Reverse osmosis process and equipment

Details Reverse osmosis process and equipment Reverse osmosis process and equipment

1994-12-14

2001-10-16

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S651000, C210S257200, C210S195200, C210S739000, C210S743000, C210S900000, C210S638000

Reexamination Certificate

active

06303037

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reverse osmosis process and equipment which are suitable for production of high-purity water for use in semiconductor and pharmaceutical industries, etc.
2. Related Art
After a pretreatment of industrial water or city water, such as coagulation and filtration, membrane filtration, activated carbon filtration, or decarbonation treatment thereof, a reverse osmosis treatment of the pretreated water has heretofore been effected using reverse osmosis equipment to produce high-purity water. A further treatment of permeate water resulting from permeation of the pretreated water through a reverse osmosis membrane in the reverse osmosis equipment has also heretofore been effected using ion exchange equipment, electrodeionization (EDI) deionized water production equipment, reverse osmosis equipment, or the like to produce higher-purity water.
Conventional reverse osmosis equipment is operated, or run, in such a state that the silica concentration of concentrate is equal to or lower than the solubility therein of silica. The reason for this is as follows. When the reverse osmosis equipment is run at a silica concentration of concentrate exceeding the solubility therein of silica, silica either alone or in chemical and/or physical combination with hardness ions [i.e., a hardness component(s)] such as calcium ions and/or magnesium ions, and/or other component(s) in the concentrate is precipitated along the flow path of the concentrate and on the surface of a reverse osmosis membrane to bring about a decrease in the flux of permeate water, an increase in pressure differential across the reverse osmosis membrane, etc. to thereby make the stable operation of the equipment difficult. Herein, the term “flux of permeate water” means the flow rate, per unit area of the reverse osmosis membrane, of permeate water flowed through the reverse osmosis membrane.
Additionally stated, the solubility of silica can be found, for example, using known data as shown on page 29 of “GYAKUSINTOHO-GENGAIROKAHO II OYO, MAKU RIYO GIJUTU HANDBOOD (REVERSE OSMOSIS-ULTRAFILTRATION II APPLICATION, HANDBOOK OF MEMBRANE UTILIZATION TECHNOLOGIES)” (published by Saiwai Shobo K. K. on Jun. 30, 1978). An example of such known data are shown in FIG.
1
. Additionally stated, the values of the solubility of silica as shown in
FIG. 1
are data on a system wherein silica alone is present in water. The values of the solubility of silica in the presence of hardness ions coexisting therewith are believed to be lower than those as shown in FIG.
1
. Methods of lowering the silica concentration of concentrate to or below the solubility therein of silica include a method wherein the recovery of permeate water (amount of permeate water/amount of feed water) is controlled in such a way as to lower the silica concentration of concentrate to or below the solubility therein of silica. The silica concentration of industrial water as well as city water in Japan is around 20 ppm (as SiO
2
, the same will apply hereinbelow) in most cases, where the upper limit of the recovery of permeate water is 75 to 80% and concentrate in an amount corresponding to 20 to 25% of feed water is therefore discarded. This is problematic from the standpoint of effective utilization of feed water.
Alternatively, a reverse osmosis membrane having a low rejection of silica of, e.g., at most 50% may be used to run the reverse osmosis process while maintaining the silica concentration of concentrate at or below the solubility therein of silica. In this case, although the recovery of permeate water can be increased, the silica concentration of the permeate water turns out to be comparatively high, thus sacrificing a high degree of purification of the permeate water, attainment of which is the primary purpose of reverse osmosis equipment.
Accordingly, an object of the present invention is to provide reverse osmosis process and equipment which are stably operable without precipitation of silica even if the silica concentration of concentrate exceeds the solubility therein of silica (this solubility being one which has heretofore been believed to prevail, hereinafter referred to as “standard solubility”), and are therefore operable at a high recovery of permeate water.
SUMMARY OF THE INVENTION
As a result of extensive investigations with a view to solving the foregoing problems, the inventors of the present invention have unexpectedly found out that, when reverse osmosis equipment is operated while maintaining the pH of concentrate at a level of at most 6, no silica is precipitated along the flow path of the concentrate and on the surface of a reverse osmosis membrane to enable a stable operation of the equipment even if the silica concentration of the concentrate exceeds the standard solubility therein of silica and even if silica coexists with concentrated hardness ions in the concentrate. The present invention has been completed based on this finding.
More specifically, in accordance with the present invention, there is provided a reverse osmosis process comprising treating feed water containing at least silica and hardness ions with a reverse osmosis membrane to separate the feed water into permeate water and concentrate, while maintaining the pH of the concentrate at a level of at most 6. The silica concentration of the concentrate may exceed the standard solubility therein of silica.
In accordance with the present invention, there also is provided a reverse osmosis process comprising treating feed water containing at least silica and hardness ions with a first reverse osmosis membrane to separate the feed water into first permeate water and first concentrate; and treating the first concentrate with a second reverse osmosis membrane to separate the first concentrate into second permeate water and second concentrate, while maintaining the pH of the second concentrate at a level of at most 6. The silica concentration of the second concentrate may exceed the standard solubility therein of silica.
In accordance with the present invention, there is further provided reverse osmosis equipment comprising at least one reverse osmosis membrane module having a reverse osmosis membrane for treating therewith feed water containing at least silica and hardness ions to separate the feed water into permeate water and concentrate; a feed water pumping means for pumping the feed water into said at least one reverse osmosis membrane module; a pH sensing means for measuring the pH of the concentrate; and a pH control means having a built-in feedback control system for controlling the pH of the feed water through feedback of the pH value of the concentrate measured with the pH sensing means in such a way as to maintain the pH of the concentrate at a level of at most 6.
In accordance with the present invention, there is still further provided reverse osmosis equipment comprising at least one first reverse osmosis membrane module having a first reverse osmosis membrane for treating therewith feed water containing at least silica and hardness ions to separate the feed water into first permeate water and first concentrate; at least one second reverse osmosis membrane module having a second reverse osmosis membrane for treating therewith the first concentrate to separate the first concentrate into second permeate water and second concentrate; a feed water pumping means for pumping the feed water into said at least one first reverse osmosis membrane module;. a pH sensing means for measuring the pH of the second concentrate; and a pH control means having a built-in feedback control system for controlling the pH of the feed water or the first concentrate through feedback of the pH value of the second concentrate measured with the pH sensing means in such a way as to maintain the pH of the second concentrate at a level of at most 6.
The present invention will now be described in detail.
In the present invention, the feed water may be a variety of water containing silica and hardness ions, specific e

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