Abstract:
An assembly for removing grit from a fluid comprising a tank with a fluid inlet into the tank and a discharge from the tank. A flow directing assembly is positioned in the tank adjacent the fluid inlet and includes a substantially U-shaped surface opposite the fluid inlet such that fluid entering the tank through the inlet is caused to have a reversed flow in a direction more than 90 degrees from an inlet flow direction. At least one embodiment of the assembly further comprises a baffle positioned proximate the tank discharge to control flow adjacent the discharge. Yet a further embodiment of the assembly includes a screw conveyor positioned in a trough of the tank and a baffle is positioned proximate the trough to minimize fluid flow velocity adjacent to the screw conveyor.

Description:
BACKGROUND  
       [0001]     All wastewater treatment facilities generally utilize grit handling and removal equipment to isolate and/or remove the harmful coarse solids contained in the waste stream flow and eliminate them from downstream processes. Utilizing this equipment also aids in reducing maintenance costs and grit related operational difficulties. This includes protecting sludge pumps, piping, centrifuges, etc., from the scouring action of grit, and preventing grit from reducing a plant&#39;s overall efficiency due to clogged sumps and pipes in addition to build-ups in fluid channels, settling basins, flocculation tanks and digestion tanks.  
         [0002]     Grit contained in grit slurries, whether removed by mechanical means such as bucket elevators, airlifts, water eductors or pumps must be dewatered and washed to produce a product which is inoffensive and can be easily handled and removed from the facility. Two pieces of equipment were commonly used in plants for this purpose, grit washers and grit separators or classifiers.  
         [0003]     SW grit washer units dewater and wash grit particles to a low putrescible content, primarily free of bacteria, fungi, foul smelling odors. These units were traditionally offered in various sizes related to maximum flow rate the units could handle and still provide effective solids separation and removal. The sizes were developed to handle the commonly captured grit size of 65 mesh, approximately 235 microns, or greater and utilized tank surface area and settling rate (related to tank detention time) methodology for sizing specific projects.  
         [0004]     A second design called a SW-C grit washer allowed much higher flow rates to the washer unit by using a first stage cyclone grit separator to deliver the grit slurry to the washer. The SW tank sizes (5 GPM TO 40 GPM) could then remain smaller but the overall system could handle the higher flow rates. The overflow from the cyclone separation process is typically returned to the upstream process flow. Since the underflow from the cyclone is at a significantly lower flow rate of approximately 5-10% and has significantly higher grit content per gallon of flow, the tanks typically require additional make-up water to be added to enable proper washing.  
         [0005]     Grit classifier units were designed to handle low to moderate solids loads in a slurry, which is being de-slimed, dewatered or washed. The surface area required to effect the desired separation determines the size of the tank. As slurry is discharged into the tank, the flow contained solids settle out of suspension and the clarified overflow passes over an adjustable weir into an effluent box. Again, tank sizes were originally based on handling grit sizes of 65 mesh or greater.  
         [0006]     In both grit washers and grit classifiers, a screw is utilized to convey solids to a discharge point, and to agitate the fluidized bed of materials, releasing entrapped slimes for removal. Sizing of the screws and speeds utilized are dependent upon expected volumes of grit and capacity output as required by the application. Variable speed drive units are commonly utilized. Both designs targeted removing 95% of the grit contained within the slurry. Traditionally, these units only run about 15-20 minutes every 3 to 4 hours, with timing based upon how often the upstream process equipment flowing into the unit was set or required to run.  
         [0007]     More recently, wastewater plants are requiring that smaller grit sizes of 100 mesh or even 150 mesh (approximately 149 to 96 microns, respectively) be removed from the influent waste stream, especially with Combined Sewer and Overflow (CSO) plants and combined systems becoming more prevalent. Plants have also dramatically cut back on the maintenance staffs, so the need to reduce maintenance efforts is increased. CSO systems deliver significantly higher flow at more common intervals and for longer periods of time. The very small grit particles such as 100 or 150 mesh are much more difficult to remove as they tend to stay suspended and do not settle out unless very large tank liquid volumes are utilized to allow enough detention time within the tank for separation to occur. Also over the last 10-15 years, Vortex grit separation has now moved into becoming the more commonly utilized first stage method to handle larger flows and smaller grit particles and is run generally on a continuous basis during storm events. What this has also done is to begin replacing the previously popular and commonly used grit cyclones as the first stage separator of the grit slurry. The Vortex systems typically preferred in CSO applications, do not have many of the problems associated with cyclones, such as routine plugging and rapid wear of the underflow (apex) end of the cyclone.  
         [0008]     Without the use of cyclones, the inlet feed rates to the washers and classifiers can commonly become 200-400 GPM. In order to keep overall equipment (tank and screw) size within reasonable limits and to be able to handle the 100-150 mesh particles, a new design for both the classifiers and washers is desired.  
       SUMMARY  
       [0009]     The present invention provides an assembly for removing grit from a fluid. The assembly comprises a tank with a fluid inlet into the tank and a discharge from the tank. A flow directing assembly is positioned in the tank adjacent the fluid inlet. The flow directing assembly includes a substantially U-shaped surface opposite the fluid inlet such that fluid entering the tank through the inlet is caused to have a reversed flow in a direction more than 90 degrees from an inlet flow direction. At least one embodiment of the assembly further comprises a baffle positioned proximate the tank discharge to control flow adjacent the discharge. Yet a further embodiment of the assembly includes a screw conveyor positioned in a trough of the tank and a baffle is positioned proximate the trough to minimize fluid flow velocity adjacent to the screw conveyor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIGS. 1-3  are side, top and rear, respectively, plan views of an illustrative grit washer tank assembly useable with the present invention;  
         [0011]      FIGS. 4 and 5  are side and top, respectively, plan views of an illustrative grit classifier tank assembly useable with the present invention;  
         [0012]      FIGS. 6 and 7  are top and side, respectively, plan views of an a first embodiment of the present invention utilized with a grit washer tank assembly;  
         [0013]      FIG. 8  is a front plan view of a plate that is part of the present embodiment of the present invention;  
         [0014]      FIGS. 9 and 10  are surface and end, respectively, plan views of another plate of the present embodiment of the present invention;  
         [0015]      FIGS. 11 and 12  are surface and end, respectively, plan views of another plate of the present embodiment of the present invention;  
         [0016]      FIGS. 13-15  are end and opposite surface, respectively, plan views of another plate of the present embodiment of the present invention;  
         [0017]      FIGS. 16 and 17  are surface and end, respectively, plan views of a discharge baffle of the present embodiment of the present invention;  
         [0018]      FIG. 18  is an end plan view of the flow directing assembly of the present embodiment of the present invention;  
         [0019]      FIG. 19  is a top plan view of a screw conveyor baffle of the present embodiment of the present invention;  
         [0020]      FIGS. 20 and 21  are top and side, respectively, plan views of an a second embodiment of the present invention utilized with a grit classifier tank assembly. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The present invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Certain terminology, for example, “top”, “bottom”, “right”, “left”, “front”, “frontward”, “forward”, “back”, “rear” and “rearward”, is used in the following description for relative descriptive clarity only and is not intended to be limiting.  
         [0022]     Referring to  FIGS. 1-3 , an illustrative grit washer tank assembly  10  useable with the present invention is shown. The grit washer tank assembly  10  includes inwardly sloped side walls  14 ,  16  and an inwardly sloped rear wall  18 . The side walls  14 ,  16  and the rear wall  18  are joined by a trough  20  extending from the rear wall  18  along the length of and beyond the side walls  14 ,  16 . The trough  20  is configured to support a screw conveyor  22  therein. The screw conveyor  22  is driven by a motor  24  or other drive to move material along the trough  20  and out a discharge  26  near the forward end of the trough  20 . The discharge  26  is aligned with a disposal mechanism (not shown), for example, a bin or a conveyor, to remove and dispose of the discharged material.  
         [0023]     The grit washer tank assembly  10  includes a cover  19  extending over the tank  12 . A fluid inlet pipe  30  extends through the cover  19  into the tank  12 . Fluid to be washed flows through the inlet pipe  30  and enters the tank  12  at the pipe outlet  32 . The inlet pipe  30  preferably extends sufficiently in to the tank  12  such that the pipe outlet  32  is below the water level WL in the tank  12 . Discharge pipes  40  and  42  are provided on opposite sides of the tank  12  to discharge fluid that has been washed. A weir plate  44  is preferably positioned adjacent the discharge pipes  40 ,  42  to prevent suspended particles from flowing to the discharge pipes  40 ,  42 .  
         [0024]     Referring to  FIGS. 4 and 5 , an illustrative grit classifier tank assembly  50  useable with the present invention is shown. The grit classifier tank assembly  50  includes a tank  12  defined by walls  14 ,  16  and  18  and trough  20 . A motor  24  driven screw conveyor  22  extends along the trough and moves material to a discharge  26 . As with the washer assembly  10 , an inlet pipe  30  extends through the cover  19  and discharges fluid through a pipe outlet  32  positioned below the water level in the tank  12 . In distinction from the washer assembly  10 , the grit classifier tank assembly  50  includes a discharge pipe  56  extending from a discharge box  54  adjacent the rear wall of the tank  18 . A weir plate  52  extends adjacent the rear wall  18  to control flow into the discharge box  54 .  
         [0025]     The present invention includes a fluid directing assembly and can be utilized with both the grit washer tank assembly  10  and the grit classifier tank assembly  50 . A first embodiment of the present invention utilized in conjunction with a grit washer tank assembly  10  will be described with reference to  FIGS. 6-19 .  
         [0026]     Referring to  FIGS. 6 and 7 , the present invention includes a flow directing assembly  60  positioned adjacent the inlet pipe outlet  32  and a screw conveyor baffle plate  90 . As described in conjunction with  FIGS. 1-3 , a weir plate  44  is positioned adjacent to the discharge pipes  40  and  42  and the inlet pipe outlet  32  is below the water level WL. As can be seen in  FIGS. 6 and 7 , the inlet pipe  30  is approximately centered, front to rear. Such positioning is forward the typical inlet position of prior art washer assemblies, and has been found to be more effective at providing a controlled flow within the tank  12 .  
         [0027]     The preferred flow directing assembly  60  includes a plurality of plates positioned within the tank to direct flow of the fluid within the tank  12 . As will be described, several of the plates are interconnected, and may be formed as individual components connected to one another, for example, via bolting, welding or the like, or may be formed from a single sheet bent or otherwise contoured to define the various plates, or a combination of both.  
         [0028]     A plate  62 , illustrated in  FIG. 8 , extends between the side walls  14  and  16  and provides a solid surface substantially parallel to the inlet pipe  30 . Plate  62  extends from above the pipe outlet  32  to below the pipe outlet  32 , thereby preventing direct horizontal, rearward flow from the pipe outlet  32 .  
         [0029]     A plate  64  is positioned adjacent the pipe outlet  32  and is illustrated in  FIGS. 9 and 10 . The plate  64  includes first and second portions  66  and  68 , each having a solid surface. The first portion  66  is configured to extend between the side walls  14  and  16 , substantially parallel to the inlet pipe  30  and opposite of plate  62 . First portion  66  preferably extends from above the water line WL to below the pipe outlet  32 , thereby deterring direct fluid flow toward the front of the tank  12 . The second portion  68  extends from the first portion  66  at an angle thereto. The second portion  68  is preferably angled such that portion  68  is angled, rather than parallel, to the pipe outlet  32 .  
         [0030]     The opposite end of the second portion  68  is connected to an angled portion  72  of a rearward extending plate  70 . The rearward extending plate  70  is illustrated in  FIGS. 11 and 12 . The rearward extending plate  70  includes the angled portion  72  and a horizontal portion  74  extending therefrom. The horizontal portion  74  extends substantially parallel to and below the water level WL to define a rearward path for the fluid flow. As shown in  FIG. 11 , the angled portion  72  includes an the upper solid portion  73  and a lower portion  75 . The interconnection line of plate portion  68  is shown in phantom in  FIG. 11 . As can be seen from  FIGS. 7 and 11 , the portion  73  of angled portion  72  above the interconnection with plate portion  68  is solid.  
         [0031]     As such, portions  66 ,  68 ,  73  and  74 , which are all solid as illustrated by the thick solid line in  FIG. 18 , define a substantially U-shaped solid surface into which flow from the pipe outlet  32  is directed. Flow into the U-shaped solid surface causes the fluid to be rapidly redirected upward and outward, causing a rolling action in the fluid, with the majority of the fluid being forced to flow toward the rear, larger portion of the tank  12 .  
         [0032]     The inlet pipe  30  is preferably sized to provide an inlet flow velocity into the U-shaped flow directing area within a desired range, for example, between 2.5 and 3 ft/sec. The inlet velocity range is selected such that the flow is sufficiently slow to allow a short period of fluid retention in the flow directing area to allow separation to begin, but fast enough to prevent substantial material settling inside the solid U-shaped flow directing area. While material will generally not settle in the U-shaped flow directing area during operation, some material may settle, for example, during shutdown. As such, clean out pipes  46  preferably extend in to the tank  12  within the U-shaped flow directing area, as shown in  FIGS. 6 and 7 . The clean out pipes  46  are capped or otherwise plugged during normal operation.  
         [0033]     Plate  76 , illustrated in  FIGS. 13-15 , extends from rearward plate  70 , with an upper solid portion  78  and a downward extending portion  80 . The solid upper portion  78  helps to maintain the rolling flow in the upper portion of the tank  12  for a longer period.  
         [0034]     Referring to  FIGS. 11 and 15 , the lower portion  75  of angled plate portion  72  and the downward extending portion  80  of plate  76  include a plurality of through holes  65 ,  67 , respectively. The through holes  65  and  67  allow fluid to flow toward the discharge pipes  40  and  42 , but in a controlled manner. Furthermore, fluid contact with the plate portions  80  and  75  causes additional separation of any materials that may remain suspended in the fluid. A discharge baffle  82 , illustrated in  FIGS. 16 and 17 , provides two portions  84  and  86  with through holes  69  adjacent to the discharge pipes  40  and  42 . Again, through holes  69  allow controlled flow to the discharge pipes  40 ,  42  while fluid contact with the plate portions  84  and  86  provides additional separation. The configuration of the discharge baffle  82 , including the size of the through holes  69 , is selected to control the flow velocity to the discharge pipes  40 ,  42 . The flow velocity at the discharge pipes  40 ,  42  is preferably greater than the flow velocity in the quiescent zone  95  of the tank  12 , as will be described hereinafter.  
         [0035]     Screw conveyor baffle plate  90  extends below the flow directing assembly  60 , above the screw conveyor trough  20 . Referring to  FIG. 19 , the screw conveyor baffle plate  90  includes a portion with a plurality of through holes  91  and a solid portion  92 . The screw conveyor baffle plate  90  is positioned such that through holes  91  are proximate the plate portions  72  and  80 . As such, the rolling flow in the rear of the tank  12  can flow through the holes  91  toward the discharge pipes  40 ,  42 . The solid portion  92  covers the screw conveyor  22  adjacent to the discharge pipes  40 ,  42 , thereby preventing the turbulent flow proximate the discharge pipes  40 ,  42  from disturbing the material being conveyed by the screw conveyor  22 . In view of the smaller material size, 100 to 150 mesh, it has also been found to be beneficial to reduce the flight clearance of the screw conveyor  22 , for example, to approximately one-quarter inch.  
         [0036]     The forced flow patterns and rolling action caused by the flow directing assembly  60  causes the inlet flow momentum to disperse rapidly. The flow directing assembly  60 , the plates with through holes  65 ,  67 ,  69 ,  91  and the baffle plate  90 , positioned properly in the tank provides a quiescent zone  95  with flow velocities within established target ranges which allows settling of finer mesh materials to take place in the bottom rear of the tank. Additionally, the target velocities allow the screw conveyor  22  to remove the settled grit particles without disruption caused by high velocity or turbulence in the screw conveyor removal zone.  
         [0037]     For example, the target velocity for removing 100 mesh particles is 0.35 ft/sec while the target velocity for removing 150 mesh particles is 0.25 ft/sec. The flow directing assembly  60 , the plates with through holes  65 ,  67 ,  69 ,  91  and the baffle plate  90  allow the target velocities to be achieved to allow desired separation without the washer assembly  10  being dependent upon large surface areas to diffuse the inlet velocity. The flow directing assembly  60  creates the desired flow pattern with the desired flow velocities. A tank  12  with a sufficient depth will provide the appropriate quiescent zone  95  to allow separation at the slower flow velocities. For example, a tank  12  including a flow directing assembly  60  and having a rear depth of 48 inches has been found sufficient to separate 100 mesh material at 200 GPM. The flow directing assembly  60  and baffles allow separation to be achieved while utilizing smaller tank volumes. For example, a tank including a flow directing assembly  60  and a volume of 160 gallons has been found sufficient to separate 100 mesh material at 200 GPM. The depths and volumes are for example purposes only and may be adjusted based on numerous variables, including particle size and flow rates.  
         [0038]     Referring to  FIGS. 20 and 21 , an embodiment of the present invention in use with a grit classifier tank assembly  50  will be described. The present invention includes a flow directing assembly  100  positioned adjacent the inlet pipe outlet  32  and a screw conveyor baffle plate  116  positioned above the screw conveyor  22 . As described in conjunction with  FIGS. 4 and 5 , a weir plate  52  is positioned adjacent to the discharge box  54  and the inlet pipe outlet  32  is below the water level WL. As can be seen in  FIGS. 20 and 21 , the inlet pipe  30  is forward of center, front to rear. Such positioning is forward the typical inlet position of prior art classifier assemblies, and has been found to be more effective at providing a controlled flow within the tank  12 .  
         [0039]     The flow directing assembly  100  includes opposed solid plates  102  and  104  extending parallel to and on opposite sides of the inlet pipe  30 . The plates  102  and  104  extend between the side walls  14 ,  16  of the tank  12  and prevent fluid flow directly forward or rearward. An angled plate portion  114  extends from plate  104  and extends opposite from the pipe outlet  32 . As in the previous embodiment, the solid portions  102 ,  104  and  114  define a substantially U-shaped solid surface into which flow from the pipe outlet  32  is directed. Flow into the U-shaped solid surface causes the fluid to be rapidly redirected upward and outward, causing a rolling action in the fluid, with the majority of the fluid being forced to flow toward the rear, larger portion of the tank  12 .  
         [0040]     A rearwardly angled plate portion  108  extends from plate portion  114  and includes a plurality of through holes  109  except for a solid region  111  aligned with the inlet pipe  30 . The solid region  111  defines a part of the U-shaped solid surface and prevents direct, uncontrolled flow out of the flow directing assembly  100 . The flow out of the pipe outlet  32  is rapidly redirected by the solid portions  104 ,  111  and  114 . The through holes  109  allow the redirected, dispersed flow to flow therethrough in a controlled manner with fluid contact with the plate portion  108  surface causing separation of some of the suspended material.  
         [0041]     A solid plate portion  110  extends from the plate portion  108  to further encourage rearward flow of the fluid. A screw conveyor cover baffle portion  116  having through holes  117  extends from plate portion  108  and covers a portion of the screw conveyor trough  20 . The through holes  117  allow controlled circulating flow of fluid in the quiescent zone  95  in the rear lower portion of the tank  12 .  
         [0042]     A solid baffle plate  120  is positioned adjacent the rear tank wall  18 . The solid baffle plate  120  includes a portion  122  parallel to and spaced from the rear wall  18  to define a narrow flow path toward the discharge box  54 . A lower portion of the plate  124  is preferably angled away from the wall  18  to define a larger entry area into the flow path. The inward angle of plate  124  also helps facilitate the rolling action of the fluid in the tank  12 . An opposite end of the plate  126  extends substantially perpendicular to and above the water level WL to minimize the amount of direct flow into the discharge box  54 . The configuration of the plate portions  124 ,  122 ,  126  define an inward concave shape that helps to promote the circular rolling flow within the tank  12 .  
         [0043]     The forced flow patterns and rolling action caused by the flow directing assembly  100  causes the inlet flow momentum to disperse rapidly. The flow directing assembly  100 , the plates with through holes  109 ,  117  and the baffle plate  120 , positioned properly in the tank provides a quiescent zone  95  with flow velocities within established target ranges which allows settling of finer mesh materials to take place in the bottom rear of the tank. The screw conveyor  22  is able to remove the settled grit particles without disruption caused by high velocity or turbulence in the removal zone. The classifier assembly  50  is not dependent upon large surface areas to diffuse the inlet velocity. As with the washer assembly  10 , a tank  12  with a sufficient depth will provide the appropriate quiescent zone  95  to allow separation at the slower flow velocities while utilizing smaller tank volumes.