Abstract:
Provided are a distributor capable of feeding a liquid in a state of evenly flowing out from an entire opening while avoiding the opening from being blocked by sludge and keeping a pressure loss small, a settling tank that employs the distributor, and a method for operating the settling tank. A distributor  10  being in the form of a rectangular frame and having straight tubular sides  11  to  14  is disposed in a lower space within a settling tank  1 . A liquid outlet opening  16  extending in a lengthwise direction of the distributor  10  is formed in a bottom surface of the distributor  10 . An open angle θ of the opening  16  with respect to a tube axis is 60 to 180°, and preferably 90 to 150°.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a distributor for introducing water to be treated into a settling tank, such as a coagulative settling tank. The present invention further relates to a settling tank that employs the distributor, and a method for operating the settling tank. 
       BACKGROUND OF INVENTION 
       [0002]    Hitherto, precipitation separation using a solid-liquid separation tank (settling tank) as a means for separating a sludge-mixed liquid into treated water and sludge has been generally employed in active sludge treatment facilities, coagulative settling treatment facilities, and the like. In the precipitation separation, the so-called sludge blanket filtering technique of forming a sludge zone (sludge blanket layer) inside the settling tank is employed to efficiently remove pollutants and minute SS in the sludge-mixed liquid and to obtain satisfactory treated water (Patent Literatures 1 and 2). The sludge-mixed liquid is introduced from a distributor to flow into a region under the sludge zone and to pass through the sludge zone, whereby the pollutants and the minute SS in the sludge-mixed liquid are filtered and separated. 
         [0003]    In the settling tanks disclosed in Patent Literatures 1 and 2, tubular distributors are disposed to extend radially from a lower end of a feed well, and raw water (sludge-mixed liquid) flows out from the distributors through openings formed at the distributors. 
         [0004]    In Patent Literature 1, each opening is a circular opening, and many openings are formed in a lateral surface of the distributor at intervals. In Patent Literature 2, the opening is formed in a bottom surface of the tubular distributor in a slit-like shape extending in the lengthwise direction of a tube. When the opening is formed in the bottom surface of the distributor as in Patent Literature 2, sludge is prevented from depositing in the distributor. 
       LIST OF LITERATURES 
       [0005]    Patent Literature 1: Japanese Patent Publication 10-202009 A 
         [0006]    Patent Literature 2: Japanese Patent Publication 2000-334214 A 
       SUMMARY OF INVENTION 
       [0007]    In order to cause a liquid to evenly flow out from the many circular raw-water outlet openings as in Patent Literature 1, a pressure loss needs to be generated in the opening. Accordingly, the power cost for feeding water is increased, and there is a risk that the opening may be blocked by sludge or foreign matters. 
         [0008]    In Patent Literature 2, because the width of the slit-like opening is large at the distal end side of the distributor, but it is small at the proximal end side, there is a risk that the opening may be blocked by sludge at the proximal end side of the distributor. 
         [0009]    An object of the present invention is to solve the above-mentioned problems in the related art, and is to provide a distributor capable of feeding a liquid in a state of evenly flowing out from an entire opening while avoiding the opening from being blocked by sludge and keeping a pressure loss small, a settling tank that employs the distributor, and a method for operating the settling tank. 
       Solution to Problem 
       [0010]    The distributor according to the present invention comprises a tubular water feed member, and a liquid outlet opening that is formed in a bottom surface of the water feed member, and that extends in a lengthwise direction of the water feed member. An open angle θ of the opening with respect to a tube axis of the water feed member is 60 to 180°. 
         [0011]    The open angle θ is preferably 90 to 150°. 
         [0012]    The settling tank according to the present invention comprises a tank body, and the distributor mounted in a lower space within the tank body, wherein the distributor is mounted with the opening directed downwards. 
         [0013]    In the settling tank described above, preferably, the distributor has a frame shape or a ring shape when it is looked at in a plan view. 
         [0014]    A method for operating a settling tank according to the present invention operates the above-described settling tank. A difference between a specific gravity d 1  of a liquid supplied to the distributor and a specific gravity d 2  of a liquid within the tank body is 0.0001 to 0.1, and d 2 &gt;d 1  is satisfied. 
       Advantageous Effects of Invention 
       [0015]    The distributor and the settling tank of the present invention are used to supply, into the distributor, raw water (liquid to be treated) having a specific gravity smaller than that of a tank liquid within the settling tank (in particular, that of a sludge blanket layer) by 0.0001 to 0.1. Since the raw water introduced to the distributor has a smaller specific gravity than the tank liquid, the raw water flows through the distributor in a lengthwise direction along its ceiling surface, and gradually flows out from the opening into the settling tank along the way. The opening of the distributor has a uniform width in the lengthwise direction of the distributor, and the raw water evenly flows out from the opening into the settling tank with no necessity of generating a large pressure loss when the raw water flows through the distributor. Furthermore, since the opening is formed in the bottom surface of the distributor, sludge is also allowed to flow out from the opening into the settling tank without being deposited in the distributor. 
         [0016]    When the central angle θ of the opening is smaller than 60°, there would be risk that the opening is blocked by sludge. When the central angle θ of the opening is larger than 180°, the raw water introduced to the distributor would all flow out from the distributor into the settling tank before reaching a distal end of the distributor. By setting the central angle θ to be 60 to 180° and, in particular, 90 to 150° C., the raw water can be fed in a state of evenly flowing out from the distributor into the settling tank without making the opening blocked by sludge. 
         [0017]    A liquid flow in the distributor is disturbed at least on the downstream side near a bent portion in which a water flowing direction in the water feed member is changed at 45° or more. For that reason, preferably, the opening is not formed in the bent portion and the vicinity thereof on the downstream side. 
         [0018]    The distributor and the settling tank of the present invention are suitably applied to a sludge-blanket type settling tank, particularly a granulation coagulative settling tank. A flow rate in the distributor is selected depending on the difference in specific gravity between the raw water and the tank liquid, the central angle θ of the opening, and respective lengths of the opening and the distributor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a vertical sectional view of a settling tank according to an embodiment. 
           [0020]      FIG. 2  is a sectional view taken along a line II-II in  FIG. 1 . 
           [0021]      FIG. 3  is a sectional view taken along a line in  FIG. 2 . 
           [0022]      FIG. 4  is a bottom view of a distributor. 
           [0023]      FIG. 5  is a bottom view of a distributor in a settling tank according to another embodiment. 
           [0024]      FIG. 6  is a bottom view of a distributor in a settling tank according to still another embodiment. 
           [0025]      FIG. 7  is a bottom view of a distributor in a settling tank according to still another embodiment. 
           [0026]      FIG. 8  is a bottom view of a distributor in a settling tank according to still another embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    One embodiment of the present invention will be described below with reference to  FIGS. 1 to 4 . 
         [0028]    A settling tank  1  includes a cylindrical tank body  2  with a direction of its axial center line set to the vertical direction, a distributor  10  mounted in a lower space within the tank body  2  (near the bottom of thereof), a shaft  3  arranged at an axial center position of the tank body  2 , a driving unit  4  including a motor to rotate the shaft  3 , and a rake  5  attached to a lower end of the shaft  3  (see  FIG. 1 , the rake  5  being not illustrated in  FIG. 2 ). 
         [0029]    In this embodiment, the distributor  10  is in the form of a square frame, when it is looked at in a plan view, which has a first side  11 , a second side  12 , a third side  13 , and a fourth side  14 . The sides  11  to  14  are each formed of a cylindrical tube having an inner space that serves as a flow path. A raw water introducing tube  15  is connected to a junction between the first side  11  and the fourth side  14 . The raw water introducing tube  15  extends in a direction in which a diagonal line of the square distributor  10  extends. 
         [0030]    In bottom surfaces of the sides  11  to  14  of the distributor  10 , outlet openings  16  are formed to extend in lengthwise directions of the sides  11  to  14 . In this embodiment, one opening  16  is formed in each of the sides  11  to  14 . The opening  16  extends from a position near one end to a position near the other end of each of the sides  11  to  14 . An opening width of the opening  16  is uniform in the lengthwise direction of each side. Therefore, each opening  16  appears as an elongate rectangular shape in  FIG. 4  that is a bottom view of the distributor  10 . Both the ends of the opening  16  in the lengthwise direction may be rounded. 
         [0031]    The sides  11  to  14  of the distributor  10  are each formed of a linear cylindrical tube. An open angle θ of the opening  16  in the bottom surface (i.e., an angle at which the opening  16  is opened in a widthwise direction thereof with respect to an axial center of the tube) is 60° to 180°, and preferably 90° to 150°. 
         [0032]    An overall length L 1  of each of the sides  11  to  14  of the distributor  10  is preferably about 50 to 90% and, in particular, 60 to 80% of a diameter A of the tank body  2 . A length L 2  of each opening  16  in the lengthwise direction thereof is given by L 1 −2L 3 . Near corners at which adjacent two of the sides  11  to  14  of the distributor  10  contact each other, the opening  16  is not present over a range of a distance L 3  from an end of each of the sides  11  to  14 . L 3  is preferably about 1 to 3 times and, in particular, about 1.2 to 2 times an inner diameter D of the tube. 
         [0033]    In the settling tank  1  including the distributor  10  constituted as described above, raw water is introduced to the distributor  10 , the raw water having a specific gravity smaller than that of a tank liquid within the settling tank  1  (or that of a sludge blanket when the sludge blanket layer is formed in the settling tank  1 ) by 0.0001 to 0.1 and, in particular, 0.0005 to 0.05. The introduced raw water flows through the distributor  10  along its ceiling surface, and gradually flows out from the opening  16  into the settling tank  1  along the way. Since the central angle θ of the opening  16  is 60 to 180° and preferably 90 to 150°, sludge is not deposited in the distributor  10 , and the opening  16  is prevented from being blocked by the sludge. 
         [0034]    In this embodiment, the distributor  10  is bent at 90° near the corners of the distributor  10  at which the sides  11  and  12 , the sides  12  and  13 , and the sides  13  and  14  intersect each other. Therefore, the flow in the distributor  10  is disturbed near each of the corners, particularly at the corner and in a region near the corner on the downstream side. The sides  11  and  14  are branched at an angle of 45° relative to the raw water introducing tube  15 . Hence the flow in the distributor  10  is also disturbed near a branched portion, particularly on the downstream side. In this embodiment, since the opening  16  is not formed over the range of the distance L 3  from each of the corners on both the upstream side and the downstream side, the raw water is avoided from flowing out in a large amount from the distributor  10  into the settling tank  1  near the corners. Since the flow is disturbed near the corners (bent portions), sludge is prevented from being deposited near the corners (bent portions). The bent portion means, for example, a portion where a flow path is bent at 45° or more on its way over a length that is about not more than 5 times an inner diameter D of the tube, or a portion where the flow path is branched steeply at an angle in such a range. When the bent portion is formed as the branched portion, the downstream side of the bent portion means the downstream side of the branched portion. 
       OTHER EMBODIMENTS 
       [0035]    While, in the above-described embodiment, the opening  16  is provided in continuous form in the lengthwise direction of each of the sides  11  to  14 , the opening  16  may be provided plural in intermittent form for each side. In that case, the width of each of the plural openings is also uniform in the lengthwise direction thereof. The openings are preferably provided at equal intervals in each side. 
         [0036]    In the following embodiments as well, the opening may be provided plural in intermittent form. 
         [0037]    While, in the above-described embodiment, the raw water introducing tube  15  is connected to the portion at which the sides  11  and  14  of the distributor  10  intersect each other, the raw water introducing tube  15  may be connected to an intermediate portion of one side  11  as in a distributor  10 A of  FIG. 5 . In that case, because a portion near a junction point between the raw water introducing tube  15  and the side  11  is regarded as the bent portion, the opening  16  is not provided in such a portion. 
         [0038]    In the present invention, as in a distributor  10 B of  FIG. 6 , a square ring-shaped tube  19  connected to the side  13  through a tube  18  may be added to the distributor  10 A of  FIG. 5  to provide a double ring shape when looked at in a plan view. Openings  16  are also formed in a lower surface of the square ring-shaped tube  19 . However, the openings  16  are not formed near bent portions at four corners of the square ring-shaped tube  19  and near a portion of the square ring-shaped tube  19  to which the tube  18  is connected. 
         [0039]    A distributor  20  of  FIG. 7  has a circular ring shape when looked at in a plan view. An opening  16  is formed over an entire bottom surface of the distributor  20  except for the vicinity of a portion where the raw water introducing tube  15  and the distributor  20  are connected to each other. 
         [0040]    A distributor  30  of  FIG. 8  includes three straight tubes  31 ,  32  and  33  radially extending in three directions. Of those tubes  31 ,  32  and  33 , one  31  is shorter than the other tubes  32  and  33 . The raw water introducing tube  15  is connected to an end of the tube  31 . In this embodiment, an angle of the tube  31  with respect to a water flowing direction in a portion branched from the tube  31  to the tubes  32  and  33  is 30° that is smaller than 45°. Thus, a portion where the tubes  31 ,  32  and  33  intersect one another is not regarded as the bent portion. Therefore, an opening  16  is formed in an entire bottom surface of each of the tubes  32  and  33 . 
         [0041]    It is to be noted that, in the distributor  30  of  FIG. 8 , distal ends of the straight tubes  32  and  33  are closed. 
         [0042]    Among the distributors described above, those ones illustrated in  FIGS. 2 ,  5  and  8  are relatively easy to manufacture and hence are preferable. The distributors  10  and  10 A of  FIGS. 2 and 5  are preferable in such a point that the raw water is more apt to evenly flow out from the openings  16 . 
       EXAMPLES 
     Experiment 1 
     Raw Water Outflow Experiment at Different Open Angles θ of Distributor 
       [0043]    The distributor illustrated in  FIG. 5  was mounted in a settling tank with a diameter of 150 cm and a water depth of 150 cm, which was installed outdoor and which was not provided with a stirrer, at a height of 10 cm from the bottom of the settling tank. A tube constituting the distributor had an outer diameter of 114 mm and an inner diameter of 107 mm. A length of each of the sides  11  to  14  was 110 cm, a length of the opening  16  in each of the sides  12  to  14  was 70 cm, and a length of each opening  16  in the side  11  was 20 cm. The open angle θ of the opening  16  was set as listed in Table 1. The tube was made of vinyl chloride. An inner space of the settling tank under the distributor was partitioned by a baffle plate into a region at the proximal end side (i.e., the side closer to the raw water introducing tube  15 ) and a region at the distal end side, thus enabling an operator to confirm that the raw water was evenly supplied to the front side of the baffle plate and the rear side thereof. 
         [0044]    Raw water was prepared by adding 300 mg/L of aluminum sulfate and 1 mg/L of a cation polymer (Kurifarm PC728 made by Kurita Water Industries Ltd.) to kaolin-dispersed water, which was prepared by adding 500 mg/L of kaolin to tap water, thereby forming flock, and then adding 3 mg/L of an anion polymer (Kurifarm PA465 made by Kurita Water Industries Ltd.). The prepared raw water was supplied to the distributor at a flow rate of 30 m 3 /h. The specific gravity of a liquid inside tank was set to be larger than that of the supplied raw water by about 0.0052. 
         [0045]    The occurrence of blockage of the openings  16  and conditions of outflow from the openings  16  were observed while the raw water was supplied continuously for 48 h. Furthermore, measurement was performed on an outflow rate of the raw water from the openings  16  at the proximal end side of the distributor (i.e., at the side closer to the raw water introducing tube  15 , namely at the left-half side with respect to a center of the tank body  2  in  FIG. 5 ) and an outflow rate of the raw water from the openings  16  at the distal end side of the distributor (i.e., at the right-half side in  FIG. 5 ). The obtained results are listed in Table 1. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                   
                   
                   
                 Outflow Rate of 
               
               
                   
                   
                 Occurrence of 
                 Outflow Conditions of 
                 Raw Water (m 3 /hr) 
               
             
          
           
               
                   
                 Open 
                 Blockage of 
                 Raw Water from 
                 Proximal 
                 Distal 
               
               
                   
                 Angle θ 
                 Openings 
                 Openings 
                 End Side 
                 End Side 
               
               
                   
               
             
          
           
               
                 COMPARATIVE 
                 45° 
                 Blocked after 
                 Even outflow into 
                 15 
                 15 
               
               
                 EXAMPLE 1 
                   
                 24 h 
                 entire tank 
                   
                   
               
               
                 EXAMPLE 1 
                 60° 
                 Not blocked 
                 Even outflow into 
                 15 
                 15 
               
               
                   
                   
                   
                 entire tank 
                   
                   
               
               
                 EXAMPLE 2 
                 90° 
                 Not blocked 
                 Even outflow into 
                 15 
                 15 
               
               
                   
                   
                   
                 entire tank 
                   
                   
               
               
                 EXAMPLE 3 
                 120° 
                 Not blocked 
                 Even outflow into 
                 15 
                 15 
               
               
                   
                   
                   
                 entire tank 
                   
                   
               
               
                 EXAMPLE 4 
                 150° 
                 Not blocked 
                 Even outflow into 
                 16.1 
                 13.9 
               
               
                   
                   
                   
                 entire tank 
                   
                   
               
               
                 EXAMPLE 5 
                 180° 
                 Not blocked 
                 Almost even outflow into 
                 18 
                 12 
               
               
                   
                   
                   
                 entire tank 
                   
                   
               
               
                 COMPARATIVE 
                 210° 
                 Not blocked 
                 Outflow rate is larger at 
                 22.5 
                 7.5 
               
               
                 EXAMPLE 2 
                   
                   
                 proximal end side 
                   
                   
               
               
                   
               
             
          
         
       
     
         [0046]    As seen from Table 1, in EXAMPLES 1 to 5 in which the open angle θ is 60 to 180°, the raw water evenly flows out from the distributor between the distal end side and the proximal end side, and the openings are not blocked. On the other hand, in COMPARATIVE EXAMPLE 1 in which the open angle θ is 45°, the openings are clogged with and blocked by sludge attributable to kaolin, leaves of trees, and dead branches in a short time. In COMPARATIVE EXAMPLE 2 in which the open angle θ is 210°, the raw water does not evenly flow out from the distributor. 
       Experiment 2 
     Raw Water Outflow Experiment Using Distributor of FIG.  8   
       [0047]    An experiment was performed under the same conditions as those in EXAMPLE 3 except for using the distributor illustrated in  FIG. 8  (the open angle θ being 120° and equal to that in EXAMPLE 3). The experiment result showed that the outflow rate from the proximal end side of the distributor was 4.5 m 3 /hr while the outflow rate from the distal end side thereof was 25.5 m 3 /hr, and that the raw water flowed out at a much larger flow rate from the distal end side. 
       Experiment 3 
     Experiment on Outflow from Distributor with Openings  16  Formed all Continuously 
       [0048]    In EXAMPLE 3, the openings  16  in the four sides of the distributor were all formed continuously. The raw water was supplied under the same conditions as those in EXAMPLE 3 except for using that distributor. The experiment result showed that the outflow rate from the proximal end side was 25 m 3 /hr while the outflow rate from the distal end side was 5 m 3 /hr, and that the raw water flowed out at a much larger flow rate from the proximal end side. 
         [0049]    From the experiments described above, it was confirmed that the distributor according to the present invention can feed a liquid in a state of evenly flowing out from the entirety of one or more openings without causing blockage of the openings. 
         [0050]    While the present invention has been described in detail in connection with specific embodiments, it is apparent to those skilled in the art that the present invention can be variously modified without departing from the intent and the scope of the present invention. 
         [0051]    This application is on the basis of Japanese Patent Application No. 2012-254492 filed Nov. 20, 2012, which is incorporated by reference herein in its entirety.