Patent Publication Number: US-2012031274-A1

Title: Aeration apparatus, seawater flue gas desulphurization apparatus including the same, and humidification method for aeration apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to wastewater treatment in a flue gas desulphurization apparatus used in a power plant such as a coal, crude oil, or heavy oil combustion power plant. In particular, the invention relates to an aeration apparatus for aeration used for decarboxylation (air-exposure) of wastewater (used seawater) from a flue gas desulphurization apparatus for desulphurization using a seawater method. The invention also relates to a seawater flue gas desulphurization apparatus including the aeration apparatus and to a humidification method for the aeration apparatus. 
     BACKGROUND ART 
     In conventional power plants that use coal, crude oil, and the like as fuel, combustion flue gas (hereinafter referred to as “flue gas”) discharged from a boiler is emitted to the air after sulfur oxides (SO X ) such as sulfur dioxide (SO 2 ) contained in the flue gas are removed. Known examples of the desulphurization method used in a flue gas desulphurization apparatus for the above desulphurization treatment include a limestone-gypsum method, spray dryer method, and seawater method. 
     In a flue gas desulphurization apparatus that uses the seawater method (hereinafter referred to as a “seawater flue gas desulphurization apparatus”), its desulphurization method uses seawater as an absorbent. In this method, seawater and flue gas from a boiler are supplied to the inside of a desulfurizer (absorber) having a vertical tubular shape such as a vertical substantially cylindrical shape, and the flue gas is brought into gas-liquid contact with the seawater used as the absorbent in a wet process to remove sulfur oxides. The seawater (used seawater) used as the absorbent for desulphurization in the desulfurizer flows through, for example, a long water passage having an open upper section (Seawater Oxidation Treatment System: SOTS) and is then discharged. In the long water passage, the seawater is decarbonated (exposed to air) by aeration that uses fine air bubbles ejected from an aeration apparatus disposed on the bottom surface of the water passage (Patent documents 1 to 3). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent document 1: Japanese Patent Application Laid-open No. 2006-055779 
         Patent document 2: Japanese Patent Application Laid-open No. 2009-028570 
         Patent document 3: Japanese Patent Application Laid-open No. 2009-028572 
       
    
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     Aeration nozzles used in the aeration apparatus each have a large number of small slits formed in a rubber-made diffuser membrane that covers a base. Such aeration nozzles are generally referred to as “diffuser nozzles.”These aeration nozzles can eject many fine air bubbles of substantially equal size from the slits with the aid of the pressure of the air supplied to the nozzles. 
     When aeration is continuously performed in seawater using the above aeration nozzles, precipitates such as calcium sulfate in the seawater are deposited on the wall surfaces of the slits of the diffuser membranes and around the openings of the slits, causing the gaps of the slits to be narrowed and the slits to be clogged. This results an increase in pressure loss of the diffuser membranes, and the discharge pressure of discharge unit, such as a blower or compressor, for supplying the air to the diffuser is thereby increased, so that disadvantageously the load on the blower or compressor increases. 
     The occurrence of the precipitates may be due to the following reason. Seawater present outside a diffuser membrane permeates inside the diffuser membrane through its slits and comes into continuous contact with air passing through the slits for a long time. Drying (concentration of the seawater) is thereby facilitated, and the precipitates are deposited. 
     In view of the above problem, it is an object of the present invention to provide an aeration apparatus that can suppress the occurrence of precipitates in the slits of diffuser membranes, a seawater flue gas desulphurization apparatus including the aeration apparatus, and a humidification method for the aeration apparatus. 
     Means for Solving Problem 
     According to an aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, the aeration apparatus includes: an air supply pipe for supplying air through discharge unit; moisture supplying unit for supplying moisture to the air supply pipe; and an aeration nozzle including a diffuser membrane having a slit, the air containing the moisture being supplied to the aeration nozzle. 
     Advantageously, in the aeration apparatus, the moisture is one of fresh water and seawater. 
     According to another aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, the aeration apparatus includes: an air supply pipe for supplying air through discharge unit; water vapor supplying unit for supplying water vapor to the air supply pipe; and an aeration nozzle including a diffuser membrane having a slit, the air containing the water vapor being supplied to the aeration nozzle. 
     Advantageously, the aeration apparatus further includes a filter and a cooling unit that are disposed in the air supply pipe. 
     Advantageously, in the aeration apparatus, the moisture is supplied near an air inlet of the discharge unit. 
     Advantageously, in the aeration apparatus, the aeration nozzle further includes the diffuser membrane covering a support body into which the air is introduced and a large number of the slits formed therein, the fine air bubbles being ejected from the large number of slits. 
     According to still another aspect of the present invention, a seawater flue gas desulphurization apparatus includes: a desulfurizer that uses seawater as an absorbent; a water passage for allowing used seawater discharged from the desulfurizer to flow therethrough and be discharged; and the aeration apparatus described above that is disposed in the water passage, the aeration apparatus generating fine air bubbles in the used seawater to decarbonate the used seawater. 
     According to still another aspect of the present invention, a humidification method for an aeration apparatus includes: using an aeration apparatus that is immersed in water to be treated and used to generate fine air bubbles in the water to be treated; adding moisture or water vapor to air when the air is supplied through discharge unit; and supplying the air containing the moisture to a slit of a diffuser membrane. 
     Effect of the Invention 
     According to the present invention, the occurrence of precipitates in the slits of the diffuser membranes of the aeration apparatus can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a seawater flue gas desulphurization apparatus according to an embodiment. 
         FIG. 2-1  is a plan view of aeration nozzles. 
         FIG. 2-2  is a front view of the aeration nozzles. 
         FIG. 3  is a schematic diagram of the inner structure of an aeration nozzle. 
         FIG. 4  is a schematic diagram of an aeration apparatus according to an embodiment. 
         FIG. 5  is a schematic diagram of another aeration apparatus according to the embodiment. 
         FIG. 6  is a schematic diagram of another aeration apparatus according to the embodiment. 
         FIG. 7  is a schematic diagram of another aeration apparatus according to the embodiment. 
         FIG. 8-1  is a diagram illustrating the states of the outflow of air (a mixture of moisture-saturated air and water mist) and the inflow of seawater in a slit of a diffuser membrane. 
         FIG. 8-2  is a diagram illustrating the states of the outflow of air (moisture-saturated air) and the inflow of seawater in the slit of the diffuser membrane. 
         FIG. 8-3  is a diagram illustrating the states of the outflow of air (humid air, relative humidity: 100% or less), the inflow of seawater, and concentrated seawater in the slit of the diffuser membrane. 
         FIG. 8-4  is a diagram illustrating the states of the outflow of air, the inflow of seawater, and concentrated seawater in the slit of the diffuser membrane. 
         FIG. 8-5  is a diagram illustrating the states of the outflow of air, the inflow of seawater, concentrated seawater, and precipitates in the slit of the diffuser membrane. 
         FIG. 9  is a set of graphs showing a change in the salt concentration in seawater entering the slits of aeration nozzles and the operating condition of an aeration apparatus when moisture is intermittently supplied to an air supply pipe. 
     
    
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION 
     Hereinafter, the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to embodiments described below. The components in the following embodiments include those readily apparent to persons skilled in the art and those substantially similar thereto. 
     EMBODIMENTS 
     An aeration apparatus and a seawater flue gas desulphurization apparatus according to embodiments of the present invention will be described with reference to the drawings.  FIG. 1  is a schematic diagram of the seawater flue gas desulphurization apparatus according to one embodiment. 
     As shown in  FIG. 1 , the seawater flue gas desulphurization apparatus  100  includes: a flue gas desulphurization absorber  102  in which flue gas  101  and seawater  103  comes in gas-liquid contact to desulphurize SO 2  into sulfurous acid (H 2 SO 2 ); a dilution-mixing basin  105  disposed below the flue gas desulphurization absorber  102  to dilute and mix used seawater  103 A containing sulfur compounds with dilution seawater  103 ; and an oxidation basin  106  disposed on the downstream side of the dilution-mixing basin  105  to subject the diluted used seawater  103 B to water quality recovery treatment. 
     In the seawater flue gas desulphurization apparatus  100 , the seawater  103  is supplied through a seawater supply line L 1 , and part of the seawater  103  is used for absorption, i.e., is brought into gas-liquid contact with the flue gas  101  in the flue gas desulphurization absorber  102  to absorb SO 2  contained in the flue gas  101  into the seawater  103 . The used seawater  103 A that has absorbed the sulfur components in the flue gas desulphurization absorber  102  is mixed with the dilution seawater  103  supplied to the dilution-mixing basin  105  disposed below the flue gas desulphurization absorber  102 . The diluted used seawater  103 B diluted and mixed with the dilution seawater  103  is supplied to the oxidation basin  106  disposed on the downstream side of the dilution-mixing basin  105 . Air  122  supplied from an oxidation air blower  121  is supplied to the oxidation basin  106  from aeration nozzles  123  to recover the quality of the seawater, and the resultant water is discharged to the sea as treated water  124 . 
     In  FIG. 1 , reference numeral  102   a  represents spray nozzles for injecting seawater upward as liquid columns;  120  represents an aeration apparatus;  122   a  represents air bubbles; L 1  represents a seawater supply line; L 2  represents a dilution seawater supply line; L 3  represents a desulphurization seawater supply line; L 4  represents a flue gas supply line; and L 5  represents an air supply line. 
     The structure of the aeration nozzles  123  is described with reference to  FIGS. 2-1 ,  2 - 2 , and  3  when a diffuser membrane is made of rubber. 
       FIG. 2-1  is a plan view of the aeration nozzles;  FIG. 2-2  is a front view of the aeration nozzles; and  FIG. 3  is a schematic diagram of the inner structure of an aeration nozzle. 
     As shown in  FIG. 3 , each aeration nozzle  123  has a large number of small slits  12  formed in a rubber-made diffuser membrane  11  that covers the circumference of a base and is generally referred to as a “diffuser nozzle.” In such an aeration nozzle  123 , when the diffuser membrane  11  is expanded by the pressure of the air  122  supplied from the air supply line L 5 , the slits  12  open to allow a large number of fine air bubbles of substantially equal size to be ejected. 
     As shown in  FIGS. 2-1  and  2 - 2 , the aeration nozzles  123  are attached through flanges  16  to headers  15  provided in a plurality of (eight in the present embodiment) branch pipes (not shown) branched from the air supply line L 5 . In consideration of corrosion resistance, resin-made pipes, for example, are used as the branch pipes and the headers  15  disposed in the diluted used seawater  103 B. 
     For example, as shown in  FIG. 3 , each aeration nozzle  123  is formed as follows. A substantially cylindrical support body  20  that is made of a resin in consideration of corrosion resistance to the used seawater  103 B is used, and a rubber-made diffuser membrane  11  having a large number of slits  12  formed therein is fitted on the support body  20  so as to cover its outer circumference. Then the left and right ends of the diffuser membrane  11  are fastened with fastening members  22  such as wires or bands. 
     The slits  12  described above are closed in a normal state in which no pressure is applied thereto. 
     In the seawater flue gas desulphurization apparatus  100 , when the air  122  is continuously supplied, the slits  12  are constantly in an open state. 
     A first end  20   a  of the support body  20  is attached to a header  15  and allows the introduction of the air  122 , and the support body  20  has an opening at its second end  20   b  that allows the introduction of the seawater  103 . 
     In the support body  20 , the side close to the first end  20   a  is in communication with the inside of the header  15  through an air inlet port  20   c  that passes through the header  15  and the flange  16 . The inside of the support body  20  is partitioned by a partition plate  20   d  disposed at some axial position in the support body  20 , and the flow of air is blocked by the partition plate  20   d . Air outlet holes  20   e  and  20   f  are formed in the side surface of the support body  20  and disposed on the header  15  side of the partition plate  20   d . The air outlet holes  20   e  and  20   f  allow the air  122  to flow between the inner circumferential surface of the diffuser membrane  11  and the outer circumferential surface of the support body, i.e., into a pressurization space  11   a  for pressurizing and expanding the diffuser membrane  11 . Therefore, the air  122  flowing from the header  15  into the aeration nozzle  123  flows through the air inlet port  20   c  into the support body  20  and then flows through the air outlet holes  20   e  and  20   f  formed in the side surface into the pressurization space  11   a , as shown by arrows in  FIG. 3 . 
     The fastening members  22  fasten the diffuser membrane  11  to the support body  20  and prevent the air flowing through the air outlet holes  20   e  and  20   f  from leaking from the opposite ends. 
     In the aeration nozzle  123  configured as above, the air  122  flowing from the header  15  through the air inlet port  20   c  flows through the air outlet holes  20   e  and  20   f  into the pressurization space  11   a . Since the slits  12  are closed in the initial state, the air  122  is accumulated in the pressurization space  11   a  to increase the inner pressure. The increase in the inner pressure of the pressurization space  11   a  causes the diffuser membrane  11  to expand, and the slits  12  formed in the diffuser membrane  11  are thereby opened, so that fine bubbles of the air  122  are injected into the diluted used seawater  103 B. Such fine air bubbles are generated in all the aeration nozzles  123  to which air is supplied through branch pipes L 5A  to L 5H  and the headers  15 . 
     Aeration apparatuses according to an embodiment will next be described. The present invention provides means for avoiding deposition of precipitates such as calcium sulfate by preventing drying and concentration of seawater in the slits  12  of the diffuser membranes  11 . To prevent the seawater  103  from being dried and concentrated in the slits  12  by the air  122  supplied thereto, wet air with a high moisture content (a high relative humidity) is used as the supplied air  122 . Preferably, the air  122  having a high relative humidity is moisture-saturated air with a relative humidity of 100% or moisture-saturated air containing water mist, and measures are taken to obtain such air. 
     The present invention will next be described specifically. 
       FIGS. 4 to 7  are schematic diagrams of the aeration apparatuses according to the present embodiment. 
     As shown in  FIG. 4 , an aeration apparatus  120 A according to the present embodiment is immersed in diluted used seawater (not shown), which is water to be treated, and generates fine air bubbles in the diluted used seawater  103 B. This aeration apparatus includes: an air supply line L 5  for supplying air  122  from blowers  121 A to  121 D (discharge unit); a fresh water tank  140  and a supply pump P 1 , which are moisture supplying unit for supplying fresh water  141  being moisture to the air supply line L 5 ; and aeration nozzles  123  each including a diffuser membrane  11  having slits for supplying air containing moisture. 
     Two cooling units  131 A and  131 B and two filters  132 A and  132 B are provided in the air supply line L 5 . The air compressed by the blowers  121 A to  121 D is thereby cooled and then filtrated. 
     Normally, three of the four blowers are used for operation, and one of them is a reserve blower. Since the aeration apparatus must be continuously operated, only one of the two cooling units  131 A and  131 B and only one of the two filters  132 A and  132 B are normally used, and the others are used for maintenance. 
     In the present embodiment, fresh water is used to supply moisture. However, instead of the fresh water, seawater (such as seawater  103  from the dilution seawater supply line L 2 , used seawater  103 A in the dilution-mixing basin  105 , or diluted used seawater  103 B in the oxidation basin  106 ) may be used. 
     In the present embodiment, since moisture (the fresh water  141  or seawater) is supplied, the air  122  supplied to the aeration nozzles  123  can be humidified (the partial pressure of water vapor in the air  122  can be increased). 
     In the aeration apparatus  120 A shown in  FIG. 4 , moisture is supplied by spraying, for example, the fresh water  141  into the supplied air  122  using single-fluid nozzles (arrow portions in the figure). 
     In an aeration apparatus  120 B shown in  FIG. 5 , an air supply line L 7  is separately provided to supply air  122  to sections to which the moisture is supplied. 
     This air  122  is used as assist gas when the moisture (the fresh water  141  or seawater) is supplied. More specifically, the moisture is finely sprayed with the assist gas into the air  122  supplied from the air supply line L 5  using two-fluid nozzles (in order to facilitate evaporation of the moisture). Reference symbol P 2  represents an air supply pump. 
     In the air supply systems shown in  FIGS. 4 and 5  above, the cooling units  131 A and  131 B may be omitted. In this case, a predetermined amount of moisture (fresh water or seawater) is injected into the air  122  pressurized by the blowers  121 A to  121 D and increased in temperature to reduce the temperature of the air  122  to be supplied so that the air in the slits  11  of the aeration nozzles  123  is saturated with moisture. 
     In an aeration apparatus  120 C shown in  FIG. 6 , water vapor  142  is supplied from a water vapor supply line L 8 . Reference symbol P 3  represents a water vapor supply pump. 
     In an aeration apparatus  120 D shown in  FIG. 7 , intake spray nozzles (not shown) for supplying moisture  143  are provided near the air inlets of the blowers  121 A to  121 D, which serve as discharge unit. In this case, the moisture  143  is added to intake air (the moisture is vaporized before it enters the blowers), and the amount of cooling in the cooling unit  131 A on the outlet side of the blowers is controlled so that the air passing through the slits of the aeration nozzles is moisture-saturated air. 
     More specifically, the temperature of the air  122  pressurized and compressed by the blowers  121 A to  121 D is as high as, for example, about 100° C. However, when an excess amount of the moisture  143  is supplied before pressurization and compression, the air  122  to be supplied is moisture rich. Then the temperature of the air is reduced by the cooling unit  131  (to, for example, 40° C.) Since the amount of moisture in the air  122  is unchanged, the degree of moisture saturation (the relative humidity) of the cooled air  122  increases. Therefore, the relative humidity of the air in the slits  12  of the aeration nozzles  123  is 100%. When the amount of water added to the intake air is further increased, moisture-saturated air containing water mist is formed, and a gas-liquid two-phase state is formed. 
     Even when the relative humidity of the air sucked by the blowers  121 A to  121 D is 100% on the inlet side of the blowers, the relative humidity of the air in the slits  11  of the aeration nozzles  123  may not be 100% because the air is compressed and cooled. In such a case, if the shortage of the moisture  143  is supplied at the inlets of the blowers, unevaporated moisture enters the blowers, which is not preferred. In this case, moisture such as fresh water or seawater is supplied on the outlet side of the blowers  121 A to  121 D or the downstream side of the cooling units  131 A and  131 B. 
     When moisture is supplied to the air  122  in each of the cases shown in  FIGS. 4 to 7  described above, the amount of moisture supplied and the amount of cooling in the cooling unit are adjusted according to the air conditions (pressure, temperature, and relative humidity) at the inlets of the blower and in consideration of pressure loss and heat exchange between the air supply pipe and the outside such that the air passing through the slits  11  of the aeration nozzles  123  is moisture-saturated air or moisture-saturated air entraining water mist. 
     As described above, moisture-saturated air or moisture-saturated air entraining water mist is supplied to the aeration nozzles  123 . This prevents drying (concentration) of seawater that enters the slits  12  of the diffuser membranes  11  and thereby prevents the deposition of salts, such as calcium sulfate, in the seawater. When concentrated seawater (salt concentration: about 3.4% or more and about 14% or less) is formed in the slits, the water mist contributes to relaxation of the concentration of the seawater (a reduction in salt concentration). 
     By supplying moisture (fresh water, water vapor, or seawater) as described above, the air  122  supplied to the aeration nozzles  123  is saturated with water vapor. This prevents drying (concentration) of the seawater that enters the slits  12  of the diffuser membranes  11  and thereby prevents the deposition of calcium sulfate and the like. In this manner, the pressure loss of the diffuser membranes  11  can be prevented. 
     Preferably, the amount of moisture supplied is set such that the air passing through the slits  12  of the aeration nozzles  123  is air fully saturated with moisture. More preferably, the amount of moisture supplied is set such that the air is moisture-saturated air entraining water mist (in a gas-liquid two-phase state). The relative humidity of the air  122  flowing into the slits  12  of the aeration nozzles  123  is 40% or more, preferably 60% or more, and more preferably 80% or more. Conditions under which the concentrating rate of seawater in the slits is slow may be used depending on the maintenance time of the apparatus. 
     The humidity condition of the air passing through the slits  12  of the aeration nozzles  123  is controlled by adjusting the humidity of air sucked by the blowers, the amount of moisture supplied, the amount of cooling in the cooling unit, and the like. 
     In this manner, the seawater entering the slits  12  of the diffuser membrane  11  is prevented from being dried, and the degree of concentration of the seawater (an increase in salt concentration) is suppressed, so that the salt concentration in the seawater can be maintained at about 14% or less. 
     The salt concentration in seawater is generally about 3.4%, and 3.4% of salts are dissolved in 96.6% of water. The salts include 77.9% of sodium chloride, 9.6% of magnesium chloride, 6.1% of magnesium sulfate, 4.0% of calcium sulfate, 2.1% of potassium chloride, and 0.2% of other salts. 
     Of these salts, calcium sulfate is deposited first as seawater is concentrated (dried), and the deposition threshold value of the salt concentration in seawater is about 14%. 
     Therefore, by injecting moisture such as the fresh water  141  into the air  122  to be supplied to the aeration nozzles  123  by the moisture supplying unit to form moisture-rich air and then supplying the moisture-rich air to the slits  12  of the diffuser membranes  11 , the concentration of the seawater (an increase in the salt concentration) in the slits  12  can be prevented, and the deposition of calcium sulfate and the like can thereby be prevented. 
     This can prevent the narrowing of the gaps of the slits  12  due to the deposition of calcium sulfate and the like and the clogging of the slits  12 , and the pressure loss of the diffuser membranes  11  can thereby be prevented. 
       FIGS. 8-1  to  8 - 5  are diagrams illustrating the outflow of air (to which moisture has been supplied) and the inflow of the seawater  103  in a slit  12  of a diffuser membrane  11 . 
     In the present invention, the slits  12  are cuts formed in the diffuser membranes  11 , and the gap of each slit  12  serves as a discharge passage of air. 
     The seawater  103  is in contact with slit wall surfaces  12   a  that form the passage. The introduction of the air  122  causes the seawater  103  to be dried and concentrated to form concentrated seawater  103   a . Then a precipitate  103   b  is deposited on the slit wall surfaces and clogs the passage in the slit. 
     In the state shown in  FIG. 8-1 , the relative humidity of the air  122  is 100% (moisture-saturated air), and the air  122  entrains water mist  150  to form a gas-liquid two-phase state. Therefore, the seawater  103  entering the slit  12  is not dried (concentrated), and the salt concentration is reduced, so that the drying (concentration) of the seawater is prevented. 
     In the state shown in  FIG. 8-2 , the relative humidity of the air  122  is 100%. Therefore, the salt concentration in seawater is unchanged, and the drying of the seawater is prevented. 
     In the state shown in  FIG. 8-3 , the relative humidity of the air  122  is, for example, 80%. Therefore, the drying of the seawater is suppressed. The salt concentration in the seawater increases gradually, and concentrated seawater  103   a  is formed. However, even if the concentration of the seawater is initiated, deposition of calcium sulfate and the like does not occur when the salt concentration in the seawater is about 14% or less. Therefore, in this state, by intermittently introducing moisture-saturated air entraining water mist  150  to force the formation of a moisture-rich state, the salt concentration increased to some extent is reduced, and the deposition is thereby avoided. In this manner, the apparatus can be operated for a long time. 
       FIGS. 8-4  and  8 - 5  show the growth states of the precipitate in the slit  12  of the diffuser membrane  11  as the drying and concentration of the seawater due to the air proceed. 
     In the state shown in  FIG. 8-4 , the precipitate  103   b  is generated in portions of the concentrated seawater  103   a  in which the salt concentration in the seawater locally exceeds 14%. In this state, the amount of the precipitate  103   b  is very small. Therefore, although the pressure loss when the air  122  passes through the slit increases slightly, the air can pass through the slit. 
     However, in the state shown in  FIG. 8-5 , since the concentration of the concentrated seawater  103   a  has proceeded further, a clogged (plugged) state due to the precipitate  103   b  is formed, and the pressure loss is high. Even in this state, the passage of the air  122  remains present, but the load on the discharge unit is considerably large. 
       FIG. 9  is a set of graphs showing a change in the salt concentration in seawater and the operating condition of an aeration apparatus. 
     As shown in  FIG. 9 , when air with a relative humidity of 100% or less is supplied, moisture-rich moisture-saturated air having a humidity of 100% and containing water mist  150  or moisture-saturated air entraining water mist is intermittently introduced after normal operation is performed for a predetermined time (the introduction period is illustrated as a peak). In this manner, the operation can be performed without deposition of calcium sulfate and the like. 
     In the present embodiment, plugging caused by deposition of seawater components and contamination components such as sludge on diffuser slits (membrane slits) can be prevented in the aeration apparatuses for aeration of seawater. Therefore, an increase in pressure loss in the aeration apparatuses can be prevented, and the aeration apparatuses can be stably operated for a long time. 
     In the description in the present embodiment, seawater is exemplified as water to be treated, but the invention is not limited thereto. For example, in an aeration apparatus for aerating polluted water in polluted water treatment (such as sewage treatment), plugging caused by deposition of contamination components such as sludge on diffuser slits (membrane slits) can be prevented, and the aeration apparatus can be stably operated for a long time. 
     In the description of the present embodiment, tube-type aeration nozzles are used in the aeration apparatuses, but the present invention is not limited thereto. For example, the invention is applicable to disk-type and flat-type aeration apparatuses having diffuser membranes and to diffusers including ceramic or metal diffuser membranes having slits that are open at all times. 
     INDUSTRIAL APPLICABILITY 
     As described above, in the aeration apparatus according to the present invention, the occurrence of precipitates in the slits of the diffuser membranes of the aeration apparatus can be suppressed. For example, when applied to a seawater flue gas desulphurization apparatus, the aeration apparatus can be continuously operated in a stable manner for a long time. 
     EXPLANATIONS OF LETTERS OR NUMERALS 
     
         
         
           
               11  diffuser membrane 
               12  slit 
               100  seawater flue gas desulphurization apparatus 
               102  flue gas desulphurization absorber 
               103  seawater 
               103   a  concentrated seawater 
               103   b  precipitate 
               103 A used seawater 
               103 B diluted used seawater 
               105  dilution-mixing basin 
               106  oxidation basin 
               120 A to  120 D aeration apparatus 
               122  air 
               123  aeration nozzle