Patent Publication Number: US-2022212127-A1

Title: Mixing/clarifying device

Description:
TECHNICAL FIELD 
     The present invention relates to a mixing/clarifying device. 
     BACKGROUND 
     In various water treatment facilities, membrane filtration devices have been adopted in which the water to be treated is filtered by a membrane to obtain treated water. As a membrane filtration device is used, a filtration membrane is clogged with solids or the like contained in the water to be treated. Therefore, it is necessary to periodically clean the filtration membrane in order to clear the clogging and regenerate the filtration function of the filtration membrane. 
     In recent years, the frequency of torrential rains has increased in many areas. For example, when the water to be treated is collected from lakes, rivers, etc., the turbidity concentration in the water to be treated increases sharply in response to the occurrence of torrential rain. If the water to be treated with a high turbidity concentration is supplied to the membrane filtration treatment, the solid load on the filtration membrane will increase and the washing cycle of the filtration membrane will be shortened. 
     A solid-liquid separation system in which a solid-liquid separation device is placed upstream of a filter device has been considered (see, for example, Patent Literature 1 (PTL 1)). According to the solid-liquid separation system disclosed in PTL 1, drinking water can be obtained by separating solids from suspended water with a solid-liquid separation device to obtain clean water, and treating the clean water with a filter device. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP2017047368(A) 
     SUMMARY 
     Technical Problem 
     However, in the above-mentioned known solid-liquid separation system, there is room for further improvement in the solid-liquid separation ability by the solid-liquid separation device. It is therefore an object of the present invention to provide a device with an excellent solid-liquid separation ability. 
     Solution to Problem 
     An object of the present invention is to solve the above-mentioned problem advantageously. The mixing/clarifying device according to the present invention includes a coagulant feeder that feeds coagulant to the water to be treated to obtain coagulant-containing water and a tank in which the coagulant-containing water is mixed to form flocs and solid-liquid separation is performed. The tank has an outer cylinder with an inflow port through which the coagulant-containing water is flowed into the tank and an inner cylinder arranged inserted from the upper side of the tank to the lower side of the inflow port of the outer cylinder and having a lower end open in the tank. According to the mixing/clarifying device of the present invention in which flocs are formed by mixing coagulant-containing water obtained by feeding coagulant to the water to be treated and separating the flocs from the liquid to clarify, a high solid-liquid separation ability can be exhibited. 
     Here, in the mixing/clarifying device according to the present invention, it is preferable that the tank includes a rapid stirrer that is located between an upper end of the outer cylinder and a lower end of the inner cylinder in a space between the outer cylinder and the inner cylinder and rapidly stirs the coagulant-containing water. If the mixing/clarifying device has a rapid stirrer located between the upper end of the outer cylinder and the lower end of the inner cylinder, floc forming efficiency in the tank can be increased effectively and as a result, a high solid-liquid separation ability can be exhibited. 
     Further, in the mixing/clarifying device according to the present invention, it is preferable that an inner wall of the outer cylinder is tapered toward the upper side of the tank, and the rapid stirrer is formed of a flow path defined by the inner wall of the outer cylinder and an outer wall of the inner cylinder. Alternatively, in the mixing/clarifying device according to the present invention, it is preferable that the outer wall of the inner cylinder is tapered toward the lower side of the tank, and the rapid stirrer is formed of a flow path defined by the inner wall of the outer cylinder and the outer wall of the inner cylinder. This is because, if the rapid stirrer is mounted as a flow path defined by the inner wall of the outer cylinder and the outer wall of the inner cylinder, at least one of them is formed into a tapered shape, the coagulant-containing water can be stirred with different stirring intensities in the tank, and thus a floc forming efficiency in the tank can be increased even more effectively. Then, as a result of increased floc formation efficiency, even higher solid-liquid separation ability can be exhibited by the mixing/clarifying device according to the present invention. 
     Further, in the mixing/clarifying device according to the present invention, it is preferable that the inner wall of the outer cylinder has a narrow flow path formed along a peripheral surface over at least one round of the peripheral surface of the inner wall, and that the narrow flow path forms the rapid stirrer. If the inner wall of the outer cylinder has a narrow flow path over at least one round of the peripheral surface, the coagulant-containing water can be stirred at different stirring intensities in the tank, and thus a floc formation efficiency in the tank can be increased even more effectively. Then, as a result of increased floc formation efficiency, even higher solid-liquid separation ability can be exhibited by the mixing/clarifying device of the present invention. 
     Advantageous Effect 
     According to the present invention, a device with an excellent solid-liquid separation ability can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is an image diagram of a mixing/clarifying device according to the present invention installed in a water treatment facility; 
         FIG. 2  is a diagram illustrating a schematic configuration of an example of the mixing/clarifying device according to the present invention; 
         FIG. 3  is a diagram schematically illustrating an aspect according to a first example of a rapid stirrer that can be provided to the mixing/clarifying device of the present invention; 
         FIG. 4  is a diagram schematically illustrating an aspect according to a second example of the rapid stirrer that can be provided to the mixing/clarifying device of the present invention; 
         FIG. 5  is a diagram schematically illustrating an aspect according to a third example of the rapid stirrer that can be provided to the mixing/clarifying device of the present invention; 
         FIG. 6  is a cross-sectional view taken along I-I in  FIG. 5 ; 
         FIG. 7  is a schematic configuration diagram of an example of an inner cylinder provided with helical ribs at its lower part; and 
         FIG. 8  is a graph showing the turbidity of the water flowed out of the mixing/clarifying device with respect to the turbidity of raw water when the water to be treated (raw water) is treated by using the water treatment facility illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the present invention will be described in detail below with reference to the drawings. In each drawing, the same reference sign is used to indicate the same component. 
     The mixing/clarifying device according to the present invention can be used when water to be treated containing solids is treated, without being particularly limited. 
     Here, examples of solid include sediment, sludge, organic substances, and the like, without being particularly limited. 
     Further, examples of the water to be treated include water collected from lakes, rivers, and the like, industrial wastewater generated in various plants, and wastewater generated in various treatment plants such as sewage treatment plants, urine treatment plants, waste-disposal facilities, and the like, without being particularly limited. 
       FIG. 1  is an image diagram of a mixing/clarifying device according to the present invention when it is installed in a water treatment facility. The water treatment facility  200  illustrated in  FIG. 1  includes a primary water tank  201 , a mixing/clarifying device  100 , a membrane filtration device  202  and a secondary water tank  203 . The primary water tank  201  stores water to be treated and supplies it to the mixing/clarifying device  100 . Then, the water passed through and flowed out of the mixing/clarifying device  100  is supplied to the membrane filtration device  202  located downstream and filtered by the membrane filtration device  202 . Then, the treated water passed through the membrane filtration device  202  is stored in the secondary water tank  203 . The difference in height between the water level of the secondary water tank  203  and the water level of the primary water tank  201  (i.e., the difference in water level) causes flow of the water to be treated from the primary water tank  201  toward the secondary water tank  203 . It is to be noted that the flow of water to be treated may be artificially generated by using a pump or other power sources, for example, without being limited to the aspect illustrated in  FIG. 1 . 
     In the water treatment facility  200 , the mixing/clarifying device  100  is placed upstream of the membrane filtration device  202 , and at least a part of the suspended solids in the water to be treated is removed at the stage before the water to be treated is flowed into the membrane filtration device  202 . Thus, even if torrential rain occurs and the water to be treated with a temporarily high turbidity concentration flows into the primary water tank  201 , the water to be treated can be supplied to the membrane filtration device  202  after turbidity is removed in the mixing/clarifying device  100 , which can prevent excessively high solid load from being imposed on the membrane filtration device  202 . Therefore, in the water treatment facility  200  including the mixing/clarifying device  100 , it is possible to prevent the cleaning cycle of the membrane filtration device  202  from being shortened depending on the change in the turbidity concentration of the water to be treated. As a result of this, even if the water to be treated with a high turbidity concentration is flowed, the need to clean the membrane filtration device  202  at a high frequency is reduced, and the water treatment efficiency of the water treatment facility  200  can be increased. 
     An example of the mixing/clarifying device  100  according to the present invention will be described in detail with reference to  FIG. 2 . The mixing/clarifying device  100  according to the present invention includes a coagulant feeder  10  and a tank  20 . The tank  20  further includes an outer cylinder  21  having an inflow port  210  through which the coagulant-containing water is flowed into the tank  20  and an inner cylinder  22  arranged inserted from the upper side of the tank  20  to the lower side of the inflow port  210  of the outer cylinder  21  and having a lower end open in the tank. The mixing/clarifying device  100  forms a floc by mixing the coagulant-containing water obtained by feeding coagulant to the water to be treated, and clarifies the water to be treated by separating the floc from the liquid through the solid-liquid separation. Thus, according to the mixing/clarifying device  100 , a high solid-liquid separation ability can be exhibited. 
     The coagulant feeder  10  has a function of feeding coagulant to the water to be treated. The coagulant feeder  10  can be mounted by known members such as a storage tank, an injection pipe and a pump, without being particularly limited. More specifically, the coagulant feeder  10  can be mounted by a storage tank for storing the coagulant, an injection pipe connected to the storage tank and a pump that is attached to the injection pipe and can switch between injection and stop injection of the coagulant. 
       FIG. 2  illustrates an aspect in which the coagulant feeder  10  is attached to the water to be treated inflow line  30  connected to the inflow port  210 . As illustrated, the position at which the coagulant feeder  10  feeds coagulant to the water to be treated is preferably the upstream side of the inflow port  210 . It is to be noted that the “upstream side” is a side closer to the supply source of the water to be treated (e.g., the primary water tank  201  illustrated in  FIG. 1 ) based on the flow direction of the water to be treated. If the coagulant feeder  10  is placed such that the coagulant is fed to the water to be treated at the position on the upstream side of the inflow port  210 , the time of mixing the water to be treated and the coagulant can be lengthened, and the solid-liquid separation ability of the mixing/clarifying device  100  can be further enhanced. 
     It is to be noted that, without being limited to the aspect illustrated in the figure, in the other example of the mixing/clarifying device according to the present invention, the coagulant feeder may be attached such that the coagulant is fed to the water to be treated in the tank. In such an aspect, it is preferable that the position at which the coagulant feeder feeds the coagulant to the water to be treated is close to the inflow port in the tank. More specifically, it is preferable that the coagulant feeder is placed at a position at which the coagulant can be fed to the water to be treated immediately after flowing into the tank. With such a disposition, the time of mixing the water to be treated with the coagulant can be increased, and the solid-liquid separation ability of the mixing/clarifying device can be further enhanced. 
     Examples of the coagulant include an aluminum-based coagulant such as aluminum sulfate and polyaluminum chloride; and an iron-based coagulant such as ferric chloride, ferric sulfate, and polysilicate iron, without being particularly limited. The injection amount and the like of the coagulant can be controlled as desired according to the turbidity or the like of the water to be treated. 
     The tank  20  has a function of mixing the coagulant-containing water flowed into the tank  20  through the inflow port  210  to form a floc and then performing solid-liquid separation. The coagulant-containing water flowed through the inflow port  210  into the tank  20  can flow down in the tank  20  through the flow path defined by the inner periphery of the outer cylinder  21  and the outer periphery of the inner cylinder  22  while a part thereof forming a flow as schematically illustrated by the arrow F and circling around the inner cylinder  22 . The tank  20  can be configured by a water tank configured to allow the coagulant-containing water flowed into the tank through the inflow port  210  to flow, as treated water, out of the tank by the inner cylinder  22 . The tank  20  can be mounted by, for example, a pressure-resistant water tank with a cylindrical body portion (corresponding to the outer cylinder  21 ) having a round-shaped cross section, without being particularly limited. In this case, the inner cylinder  22  may also have a cylindrical body portion having a round-shaped cross section, and the outer cylinder  21  and the inner cylinder  22  may share the same axis, or each axis of them may not be aligned. The tank  20  may include a solid matter extraction mechanism at the bottom thereof. As illustrated, the outer cylinder  21  of the tank  20  is closed by being connected to the outer wall of the inner cylinder  22  at the top. In other words, the space between the outer cylinder  21  and the inner cylinder  22  is closed at the top of the tank  20 . 
     In the tank  20 , the space between the outer cylinder  21  and the inner cylinder  22  acts as a floc formation region for stirring the coagulant-containing water to form a floc. In the floc formation region, stirring action may not only form a floc but also increase the size of the floc. On the other hand, the flow rate of the coagulant-containing water decreases near the lower end of the inner cylinder  22  and the region lower than the lower end of the inner cylinder  22 . Then, finally, the floc formed in the floc formation region settles down at a settling velocity V 2 . At the near lower end of the inner cylinder  22 , although the liquid is attracted by the upward flow flowing at a flow velocity V 1  in the inner cylinder  22  and tries to flow out of the tank  20 , most of the floc that is heavier than the liquid and settles down at a settling velocity V 2  faster than the flow velocity V 1  goes down to the bottom of the tank  20  against the flow velocity V 1 . Then, the liquid flows out of the tank  20  and flocculated solids accumulate at the bottom of the tank  20 , and as a result the coagulant-containing water is clarified. In this manner, the region lower than the lower end of the inner cylinder  22  acts as a solid-liquid separation region. The liquid flowed out of the tank  20  may accompany solids that cannot be separated from the liquid, but its amount is much less than that of the solids of the coagulant-containing water flowed into the tank  20 . 
     It is effective to increase the time for stirring the coagulant-containing water to enhance the solid-liquid separation ability. As described above, since the space between the outer cylinder  21  and the inner cylinder  22  acts as a floc formation region, the longer the inner cylinder  22 , the larger the size of the floc formation region can be. On the other hand, the solid-liquid separation region needs to have a length enough for settling down the floc formed in the floc formation region. The length of the inner cylinder  22  can be determined such that the floc forming ability and the solid-liquid separation ability is balanced. 
     As illustrated in  FIG. 2 , it is preferable that the lower portion of the tank  20  is tapered downward. More specifically, as illustrated in  FIG. 2 , the outer cylinder  21  forming the tank  20  may include a straight body portion and further a tapered portion being continuous from the straight body portion. According to the tank  20  having the above-described shape, floc settlement can be promoted, and as a result, the solid-liquid separation ability of the mixing/clarifying device  100  can be further enhanced. 
     Furthermore, the tank  20  may include a solid discharge mechanism  40  underneath. In the example illustrated, the solid discharge mechanism  40  is composed of a solid discharge pipe  41  and a solid discharge valve  42 . The solid discharge valve  42  may be open when the solids accumulated on the bottom of the tank  20  are discharged, and is closed at other times. When to open the solid discharge valve  42  can be set as desired. 
     From the viewpoint of further enhancing the solid-liquid separation ability of the mixing/clarifying device  100 , it is preferable that the tank  20  includes a rapid stirrer for rapidly stirring the coagulant-containing water in the space between the outer cylinder  21  and the inner cylinder  22  and in the region between the upper end of the outer cylinder  21  and the lower end of the inner cylinder  22 . Such a rapid stirrer may be mounted to any structural parts, without being particularly limited, as long as the velocity (first velocity) of the flow of the coagulant-containing water immediately after it is flowed into the tank  20  is faster than a predetermined velocity. The “predetermined velocity” is preferably a velocity that is faster than the velocity at which the coagulant-containing water flows into the tank  20  through the inflow port  210 , and the “predetermined velocity” is more preferably a velocity that is faster than the flow rate sufficient for circling around the space between the outer cylinder  21  and the inner cylinder  22  at least once. Various mounting aspects of the rapid stirrer will be described below with reference to  FIGS. 3 to 6 . It is to be noted that, in  FIGS. 3 to 5 , although the coagulant feeder is not illustrated, each mixing/clarifying device illustrated in the figures includes a coagulant feeder. 
       FIG. 3  is a diagram schematically illustrating an aspect according to a first example of a rapid stirrer that can be provided to the mixing/clarifying device of the present invention. In the mixing/clarifying device  101  illustrated in  FIG. 3 , the inner wall of the outer cylinder  21 A is tapered toward the upper side of the tank  20 A, and the rapid stirrer is formed by the flow path defined by the inner wall of the outer cylinder  21 A and the outer wall of the inner cylinder  22 . The coagulant-containing water flowed into the tank  20 A through the water to be treated inflow line  30  gradually flows down toward the lower portion of the tank  20 A while circling around the flow path. At this time, as the cross-sectional area of the flow path increases toward the lower portion, the flow rate of the coagulant-containing water flowing through the flow path also decreases. Therefore, compared with the flow rate immediately after flowing into the tank  20 A, the flow rate gradually slows down. As a result, the coagulant-containing water, which is rapidly stirred by flowing through the flow path with a rapid flow rate immediately after its inflow, is slowly stirred as it flows down the flow path. Such a change in the stirring intensity effectively contributes to an increase in the size of the floc. Thus, the mixing/clarifying device  101  according to the present invention has even better solid-liquid separation ability. 
       FIG. 4  is a diagram schematically illustrating an aspect according to a second example of the rapid stirrer that can be provided to the mixing/clarifying device of the present invention. In the mixing/clarifying device  102  illustrated in  FIG. 4 , the outer wall of the inner cylinder  22 B is tapered toward the lower side of the tank  20 B, and the rapid stirrer is formed by the flow path defined by the inner wall of the outer cylinder  21  and the outer wall of the inner cylinder  22 B. As with the aspect illustrated in  FIG. 3 , the coagulant-containing water flowed into the tank  20 B through the water to be treated inflow line  30  gradually flows down toward the lower portion of the tank  20 B while circling around the flow path. According to the same principle as that illustrated in  FIG. 3 , the rapid stirring action on the coagulant-containing water that occurred immediately after inflow gradually disappears as the water flows down the flow path, and is switched to the slow stirring action. Such a change in stirring intensity can effectively contribute to an increase in floc size. Thus, the mixing/clarifying device  102  according to this example also has an even better solid-liquid separation ability. 
       FIG. 5  is a diagram schematically illustrating an aspect according to a third example of the rapid stirrer that can be provided to the mixing/clarifying device of the present invention. In the mixing/clarifying device  103  illustrated in  FIG. 5 , the inner wall of the outer cylinder  21 C has a narrow flow path  23  formed along the peripheral surface over at least one round of the peripheral surface of the inner wall. The narrow flow path  23  rapidly flows the coagulant-containing water flowed in through the inflow port  210  and acts as a rapid stirrer. The narrow flow path  23  can be defined by a narrow flow path wall  23   w , a narrow flow path support portion  23   s  and an inner wall surface of the outer cylinder  21 C. The narrow flow path wall  23   w  may be parallel with the inner wall surface of the outer cylinder  21 C or may be inclined with respect to the inner wall surface of the outer cylinder  21 C. When the narrow flow path wall  23   w  is inclined with respect to the inner wall surface of the outer cylinder  21 C, it is preferable that the narrow flow path wall  23   w  is inclined such that the distance between the inner wall surface of the outer cylinder  21 C and the narrow flow path wall  23   w  on the upper side of the narrow flow path wall  23   w  is larger than the distance between the inner wall surface of the outer cylinder  21 C and the narrow flow path wall  23   w  on the lower side of the narrow flow path wall  23   w  (e.g., the angle between them is more than 0° and 30° or less). In the example illustrated in  FIG. 5 , when the distance between the inner wall surface of the outer cylinder  21 C and the narrow flow path wall  23   w  near the inflow port  210  is defined as L 1  and the distance between the inner wall surface of the outer cylinder  21  and the narrow flow path wall  23   w  at the upper end of the narrow flow path wall  23   w  is defined as L 2 , L 1 &lt;L 2 . Furthermore, in the example illustrated in  FIG. 5 , when the distance between the wall surface on the inner cylinder  22 C side of the narrow flow path wall  23   w  and the outer wall surface of the inner cylinder  22 C at the upper end of the narrow flow path wall  23   w  is defined as L 3 , L 2  equals L 3  or L 3  is larger than L 2 . That is, in the example illustrated in  FIG. 5 , the relationships of L 1 &lt;L 2  and L 2 ≤L 3  are established. 
     The narrow flow path support portion  23   s  is attached by adhering to the inner peripheral surface over the entire circumference of the inner wall surface of the outer cylinder  21 C. This can avoid short circuit of flow of the coagulant-containing water at the rapid stirrer. Therefore, the time for stirring the coagulant-containing water in the tank  20 C can be increased, and as a result, flocs are formed, and further, the growth of flocs can be further promoted. 
     In order to describe the structure of the narrow flow path  23 , a cross-sectional view taken from the line I-I in  FIG. 5  is illustrated in  FIG. 6 . The I-I cross section is a plane that passes through the center of the inflow port  210  and is perpendicular to the vertical direction of the tank  20 C. As obvious from  FIG. 6 , the narrow flow path  23  is formed over the entire circumference of the inner wall of the outer cylinder  21 C without interruption. Here, when comparing, on the cross-section illustrated in  FIG. 6 , the distance D a  between the narrow flow path wall  23   w  and the inner wall surface of the outer cylinder  21 C and the distance D b  between the narrow flow path wall  23   w  and the outer wall surface of the inner cylinder  22 C, D a &lt;D b . If the relationship of D a &lt;D b  is satisfied, after the coagulant-containing water flowed into the tank  20 C through the inflow port  210  is flowed into the rapid stirring action in the narrow flow path  23 , the coagulant-containing water can be slowly stirred in a region extending between the outer wall of the inner cylinder  22 C and the narrow flow path wall  23   w  (hereinafter referred to also as “slow stirring portion”), and as a result floc formation can be efficiently promoted. 
     The coagulant-containing water flows out of the open end on the top side of the narrow flow path  23  after circling around the narrow flow path  23  and reaches the slow stirring portion. Further, the coagulant-containing water flows down while circling around the inner cylinder  22 C at the slow stirring portion. 
     Note that, in  FIG. 6 , D a  and D b  have constant values, respectively, but are not limited to the aspect illustrated in  FIG. 6 . For example, in the mixing/clarifying device according to a variation, the narrow flow path  23  may be designed such that the value of D a  near the inflow port  210  will be the smallest and that of near the opposite side of the inflow port  210  will be the largest. In this case, the stirring intensity inside the narrow flow path  23  can be changed in a plane perpendicular to the vertical direction of the tank  20 C, and thus floc formation can be further promoted. 
     Furthermore, as illustrated in  FIG. 5 , the inner cylinder  22 C has a tapered portion  22 Ct on the lower side, more preferably, on the lower side of the narrow flow path supporting portion  23   s . With the tapered portion  22 Ct, the tapered portion  22 Ct exhibits an action to promote settling of flocs, and thus settling efficiency of flocs contained in the coagulant-containing water that flows down through the slow stirring portion can be further increased. 
     Moreover, as illustrated in  FIG. 5 , the inner cylinder  22 C has a large-bore end portion  22 Ca provided continuously over the tapered portion  22 Ct. The large-bore end portion  22 Ca makes it difficult for small-sized flocs, and the like, to be sucked into the inner cylinder  22 C. The bore size of the large-bore end portion  22 Ca is not particularly limited, and may be selected, for example, from those small enough to form upward flow in the inner pipe  22 C and large enough to create flow lower than the settling velocity V 2  as desired. 
     As described with reference to  FIGS. 3 to 6 , in the mixing/clarifying device according to the present invention, when a rapid stirrer that can be mounted by various aspects is adopted, floc formation and growth can be promoted in the coagulant-containing water, and the solid-liquid separation ability of the device can be further enhanced. Other configuration examples of enhancing the solid-liquid separation ability of the device include helical ribs as illustrated with reference to  FIG. 7 . 
       FIG. 7  illustrates a schematic configuration of an inner cylinder  22 D provided with helical ribs at its lower portion. The inner cylinder  22 D having a configuration as illustrated can be adopted by a mixing/clarifying device of any aspect illustrated with reference to  FIGS. 2 to 6 , without being particularly limited. As illustrated in  FIG. 7 , the inner cylinder  22 D is provided with a helical first rib  24   a  and second rib  24   b  at its lower portion. It should be noted that, without being limited to the aspect illustrated, the inner cylinder may have three or more ribs or one rib. The first rib  24   a  and the second rib  24   b  of such shape act each to promote floc settling. Therefore, the solid-liquid separation ability of the mixing/clarifying device including the first rib  24   a  and the second rib  24   b  can be further enhanced. Further, in  FIG. 7 , although the helical first rib  24   a  and second rib  24   b  are placed with respect to the tapered outer wall of the inner cylinder  22 D, the shape of the inner cylinder  22 D is not limited to the tapered shape. More specifically, at least one helical rib may be provided to the straight body type inner cylinder  22 D. 
     Although not illustrated, the helical rib may be provided to the inner wall of the outer cylinder forming the tank, not to the inner cylinder. Also in this case, the helical rib can exhibit floc settling promotion effect as with the case where the helical rib is provided to the inner cylinder. Further, the position of the helical rib in the vertical direction of the mixing/clarifying device is not particularly limited, and, for example, it may be provided below the inflow port through which the coagulant-containing water is flowed into the tank. 
     Although some examples of the mixing/clarifying device according to the present invention have been described, the mixing/clarifying device according to the present invention is not limited to the above described content. 
     Although the present invention will be further described in detail below using an example, the present invention is not limited to the aspect adopted by the example. 
     Example 
     The water to be treated was treated, under the following conditions, by using a test machine having the same structure as that of the water treatment facility  200  illustrated in  FIG. 1 . Note that the structure of the mixing/clarifying device adopted by this example was according to the structure illustrated in  FIG. 2 . More specifically, the mixing/clarifying device has the following structure. 
     &lt;Structure of the Mixing/Clarifying Device&gt; 
     Inflow line of water to be treated: 15 mm in diameter 
     Inflow port diameter: 15 mm 
     Inner cylinder diameter: 65 mm 
     Outer cylinder diameter: 100 mm 
     Inner cylinder length: 400 mm 
     Distance from the lower end of the inner cylinder to the lower end of the straight body portion of the outer cylinder: 100 mm 
     &lt;Treatment Conditions&gt; 
     Coagulant: Polyaluminum chloride (injection rate: 80 mg/L) 
     Inflow velocity of coagulant-containing water to the mixing/clarifying device: 1.34 L/min. 
     &lt;Turbidity Measurement&gt; 
     The water to be treated (before the coagulant was added) and the treated water immediately after flowing out of the mixing/clarifying device were sampled, diluted, and measured for turbidity with a turbidity meter (“WA6000” by NIPPON DENSHOKU INDUSTRIES Co., Ltd.). The results obtained according to the above are illustrated in  FIG. 8 . 
     From  FIG. 8 , according to the mixing/clarifying device of the present invention, it can be seen that the turbidity of the water flowing out of the mixing/clarifying device could be maintained substantially constant even if the turbidity of the raw water fluctuated. Therefore, it can be seen that a high solid-liquid separation ability could be exerted regardless of the raw water turbidity. Therefore, in such a water treatment facility, even if the turbidity concentration in the raw water changes irregularly due to occurrence of torrential rain or the like, the solid content load of the membrane filtration device does not temporarily increase excessively. Thus, according to the mixing/clarifying device, it can be seen that water treatment efficiency of the water treatment facility can be increased. 
     INDUSTRIAL APPLICABILITY 
     According to the mixing/clarifying device of the present invention, a high solid-liquid separation ability can be exhibited. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  Coagulant feeder 
               20 ,  20 A, 20 B, 20 C Tank 
               21 , 21 A, 21 C Outer cylinder 
               22 , 22 B, 22 C, 22 D Inner cylinder 
               22 Ca Large-bore end portion 
               22 Ct Tapered portion 
               23  Narrow flow path 
               23   s  Narrow flow path support portion 
               23   w  Narrow flow path wall portion 
               24   a  First rib 
               24   b  Second rib 
               30  Water to be treated inflow line 
               40  Solid discharge mechanism 
               41  Solid discharge pipe 
               42  Solid discharge valve 
               100 , 101 , 102 , 103  Mixing/clarifying device 
               200  Water treatment facility 
               201  Primary water tank 
               202  Membrane filtration device 
               203  Secondary water tank 
               210  Inflow port