Method for maximizing uniform effluent flow through a waste water treatment system

Abstract

A screen decanter for decanting liquid from a reservoir, comprising at least one rack comprising screens and baffles forming the sides of a cavity; a frame attached to the screens and baffles and providing a barrier so that liquid cannot pass from outside into the cavity without passing through the screens; a patterned perforated drain pipe inside the cavity and leading to an opening through which liquids may drain out from the cavity. The pattern of the openings counteracts the hydrostatic head within the rack such that flow through the screens is uniform at all depths of immersion in the liquid reservoir. Preferably, the screens have a porosity of about 50 micrometers.

Claims

1. A method for assuring substantially equal rates of flow per unit area through all immersed portions of the decanter screens of a screen decanter, the method comprising the steps of: a) providing a screen decanter having a three-dimensional structure at least partially submersible in a liquid reservoir, including a bottom, sides each of which comprises a decanter screen, and a drain apparatus disposed in an opening in said bottom for draining liquid from within said structure, wherein said drain apparatus includes a plurality of openings; and b) arranging said plurality of openings in a pattern along the vertical length and partial circumference of said drain apparatus to provide substantially equal rates of flow per unit area through all immersed portions of each decanter screen of said screen decanter when immersed to any depth in said liquid reservoir.

2. The method of claim 1, further comprising the step of orienting a circumferential center of said pattern of said openings in a direction normal to the direction of flow of liquid through said screens.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a first isometric view from above of an embodiment of a screen decanter (SBX) in accordance with the present application;

(3) FIG. 2 is a second isometric view in partial cutaway of the screen decanter shown in FIG. 1;

(4) FIGS. 3-9 are elevational views of alternate configurations of perforations in a drain standpipe; and

(5) FIG. 10 is a schematic horizontal cross-sectional view of a portion of a screen decanter showing a currently preferred orientation of a drain standpipe having a pattern of perforations with respect to the direction of effluent flow through a decanter screen.

DETAILED DESCRIPTION OF THE INVENTION

(6) Throughout the following description, specific elements are set forth in order to provide a more thorough understanding of the invention. However, in some embodiments the invention may be practiced without some of these elements. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive. It is to be further noted that the drawings may not be to scale.

(7) Referring now to FIGS. 1 and 2, a preferred embodiment 01 of a screen decanter in accordance with the present application comprises at least one rack 10, with the embodiment shown comprising three racks 10. Each rack 10 comprises first and second screens 12 forming opposite sides of a cavity 14 within which one or more drain standpipes 16 are disposed to draw waste water through screens 12. A pattern of openings along the length of each of the drain standpipes 16 is formed to counteract the range of hydraulic head within the cavity to provide essentially equal flow through the screen and drain standpipe at all depths of immersion of the screen. Exemplary patterns of openings are shown in FIGS. 3-9 and are described in greater detail below.

(8) Screens 12 are preferably mounted as coplanar pairs, two to a side, and repeated on opposite sides of each cavity. The screens on opposite sides of cavity 14 are parallel and in a currently preferred embodiment are eight inches apart, although size and spacing of screens 12 may vary depending on the application. To promote uniform flow of liquid between racks 10, the racks also are preferably eight inches apart. In a preferred embodiment, the screens 12 are ultrafine screens of approximately 50 micrometers porosity.

(9) Baffles 18 are positioned at the ends of the cavity 14 defined by the screens 12. In a preferred embodiment, the baffles 18 are semi-elliptical, with the combination of screens 12 and baffles 18 defining cavity 14 with an elongate base and parallel sides. This shape of screens and baffles smoothes out the flow of waste water in the vicinity of rack 10 in a manner that minimizes turbulent flows in the vicinity of the ends. A frame 20, attached to and supporting the screens 12 and baffles 18, forms the bottom and the remaining sides of the rack 10 and provides a barrier so that liquid passing from outside of the rack 10 into the cavity 14 passes exclusively through the screens 12 during decanting.

(10) In operation, a currently preferred vertical placement of the screen decanter 01 relative to the surface of liquid in a tank in a waste water treatment system is such that the screen decanter is submerged only to the depth necessary to bring liquid approximately to the top of the screens 12 but no further, to prevent unfiltered fluid from spilling over the top of the frame 20 into cavity 14.

(11) Referring to FIGS. 1-2, at least one drain opening 22 is located at the bottom of cavity 14 through which liquids may flow from cavity 14.

(12) At least one drain apparatus 16, currently preferred as a cylindrical standpipe, is located inside the cavity 14 within the rack 10, forming a channel for the flow of liquid from inside the cavity 14 to drain opening 22. In a preferred embodiment, several such drain standpipes 16 comprising tubes of constant diameter are evenly spaced within the rack 10 to promote uniform flow of liquid through each screen 12. In a 20, preferred embodiment, the drain standpipes 16 are positioned on or near a line down the center of the rack 10. Fluid dynamic modeling shows that the combination of curved end baffles 18, parallel coplanar screens 12, and evenly spaced patterned drain standpipes 16, all being properly dimensioned for the chosen operating conditions, provides uniform flow of effluent through screens 12 across the width and height thereof.

(13) Referring to FIGS. 3-10, various patterns of openings 24 (holes 24a, vertical slots 24b, tapering or of variable length horizontal slots 24c, and screening 24d) are shown. The cross-sectional areas of the openings 24 increase with increasing height along the length of the drain pipe 16 to promote uniform flow top to bottom of each screen 12 by counteracting the hydraulic head of filtered effluent within cavity 14.

(14) Referring to FIG. 7, in a presently preferred embodiment, a plurality of circular openings 24a of differing diameter are vertically spaced along the length of each drain standpipe 16, graduated by height to receive decanted fluid, and increasing in diameter, with smaller diameter holes 24a at the bottom and larger diameter holes 24a at the top, promoting uniform flow vertically across each screen 12.

(15) Referring to FIG. 8, in an alternate embodiment, a plurality of circular openings 24a of approximately the same diameter are vertically spaced along the length of each drain standpipe 16, with the cross-sectional area of the perforations 24 increasing vertically with increasing height along the length of the pipe 16 (i.e., with the number and/or diameter openings 24a generally increasing with increasing height along the length of the drain pipe 16). As shown in FIG. 10, in a currently preferred embodiment, the centerline 28 of openings 24a is generally oriented at approximately ninety degrees (normal) to the direction of effluent flow 30 through the screens 12; that is, the openings do not face the screens 12 directly, promoting uniform flow horizontally through each screen 12. The diameter and orientation of the drain standpipes 16 is such that they create minimal turbulence within the rack 10 during decanting.

(16) Referring further to FIGS. 1 and 2, in a preferred embodiment, screen decanter 01 further comprises a deflector plate 26 disposed at the bottom of racks 10 to suppress vertical motion of liquid in a tank below the decanter 01, in effect preventing larger previously-settled BOD particles from moving up in the tank and fouling the screens 12.

(17) In a preferred embodiment, an air plenum 28 is attached to a lower region of each frame 20, each air plenum 28 being supplied from a source of compressed gas and also provided with exit slots 30 so that gas bubbles exiting the air plenum 28 through the exit slots 30 flow along, near, and through the surfaces of the screens 12 of the screen box assembly to scour and clean the screens 12.

(18) While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.