Water treatment system and water treatment method

Abstract

To provide a water treatment system and a water treatment method capable of stably obtaining treated water of good quality at low operating pressure with a high recovery rate.SOLUTION: A water treatment system in which treated water is treated by a first separation membrane to obtain first concentrated water and first permeate, and then the first permeate is treated by a second separation membrane to obtain second concentrated water and second permeate, the operating pressure of a first membrane separation treatment unit and the operating pressure of a second membrane separation treatment unit are set below a specific value, the salt removal rate of the first separation membrane and the salt removal rate of the second separation membrane are in a specific relationship, and the first separation membrane and the second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane. The water treatment system is provided with return means for returning at least one of the following A and B to the treated water. The water treatment system is provided with discharge means for discharging at least one of the following A and B. A: at least a portion of the first concentrated water. B: at least a portion of the second concentrated water.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a water treatment system and a water treatment method. [Background technology]

[0002] A method of passing water to be treated through a membrane separation treatment device such as a reverse osmosis membrane (RO membrane) to obtain concentrated water and permeate is used in various fields such as desalination of seawater, production of pure water and ultrapure water, and wastewater treatment. In order to ensure sufficient water quality of the treated water, a method of performing treatment using a reverse osmosis membrane in two stages has been proposed (for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-075219 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-120970 Summary of the Invention [Problem to be solved by the invention]

[0004] When treating water with a high concentration of ions, i.e., a high total dissolved solids (TDS), membranes with a high TDS removal rate are used, such as the reverse osmosis membranes used for seawater desalination in Patent Document 2. However, equipment using membranes with a high TDS removal rate requires very high operating pressure for filtration. As a result, the concentrate becomes very concentrated, which is prone to causing scaling, where dissolved substances precipitate and adhere.

[0005] Scaling can cause a further increase in operating pressure by narrowing the supply water flow path or a decrease in the filtration function due to damage to the membrane surface. To suppress scaling, generally, the reduction of the final permeate recovery rate or the addition of a scale inhibitor is carried out. For example, the water recovery rate in seawater desalination equipment is about 20% by volume even when a scale inhibitor is used in combination. To further increase the recovery rate of the entire equipment, it is necessary to treat the discharged concentrated water again in two or three stages with a reverse osmosis membrane. In that case, it is necessary to further increase the operating pressure.

[0006] Also, when treating the discharged concentrated water in three or more stages, the required operating pressure becomes even higher, and the strength of the reverse osmosis membrane is insufficient, making it difficult to apply. In that case, it is necessary to combine different treatment methods such as electrodialysis to recover the concentrated water, and the required cost increases due to the complexity of the system.

[0007] As described above, when treating water to be treated with a certain level of TDS, the conventional method still has problems in stably obtaining treated water with good water quality at a high recovery rate with a low operating pressure. The present disclosure provides a water treatment system and a water treatment method capable of stably obtaining treated water with good water quality at a high recovery rate with a low operating pressure.

Means for Solving the Problems

[0008] [1] A first membrane separation treatment device that performs membrane separation treatment on the water to be treated with a first separation membrane to obtain a first concentrated water and a first permeate, A second membrane separation treatment device that performs membrane separation treatment on the first permeate with a second separation membrane to obtain a second concentrated water and a second permeate, A water treatment system comprising at least: When the operating pressure (MPa) of the first membrane separation treatment device is P1 and the operating pressure (MPa) of the second membrane separation treatment device is P2, P{1} and P{2} are 2.0 MPa or less, When the salt rejection rate (weight %) of the first separation membrane is S1 and the salt rejection rate (weight %) of the second separation membrane is S2, the relationship S1 < S2 is satisfied. The first separation membrane and the second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane, The water treatment system includes a return means for returning at least one of the following A and B to the water to be treated, The water treatment system includes a discharge means for discharging at least one of the following A and B, a water treatment system. A: At least a part of the first concentrated water B: At least a part of the second concentrated water [2] The water treatment system according to [1], wherein the value of the total dissolved solids TDS of the water to be treated is 1000 mg / L or more. [3] The water treatment system according to [1] or [2], wherein S1 and S2 are 70% by weight or more. [4] The water treatment system according to any one of [1] to [3], wherein P1 and P2 satisfy the relationship P1 ≤ P2. [[ID=?]] [5] The water treatment system includes a return means for returning the B to the water to be treated, The water treatment system according to any one of [1] to [4], which includes a discharge means for discharging the A. [6] A first membrane separation treatment step of subjecting the water to be treated to membrane separation treatment by a first separation membrane to obtain a first concentrated water and a first permeated water, A second membrane separation treatment step of subjecting the first permeated water to membrane separation treatment by a second separation membrane to obtain a second concentrated water and a second permeated water, A water treatment method having at least When the operating pressure (MPa) of the first membrane separation treatment is P1 and the operating pressure (MPa) of the second membrane separation treatment is P2, P1 and P2 are 2.0 MPa or less, When the salt removal rate (% by weight) of the first separation membrane is S1 and the salt removal rate (% by weight) of the second separation membrane is S2, the relationship S1 < S2 is satisfied, The first separation membrane and the second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane, Return at least one of the following A and B to the water to be treated, A water treatment method for discharging at least one of the following A and B. A: at least a portion of the first concentrated water B: at least a portion of the second concentrated water [Effects of the Invention]

[0009] The present disclosure provides a water treatment system and a water treatment method that can stably obtain treated water of good quality at a high recovery rate even under low operating pressure. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic configuration diagram showing an example of a water treatment system according to an embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram illustrating an example of a conventional water treatment system. DETAILED DESCRIPTION OF THE INVENTION

[0011] Unless otherwise specified, the expressions "XX to YY" or "XX to YY" representing a numerical range mean a numerical range including the lower and upper limits, which are the endpoints. When a numerical range is described in stages, the upper and lower limits of each numerical range can be combined in any way.

[0012] [Water treatment system] One embodiment of the water treatment system of the present invention is a water treatment system including at least a first membrane separation treatment device that performs membrane separation treatment on water to be treated using a first separation membrane to obtain first concentrated water and first permeate, and a second membrane separation treatment device that performs membrane separation treatment on the first permeate using a second separation membrane to obtain second concentrated water and second permeate. According to the water treatment system of this embodiment, treated water of good quality can be stably obtained at a high recovery rate under low operating pressure. A preferred embodiment of the water treatment system of this embodiment will be described below.

[0013] Fig. 1 is a schematic diagram showing an example of a water treatment system according to the present embodiment. The water treatment system 100 shown in Fig. 1 includes a first membrane separation treatment device 10 that passes water to be treated through a first separation membrane to obtain a first concentrate and a first permeate, and a second membrane separation treatment device 20 that passes the first permeate through a second separation membrane to obtain a second concentrate and a second permeate.

[0014] A liquid feed pipe 11 is connected to the inlet of the first membrane separation treatment device 10 for water to be treated. The water to be treated is fed to the first membrane separation treatment device 10 through the liquid feed pipe 11, and in the first membrane separation treatment device 10, the water is passed through a first separation membrane 12 and separated into a first concentrated water that did not permeate through the first separation membrane 12 and a first permeate that permeated through the first separation membrane 12. The first permeate outlet of the first membrane separation treatment device 10 and the first permeate inlet of the second membrane separation treatment device 20 are connected by a liquid feed pipe 21. The first permeate is fed from the first membrane separation treatment device 10 to the second membrane separation treatment device 20 through the liquid feed pipe 21, and is passed through a second separation membrane 22 in the second membrane separation treatment device 20 to be separated into a second concentrate that did not permeate the second separation membrane 22 and a second permeate that permeated the second separation membrane 22. The second permeate is discharged from the system through a final permeate pipe 25, and a final permeate is obtained.

[0015] The first concentrated water outlet is connected to a return pipe 13 and a discharge pipe 14, and the second concentrated water outlet is connected to a return pipe 23 and a discharge pipe 24. At least a portion of the first concentrated water and the second concentrated water is returned as water to be treated through the return pipe 13 and the return pipe 23. In addition, a portion of one or both of the first concentrated water and the second concentrated water is discharged from the system through a portion of one or both of the discharge pipe 14 and the discharge pipe 24.

[0016] When the operating pressure (MPa) of the first membrane separation treatment device is P1, P1 is 2.0 MPa or less, preferably 1.75 MPa or less, and more preferably 1.5 MPa or less. Also, P1 is preferably 0.25 MPa or more, and more preferably 0.5 MPa or more.

[0017] Furthermore, when the operating pressure (MPa) of the second membrane separation treatment apparatus is P2, P2 is 2.0 MPa or less, preferably 1.75 MPa or less, and more preferably 1.5 MPa or less. Also, P2 is preferably 0.25 MPa or more, and more preferably 0.5 MPa or more.

[0018] In addition, it is preferable that P1 and P2 satisfy the relationship P1 ≦ P2. By P1 and P2 satisfying the above conditions, treated water with better water quality can be obtained.

[0019] When the salt removal rate (weight %) of the first separation membrane is S1, S1 is preferably 70% by weight or more, more preferably 75% by weight or more, and even more preferably 80% by weight or more. Also, S1 may be 100% by weight or less.

[0020] Furthermore, when the salt removal rate (weight %) of the second separation membrane is S2, S2 is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 90% by weight or more. Also, S2 may be 100% by weight or less.

[0021] In addition, S1 and S2 satisfy the relationship S1 < S2. By S1 and S2 satisfying the above conditions, treated water with better water quality can be stably obtained at a high recovery rate. In order for S1 and S2 to satisfy the above relationship, commercially available products of separation membranes having appropriate salt removal rates may be selected.

[0022] The salt removal rate of the separation membrane can be measured according to JIS K 3805-2:1990 "Performance Test Methods for Reverse Osmosis Elements and Modules".

[0023] When the recovery rate (volume %) of the permeated water in the membrane separation treatment apparatus is defined as in the following formula (1) Recovery rate (volume %) = (amount of permeated water / amount of feed water passed through the membrane separation treatment apparatus) × 100 ··· Formula (1) When the recovery rate (volume %) of the first permeated water in the above first membrane separation treatment device is defined as R1, it is preferably 50% by volume or less, and more preferably 40% by volume or less. Also, R1 is preferably 5% by volume or more, and more preferably 15% by volume or more.

[0024] Furthermore, when the recovery rate (volume %) of the second permeated water in the above second membrane separation treatment device is defined as R2, R2 is preferably 70% by volume, and more preferably 50% by volume or less. Also, R2 is preferably 5% by volume or more, and more preferably 15% by volume or more.

[0025] In addition, it is preferable that the above R1 and the above R2 satisfy the relationship R1 < R2. By R1 and R2 satisfying the above conditions, treated water with better water quality can be stably obtained at a high recovery rate. In order for R1 and R2 to satisfy the above relationship, R1 and R2 may be controlled by a mechanical or electrical control method. The mechanical control method is a method of controlling to a constant flow rate by a constant flow rate valve, and the electrical control method is a method of automatically adjusting the opening degree of a flow rate regulating valve or a method of controlling the rotation speed of a supply pump.

[0026] The above first separation membrane and the above second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane. Furthermore, it is preferable that the pore diameter of the first separation membrane is larger than the pore diameter of the second separation membrane. By the above first separation membrane and the above second separation membrane satisfying the above conditions, treated water with better water quality can be stably obtained at a high recovery rate.

[0027] Examples of water to be treated by this system include wastewater from factories, seawater, brackish water, and recycled water from factories. The water may also contain metal components such as sodium, calcium, magnesium, iron, copper, and zinc, as well as chelating agents. The total dissolved solids (TDS) of the water to be treated is preferably 1,000 mg / L or higher, and more preferably 5,000 mg / L or higher. The TDS value is preferably 100,000 mg / L or lower, and more preferably 50,000 mg / L or lower. Total dissolved solids (TDS) can be measured according to JIS K 0102:2019 "Testing Methods for Industrial Wastewater."

[0028] The water treatment system of this embodiment is provided with a return means for returning at least one of the following A and B as the water to be treated, and further provided with a discharge means for discharging at least one of the following A and B. The water treatment system of this embodiment is also preferably provided with a return means for returning the following B as the water to be treated, and is also preferably provided with a discharge means for discharging the following A. Concentration Returning the water increases the recovery rate of treated water but worsens its quality, while discharging the concentrated water decreases the recovery rate of treated water but improves its quality. By returning the second concentrated water with a lower TDS and discharging the first concentrated water with a higher TDS, it is possible to achieve both a high recovery rate and good water quality. A: at least a portion of the first concentrated water B: at least a portion of the second concentrated water

[0029] [Water treatment method] One embodiment of the water treatment method of the present invention is a water treatment method comprising at least a first membrane separation treatment step in which water to be treated is subjected to membrane separation treatment using a first separation membrane to obtain a first concentrate and a first permeate, and a second membrane separation step in which the first permeate is subjected to membrane separation treatment using a second separation membrane to obtain a second concentrate and a second permeate. According to the water treatment method of this embodiment, treated water of good quality can be stably obtained at a high recovery rate under low operating pressure.

[0030] When the operating pressure (MPa) of the first membrane separation treatment is P1 and the operating pressure (MPa) of the second membrane separation treatment is P2, P1 and P2 are 2.0 MPa or less. When the salt removal rate (% by weight) of the first separation membrane is S1 and the salt removal rate (% by weight) of the second separation membrane is S2, the relationship S1 < S2 is satisfied. The first separation membrane and the second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane. At least one of the following A and B is returned as the water to be treated, and at least one of the following A and B is discharged. A: At least a part of the first concentrated water B: At least a part of the second concentrated water

[0031] Preferred embodiments of other water treatment methods are the same as those of the water treatment system of the present invention.

Example

[0032] The present invention will be described more specifically below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0033] In the present examples and comparative examples, the following separation membranes were used as the first to third separation membranes. Membrane A: Ultra-low pressure reverse osmosis membrane (manufactured by Toray Industries, Inc., TMG series) Membrane B: Low pressure reverse osmosis membrane (manufactured by Toray Industries, Inc., TM series) Membrane C: Reverse osmosis membrane for seawater desalination (manufactured by Toray Industries, Inc., TM800 series)

[0034] [Examples 1 to 4] In Examples 1 to 4, water treatment was carried out as follows using the water treatment system having the overall configuration shown in FIG. 1. Commercially available reagents such as sodium salts, calcium salts, and magnesium salts were added to the final treated water from the wastewater treatment facility in our own factory, and after adjusting the TDS, pretreatment was performed using an ultrafiltration membrane to obtain treated water. The obtained treated water was pressurized to pass through a first separation membrane (membrane A) to obtain a first concentrated water and a first permeate. The obtained first permeate was pressurized to pass through a second separation membrane (membrane B) to obtain a second concentrated water and a second permeate. 30% by volume of the obtained first concentrated water was discharged, and the remaining 70% by volume of the first concentrated water and the second concentrated water were returned as treated water, and the above water treatment was carried out again. The finally obtained second permeate The total amount was used as the final permeate.

[0035] Total dissolved solids (TDS) (mg / L) of the treated water, types of first and second separation membranes, operating pressures P1-P2 (MPa), salt rejection rates S1-S2 (weight%), permeate recovery rates R1-R2 (volume%), amount of treated water (m 3 / day), final permeate amount (m 3 / day) and TDS (mg / L) of the final permeate are shown in Table 1.

[0036] [Comparative Examples 1 to 3] In Comparative Examples 1 to 3, water treatment was carried out as follows using a water treatment system having the overall configuration shown in FIG. The water to be treated, obtained in the same manner as in Examples 1 to 4, was pressurized to pass through a first separation membrane (Membrane C), yielding a first concentrate and a first permeate. The obtained first concentrate was pressurized to pass through a second separation membrane (Membrane C), yielding a second concentrate and a second permeate. The obtained first permeate and second permeate were mixed, and pressure was applied to 50% of the mixture to pass through a third separation membrane (Membrane C), yielding a third concentrate and a third permeate. 50% of the second concentrate and the third concentrate were discharged, and the remaining 50% of the second concentrate was returned as the water to be treated, and the above water treatment was carried out again. The remaining 50% of the mixture of the first permeate and the second permeate that was not passed through the third separation membrane and the total amount of the third permeate were combined to form the final permeate.

[0037] [Reference example 1] In Reference Example 1, water treatment was carried out as follows using a water treatment system having the overall configuration shown in FIG. The water to be treated, obtained in the same manner as in Examples 1 to 4, was pressurized to pass through a first separation membrane (Membrane C), yielding a first concentrate and a first permeate. The first permeate thus obtained was pressurized to pass through a second separation membrane (Membrane B), yielding a second concentrate and a second permeate. 5% by volume of the first concentrate thus obtained was discharged, and the remaining 95% by volume of the first concentrate and the second concentrate were returned as the water to be treated, and the above-mentioned water treatment was carried out again. The total amount of the second permeate finally obtained was designated as the final permeate.

[0038] [Reference example 2] In Reference Example 2, water treatment was carried out as follows using a water treatment system having the overall configuration shown in FIG. The water to be treated, obtained in the same manner as in Examples 1 to 4, was pressurized to pass through a first separation membrane (Membrane B), yielding a first concentrate and a first permeate. The first permeate thus obtained was pressurized to pass through a second separation membrane (Membrane B), yielding a second concentrate and a second permeate. 10% by volume of the first concentrate thus obtained was discharged, and the remaining 90% by volume of the first concentrate and the second concentrate were returned as the water to be treated, and the above-mentioned water treatment was carried out again. The total amount of the second permeate finally obtained was designated as the final permeate.

[0039] Total dissolved solids (TDS) (mg / L) of the treated water, types of first to third separation membranes, operating pressures P1 to P3 (MPa), salt rejection rates S1 to S3 (weight%), permeate recovery rates R1 to R3 (volume%), amount of treated water (m 3 / day), final permeate amount (m 3 / day) and TDS (mg / L) of the final permeate are shown in Table 1.

[0040] [Table 1]

[0041] As shown in Table 1, in Examples 1 to 4, a large amount of permeate was obtained, and the TDS of the permeate was also good. On the other hand, in Comparative Examples 1 to 3, the permeate volume was smaller and the permeate TDS was higher than in Examples 1 to 4. In Comparative Examples 1 to 3, the liquid feed pipe 33 was used as a bypass to ensure the final permeate volume, and some of the water to be treated was recovered without passing through the third separation membrane, which is thought to have worsened the permeate TDS. Here, it is presumed that if the liquid feed pipe 33 were not used, the permeate TDS would improve but the final permeate volume would decrease. In Reference Examples 1 and 2, the TDS of the permeate was good, but the operating pressure was high, resulting in high power costs. Furthermore, the permeate volume decreased due to the high concentration of the target substances on the membrane surface, so it was necessary to increase the number of membranes to ensure the final permeate volume. [Explanation of symbols]

[0042] 100...water treatment system, 10...first membrane separation treatment device, 11...liquid transfer piping, 12...first separation membrane, 13...return piping, 14...discharge piping, 20...second membrane separation treatment device, 21...liquid transfer piping, 22...second separation membrane, 23...return piping, 24...discharge piping, 25...final permeate piping, 26...liquid transfer piping, 30...third membrane separation treatment device, 31...liquid transfer piping, 32...third separation membrane, 33...liquid transfer piping, 34...discharge piping

Claims

1. a first membrane separation treatment device that performs membrane separation treatment on water to be treated using a first separation membrane to obtain first concentrated water and first permeate; a second membrane separation treatment device that performs membrane separation treatment on the first permeate using a second separation membrane to obtain a second concentrate and a second permeate; A water treatment system comprising at least The total dissolved solids (TDS) value of the treated water is 1000 mg / L or more, When the operating pressure (MPa) of the first membrane separation treatment device is P1 and the operating pressure (MPa) of the second membrane separation treatment device is P2, P1 and P2 are 2.0 MPa or less, When the salt rejection rate (wt%) of the first separation membrane is S1 and the salt rejection rate (wt%) of the second separation membrane is S2, the relationship S1 < S2 is satisfied, the first separation membrane and the second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane; The water treatment system includes a return means for returning at least one of the following A and B to the water to be treated: The water treatment system comprises a discharge means for discharging at least one of the following A and B: A: at least a portion of the first concentrated water B: at least a portion of the second concentrated water

2. The water treatment system according to claim 1 , wherein the S1 and S2 are 70% by weight or more.

3. The water treatment system according to claim 1 or 2, wherein P1 and P2 satisfy a relationship of P1≦P2.

4. The water treatment system includes a return means for returning the B to the water to be treated, The water treatment system according to any one of claims 1 to 3, further comprising a discharge means for discharging the A.

5. a first membrane separation treatment step in which the water to be treated is subjected to membrane separation treatment using a first separation membrane to obtain a first concentrate and a first permeate; a second membrane separation treatment step in which the first permeate is subjected to membrane separation treatment using a second separation membrane to obtain a second concentrate and a second permeate; A water treatment method comprising at least The total dissolved solids (TDS) value of the treated water is 1000 mg / L or more, When the operating pressure (MPa) of the first membrane separation treatment step is P1 and the operating pressure (MPa) of the second membrane separation treatment step is P2, P1 and P2 are 2.0 MPa or less, When the salt rejection rate (wt%) of the first separation membrane is S1 and the salt rejection rate (wt%) of the second separation membrane is S2, the relationship S1 < S2 is satisfied, the first separation membrane and the second separation membrane are at least one selected from a nanofiltration membrane and a reverse osmosis membrane; At least one of the following A and B is returned to the water to be treated: A water treatment method comprising discharging at least one of the following A and B: A: at least a portion of the first concentrated water B: at least a portion of the second concentrated water

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