Reverse osmosis membrane treatment method

Abstract

To appropriately control an addition amount of a scale inhibitor in a method of adding the 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: An RO membrane treatment method adds a scale inhibitor to RO membrane feed water and feeds to a RO membrane device. The RO membrane treatment method decides an addition amount of the scale inhibitor according to a measured value of a permeability indicator of a precision filtration membrane or an ultrafilter membrane measuring with regard to the RO membrane feed water or concentrated water of the RO membrane, or a measured value of a flux of the RO membrane device.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for RO membrane treatment by adding a 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 scale inhibitor added when recovering wastewater using an RO membrane, particularly wastewater discharged from the electronics industry or the like, adding a scale inhibitor to the wastewater, treating it with an RO membrane, and recovering and reusing the permeated water. [Background technology]

[0002] When wastewater discharged from the electronics industry and the like is treated with an RO membrane to be recovered and reused, the scale components (Ca, Mg, Fe, etc.) contained in the wastewater are concentrated in the RO membrane, increasing in concentration, and exceeding their solubility, causing scale formation on the RO membrane surface. To prevent this, a scale dispersant is added to the RO membrane feed water. Various scale inhibitors have been proposed for use here (e.g., Patent Documents 1 to 3).

[0003] Conventionally, the amount of anti-scalant added to RO membrane feed water has been determined based on the expected maximum concentration of scale components contained in the wastewater.

[0004] If a fixed amount of antiscalant is added regardless of fluctuations in the quality of the wastewater being treated, the excess amount of antiscalant will be wasted, resulting in additional chemical costs. In addition, the chemical treatment load of the antiscalant contained in the RO membrane concentrated water and discharged is large, increasing wastewater treatment costs and recovery costs. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5884730 [Patent Document 2] Patent No. 6057002 [Patent Document 3] Patent No. 6146075 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide an RO membrane treatment method in which a scale inhibitor is added to RO membrane feed water and the amount of scale inhibitor added is precisely controlled to reduce chemical costs, treatment costs, and recovery costs. [Means for solving the problem]

[0007] As a result of extensive investigations aimed at solving the above-mentioned problems, the inventors have discovered that by determining the amount of scale inhibitor to be added based on the measured permeability index of a microfiltration (MF) membrane or ultrafiltration (UF) membrane measured on RO membrane feed water or concentrated water, or the measured flux (permeation flux) of an RO membrane device, specifically, by setting a threshold value for the permeability index or flux and administering the scale inhibitor within a range that does not exceed this threshold, it is possible to reduce the amount of scale inhibitor to be administered while maintaining the same treated water quality and amount as before the reduction, and have completed the present invention. That is, the present invention relates to the following.

[0008] [1] A reverse osmosis membrane treatment method in which a 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 scale inhibitor to be added is determined based on a measured value of the permeability index of a microfiltration membrane or ultrafiltration membrane measured on the reverse osmosis membrane feed water or the concentrated water of the reverse osmosis membrane, or a measured value of the flux of the reverse osmosis membrane device.

[0009] [2] The reverse osmosis membrane treatment method according to [1], wherein the amount of scale inhibitor to be added is determined based on the measured value of the permeability index, and the amount of scale inhibitor to be added is increased by a predetermined amount when the measured value of the permeability index exceeds a predetermined threshold value.

[0010] [3] The reverse osmosis membrane treatment method according to [1] or [2], wherein the measured value of the permeability index is an SDI value, an FI value, an MF value or an MFF value.

[0011] [4] The reverse osmosis membrane treatment method according to [1], wherein the amount of scale inhibitor to be added is determined based on a measured value of flux of the reverse osmosis membrane device, and the amount of scale inhibitor to be added is increased by a predetermined amount when the measured value of the flux falls below a preset threshold. Effect of the Invention

[0012] According to the present invention, in a method for performing RO membrane treatment by adding a scale inhibitor to RO membrane feed water, the amount of scale inhibitor added can be accurately controlled without affecting the treated water quality or amount, thereby reducing chemical costs, treatment costs, and recovery costs. [Brief description of the drawings]

[0013] [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 changes over time in RO membrane flux in Examples 1 to 3 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] 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.

[0015] The RO membrane treatment method of the present invention is an RO membrane treatment method in which a scale inhibitor is added to RO membrane feed water (hereinafter sometimes simply referred to as "feed water") and the resulting water is passed through an RO membrane device, and is characterized in that the amount of the scale inhibitor to be added is determined based on a measured value of the permeability index (hereinafter sometimes simply referred to as "permeability index") of the MF membrane or UF membrane measured on the RO membrane feed water or the concentrated water of the RO membrane (hereinafter sometimes simply referred to as "concentrated water"), or a measured value of the flux of the RO membrane device (hereinafter sometimes referred to as "RO membrane flux").

[0016] The permeability index of the feed water or concentrated water or the RO membrane flux, which are indicators of the raw water (RO membrane feed water) conditions of the RO membrane, change when scale occurs in the RO membrane. However, if there is no change in the permeability index of the feed water or concentrated water or the RO membrane flux, it can be assumed that no scale has occurred.

[0017] Therefore, by setting threshold values ​​as management values ​​for these measured values ​​and controlling the dosage of scale inhibitor so that the threshold values ​​are not exceeded, it is possible to reduce the amount of scale inhibitor added and perform stable RO membrane treatment.

[0018] For this reason, in the present invention, the amount of scale inhibitor to be added is determined based on the permeability index of the feed water or concentrated water or the measured value of the RO membrane flux.

[0019] In the present invention, the permeability indexes used as an index of the water supply conditions include the SDI (silt density index) value defined in ASTM D4189, the FI (fouling index) value, MF value (Desalination, vol. 20, p. 353-364, 1977), and MFF (MF factor) value defined in JIS K 3802. These values ​​are usually values ​​obtained based on the filtration time measured when RO membrane feed water is subjected to a predetermined filtration with an MF membrane, which is a cellulose-based filter with a pore size of 0.45 μm, but the present invention is not limited to this, and values ​​obtained based on the filtration time when feed water or concentrated water is filtered with an MF membrane or UF membrane with a pore size other than the above may be used. Note that the SDI value and the FI value are synonymous.

[0020] The measurement methods for SDI (FI), MF, and MFF are as follows (JP 2017-113681 A).

[0021] <SDI(FI)> Under pressure of 0.2 MPa, the sample water is passed through an MF membrane with a maximum pore size of 0.45 μm and 47 mmφ, and the permeation time T0 of the first 500 mL is measured. After passing the water for 15 minutes, the time T at which the next 500 mL of permeated water is obtained is measured. 15is measured and SDI (FI) is calculated from the measured value using the following formula. SDI(FI)=(T 15 -T0) / T 15 ×100 / 15(min) Here, (T 15 -T0) / T 15 × 100 indicates the percentage decrease in filtration rate after 15 minutes of water flow, and SDI (FI) is calculated by dividing this by 15 minutes, and indicates the percentage decrease in filtration rate per minute.

[0022] <MF·MFF> The MF value is the time, expressed in seconds, for 1 L of sample water to pass through the MF membrane under a reduced pressure of 500 mmHg (-67 kPa). However, the viscosity of water, which is related to the resistance to water flow, varies greatly depending on the water temperature, and there are individual differences in the water permeability of MF membranes. The MFF value offsets this.

[0023] The MFF value is calculated by dividing 1 L of sample water into 500 mL portions, passing them through the MF membrane, measuring the filtration times T0 and T1 for each, and using these measurements to calculate the MFF value using the following formula. MFF=T1 / T0 If the sample water temperature and the temperature inside the measurement room do not differ significantly, the sample water temperatures can be considered to be the same, and therefore temperature-viscosity coefficient correction does not need to be performed.

[0024] RO membrane flux (unit: m 3 / Day element) is the value measured for the RO membrane.

[0025] The permeability indexes such as SDI value (FI value), MF value, and MFF value or the threshold value of RO membrane flux differ depending on each wastewater treatment system. Therefore, it is sufficient to conduct water flow tests under various conditions in advance and determine appropriate values ​​that can maintain the treated water quality and treated water volume based on the results.

[0026] Specific examples of methods for determining the amount of scale inhibitor to be added based on the threshold value of the measured value of the permeability index or RO membrane flux include the following methods (1) and (2). (1) When the measured value of the permeability index exceeds a preset threshold, the amount of scale inhibitor added is increased by a predetermined amount. In this case, when the measured value of the permeability index falls below the threshold, the amount of scale inhibitor added is returned to the amount before the increase. (2) When the measured value of RO membrane flux falls below a preset threshold, the amount of scale inhibitor added is increased by a predetermined amount. In this case, when the measured value of RO membrane flux becomes equal to or greater than the threshold, the amount of scale inhibitor added is returned to the amount before the increase.

[0027] In the above control method (1) or (2), there is no particular restriction on the amount of scale inhibitor to be increased when the threshold value is exceeded or fallen below the threshold value. For example, a control method may be used in which the amount of scale inhibitor added is increased by 5 to 100%, preferably by about 10 to 30%, of the normal amount of scale inhibitor.

[0028] In the present invention, there are no particular limitations on the scale inhibitor added to the feed water, and examples include general scale inhibitors for RO membrane treatment such as those described in Patent Documents 1 to 3 listed above. From these, an agent effective for preventing scale in the feed water can be selected and used depending on the water quality of the feed water (scale components in the feed water), and the amount of the agent to be added under normal circumstances can be determined based on the average concentration of scale components in the feed water.

[0029] 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.

[0030] Raw water in a raw water tank 1 is sent by a pump 2 to a pH adjustment tank 3, 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.

[0031] The water in the pH adjustment tank 3 is transferred to a coagulation tank 4 where a coagulant such as polyferric iron, ferric chloride, aluminum sulfate, PAC, etc. is added and the water is subjected to a coagulation treatment, after which it is transferred to a pressure flotation tank 5 where it is subjected to a pressure flotation treatment. The water from which the floating matter has been removed is transferred to a gravity type two-layer filter 6 and filtered. The filtered water is transferred to a recovery water tank 7.

[0032] The water in the recovery water tank 7 is sent to the MF membrane device 11 by a pump 8 and a pipe 9. During this water sending, a scale inhibitor is added to the pipe 9.

[0033] The permeate from the MF membrane device 11 is pressurized by a high-pressure pump 12 and supplied to an RO membrane device 13, where the permeate is extracted as treated water and sent to a location where the recovered water is reused. On the other hand, the concentrated water is discharged outside the system and is separately treated as wastewater.

[0034] In such a wastewater treatment system, a permeability index such as the SDI value of the feed water or concentrated water of the RO membrane device 13 is measured, the measured value is compared with a preset threshold value, and if the measured value exceeds the threshold value, the amount of scale inhibitor added is increased, and if the measured value falls below the threshold value, the amount of scale inhibitor added is returned to the original value. Alternatively, the RO membrane flux of the RO membrane device 13 is measured, the measured value is compared with a preset threshold value, and if the measured value falls below the threshold value, the amount of scale inhibitor added is increased, and if the measured value exceeds the threshold value, the amount of scale inhibitor added is returned to the original value.

[0035] Such control of the dosing of the scale inhibitor can be performed automatically by a measuring means for measuring the permeability index of the feed water or concentrated water of the RO membrane device 13 or the RO membrane flux of the RO membrane device 13, and a control means for inputting the measured values ​​and controlling the dosing pump of the scale inhibitor based on the inputted measured values.

[0036] In FIG. 1, the MF membrane device 9 is installed in front of the RO membrane device 13, but a UF membrane device may be provided instead of the MF membrane device 9.

[0037] In the present invention, the wastewater to be subjected to RO membrane treatment is not particularly limited. However, the RO membrane treatment of the present invention is suitably applied, for example, to a wastewater treatment system that treats and recovers and reuses wastewater discharged from the electronic industry field, the steel manufacturing field, petrochemical complexes, etc.

Example

[0038] The present invention will be described more specifically with reference to the following examples.

[0039] In the following examples and comparative examples, electronic industry wastewater with the following water quality was treated using the wastewater treatment system shown in FIG. 1. <Electronic industry wastewater quality> Ca concentration: 2 mg / L NH4: 15 mg / L NO3: 10 mg / L TOC: 5 mg / L pH: 6.5

[0040] Sulfuric acid was used as a pH adjuster to adjust the pH to 5.8. Polyiron was used as a flocculant and added at an addition amount of 30 mg / L.

[0041] The scale inhibitor added, the specifications and treatment conditions of the MF membrane device and RO membrane device used were as follows. <Scale inhibitor> Scale inhibitor "Clifloat D603" manufactured by Kurita Water Industries, Ltd. <MF membrane device> 36FPC-W500V200 manufactured by Ameroid Co., Ltd. <RO membrane device> KROA-98-8HN manufactured by Kurita Water Industries, Ltd. <Treatment conditions> RO feed water pressure: 0.25 MPa RO feed water pH: 7.5 RO feed water temperature: 25 °C Water recovery rate: 90%

[0042] [Example 1] The amount of scale inhibitor added was set to 1 mg / L, and the threshold SDI value of the feedwater to the RO membrane device 13 was set to 4. When the SDI value of the RO feedwater sampled and measured exceeded 4, the amount of scale inhibitor added was increased by 1 mg / L to 2 mg / L, and when the SDI value fell below the threshold, the amount of scale inhibitor added was returned to 1 mg / L, as a chemical dosing control.

[0043] [Example 2] In the first embodiment, chemical dosing control was performed based on the measured value of the RO membrane flux of the RO membrane device 13 instead of the SDI value of the feed water of the RO membrane device 13. The threshold value of the RO membrane flux was set to 24 m 3 / Day·element, and the RO membrane flux is 24m 3 When the value fell below the threshold, the amount of scale inhibitor added was increased by 1 mg / L to 2 mg / L, and when the value rose above the threshold, the amount of scale inhibitor added was returned to 1 mg / L.

[0044] [Example 3] The amount of scale inhibitor added was set to 1 mg / L, and the threshold MFF value of the feedwater to the RO membrane device 13 was set to 1.2. When the MFF value of the RO feedwater sampled and measured exceeded 1.2, the amount of scale inhibitor added was increased by 1 mg / L to 2 mg / L, and when the MFF value fell below the threshold, the amount of scale inhibitor added was returned to 1 mg / L, as a chemical dosing control.

[0045] [Comparative Example 1] Amount of scale inhibitor added: A constant amount of 1 mg / L was added.

[0046] [Comparative Example 2] Amount of scale inhibitor added: A constant amount of 9 mg / L was added.

[0047] [Results / Discussion] The changes over time in RO membrane flux in Examples 1 to 3 and Comparative Examples 1 and 2 are shown in FIG. Table 1 also shows the change over time in the SDI value of the RO feedwater in Examples 1 and 2 and Comparative Examples 1 and 2, and the change over time in the MFF value of the RO feedwater in Example 3. Table 1 also shows the average amount of scale inhibitor added per liter of feed water during the test period for each example.

[0048] [Table 1]

[0049] 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 scale inhibitor added. [Explanation of symbols]

[0050] 1 Raw water tank 3 pH adjustment tank 4 Coagulation tank 5. Pressure flotation tank 6 Gravity double layer filter 7. Collection tank 11 MF membrane device 13 RO membrane equipment

Claims

1. A reverse osmosis membrane treatment method in which a 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 scale inhibitor to be added is determined based on a measured value of the permeability index of a microfiltration membrane or ultrafiltration membrane measured on the reverse osmosis membrane feed water or the concentrated water of the reverse osmosis membrane, or a measured value of the flux of the reverse osmosis membrane device.

2. 2. The reverse osmosis membrane treatment method of claim 1, wherein the amount of scale inhibitor to be added is determined based on the measured value of the permeability index, and the amount of scale inhibitor to be added is increased by a predetermined amount when the measured value of the permeability index exceeds a preset threshold value.

3. 3. The reverse osmosis membrane treatment method according to claim 1, wherein the measured value of the permeability index is an SDI value, an FI value, an MF value, or an MFF value.

4. 2. The reverse osmosis membrane treatment method according to claim 1, wherein the amount of scale inhibitor to be added is determined based on a measured value of flux of the reverse osmosis membrane device, and the amount of scale inhibitor to be added is increased by a predetermined amount when the measured value of flux falls below a preset threshold value.

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