ULTRAPURE WATER PRODUCING METHOD

Abstract

A method for producing ultrapure water includes supplying raw water (industrial water, tap water, well water, or used ultrapure water discharged from semiconductor plants) to a pretreatment system for treating the raw water to produce water, supplying the water to a primary water purification system having a reverse osmosis membrane separation unit to produce a primarily purified water, and supplying the primarily purified water to a secondary purification system to produce ultrapure water.

Claims

1. A method for producing ultrapure water, comprising: supplying raw water to a pretreatment system for treating the raw water, thereby producing water, the raw water being selected from the group consisting of industrial water, tap water, well water, and used ultrapure water discharged from semiconductor plants, supplying the water to a primary water purification system comprising a reverse osmosis membrane separation unit, thereby producing a primarily purified water; and supplying the primarily purified water to a secondary purification system, thereby producing ultrapure water, wherein the reverse osmosis membrane separation unit installed in the primary water purification system is a high-pressure reverse osmosis membrane separation unit installed in a single stage, and the high-pressure reverse osmosis membrane separation unit has a pure water flux of 0.5 m.sup.3/m.sup.2.Math.D or more and a NaCl rejection of 99.5% or more (32,000 mg/L NaCl) at an operating pressure of 5.52 MPa.

2. The method for producing ultrapure water according to claim 1, wherein the water supplied to the high-pressure reverse osmosis membrane separation unit has a TDS of 1,500 mg/L or less.

3. The method for producing ultrapure water according to claim 1, wherein a pressure difference between the high-pressure reverse osmosis membrane separation unit on a side of the primary water purification system and the high-pressure reverse osmosis membrane separation unit on a side of the secondary purification system is 1.5 to 3 MPa.

4. The method for producing ultrapure water according to claim 1, wherein the pretreatment system includes a flocculation unit, a pressure flotation unit, a sedimentation unit, or a filtration unit.

5. The method for producing ultrapure water according to claim 4, wherein the primary water purification system further comprises an ion exchange unit and a degasification unit.

6. The method for producing ultrapure water according to claim 5, wherein the reverse osmosis membrane separation unit, the ion exchange unit, and the degasification unit are connected in this order.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a system diagram of an example embodiment of an ultrapure water producing apparatus according to the present invention.

[0020] FIG. 2 is a system diagram of a conventional ultrapure water producing apparatus.

DESCRIPTION OF EMBODIMENTS

[0021] Embodiments of ultrapure water producing apparatuses of the present invention will now be described in detail.

[0022] In the present invention, as shown in FIG. 1, ultrapure water is preferably produced by sequentially treating raw water through a pretreatment system 1, a primary water purification system 2, and a subsystem 3. High-pressure reverse osmosis membrane separation units serving as reverse osmosis membrane separation units (RO units) are installed in a single stage in the primary water purification system 2.

[0023] A high-pressure reverse osmosis membrane separation unit has been used in seawater desalination and has a standard operating pressure of 5.52 MPa or more and has a pure water flux of 0.5 m.sup.3/m.sup.2.Math.D or more and a NaCl rejection of 99.5% or more (32,000 mg/L NaCl) at the standard operating pressure. The NaCl rejection is measured at 25° C. using an aqueous NaCl solution with a NaCl concentration of 32,000 mg/L. High-pressure, low-pressure, and ultra-low-pressure reverse osmosis membranes can be distinguished based on data from catalogues (including technical documents) available from membrane manufacturers that list the specifications of their reverse osmosis membranes.

[0024] A high-pressure reverse osmosis membrane includes a denser skin layer, which forms the outer surface thereof, than a low-pressure or ultra-low-pressure reverse osmosis membrane used in a primary water purification system of a conventional ultrapure water producing apparatus. Thus, a high-pressure reverse osmosis membrane has a lower membrane permeate flow rate per unit operating pressure and an extremely higher organic rejection than a low-pressure or ultra-low-pressure reverse osmosis membrane. When a reverse osmosis membrane is used to treat feed water with a salt concentration of 1,500 mg/L or less TDS (total dissolved solids), a maximum osmotic pressure applied thereto is about 1.0 MPa under an operating condition of a recovery of 90%. Accordingly, when a high-pressure reverse osmosis membrane separation unit is used to treat feed water with a TDS of 1,500 mg/L or less, the unit is preferably used at an effective transmembrane pressure (the difference in pressure between the primary and secondary sides) of about 1.5 to 3 MPa, more preferably about 2 to 3 MPa, to achieve a flow rate similar to that of a low-pressure or ultra-low-pressure reverse osmosis membrane. As a result, water can be treated only by one-stage RO membrane treatment with a quality and flow rate similar to those of conventional two-stage RO membrane treatment. This requires fewer membrane units, vessels, and pipes and therefore contributes to cost reduction and space saving.

[0025] The reverse osmosis membranes may be membranes of any shape, such as spiral wound membranes, hollow fiber membranes, 4 inch RO membranes, 8 inch RO membranes, or 16 inch RO membranes.

[0026] Although raw water is treated by the pretreatment system 1 before being supplied to the primary water purification system 2 in FIG. 1, a dilute wastewater treatment system (not shown) may be installed in parallel with the pretreatment system 1, and water treated by the dilute wastewater treatment system may be supplied to the primary water purification system. In this case, in the flow in FIG. 1, a tank is preferably installed upstream of the primary water purification system 2 such that both the treated water from the pretreatment system 1 and the treated water from the dilute wastewater treatment system flow into the tank.

EXAMPLES

Experiment 1

[0027] Electronic device factory wastewater (electrical conductivity: 100 mS/m, TDS: 600 mg/L, TOC: 10 mg/L) was passed through a high-pressure reverse osmosis membrane separation unit (RO membrane: SWC4+ available from Nitto Denko Corporation, flux at operating pressure of 5.52 MPa: 24.6 m.sup.3/m.sup.2.Math.D, NaCl rejection: 99.8% (32,000 mg/L NaCl)) installed in a single stage at a recovery of 73%. As a result, the permeate water had a TOC of 0.85 mg/L. The effective transmembrane pressure was 2.0 MPa.

Experiment 2

[0028] The same electronic device factory wastewater used in Experiment 1 was passed through RO units installed in two stages and equipped with an ultra-low-pressure RO membrane (ES-20 available from Nitto Denko Corporation) at a condition where an upstream RO recovery is 75%, a downstream RO recovery is 90%, and a total water recovery is 73% (the downstream RO concentrate water was returned to the upstream RO feed water). As a result, the first-stage RO permeate water had a TOC concentration of 1.35 mg/L, and the second-stage RO permeate water had a TOC concentration of 0.9 mg/L. The effective transmembrane pressure was 0.5 MPa in the first stage and was 0.75 MPa in the second stage.

[0029] Experiments 1 and 2 demonstrated that the quality of permeate water produced by the high-pressure reverse osmosis membrane separation unit installed in a single stage was similar to that of permeate water produced by the ultra-low-pressure reverse osmosis membrane separation units installed in two stages. In Experiment 2, the first-stage RO permeate water had a TOC concentration as high as 1.35 mg/L, demonstrating that the ultra-low-pressure reverse osmosis membrane separation unit installed in a single stage was less effective in removing TOC and TDS than the high-pressure reverse osmosis membrane separation unit.

[0030] Next, further experiment was conducted. This experiment used an ultrapure water producing apparatus same as that shown in FIG. 2 except that the primary water purification system thereof was replaced by the primary water purification system shown in FIG. 1 having the above high-pressure reverse osmosis membrane separation unit installed in a single stage. This ultrapure water producing apparatus was operated at an effective transmembrane pressure of 2.0 MPa of the high-pressure reverse osmosis membrane separation unit. By this operation, ultrapure water was produced with a quality similar to that of water produced by a conventional apparatus (having ROs in a two-stage, first-stage effective transmembrane pressure: 0.5 MPa, second-stage effective transmembrane pressure: 0.75 MPa) at substantially the same product flow rate.

[0031] Whereas particular embodiments of the present invention have been described in detail, a person skilled in the art would appreciate that various modifications can be made without departing from the spirit and scope of the present invention.

[0032] This application is based on a Japanese patent application 2011-117142 filed on May 25, 2011, the entire content of which is herein incorporated by reference.