Abstract:
Underdrain units in a sand medium or dual medium filtration system include efficient air, water and combined air/water cleaning. Air, water or both are delivered in backflow direction from a very low position on the underdrain unit, as well as water from a higher position, so as to effectively remove blocking contaminants on the outside the underdrain unit and in the surrounding filter media.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention concerns underdrain filtration systems, especially sand filters wherein metal underdrain units extend longitudinally at the bottom of the medium. More specifically it is concerned with backflushing of the underdrain units and surrounding medium to remove particulates or cake on the outside of the underdrain unit and in the surrounding filter medium. 
         [0002]    Current stainless steel underdrain products are essentially limited to one of two configurations: (1) an air scour lateral is internal but positioned at the top of the underdrain compartment; or (2) the air laterals are “outside”, separate from the underdrain unit, either a pipe or rectangular tube (such as shown in U.S. Pat. No. 6,090,284). Fine openings that make the underdrain unit “media retaining” are typically formed in separate panel pieces bolted or welded to the outside of the underdrain body, although in some underdrain systems the small openings were slits formed in the folded metal underdrain structure itself. There are currently three products fitting the above description. Two current stainless steel underdrain products can have the air feed from the gullet, from below the underdrain; one cannot. Two of the current stainless steel products do not scour the bottom four inches of filter media in the filter with air. The air scour chamber and ports are located too high in the product to allow full depth scouring. 
         [0003]    The following U.S. patents and publications have some relevance to the invention: U.S. Pat. Nos. 2,710,692, 4,065,391, 4,214,992, 4,322,299, 5,019,259, 5,269,920, 5,328,608, 5,639,384, 6,090,084, 6,325,931, 6,569,328, 6,691,413, 6,797,166, 2002/0096463, 2004/007541, 2008/0245750 and 2014/0021121. 
         [0004]    Prior underdrain units have not exhibited the efficiency and overall effectiveness in cleaning the exterior underdrain units and surrounding filter medium as in the current invention. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention described herein distributes backflow air and water, separately and/or together, in such a way as to clear all perforations of a metal underdrain unit of particulate matter gathered during filtration, and to clean the filter medium outside the units, carrying the particles away. 
         [0006]    One important feature of the new underdrain unit design is improved air/water cleaning at the lowest level of the filter, as compared to other current designs of underdrain units. Air for scouring enters the filter medium at the bottom of the underdrain unit, preferably through air inlet orifices in the floor. Air and water enter the medium together as simultaneous air/water cleaning through air and water orifices, again up through the floor of the unit. A final stage preferably is a water only cleaning stage that completes the removal of particles. 
         [0007]    Another important benefit of the current underdrain configuration is that it allows floor air inlet orifices to be used for water orifices when not used for air. In conventional underdrain systems air and water compartments were completely separate, with the inlet orifice being exclusive for only air or only water. 
         [0008]    The invention also encompasses a preferred method of operation for cleaning the filtration units and surrounding medium in the system as described above. 
         [0009]    First is an air only step, agitating the medium banging the medium grains together to accomplish a scrubbing action detaching the trapped particles from the grains of medium. This makes it easier to flush the detached particles from the medium and to accomplish this using less water. 
         [0010]    Second is a combination air and water step, continuing to agitate the medium but also introducing water at a low “sub-fluidization” rate. The water rate is too low to suspend grains of medium by using the velocity of the water moving through it but it is high to begin moving the detached particles from the filter medium. The water continues to carry the medium out of the filter bed. 
         [0011]    Third is the water-only step; air scour is terminated. Water flow is gradually increased to a high “fluidization” rate expanding the medium bed and flushing all the detached particles from the bed. This step is accomplished using less water than other systems of the prior art. 
         [0012]    With the system and method of the invention, there are no “dead zones” of air or water for cleaning the filter medium bed, unlike previous and conventional designs. 
         [0013]    It is among the objects of the invention to more efficiently and thoroughly clean particulate material from an underdrain filtration system during backflow cleaning cycles as compared to prior systems, with savings in water and energy costs. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view, partially cut away, showing an underdrain filtration unit according to the invention. 
           [0015]      FIG. 2  is a plan view showing an underdrain unit of the invention. 
           [0016]      FIGS. 3, 4 and 5  are schematic elevation views in cross section showing three different stages of cleaning in a preferred method of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]      FIG. 1  shows in perspective, and somewhat schematically, a portion of an underdrain unit  10  for an underdrain filtration system. As is typical, a granular filter medium, typically sand (sometimes gravel is placed below the sand), surrounds a series of underdrain units  10  arranged in parallel in a basin floor. The illustrated underdrain unit  10  is a folded sheet metal unit typically formed of stainless steel and having a multiplicity of perforations (indicated as slits as an example) at least in its two sides  12 , and they can also be included in the top surface  14 . What is illustrated here is a trapezoidal folded underdrain unit, which is preferred, although the unit could be rectangular if desired. These folded metal underdrain units are shaped essentially as shown in prior U.S. Pat. Nos. 6,090,284, 6,569,328, 6,691,413 and 6,797,166. The perforations for passing filtrate into the interior of the unit, and for holding back filter medium, can be formed in accordance with U.S. Pat. Nos. 6,569,328 and 6,691,413, both owned by the applicant corporation. 
         [0018]    As described above, the underdrain filtration unit  10  is formed with side ducts  16  inside the interior of the trapezoidal filtration unit. These are air and water distribution ducts for cleaning the filter medium. The ducts  16  are secured to the walls of the underdrain plenum by welding, rivets or other suitable efficient securing means, with a top horizontal part  18  of the duct structure preferably having a lip  20  that extends down at a slightly oblique angle to match the angle of the side walls  12  of the structure. A vertical wall  21  extends down from the horizontal plate  18  as shown, with the parts  18 ,  20  and  21  all being an integral folded metal component. Each unit  10  preferably has a floor  22 , and can be adequately and reliably secured to the floor. As illustrated, the unit includes “foot” flanges  24  at each side, for securing to the basin floor. Typically the unit will be secured to the floor in sealed relationship. 
         [0019]    As shown in  FIG. 1 , an air inlet orifice is positioned as shown at  26 , from a duct or conduit below the floor. Other inlet orifices are shown at  28 . The air inlet  26  enters a space  31  defined by the floor, a top cover plate  30  and two walls  32  positioned at longitudinal ends of this space. The entering air (or water, as seen below), is thus directed in outward directions, left and right with respect to the longitudinally extending unit  10 , into the side ducts  16 . The air is distributed through the length of the side ducts, so as to distribute the flow of air generally evenly through the perforations or slits in the walls  12  of the unit at the lower portion of those walls. 
         [0020]    The other two fluid delivery orifices  28  in the floor are for delivery of water. Water enters these orifices between the side ducts  16 , flows upwardly and reaches only the filtrate perforations above the ducts  16 . 
         [0021]      FIG. 2  shows the underdrain filtration unit  10  in plan view. A single unit can be, for example, about 22 feet in length (or a range of about 10-30 feet), and about 11 inches in overall width (or a range of about 8 inches to 14 inches). The width includes the outwardly projecting “feet” or anchoring flanges  24 .  FIG. 2  does not show the side walls or top of the underdrain unit (which can be about 6 inches in height, or a range of about 5 to 8 inches), but shows the duct walls  21  and the walls  32  that define the air inlet atrium space  31 . The drawing indicates air flow patterns from the air inlet manifold or atrium  31  out into the air-distributing side ducts  16 , indicated with arrows  35 . 
         [0022]    In one preferred embodiment, underdrain units  10  having the approximate dimensions mentioned above can have side ducts with approximately two inch width at bottom and about two inches high (height of walls  21 ), leaving about five inches between the walls  21 , defining the width of the air inlet chamber  31  and the water inlet space  36 . These dimensions can vary, keeping the approximate proportions as with the dimensions above. 
         [0023]    The underdrain units are retained down to the floor in a way similar to that of U.S. Pat. No. 6,090,284, by clamp channels  38  secured to the floor, or another suitable connection. The underdrain units  10  are arrayed side by side as shown in  FIG. 2 , and they can be secured in tandem end to end (in sealed relationship) if needed for the size of the filter basin in which they are installed.  FIG. 2  indicates in dashed lines a gullet  40  extending laterally beneath and transverse to the underdrain units. The gullet transports filtrate water out, and also serves to deliver cleaning water into the plenum and, via a pipe  42  ( FIGS. 3-5 ) laid within the plenum, scour air into the plenum. Where the underdrain units bridge the gullet  40  the clamp channel  38  cannot be bolted to the floor but instead is bolted to a lower clamp channel  44  immediately below the clamp channel  38 . For clarity the lower channel  44  is shown at a different location on  FIG. 2 , with a length to extend the width of the gullet  40 . Between adjacent underdrain units  10  the underdrain foot flanges  24  are sandwiched between the upper and lower clamp flanges  38  and  44  with a gasket  45 , sealing the two underdrain units at bottom over the gullet. 
         [0024]      FIGS. 3, 4 and 5  are schematic views in sectional elevation showing an underdrain unit  10  of the invention. The unit  10  in  FIG. 3  lies on a concrete floor surface, but the floor is not shown in these schematic views. 
         [0025]    As explained and shown above, air coming up through the air opening  26  in the sheet metal floor  22  of the unit is distributed via the entry atrium  31  to the side ducts  16  for distribution of this scour air through lower perforations of the unit, perforations within the area where these ducts  16  are positioned. As described above, this air scour is a first step in cleaning the perforations and surrounding filter medium outside the unit  10 . The pressurized air is delivered through a duct or pipe  42  that is laid within the gullet  40  discussed above. The gullet is substantially filled with water. The air under pressure passes through any gullet water above the upper end of the pipe  42  and is distributed up through the ducts  16  to agitate the medium to accomplish a scrubbing action that detaches the trapped particles from the grains of medium. As explained above, once this scrubbing action has occurred, the detached particles can be flushed from the medium using less water than would otherwise be required. 
         [0026]    The next stage or step of cleaning, indicated in  FIG. 4 , involves both air and water being flushed simultaneously up through the perforations of the underdrain unit. Air and water are delivered together, through the central floor in the orifice  26 . Water in the gullet is put under pressure for this step and air is delivered through the conduit or pipe  42  simultaneously, so that air and water are driven out the lower underdrain perforations, from the side ducts  16 . At the same time, the pressurized water is admitted through the other floor inlet orifices  28 , so that water also is flushed through the remaining underdrain perforations, above the ducts  16  and above the cover plate  30  that defines the top of the inlet atrium  30 ,  31 . As characterized above, this air/water flush step continues to agitate the filter medium while introducing water at a low rate, below a fluidization flow rate. The flow of water is sufficient to move the attached particles from the filter medium, and the water continues to carry the medium out of the filter bed. 
         [0027]    In  FIG. 5 , the third stage of cleaning pursuant to the invention, only water is delivered through all floor inlet orifices  26  and  28 . The pipe  42  is not shown but remains in place without any air delivery, i.e. without air scouring. As noted above, the flow of water is gradually increased to a high rate for fluidization of the medium, expanding the medium bed and flushing substantially all the detached particles from the bed. Because of the action of the previous steps, considerably less water is required than in prior underdrain backflow cleaning systems and methods. 
         [0028]    The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.