Abstract:
A scrubber wastewater treatment method, according to one possible embodiment, includes obtaining a measurement of a turbidity or of a suspended substance concentration of scrubber wastewater and, upon determining that measurement of turbidity or suspended substance concentration is within a certain range, performing treatment. A scrubber wastewater treatment device, according to one possible embodiment, includes a magnetic powder adding device controllable to add a magnetic powder to be added to scrubber wastewater having been generated by treating combustion exhaust gas in a scrubber, and a controller configured to control an amount of the magnetic powder added by the magnetic powder adding device in accordance with a measurement value obtained by a sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application, filed under 35 U.S.C. §111(a), of International Application PCT/JP2016/052455 filed on Jan. 28, 2016 and claims foreign priority to Japanese Patent Application No. 2015-050228, filed Mar. 13, 2015, the content of each of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present disclosure relates to a method of treating scrubber wastewater generated when treating combustion exhaust gas with a scrubber, and to a scrubber wastewater treatment device. 
         [0004]    2. Related Art 
         [0005]    Combustion exhaust gas generated in a diesel engine of a ship, a furnace, or the like, includes black carbon, soot, oil droplets, and the like, because of which the exhaust gas is passed through a dirt collecting device (scrubber), cleaning water is brought into contact with the gas by spraying, and the toxic substances are removed by being shifted to the cleaning water side. Meanwhile, scrubber wastewater generated when treating with this kind of dirt collecting device (scrubber) includes collected toxic substances such as black carbon, soot, and oil droplets, and these too need to be removed so as not to be released into the environment. To date, methods such as centrifugal separation (see JP-A-2004-81933), filtration treatment (see JP-A-11-165180 (paragraph 0019)), coagulation-sedimentation (see JP-A-2001-129596 (paragraph 0004)), and the like, have been employed in the removal of toxic substances from scrubber wastewater. 
       SUMMARY 
       [0006]    However, there is a problem with centrifugal separation in that a device consumes a large amount of power, and power generating equipment is needed for this, meaning that large-scale facilities are needed. Also, there is a problem with safety in a rolling ship or the like. In the case of filtration treatment, maintenance involving filter material replacement or cleaning due to clogging is troublesome. There are problems with coagulation-sedimentation in that coagulated floc needs to be left standing for a long time in order to cause the floc to precipitate, meaning that efficiency is poor, and a precipitate that has once been formed is liable to be stirred up in a rolling ship or the like, meaning that stable and efficient solid-liquid separation is difficult. 
         [0007]    Therefore, the disclosure provides a scrubber wastewater treatment method and scrubber wastewater treatment device such that solid-liquid separation of a once formed floc is easy, with no need for large-scale facilities or troublesome maintenance. 
         [0008]    In order to achieve the benefits described in the above paragraph, a scrubber wastewater treatment method of the disclosure is characterized in that a magnetic powder is added to scrubber wastewater generated when treating combustion exhaust gas in a scrubber, a bound substance of a suspended substance included in scrubber wastewater and the magnetic powder is formed, and the bound substance is magnetically separated, thereby cleaning the wastewater. 
         [0009]    According to the scrubber wastewater treatment method of the disclosure, even in the case of a suspended substance included in scrubber wastewater such as black carbon, soot, or oil, which has a low specific gravity and precipitates with difficulty, the suspended substance can be efficiently removed, and the wastewater cleaned, by magnetically separating the suspended substance together with a magnetic powder. 
         [0010]    The scrubber wastewater treatment method of the disclosure is preferably such that a coagulating agent is further added together with the magnetic powder to the scrubber wastewater, a bound substance of a suspended substance included in the scrubber wastewater and the magnetic powder is formed, and the bound substance is magnetically separated. By so doing, the efficiency of a bound substance of a suspended substance included in the scrubber wastewater and a magnetic powder being formed increases, because of which wastewater can be cleaned more efficiently. 
         [0011]    Also, preferably, a range A wherein no suspended substance removal process is necessary, a range B wherein treatment can be carried out by an addition of a magnetic powder alone, and a range C wherein a magnetic powder and a coagulating agent need to be used concurrently, are set in advance with regard to a turbidity or suspended substance concentration of the scrubber wastewater, the turbidity or suspended substance concentration is measured, the scrubber wastewater is discharged as it is, or circulated and used, when the turbidity or suspended substance concentration is in range A, treatment is carried out by adding magnetic powder only to the scrubber wastewater when the turbidity or suspended substance concentration is in range B, and treatment is carried out by adding magnetic powder and a coagulating agent to the scrubber wastewater when the turbidity or suspended substance concentration is in range C. By so doing, magnetic powder or a coagulating agent is added in accordance with scrubber wastewater turbidity or suspended substance concentration, excessive use of magnetic powder or a coagulating agent is restricted, and running costs can be restricted. 
         [0012]    Also, an amount of the magnetic powder added and/or an amount of the coagulating agent added when the range of the turbidity or suspended substance concentration is B or C is preferably an amount in accordance with the turbidity or suspended substance concentration. By so doing, excessive use of magnetic powder or a coagulating agent is effectively restricted, and running costs can be further restricted. 
         [0013]    Also, a boundary value of the range A and range B is preferably set in a range of 25 to 100 NTU turbidity. 
         [0014]    Also, a boundary value of the range A and range B is preferably set in a range of 10 to 50 mg/L suspended substance concentration. 
         [0015]    Also, a boundary value of the range B and the range C is preferably set in a range of 50 to 150 NTU turbidity, and is a value 25 NTU or more greater than a boundary value of the range A and the range B. 
         [0016]    Also, a boundary value of the range B and the range C is preferably set in a range of 20 to 100 mg/L suspended substance concentration, and is a value 10 mg/L or more greater than a boundary value of the range A and the range B. 
         [0017]    Meanwhile, a scrubber wastewater treatment device of the disclosure is characterized by including a magnetic powder adding device that adds a magnetic powder to scrubber wastewater generated when treating combustion exhaust gas in a scrubber, and a magnetic separation device that magnetically separates a bound substance of a suspended substance included in the scrubber wastewater and the magnetic powder. 
         [0018]    According to the scrubber wastewater treatment device of the disclosure, even in the case of a suspended substance included in scrubber wastewater such as black carbon, soot, or oil, which has a low specific gravity and precipitates with difficulty, the suspended substance can be efficiently removed, and the wastewater cleaned, by magnetically separating the suspended substance together with a magnetic powder. 
         [0019]    The scrubber wastewater treatment device of the disclosure preferably further includes a coagulating agent adding device that adds a coagulating agent to the scrubber wastewater. By so doing, the efficiency of a bound substance of a suspended substance included in scrubber wastewater and a magnetic powder being formed by using a coagulating agent increases, because of which wastewater can be cleaned more efficiently. 
         [0020]    Also, the scrubber wastewater treatment device of the disclosure preferably further includes a measuring device that measures a turbidity or suspended substance concentration of the scrubber wastewater, and a control device that controls an amount of magnetic powder added from the magnetic powder adding device in accordance with a measurement value from the measuring device. By so doing, magnetic powder is not used more than necessary, because of which running costs can be restricted. 
         [0021]    Also, the scrubber wastewater treatment device of the disclosure preferably further includes a measuring device that measures a turbidity or suspended substance concentration of the scrubber wastewater, and a control device that controls an amount of magnetic powder added from the magnetic powder adding device and an amount of coagulating agent added from the coagulating agent adding device in accordance with a measurement value from the measuring device. By so doing, magnetic powder and coagulating agent are not used more than necessary, because of which running costs can be restricted. 
         [0022]    Also, preferably, a range A wherein no suspended substance removal process is necessary, a range B wherein treatment can be carried out by an addition of a magnetic powder alone, and a range C wherein a magnetic powder and a coagulating agent need to be used concurrently, are input in advance into the control device with regard to a turbidity or suspended substance concentration of the scrubber wastewater, the turbidity or suspended substance concentration is measured by the measuring device and compared with the ranges input in advance into the control device, the scrubber wastewater is discharged as it is, or circulated and used, when the turbidity or suspended substance concentration is in range A, treatment is carried out by adding magnetic powder only to the scrubber wastewater when the turbidity or suspended substance concentration is in range B, and treatment is carried out by adding magnetic powder and a coagulating agent to the scrubber wastewater when the turbidity or suspended substance concentration is in range C. 
         [0023]    According to the disclosure, even in the case of a suspended substance included in scrubber wastewater such as black carbon, soot, or oil, which has a low specific gravity and precipitates with difficulty, the suspended substance can be efficiently removed, and the wastewater cleaned, by magnetically separating the suspended substance together with a magnetic powder. Also, by using a coagulating agent concurrently, the efficiency of a bound substance of a suspended substance included in scrubber wastewater and a magnetic powder being formed increases, because of which wastewater can be cleaned more efficiently. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic configuration diagram of a device according to a first embodiment for implementing a method of the disclosure. 
           [0025]      FIG. 2  is an illustration of a first operation example of the aforementioned device. 
           [0026]      FIG. 3  is an illustration of a second operation example of the aforementioned device. 
           [0027]      FIG. 4  is an illustration of a third operation example of the aforementioned device. 
           [0028]      FIG. 5  is a flowchart representing an example of the first to third operation examples being reciprocally controlled. 
           [0029]      FIG. 6  is a schematic configuration diagram showing an example of a magnetic separation device included in the device according to the first embodiment. 
           [0030]      FIG. 7  is a schematic configuration diagram showing another example of a magnetic separation device included in the device according to the first embodiment. 
           [0031]      FIG. 8  is a schematic configuration diagram of a device according to a second embodiment for implementing the method of the disclosure. 
           [0032]      FIG. 9  is a schematic configuration diagram showing an example of a magnetic separation device included in the device according to the second embodiment. 
           [0033]      FIG. 10  is a schematic configuration diagram of a device according to a third embodiment for implementing the method of the disclosure. 
           [0034]      FIG. 11  is a schematic configuration diagram showing an example of a magnetic separation device included in the device according to the third embodiment. 
           [0035]      FIG. 12  is a schematic configuration diagram of a device according to a fourth embodiment for implementing the method of the disclosure. 
           [0036]      FIG. 13  is a schematic configuration diagram showing an example of a magnetic separation device included in the device according to the fourth embodiment. 
           [0037]      FIG. 14  is a schematic configuration diagram showing another example of a magnetic separation device included in the device according to the fourth embodiment. 
           [0038]      FIG. 15  is a graph showing results of a height of an interface between a transparent portion and a suspended portion being measured over time in a scrubber wastewater coagulation-sedimentation test. 
           [0039]      FIG. 16  is a graph showing results of sampling being carried out in a position 2 cm from a water surface, and turbidity being measured, after coagulation-sedimentation in a scrubber wastewater coagulation-sedimentation test. 
           [0040]      FIG. 17  is a graph showing results of that which remains after a supernatant is removed by decantation after coagulation-sedimentation being taken to be sludge, and a moisture content being measured using an infrared moisture meter, in a scrubber wastewater coagulation-sedimentation test. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    Hereafter, while referring to the drawings, a description will be given of embodiments of the disclosure. 
         [0042]      FIG. 1  shows a device according to a first embodiment for implementing a method of the disclosure. In this embodiment, exhaust gas generated in an engine  1  is introduced into a scrubber  2  from an air inlet  2   a  and comes into gas-liquid contact with scrubber cleaning water, and after toxic substances such as black carbon, soot, and oil droplets are removed by shifting to the cleaning water side, the exhaust gas is discharged from an exhaust port  2   b . Also, scrubber wastewater generated at this time passes from a drain port  2   c  into a pipe L 1 , and is introduced into a pipe L 2  or pipe L 3  via a valve  3 . Further, the pipe L 2  communicates with a mixing tank  7  and a magnetic separation device  11  downstream of the mixing tank  7 , while the pipe L 3  bypasses the mixing tank  7  and magnetic separation device  11 , and communicates with a pipe L 6  downstream of the mixing tank  7  and magnetic separation device  11 . Owing to this bypass route, wastewater is discharged without receiving treatment in the mixing tank  7  or the magnetic separation device  11 , or is circulated and utilized as scrubber cleaning water. Also, the pipe L 1  branches on an upstream side of the valve  3  into a pipe L 1   a  that communicates with a measurement tank  4 , one portion of the wastewater is transferred through the pipe L 1   a  to the measurement tank  4  and accumulates there, and the wastewater can be inspected using a pH measuring device  5  or a water quality measuring device  6 . In this embodiment, one portion of the wastewater is accumulated in the measurement tank  4  and inspected, but the pipe L 1   a  may stand alone without the tank being provided, and wastewater flowing inside the pipe may be inspected directly. Also, although the piping is branched and one portion of the wastewater is removed for inspection, wastewater flowing inside the pipe L 1  may be inspected directly without removing one portion of the wastewater. Also, wastewater after inspection is transferred through a pipe L 4  to the mixing tank  7 , but the wastewater that has been inspected too, when not needed, may be returned to the pipe L 1  upstream of the valve  3  and downstream of a branch point, and be able to bypass the mixing tank  7  and magnetic separation device  11  through the pipe L 3 . 
         [0043]    Water quality categories inspected by the water quality measuring device (or sensor)  6  include wastewater turbidity or suspended substance (SS) concentration (“suspended substance concentration” in this specification) as representative indices of suspended substances such as black carbon, soot, and oil droplets included in wastewater. For example, wastewater turbidity can be measured using a method compliant with 9.3 of JIS K0101, or the like. Also, wastewater suspended substance concentration can be measured using a method compliant with 14.1 of JIS K0102, or the like. 
         [0044]    An agitating device  15  having an agitating blade  15   a  rotated via a pivot  15   b  by a motor M is provided in the mixing tank  7 , as shown in  FIGS. 2 to 4 . Further, wastewater is mixed by the agitating device  15  in the mixing tank  7  with an alkaline agent or acid agent added from a pH regulating device  8 , magnetic powder added from a magnetic powder adding device  9 , and a coagulating agent added from a coagulating agent adding device  10 , whereby a bound substance (hereafter referred to as “magnetic floc”) of suspended substances such as black carbon, soot, and oil droplets included in the wastewater and magnetic powder can be formed. The pH of wastewater is preferably regulated to 4 to 11. By so doing, alkali consumed by the coagulating agent can be supplied, because of which magnetic floc can be efficiently formed. The pH is preferably regulated in accordance with the pH state of wastewater measured by the pH measuring device  5 . 
         [0045]    The magnetic powder can be added in powder form, or added in a state wherein magnetic powder is dispersed in a dispersion medium such as water. A material of the magnetic powder may be either a normal magnetic powder or strong magnetic powder, including, for example, a ferric oxide such as triiron tetraoxide, cobalt, chromium oxide, ferrite, and the like. Preferably, a particle diameter of the magnetic powder is normally within a range of 0.05 μm or greater, 10 μm or less, and more preferably still, within a range of 0.05 μm or greater, 5 μm or less. Also, preferably, a coercive force of the magnetic powder is normally within a range of 10 4 /4πA/m or greater, 4×10 5 /4πA/m or less, and more preferably still, within a range of 2×10 5 /4πA/m or greater, 3×10 5 /4πA/m or less. With regard to an amount of magnetic powder added, the magnetic powder is preferably added so that a magnetic powder mass is 0.1 to 10 with respect to a suspended substance mass of 1, and more preferably still, added so that the magnetic powder mass is 0.5 to 5. When the amount added is less than this range, there is a tendency for the magnetic floc forming efficiency to worsen, and when the amount added exceeds the range, the magnetic powder is unnecessarily used excessively, and a cost thereof increases, meaning that both cases are undesirable. Also, the amount of magnetic powder added is preferably regulated in accordance with a turbidity or suspended substance concentration of wastewater measured by the water quality measuring device  6 . In this case, for example, a table of a relationship between measurement value and amount of magnetic powder added may be stored in advance in a control device, to be described hereafter, and the amount added determined by referring to the table. 
         [0046]    The coagulating agent can be added in powder form, or added in a state wherein a coagulating agent is dispersed in a dispersion medium such as water. For example, polyaluminum chloride (PAC), ferric polysulfate, aluminum sulfate (alum), a polymer (nonionic, cationic, anionic, dipolar), and the like, are included as coagulating agent components, and the coagulating agent can be selected in accordance with wastewater properties. Wth regard to an amount of coagulating agent added, the coagulating agent is preferably added so that a coagulating agent mass is 0.005 to 1 with respect to a suspended substance mass of 1, and more preferably still, added so that the coagulating agent mass is 0.01 to 0.5. When the amount added is less than this range, there is a tendency for the efficiency of aiding magnetic floc formation to worsen, and when the amount added exceeds the range, the coagulating agent is unnecessarily used excessively, and a cost thereof increases, meaning that both cases are undesirable. Also, the amount of coagulating agent added is preferably regulated in accordance with a turbidity or suspended substance concentration of wastewater measured by the water quality measuring device  6 . In this case, for example, a table of a relationship between measurement value and amount of coagulating agent added may be stored in advance in a control device, to be described hereafter, and the amount added determined by referring to the table. 
         [0047]    Wastewater including magnetic floc formed by mixing with magnetic powder, a coagulating agent, and the like in the mixing tank  7  is introduced through a pipe L 5  into the magnetic separation device (magnetic separator)  11  and subjected to a magnetic separation process, and sludge collected as an aggregate (lump of magnetic floc) by the magnetic separation process is stored in a sludge tank  12 . The magnetic separation process in the magnetic separation device  11  will be described hereafter. 
         [0048]    A liquid portion (a clear supernatant) after magnetic separation is discharged from the pipe L 6  via a valve  13  and through a pipe L 7 , or introduced through a pipe L 8  into the scrubber  2  from a liquid inlet  2   d , and reutilized as scrubber cleaning water. 
         [0049]    The device of this embodiment further includes a control device  14 , wherein the control device  14  is connected by wiring T to each of the pH measuring device  5 , the water quality measuring device  6 , the pH regulating device  8 , the magnetic powder adding device  9 , the coagulating agent adding device  10 , the magnetic separation device  11 , and valves  3  and  13 , and can send and receive signals, store determination data, and determine by comparing with the determination data. That is, the control device  14  can, for example, receive a signal relating to a measurement value from the pH measuring device  5  or the water quality measuring device  6 , determine whether or not the received measurement value is within a predetermined range, send a signal relating to the amount of alkaline agent or acid agent to be added to the pH regulating device  8 , a signal relating to the amount of magnetic powder to be added to the magnetic powder adding device  9 , and a signal relating to the amount of coagulating agent to be added to the coagulating agent adding device  10 , send a signal relating to the necessity or otherwise of operating to the magnetic separation device  11 , and send a signal relating to pipe connection by the valves  3  and  13 . 
         [0050]    The foregoing description has as a precondition an aspect wherein all wastewater is treated, but a configuration may be such that wastewater is caused to diverge using a pipe, valve, or the like, one portion is treated, and the treated water and untreated wastewater are mixed and discharged, or reutilized as scrubber cleaning water. In this case, the valve  3  may be of a configuration such that the valve  3  can arbitrarily change the ratios of the amounts of wastewater passing into the pipe  2  and the pipe L 3 , and can arbitrarily select the ratios of the amount of wastewater to be treated and the amount of wastewater not to be treated. 
         [0051]      FIG. 1  can also be understood as illustrating a system comprising the various elements illustrated in this figure, which include the scrubber  2  and the various wastewater treatment features discussed above. 
         [0052]      FIG. 2  shows a first operation example of the heretofore described device. In this operation example, wastewater from the pipe L 1  flows into the pipe L 3  owing to a connection of the valve  3 , which is formed of a three-way valve, bypasses the mixing tank  7  and magnetic separation device  11 , and is transferred to the pipe L 6  downstream of the mixing tank  7  and magnetic separation device  11 . This kind of operation example is appropriate when the amount of suspended substances in the wastewater is within a range such that a suspended substance removal process is unnecessary (hereafter referred to as “range A”), and there is no need to remove the suspended substances in the wastewater. An upper limit value of the range A (hereafter referred to as a “boundary value AB”) differs in accordance with a scrubber device configuration, application, discharge regulation standard, and the like, but is preferably set, for example, within a range of 25 to 100 NTU when taking wastewater turbidity as an index, or within a range of 10 to 50 mg/L when taking wastewater suspended substance concentration as an index. In this case, there is no addition of magnetic powder from the magnetic powder adding device  9 , no addition of the coagulating agent from the coagulating agent adding device  10 , or the like, and the agitating device  15  and the magnetic separation device  11  do not operate either. Addition of an alkaline agent or an acid agent from the pH regulating device  8  is arbitrary. 
         [0053]      FIG. 3  shows a second operation example of the heretofore described device. In this operation example, wastewater from the pipe L 1  flows into the pipe L 2  owing to a connection of the valve  3 , which is formed of a three-way valve, and is transferred to the mixing tank  7 . In the mixing tank  7 , magnetic powder is added from the magnetic powder adding device  9  to wastewater that is stored in the tank for a certain time, and the wastewater is agitated by the agitating device  15 , thereby causing magnetic floc to be formed. Addition of an alkaline agent or an acid agent from the pH regulating device  8  is arbitrary. This kind of operation example is appropriate in a range wherein wastewater suspended substances can be treated with magnetic powder alone to form magnetic floc (hereafter referred to as “range B”), and there is no need to go as far as adding a coagulating agent. An upper limit value of the range B (hereafter referred to as a “boundary value BC”) differs in accordance with a scrubber device configuration, an application, a discharge regulation standard, and the like, but is preferably, for example, a value within a range of 50 to 150 NTU, and 25 NTU or more greater than the boundary value AB, when taking wastewater turbidity as an index, or a value within a range of 20 to 100 mg/L, and 10 mg/L or more greater than the boundary value AB, when taking wastewater suspended substance concentration as an index. Provided that wastewater suspended substances are within the range B, the wastewater suspended substances can be treated with magnetic powder alone to form magnetic floc. Therefore, no addition of coagulating agent from the coagulating agent adding device  10  is carried out. 
         [0054]      FIG. 4  shows a third operation example of the heretofore described device. In this operation example, wastewater from the pipe L 1  flows into the pipe L 2  owing to a connection of the valve  3 , which is formed of a three-way valve, and is transferred to the mixing tank  7 . In the mixing tank  7 , together with magnetic powder being added from the magnetic powder adding device  9 , a coagulating agent is added from the coagulating agent adding device  10  to wastewater that is stored in the tank for a certain time, and the wastewater is agitated by the agitating device  15 , thereby causing magnetic floc to be formed. Addition of an alkaline agent or an acid agent from the pH regulating device  8  is arbitrary. This kind of operation example is appropriate in a range wherein wastewater suspended substances cannot be treated with magnetic powder alone, and it is necessary to use a coagulating agent concurrently (hereafter referred to as “range C”). Although differing in accordance with a scrubber device configuration, an application, a discharge regulation standard, and the like, the range C is preferably a range exceeding the boundary value BC, that is, a range exceeding 50 to 150 NTU when taking wastewater turbidity as an index, or a range exceeding 20 to 100 mg/L when taking wastewater suspended substance concentration as an index. When adding the coagulating agent, an agitation operation is desirably regulated so as to agitate strongly at first in order to form a minute micro-floc, and next to agitate weakly in order to cause a larger floc to grow, and these operations may be carried out separately by dividing the mixing tank  7  into two or more receptacles. 
         [0055]    The first to third operation examples can be controlled by, for example, the water quality measuring device  6  and the control device  14 .  FIG. 5  shows a flowchart representing one example of the control. In this control example, firstly, wastewater is inspected by the water quality measuring device  6 , a measurement value thereof is compared with the determination data stored in advance in the control device  14 , and it is determined whether the measurement value is within the range A wherein a suspended substance removal process is unnecessary, in other words, whether or not the measurement value is equal to or lower than the boundary value AB (step S 1 ). When the measurement value is equal to or lower than the boundary value AB, the valve  3  is connected to the pipe L 3  (step S 2 ), and the wastewater is discharged or recirculated (step S 3 ), according to the first operation example. 
         [0056]    Next, when the measurement value exceeds the boundary value AB, the valve  3  is connected to the pipe L 2  (step S 4 ), the measurement value from the water quality measuring device  6  is compared with the determination data stored in advance in the control device  14 , and it is determined whether the measurement value is within the range B wherein processing can be carried out by adding only magnetic powder, in other words, whether or not the measurement value is equal to or lower than the boundary value BC (step S 5 ). When the measurement value is equal to or lower than the boundary value BC, magnetic floc is caused to form by adding magnetic powder from the magnetic powder adding device  9  (step S 6 ), the wastewater is magnetically separated into an aggregate (lump of magnetic floc) and a liquid portion (clear supernatant) by the magnetic separation device  11  (step S 7 ), and the wastewater after treatment is discharged or recirculated (step S 8 ), according to the second operation example. 
         [0057]    Next, when the measurement value exceeds the boundary value BC, it is determined that the measurement value is within the range C wherein magnetic powder and a coagulating agent need to be used concurrently (step S 9 ), and together with magnetic powder being added from the magnetic powder adding device  9  (step S 10 ), a coagulating agent is added from the coagulating agent adding device  10 , whereby magnetic floc is caused to form (step S 11 ), the wastewater is magnetically separated into an aggregate (lump of magnetic floc) and a liquid portion (clear supernatant) by the magnetic separation device  11  (step S 12 ), and the wastewater after treatment is discharged or recirculated (step S 13 ), according to the third operation example. 
         [0058]    When carrying out the heretofore described kind of control, range A, range B, and range C need to be set as ranges that do not overlap each other. For example, setting is such that range A is equal to or lower than the boundary value AB, range B exceeds the boundary value AB and is equal to or lower than the boundary value BC, range C exceeds the boundary value BC, and the like. 
         [0059]    It is noted that range A may be referred to as a first range, and ranges B and C may collectively be referred to as a second range, where ranges B and C are respective portions of the second range. 
         [0060]    Hereafter, while referring to  FIGS. 6 and 7 , a description will be given of the magnetic separation process in the magnetic separation device  11 . 
         [0061]      FIG. 6  shows an example of a magnetic separation device included in the device according to the first embodiment. A channel  21  that communicates with the pipe L 5  extending from the mixing tank  7  is provided in this magnetic separation device  20 , and the channel  21  has a predetermined capacity, because of which, when wastewater that has finished treatment in the mixing tank  7  passes through the channel  21 , the wastewater passes through while being retained for a predetermined time. A rotating disk  22  is disposed above the channel  21  so that approximately a lower half portion of the rotating disk  22  is immersed in the wastewater flowing through the channel  21 , and the rotating disk  22  is rotated by unshown drive means. In  FIG. 6 , a direction of rotation in a circumferential direction of the rotating disk  22  is a forward direction with respect to the flow in a portion immersed in the wastewater. Also, a multiple of blades of the rotating disk  22  are coaxially arrayed in parallel at predetermined intervals in an unshown depth direction, and are disposed over practically the whole of a width direction of the channel  21 , in a state such as to leave a slight gap. A magnet is disposed over the whole of a surface of the rotating disk  22 , or within a range that can practically cover the depth of water flowing through the channel  21 , and magnetic floc formed in the wastewater adheres to the surface of the rotating disk  22  owing to magnetic attraction force of the magnet. A scraper  23  is disposed along a surface on either side of the rotating disk  22  in a portion of the rotating disk  22  exposed on a surface of the wastewater, and sludge wherein magnetic floc forms a lump is scraped off and stored in the sludge tank  12 . 
         [0062]      FIG. 7  shows another example of a magnetic separation device included in the device according to the first embodiment. A rotating drum  32 , on a cylindrical side surface of which a magnet is disposed, is used in this magnetic separation device  30  instead of the rotating disk  22  shown in  FIG. 6 . The rotating drum  32  is disposed so that a lower half portion is immersed in the wastewater, and a scraper  33  is disposed along a side surface of the rotating drum  32  in a portion of the rotating drum  32  exposed on the surface of the wastewater. Magnetic floc formed in the wastewater adheres to the side surface of the rotating drum  32  owing to the magnet disposed on the rotating drum  32 , and sludge wherein magnetic floc forms a lump is scraped off by the scraper  33  and stored in the sludge tank  12 . 
         [0063]      FIG. 8  shows a device according to a second embodiment for implementing the method of the disclosure. The device according to this embodiment differs from the device according to the first embodiment in that a mixing tank  7   a  and a magnetic separation device  11   a  are configured integrally. 
         [0064]      FIG. 9  shows an example of a magnetic separation device included in the device according to the second embodiment. This magnetic separation device  40  shares a basic configuration with the magnetic separation device  30  illustrated in  FIG. 7 , but is installed directly in the mixing tank  7   a  rather than being a channel extending from the mixing tank. Further, a lower half portion of a rotating drum  41  of the magnetic separation device  40  is disposed so as to be immersed in wastewater in the mixing tank  7   a , the magnetic floc  17  formed by adding magnetic powder, or magnetic powder and a coagulating agent, to a suspended substance  16  such as black carbon, soot, or oil and mixing in the mixing tank  7   a  is caused to adhere to a side surface of the rotating drum  41  by a magnet disposed on the rotating drum  41 , and sludge wherein magnetic floc forms a lump is scraped off by the scraper  42  and stored in the sludge tank  12 . 
         [0065]      FIG. 10  shows a device according to a third embodiment for implementing the method of the disclosure. The device according to this embodiment differs from the device according to the first embodiment in that a mixing tank  7   b  and a magnetic separation device  11   b  are configured integrally, and furthermore, the mixing tank  7   b  also performs a function of a sludge tank. That is, in this embodiment, sludge accumulates in a bottom portion of the mixing tank  7   b , and the sludge is regularly discharged from an unshown outlet. 
         [0066]      FIG. 11  shows an example of a magnetic separation device included in the device according to the third embodiment. This magnetic separation device  50  is formed of an electromagnet, and is disposed partitioning a bottom surface of the mixing tank  7   b  on an exterior thereof. The magnetic floc  17  formed by adding magnetic powder, or magnetic powder and a coagulating agent, to the suspended substance  16  such as black carbon, soot, or oil and mixing in the mixing tank  7   b  is attracted by the electromagnet, and accumulated on an interior bottom surface of the mixing tank  7   b . By providing a separate operating mode for sludge discharge, sludge wherein the magnetic floc forms a lump can be discharged from the mixing tank  7   b  via an unshown sludge removal route by causing clean water to flow in place of the wastewater while carrying out agitation in the agitation device  15 , with the electromagnet in an off state. 
         [0067]      FIG. 12  shows a device according to a fourth embodiment for implementing the method of the disclosure. The device according to this embodiment differs from the device according to the first embodiment in that a mixing pipe  7   c  including an in-line mixer is provided in place of the mixing tank, the mixing pipe  7   c  and a magnetic separation device  11   c  are configured integrally, and furthermore, these structures also perform a function of a sludge tank. 
         [0068]      FIG. 13  shows an example of a magnetic separation device included in the device according to the fourth embodiment. This magnetic separation device  60  is formed of an electromagnet, and is disposed partitioning a side surface of the mixing pipe  7   c  including the in-line mixer, on an exterior thereof. The magnetic floc  17  formed by adding magnetic powder, or magnetic powder and a coagulating agent, to the suspended substance  16  such as black carbon, soot, or oil and mixing with the unshown in-line mixer in the mixing pipe  7   c  is attracted by the electromagnet, and accumulated on an interior side surface of the mixing pipe  7   c . By providing a separate operating mode for sludge discharge, sludge wherein the magnetic floc forms a lump can be discharged from the mixing pipe  7   c  via an unshown sludge removal route by causing clean water to flow in place of the wastewater, with the electromagnet in an off state. 
         [0069]      FIG. 14  shows another example of a magnetic separation device included in the device according to the fourth embodiment. This magnetic separation device  70  is configured of a sludge storage pipe  71  branching off the mixing pipe  7   c  including the in-line mixer, a pipeline induction guide  72  disposed in an intersection portion on an interior side surface of a portion that branches off from the mixing pipe  7   c  to the sludge storage pipe  71 , a magnetism generating drum  73  disposed in a position on a pipe exterior opposing the pipeline induction guide  72  and rotating with a predetermined axis of rotation, and an electromagnet  74  disposed partitioning a side surface of the sludge storage pipe  71 , on an exterior thereof. The magnetic floc  17  formed by adding magnetic powder, or magnetic powder and a coagulating agent, to the suspended substance  16  such as black carbon, soot, or oil and mixing with the unshown in-line mixer in the mixing pipe  7   c  is attracted to the sludge storage pipe  71  side by actions of the pipeline induction guide  72  and the magnetism generating drum  73 , transferred to the sludge storage pipe  71  together with a flow of wastewater, attracted to the electromagnet  74  disposed on the exterior side surface of the sludge storage pipe  71 , and accumulated on an interior side surface of the sludge storage pipe  71 . By providing a separate operating mode for sludge discharge, sludge wherein the magnetic floc forms a lump can be discharged from the sludge storage pipe  71  and the mixing pipe  7   c  via an unshown sludge removal route by causing clean water to flow in place of the wastewater, with the electromagnet in an off state. 
       Working Example 1 
       [0070]    In order to verify an operational advantage of the disclosure, a scrubber wastewater coagulation-sedimentation test has been carried out. 
       Test Example 1 
       [0071]    Exhaust gas of a diesel engine is treated using a scrubber, whereby scrubber wastewater of 240 NTU (formazin) turbidity and 80 mg/L suspended substance concentration is prepared, and polyaluminum chloride (PAC) is added as a coagulating agent so that a coagulating agent mass with respect to suspended substance mass is 100:5. High-speed agitation at 150 rpm is carried out for 20 seconds using a bell-jar stator, after which low-speed agitation at 40 rpm is carried out for 20 minutes, and the wastewater is moved to a measuring cylinder of 500 mL capacity. A height of an interface between a transparent portion and a suspended portion is measured over time as a treatment speed index. Also, as a treatment water quality index, sampling is carried out in a position 2 cm from the water surface after coagulation-sedimentation, and turbidity is measured using a portable torpidimeter 2100Q (made by HACH). Furthermore, that which remains after a supernatant is removed by decantation after coagulation-sedimentation is taken to be sludge, and a moisture content is measured using an infrared moisture meter. 
       Test Example 2 
       [0072]    Other than using ferric polysulfate in place of polyaluminum chloride (PAC) as a coagulating agent, a test example 2 has been carried out in the same way as test example 1. 
       Test Example 3 
       [0073]    Triiron tetraoxide is added as magnetic powder to the scrubber wastewater so that a magnetic powder mass is 1:1 with respect to a suspended substance mass. High-speed agitation at 150 rpm is carried out for 20 seconds using a bell-jar stator, after which the wastewater is moved to a measuring cylinder of 500 mL capacity, and the measuring cylinder is placed on a neodymium magnet. In the same way as in test example 1, the height of an interface between a transparent portion and a suspended portion is measured over time. Also, sampling is carried out in a position 2 cm from the water surface after coagulation-sedimentation, and turbidity is measured. Furthermore, the sludge moisture content is measured. 
       Test Example 4 
       [0074]    Triiron tetraoxide is added as magnetic powder to the scrubber wastewater so that a magnetic powder mass is 1:1 with respect to a suspended substance mass, and furthermore, ferric polysulfate is added as a coagulating agent so that a coagulating agent mass is 100:5 with respect to the suspended substance mass. High-speed agitation at 150 rpm is carried out for 20 seconds using a bell-jar stator, after which the wastewater is moved to a measuring cylinder of 500 mL capacity, and the measuring cylinder is placed on a neodymium magnet. In the same way as in test example 1, the height of an interface between a transparent portion and a suspended portion is measured over time. Also, sampling is carried out in a position 2 cm from the water surface after coagulation-sedimentation, and turbidity is measured. Furthermore, the sludge moisture content is measured. 
       Comparison Example 1 
       [0075]    Scrubber wastewater to which nothing is added is taken as a comparison example 1. 
       (Results) 
       [0076]      FIG. 15  shows results of a height of an interface between a transparent portion and a suspended portion. As shown in  FIG. 15 , hardly any decrease in interface height occurs within the testing time in comparison example 1, or in test examples 1 and 2 in which a coagulating agent is added. As opposed to this, it is seen that in test example 3, in which metallic powder is added, and in test example 4, in which metallic powder and a coagulating agent are added, the interface reaches the vicinity of the bottom of the measuring cylinder in around four minutes, and coagulation-sedimentation is swiftly completed by magnetic separation. 
         [0077]      FIG. 16  shows results of turbidity in a position 2 cm from the water surface. As shown in  FIG. 16 , hardly any decrease in turbidity in the vicinity of the water surface occurs in comparison example 1. As opposed to this, turbidity in the vicinity of the water surface decreases in test examples 1 and 2, in which a coagulating agent is added, and an improvement in water quality can be confirmed. Meanwhile, turbidity in the vicinity of the water surface also decreases in test example 3, in which metallic powder is added, and in test example 4, in which metallic powder and a coagulating agent are added, and an improvement in water quality can be confirmed. For example, the turbidity after treatment is in the region of 60 NTU in test example 3, in which metallic powder is added, and although the turbidity is a little too high for discharging, it is of a level such that there is no problem in reutilizing in the scrubber. Also, the turbidity decreases to 12 NTU in test example 4, in which metallic powder and a coagulating agent are added, which is a level such that discharging can be carried out. 
         [0078]      FIG. 17  shows results of collected sludge moisture content. In comparison example 1, collecting sludge is difficult, because of which the moisture content of the wastewater itself is measured for the sake of convenience, as a result of which the moisture content is taken to be 100%. 
         [0079]    As shown in  FIG. 17 , the sludge moisture content is high at 99.5% both when using polyaluminum chloride (PAC) as a coagulating agent (test example 1) and when using ferric polysulfate (test example 2). As opposed to this, according to magnetic separation, the moisture content is 89% both in the case of magnetic powder alone in test example 3, and in the case of using magnetic powder and ferric polysulfate concurrently in test example 4. When converting from the moisture content, it is seen that sludge derived from magnetic separation has a volume in the region of one-two hundredth compared with sludge collected using a coagulating agent alone. Also, it is seen that sludge collected using a coagulating agent alone has a low density, because of which the sludge is extremely weak against the power of the water flow and the like, and the sludge is stirred up when decanting, or the like. As opposed to this, it is seen that sludge derived from magnetic separation is in a state attracted to a magnet across a bottom of a receptacle, does not become detached even when the receptacle is inverted, and is extremely strong against the power of the water flow and the like. 
         [0080]    Inclusion in this disclosure of any characterization of any product or method of the related art does not imply or admit that such characterization was known in the prior art or that such characterization would have been appreciated by one of ordinary skill in the art at the time a claimed invention was made, even if the product or method itself was known in the prior art at the time of invention of the present disclosure. For example, if a related art document discussed in the foregoing sections of this disclosure constitutes prior art, the inclusion of any characterization of the related art document does not imply or admit that such characterization of the related art document was known in the prior art or would have been appreciated by one of ordinary skill in the art at the time a claimed invention was made, especially if the characterization is not disclosed in the related art document itself. 
         [0081]    Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents. 
         [0082]    Reference signs and numerals are as follows: 
         [0083]      1 : Engine 
         [0084]      2 : Scrubber 
         [0085]      2   a : Air inlet 
         [0086]      2   b : Exhaust port 
         [0087]      2   c : Drain port 
         [0088]      2   d : Liquid inlet 
         [0089]      3 ,  13 : Valve 
         [0090]      4 : Measurement tank 
         [0091]      5 : pH measuring device 
         [0092]      6 : Water quality measuring device 
         [0093]      7 ,  7   a ,  7   b : Mixing tank 
         [0094]      7   c : Mixing pipe 
         [0095]      8 : pH regulating device 
         [0096]      9 : Magnetic powder adding device 
         [0097]      10 : Coagulating agent adding device 
         [0098]      11 ,  11   a ,  11   b ,  11   c ,  20 ,  30 ,  40 ,  50 ,  60 ,  70 : Magnetic separation device 
         [0099]      12 : Sludge tank 
         [0100]      14 : Control device 
         [0101]      15 : Agitating device 
         [0102]      16 : Suspended substance 
         [0103]      17 : Magnetic floc 
         [0104]      21 : Channel 
         [0105]      22 : Rotating disk 
         [0106]      23 ,  33 ,  42 : Scraper 
         [0107]      32 ,  41 : Rotating drum 
         [0108]      71 : Sludge storage pipe 
         [0109]      72 : Pipeline induction guide 
         [0110]      73 : Magnetism generating drum 
         [0111]      74 : Electromagnet 
         [0112]    L 1 , L 1   a , L 2 , L 3 , L 4 , L 5 , L 6 , L 7 , L 8 : Pipe 
         [0113]    M: Motor 
         [0114]    T: Wiring