WO2012033257A1

WO2012033257A1 - Two-pass reverse osmosis desalination apparatus and method

Info

Publication number: WO2012033257A1
Application number: PCT/KR2010/008292
Authority: WO
Inventors: Yong Gyun Park; Hyo Sang Kim; Sung Hyuk Park; Young Khee Oh
Original assignee: GS Engineering and Construction Corp
Current assignee: GS Engineering and Construction Corp
Priority date: 2010-09-09
Filing date: 2010-11-23
Publication date: 2012-03-15
Anticipated expiration: 2013-03-09
Also published as: KR101051345B1

Abstract

A two-pass reverse osmosis (RO) desalination apparatus includes a first RO module for receiving feed water through an inlet pipe to desalinate the feed water; and a second RO module for receiving primarily treated water to desalinate the primarily treated water. Further, the two-pass RO desalination apparatus includes a bypass pipe through which bypass water branched from the first RO module or the inlet pipe bypasses such that the bypass water may not be desalinated in the first or the second RO module. Furthermore, the two-pass RO desalination apparatus includes a product water pipe through which a product water in which the secondarily treated water is blended with the bypass water passes; and a controller for controlling the flow rate of the water passing through the first or second RO module, or the pipes.

Description

TWO-PASS REVERSE OSMOSIS DESALINATION APPARATUS AND METHOD

The present invention relates to reverse osmosis (RO) desalination apparatus and method; and more particularly to, two-pass RO desalination apparatus and method for controlling concentration of a specific material through two-pass RO process.

Seawater desalination is a representative technology for solving domestic and foreign problems of fresh water shortage and securing alternative water resource as well as developing global markets and creating high value industrial products. The size of seawater desalination market is currently about 3 million ton/day and it is expected to grow up to 6.2 million ton/day. The technology is becoming a solution to provide alternative water resource to domestic and foreign water stressed regions. Particularly, it is possible to save water cost and improve environmental problem by substituting plans for securing water resource by dam constructions which causes environmental disputes. Therefore, demands for the seawater desalination are gradually growing. There are several methods such as a thermal and a RO desalination. Recently, the RO is widely used because the energy for producing a unit volume of water is less than the thermal method. RO system separates components contained in seawater and brackish water using polymeric membranes, dilutes the product water to use it as any objective water or drinking water, and discharges the concentrated water back to the sea.

One example of a RO seawater desalination system is disclosed in a U.S. Patent Application Publication No. 2009/0194471, entitled REVERSE OSMOSIS SEA WATER DESALINATION SYSTEM.

In the seawater desalination using RO method, one of troubles in real practice is a removal of boron. Boron exists with an average concentration of 5 to 7 mg/L in seawater and known to have bad effects to human body when ingested large amount. Boron causes acute toxic symptoms such as bipolar disorder, convulsion, atrophic kidney, and shrinking testicles and chronicle toxic symptoms such as stomach stimulation, anorexia, vomiting, and travel sickness.

Thus, global regulations of boron in drinking water range between 0.3 and 1 mg/L. In general, a RO membrane has a rejection rate for NaCl equal to or larger than 99.9% but has a relatively low rejection rate for boron about 80 to 90%. Consequently, the filtered water through the RO membrane possibly contains a relatively high concentration of boron.

In order to remove boron effectively, a two-pass RO process is used. That is, once treated water with an efficiency of 80 to 90% in a primary RO process is treated in a secondary RO process to maintain concentration of boron under standard water quality.

However, the rejection rate for boron in the RO process varies according to concentration of boron, temperature, and operating time of the RO process, concentration of boron in the treated water is irregular.

In view of the above, the present invention provides two-pass RO desalination apparatus and method for producing fresh water in which concentration of a specific material is regulated under the present regulations by controlling a flow rate of the water treated by the RO method while measuring concentration of the specific material in the water.

In accordance with a first aspect of the present invention, there is provided a two-pass RO desalination apparatus. The two-pass RO desalination apparatus includes a first RO module for receiving feed water through an inlet pipe to desalinate the feed water; and a second RO module for receiving primarily treated water to desalinate the primarily treated water. Further, the two-pass RO desalination apparatus includes a bypass pipe through which bypass water (i.e., some amount of the feed water or some amount of the primarily treated water) branched from the first RO module or the inlet pipe bypasses such that the bypass water may not be desalinated in the first or the second RO module. Furthermore, the two-pass RO desalination apparatus includes a product water pipe through which a product water in which the secondarily treated water is blended with the bypass water ; and a controller for controlling the flow rate of the water passing through the first or second RO module, or the pipes.

The controller may measure concentrations of specific materials in the product water and control the flow rate of the water to maintain the measured concentrations of the specific materials in the product water under a preset regulation range.

Further, the specific material may comprise boron and the concentration of the specific material may be regulated to range from about 0.3 to 1.0 mg/L.

The bypass pipe may allow some amount of the feed water to bypass the first RO module or some amount of the primarily treated water to bypass the second RO module, and the controller may control the ratio of the bypass water.

The two-pass RO desalination apparatus may further include feed water sensor for measuring properties of the feed water, the feed water sensor being provided on the inlet pipe; and a product water sensor for measuring properties of the product water, the product water sensor being provided on the product water pipe. The controller controls flow rate of the water based on the properties measured by the feed water sensor and the product water sensor.

Further, the properties of the feed water and the product water may comprise at least one of conductivity, temperature, pH, and a flow rate. The controller may calculate the concentration of the specific material in the product water based on the measured values to control the flow rate of the water based on the calculated concentration of the specific material.

The controller may calculate a total concentration of ions using the measured conductivity, a recovery rate using the measured flow rate, a water velocity through RO membrane using the total concentration of the ions and the flow rate, and the concentration of the specific material using the total concentration of the ions, the recovery rate, and the water velocity.

The controller may calculate the concentration of the specific material using a pre-calculated constant according to properties of the first RO module and the second RO module, and may determine whether the calculated concentration of the specific material satisfies a regulation standard of treated water.

Further, the two-pass RO desalination apparatus may further include a primarily treated water sensor for measuring a flow rate of a primarily treated water supplied to the second RO module; and a bypass water sensor for measuring a flow rate of the bypass water. The controller may control a ratio of the primarily treated water and the bypass water based on the measured values by the primarily treated water sensor and the bypass water sensor.

The two-pass RO desalination apparatus may further include a primarily treated water pump for controlling the flow rate of the primarily treated water or a bypass water pump for controlling the flow rate of the bypass water. The controller controls the primarily treated water pump or bypass water pump to control the flow of the water.

In accordance with a second aspect of the present invention, there is provided a two-pass RO desalination method. The two-pass RO desalination method includes desalinating feed water into a primarily treated water in a first RO module; and desalinating the primarily treated water into a fresh water in a second RO module. Further, the two-pass RO desalination method includes allowing the bypass water to bypass the first or the second RO module such that the bypass water may not be desalinated in the first or the second RO module; and blending the primarily or secondarily treated water with the bypass water to produce a product water. Furthermore, the two-pass RO desalination method includes controlling a flow of the water passing through the first or second RO module, or the pipes.

Further, said controlling the flow of the water may include measuring a concentration of a specific material of the product water; and controlling a flow rate of the water based on the measured concentration such that the concentration of the specific material in the product water is maintained within a preset range.

Further, said measuring the concentration may include measuring a concentration of boron and said controlling the flow rate of the water may regulate the concentration of the boron to range from about 0.3 to 1.0 mg/L.

Furthermore, said controlling the flow rate of the water may control a ratio of a flow rate of the bypass water to maintain a concentration of the product water constant.

Further, said controlling the flow of the water may include measuring properties of the feed water and the product water; calculating a concentration of the specific material in the product water based on the measured values; and determining whether the calculated concentration of the specific material satisfies a regulation standard for treated water.

Furthermore, said measuring the properties may include measuring at least one of conductivity, temperature, pH and flow rates of the feed water and the product water, and calculating the concentration of the specific material using a pre-calculated constant according to properties of the first and the second RO modules .

Further, said controlling the flow of the water may include measuring flow rates of the primarily treated water supplied to the second RO module and the bypass water, and controlling a ratio between the flow rate of the primarily treated water and the bypass water based on the measured flow rates.

In accordance with the two-pass RO desalination apparatus and method of the present invention, a flow rate of water desalinated in the RO desalination method is controlled by measuring concentration of a specific material in the water. As a result, fresh water that concentration of the specific material therein is maintained while satisfying a regulation standard of treated water may be produced.

Further, in the two-pass RO desalination method, when some amount of the feed water or the primarily water, i.e., the bypass water is separated to be blended with a primarily or secondary treated water to maintain a constant concentration of boron, the load applied to the two-pass RO desalination method is reduced. Consequently, the number of needed modules decreases and the amount of the product water increases.

Concentrations of separated water and treated water are controlled in association with concentration of the boron in the product water to maintain a preset concentration of boron in the product water. Consequently, the seawater desalination method may be stably managed and maximum operation efficiency may be obtained.

The objects and features of the present invention will become apparent from the following description of embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic diagram illustrating a two-pass RO desalination apparatus in accordance with an embodiment of the present invention;

Fig. 2 is a schematic diagram illustrating a two-pass RO desalination apparatus in accordance with another embodiment of the present invention;

Fig. 3 is a flow chart illustrating a two-pass RO desalination method in accordance with an embodiment of the present invention; and

Fig. 4 is a flow chart illustrating a method of controlling a flow rate of water in accordance with an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings which form a part hereof.

Fig. 1 shows a schematic diagram illustrating a two-pass RO desalination apparatus in accordance with an embodiment of the present invention.

Referring to Fig. 1, a two-pass RO desalination apparatus in accordance with an embodiment of the present invention includes an inlet pipe 1 through which a feed water passes; and a first RO module 11 receiving the feed water and desalinating the received feed water into a primarily treated water. Further, the two-pass RO desalination apparatus includes a primarily treated water pipe 2 through which the primary treated water desalinated in the first RO module 11 passes; and a second RO module 12 receiving the primarily treated water and desalinating the received primary treated water into a secondarily treated water. Further, the two-pass RO desalination apparatus includes a secondarily treated water pipe 3 through which the secondarily treated water desalinated in the second RO module 12 passes. Further, the two-pass RO desalination apparatus includes a bypass pipe 4 branched from the first RO module 11 to allow some amount of the primarily treated water (i.e., bypass water) to bypass the second RO module 12 such that the bypass water may not be desalinated in the second RO module 12. Furthermore, the two-pass RO desalination apparatus includes a product water pipe 5 through which a product water in which the secondarily treated water is blended with the bypass water passes.

The two-pass RO desalination apparatus may include a feed water sensor 23 provided in the inlet pipe 1 to measure properties of the feed water and a product water sensor 24 provided in the product water pipe 5 to measure properties of the product water. The RO desalination apparatus may further include a primarily treated water sensor 22 measuring a flow rate of the primarily treated water supplied to the second RO desalination module 12 and a bypass water sensor 21 measuring a flow rate of the bypass water supplied to the bypass pipe 4. Further, the two-pass RO desalination apparatus may include a primarily treated water pump 28 controlling the flow rate of the primarily treated water or a bypass water pump 27 controlling the flow rate of the bypass water. The concentrated water from the first RO desalination module 11 and the second RO desalination module 12 is discharged through an outlet pipe 6. The bypass water pump 27 and the primarily treated water pump 28 may be substituted with other control instrument such as valves.

The feed water may be seawater or brackish water. A specific material of which is subject to be controlled may be boron (B) or other various materials such as chlorine Cl or sulfide S. The concentration of the boron may be regulated to range from about 0.3 to 1.0 mg/L according to the international standard.

A controller 14 controls a flow of the water passing through the first and second

RO desalination modules

11 and 12 and the

pipes

2, 3, 4, and 5. The controller 14 measures the concentration of the specific material in the product water and controls the flow rate of the water such that the concentration of the specific material in the product water is maintained at a preset level or under a specific value based on the measured concentration. The controller 14 calculates the concentration of the specific material in the product water based on measured properties of the feed water and the product water to determine whether the calculated concentration satisfies the regulation standard of treated water.

The properties of the feed water and the product water measured by the feed water sensor 23 and the product water sensor 24 include at least one of conductivities, temperatures, pH, and flow rates of the feed water and the product water. The concentration of the specific material is calculated using the properties such as the measured conductivities, temperatures, pH, and flow rates of the feed water and the product water. The measured conductivities are used to calculate total concentration of ions, the measured flow rates are used to calculate a recovery rate and the total concentration of ions and the flow rates are used to calculate a water velocity through RO membranes. Further, the concentration of the specific material may be calculated based on the total concentration of ions, the recovery rate, the water velocity through the RO membranes, and the measured temperatures. The pre-calculated coefficient is used to calculate the concentration of the specific material according to properties of the first RO module 11 and the second RO module 12.

Based on the flow rates measured by the primarily treated water sensor 22 and the bypass water sensor 21, a ratio between the flow rate of the primarily treated water supplied to the second RO module 12 and the flow rate of the bypass water supplied to the bypass pipe 4 is controlled. As a result, the concentration of the product water may be controlled. The primarily treated water pump 28 or the bypass water pump 27 is controlled to control the ratio between the primarily treated water and the bypass water, thus a flow of the water can be controlled. For example, current regulations of the boron in the treated water are between 0.5 and 1.0 mg/L. When the concentration of the boron in the product water is equal to or greater than 1.0 mg/L, the bypass water pump 27 and the primarily treated water pump 28 are controlled to increase the flow rate of the primarily treated water and to decrease the flow rate of the bypass water. When the concentration of the boron in the product water is less than 0.5 mg/L, the flow rate of the bypass water is relatively increased.

Fig. 2 is a schematic diagram illustrating a two-pass RO desalination apparatus in accordance with another embodiment of the present invention. In this embodiment of the present invention, same reference numerals are assigned to the same elements as or elements corresponding to the elements in Fig. 1 and descriptions thereof will be omitted.

In the two-pass RO desalination apparatus in accordance with another embodiment of the present invention, unlike the above embodiment, the bypass pipe 4 is branched from the feed water pipe 1 to allow some amount of the feed water (i.e., bypass water) to bypass the first RO module 11 such that the bypass water may pass through the bypass pipe 4 and may not be desalinated in the first RO module 11. The bypass pipe 4 includes a bypass water sensor 21 measuring a flow rate of the bypass water and a bypass pump 27 controlling the flow rate of the bypass water.

Hereinafter, a two-pass RO desalination method performed by the two-pass RO desalination apparatus will be described. In the foregoing description, reference numerals are assigned to operating steps only for convenient description but do not limit the order of performing the operating steps. The operating steps may be performed in a sequential order or in parallel.

Fig. 3 is a flow chart illustrating a two-pass RO desalination method in accordance with an embodiment of the present invention. Fig. 4 is a flow chart illustrating a method of controlling flow of water in accordance with the embodiment of the present invention.

The two-pass RO desalination method in accordance with the embodiment of the present invention includes desalinating a feed water into a primarily treated water in the first RO module 11 in step S41; and desalinating the primarily treated water into a secondarily treated water in the second RO module 12 in step S42. Further, the two-pass RO desalination method includes allowing the bypass water to bypass the first RO module 11 or the second RO module 12 such that the bypass water may not be desalinated in the first or the second RO module in step S43. Further, the two-pass RO desalination method includes blending the primarily treated water or the secondarily treated water with the bypass water to produce a product water in step S4. Furthermore, the two-pass RO desalination method includes controlling a flow of the water passing through the first RO module 11 or the second RO module 12 and the

pipes

2,3,4 and 5 in step S45.

In step S45 of controlling the flow of the water, concentration of the specific material in the product water is measured and a flow rate of the water may be controlled to maintain the concentration of the specific material in the product water within a preset range based on the measured concentration of the specific material. At least one of properties such as conductivities, temperatures, pHs, and flow rates of the feed water and the product water is measured in step S45a and concentration of the specific material in the product water is calculated based on the measured properties in step S45b. In addition, the flow rate of the water is controlled by determining whether the calculated concentration of the specific material satisfies the regulation standard of treated water within a preset range in step S45c.

Further, in step S45 of controlling the flow of the water, flow rates of the feed water supplied to the first RO module 11 or the primarily treated water supplied to the second RO module 12, and the bypass water are measured in step S45d. Further, a ratio between the primarily treated water or the secondarily treated water and the bypass water is controlled based on the measured flow rates in step S45e so that concentration of the boron in the product water may be controlled.

Hereinafter, for an example, a method in step S45b of calculating the concentration of the boron based on the measured properties of the water will be described in detail. Here, term "treated water" means "product water".

First, total concentration of the water may be indirectly measured using the following math figure 1. Here, C is total concentration of ions in the water and CD is conductivity of the water.

MathFigure 1

A recovery rate R is a ratio value of a flow rate of the treated water with respect to the feed water, that is, a value of the flow rate of the treated water over the flow rate of the feed water. Therefore, the recovery rate is obtained using a measured flow rate of the feed water and a measured flow rate of the treated water from the following math figure 2.

MathFigure 2

A water velocity of ions through a RO membrane J is calculated. The water velocity through the RO membrane may be obtained from the following math figure 3. Here, Q_p is a flow rate of the treated water, C_p is a total concentration of ions in the treated water, and A is a sectional area of the RO membrane.

MathFigure 3

The water velocity of the total ions and concentration of the boron have a proportional relationship and a proportional coefficient of the relationship is expressed by the following math figure 4.

MathFigure 4

In the preliminary experiment, conductivity C_f, temperature T_f, pH, and flow rate Q_fof the feed water are measured by sensors and conductivity C_p, temperature T_p, pH, and flow rate Q_p of the treated water are measured by sensors. CD_f and T_fsubstitute for T and CD in math figure 1 to obtain the total concentration C_f of the feed water, and CD_p substitute for CD in math figure 1 to obtain the total concentration of ions. The recovery rate R of the treated water is obtained from math figure 2 and the water velocity J of ions through a RO membrane is obtained from math figure 3, and then the proportional coefficient B₀ with respect to the total concentration of ions is obtained from math figure 4.

Further, since a relative concentration of a boric acid and a borate having different chemical structures has a mathematical relationship as shown in math figures 5 and 6, a relative concentration of the boric acid and the borate may be calculated if pH and temperature are given.

MathFigure 5

MathFigure 6

In math figures 5 and 6, K_a1 indicates an equilibrium constant between a boric acid and a borate, [H₂BO₃ ^-] indicates concentration of the borate, [H₃BO₃] indicates concentration of the boric acid, [H⁺] indicates an activity of hydrogen ion. Units of the concentrations of the borate and the boric acid, the activity of hydrogen ion, and concentration of chlorine ion S are ML^-3. Moreover, T in math figures 2 and 3 indicates absolute temperature (degree K).

In the preliminary experiment, when the boron is input in the feed water of pH 7, boric acid only exists in the feed water. Here, the total concentration of ions in the feed water C_f and the total concentration of ions in the treated water C_pwhich are obtained through conductivity, temperature, pH, and flow rate become concentrations of ions of the boric acid C_{f, Boric acid} and C_{p, Boric acid}. The obtained concentration of ions of the boric acid in the feed water C_{f, Boric acid} and the obtained concentration of ions of the boric acid in the treated water C_{p, Boric acid} are substituted in math figure 5 to obtain a proportional coefficient B₁ with respect to the boric acid.

MathFigure 7

Moreover, in the preliminary experiment, when the boron is input in the feed water of pH 11, borate only exists in the feed water. Here, the total concentration of ions in the feed water C_f and the total concentration of ions in the treated water C_p, which are obtained through conductivity, temperature, pH, and flow rate become concentrations of ions of the borate C_{f, Borate} and C_{p, Borate}. The obtained concentration of ions of the borate in the feed water C_{f, Borate} and the obtained concentration of ions of the borate in the treated water C_{p, Borate} are substituted in math figure 5 to obtain a proportional coefficient B₂ with respect to the borate. That is, the proportional coefficient B₂ of the borate is obtained from math figure 8.

MathFigure 8

In math figures 7 and 8, terms J, R, and T_p are the above-described water velocity J through a RO membrane and the recovery rate R and the temperature T_p of the treated water.

In the preliminary experiment, f₁ and f₂ are finally calculated from the following math figures 9 and 10. Here, f₁ is a ratio between the proportional coefficient B₁ of the boric acid and a proportional coefficient B₀ with respect to the total concentration of ions, and f₂ is a ratio between a proportional coefficient B₂of the borate and the proportional coefficient B₀ with respect to the total concentration of ions.

The constants f₁ and f₂ calculated in the preliminary experiment are used to calculate concentration of the boron during the operation of the two-pass RO desalination apparatus.

MathFigure 9

MathFigure 10

During the actual operation of the two-pass RO desalination apparatus, conductivity CD_f,real, temperature T_{f, real}, pH, and a flow rate Q_{f, real}of the feed water and conductivity CD_{p, real}, temperature T_{p, real}, and a flow rate Q_{p, real} of the treated water are measured in real time. Total concentration of ions C_{f, real} of the feed water and total concentration of ions C_{p, real} of the treated water are obtained using the measured values from math figure 1, the recovery rate of the treated water R is obtained from math figure 2. Further, the water velocity J of ions through a RO membrane is obtained from math figure 3. Resultantly, the proportional coefficient B_{0, real} with respect to the total concentration of ions is obtained from math figure 4.

Next, B_{1, real} and B_{2, real} are calculated by math figures 7 and 8 using the obtained B_{0, real} and f₁ and f₂ obtained in the preliminary experiment.

Moreover, the total concentration of ions C_{f, real} and the temperature T_{f, real} of the feed water are substituted for C_f and T_f1 of math figure 6 to calculate the equilibrium constant K_a1 between the boric acid and the borate, and finally the concentration C_{p, boron} of the boron in the treated water is calculated by the following math figure 11.

MathFigure 11

While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

A two-pass reverse osmosis (RO) desalination apparatus comprising:

a first RO module for receiving feed water through an inlet pipe to desalinate the feed water;

a second RO module for receiving primarily treated water in which the feed water is desalinated by the first RO module to desalinate the primarily treated water;

a bypass pipe through which bypass water branched from the first RO module or the inlet pipe bypasses such that the bypass water may not be desalinated in the first or the second RO module;

a product water pipe through which a product water in which the secondarily treated water is blended with the bypass water passes; and

a controller for controlling the flow rate of the water passing through the first or second RO module, or the pipes.
The apparatus of claim 1, wherein the controller measures concentrations of specific materials in the product water and controls the flow rate of the water to maintain the measured concentrations of the specific materials in the product water under a preset regulation range.
The apparatus of claim 2, wherein the specific material comprises boron and the concentration of the specific material is regulated to range from about 0.3 to 1.0 mg/L.
The method of claim 2, wherein the bypass pipe allows some amount of the feed water to bypass the first RO module or some amount of the primary treated water to bypass the second RO module, and the controller controls a ratio of the bypass water.
The apparatus of any one of claims 1 to 4, further comprising:

feed water sensor for measuring properties of the feed water, the feed water sensor being provided on the inlet pipe; and a product water sensor for measuring properties of the product water, the product water sensor being provided on the product water pipe,

wherein the controller controls flow rate of the water based on the properties measured by the feed water sensor and the product water sensor.
The apparatus of claim 5, wherein the properties of the feed water and the product water may comprise at least one of conductivity, temperature, pH, and a flow rate,

the controller calculates the concentration of the specific material in the product water based on the measured values to control the flow rate of the water based on the calculated concentration of the specific material.
The apparatus of claim 5, wherein the controller calculates a total concentration of ions using the measured conductivity, a recovery rate using the measured flow rate, a water velocity through RO membrane using the total concentration of the ions and the flow rate, and the concentration of the specific material using the total concentration of the ions, the recovery rate, and the water velocity.
The apparatus of claim 5, wherein the controller calculates the concentration of the specific material using a pre-calculated constant according to the properties of the first RO module and the second RO module, and determines whether the calculated concentration of the specific material satisfies a regulation standard of treated water.
The apparatus of any one of claims 1 to 4, further comprising:

a primarily treated water sensor for measuring a flow rate of a primarily treated water supplied to the second RO module; and

a bypass water sensor for measuring a flow rate of the bypass water,

wherein the controller controls a ratio between a flow rate of the RO water and a flow rate of the bypass water based on the measured results by the feed water parameter sensor and the bypass water parameter sensor.
The apparatus of claim 9, further comprising:

a primarily treated water pump for controlling the flow rate of the primarily treated water or a bypass water pump for controlling the flow rate of the bypass water,

wherein, the controller controls the primarily treated water pump or the bypass water pump to control the flow of the water.
A two-pass reverse osmosis (RO) desalination method comprising:

desalinating a feed water into a primarily treated water in a first RO module;

desalinating the primarily treated water into a fresh water in a second RO module;

allowing the bypass water to bypass the first or the second RO module such that the bypass water may not be desalinated in the first or the second RO module;

blending the primarily or secondarily treated water with the bypass water to produce a product water; and

controlling a flow of the water passing through the first or second RO module, or the pipes.
The method of claim 11, wherein said controlling the flow of the water includes:

measuring a concentration of a specific material of the product water; and

controlling a flow rate of the water based on the measured concentration such that the concentration of the specific material in the product water is maintained within a preset range.
The method of claim 12, wherein said measuring the concentration includes measuring a concentration of boron and said controlling the flow rate of the water may regulate the concentration of the boron to range from about 0.3 to 1.0 mg/L.
The method of claim 12, wherein said controlling the flow rate of the water further includes controlling a ratio of a flow rate of the bypass water to maintain a concentration of the product water constant.
The method of any one of claims 11 to 14, wherein said controlling the flow of the water includes:

measuring properties of the feed water and the product water;

calculating a concentration of the specific material in the product water based on the measured values; and

determining whether the calculated concentration of the specific material satisfies a regulation standard for treated water.
The method of claim 15, wherein said measuring the properties includes:

measuring at least one of conductivity, temperature, pH and flow rates of the feed water and the product water; and

calculating the concentration of the specific material using a pre-calculated constant according to properties of the first and the second RO modules.
The method of any one of claims 11 to 14, wherein said controlling the flow of the water further includes:

measuring flow rates of the primarily treated water supplied to the second RO module and the bypass water; and

controlling a ratio between the flow rate of the primarily treated water and the bypass water based on the measured flow rates.