JPH08294688A - Seawater desalting plant - Google Patents

Abstract

PURPOSE: To perform proper injection of a pH controller corresponding to the properties of raw water and to suppress the precipitation of a soluble substance on the membrane surface of a reverse osmosis membrane module by adjusting the pH of raw water. CONSTITUTION: The buffer action of raw water is operated on the basis of the measured value by a measuring means in a buffer action operation means 27. In a pH constant control means 28, the injection rate of a pH controller is determined from the pH of raw water after pH adjustment and the buffer action of raw water so that the pH of raw water after pH adjustment becomes an objective value. A sulfuric acid injection machine 1 injects sulfuric acid on the basis of the injection rate calculated by the pH constant control means 28 to adjust the pH of raw water. The raw water after pH adjustment is supplied to a reverse osmosis membrane module.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a seawater desalination plant using a reverse osmosis membrane module.
The present invention relates to a seawater desalination plant having a regulating function.

[0002]

2. Description of the Related Art Conventionally, a seawater desalination plant for desalinating raw water which is seawater using a reverse osmosis membrane module is known. In controlling the operation of this seawater desalination plant, efficient operation is desired by performing appropriate operation according to the conditions of raw water and the state of the membrane.

However, conventionally, raw water temperature, raw water concentration, raw water pH, raw water flow rate, permeated water flow rate, recovery rate, raw water pressure, permeated water concentration, which are various operating factors of a seawater desalination plant using a reverse osmosis membrane module, The relationship between the number of modules and the operating cost is complicated, and it is difficult to operate at the target optimum operating point. Particularly in the case of the pH of raw water, since the acid-base equilibrium of seawater is complicated, it is difficult to control it properly, and the pH cannot be kept constant at the target value. Therefore, it is not possible to effectively use the expensive membrane or perform efficient operation.

[0004]

In a seawater desalination apparatus having a reverse osmosis membrane module, since raw water is concentrated on the membrane surface, dissolved substances in the raw water are likely to be deposited on the membrane surface, which causes the amount of water produced. It was not possible to effectively use the expensive membrane and to operate efficiently.

[0005] Seawater contains a large amount of carbonic acid, and while maintaining equilibrium with carbon dioxide in the atmosphere, alkalinity, temperature, and pH.
As a result, carbonic acid changes its dissolved state between bicarbonate ion and carbonate ion. Since there are many carbonate ions at a high pH, if the concentration on the membrane surface progresses, the solubility limit with concentrated calcium ions and the like will be exceeded, resulting in precipitation of calcium carbonate and the like on the membrane surface. Since calcium carbonate is the substance that is most likely to precipitate, it is necessary to adjust the pH of the raw water to a pH that can prevent the precipitation.

Since the relationship between the injection amount of the pH adjusting agent and the pH is non-linear, it is necessary to control the injection of the pH adjusting agent according to the pH of the raw water and the pH buffering capacity (buffering capacity). Here, the pH buffering ability means an action of preventing a drastic change in pH when an acid or a base is added to the solution.

The present invention has been made in consideration of the above points. The raw water buffer capacity is determined from the raw water temperature and the raw water quality, and the injection of an appropriate pH adjusting agent according to the raw water quality is controlled. An object of the present invention is to provide a seawater desalination plant using a reverse osmosis membrane module capable of preventing the deposition of soluble substances in raw water on the membrane.

[0008]

According to the first aspect of the present invention,
Reverse osmosis membrane module that makes fresh water through raw water that is seawater, and injects a pH adjuster to adjust the pH of raw water p
H adjusting device, buffer capacity calculating means for calculating the buffer capacity of raw water by the measurement value measured by the measuring means, pH after pH adjustment measured by the measuring means, and buffer of raw water obtained by the buffer capacity calculating means And a constant pH control means for determining the injection rate of the pH adjusting agent by the pH adjusting device so that the pH after the pH adjustment becomes constant at the target value. is there.

The invention according to claim 2 is characterized in that the buffer capacity calculating means calculates the buffer capacity of the raw water using the following formula.

[0010]

(Equation 15) The invention according to claim 3 is characterized in that the buffer capacity calculating means calculates the buffer capacity of the raw water by the following formula.

[0011]

[Equation 16]

[0012]

[Equation 17] The invention according to claim 4 is characterized in that the buffer capacity calculating means calculates the buffer capacity of the raw water by the following formula.

[0013]

(Equation 18)

[0014]

[Formula 19]

[0015]

(Equation 20) The invention according to claim 5 is characterized in that the buffer capacity calculating means calculates the buffer capacity of the raw water by the following formula.

[0016]

[Equation 21]

[0017]

[Equation 22]

[0018]

(Equation 23) The invention according to claim 6 is characterized in that the buffer capacity calculating means calculates the buffer capacity of the raw water by the following formula.

[0019]

[Equation 24]

[0020]

(Equation 25) The invention according to claim 7 is characterized in that the buffer capacity calculating means calculates the buffer capacity of the raw water by the following formula.

[0021]

(Equation 26)

[0022]

[Equation 27] In the invention according to claim 8, the constant pH control means uses the second carbonate dissociation equilibrium constant in seawater and the calcium carbonate solubility product from the raw water calcium concentration, raw water alkalinity and raw water pH measured by the measuring means. Then, the Langeria coefficient is calculated by the following formula, and p is calculated so that calcium carbonate does not precipitate in the film.
It is characterized in that it has a target value determining means based on a Langerian index for determining the H target value.

PHsv = pHs-KpHs pHsv: target pH value pHs: Langerian index KpH: target pH value safety range pHs = (pK ₂ -pK _s ) -log [Ca ⁺ ] -lo
gTA + TS K ₂ : Carbonic acid second dissociation constant K _s : Solubility product of calcium carbonate [Ca ⁺ ]: Calcium ion concentration TA: Alkalinity TS: Correction value by soluble substance

[0024]

[Equation 28]

[0025]

According to the first aspect of the invention, the buffering capacity calculating means calculates the buffering capacity of the raw water based on the measurement value measured by the measuring means. In the constant pH control means, from the pH of the raw water after pH adjustment measured by the measuring means and the buffer capacity of the raw water, p
The injection rate of the pH adjuster is determined so that the pH of the raw water after H adjustment becomes the target value. The pH adjuster injects the pH adjuster at the injection rate determined by the pH constant control means to adjust the pH of the raw water.
To adjust. The raw water after pH adjustment is supplied to the reverse osmosis membrane module.

According to the second aspect of the invention, the acid of seawater
The buffer capacity of raw water can be calculated using the base equilibrium formula.

According to the third aspect of the present invention, the buffer capacity of the raw water can be calculated in consideration of the buffer effect of carbonic acid.

According to the fourth aspect of the invention, the buffer capacity of the raw water can be calculated in consideration of the buffer effect of carbonic acid.

According to the fifth aspect of the present invention, in addition to the buffering effect of carbonic acid, the buffering effect of boric acid and hydrogen ions can be calculated using the equilibrium equation, and the buffering capacity of raw water can be calculated more accurately. it can.

According to the sixth aspect of the invention, the buffer capacity of the raw water can be calculated in consideration of the buffer effect of carbonic acid.

According to the seventh aspect of the invention, in addition to the buffering effect of carbonic acid, the buffering effect of boric acid and hydrogen ions can be calculated using the equilibrium equation, and the buffering capacity of raw water can be calculated more accurately. it can.

According to the present invention, the target value determining means determines the second carbonic acid dissociation equilibrium constant in seawater and the calcium carbonate from the raw water calcium concentration, the raw water alkalinity and the raw water pH measured by the measuring means. The solubility product can be used to determine the Langerian coefficient to determine a pH target value at which calcium carbonate does not precipitate in the film.

[0033]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of a seawater desalination plant using a reverse osmosis membrane module according to the present invention.

In FIG. 1, the seawater desalination plant pressure-feeds a plurality of reverse osmosis membrane modules 3 having a reverse osmosis membrane 3a and raw water (seawater) supplied from a supply water inflow path 4 to the reverse osmosis membrane module 3. High-pressure pump 2 and feed water inflow path 4
And a sulfuric acid injector 1 for injecting sulfuric acid into the raw water therein.

A flow meter 5 and a sampling pump 6 connected to a sampling pipe 7 are installed in the supply water inflow path 4. Raw water is pressure-fed by a sampling pump 6 and passes through a sampling pipe 7 to measure a plant state value, for example, a water temperature meter 8, a conductivity meter 9, a pH meter 10, an alkalinity meter 11, an SDI meter 12, and a calcium concentration measurement. It is designed to be sent to the device 13.

A sampling pump 14 connected to a sampling pipe 15 is installed in the feed water inflow passage 4 to measure the pH after the sulfuric acid injection. The raw water after the sulfuric acid injection is pumped by the sampling pump 14, and the feed water is sent to the pH meter 16 through the sampling pipe 15. A supply water pressure gauge 17 is installed in the supply water inflow path 4.

The reverse osmosis membrane module 3 includes a reverse osmosis membrane 3a.
Permeate outflow passage 18 for permeate that permeates water and reverse osmosis membrane 3
Condensed water outflow paths 19 for concentrated water in which salt is concentrated without passing through a are connected to each other.

A sampling pump 20 is installed in the permeated water outflow passage 18, and the permeated water is sampled by the sampling pipe 2.
1 to the permeated water conductivity meter 22. Further, a permeated water flowmeter 23 is installed in the permeated water outflow path 18, and a concentrated water pressure gauge 24 is installed in the concentrated water outflow path 19.

As shown in FIG. 1, the seawater desalination plant is further provided with a pH control system 25. The pH control system 25 measures with a man-machine interface means 26 for exchanging signals with an operator and a water temperature gauge 8. Raw water temperature T and the raw water conductivity TDS measured by the conductivity meter 9
And a buffer for calculating pH buffering capacity β of raw water using the acid-base equilibrium equation of seawater and the equilibrium constant from the raw water pHff measured by the pH meter 10 and the raw water alkalinity TA measured by the alkalimeter 11. Based on the capacity calculation means 27, the pHpv after pH adjustment measured by the pH meter 16, and the buffering capacity β, p after pH adjustment
PH constant control means 28 that determines the injection rate of the pH adjusting agent (sulfuric acid) by the sulfuric acid injector 1 so that Hpv becomes constant at the target value pHsv, and injection amount calculation means 29 that calculates the injection rate from the injection rate and the flow rate. have.

The buffer capacity calculating means 27 is a pH change graph displaying means for assisting in determining a target pH value by obtaining the relationship between the pH adjusting agent injection amount and pH from the acid-base equilibrium equation of seawater and displaying the result in a graph. 30 is connected.

The constant pH control means 28 uses the second water carbonate dissociation equilibrium constant in seawater and the calcium carbonate solubility product to calculate the Langerian coefficient pHs from the raw water calcium concentration, raw water alkalinity and raw water pH. The target value determining means 31 based on the Langerian index for determining the pH target value pHsv so that calcium carbonate does not precipitate in the film is connected.

Further, the target value determining means 31 has the Langerian index pHs, the SDI measured by the SDI meter 12, and the pH.
Adjusted pHpv and raw water temperature T measured with a water temperature gauge 8,
The permeated water flow rate Qppv measured by the permeated water flow meter 23, the raw water pressure Pfpv measured by the feed water pressure gauge 17, the concentrated water pressure Pbpv measured by the concentrated water pressure gauge 24, and the permeated water conductivity meter 22 were measured. Water permeation coefficient A and salt permeation coefficient B of the RO membrane converted to the standard state calculated from permeated water concentration Cppv
, And the time series change of Langeria index pHs, the range of change of water permeation coefficient A, and the average of Langeria index pHs and SDI are displayed, and different SDI, Langereria index pHs, and pH are displayed.
Membrane fouling diagnostic means 32 is connected to compare the degree of fouling of the membrane when the target value is pHsv and to support the proper pH target value safety range used when the pH target value is determined from the Langerian index.

Next, the operation of this embodiment having such a configuration will be described.

When an acid or base (alkali) is injected into water, p
H changes, but the magnitude of the change depends on the nature of water. For example, a strong base such as sodium hydroxide (N
Addition of aOH) causes complete dissociation of hydroxide ions (OH
^- ) Remains in the water, so the pH rises sharply. Where p
The definition of H is shown in Formula (1). The pH is a value obtained by multiplying the logarithm of the hydrogen ion concentration by minus. In equation (1),
[] Represents the concentration.

In water, since the hydroxide ion and the hydrogen ion are in equilibrium according to the equation (2), the equation (3) has a constant value. Now, when sodium hydroxide is injected and OH ⁻ increases, H ⁺ decreases and pH increases.

[0047]

[Equation 29] When a strong base or a strong acid is added, the pH sharply rises or falls as described above, but the pH does not easily change even if a strong base is added to a weak acid solution such as carbonic acid.

If the weak acid is HA and the strong base is BOH,
As shown in formula (4), BOH is H generated by dissociation of HA.
To disappear neutralized by ^+, of the total solution H ⁺
This is because the concentration does not change significantly.

HA + B ⁺ + OH ⁻ → B ⁺ + A ⁻ + H ₂ O (4) As described above, the action of preventing a drastic change in pH when an acid or a base is added to a solution is called a buffer action and is quantified. The thing is called Buffer value β. β is the amount of a strong acid or a strong base to be added to a solution to generate a pH change of 1 unit and is represented by the formula (5).

[0050]

[Equation 30] Since the equation (5) cannot be calculated as it is, it is developed as the following equation (6) and β of the target solution is obtained.

[0051]

[Equation 31] The base component, which is [B ⁺ ], is determined according to seawater, and [H ⁺ ]
Differentiate with and calculate. The basic components of seawater are carbonic acid and boron (B).

Specifically, in the buffer capacity calculating means 27,
Raw water temperature T measured with a water temperature meter 8, raw water conductivity TDS measured with a conductivity meter 9, and raw water pH measured with a pH meter 10.
ff and raw water alkalinity TA measured by alkalinity meter 11
From the above, the buffer capacity β of the raw water is calculated using the acid-base equilibrium equation shown in equation (6) of seawater.

In the pH constant control means 28, the pH pv after pH adjustment is made constant at the target value pHsv from the pH pv after pH adjustment measured by the pH meter 16 and the buffer capacity β calculated by the buffer capacity calculation means 27. Therefore, the injection rate of sulfuric acid is determined as follows.

It is considered that the tuning of the proportional gain Kp based on the buffering capacity β is different for each site, so that it is obtained by conducting an experiment for each site and input in advance.

S (n) = ΔSb + S (n-1) (7) S (n): Sulfuric acid injection rate set value [mg / L] S (n-1): Control output of the previous control cycle [mg / L] ΔSb: Control output deviation of current control cycle ([mg / L])

[0056]

[Equation 32] On the other hand, as a method of calculating the buffer capacity of seawater in the buffer capacity calculator 27, the basic component of seawater, that is, alkalinity (TA) can be obtained as a function of [H ⁺ ]. In general, TA of seawater can be expressed by equation (10).

[B ⁺ ] = TA = [HCO ₃ ⁻ ] +2 [CO ₃ ^{2 −} ] + [B (OH) ₄ ⁻ ] + [OH ⁻ ] + [R ⁻ ] − [H ⁺ ] (10 ) Here, [R ⁻ ] is the ion amount of the weak base in seawater.

Further, the respective ion concentrations on the right side of the equation (10) can be calculated by using the equilibrium equations of carbonic acid type, boron type and hyposulfite type [H ⁺ ]
By deriving an equation having only a measurable term and a term of a known equilibrium constant, TA can be obtained and the buffering capacity β can be obtained.

In this case, the buffer capacity β is calculated according to the equation (11). The total carbon dioxide concentration TCO ₂ that cannot be measured in the equation is calculated by the equation (12).

[0060]

[Expression 33]

[0061]

(Equation 34) Since the equilibrium constant in the calculation formula changes depending on the water temperature and the salt content, it is calculated by the following formula.

PK ₁ (t, S) = 6320.81 / T-126.3405 + 19.568 (1n T) + (19.894-840.39 / T-3.0189 (1n T)) S ^1/2 + 0.0068S ...... (13) pK ₂ (t, S) = 5143.69 / T-90.1833 + 14.613 (1n T) + (17.176-690.59 / T-2.6719 (1n T)) S 1/2 + 0.0217S ...... (14) 1n K B (t, S ) ＝ 148.0248-8966.90 / T-24.4344 (1n T) + (0.5998-75.25 / T) S ^1/2 + 0.01767S …… (15) 1n K _W (t, S) ＝ 148.9802-13847.26 / T-23.6521 ( 1n T) + (-79.2447 + 3298.720 / T + 12.0408 (1n T)) S ^1/2 + 0.019813S …… (16) T: Temperature [K] S: Salinity [g / l] T (k) = T [° C.] + 273 S is obtained from the conductivity, and S = f (TDS).

When the above equation is corrected by pressure, the following equation is used. The coefficients of the equation are listed in Table 1.

[0064] _{K 1 (t, S, P} ) = K 1 (t, S) exp ((- ΔV 1 P + ΔK 1 P 2/2) / RT) ...... (17) -ΔV 1 = a 0 + a 1 ( S-34.8) + a ₂ t + a ₃ t ² (18) -ΔK ₁ = (b ₀ + b ₁ (S-34.8) + b ² t) × 10 ^-3 (19) R = 82.06 cm ³ atm / (Mol K)

[0065]

[Table 1] Further, the buffer capacity calculating means 27 can also calculate the buffer capacity as follows. However, since carbonic acid controls the pH of seawater, only the equilibrium of carbonic acid is considered.

First,

[0067]

[Equation 35] Is.

Next, the carbonate system of seawater will be described.

The following equilibrium exists between gaseous carbon dioxide CO ₂ (g) and dissolved carbon dioxide CO ₂ (aq).

[0070]

[Equation 36] Here, CO ₂ (aq) is the sum of molecular CO ₂ and H ₂ CO ₃ produced by the reaction of water, but since it cannot be measured separately, the sum of both is represented by [CO ₂ ].

The carbon dioxide partial pressure PCO ₂ of seawater is defined by the following equation.

[0072]

(37) Here, K ₀ is the saturated solubility of carbon dioxide under 1 atm given by the Weiss equation.

In seawater, carbon dioxide reacts with water to produce the following equilibrium. K ₁ and K ₂ are the first and second dissociation constants of carbonic acid.

[0074]

(38)

[0075]

[Formula 39]

[0076]

(Equation 40) Further, the buffer capacity calculating means 27 can calculate the buffer capacity as follows. In order to improve the calculation accuracy of the buffer capacity, dissociation of boric acid, hydrogen ion, and hydroxide ion is also considered.

The boric acid system of seawater will be described.

Borate ion concentration in seawater [B (OH) ₄
^- ] Is the ion dissociation constant K _{B of} boric acid [B (OH) ₃ ].
From the total concentration TB of boric acid and TB, the following equation (37) can be used. TB can also be represented by the salt concentration S.

[0079]

[Formula 41]

[0080]

(Equation 42) Further, in the buffer capacity calculating means 27, the buffer capacity can be calculated in consideration of only the carbonic acid equilibrium as follows. However, the buffering capacity is calculated by a different calculation method from the formula (26).

[0081]

[Equation 43] In addition, the buffer capacity calculating means 27 can calculate the buffer capacity in consideration of not only the equilibrium of carbonic acid but also the equilibrium of boric acid and hydrogen ion. However, the formula (31) is
The buffer capacity is calculated by different calculation methods.

[0082]

[Equation 44] Further, the pH change graph display means 30 obtains the relationship between the pH adjusting agent injection amount and pH from the acid-base equilibrium equation of seawater and displays it in a graph as shown in FIG. 2 to assist in determining the target pH value.

Furthermore, in the target value determining means 31 based on the Langerian index, the second carbonate dissociation equilibrium constant in seawater and the calcium carbonate solubility product are used from the measured raw water calcium concentration, raw water alkalinity and raw water pH to obtain Langerlia. The coefficient is calculated to determine a pH target value such that calcium carbonate does not precipitate in the film.

PHsv = pHs-KpHs (39) pHsv: pH target value pHs: Langeria index KpH: pH target value safety range pHs = (pK ₂ -pK _s ) -log [Ca ⁺ ] -log TA + TS ...... (40) ) K _2: carbonic second dissociation constant K _s: the solubility product of calcium carbonate [Ca ^+]: calcium ion concentration TA: alkalinity TS: correction value by soluble substances

[0085]

[Equation 45] During this period, the membrane fouling diagnostic means 32 is a transport equation of the water permeation system reverse osmosis process showing the influence of the water temperature, based on the raw water temperature Tpv measured by the water temperature gauge 8 and the water permeation coefficient in the standard state, which is the equation (42). And the coefficient water temperature correction equation (43) are used for the calculation. Membrane surface osmotic pressure πm between the membranes of formula (42)
Is calculated using the equations (44), (45), and (46). It is assumed that the temperature correction coefficient At is obtained by conducting an experiment for each site and is input in advance because the characteristics of the water permeation coefficient water temperature correction formula (43) are considered to be different for each site.

The film fouling diagnostic means 32 is shown in FIG.
As shown in (b), (c), (d), (e), (f), and (g),
Langerian index (a) obtained by the equation (40), measured SDI (b), pH (c) after pH adjustment, and water permeability coefficient (RO) of the RO membrane obtained as described above and converted to the standard state. d) and the time series change of the salt permeation coefficient (e), the change width of the water permeation coefficient, the Langerian index, and the average of SDI are displayed.

The operator can compare the fouling conditions of the membrane by comparing different SDI, Langerian index, water permeability coefficient at different pH target values, and salt permeability coefficient. This function is used by the operator to determine the pH target value from the Langerian index.
It will help to make the H target value safe range appropriate.

[0088]

[Equation 46]

[0089]

[Equation 47]

[0090]

[Equation 48] (Permeated water concentration target value Cfsv, module number target value Nsv)
The raw water flow rate target value Qfsv other than the next input, the permeate flow rate target value Qpsv, the recovery rate target value RRsv, the raw water pressure target value Pfsv, the permeate concentration target value Cfsv, and the module number target value Nsv In this case, the salt permeation coefficient is calculated as the transport equation (43), (47) of the reverse osmosis process.
Calculate using a formula.

The B value temperature correction coefficient Bt is obtained by conducting an experiment for each site and preliminarily input because the characteristic of the salt permeation coefficient water temperature correction expression (48) is considered to be different for each site. I shall.

[0092]

[Equation 49] As described above, according to the present embodiment, the buffer capacity calculating means 27 can calculate the buffer capacity of the raw water, so that it is possible to know how much sulfuric acid is added to the raw water to reach the target pH, Can be injected with sulfuric acid. Further, by continuously obtaining the buffering capacity, it is possible to inject appropriately according to the change in the water quality of the raw water. In the constant pH control means 28, p
Since the injection rate of the sulfuric acid injector 1 is determined so that the pH after pH adjustment becomes constant at the target value from the pH after H adjustment and the buffer capacity obtained by the buffer capacity calculation means 27, the target pH is always maintained.
It is possible to supply raw water to the reverse osmosis membrane module. Therefore, it is possible to prevent the deposition of a soluble substance such as calcium carbonate on the surface of the film and prolong the life of the film.

Further, by means of the pH change graph display means 30,
Since the relationship between the injection amount of the sulfuric acid injector 1 and the pH can be obtained and displayed as a graph, the operator can determine whether or not the current injection is appropriate and can assist the determination of the pH target value. This makes it possible to perform more appropriate injection, prevent the deposition of soluble substances such as calcium carbonate on the film surface, and prolong the life of the film.

Further, by means of the target value determination means 31 using the Langerian index, the Langerian coefficient is calculated from the measured raw water calcium concentration, raw water alkalinity and raw water pH using the second carbonic acid dissociation equilibrium constant in seawater and the calcium carbonate solubility product. P, such that calcium carbonate does not precipitate in the film.
The H target value can be determined. Therefore, the acid of the raw water
Since the pH target value can be determined in consideration of not only the property of base equilibrium but also the calcium concentration, it is possible to more reliably prevent the deposition of soluble substances such as calcium carbonate on the film surface on the film surface.

According to the membrane fouling diagnostic means 32, the SDI measured as the Langerian index, the pH after pH adjustment, the water permeation coefficient of the RO membrane converted into the standard state obtained as described above, and the salt permeation Coefficient, time series change of water permeation coefficient, change range of water permeation coefficient, Langeria index, the average of SDI can be displayed, so that the operator has different SDI, Langeria index, water permeation coefficient at the pH target value, The salt permeation coefficient can be compared to compare the degree of fouling of the membrane. This function assists the operator in adjusting the pH target value safety range used when determining the pH target value from the Langerian index. In consideration of the water permeation coefficient, salt permeation coefficient and SDI, pH injection control considering the entire plant can be realized and optimum pH adjustment can be performed.

[0096]

EFFECTS OF THE INVENTION Generally, in a seawater desalination plant having a reverse osmosis membrane module, when high-pH feed water is supplied to the membrane module, soluble substances such as calcium carbonate dissolved in seawater reach the membrane surface. It causes the phenomenon of precipitation. According to the present invention, the buffer capacity of raw water, which is the amount of change in pH per unit pH adjuster injection, is determined, and constant pH control can be realized according to that value, so that an appropriate pH injection according to the nature of raw water can be achieved. It can be carried out. Also, since most of the precipitates are calcium, the calcium concentration of the raw water can be measured and the pH target value at which calcium salts do not precipitate can be set, so that the precipitation of substances on the film surface can be suppressed and the life of the film can be prolonged.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of a seawater desalination plant according to the present invention.

FIG. 2 is a diagram showing changes in pH of raw water due to injection of sulfuric acid.

FIG. 3 is a diagram showing an operating condition of a seawater desalination plant.

[Explanation of symbols]

 1 Sulfuric Acid Injector 2 High Pressure Pump 3 Reverse Osmosis Membrane Module 4 Feed Water Raw Water Inflow Channel 8 Water Temperature Meter 9 Conductivity Meter 10, 16 pH Meter 11 Alkalinity Meter 12 SDI Meter 13 Calcium Concentration Measuring Device 14 Sampling Pump 15 Sampling Pipe 18 Permeation Water outflow path 19 Concentrated water outflow path 27 Buffer capacity calculation means 28 pH constant control means 29 Injection amount calculation means 30 pH change graph display means 31 Target value determination means 32 Membrane fouling diagnosis means

Claims (8)

[Claims] 1. A reverse osmosis membrane module for converting raw water, which is seawater, into fresh water, a pH adjusting device for injecting a pH adjusting agent for adjusting the pH of the raw water, and a buffering ability of the raw water based on the measurement value measured by the measuring means. The pH after pH adjustment is calculated from the buffering capacity calculating means for calculating, the pH after pH adjustment measured by the measuring means, and the buffering capacity of the raw water obtained by the buffering capacity calculating means.
A seawater desalination plant, comprising: a constant pH control means for determining an injection rate of a pH adjusting agent by a pH adjusting device so that the value becomes constant at a target value. 2. The seawater desalination plant according to claim 1, wherein the buffer capacity calculating means calculates the buffer capacity of the raw water using the following formula. [Equation 1] 3. The seawater desalination plant according to claim 1, wherein the buffer capacity calculating means calculates the buffer capacity of the raw water according to the following formula. [Equation 2] (Equation 3) 4. The seawater desalination plant according to claim 1, wherein the buffer capacity calculating means calculates the buffer capacity of the raw water according to the following formula. [Equation 4] (Equation 5) (Equation 6) 5. The seawater desalination plant according to claim 1, wherein the buffer capacity calculating means calculates the buffer capacity of the raw water according to the following formula. (Equation 7) (Equation 8) [Equation 9] 6. The seawater desalination plant according to claim 1, wherein the buffer capacity calculating means calculates the buffer capacity of the raw water according to the following formula. [Equation 10] [Equation 11] 7. The seawater desalination plant according to claim 1, wherein the buffer capacity calculating means calculates the buffer capacity of the raw water by the following formula. (Equation 12) (Equation 13) 8. The pH constant control means uses the second carbonic acid dissociation equilibrium constant in seawater and the calcium carbonate solubility product from the raw water calcium concentration, raw water alkalinity and raw water pH measured by the measuring means to determine the Find the Langerian coefficient by the formula,
2. The seawater desalination plant according to claim 1, further comprising a target value determination means based on a Langerian index for determining a pH target value such that calcium carbonate does not precipitate in the film. pHsv = pHs-KpHs pHsv: pH target value pHs: Langelier index KPH: pH target value safety margin _{pHs = (pK 2 -pK s)} -log [Ca +] -lo
gTA + TS K ₂ : Carbonic acid second dissociation constant K _s : Solubility product of calcium carbonate [Ca ⁺ ]: Calcium ion concentration TA: Alkalinity TS: Corrected value by soluble substance [Equation 14]