Patent Publication Number: US-2012043283-A1

Title: Aeration apparatus, seawater flue gas desulfurization apparatus including the same, and operation method of aeration apparatus

Description:
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 an operation method of the aeration apparatus. 
     BACKGROUND 
     In conventional power plants that use coal, crude oil, and the like as fuel, combustion flue gas (hereinafter referred to as “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). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open No. 2006-055779 
         Patent Literature 2: Japanese Patent Application Laid-open No. 2009-028570 
         Patent Literature 3: Japanese Patent Application Laid-open No. 2009-028572 
       
    
     SUMMARY 
     Technical Problem 
     Aeration diffusers 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 diffusers are generally referred to as “diffuser nozzles.” These aeration diffusers 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. Conventionally, in the case of a rubber-made diffuser membrane, the length of the slit is about 1 to 3 millimeters. 
     When aeration is continuously performed in seawater using the above aeration diffusers, 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 discharge precipitates generated in the slits of diffuser membranes to the outside of the diffuser membranes, a seawater flue gas desulfurization apparatus including the aeration apparatus, and an operation method of an aeration apparatus. 
     Solution to 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, includes: an air supply pipe for supplying air through a discharge unit; and an aeration diffuser including a diffuser membrane having a slit, the air being supplied through the slit to the aeration diffuser. An opening shape of the slit is deformed due to pressure of air supplied through the slit. 
     Advantageously, in the aeration apparatus, the slit has at least a bent portion. 
     Advantageously, the aeration apparatus further includes a control unit that controls a temporal increase in supply of air at every predetermined time. 
     Advantageously, in the aeration apparatus, the control unit controls a temporal increase in the supply of air and supply of water to the air supply pipe. 
     According to another aspect of the present invention, a seawater flue gas desulfurization apparatus includes: a desulfurizer that uses seawater as an absorbent; a water passage that discharges used seawater discharged from the desulfurizer; and any one of the aeration apparatus described above that is disposed in the water passage, and generate fine air bubbles in the used seawater to decarbonate the used seawater. 
     According to still another aspect of the present invention, an operation method of an aeration apparatus includes: using the aeration apparatus according to any one of claims  1  to  4  that is immersed in water to be treated and used to generate fine air bubbles in water to be treated; and temporarily increasing supply of air at every predetermined time to prevent clogging, when air is supplied through a discharge unit. 
     Advantageously, in the operation method of an aeration apparatus, water is supplied to an air supply pipe when the supply of the air is temporarily increased or separately. 
     Advantageous Effects of Invention 
     According to the present invention, discharge of precipitates to the outside of diffuser membranes can be facilitated in the slits of the diffuser membranes of the aeration apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a seawater flue gas desulphurization apparatus according to an embodiment. 
         FIG. 2A  is a plan view of aeration diffusers. 
         FIG. 2B  is a front view of the aeration diffusers. 
         FIG. 3  is a schematic diagram of the inner structure of an aeration diffuser. 
         FIG. 4A  is a schematic diagram of a shape of a first slit of the aeration diffuser according to the embodiment. 
         FIG. 4B  is a schematic diagram of a shape of a second slit of the aeration diffuser according to the embodiment. 
         FIG. 4C  is a schematic diagram of a shape of a third slit of the aeration diffuser according to the embodiment. 
         FIG. 4D  is a schematic diagram of a shape of a fourth slit of the aeration diffuser according to the embodiment. 
         FIG. 4E  is a schematic diagram of a shape of a fifth slit of the aeration diffuser according to the embodiment. 
         FIG. 4F  is a schematic diagram of a shape of a sixth slit of the aeration diffuser according to the embodiment. 
         FIG. 4G  is a schematic diagram of a shape of a seventh slit of the aeration diffuser according to the embodiment. 
         FIG. 4H  is a schematic diagram of a shape of an eighth slit of the aeration diffuser according to the embodiment. 
         FIG. 4I  is a schematic diagram of a shape of a ninth slit of the aeration diffuser according to the embodiment. 
         FIG. 5A  depicts the outflow of air (humid air having a low degree of saturation), the inflow of seawater, and a state of concentrated seawater in a slit of a diffuser membrane. 
         FIG. 5B  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates in the slit of the diffuser membrane. 
         FIG. 5C  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates (when precipitates grow) in the slit of the diffuser membrane. 
         FIG. 6  is a schematic diagram of an aeration apparatus according to the embodiment. 
         FIG. 7  is a schematic diagram of the aeration apparatus according to the embodiment. 
         FIG. 8  is a graph of a relation between a passage of time and pressure loss fluctuation of a diffuser membrane when an amount of air is temporarily increased. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     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 , a 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 3 ); 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 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 diffusers  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 diffusers  123  is described with reference to  FIGS. 2A ,  2 B, and  3 . 
       FIG. 2A  is a plan view of the aeration diffusers; FIG.  2 B is a front view of the aeration diffusers; and  FIG. 3  is a schematic diagram of the inner structure of an aeration diffuser. 
     As shown in  FIGS. 2A and 2B , each aeration diffuser  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 diffuser  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. 2A and 2B , the aeration diffusers  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 diffuser  123  is formed as follows. A substantially cylindrical support body  20  that is made of a resin in consideration of corrosion resistance to the diluted 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 , because 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 diffuser  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 diffuser  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 diffusers  123  to which air is supplied through branch pipes L 5 A to L 5 H and the headers  15  (see  FIGS. 6 and 7 ). 
     The aeration apparatus according to the present embodiment will next be described. In the present invention, an opening shape of the slit  12  formed in the diffuser membrane  11  is deformed due to the pressure of air (an amount of air) to be supplied, thereby discharging precipitates generated in the slits  12  to the outside of the diffuser membrane  11 . 
       FIGS. 4A to 4I  depict shapes of various slits formed in the diffuser membrane of the aeration diffuser according to the present embodiment. 
       FIG. 4A  is a schematic diagram of a shape of a first slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4A , the shape of a first slit  12 A is formed by a linear reference slit  12   a  and a branched slit  12   b  crossing the linear reference slit  12   a  at a center thereof. An opening amount of the first slit  12 A changes due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at a bent portion of a crossing  12   c  of the linear reference slit  12   a  and the branched slit  12   b  increases, when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
     The salt concentration in seawater is 3.4%, and 3.4% of salts are dissolved in 96.6% of water. The salt includes 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 precipitation threshold value of the salt concentration in seawater is about 14%. 
     A mechanism in which precipitates are deposited in the slits  12  is explained with reference to  FIGS. 5A to 5C . 
       FIG. 5A  depicts the outflow of air (humid air having a low degree of saturation), the inflow of seawater, and a state of concentrated seawater in the slit of the diffuser membrane.  FIG. 5B  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates in the slit of the diffuser membrane.  FIG. 5C  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates (when precipitates grow) in the slit of the diffuser membrane. 
     In the present invention, the slits  12  are cuts formed in the diffuser membrane  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× 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 . A precipitate  103   b  is then deposited on the slit wall surfaces  12 × and clogs the passage in the slits  12 . 
       FIG. 5A  depicts a state in which salt content in seawater is gradually concentrated to form the concentrated seawater  103   a  due to low relative humidity of the air  122  (low degree of saturation). 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. 
     In the state shown in  FIG. 5B , 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 slits  12  increases slightly, the air  122  can pass through the slits  12 . 
     Therefore, in this state, precipitation is forcibly removed by generating pressure fluctuation as described later, thereby enabling an operation for a long time. 
     On the other hand, in the state shown in  FIG. 5C , because 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 becomes high. Even in this state, the passage of the air  122  remains. Even in this state, precipitates are forcibly removed by generating pressure fluctuation as described later, thereby enabling an operation for a long time. 
     Therefore, in the present embodiment, as shown in  FIG. 4A , it is set that an opening shape of the slit can be deformed due to the pressure of air (an amount of air) to be supplied, thereby preventing clogging. 
       FIG. 4B  is a schematic diagram of a shape of a second slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4B , the shape of a second slit  12 B is formed by the linear reference slit  12   a  and branched slits  12   b  formed so as to be orthogonal to the opposite ends of the linear reference slit  12   a . The opening shape of the second slit  12 B is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at the bent portions of the crossings  12   c  of the linear reference slit  12   a  and the branched slits  12   b  formed at the ends thereof increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4C  is a schematic diagram of a shape of a third slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4C , the shape of a third slit  12 C is formed by the linear reference slit  12   a  and branched slits  12   b  formed so as to be branched just before the opposite ends of the linear reference slit  12   a . The opening shape of the third slit  12 C is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at the bent portions of the crossings  12   c  of the linear reference slit  12   a  and the branched slits  12   b  formed at the ends thereof increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4D  is a schematic diagram of a shape of a fourth slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4D , the shape of a fourth slit  12 D is formed by the linear reference slit  12   a  and branched slits  12   b ,  12   b  formed so as to be branched in a V shape at the opposite ends of the linear reference slit  12   a . The opening shape of the fourth slit  12 D is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at the bent portions of the crossings  12   c  of the linear reference slit  12   a  and the V-shaped branched slits  12   b ,  12   b  formed at the ends thereof increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4E  is a schematic diagram of a shape of a fifth slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4E , the shape of a fifth slit  12 E is formed by the linear reference slit  12   a  and branched slits  12   b ,  12   b  formed so as to be branched at a sharp angle at the opposite ends of the linear reference slit  12   a . The opening shape of the fifth slit  12 E is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at bent portions  12   f  at the opposite ends of the linear reference slit  12   a  increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4F  is a schematic diagram of a shape of a sixth slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4F , the shape of a sixth slit  12 F is formed by the linear reference slit  12   a  and branched slits  12   b ,  12   b  formed so as to be branched in an L-shape at the opposite ends of the linear reference slit  12   a . The opening shape of the sixth slit  12 F is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at the bent portions  12   f  of the linear reference slit  12   a  and the L-shaped branched slits  12   b ,  12   b  formed at the ends thereof increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4G  is a schematic diagram of a shape of a seventh slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4G , the shape of a seventh slit  12 G is formed by the linear reference slit  12   a  and branched slits  12   b ,  12   b  formed so as to be branched in a V-shape at the end of the linear reference slit  12   a . The opening shape of the seventh slit  12 G is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at the crossing  12   c  of the linear reference slit  12   a  and the V-shaped branched slits  12   b ,  12   b  formed at the end thereof increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4H  is a schematic diagram of a shape of an eighth slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4H , the shape of an eighth slit  12 H is formed by an S-shaped slit  12   d . The opening shape of the eighth slit  12 H is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     In this manner, because the opening amount at a bent portion of a curve of the S-shaped slit  12   d  increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIG. 4I  is a schematic diagram of a shape of a ninth slit of the aeration diffuser according to the present embodiment. 
     As shown in  FIG. 4I , the shape of a ninth slit  12 I is formed by a U-shaped slit  12   e . The opening shape of the ninth slit  12 I is deformed due to the pressure of the air  122  (an amount of air) to be supplied. 
     At the bent portion, because the opening amount at a curved portion of the U-shaped slit  12   e  increases when the pressure of air to be supplied becomes high (when the amount of air increases), discharge of the precipitates to the outside of the diffuser membrane is facilitated, differently from conventional cases having only linear slits. 
       FIGS. 6 and 7  are schematic diagrams of the aeration apparatus according to the present embodiment. 
     As shown in  FIG. 6 , 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. 
     This aeration apparatus includes: an air supply line L 5  that supplies the air  122  from blowers  121 A to  121 D serving as discharge units; aeration diffusers  123  each including the diffuser membrane  11  having slits for supplying air including water, and a control unit (not shown) that controls a temporal increase in the supply of the air  122  at every predetermined time. 
     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, the control unit issues a command to temporarily increase the supply of the air  122  at every predetermined time. 
       FIG. 8  is a graph of a passage of time and pressure fluctuation. 
     As shown in  FIG. 8 , during a stable operation, after a predetermined time has passed, a purge operation for increasing the amount of air is performed for a predetermined time. 
     In this manner, because the supply of the air  122  is increased at every predetermined time and pressure fluctuation occurs (the amount of air temporarily increases) to increase expansion of the diffuser membrane  11 , precipitates of calcium sulfate deposited in the slits  12  are discharged to the outside, and the slit returns to a normal state. 
     As a result, it can be prevented that the slits  12  are clogged and the gap of the slits  12  becomes narrow due to precipitation of calcium sulfate in a continuous operation, thereby preventing pressure loss of the diffuser membrane  11 . 
     An interval of the increase can be appropriately changed corresponding to the precipitation state of precipitates; however, preferably, the increase is made once in one or two days. 
     This is because precipitates can be easily discharged to the outside of the diffuser membrane by increasing the supply of air at an early stage of initial precipitation to cause pressure fluctuation passing through the slits  12 . 
     To realize the temporal increase, for example, in the aeration apparatus  120 A shown in  FIG. 6 , when three blowers  121 A to  121 C are normally operated, a reserve blower  121 D can be further driven to supply a large amount of air  122  to the air supply line L 5 . 
     That is, the amount of air to be introduced into the aeration diffusers  123  is increased by activating the reserve blower  121 D. As a result, the slits  12  of the diffuser membrane  11  open largely, and calcium sulfate can be discharged to the seawater side and removed. 
     Accordingly, it can be prevented that the slits  12  are clogged and the gap of the slits  12  becomes narrow due to precipitation of calcium sulfate, thereby preventing pressure loss of the diffuser membrane  11 . 
     When a capacity of blower is not sufficient, a predetermined purge condition can be set so that precipitates are pushed out and blown away from the slits  12  by using an additional blower. 
     Further, as shown in  FIG. 7 , in an aeration apparatus  120 B according to the present embodiment, a water supply line L 6  that supplies fresh water  141  to the air supply line L 5  is provided. It suffices that a control unit (not shown) then controls a temporal increase in the supply of the air  122  and controls the supply of the fresh water  141  to the air supply line L 5 . 
     In this manner, by supplying the fresh water  141 , it is introduced into the aeration diffusers  123 . Accordingly, the slits  12  of the diffuser membrane  11  are cleaned, and precipitates such as calcium sulfate adhered to the slits  12  can be dissolved and removed. 
     As a result, it can be prevented that the slits  12  are clogged and the gap of the slits  12  becomes narrow due to precipitation of calcium sulfate, thereby preventing pressure loss of the diffuser membrane  11 . 
     In the present embodiment, while the fresh water  141  is used as the supplied water, instead of fresh water, seawater (for example, the seawater  103  from the diluted seawater supply line L 2 , the used seawater  103 A in a dilution-mixing basin  105 , or the diluted used seawater  103 B in the oxidation basin  106 ) and water vapor can be used. 
     In the present embodiment, while seawater has been exemplified as the water to be treated, the present invention is not limited thereto. For example, plugging caused by deposition of contamination components such as sludge on diffuser slits (membrane slits) can be prevented in the aeration apparatus for aeration of contaminated water in decontamination processing, and thus the aeration apparatus can be stably operated for a long time. 
     In the present embodiment, while tube-type aeration diffusers have been exemplified for explaining the aeration apparatus, the present invention is not limited thereto. For example, the invention is applicable to disk-type and flat-type aeration apparatuses and to diffusers made of ceramic or metal. 
     INDUSTRIAL APPLICABILITY 
     As described above, in the aeration apparatus according to the present invention, precipitates generated in the slits of the diffuser membrane of the aeration apparatus can be discharged to the outside of the diffuser membranes. 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. 
     REFERENCE SIGNS LIST 
     
         
         
           
               11  diffuser membrane 
               12  slit 
               12 A to  12 I first slit to ninth slit 
               100  seawater flue gas desulphurization apparatus 
               102  flue gas desulphurization absorber 
               103  seawater 
               103 A used seawater 
               103 B diluted used seawater 
               105  dilution-mixing basin 
               106  oxidation basin 
               120 ,  120 A,  120 B aeration apparatus 
               123  aeration diffuser