Abstract:
Apparatus and method are provided for separating grit from liquid sewage while retaining organic solids in suspension. A sustained rotational liquid sewage first fluid flow, an induced upward liquid sewage second fluid flow, and a sustained rotational liquid sewage third fluid flow are employed in an apparatus having a cylindrical grit settling main chamber, a grit storage secondary chamber including a central grit settling access top mouth opening, a vertical shaft, a means for causing rotation of the vertical shaft, and a partition extending transversely through the main chamber and forming upper and lower subchambers. A fluid flow speed gradient is established between the liquid sewage third fluid flow and the liquid sewage first fluid flow. In this manner, grit is separated from liquid sewage.

Description:
CROSS-REFERENCE DATA 
     This application claims the conventional priority of U.S. Provisional patent application No. 61/200,560 filed on Dec. 1, 2008. 
     BACKGROUND OF THE INVENTION 
     In sewage treatment plants, heavy mineral matter called “grit,” forms part of the fluids that need to be processed and segregated from other fluid material. Grit is principally made up of sand and soil, but can also contain cinders, coffee grounds, seeds, corn, and other coarse sediments. As grit cannot be treated, reduced in size or eliminated by treatment methods, it needs to be physically removed. Grit presents a problem to wastewater treatment as it is hard and abrasive; it wears pumps and other mechanical devices; it is heavy and accumulates in clarifiers, treatment basins, digesters, et al, where it must often be removed by hand. 
     U.S. Pat. No. 4,767,532 issued in Aug. 30, 1988 to Smith &amp; Loveless inc., discloses a grit selector having an upper settling chamber and a lower grit storage chamber. The settling chamber communicates with the grit storage chamber through an opening in a transition surface there between. An influent flume directs influent liquid directly into a lower portion of the settling chamber. An effluent flume withdraws effluent liquid from an upper portion of the settling chamber. The influent flume and effluent flume have a common centerline with the effluent flume being positioned at an elevation above the influent flume. A baffle member extends into the settling chamber for directing the influent liquid stream outwardly towards a lower portion of the periphery of the settling chamber. Influent fluid forcibly flows into the settling chamber in a tangential fashion, which induces rotational circulation inside the settling chamber. A rotating blade sustains the rotational circulation brought about initially by the incoming tangential fluid flow. Evacuation of sand and other grit material is done mainly under gravity into bottom grit pit, while water escape is performed once again under tangential flow bias. 
     A problem with such prior art grit removal apparatuses relates to design limitations in the orientation and size of the effluent flume liquid flow channel exiting from the apparatus settling chamber, compared to the influent flume liquid sewage flow channel. In particular, design borne flow load limitations require that: 
     1. the inner diameter of the effluent flume flow channel be substantially the same as the inner diameter of the influent flume flow channel; and 
     2. the general orientation and flow direction of the effluent flume flow channel be the same as that of the influent flume flow channel, i.e. no angular deviation (e.g. a right angle deviation) from the flow direction of the influent flume flow channel is allowed relative to the flow direction of the effluent flume flow channel, for the prior art grit removal apparatus to remain operational. 
     SUMMARY OF THE INVENTION 
     The invention relates to an apparatus for separating grit from liquid sewage while retaining organic solids in suspension including inlet means for admitting liquid sewage into the apparatus, outlet means for removing liquid from which grit has been separated from the apparatus, and means for removing separated grit from the apparatus, the apparatus further comprising:—a cylindrical grit settling main chamber defining a bottom end portion, a top end and a peripheral wall;—a grit storage secondary chamber positioned below the main chamber bottom end portion such that grit settling out of the liquid will settle into said secondary chamber, said secondary chamber including a central grit settling access top mouth opening through said main chamber bottom end portion;—a vertical shaft positioned centrally in said main chamber and in said secondary chamber, said shaft having a longitudinal axis;—means for causing rotation of said vertical shaft about said longitudinal axis;—a partition extending transversely through said main chamber intermediate said top end and said bottom end thereof spacedly therefrom wherein an upper subchamber is formed in said main chamber above said partition and a lower subchamber is formed in said main chamber below said partition, said liquid sewage inlet means in direct fluid communication with said lower sub-chamber, said liquid outlet means in direct fluid communication with said upper sub-chamber, said partition having a peripheral edge integrally mounted in substantially fluid tight fashion to said peripheral wall of said main chamber; said partition including a bottom central aperture housing said shaft, said partition bottom central aperture being spaced from said shaft to define an annular opening between said shaft and said partition to provide for upward flow of liquid from said lower subchamber to said upper subchamber; and mechanical means positioned within said main chamber and enabling sustained rotational liquid sewage first fluid flow within said lower subchamber, enabling inducing upward liquid second fluid flow from said lower subchamber through said partition annular opening and into said upper subchamber, and enabling sustaining rotational liquid third fluid flow within said upper subchamber for escape through said outlet means, wherein a fluid flow speed gradient is established between said third fluid flow and said first fluid flow. 
     According to one embodiment, said fluid flow speed gradient is preferably such that said third fluid flow is at substantially smaller speed than said first fluid flow, with said third fluid flow speed being preferably about four times smaller than that of said first fluid flow. 
     According to one embodiment, said mechanical means for causing said second fluid flow and said third fluid flow includes a plurality of vanes fixed to said shaft and rotatable therewith, said vanes located within said lower subchamber; wherein said fluid flow speed gradient enables omnidirectional radial or tangential escape flow of the liquid from which grit has been separated from said upper subchamber through said outlet means, and furthermore accommodates differential fluid flow loads between said inlet means and said outlet means. 
     According to an alternate embodiment, said vanes are located within said upper subchamber. 
     According to one embodiment, said partition is a downwardly convex cone with a diametrally smaller bottom mouth and a diametrally larger top mouth. Preferably, the bottom mouth diameter of said conical partition represents between 40 and 60% of the diameter of said conical partition top mouth, and preferably about 50% thereof. The angular slope of said conical partition could range between 15° and 30°, with optimal value at 20°. 
     The main chamber bottom end portion is preferably funnel shaped with an angular slope substantially matching that of said conical partition, preferably having an angular slope of about 20°. 
     In one embodiment, said inlet means includes an access port made in said lower sub-chamber peripheral wall and opening into said lower subchamber, and a liquid sewage supply channel tangentially projecting from said lower subchamber, said supply channel having an angular slope ranging between 10° and 30° (optimal value being 15° relative to a plane at right angle to said lower subchamber peripheral wall. 
     Alternately, said partition is a flat panel. 
     Alternately, said vanes are circumscribed within said funnel shaped main chamber bottom end portion and mounted to a registering portion of said shaft. 
     The invention also relates to a method for removing grit from liquid sewage while retaining organic solids in suspension including inlet means for admitting liquid sewage into the apparatus, outlet means for removing liquid from which grit has been separated from the apparatus, and means for removing separated grit from the apparatus, the method comprising the following steps:—providing a cylindrical grit settling main chamber defining a bottom end, a top end and a peripheral wall, a grit storage secondary chamber positioned below the main chamber such that grit settling out of the liquid will settle into said secondary chamber, said secondary chamber including a peripheral wall having an upper mouth; a vertical shaft positioned centrally in said main chamber and in said secondary chamber, said shaft having a longitudinal axis;—causing rotation of said vertical shaft about said longitudinal axis;—providing a partition extending transversely through said main chamber intermediate said top end and said bottom end thereof spacedly from said secondary chamber wherein an upper subchamber is formed in said main chamber above said partition and a lower subchamber is formed in said main chamber below said partition, wherein said liquid sewage inlet means is in fluid communication with said lower sub-chamber, and said liquid outlet means is in fluid communication with said upper sub-chamber, said partition having a peripheral edge integrally mounted to said peripheral wall of said main chamber; said partition including a bottom central aperture housing said shaft, said bottom aperture being spaced from said shaft to define an annular opening between said shaft and said partition to provide for upward flow of liquid from said lower subchamber to said upper subchamber;—generating sustained rotational liquid sewage first fluid flow within said lower subchamber;—inducing vertical upward liquid second fluid flow from said lower subchamber through said partition annular opening and into said upper subchamber;—sustaining rotational liquid third fluid flow within said upper subchamber for escape through said outlet means; and—generating a fluid flow speed gradient between said third fluid flow and said first fluid flow. 
     Preferably, the step of generating a fluid flow speed gradient between said third fluid flow and said first fluid flow, is of such a degree that about a 75% decrease in speed of third fluid flow is achieved relative to that of said first fluid flow. 
     Preferably, there is further included the step of radial liquid escape from said upper subchamber through said outlet means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of an embodiment of grit removing apparatus according to the present invention; 
         FIGS. 2-4  are elevational views from three different perspectives of the apparatus of  FIG. 1 ; 
         FIGS. 5-6  are enlarged views of  FIGS. 1 and 3 , respectively, showing further detail; 
       FIGS.  7 A, 7 B and  7 C are views similar to  FIG. 2  but at an enlarged scale and showing three different alternate fluid propeller mountings on the vertical shaft relative to the conical partition; 
         FIGS. 8 and 9  are a top plan view similar to  FIG. 1  and an elevational view similar to  FIG. 3 , but showing an alternate embodiment of the invention where the partition is a flat panel; 
         FIGS. 10 to 13  are views similar to  FIG. 1 , but at a larger scale and showing four different alternate orientations of liquid outlet means channels enabled by the present design of grit removal apparatus; 
         FIG. 14  is an enlarged cut-out view of the central section of  FIG. 6 , showing how the conical partition is fixedly connected to the peripheral wall of the main grit settling chamber; 
         FIG. 15  is a view similar to  FIG. 7A , but at a slightly enlarged scale and further showing an embodiment of the invention where the propeller blades are mounted within the funnel of the settling chamber bottom floor; 
         FIG. 16  is a view similar to  FIG. 14 , but further showing the shaft mounting to the top end of the grit settling chamber; and 
         FIG. 17  is a comparative efficiency graph showing the performance of the present grit removal apparatus relative to increasing grit particle size, compared to prior art apparatuses. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-6  and  14  show a first embodiment of apparatus for separating grit from incoming grit sewage,  100 . Apparatus  100  includes a main cylindrical settling chamber  102 , disposed immediately above and concentric to a lower diametrally smaller secondary cylindrical grit storage chamber  104 . The bottom grit storage chamber  104  is for storing grit removed under centrifugal and gravity forces from the grit sewage fluid having engaged the settling chamber  102 . Chamber  102  defines an upright peripheral wall  106 , a top wall  108  and a bottom wall  110 . A funnel shape flooring  112  is mounted above bottom wall  110 , with the diametrally smaller bottom mouth  112 A of funnel shape flooring  112  registering with and opening into grit storage chamber  104  through a corresponding aperture  110 A in flooring  110 , and with the diametrally larger top mouth  112 B thereof merging with wall  106 . Preferably, the slope of funnel shape flooring  112  ranges between 15° to 30°, and most preferably is about 20° for optimal value. 
     In the preferred embodiment of  FIGS. 1-6  and  14 , a downwardly convex conical partition  114  is mounted into main chamber  102  spacedly above flooring  112  and below main chamber top wall  108 . Conical partition  114  defines a main conical body  116 , with a top annular flange  118  and a bottom mouth  120  circumscribed by a bottom annular rim  122 . Preferably, the diameter of the partition bottom mouth  120  ranges between 40% and 60% of that of top flange  118 , with optimal value being about 50%. 
     Top flange  118  is fixedly connected in substantially fluid tight fashion to upright wall  106 , wherein bottom mouth  120  forms a plane generally orthogonal to the main chamber upright wall  106 . However, for practical purposes, a functional tolerance of a few millimeters between the partition top flange  118  and the main chamber upright wall  106  may be found to be operationally acceptable for mounting purposes. 
     Preferably, the slope of conical body  116  matches that of funnel shape flooring  112 , with an optimal value of about 20°. A greater angular conicity of the conical partition  114 , for example of between 30° to 45°, could theoretically be effective, however that would create substantial increase in grit removal device size and thus in fixed costs, that would reduce or eliminate the cost-savings associated with the improved grit removal capability. 
     Accordingly, an upper subchamber  102 A is formed between the conical body  116  of partition  114  and the top wall  108  of main chamber  102 , and a lower subchamber  102 B is formed between the conical body  116  of partition  114  and the funnel shaped flooring  112  of chamber  102 , wherein subchambers  102 A and  102 B come in fluid communication only through radially inward bottom mouth  120  of conical partition  114 . Attachment brackets  122  are fixedly provided edgewisely on flange  118  and are anchored to wall  102  by anchor fasteners  124  in substantially fluid tight fashion with elastomeric strips  126  lodged into a peripheral cavity  122 A of brackets  122 . 
     It is thus understood that conical partition  114  is sized and shaped relative to grit settling chamber  102  in such a fashion as to restrict all vortex induced upward flow of partially grit-removed water to a water flow only partition bottom central mouth  120 . Water partially purged from grit is not allowed to flow upwardly between the sealed peripheral edge portion of conical partition  114  and the peripheral inner wall  106  of settling chamber  102 , so that all water flow between sub-chambers  102 A and  102 B occur only through central bottom mouth  120 . 
     A fluid intake port  128  transversely opens through upright wall  106  and into lower subchamber  102 B. A liquid sewage intake channel  130  opens at one end into intake port  128 , for ingress into subchamber  102 B of liquid sewage. Channel  130  tangentially intersects the lower portion of main settling chamber wall  106  so as to cause the incoming influent sewage liquid to flow tangentially into lower subchamber  102 B. A centrifugal force is generated for the sewage fluid engaging inside cylindrical lower subchamber  102 B, which brings about sewage fluid forcibly radially outwardly against the interior wall of chamber  102 B. 
     Channel  130  has at is upstream end a generally horizontal main feeder segment  130 A, connecting with channel  130  via an intermediate downwardly inclined elbowed section  130 B, wherein channel  130  forms a non-orthogonal angular value with wall  106 . Preferably, the angular value of channel section  130  relative to a plane orthogonal to wall  106  ranges between 10° and 30°, and most preferably having an optimal value of 15°. Accordingly, liquid sewage is designed to flow through inlet port  128  and into subchamber  102 B at a substantial flow speed. The diametral size of fluid inlet port  128  is preferably substantially equal to the distance between top flange  118  of conical partition  114  and the top mouth  112 B of funnel shape flooring  112 . 
     A fluid outlet port  132  transversely open through upright wall  106  and into upper subchamber  102 A. A liquid channel  134  transversely opens at one end into fluid outlet port  132 , for outflow escape of liquid separated from grit from upper subchamber  102 A and into channel  134 . As suggested in  FIG. 6 , the inner diameter of liquid outflow channel  134  may be substantially larger than that of fluid intake channel  130  and may remain in the same general direction than the latter in this operational design. 
     Alternately, as suggested by the embodiment of  FIG. 10 , liquid outflow channel  134 ′ of grit removal apparatus  100 ′ may operationally become reoriented by 180° relative to the direction of intake channel  130 . Moreover, as also illustrated in  FIG. 10 , the liquid outflow channel  134 ′ need not escape tangentially from wall  106 ′, as with the previous embodiment, but may escape radially therefrom and for example in parallel counterflow fashion to channel  130  while grit removing apparatus  100 ′ remains fully operational. 
     Still alternately, as suggested by the embodiment of  FIG. 11 , channels  130 ″,  134 ″ of grit removal apparatus  100 ″ may be coaxial. The alternate operational embodiment of  FIG. 12  is similar to  FIG. 10 , except that the channels  130 ′″,  134 ′″ of grit removal apparatus  100 ′″ have substantially same inner diameter. The alternate embodiment of grit removal apparatus  100 ″″ of  FIG. 13  shows a operational design where the liquid outlet channel  134 ″″ escapes tangentially from main chamber wall  106 ″″ at right angle relative to the direction of sewage intake channel  130 ″″. 
     A hollow shaft  140  is mounted in upright condition within main chamber  102 , defining a top end portion  140 A journalled into top wall  108  through an aperture  108 A, and sized so that its bottom end mouth  108 B open freely into grit storage chamber  104  in such a way as to be able to reach most of the grit material sedimentation therein. Shaft  140  extends freely through mouths  118  and  120  of conical partition  114 . A motor  142  carried over wall  108  is operatively connected to shaft  140  and drives same into rotation. The gear box of the shaft motor  142  will preferably be manufactured from a heavy bearing support plate and structural members. It shall be designed so that the gears and bearings be easily grease lubricated. The lower portion of the case could be closed with an anti-splash plate. The gear case could include a pinion mounted directly on the gear motor&#39;s output shaft and riding on for example a 495 mm pitch diameter slewing ring having external gearing. Preferably, the motor  142  is of the constant speed type, but could alternately be of the variable speed type. 
     A fluid pump  144  is also carried by top wall  108  adjacent motor  142 , and is operatively connected to hollow shaft  140  and generates negative pressure therein for upwardly pulling grit material from grit storage chamber  104  through the hollow of shaft  140  and outwardly at the top mouth of shaft  140  to a channel  146  leading to an external refuse collector. Operation of fluid pump  144  may be cyclical, for example 15 minutes each hour. 
     A multibladed propeller  150  having a number of peripherally mounted blades  152  is transversely fixedly mounted onto shaft  140  for rotation about a vertical axis centered in settling chamber  102 . In the preferred embodiment of  FIG. 6 , propeller  150  is mounted into lower subchamber  102 B, above funnel shape flooring top mouth  112 B and below the bottom mouth  120  of conical partition  114 , in transverse register with the fluid inlet port  128 , wherein the liquid sewage flow from channel  130  is directed tangentially toward the propeller blades  152 . Preferably, propeller  150  is sized so that it diametrally matches the diameter of conical partition bottom mouth  120 . The size of the partition mouths  118  and  120  should be such as to allow manual access to propeller  150  by removal of top wall  108  of main chamber  102 . The blades  152  are mounted in slightly tilted fashion, for example by about 30° relative to the horizontal plane. 
     The preferred embodiment of grit removal apparatus  100  shown in  FIG. 6  operatively enables the various angular tangential or radial mountings of the liquid outlet channel  134 , in view in particular of the location of the propeller  150  being located in the lower subchamber  102 B. The propeller  150  thus induces a turbine effect in the lower subchamber  102 B, generating a rising central vortex (along arrows R 1  in  FIG. 15 ). In a rising vortex, the liquid part of the fluid rises along arrows R 2  in  FIG. 15 ) but the coarse solids slide toward the bottom along the downwardly inwardly inclined slope of the funnel shape flooring  112  toward the grit storage chamber  104 . The tangential speed of the blades  152  of rotating propeller  150  should preferably be the same as that of the liquid sewage flow coming from the inlet channel  130 , for example by about one meter per second flow speed and 1.2 cubic meter per second flow volume. Alternately, the propeller  150  may rotate at a greater speed than that of the sewage flow from the inlet channel  130 , for example up to several times the sewage flow speed from inlet channel  130 , while still remaining at least partially effective to enhance the rising vortex motion of not only the liquid part but also the organic solids having a lower density than sand (e.g., corn particles). Coarse particles may rotate for example 5 to 6 times or more in the lower sub-chamber  102 B, before escaping upwardly through the conical partition mouths  118  and  120  toward and into the upper subchamber  102 A, (arrows R 3  in  FIG. 15 ) and one important function of the propeller  150  is to provide optimization of this rising vortex fluid motion. The direction of rotation of propeller  150  should be in the same direction as the sewage liquid flow direction. 
       FIGS. 7A ,  7 B and  7 C show alternate mountings for propeller  150 . 
     In the embodiment of  FIG. 7A , propeller  150 ′ is mounted within upper subchamber  102 A, above conical partition  114  and below top wall  108 . Propeller  150 ′ includes rocker mountings  151  for each of the blades  152 ′, with said rocker mountings  151  enabling partial radially outward tilting of the blades  152 ′ from a stationary downwardly extending condition (as illustrated) to a partly radially outwardly extended operative condition, for example by up to 60° from the horizontal plane. The purpose of such blade rocker mountings  151  is to mitigate drag inertia at the start of the operating cycle, and accordingly, such blade rocker mountings  151  can operate only in an environment corresponding to the upper subchamber  102 A. In this embodiment of  FIG. 7A , the speed gradient between the upper subchamber  102 A and the lower subchamber  102 B is substantially smaller than with the embodiment of  FIG. 6  where the propeller is mounted within the lower subchamber  102 B. The embodiment of grit removal apparatuses of  FIGS. 10-13  are therefore not suitable for use with the propeller mounting of  FIG. 7A . 
     In the second embodiment of  FIG. 7B , propeller  150 ″ is again mounted into upper subchamber  102 A, with similar limitations as with  FIG. 7A , but now substantially coplanar to the top flange  118  of conical partition  114 . 
     In the third embodiment of  FIG. 7C , propeller  150 ′″ is mounted into lower subchamber  102 B, but now substantially with the bottom mouth  120  of conical partition  114 . Limitations as to speed gradients are similar to those of  FIG. 7A . 
     As suggested by computer generated fluid dynamic simulation graph illustrated in  FIG. 17  of the drawings, it has been found that improved efficiency—of the order of 10 to 15%—in grit removal capability relative to prior art grit removal apparatuses, can be obtained with such a grit removal apparatus of the present invention, in particular with the embodiment having a downwardly conical partition  114  and a propeller  150  mounted intermediately into the lower subchamber  102 B. The efficiency level relates to the difference in grit content in the influent channel, as compared to that in the effluent channel. 
     Alternately, and as illustrated in  FIGS. 8-9  of the drawings, the partition  114 ′ could be planar, instead of conical, but at a cost of added structural construction difficulty but still unexpected improvement of efficiency compared to prior art, namely, of about 10 to 15% improved efficiency relative to prior art grit removal devices. When the partition is conical,  114 , a substantial unexpected 10 to 15% improvement in efficiency is achieved compared to prior art grit removal apparatuses. An important consideration here is to have a new partition mounted into a grit removal device settling chamber that separates the main settling chamber  102  into two sub-chambers  102 A and  102 B: a lower sub-chamber  102 B, into which the water and grit sewage influent engages; and an upper sub-chamber  102 A, from which escapes the partially grit-removed water, wherein substantially all water flow from the lower sub-chamber to the upper sub-chamber is enabled through the central mouths  118   120 , only of the partition  114 . 
     It has been found that unexpectedly, a fluid flow speed gradient is established between the liquid flow inside the upper subchamber  102 A and the liquid flow inside the lower subchamber  102 B. In particular, when the propeller  150  is located within the lower subchamber  102 B, optimal results are achieved wherein the fluid flow speed gradient enables omnidirectional radial or tangential escape flow of the liquid from the upper subchamber  102 A through the outlet port  132 , and furthermore accommodates differential fluid flow loads between the inlet channel  1309  and outlet channel  134 . For optimal values, the fluid flow speed gradient is such that the fluid flow speed inside the upper subchamber  102 A (arrows R 3  in  FIG. 15 ) is about four times smaller than that of the fluid flow speed inside the lower subchamber  102 B (arrows R 1  in  FIG. 15 ). It is further noted that this speed gradient promotes final gravity-borne sedimentation of sand particles which may have accidentally escaped into upper subchamber  102 A, through the rising vortex and through the partition central mouths  118 ,  120 , thus still further enhancing the grit separation effect sought with the present apparatus  100 . 
     It is also noted that the present apparatus  100  easily accommodates up to 25% increase in sewage fluid flow speed relative to constant speed of propeller  150 , without significant decrease in grit removal operational efficiency or without significant backflow. The present apparatus has high adaptability to accidental fluctuations in fluid flow parameters or liquid outflow configurations. 
     Another improvement over prior art grit removal apparatuses relates to fluid level controls inside the main grit settling chamber  102 . In the prior art apparatus, such control was critical in view of avoiding substantial decrease in effectiveness. However, in the present invention apparatus, fluid level control in the main grit settling chamber  102  is far less important. 
     The present grit removal apparatus should be able to provide the following performance: 
     a) removal of at least 95% of particulate grit equal to or greater than 300 micrometers in size; 
     b) removal of at least 85% of particulate grit equal to or greater than 210 micrometers in size; and most importantly, 
     c) removal of at least 65% of particulate grit equal to or greater than 150 micrometers in size. 
     The present grit removal apparatus is particularly well suited for wastewater treatment plants, but is not limited thereto.