APPARATUS FOR REMOVING GRIT

Abstract: A grit selector having an upper settling chamber (11) and a lower grit storage chamber (12). The settling chamber (11) communicates with the grit storage chamber (12) through an opening (15) in a transition surface (13) there­ between. An influent flume (41) directs influent liquid directly into a lower portion of the settling chamber (11). An effluent flume (52) withdraws effluent liquid from an upper portion of the settling chamber (11). The influent flume (41) and effluent flume (52) have a common centerline with the effluent flume (52) being positioned at an elevation above the influent flume (41). A baffle member (45) extends into the settling chamber (11) for directing the influent liquid stream outwardly towards a lower portion of the peri­ phery of the settling chamber (11) (Fig. 1).

Description: The present invention relates to an apparatus for separating grit from a liquid stream while retaining organic solids therein, according to the preamble of claim 1. An apparatus of the above mentioned type is known from US-A-4 107 038. According to this known apparatus, the sewage is caused to flow in a toroidal pattern around the periphery of the chamber. Grit falls to the floor of the settling chamber and passes radially to and then through the central opening into a grit collection chamber therebeneath. Organic solids are kept in motion in the toroidal flow pattern or lifted from the settling chamber floor into such flow pattern, and are discharged through an outlet adjacent the upper surface of the liquid in the settling chamber. Toroidal flow is created and maintained by obstructing the tangential flow path of incoming sewage with a baffle which blocks the tangential inlet and has an edge extending above the upper surface of the liquid. A propeller rotating about a vertical axis in the center of the chamber is adjustable so as to keep the contents of the chamber impinging against the baffle as they flow around the tank in the toroidal pattern. From US-A-2 020 617 is known a grit separator having at each side of a basis a concrete pier. I-beams extend over the tank between the piers. Theses beams support a baffle drum within the tank. The drum consists of a sheet metal cylinder, suitably stiffened and open at its top and bottom. It is supported from a series of spokes extending from hub flanges formed upon a tubular shaft. The shaft is supported on a thrust bearing and is guided by a suitable guide bearing so that it will maintain a rigid vertical position and support the weight of the baffle drum. The above-described prior art devices operate on the forced vortex principle. In these devices the head at the periphery of the settling chamber is higher than at the center of the settling chamber. This causes liquid to flow down the wall of the settling chamber to the bottom thereof and across the bottom to the point of lower head at the center thereof. It is this transverse circulatory flow pattern which permits the device to work. The particulate matter in suspension must follow this path to reach the bottom of the settling chamber and be carried to the center of the transition surface to the storage chamber. This takes some time and some of the particulate matter may not travel the full circuit before it is caught in the flow passing out the effluent, which results in a lowering of grit removal efficiency. The influent flume and the effluent flume of the above-described devices are at right angles to each other and in most installations require a 90° bend to get them back in line. As discussed hereinabove, the prior art grit removal devices do not direct all of the sand and grit immediately to the bottom of the settling chamber causing a lower efficiency of separation. Also, the inlet and outlet flumes thereof do not lend themselves to small space requirement. Briefly stated, the present invention is directed to improve upon the above-described grit removal devices by causing the sand and grit in the influent stream to come in contact with the bottom of the settling chamber and confine their movement to a plane parallel to the bottom of the chamber and in close proximity to the transition surface. Also the influent and effluent flumes are arranged so that they have the same centerlines. The afore-mentioned object was achieved by the features of the characterizing portion of claim 1. More specifically, in accordance with the present invention the influent stream is directed into a lower portion of the settling chamber along the outer periphery thereof. A baffle member is positioned in a lower portion of the settling chamber for directing the lower portion of the influent stream towards the periphery of the settling chamber and thereby preventing the spreading thereof across the floor of the settling chamber. The influent flume and the effluent are substantially parallel to one another, with the effluent flume being elevated above the influent flume. As will be discussed hereinbelow, the flow patterns within the settling chamber are not a forced vortex and the settling solid particulates substantially avoid the influence of the effluent stream and are not affected by the exit velocity. This results in an increased solids removal efficiency. Description of the Drawing FIG. 1 is a perspective view, partially broken away, illustrating a grit removal device constructed in accordance with the present invention. FIG. 2 is a perspective schematic representation of the settling chamber of the invention showing the flow patterns therein. FIG. 3 is a top plan view of the grit separator as shown in FIG. 1. FIG. 4 is a sectional view taken along line 4-4 in FIG. 3. FIG. 5 is a sectional view taken along line 5-5 in FIG. 3. Description of a Preferred Embodiment The drawing shows an apparatus 10 for separating grit from incoming liquid sewage in accordance with the present invention. A right circular cylindrical settling chamber 11 is disposed immediately above and concentric with a cylindrical grit storage chamber 12 of a lesser diameter. A substantially flat transition surface or floor 13, including a removable plate 14 centered in chamber 12, separates chambers 11 and 12. A circular opening 15 in the center plate 14 permits communication between chambers 11 and 12. The surfaces of apparatus 10 defining chambers 11 and 12 may be made from concrete or steel. A multi-bladed propeller 17 is mounted on a hollow cylindrical shaft 18 for rotation about a vertical axis centered in opening 15, the diameter of shaft 18 being substantially less than that of opening 15. The upper portion of shaft 18 is connected to a conventional variable speed gear box and motor assembly 19 supported on a platform 20 for rotating propeller 17 at various speeds. However, the propeller is preferably shorter in length so as to cause rotation at a higher tip speed. In accordance with a preferred embodiment of the invention, the blades are approximately 81.28 cm (thirty-two inches) in diameter for settling chamber diameters of 213.36 to 822.96 cm (seven to twenty-eight feet). An inlet trough 40 has a downwardly extending influent flume portion 41 to introduce an influent liquid stream directly into a lower portion of settling chamber 11. Flume portion 41 has an outer wall 42 that tangentially intersects the lower portion of settling chamber 11 so as to cause the incoming influent liquid stream to flow tangentially into chamber 11. A baffle member 45 is positioned within chamber 11 and extends upwardly from floor 13. Baffle member 45 extends into chamber 11 in a substantially common vertical plane to an inner wall 44 of flume portion 41 so as to direct the influent stream towards a lower portion of the peripheral wall 22 of chamber 11 and thereby precluding the influent stream from spreading inwardly across floor 13. Baffle member 45 is preferably of a height (H) approximately equal to about twenty-five percent of the height of the liquid flowing through inlet trough 40 at maximum flow (H₁). Baffle member 45 preferably extends into chamber 11 a distance such that the included angle between imaginary lines from the center axis of chamber 11 to the outer end of baffle member 45 and the center axis to the inner end of baffle member 45 is approximately sixty degrees. An outlet trough 50 has an effluent flume portion 52 which extends into an upper portion of settling chamber 11 for removing an effluent liquid stream from an upper portion of chamber 11 adjacent its outer periphery. Influent flume portion 41 and effluent flume portion 52 are generally parallel to one another with the effluent flume 52 being elevated above the influent flume 41. The outer wall 53 of the effluent flume is substantially parallel and in the same vertical plane as outer wall 42 and tangentially intersects the upper portion of settling chamber 11. The inner wall 54 of effluent flume 52 is preferably substantially parallel and in the same vertical plane as the inner wall 44 of influent flume 41. The floor 55 of effluent flume 52 extends into settling chamber 11 such that the outer edge thereof contacts the peripheral wall 22 of chamber 11 at an intermediate elevation between the upper and lower portions of chamber 11. The points of intersection between flumes 41 and 52 and the settling chamber 11 are preferably spaced apart by about 90° in the direction of liquid flow within chamber 11, as shown in FIG. 1. The flow path of the liquid stream within chamber 11 is approximately 450° during its flow path from its entry through the influent flume 41 and its exit through the effluent flume 52. An air lift discharge pipe 47 extends downwardly from shaft 18 towards the bottom of chamber 12. Means such as a turbo pump 49 are provided to lift settled solid particles from the bottom of chamber 12 through pipe 47 for removal in a suitable manner. In accordance with a preferred embodiment of the invention, a helix flight 48 may be provided to rotate with discharge pipe 47 to keep the settled solid particles from compacting on the bottom of chamber 12. The description of the operation of the apparatus 10 in accordance with the invention, which hereinbelow follows, points out, in addition to the method and theory of operation, a disclosure of various optimum relationships which optimize its performance. Water or sewage from which solid grit particles are to be removed is introduced into settling chamber 11 through inlet trough 40 and influent flume portion 41 as an influent stream, as indicated between broken lines 60 in FIG. 2. The flow of the influent stream is directed downwardly as it passes through influent flume portion 41 and thereby enters a lower portion of settling chamber 11. The flow of the influent stream is partially constrained by baffle member 45 which directs the influent stream towards the peripheral wall 22 of chamber 11 and prevents the flow from spreading across floor 13. As the flow of the influent stream moves around the peripheral wall, the width of the stream is narrowed by centrifugal force as it flattens against peripheral wall 22, thereby increasing its velocity as it moves around the chamber 11. As the liquid stream flattens against peripheral wall 22, liquid is drawn in to fill the void. This liquid is drawn from the liquid in the center core area of chamber 11 as is illustrated by broken line 62 in FIG. 2. This streamline moves rapidly downward as it passes around the chamber 11, resulting in a drop in velocity at the upper liquid surface and an increase in velocity at the bottom of the center core area. This is aided by the viscous drag of the entering influent stream. As the influent stream rotates around the settling chamber 11 in the direction indicated by the arrow in FIG. 2, grit and certain organics settle towards the bottom of chamber 11 onto transition surface or floor 13 and grit-free liquid moves upwardly and exits through effluent flume portion 52. As this flow passes out through flume 52, the velocity of the stream along the peripheral wall 22 drops to zero. Accordingly, the hydraulic flow pattern of the influent stream is precluded from being a forced vortex. This drop in velocity increases the flattening of the influent stream to fill this void in velocity and further tends to draw the streamline 62 downwardly towards floor 13. The solid particles in suspension are carried near the floor 13 and as they settle they contact the floor. The settled particles are urged around the floor and radially inwardly where they drop through opening 15 and pass into grit storage chamber 12. The influent liquid entering chamber 11 also moves around the peripheral wall of chamber 11 and is forced to the center by baffle member 45 in a streamline adjacent floor 13, as is illustrated by broken line 63 in FIG. 2. The solid particulates settling in this stream contact the floor 13 and move around chamber 11 and radially to opening 15. Solid particulates in the influent stream settle below the portion of floor 55 extending into chamber 11. The flattening of the influent stream against the peripheral wall of chamber 11 also causes a thin stream of liquid to move down the wall as it passes around the chamber and to move across floor 13 towards the center core area where it moves upwardly to enter the center core flow. The rotating propeller 17 is caused to rotate at a higher speed than the velocity of the liquid rotating in chamber 11 in the same direction as liquid flow. This forces the liquid in the center core to flow upwards around the axis of shaft 18, as shown by broken line 65 in FIG. 2. This increased flow also causes the radial flow along the floor to increase and alters the flow to the streamlines as shown by broken line 64 in FIG. 2. The increased velocity across the floor increases the movement of the heavier particles towards opening 15. The lighter particles and organics are picked up by the increased flow and are carried up with the center core streamline 65 and are subsequently carried out with the effluent stream. As alluded to hereinabove, the prior art grit removal devices operate on the forced vortex principle. In these devices the head at the periphery of the settling chamber is higher than at the center of the settling chamber. This causes liquid to flow down the peripheral wall of the settling chamber to the floor and across the floor to the point of lower head at the center. The solid particulate matter in suspension must follow this path to reach the floor and be carried to the center for direction to the grit storage chamber. This flow pattern takes time and some of the solid particulate matter does not travel the complete circuit before it is caught in the flow passing out the effluent flume. This results in a lowering of the solid particulates removal efficiency. In contrast, the grit removal device of the present invention, as described hereinabove, directs all of the influent flow into a lower portion of the settling chamber where the bulk of the solid particulate matter remains. The flow patterns within the settling chamber are such that the solid particulates do not have to settle as far to avoid the influence of the effluent stream and thus are not affected by the exit velocity thereof. The present invention has an inner core stream 62 which rapidly moves downwardly towards the floor and carries the solid particulates with it towards the floor. The flow pattern within the settling chamber is not a forced vortex. The propeller in the present invention is smaller in diameter and rotates at a higher rate of speed than in the prior art grit removal devices. The purpose of the propeller in accordance with the invention is to pump liquid across the floor to propel the settled solid particulates towards the hole in the center of the floor for transmission thereof to the grit storage chamber. In contrast, the purpose of the propeller in the prior art grit selector devices is to reinforce the roll of the liquid in the settling chamber. The effluent flume in accordance with the invention has its centerline the same as the centerline of the influent flume. This facilitates installation of the device.

Claims: Apparatus for separating grit from a liquid stream while retaining organic solids therein, comprising (a) a cylindrical settling chamber (11); (b) a cylindrical grit storage chamber (12) having a smaller diameter than said settling chamber (11) and disposed immediately below said settling chamber (11); (c) a transition surface (13) including a removable plate (14) centered in said grit storage chamber (12) separating said settling chamber (11) and said storage chamber (12), said removable plate (14) having a center opening (15) therethrough permitting communication between said settling chamber (11) and said storage chamber (12); (d) a multi-bladed propeller (17) mounted within said settling chamber (11) to a substantially vertical shaft (18) extending downwardly through said opening (15) into said grit storage chamber (12); (e) an inlet trough (40) having an influent flume portion (41) and having an outer wall (42) that tangentially intersects said settling chamber (11) tangentially for flowing a tangential stream of liquid sewage into said settling chamber (11); (f) an outlet trough (50) having an effluent flume portion (52) for removing grit-free liquid and organic solids from an upper portion of said settling chamber (11) along the outer periphery thereof, said outlet trough (50) being spaced from said inlet trough (40) in the direction of liquid flow; (g) a baffle member (45) being positioned within the settling chamber (11); characterized in that (h) said transition surface (13) being substantially flat; (i) said inlet trough (40) and said outlet trough (50) being spaced in proximity in the direction of liquid flow; (j) said influent flume portion (41) being downwardly extended to introduce an influent liquid stream directly into a lower portion of the settling chamber (11); (k) the exit of said influent flume portion (41) being positioned in a lower plane than and substantially parallel to said effluent flume portion (52); (l) the influent flume portion (41) and the effluent flume portion (52) ending at the periphery of the settling chamber (11) and being spaced such that the influent liquid stream travels a distance greater than 360° within said settling chamber (11) prior to entering said effluent flume portion (52) as the effluent stream; (m) said baffle means (45) being extended into said settling chamber (11) in a plane substantially common to the plane of an inner wall (44) of said influent flume portion (41). Apparatus of claim 1 wherein the floor (55) of the effluent flume portion (52) being extended to an intermediate elevation of the wall of the settling chamber (11) adjacent a portion of the periphery thereof. Apparatus as defined in claim 1 wherein said baffle means (45) being extended upwardly from said transition surface (13) a distance approximately equal to 25 % of the height (H1) of the liquid flow through said influent flume portion (41) at maximum flow. Apparatus of claim 1 or 3 wherein said baffle means (45) being extended into said settling chamber (11) a distance such that the included angle between imaginary lines from the settling chamber center axis to respective ends thereof is approximately 60°. Apparatus of claim 1 wherein said shaft (18) having a helix flight means (48) at the lower end thereof to keep settled particles from compacting on the bottom of said grit storage chamber (12).

Inventors: WEIS, FRANK G.
Applicants: SMITH & LOVELESS, INC.
Application year: 1988
Main classification: B01D21
IPC codes: B01D21