separation membrane treatment step of performing a separation membrane treatment using a first separation membrane on the water to be treated into which the chemical has been injected, to obtain a first concentrated water and a first permeate;
a downstream separation membrane treatment step of storing the first concentrated water in an intermediate water tank, and then performing a separation membrane treatment on the first concentrated water sent from the intermediate water tank using a second separation membrane to obtain a second concentrated water and a second permeate;
Including,
The water treatment method is characterized by adjusting the injection time ( _Td ) of the chemical, the chemical concentration ratio (a) in the upstream separation membrane treatment process, and the residence time ( _Tr ) in the intermediate water tank so that the maximum value of the inflow concentration ( _C2 ) of the chemical to the downstream separation membrane treatment process is equal to or greater than the inflow concentration ( _C0 ) of the chemical to the upstream separation membrane treatment process. The water treatment method according to claim 1,
The water treatment method, wherein the first separation membrane and the second separation membrane are reverse osmosis membranes. The water treatment method according to claim 1 or 2,
The water treatment method, wherein the drug concentration ratio (a) in the upstream separation membrane treatment step is in the range of 1 to 10 times. The water treatment method according to any one of claims 1 to 3,
The water treatment method, wherein the agent is a biofouling inhibitor. The water treatment method according to claim 4,
The water treatment method, wherein the biofouling inhibitor comprises at least one of a stabilized hypobromous acid composition, chlorosulfamic acid, and DBNPA. An agent injection means for intermittently injecting an agent into the water to be treated;
an upstream separation membrane treatment means for performing a separation membrane treatment using a first separation membrane on the water to be treated into which the chemical has been injected, to obtain a first concentrated water and a first permeate;
An intermediate water tank for storing the first concentrated water;
a downstream separation membrane treatment means for performing a separation membrane treatment using a second separation membrane on the first concentrated water sent from the intermediate water tank to obtain a second concentrated water and a second permeate;
an adjusting means for adjusting the injection time (T _d ) of the agent, the agent concentration ratio (a) in the upstream separation membrane treatment means, and the residence time ( _Tr ) in the intermediate water tank so that the maximum value of the inflow concentration (C ₂ ) of the agent to the downstream separation membrane treatment means is equal to or greater than the inflow concentration (C ₀ ) of the agent to the upstream separation membrane treatment means;
A water treatment device comprising: The water treatment device according to claim 6,
The water treatment device, wherein the first separation membrane and the second separation membrane are reverse osmosis membranes. The water treatment device according to claim 6 or 7,
A water treatment device, wherein the drug concentration ratio (a) in the upstream separation membrane treatment means is in the range of 1 to 10 times. The water treatment device according to any one of claims 6 to 8,
The water treatment device according to claim 1, wherein the chemical is a biofouling inhibitor. The water treatment device according to claim 9,
The water treatment device, wherein the biofouling inhibitor comprises at least one of a stabilized hypobromous acid composition, chlorosulfamic acid, and DBNPA.

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