Patent Description:
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 <NUM> (PTL <NUM>)). According to the solid-liquid separation system disclosed in PTL <NUM>, 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. Further examples of solid-liquid separation systems are disclosed in PTL <NUM> to <NUM>.

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.

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. The tank also has a rapid stirrer that includes a flow path defined by the inner periphery of the outer cylinder and the outer periphery of the inner cylinder, 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 is configured to rapidly stir the coagulant-containing water by circulating the coagulant-containing water around the inner cylinder, and has a cross-sectional area that increases toward a lower portion, wherein the tank is configured to allow the coagulant-containing water flowed into the tank through the flow path to flow, as treated water, out of the tank by the inner cylinder, and the rapid stirrer further includes a narrow flow path defined by a narrow flow path wall, a narrow flow path support portion and an inner wall surface of the outer cylinder, wherein the narrow flow path support portion connects the inner wall surface of the outer cylinder to a lower side of the narrow flow path wall over the entire circumference of the inner wall surface of the outer cylinder, and wherein the narrow flow path wall is inclined such that a distance between the inner wall surface of the outer cylinder and the narrow flow path wall on an upper side of the narrow flow path wall is larger than a distance between the inner wall surface of the outer cylinder and the narrow flow path wall on a lower side of the narrow flow path wall. 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.

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.

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.

According to the present invention, a device with an excellent solid-liquid separation ability can be provided.

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> 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 <NUM> illustrated in <FIG> includes a primary water tank <NUM>, a mixing/clarifying device <NUM>, a membrane filtration device <NUM> and a secondary water tank <NUM>. The primary water tank <NUM> stores water to be treated and supplies it to the mixing/clarifying device <NUM>. Then, the water passed through and flowed out of the mixing/clarifying device <NUM> is supplied to the membrane filtration device <NUM> located downstream and filtered by the membrane filtration device <NUM>. Then, the treated water passed through the membrane filtration device <NUM> is stored in the secondary water tank <NUM>. The difference in height between the water level of the secondary water tank <NUM> and the water level of the primary water tank <NUM> (i.e., the difference in water level) causes flow of the water to be treated from the primary water tank <NUM> toward the secondary water tank <NUM>. 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>.

In the water treatment facility <NUM>, the mixing/clarifying device <NUM> is placed upstream of the membrane filtration device <NUM>, 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 <NUM>. Thus, even if torrential rain occurs and the water to be treated with a temporarily high turbidity concentration flows into the primary water tank <NUM>, the water to be treated can be supplied to the membrane filtration device <NUM> after turbidity is removed in the mixing/clarifying device <NUM>, which can prevent excessively high solid load from being imposed on the membrane filtration device <NUM>. Therefore, in the water treatment facility <NUM> including the mixing/clarifying device <NUM>, it is possible to prevent the cleaning cycle of the membrane filtration device <NUM> 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 <NUM> at a high frequency is reduced, and the water treatment efficiency of the water treatment facility <NUM> can be increased.

An example of a mixing/clarifying device <NUM> will be described in detail with reference to <FIG>. The mixing/clarifying device <NUM> includes a coagulant feeder <NUM> and a tank <NUM>. The tank <NUM> further includes an outer cylinder <NUM> having an inflow port <NUM> through which the coagulant-containing water is flowed into the tank <NUM> and an inner cylinder <NUM> arranged inserted from the upper side of the tank <NUM> to the lower side of the inflow port <NUM> of the outer cylinder <NUM> and having a lower end open in the tank. The mixing/clarifying device <NUM> 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 <NUM>, a high solid-liquid separation ability can be exhibited.

The coagulant feeder <NUM> has a function of feeding coagulant to the water to be treated. The coagulant feeder <NUM> 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 <NUM> 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> illustrates an aspect in which the coagulant feeder <NUM> is attached to the water to be treated inflow line <NUM> connected to the inflow port <NUM>. As illustrated, the position at which the coagulant feeder <NUM> feeds coagulant to the water to be treated is preferably the upstream side of the inflow port <NUM>. 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 <NUM> illustrated in <FIG>) based on the flow direction of the water to be treated. If the coagulant feeder <NUM> 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 <NUM>, 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 <NUM> 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, 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 <NUM> has a function of mixing the coagulant-containing water flowed into the tank <NUM> through the inflow port <NUM> to form a floc and then performing solid-liquid separation. The coagulant-containing water flowed through the inflow port <NUM> into the tank <NUM> can flow down in the tank <NUM> through the flow path defined by the inner periphery of the outer cylinder <NUM> and the outer periphery of the inner cylinder <NUM> while a part thereof forming a flow as schematically illustrated by the arrow F and circling around the inner cylinder <NUM>. The tank <NUM> can be configured by a water tank configured to allow the coagulant-containing water flowed into the tank through the inflow port <NUM> to flow, as treated water, out of the tank by the inner cylinder <NUM>. The tank <NUM> can be mounted by, for example, a pressure-resistant water tank with a cylindrical body portion (corresponding to the outer cylinder <NUM>) having a round-shaped cross section, without being particularly limited. In this case, the inner cylinder <NUM> may also have a cylindrical body portion having a round-shaped cross section, and the outer cylinder <NUM> and the inner cylinder <NUM> may share the same axis, or each axis of them may not be aligned. The tank <NUM> may include a solid matter extraction mechanism at the bottom thereof. As illustrated, the outer cylinder <NUM> of the tank <NUM> is closed by being connected to the outer wall of the inner cylinder <NUM> at the top. In other words, the space between the outer cylinder <NUM> and the inner cylinder <NUM> is closed at the top of the tank <NUM>.

In the tank <NUM>, the space between the outer cylinder <NUM> and the inner cylinder <NUM> 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 <NUM> and the region lower than the lower end of the inner cylinder <NUM>. Then, finally, the floc formed in the floc formation region settles down at a settling velocity V2. At the near lower end of the inner cylinder <NUM>, although the liquid is attracted by the upward flow flowing at a flow velocity V1 in the inner cylinder <NUM> and tries to flow out of the tank <NUM>, most of the floc that is heavier than the liquid and settles down at a settling velocity V2 faster than the flow velocity V1 goes down to the bottom of the tank <NUM> against the flow velocity V1. Then, the liquid flows out of the tank <NUM> and flocculated solids accumulate at the bottom of the tank <NUM>, and as a result the coagulant-containing water is clarified. In this manner, the region lower than the lower end of the inner cylinder <NUM> acts as a solid-liquid separation region. The liquid flowed out of the tank <NUM> 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 <NUM>.

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 <NUM> and the inner cylinder <NUM> acts as a floc formation region, the longer the inner cylinder <NUM>, 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 <NUM> can be determined such that the floc forming ability and the solid-liquid separation ability is balanced.

As illustrated in <FIG>, it is preferable that the lower portion of the tank <NUM> is tapered downward. More specifically, as illustrated in <FIG>, the outer cylinder <NUM> forming the tank <NUM> may include a straight body portion and further a tapered portion being continuous from the straight body portion. According to the tank <NUM> having the above-described shape, floc settlement can be promoted, and as a result, the solid-liquid separation ability of the mixing/clarifying device <NUM> can be further enhanced.

Furthermore, the tank <NUM> may include a solid discharge mechanism <NUM> underneath. In the example illustrated, the solid discharge mechanism <NUM> is composed of a solid discharge pipe <NUM> and a solid discharge valve <NUM>. The solid discharge valve <NUM> may be open when the solids accumulated on the bottom of the tank <NUM> are discharged, and is closed at other times. When to open the solid discharge valve <NUM> can be set as desired.

From the viewpoint of further enhancing the solid-liquid separation ability of the mixing/clarifying device <NUM>, it is preferable that the tank <NUM> includes a rapid stirrer for rapidly stirring the coagulant-containing water in the space between the outer cylinder <NUM> and the inner cylinder <NUM> and in the region between the upper end of the outer cylinder <NUM> and the lower end of the inner cylinder <NUM>. 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 <NUM> 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 <NUM> through the inflow port <NUM>, 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 <NUM> and the inner cylinder <NUM> at least once. Various mounting aspects of the rapid stirrer will be described below with reference to <FIG>. It is to be noted that, in <FIG>, although the coagulant feeder is not illustrated, each mixing/clarifying device illustrated in the figures includes a coagulant feeder.

<FIG> 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. In the mixing/clarifying device <NUM> illustrated in <FIG>, the inner wall of the outer cylinder 21A is tapered toward the upper side of the tank 20A, and the rapid stirrer is formed by the flow path defined by the inner wall of the outer cylinder 21A and the outer wall of the inner cylinder <NUM>. The coagulant-containing water flowed into the tank 20A through the water to be treated inflow line <NUM> gradually flows down toward the lower portion of the tank 20A 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 20A, 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 <NUM> has even better solid-liquid separation ability.

<FIG> 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. In the mixing/clarifying device <NUM> illustrated in <FIG>, the outer wall of the inner cylinder 22B is tapered toward the lower side of the tank 20B, and the rapid stirrer is formed by the flow path defined by the inner wall of the outer cylinder <NUM> and the outer wall of the inner cylinder 22B. As with the aspect illustrated in <FIG>, the coagulant-containing water flowed into the tank 20B through the water to be treated inflow line <NUM> gradually flows down toward the lower portion of the tank 20B while circling around the flow path. According to the same principle as that illustrated in <FIG>, 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 <NUM> according to this example also has an even better solid-liquid separation ability.

<FIG> 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 <NUM> illustrated in <FIG>, the inner wall of the outer cylinder 21C has a narrow flow path <NUM> formed along the peripheral surface over at least one round of the peripheral surface of the inner wall. The narrow flow path <NUM> rapidly flows the coagulant-containing water flowed in through the inflow port <NUM> and acts as a rapid stirrer. The narrow flow path <NUM> can be defined by a narrow flow path wall 23w, a narrow flow path support portion <NUM> and an inner wall surface of the outer cylinder 21C. The narrow flow path wall 23w is inclined with respect to the inner wall surface of the outer cylinder 21C. When the narrow flow path wall 23w is inclined with respect to the inner wall surface of the outer cylinder 21C, the narrow flow path wall 23w is inclined such that the distance between the inner wall surface of the outer cylinder 21C and the narrow flow path wall 23w on the upper side of the narrow flow path wall 23w is larger than the distance between the inner wall surface of the outer cylinder 21C and the narrow flow path wall 23w on the lower side of the narrow flow path wall 23w (e.g., the angle between them is more than <NUM>° and <NUM>° or less). In the example illustrated in <FIG>, when the distance between the inner wall surface of the outer cylinder 21C and the narrow flow path wall 23w near the inflow port <NUM> is defined as L1 and the distance between the inner wall surface of the outer cylinder <NUM> and the narrow flow path wall 23w at the upper end of the narrow flow path wall 23w is defined as L2, L1 < L2. Furthermore, in the example illustrated in <FIG>, when the distance between the wall surface on the inner cylinder 22C side of the narrow flow path wall 23w and the outer wall surface of the inner cylinder 22C at the upper end of the narrow flow path wall 23w is defined as L3, L2 equals L3 or L3 is larger than L2. That is, in the example illustrated in <FIG>, the relationships of L1 < L2 and L2 ≤ L3 are established.

The narrow flow path support portion <NUM> is attached by adhering to the inner peripheral surface over the entire circumference of the inner wall surface of the outer cylinder 21C. 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 20C 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 <NUM>, a cross-sectional view taken from the line I-I in <FIG> is illustrated in <FIG>. The I-I cross section is a plane that passes through the center of the inflow port <NUM> and is perpendicular to the vertical direction of the tank 20C. As obvious from <FIG>, the narrow flow path <NUM> is formed over the entire circumference of the inner wall of the outer cylinder 21C without interruption. Here, when comparing, on the cross-section illustrated in <FIG>, the distance Da between the narrow flow path wall 23w and the inner wall surface of the outer cylinder 21C and the distance Db between the narrow flow path wall 23w and the outer wall surface of the inner cylinder 22C, Da < Db. If the relationship of Da < Db is satisfied, after the coagulant-containing water flowed into the tank 20C through the inflow port <NUM> is flowed into the rapid stirring action in the narrow flow path <NUM>, the coagulant-containing water can be slowly stirred in a region extending between the outer wall of the inner cylinder 22C and the narrow flow path wall 23w (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 <NUM> after circling around the narrow flow path <NUM> and reaches the slow stirring portion. Further, the coagulant-containing water flows down while circling around the inner cylinder 22C at the slow stirring portion.

Note that, in <FIG>, Da and Db have constant values, respectively, but are not limited to the aspect illustrated in <FIG>. For example, in the mixing/clarifying device according to a variation, the narrow flow path <NUM> may be designed such that the value of Da near the inflow port <NUM> will be the smallest and that of near the opposite side of the inflow port <NUM> will be the largest. In this case, the stirring intensity inside the narrow flow path <NUM> can be changed in a plane perpendicular to the vertical direction of the tank 20C, and thus floc formation can be further promoted.

Furthermore, as illustrated in <FIG>, the inner cylinder 22C has a tapered portion 22Ct on the lower side, more preferably, on the lower side of the narrow flow path supporting portion <NUM>. With the tapered portion 22Ct, the tapered portion 22Ct 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>, the inner cylinder 22C has a large-bore end portion 22Ca provided continuously over the tapered portion 22Ct. The large-bore end portion 22Ca makes it difficult for small-sized flocs, and the like, to be sucked into the inner cylinder 22C. The bore size of the large-bore end portion 22Ca is not particularly limited, and may be selected, for example, from those small enough to form upward flow in the inner pipe 22C and large enough to create flow lower than the settling velocity V2 as desired.

As described with reference to <FIG>, 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>.

<FIG> illustrates a schematic configuration of an inner cylinder 22D provided with helical ribs at its lower portion. The inner cylinder 22D having a configuration as illustrated can be adopted by a mixing/clarifying device of any aspect illustrated with reference to <FIG>, without being particularly limited. As illustrated in <FIG>, the inner cylinder 22D is provided with a helical first rib 24a and second rib 24b 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 24a and the second rib 24b of such shape act each to promote floc settling. Therefore, the solid-liquid separation ability of the mixing/clarifying device including the first rib 24a and the second rib 24b can be further enhanced. Further, in <FIG>, although the helical first rib 24a and second rib 24b are placed with respect to the tapered outer wall of the inner cylinder 22D, the shape of the inner cylinder 22D is not limited to the tapered shape. More specifically, at least one helical rib may be provided to the straight body type inner cylinder 22D.

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.

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 <NUM> illustrated in <FIG>. Note that the structure of the mixing/clarifying device adopted by this example was according to the structure illustrated in <FIG>. More specifically, the mixing/clarifying device has the following structure.

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. The results obtained according to the above are illustrated in <FIG>.

From <FIG>, 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.

Claim 1:
A mixing/clarifying device, comprising:
a coagulant feeder (<NUM>) that feeds coagulant to water to be treated to obtain coagulant-containing water; and
a tank (20C) in which the coagulant-containing water is mixed to form a floc and solid-liquid separation is performed, wherein
the tank (20C) includes:
an outer cylinder (21C) having an inflow port (<NUM>) through which the coagulant-containing water is flowed into the tank (20C) and an inner cylinder (22C) arranged inserted from an upper side of the tank (20C) to a lower side of the inflow port (<NUM>) of the outer cylinder (21C) and having a lower end open in the tank (20C), and
a rapid stirrer that
includes a flow path defined by the inner periphery of the outer cylinder (21C) and the outer periphery of the inner cylinder (22C),
is located between an upper end of the outer cylinder (21C) and a lower end of the inner cylinder (22C) in a space between the outer cylinder (21C) and the inner cylinder (22C) and is configured to rapidly stir the coagulant-containing water by circulating the coagulant-containing water around the inner cylinder (22C), and
has a cross-sectional area that increases toward a lower portion, wherein:
the tank (20C) is configured to allow the coagulant-containing water flowed into the tank through the flow path to flow, as treated water, out of the tank (20C) by the inner cylinder (22C), and
characterized in that:
the rapid stirrer further includes a narrow flow path (<NUM>) defined by a narrow flow path wall (23w), a narrow flow path support portion (<NUM>) and an inner wall surface of the outer cylinder (21C), wherein the narrow flow path support portion (<NUM>) connects the inner wall surface of the outer cylinder (21C) to a lower side of the narrow flow path wall (23w) over the entire circumference of the inner wall surface of the outer cylinder (21C), and
wherein the narrow
flow path wall (23w) is inclined such that a distance between the inner wall surface of the outer cylinder (21C) and the narrow flow path wall (23w) on an upper side of the narrow flow path wall (23w) is larger than a distance between the inner wall surface of the outer cylinder (21C) and the narrow flow path wall (23w) on a lower side of the narrow flow path wall (23w).