Abstract:
The enhanced deliquifying of the compacted bed of deep bed thickener/clarifier is disclosed. Such an apparatus comprises a vessel receiving a slurry of liquid and solid particles suspended in the liquid. The vessel defines a free settling zone, a hindered settling zone intermediate, an upper clarification zone and a lower compaction zone. A deliquifying member is provided in the vessel extending up between at least two zones for facilitating the upward flow of free liquid from one of the lower zones and the settling of particulates. A rotating rake shaft with pickets are so designed to release the interstitial liquid trapped in a lower zone. The pickets provide pathways for liquid to release generally vertically. The release liquid travels upward to the deliquifying member and on into the clarification zone. The lower zone, freed of the interstitial water, further collapses becoming more concentrated or dense.

Description:
This application claims the benefit of provisional application Ser. No. 60/147,652 filed Aug. 6, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to clarifiers and thickeners of the deep bed type. This invention also relates to an associated method for separating liquid and solid particles in a suspension of slurry in thickness and clarifiers. 
     Deep bed thickeners and clarifiers (or so-called thickener/clarifier devices) are shown and described, for example, in U.S. Pat. Nos. 5,433,862, 5,549,827 and 5,800,715, the disclosures of which are hereby incorporated by reference herein. 
     The device described in U.S. Pat. No. 5,433,862 comprises a vessel receiving a slurry of liquid and solid particles suspended on the liquid. The vessel defines a hindered settling zone in which the liquid and solids of the slurry tend to separate with the solids settling down into a compaction zone below the hindered settling zone. In the compaction zone, the solids concentration tends to increase, while the liquid phase rises to a clarification zone above the hindered settling zone. An overflow launder adjacent the upper end of the vessel receives the overflow phase of the slurry separated in the vessel. An underflow discharge port toward the bottom of the vessel receives the thickened or underflow phase of the slurry separated in the vessel. One or more so-called separating members are provided in the vessel extending upwardly from a lower end in the hindered settling or compaction zones to an upper end in a clarification zone. Preferably, this member includes cones or other downwardly inclined surfaces that facilitate both the settling of solids down to the compaction zone and the upward flow of free liquid from the hindered settling and/or compaction zone to the clarification zone. 
     The deep bed thickener or clarifier device described in U.S. Pat. No. 5,800,715 is similar to that disclosed in U.S. Pat. No. 5,433,862, but further has a fluid flow barrier between the overflow clarification zone and the other zones of the vessel, and one or more clarifying conduits extending through the barrier. Each such conduit carries a suspended floc bed of particulate solids that filters and further clarifies the liquid flowing up from the hindered settling zone to the clarification zone. This further filtering removes solid particles that would otherwise be carried along the “clarified” liquid to the overflow launder. These clarification conduits thus serve a significantly different purpose than do the separating members and function independently thereof. 
     Deep bed thickener/clarifiers offer certain design and operational advantages over the conventional thickeners and clarifiers while providing clarity of overflow and solids concentration of underflow comparable to that of conventional thickener/clarifiers. Such thickeners and clarifiers are typically of low-profile design (i.e., shorter in height but of much greater diameter than deep bed thickeners), but like deep bed thickeners have generally central feedwells and underflow underflow discharge ports. 
     Conventional low profile thickeners rely on the radial expanse of the horizontal settling zones to effect settling of the slurry throughout the thickener. To achieve this settling action, the influent feed is typically dispersed radially outwardly from the central feedwell and rotating rake arms, having generally plow shaped blades, gather and laterally direct the settled solids along the thickener/clarifier bottom to the central discharge opening in the bottom of the thickener/clarifier tank. These rake blades are located closely adjacent the tank bottom and are designed to move the solids across the tank bottom with minimal disruption in the downward (or settling) flow pattern of the slurry though the hindered settling and compaction zones. Consistent with this purpose, the rakes are rotated at a very slow speed for example, less than 1 revolution per minute. U.S. Pat. Nos. 4,082,671, 4,217,916 and 4,271,027, for example, describe and illsutrate rake assemblies that perform this function of moving settled solids across the tank bottom of conventional low-profile thickeners for discharge at a central underflow outlet. Indeed, some of the rake assemblies are intended to scrape the settled solids from the tank bottom, see U.S. Pat. No. 4,247,400. 
     In contrast, “deep bed” thickener/clarifiers rely on the height of the hindered settling to cause separation of the slurry into its clarified liquid and settled solids phases. Deep bed thickeners/clarifiers further have a deep, high angle conical bottom for directing the settled solids toward the discharge opening. Thus there is no need for rotating rakes to move the solids within the thickener/clarifier to effect removal of the solids. Indeed, rakes would serve little purpose in moving the solid, but rather would add expense and operating complexity. 
     Basically, deep bed thickeners rely on a conical tank bottom having a sidewall extending an angle from horizontal generally greater than the angle of repose of the settled solids to direct the solids to move by gravity alone along the tank bottom to the discharge opening. This eliminates the need for a rotating rake and thereby simplifies the operation and construction of the thickener/clarifier as well as reduces the cost of the thickener/clarifier compared to conventional thickeners/clarifiers. This cost reduction takes the form of the elimination of a capital expenditure for the rake assemblies, as well as the elimination of the associated torque driveheads and motors (often in excess of 1,000,000 foot pounds torque) and high strength bridges spanning the tank top to support the drive. 
     In contrast, conventional low profile thickener/clarifiers have tank bottoms that are flat or inclined at angles generally below the angle of repose of the settled solids and thus require rake assemblies and the associated drive and bridge (or column) structures to remove the settled solids from the thickener/clarifier. 
     SUMMARY OF THE INVENTION 
     The present invention enhances liquid-solids separation in a deep bed thickener/clarifier and thus produces an underflow product or sludge which is thickened relative to that of existing deep bed thickeners/clarifiers. 
     More particularly, the deep bed thickener/clarifier of this invention provides for the release and removal of free or interstitial liquid (typically water) trapped in the settled solids in the hindered settling zone and/or the compaction zone. This release of liquid is effected without increasing the size of the thickener/clarifier, disturbing the settling processes or the naturally occurring movement of the solids along the conical tank bottom. 
     The deep bed thickener/clarifier of the present invention comprises a vessel with an overflow launder and an underflow discharge port. The vessel has an inlet for receiving a slurry of a liquid and solid particles suspended in the liquid. In an upper portion of the vessel, a free settling zone has a low concentration of solid slurry particles, while in a lower portion of the vessel, a compaction zone has a high concentration of solid slurry particles. The overflow launder is attached to the vessel adjacent an upper end thereof for discharge of an overflow phase of the slurry that has been separated in the vessel. The underflow discharge port is adjacent to a bottom thereof for discharge of a thickened, underlow phase of the slurry that has been separated in the vessel. 
     An upper portion of the vessel holds the free settling zone and a lower portion of the vessel holds the compaction zone. A hindered settling zone extends between the free settling and compaction zones, with the hindered settling zone and compaction zone constituting lower settling zones. A deliquifying member is positioned in the vessel extending up from adjacent the lower portion of the vessel to adjacent the upper portion of the vessel for flow of liquid from at least one of the lower zones to the free settling zone. The deliquifying member further serves to isolate this flow of liquid from the flow of slurry down in the lower settling zones. A rake assembly is mounted for rotation in the lower portion of the vessel, with the rake assembly forming channels in the slurry held in the lower portion of the vessel for releasing liquid in at least one of said lower settling zones to flow to the deliquifying member and on to the free settling zone. 
     The method of this invention for operating a deep bed thickener/clarifier comprises, in accordance with the present invention feeding, into a vessel, a slurry of a liquid and solid particles suspended in the liquid and separating the slurry by gravity into different zones having respective degrees of liquid-solids separation including, at an upper portion of the vessel, a free settling zone having a low concentration of solid slurry particles and further including, at a lower portion of the vessel, a compaction zone having a high concentration of solid slurry particles. A hindered settling zone is positioned between these zones. The method also comprises discharging, into an overflow launder attached to the vessel proximately to an upper end thereof, an overflow phase of the slurry that has been separated in the clarification zone and discharging, via an underflow discharge port fixed to the vessel at least proximately to a bottom thereof, a thickened, underflow phase of the slurry that has been separated in the vessel. Liquid is directed from at least one of the lower settling zones to the free settling zone via a flow path which is isolated from the flow of slurry down in the vessel in the lower settling zones. In accordance with his invention, upwardly, extending channels are formed in the slurry held in the lower portion of the vessel to release liquid in at least one of the lower settling zones to flow to the isolated flow path and then to the free settling zone. 
     Other objects of the invention will be in part apparent and in part described and shown in the following description of the drawings and preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
     FIG. 1 illustrates a vertical section of the thickener/clarifier of this invention and its various components including the deliquifying member and a rake assembly; 
     FIG. 2 illustrates a vertical section of a second thickener/clarifier of this invention with the rake shaft passing through the inside of the deliquifying member; and 
     FIG. 3 illustrates the enlarged detail of a rake picket in the slurry for forming a water flow channel in the slurry. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in FIG. 1, a deep bed thickener/clarifier of this invention, generally indicated at  10 , comprises a vessel  11  having an inlet  12  receiving a slurry comprising liquid and solid particles suspended in the liquid. The slurry components are separated in vessel  11  into a relatively light-weight overflow phase (i.e., a primarily clarified liquid) and a relatively heavy underflow phase (i.e., a primarily high solid concentration). Vessel  11  includes a downwardly tapering frusto-conical bottom  14  having an apex (not designated) connected to an underflow discharge port  16  for the discharge of the thickened, underflow phase of the slurry that has been separated in vessel  11 . A variable speed pump and motor  18  communicate via a valve  65  with discharge port  16  for the controlled removal of the underflow from vessel  11 . Alternatively, a remotely actuated valve (not shown) may be provided as a mechanical end use device at discharge port  16  to control the removal of the underflow. 
     The thickener/clarifier  10  further includes an overflow weir or launder  20  for removal of the overflow phase separated from the slurry in vessel  11 . An upper lip (not designated) of launder  20  defines the upper level of the clarified liquid. A cylindrical wall  26  in vessel  11  forms an annular chamber  27  and defines a fluid barrier between the annular chamber and the remainder of the vessel interior. Projecting through a lower wall or panel  28  of annular chamber  27  is one or more elongate conduits  30  having generally vertically extending sidewalls which support fluidized floc beds  32  of the particulate material suspended in the different conduits, as described in greater detail in U.S. Pat. No. 5,800,715. Two such conduits  30  are shown in FIG.  1 . However, other vessel designs may have only one such conduit or more than two conduits. Conduits  30  are shown to be of square or circular shape in section, but may be of any shape in section. 
     To facilitate flocculation in vessel  11 , a settling agent such as a polyelectrolyte may be delivered as an additive to the slurry or as shown in FIG. 1 preferably to the free settling liquid in the vessel. As shown in FIG. 1. a source of settling agent such as a storage tank  36  is in fluid flow communication with the vessel  11 . A fluid flow control device, such as a remotely actuated valve  40 , regulates the dosage of settling aid delivered to the slurry and thus control the extent of floc formation within vessel  11 . 
     The slurry received and held in vessel  11  separates by gravity into a hindered settling zone  42  which is located above a compaction zone  44  and below an interior free settling clarification zone  46  and an overflow clarification zone  47  in annular chamber  27 . Within hindered settling zone  42 , the liquid and solid particles tend to separate with the solid particles tending to flow downwardly to the compaction zone  44  and with the clarified liquid flowing up to clarification zone  47 . Within compaction zone  44  the solids concentration increases to the highest level of concentration at discharge port or outlet  16  at the vessel bottom. 
     As described in U.S. Pat. No. 5,433,862, a deliquifying member  48  extends up within vessel  11  from generally adjacent the compaction zone  44 . Deliquifying member  48  is of generally tubular configuration and receives free liquid released from compaction zone  44  and delivers the liquid to the upper end of the deliquifying member  48  which is preferably positioned at the interior free settling clarification zone  46 . At its upper end, deliquifying member  48  is provided with openings  52  for feeding expressed liquid to the free settling clarification zone  46 . In the thickener/clarifier of FIG. 1, the deliquifying member  48  also serves as part of a rotary drive member or rake shaft as described more fully hereinafter. 
     The vessel comprises an upper portion, indicated generally at  81 , holding at least a portion of the free settling zone  46  and a lower portion, indicated generally at  83 , holding at least a portion of the compaction zone  44 . The hindered settling zone  42  may extend into either the upper or lower portion of the vessel. The compaction zone  44  and hindered settling zone  42  constitute lower settling zones. The deliquifying member  48  in the vessel extends up from the lower portion  83  of the vessel to the upper portion  81  of the vessel for flow of liquid from at least one of the lower settling zones to the free settling zone while isolating the flow of liquid from the flow of slurry down into the lower settling zone. A rake assembly  63  is mounted for rotation as indicated at  60  in the lower portion  83  of the vessel and has a plurality of generally vertical pickets  62 . As shown in FIG. 3, as the rake assembly rotates the pickets  62  travel through the slurry, see arrow  80 , and form temporary channels  76  in the slurry held in the lower portion of the vessel. These channels release free or interstitial liquid in pockets  75  in at least one of the lower settling zones to flow along generally vertical paths  72  to the deliquifying member  48 . From there, the liquid is free to flow on to the free settling zone  46 . The deliquifying member  48  supports and rotates the rake assembly  63 . At its upper end, deliquifying member  48  is operatively connected to a motor  70  via a drivehead  61 . The motor  70  may be of fixed or variable speed, and use any suitable motive power, such as an electric or hydraulic motor or a combustion engine. 
     Accordingly, the deliquifying member  48  facilitates removal of free liquid from at least one of the lower zones to enable solids in the compaction zone to increase to a higher concentration level and to do so more rapidly than would otherwise be possible. The deliquifying member  48  is open to hindered settling zone  42  to facilitate release of free liquid from that zone. To that end. deliquifying member  48  is provided with one or more suitable inclined members such as upwardly tapering conical deliquifying baffles  56  shown, for example, in FIG. 1 as being ganged in a sequence. Each conical deliquifying baffle  56  is open at a lower end  50  and guides released liquid upwardly along an inner surface (not shown) to a respective opening or aperture  54  provided in deliquifying member  48 . Settling solids particles are directed along the outer surfaces of conical deliquifying baffles  56  towards the bottom of vessel  11 . Thus the deliquifying member  48  isolates the upward flow of liquid from the downward flow of the slurry. 
     Deliquifying member  48  provides a conduit for the flow of free liquid thus released from hindered settling zone  42  and compaction zone  44 . This channeling of the released liquid in those zones influences the solids settling rate therein. Deliquifying member  48  is shown to be a tube of circular or square cross-section. However, deliquifying member  48  may be of other shapes as described and shown in U.S. Pat. No. 5,433,862. The word “tubular” is used herein to mean a hollow profile of any suitable cross-sectional shape. 
     In operation, deliquifying member  48  is rotated at a slow rate during the gravity settling process. The consequent movement of the pickets  62  through the thickened slurry or sludge in the lower zones releases otherwise trapped interstitial liquid  71 . This freed liquid  71  generally escapes from compaction zone  44  in laminar flow paths along pickets  62 , as illustrated in FIG.  3 . The overall process of thickening or clarification in apparatus  10  is controlled by adjusting the underflow withdrawal and overflow discharge rates relative to the inlet. Thickener/clarifier  10  as provided with a rotatable rake assembly  63  provides for higher underflow solids concentration, improved overflow clarity, increased rate of influent feed, reduced settling aid usage or any combination of these advantages or results than would be possible in treating slurries in conventional deep bed or low-profile, large diameter thickener/clarifiers. 
     Thickener/clarifier  10  includes a cover plate or upper panel  74  which carries drivehead  61  and motor  70 . An outlet port  75  is provided at the upper end of vessel  11  for discharging the clarified liquid phase which spills over weir or launder  20 . The deliquifying member  48  is rotatably suspended from cover plate or upper panel  74 . 
     FIG. 2 depicts a second embodiment of the thickener/clarifier  110  Which includes certain modifications with respect to the apparatus of FIG.  1 . Like structures in FIGS. 1 and 2 are designated with the same reference numerals. 
     In FIG. 2, the deliquifying member  148 , unlike the deliquifying member  48  in FIG. 1, does not serve to rotate the rake assembly and may be rigidly fixed to cover or upper panel  74 . The deliquifying member  148  is also shown to terminate at an upper end of a lowermost conical deliquifying baffle  56 . 
     Thickener/clarifier  110  includes a rotatable rake  163  having a plurality of generally parallel vertically extending pickets  162 . Rake  163  serves to augment or accelerate the deliquifying of the lower zones. Rake  163  is rotated by a drive shaft  166  which is generally coaxially and longitudinally disposed in deliquifying member  148 . Drive shaft  166  is rotatably journaled in cover plate or upper panel  74  and is operatively connected to drivehead  61  and motor  70 . 
     The upper three conical deliquifying baffles  56  communicate with a lumen or interior (not designated) of fluid flow member  148  via openings or apertures  154  provided in tubular fluid flow member  148 . As discussed above, settling solids particles are directed along the outer surfaces of conical deliquifying baffles  56  towards the bottom of vessel  11 . 
     The embodiment of FIG. 2 also differs from that of FIG. 1 in not incorporating clarification tubes (such as tubes  30 ) and in directing flocculant to the free settling zone at the exterior of the deliquifiing member  148 . 
     Although the invention has been described in terms of its preferred embodiments, those skilled in the an will recognize that numerous modifications and changes may be made while remaining within the scope and spirit of the invention. For example, pickets  62  and  162  of rakes  63  and  163 , respectively extend parallel to one another vertically through compaction zone  44  and into hindered settling zone  42 . Pickets  62  and  162  may be attentively inclined at an angle to the vertical. In addition, conical deliquifying baffles  56  may be greater or fever in number than four. The conical deliquifying baffles located below chamber  27  may be larger. In addition, the pickets may be positioned and be of sufficient length to extend into both the compaction zone  44  and hindered settling zone  42 . The deliquifying member  48 ,  148  may be open only adjacent its lower end to receive liquid from one of the lower zones and adjacent its upper end to discharge liquid conveyed by the member. Alternatively, the member may be open at a plurality of locations as well as for a substantial portion of its length. 
     Accordingly, it is to be understood that the drawings and description herein be offered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.