Reverse osmosis membrane treatment method

Abstract

To appropriately control an addition amount of a chelating scale inhibitor in a method of adding the chelating scale inhibitor into RO membrane feed water to apply a RO membrane treatment, so as to reduce a chemical cost, a treatment cost, and a recovery cost.SOLUTION: A RO membrane treatment method adds a chelating scale inhibitor to RO membrane feed water and feeds to a RO membrane device. The RO membrane treatment method controls an addition amount of the chelating scale inhibitor according to a measured value of an oxidation reduction potential measuring at least one of the RO membrane feed water or concentrated water of the RO membrane after adding the chelating scale inhibitor.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for RO membrane treatment by adding a chelating scale inhibitor to reverse osmosis (RO) membrane feed water, and in particular to a method for efficiently producing fresh water by accurately controlling the amount of the chelating scale inhibitor added when recovering wastewater using an RO membrane, particularly wastewater containing heavy metals, is treated with the RO membrane after adding the chelating scale inhibitor, and the permeated water is recovered and reused. [Background technology]

[0002] When wastewater is subjected to coagulation treatment and then treated with an RO membrane for recovery and reuse, a chelate-based scale inhibitor is added together with a general scale dispersant to mask heavy metals such as Mn, Zn, Ni, and Cu contained in the RO membrane feed water (for example, Patent Document 1).

[0003] For example, when an iron-based inorganic coagulant is added to the wastewater to be treated to perform coagulation treatment, and the coagulated wastewater is then subjected to solid-liquid separation and treated with an RO membrane, heavy metals such as Mn in the iron-based inorganic coagulant adhere to the surface of the RO membrane and act as an oxidation catalyst, which can deteriorate the RO membrane.

[0004] In order to prevent the oxidative deterioration of the RO membrane caused by such heavy metals, a chelate-based scale inhibitor is added to solubilize the heavy metals in the RO membrane feed water by forming a chelate compound with the chelate-based scale inhibitor. The solubilized heavy metals are contained in the RO membrane concentrate and discharged outside the system, preventing the oxidative deterioration of the RO membrane caused by the catalytic action of the heavy metals.

[0005] Conventionally, the amount of chelating scale inhibitor added to RO membrane water was determined based on the estimated maximum concentration of heavy metals contained in the wastewater.

[0006] If a fixed amount of chelate-based scale inhibitor is added regardless of fluctuations in the quality of the wastewater being treated, the excess amount of chelate-based scale inhibitor will be wasted, resulting in corresponding chemical costs. In addition, the burden of chemical treatment of the chelate-based scale inhibitor contained in the RO membrane concentrated water and discharged is large, increasing wastewater treatment costs and recovery costs. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6135511 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide an RO membrane treatment method in which a chelate-based scale inhibitor is added to RO membrane feed water and the amount of the chelate-based scale inhibitor added is precisely controlled to reduce chemical costs, treatment costs, and recovery costs. [Means for solving the problem]

[0009] As a result of extensive investigations aimed at solving the above-mentioned problems, the inventors have discovered that by controlling the amount of chelate-based scale inhibitor to be added based on the measured oxidation-reduction potential of the RO membrane feed water or concentrated water after addition of the chelate-based scale inhibitor, specifically, by setting a threshold value for the oxidation-reduction potential and administering the chelate-based scale inhibitor within a range that does not exceed this threshold, it is possible to reduce the amount of chelate-based scale inhibitor to be administered while maintaining the same treated water quality and amount as before the reduction, and have completed the present invention.

[0010] That is, the present invention relates to the following.

[0011] [1] A reverse osmosis membrane treatment method in which a chelate-based scale inhibitor is added to reverse osmosis membrane feed water and the water is passed through a reverse osmosis membrane device, The amount of the chelate-based scale inhibitor added is controlled based on a measured oxidation-reduction potential of at least one of the reverse osmosis membrane feed water and the reverse osmosis membrane concentrated water after the addition of the chelate-based scale inhibitor.

[0012] [2] The reverse osmosis membrane treatment method according to [1], wherein the amount of the chelating scale inhibitor added is controlled based on a measured oxidation-reduction potential of the concentrated water from the reverse osmosis membrane.

[0013] [3] The reverse osmosis membrane treatment method according to [1] or [2], wherein the amount of the chelate-based scale inhibitor added is controlled within a range in which the measured value of the oxidation-reduction potential does not exceed a preset threshold value. Effect of the Invention

[0014] According to the present invention, in a method of adding a chelating scale inhibitor to RO membrane feed water and performing RO membrane treatment, the amount of chelating scale inhibitor added can be accurately controlled and reduced without affecting the treated water quality or volume, thereby reducing chemical costs, treatment costs, and recovery costs. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a system diagram showing an example of a wastewater treatment system to which the RO membrane treatment method of the present invention is applied. [Diagram 2] 1 is a graph showing the change over time in the electrical conductivity of RO membrane permeate in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the preferred embodiments described below.

[0017] The RO membrane treatment method of the present invention is an RO membrane treatment method in which a chelate-based scale inhibitor is added to RO membrane feed water (hereinafter may be simply referred to as "feed water") and the water is passed through an RO membrane device, and is characterized in that the amount of the chelate-based scale inhibitor added is controlled based on a measured value of an oxidation-reduction potential (hereinafter may be simply referred to as "oxidation-reduction potential" or "ORP") measured for the RO membrane feed water to which the chelate-based scale inhibitor has been added (hereinafter may be simply referred to as "chelate-based scale inhibitor-added feed water") or the RO membrane concentrate (hereinafter may be simply referred to as "concentrate water").

[0018] The oxidation-reduction potential (ORP) of the feed water or concentrated water is an indicator of the oxidation-reduction deterioration of the RO membrane caused by heavy metals, and if there is no change in the oxidation-reduction potential, it can be considered that the oxidation-reduction deterioration of the RO membrane caused by heavy metals is suppressed by masking the heavy metals with a chelate-based scale inhibitor. Therefore, by setting a threshold value for the measured oxidation-reduction potential of the feed water and / or concentrated water to which a chelate-based scale inhibitor has been added, and controlling the amount of the chelate-based scale inhibitor added to be as small as possible within a range in which the measured oxidation-reduction potential does not exceed the threshold value, it is possible to reduce the amount of the chelate-based scale inhibitor added and perform stable RO membrane treatment.

[0019] For this reason, in the present invention, the amount of chelate-based scale inhibitor added is controlled based on the measured oxidation-reduction potential of the chelate-based scale inhibitor-added feed water and / or concentrated water.

[0020] The amount of chelate-based scale inhibitor added may be controlled based on the measured value of the oxidation-reduction potential of the chelate-based scale inhibitor-added feed water, or based on the measured value of the oxidation-reduction potential of the concentrated water, or based on both of these measured values. However, since the oxidation-reduction potential is easy to interpret as an indicator, particularly in the concentrated water, where redox substances are concentrated and have a high concentration, and is effective as an indicator of oxidative deterioration of the RO membrane, in the present invention it is preferable to control the dosing of the chelate-based scale inhibitor based on the measured value of the oxidation-reduction potential of the concentrated water.

[0021] The oxidation-reduction potential can be measured by an ORP meter provided in the water supply pipe for the chelate-based scale inhibitor-added water or in the discharge pipe for the concentrated water.

[0022] In the present invention, the threshold value of the oxidation-reduction potential, which serves as an index for controlling the chemical injection of the chelate-based scale inhibitor, differs depending on the quality of the wastewater to be treated to which the present invention is applied, the type of wastewater treatment system, and its treatment conditions, for example, operating pH. Therefore, water flow tests should be carried out in advance under various conditions, and based on the results, the threshold value should be appropriately determined as a value at which the treated water quality and treated water volume can be maintained.

[0023] In general, under neutral pH conditions, it is often preferable to set the ORP threshold value on the high side, in the range of 100 to 200 mV, whereas under alkaline conditions, such as about pH 9, it is often preferable to set the ORP threshold value on the low side, in the range of about 20 to 80 mV.

[0024] The chelate scale inhibitor used in the present invention is not particularly limited, but preferably used are ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), etc. These chelate scale inhibitors may be used alone or in combination of two or more.

[0025] In the present invention, a general scale inhibitor may be added together with the chelate-based scale inhibitor. In this case, the scale dispersant to be used in combination is not particularly limited, but may be low molecular weight polymers such as (meth)acrylic acid (co)polymers and their salts, maleic acid (co)polymers and their salts, phosphonic acids and phosphonates such as ethylenediaminetetramethylenephosphonic acid and its salts, hydroxyethylidene diphosphonic acid and its salts, nitrilotrimethylenephosphonic acid and its salts, phosphonobutane tricarboxylic acid and its salts, inorganic polymerized phosphoric acids and inorganic polymerized phosphates such as hexametaphosphoric acid and its salts, and tripolyphosphoric acid and its salts. These scale dispersants may be used alone or in combination of two or more. Here, "(meth)acrylic acid" means one or both of "acrylic acid" and "methacrylic acid". Also, "(co)polymer" means one or both of "polymer" and "copolymer".

[0026] Of these, as the scale inhibitor to be used in combination, at least one of a copolymer of isobutylene and maleic acid and a salt thereof, and a copolymer of isobutylene and acrylic acid and a salt thereof can be suitably used.

[0027] The amount of scale dispersant added is preferably about 1 to 100 mg / L relative to the RO membrane feed water.

[0028] An example of an embodiment of the RO membrane treatment method of the present invention will be described below with reference to Fig. 1. Fig. 1 is a system diagram showing an example of a wastewater treatment system to which the present invention is applied.

[0029] Raw water in a raw water tank 1 is pumped to a pH adjustment tank 3 by a pump 2, where an acid such as hydrochloric acid or sulfuric acid or an alkali such as caustic soda or hydrated lime is added to adjust the pH to about 6.0 to 9.0, particularly about 7.0 to 8.0.

[0030] The water in the pH adjustment tank 3 is transferred to a hydrogen peroxide (H2O2) decomposition tank 4, where reducing agents such as catalase and sodium bisulfite are added. After the hydrogen peroxide is decomposed, the water is transferred to a relay tank 5.

[0031] The water in the relay tank 5 is sent to a coagulation and sedimentation facility 7 by a pump 6. In this coagulation and sedimentation facility 7, a coagulant such as polyferric iron, ferric chloride, PAC, aluminum sulfate, polyacrylamide, sodium polyacrylate, etc. is added at a pH of 7.0 to 11.0, and a coagulation and sedimentation process is carried out, and Cu ions are precipitated and separated. The water from which Cu has been removed is transferred to a neutralization tank 8, where hydrochloric acid, sulfuric acid, etc. are added for neutralization to a pH of 6.0 to 10.0, and then transferred to a relay tank 9.

[0032] The water in the relay tank 9 is transferred to an aerobic biological treatment device 13 via a pump 10 and a pipe 11, where it is subjected to aerobic biological treatment. A Cu meter 12 for measuring the Cu ion concentration is installed in the pipe 11.

[0033] The water that has been aerobically treated in the aerobic biological treatment unit 13 is transferred to the coagulation and sedimentation equipment 14, where a coagulant such as polyferric iron, ferric chloride, PAC, aluminum sulfate, polyacrylamide, sodium polyacrylate, etc. is added, the water is subjected to coagulation and sedimentation treatment, and then transferred to the relay tank 15.

[0034] The water in the relay tank 15 is sent by a pump 16 to a pressure filter 17 and filtered. The filtered water is stored in a filtered water tank 18.

[0035] The water in the filtered water tank 18 is sent to an MF membrane device 22 via a pump 19 and a pipe 20. In this pipe 20, a chelate-based scale inhibitor is added by an addition means 21, and then the water is subjected to MF membrane filtration treatment in the MF membrane device 22.

[0036] The permeated water from the MF membrane device 22 is pressurized by a pump 23 and supplied to an RO membrane device 24 where it is treated by the RO membrane. The permeated water from the RO membrane device 24 is taken out as treated water via a pipe 25, and concentrated water is sent to another treatment process via a pipe 27. An ORP meter 26 is provided on the concentrated water pipe 27.

[0037] In this wastewater treatment system, based on the measured ORP value of the concentrated water of the RO membrane device 24 measured by the ORP meter 26, chemical dosing control is performed with a reduced amount of the chelating scale inhibitor added within a range in which the measured value does not exceed a threshold value. This chemical dosing control may be performed manually, or may be performed automatically by a control means which inputs the measured value of the ORP meter, compares the inputted measured value with a threshold value, and outputs a chemical dosing signal to the means for adding the chelating scale inhibitor 21 so that the threshold value is not exceeded.

[0038] In the present invention, there are no particular limitations on the wastewater to be subjected to RO membrane treatment, and the RO membrane treatment of the present invention is suitably applied to wastewater treatment systems that treat industrial water, river water, lake water, well water, etc., as well as biologically treated organic wastewater, and wastewater from various manufacturing processes and cleaning processes in the electronics industry, etc.

[0039] The wastewater to be treated in the present invention is wastewater containing various heavy metals that requires the addition of a chelate-based scale inhibitor. Examples of such wastewater include wastewaters to which an inorganic flocculant containing a heavy metal, particularly an iron-based flocculant, has been added to the various types of wastewater mentioned above.

[0040] Examples of the iron-based inorganic flocculant include ferric chloride and polyferric sulfate, and in particular, ferric chloride, which has a high heavy metal content. The present invention is effective when the total content of Mn, Ni, and Zn in the iron-based inorganic flocculant is 0.05% by weight or more and 5% by weight or less, for example, 0.1% by weight or more and 2% by weight or less. The amount of the iron-based inorganic flocculant added to the wastewater to be treated is preferably determined experimentally by a jar test or the like. The amount of the iron-based inorganic flocculant added to the wastewater to be treated varies depending on the water quality of the wastewater to be treated, but is usually about 10 to 400 mg / L.

[0041] When an iron-based inorganic flocculant is added to the wastewater to be treated, it is preferable to add a pH adjuster as necessary to adjust the pH to about 4 to 8, particularly about 5 to 8, before carrying out the flocculation treatment. As the pH adjuster, an acid such as hydrochloric acid or sulfuric acid, or an alkali such as sodium hydroxide is used, but is not limited to these. After the iron-based inorganic flocculant is added, a polymer flocculant such as an anionic polymer flocculant may be added.

Example

[0042] The present invention will be described more specifically with reference to the following examples.

[0043] In the following examples and comparative examples, an iron-based flocculant was added for flocculation treatment, and the MF membrane-treated water with the following water quality obtained by treating the flocculation-treated water with an MF membrane device was treated. <Water quality of MF membrane-treated water> Ca: 0.3 mg / L Mn: 0.1 mg / L Zn: 0.1 mg / L Ni: 0.1 mg / L pH: 9.6 Electrical conductivity: 200 mS / m

[0044] In addition, the following chelating scale inhibitors and RO membrane devices were used and treated under the following conditions.

[0045] An ORP meter was installed in the discharge pipe of the RO membrane concentrate to measure the ORP of the concentrate. <Chelating scale inhibitor> Chelating scale inhibitor A (EDTA content 40% by mass) <RO membrane device> KROA-2032 manufactured by Kurita Water Industries <Treatment conditions> RO feed water pressure: 0.39 MPa RO feed water pH: 6.0 RO feed water temperature: 25 °C Recovery rate: 70%

[0046] [Comparative Example 1] When RO membrane treatment was performed without adding a chelating scale inhibitor, the ORP of the RO membrane concentrate was 300 mV, and due to the oxidative degradation of the RO membrane, the electrical conductivity of the obtained permeate increased with time. The change over time of the RO membrane permeate at this time is shown in Fig. 2.

[0047] [Comparative Example 2] The amount of chelate scale inhibitor A added was constant at 35 mg / L. At this time, the measured ORP value of the RO membrane concentrated water was 44 mV.

[0048] No oxidative degradation of the RO membrane occurred, and the electrical conductivity of the water passing through the RO membrane was stable at 2 to 3 mS / m.

[0049] [Example 1] The ORP threshold for the RO membrane concentrated water was set to 45 mV, and the dosing control of the chelate-based scale inhibitor A was performed so that the ORP of the concentrated water did not exceed 45 mV. The average amount of chelate-based scale inhibitor A added during the operation period was 24 mg / L.

[0050] In addition, the RO membrane did not undergo oxidative degradation, and the electrical conductivity of the water passing through the RO membrane was stable at 2 to 3 mS / m.

[0051] The change over time in the RO membrane permeate water at this time is shown in Figure 2. Table 1 also shows the average amounts of chelate scale inhibitor A added during the operation period and the electrical conductivity of the RO membrane permeate water in Example 1 and Comparative Example 2.

[0052] [Table 1]

[0053] From Table 1 and FIG. 2, it is clear that according to the present invention, stable RO membrane treatment can be performed while reducing the amount of chelate scale inhibitor added. [Explanation of symbols]

[0054] 1 Raw water tank 3 pH adjustment tank 4 Hydrogen peroxide decomposition tank 5. Relay tank 7. Coagulation and Sedimentation Equipment 8 Neutralization tank 9 Relay tank 13 Aerobic biological treatment equipment 14 Coagulation and sedimentation equipment 15 Relay tank 17 Pressure filter 18 Filtration tank 22 MF membrane device 24 RO membrane equipment 26 ORP meter

Claims

1. A reverse osmosis membrane treatment method in which a chelate-based scale inhibitor is added to reverse osmosis membrane feed water and the water is passed through a reverse osmosis membrane device, The amount of the chelate-based scale inhibitor added is controlled based on a measured oxidation-reduction potential of at least one of the reverse osmosis membrane feed water and the reverse osmosis membrane concentrated water after the addition of the chelate-based scale inhibitor.

2. 2. The reverse osmosis membrane treatment method according to claim 1, wherein the amount of the chelate-based scale inhibitor added is controlled based on a measured value of an oxidation-reduction potential of the concentrated water from the reverse osmosis membrane.

3. 3. The reverse osmosis membrane treatment method according to claim 1, wherein the amount of the chelate-based scale inhibitor added is controlled within a range in which the measured value of the oxidation-reduction potential does not exceed a preset threshold value.

Images