Patent Application
20140116943
The present invention relates to a purification system for obtaining purified water from seawater, wastewater, or the like.
As background art of the present technical field, there has been one disclosed in JP-A-2010-149123. In this literature, a means is described for “providing a seawater desalination method for desalinating seawater by filtration treatment using a reverse osmosis membrane device, characterized in that seawater is desalinated by performing a mixing step for mixing biologically treated water obtained by treating wastewater containing organic materials biologically as dilution water with seawater having a salt concentration of 1.0 to 8.0 mass %; and a mixed water treating step for supplying the mixed water obtained in said mixing step to the reverse osmosis membrane device for filtration treatment”.
According to this method, the salt concentration is lowered so that pressurization to the reverse osmosis membrane device required in the conventional seawater desalination can be suppressed low and seawater desalination can be performed in an energy saving manner.
In addition, in US 2006/0144789 A1, there is disclosed a method for lowering a salt concentration of seawater using a forward osmosis membrane.
In JP-A-2010-149123, it is described to obtain fresh water by diluting seawater with biologically treated water and treating the water after dilution with reverse osmosis membrane treatment. However, in biologically treated water persistent organic substances are contained which remain since organisms cannot completely decompose and a part of the persistent organic substances are adsorbed or deposited onto the reverse osmosis membrane surface, resulting in fouling (clogging).
Once fouling occurs, increase in operating pressure is required to obtain. the same amount of purified water and increases energy consumption for operation. In the case where fouling progresses further, an operating rate of the system decreases for conducting membrane cleaning. Moreover, by repeating cleaning, performance of the membrane salt rejection rate, leading to replacement of the membrane. Due to these, fouling has become a problem of increases in fresh water generation costs (running costs),
In US 2006/0144789 A1, described is a step for lowering the salt concentration of seawater or concentrated water after desalination with wastewater or seawater through a forward osmosis membrane. In this method, while it is capable to prevent substances causing fouling contained in wastewater from flowing into reverse osmosis treatment by separating with the forward osmosis membrane, there has not been taken into consideration on substances causing fouling contained in seawater. Seawater contains metabolites of microorganisms such as plankton or microorganisms which cannot be completely removed in pretreatment, and there is a problem that they cause fouling.
In an aspect of the present invention, it is intended to provide a water purification system in which bringing in substances causing fouling to a reverse osmosis membrane step is suppressed, and thus preventing fouling.
To address the problem described above, configurations described in “What is claimed is” are adopted, for example. The present application contains plural, means for addressing the aforementioned problem and characterized, as an example, by having a flow channel of an aqueous solution, closed by being separated by a semipermeable membrane between water to be treated and a reverse osmosis membrane.
According to the aspect of the present invention, because there is no direct contact between the reverse osmosis membrane and water to be treated which contains a large quantity of substances causing fouling of the reverse osmosis membrane, it is possible to prevent fouling of the reverse osmosis membrane and to reduce fresh water generation costs.
Other objects, features, and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
Explanation is given below on embodiments according to the present invention with reference to the drawings.
A treatment flow of seawater desalination of the present embodiment is shown in
The pump 6, the reverse osmosis membrane module 3, and the forward osmosis membrane module 1 are connected with water channels. The pump 6 pressurizes the circulating water 4 which passes through the forward osmosis membrane module 1 to send it to the reverse osmosis membrane module 3.
Explanation is now given on operation of the water treatment system of the present embodiment. To-be-treated water (seawater, for example) is treated at the pretreatment equipment 5 and sent to the forward osmosis membrane module 1. In the forward osmosis membrane module 1, to-be-treated water and circulating water are opposed to each other across the forward osmosis membrane 1a, where circulating water has a higher solute concentration than to-be-treated water. Therefore, by osmotic pressure water molecules in the to-be-treated water permeate through a semipermeable membrane 1a and move to a circulating water side. Since the solutes do not move across the forward osmosis membrane 1a, the to-be-treated water is concentrated and discharged as concentrated wastewater.
The circulating water 4 which has passed through the forward osmosis membrane module 1 is pressurized by the pump 6 to be sent to the reverse osmosis membrane module 3.
In the reverse osmosis membrane module 3, the circulating water 4 and purified water are opposed with each other across the reverse osmosis membrane 3a. Since the circulating water 4 has a higher pressure than the purified water, water molecules in the circulating water 4 permeate through the reverse osmosis membrane 3a to become the purified water of an extremely low solute concentration, which is taken out from the water treatment system. Since in the reverse osmosis membrane module 3 water molecules move to the purified water but solutes do not permeate through the semipermeable membrane 3a, the circulating water 4 becomes higher in the concentration and moves to the forward osmosis membrane module 1.
Description is given in detail on processings in respective constituents, in the forward osmosis membrane module 1, a forward osmosis processing is performed. Here, the forward osmosis processing indicates a processing in which water molecules are reclaimed into the circulating water 4 at the downstream side in the directions of the dotted arrows through the forward osmosis membrane 1a by arranging the circulating water 4 of a high osmotic pressure having a higher solute concentration at the downstream side (the circulating water side) than at the upstream. side (the to-be-treated water side) across the semipermeable membrane 1a, which does not let solutes pass but does let only water molecules of a solvent permeate. Because water molecules move using a difference in osmotic pressures, it is a processing which requires no power in theory. Practically, in order to perform movement of water molecules efficiently, the upstream side (the to-be-treated. water side) may be pressurized in some cases.
As the forward. osmosis membrane 1a, one made of cellulose acetate, polyamide, or the like as a primary component is known however, it is not intended to add limitations in material thereof. It is also possible to use a semipermeable membrane commercially available as a. reverse, osmosis membrane for the forward osmosis processing.
As the circulating water 4 disposed at the downstream side of seawater across the forward osmosis membrane 1a, an aqueous solution from which organic substances possibly casing fouling are eliminated from is used. For example, there are aqueous solutions of ionic substances prepared with ultrapure water or the like. As the solutes organic substances, which may become causes of fouling, are suppressed to an extremely low concentration. As the ionic substances monovalent ions are preferable to use to divalent positive ions, which may cause scale; it is not intended to particularly limit thereto, however. Namely, as the circulating water 4 a solution having an extremely low concentration of organic substances and having a high ion concentration is desirable.
Specifically, a solution in which the amount of organic substances is 0.1 mg/L or lower in TOC equivalent and a concentration of ionic substances at the upstream side of the forward osmosis membrane is 2 to 4 times electric charge equivalent of seawater is desirable. When it is not twice or greater, there would be no sufficient difference in osmotic pressures; when it is not four times or less, too high load would be imposed on the reverse osmosis membrane. In the case of a saline solution it would be 6 to 12%; in the case of using another ionic substance, positive charges for monovalent ions (the same amount of negative charges are also present since it is neutral as a whole) are between 1 and 2 mol/L. Namely, when n-valent ions are generated, original ionic substances are dissolved by 1/n to 2/n mol/L. In the present embodiment, an aqueous solution of 10% NACl prepared with ultrapure water providing a sufficient osmotic-pressure difference relative to seawater of a salt concentration of 3.2% is used as the circulating water 4.
A high osmotic-pressure solution 4 is retained in closed water channel 2, separated from upstream and downstream across the forward osmosis membrane 1a at the seawater side and across the reverse osmosis membrane 3a at the reverse osmosis membrane side. By using a semipermeable membrane having high blocking capability of organic substances, the organic substances won't infiltrate into the circulating water 4 from the outside. Since both of the circulating water 4 and the purified water in contact with the reverse osmosis membrane 3a are low in the concentrations of organic substances, fouling is difficult to occur. Moreover, although organic substances are contained in the to-be-treated water, since the to-be-treated water is not pressurized intensively, fouling is difficult to occur on the forward osmosis membrane 1a of the forward osmosis membrane module 1. In addition, even when fouling occurs, the influence thereof on the operating pressure is small since osmosis is driven by a difference in concentrations in the forward osmosis processing.
In the reverse osmosis membrane module 1, a reverse osmosis processing is performed. Here, the reverse osmosis processing indicates a processing in which water molecules in the circulating water 4 are reclaimed into the purified water on the downstream side in the directions of the dotted arrows through the reverse osmosis membrane 3a, in spite of a higher osmotic pressure of the circulating water 4 than that of the purified water by setting pressure at the upstream side (the circulating water side) higher than at the downstream side the purified water side) across the reverse osmosis membrane (semipermeable membrane) 3a, which does not let solutes pass but does let only water molecules of a solvent permeate. Here, in order to move water molecules against the difference in osmotic pressures, power is required. Accordingly, if clogging occurs in the semipermeable membrane 3a, power loss increases; however, in the present embodiment, because the semipermeable membrane 3a is between the circulating water 4 of a low concentration of organic substances and the purified water, fouling is difficult to occur.
In a conventional water treatment system shown. in
In the water treatment system of the present embodiment, when seawater was treated by sand filtration and an ultrafiltration membrane to remove foreign elements (insoluble elements) in the solution, soluble organic components of 10 mg/L in TOC equivalent (the amount of total organic carbon) were present in the to-be-treated water. When the to-be-treated water was subjected to the forward osmosis membrane processing, an aqueous solution of 10% NaCl of the circulating water 4 was diluted to 5% and the TOC measured for the circulating water 4 sampled at the vicinity of the forward osmosis membrane 1a was found to be 0.1 mg/L or lower.
Through a reverse osmosis membrane processing of an aqueous solution of 5% NaCl purified water was obtained. However, pump power consumed in the reverse osmosis membrane processing was increased to 8 MPa to secure the amount of permeated water compared to 6 MPa of a conventional power shown in
Fouling of the reverse osmosis membrane 3a was suppressed and increase in the operating pressure to obtain the same amount of permeated water was not observed for two weeks. On the other hand, the semipermeable membrane surface of the forward osmosis processing, which contacts directly to seawater, was not pressurized and thus fouling substances were not pressed thereon with pressure; the state where fouling was hard to occur was maintained.
The circulating water was concentrated by the reverse osmosis membrane processing to a concentration of 10% again and sent back toward the side of the forward osmosis membrane module 1. Concentration variations of the high osmotic-pressure solution at respective processing positions are shown in
In addition to the first embodiment, a system of a second embodiment is shown in
In
While the amount of organic substances contained in seawater was 10 mg/L in TOC equivalent and the amount of organic substances contained in the biologically treated water was 4 mg/L in TOC equivalent, the TOC amount in. the circulating water was maintained at 0.1 mg/L or lower; further, the effect to fouling of the reverse osmosis membrane was obtained similar to in the first embodiment.
Moreover, while in the conventional method shown in
As a further effect, the water intake of seawater and the discharge of the concentrated seawater per unit amount of the fresh water could be reduced. and an effect of mitigating an influence on the environment was also obtained.
In the present embodiment, although biologically treated wastewater was chosen for the second to-be-treated water, river water, well water, primary wastewater of industrial drainage, or the like can be used as long as the salt concentration is equal to or lower than the seawater concentration; even though it is not intended to limit particularly, it is desired the salt concentration of the second treated water is 1% or lower to obtain a sufficient osmotic-pressure difference.
While in the present embodiment two kinds of to-be-treated water were adopted, such a system can also be designed that three or more kinds of to-be-treated water having different osmotic pressures are arranged in the order of high osmotic pressures to recover water via a forward osmosis membrane module.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited. thereto and various changes and modifications may he made without departing from the spirit of the invention and the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-146948 | Jul 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/003451 | 5/28/2012 | WO | 00 | 12/17/2013 |
