Abstract:
The invention recites a filtration tank including a chamber for holding liquid to be filtered, the filtration tank includes a fluid inlet for providing liquid to the chamber. The filtration tank further includes a fluid outlet and at least one filter block disposed between the inlet and the outlet, wherein the at least one filter block includes a porous cap having an average pore size of up to about 250 microns.

Description:
RELATED APPLICATION DATA 
     This application claims benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Application No. 60/296,331 filed Jun. 6, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to underdrains and gravity filters and, more particularly, to a filter media support system for underdrain blocks. 
     2. Description of Related Art 
     Granular media filtration units are typically used to filter water, wastewater and industrial fluids. Filtration units typically employ underdrain systems to collect filtrate, channel it away from the filter bed, and also to distribute backwash gases and fluids into the filter bed. Several underdrain assemblies for filter bottoms are known in the art. U.S. Pat. No. 4,619,765 to Roberts; U.S. Pat. No. 5,108,627 to Berkible; U.S. Pat. No. 5,328,608 to Bergmann and U.S. Pat. No. 5,489,388 to Brown describe known underdrain systems. A typical filter media bottom comprises a main flume with multiple parallel rows of filter blocks, also known as laterals, arranged perpendicularly to the main flume across the filter bottom. Filter media of varying particle size covers the filter bottom. Frequently a layer of gravel separates the filter media and the filter blocks to prevent the filter media from penetrating apertures in the top of the filter blocks. 
     Fluid to be filtered typically enters the filtration unit from above and flows down through the filter media by gravitational forces. The filtrate then flows into the block underdrains, through the main flume to the outlet. The filter media is typically cleansed at predetermined intervals by backwashing. During the backwash cycle, gas and/or liquid is directed through the main flume to the lateral underdrain blocks and upwardly through the filter media to loosen and remove contaminants collected by the filter media. The gravel support layer above the filter blocks may typically be up to 18 inches in height and contain several layers of varying size gravel. The gravel may be layered according to size in an hour glass configuration in which a fine size gravel layer is sandwiched between layers of progressively larger gravel sizes. The innermost fine size gravel layer prevents the filter media from penetrating the underdrain block, while the coarse size layer prevents plugging of the fine gravel layer. Gravel, however, may be difficult and costly to install and may require the use of deeper filter units. In addition, the hour-glass configuration of the gravel layers may be disturbed during backwash, necessitating restoration of the desired layered configuration. In other prior art arrangements, filter media retaining caps supported on the filter blocks may be used in conjunction with, or as a substitution for, the gravel support layer to block the media from entering the filter blocks. Several filter media retaining caps are known in the art. 
     U.S. Pat. No. 2,716,490 to Barstow discloses a means for securing porous plates to the supporting filter block so that they may be quickly and easily set and held in position to provide an integral porous partition between upper and lower chambers. 
     U.S. Pat. No. 4,882,053 to Ferri discloses a porous filter support plate of the kind used in traveling bridge filters for the support of granular filter media. The support plates are formed of porous, heat-fusible materials, for example, a thermoplastic organic material, joined together by heat fused butt joints and/or reinforced by vertical zones which extend vertically through the plates in which the material has been brought to a molten state and pressed together to form a dense, solid, nonporous mass. 
     U.S. Pat. No. 5,019,259 to Hambley discloses a filter underdrain apparatus comprising plate means and a screen arrangement which may be included to screen the liquid and gas orifices from filter media exterior of the distributor conduits. The screen may comprise perforated grids and may extend across the trough between adjacent distributor conduits. 
     U.S. Pat. No. 5,089,147 to Ross discloses a bed of granular medium such as sand supported on a screen within a filter tank cell. An underdrain structure supports the screen while a hold down grating secures the screen in place from above. The grating is held in place by adjustable hold down means secured to tank walls. 
     U.S. Pat. No. 5,149,427 to Brown discloses a cap for filter underdrain blocks, wherein the cap has a porous body. The cap is installed on a filter block having a plurality of orifices in a top wall of the filter block for channeling fluids to and from an overlying filter media. The cap eliminates the need for a separate gravel support layer to be installed between the fine grain filter media and the underdrain blocks. 
     U.S. Pat. No. 5,269,920 to Brown discloses a cap for underdrains and gravity filters which has a top surface and a bottom surface with a plurality of tapered screen members defining slots in the top surface. The slots provide communication with a filter bed without passage of the fine grain filter media therethrough. 
     International Application WO 97/40907 discloses a system for supporting fluid-treatment media above a lower surface that reduces media clogging and head loss in granular fluid-treatment media systems by providing a layered porous plate. The porous plate can have multiple layers of fine sized and course sized pores. The system for supporting fluid-treatment media is securely anchored to the infrastructure of the underdrain. 
     It is an object of the present invention to provide a cap for underdrain blocks which support fine grain filter media without resulting in an excessive pressure drop. 
     SUMMARY OF THE INVENTION 
     The invention includes a filtration tank having a plurality of underdrain filter blocks positioned beneath a media bed filter. Porous plates, used to prevent flow of media into the underdrain filter blocks, are attached to the top surfaces of the filter blocks by a grid arrangement. The grid arrangement permits the employment of thin porous plates that would otherwise be thought to be too fragile for use with the underdrain filter blocks. The grid arrangement supports the porous plates against shearing forces applied to portions of the porous plates during backwashing and that would otherwise tear the porous plates, particularly in the areas of the porous plates around screws used to secure the plates to the underdrain filter blocks. The use of a combination of a grid arrangement and thin porous plates with underdrain filter blocks successfully prevents shearing or tearing of the plates even during the backwashing operation, and the use of thin porous plates permits the employment of finer pore sizes without clogging of the pores of the thin porous plates or other permeability loss. Thin plates have the advantage of being manufactured with greater control of the pore size to thereby achieve greater media retention properties and permitting use of very fine media such as garnet sand having a media size as small as 150 microns. Use of such fine media improves the efficiency and the effectiveness of the filtration tank in removing contaminants from the liquid flowing through the filtration tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are illustrative of the preferred embodiment of this invention. 
         FIG. 1  is a sectional elevation view of a gravity filter embodying the present invention, including a plurality of filter underdrain blocks; 
         FIG. 2  is a plan view of the gravity filter shown in  FIG. 1 ; 
         FIG. 3  is an enlarged view of a portion of the gravity filter shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of a filter underdrain block shown in  FIG. 1 ; 
         FIG. 5  is an elevation view of the underdrain block shown in  FIG. 4 ; 
         FIG. 6  is a cross section view taken along line  6 — 6  in  FIG. 5 ; 
         FIG. 7  is a cross section taken along line  7 — 7  in  FIG. 6 ; and 
         FIG. 8  is an end view of the hold down grid shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a filtration tank  10  embodying the invention and which may be formed of concrete or other structural materials, such as metal. It will also be understood that although the tank, as shown, is rectangular in horizontal cross section, other shapes may be used. The filtration tank  10  includes a bottom wall  12  and two side walls  14 . A plurality of underdrain blocks  16  are placed end-to-end in parallel rows across the bottom wall  12 . As best shown in  FIG. 4 , filter media retaining caps  70  are disposed on underdrain blocks  16  to support a filter bed  20  ( FIG. 1 ). Filter bed  20  is typically comprised of several layers of filter media. The choice of filter media to be used in the filter bed is dependent on the type of liquid to be filtered and the degree of filtration desired. Some typical filter media include, but are not limited to: granular activated carbon, anthracite, coal, magnesium oxide, ilmenite and sand, including garnet, silica, and quartz. The filter bed of a preferred embodiment of the invention comprises at least one layer of garnet with a particle size of approximately 180 microns. 
     Fluid inlet  22  directs fluids into the filtration tank and fluid outlet  24  directs filtrate away from filtration tank  10 . Fluid inlet  22  may be an open pipe discharging onto the top of filter bed  20 . Additionally, fluid outlet  24  may serve as a gas and/or fluid inlet for backwashing. The term gas as used herein is defined as air or other gases suitable for backwashing filter bed  20 . The term fluid as used herein is defined as water or other liquids suitable for filtering or backwashing filter bed  20 . 
       FIGS. 4–6  show a modular underdrain block  16  with bottom wall  26 , top wall  28  and two side walls  30 . Modular underdrain blocks  16  are shown as rectangular, although it is understood that they may be constructed in any shape that is convenient for installation. Inclined wall surfaces  32  separate primary conduit  40  and secondary conduits  42 . Primary conduit  40  serves to channel filtrate to the fluid exit and away from filter unit  10 . Secondary conduits  42  serve as equalization chambers during filtration and backwash. Orifices  34  ( FIG. 4 ) through inclined wall  32  provide fluid communication between the primary conduit  40  and the secondary conduits  42 . Orifices  36  ( FIG. 6 ) through top wall  28  provide fluid communication between secondary conduits  42  and filter media retaining cap  70 . Orifices  36  serve to receive effluent from filter bed  20  when filtration tank  10  is operating in the downflow mode. Orifices  36  also serve to discharge gas and/or liquid backwash into filter bed  20  when filtration tank  10  is operating in an upflow mode. During backwash, liquid and contaminants are typically drawn from the top of filtration tank  10  via a spillway (not shown). 
     Modular underdrain blocks  16  are typically installed on the floor of the filter bed. They may be placed end-to-end in parallel adjacent rows, also called laterals, which are connected to a flume  17 . The modular underdrain blocks may first be connected to one another and then cemented or grouted in place. The connection may be a simple butt placement or a snap-fit closure. In one embodiment of the invention, the underdrain block has a male connector  62  ( FIG. 5 ) and a female connector  64  on either end of the underdrain block. Male connector  62  may have a protrusion or wedge  66  sized and shaped to be inserted into hole  68  on female connector  64 . In one embodiment, the male connector  62  of one underdrain block is placed adjacent to the female connector  64  of another underdrain block. A clamping tool, not shown, is placed on both underdrain blocks in such a way as to pull the underdrain blocks  16  together so that the protrusion  66  on the male connector  62  removably slides into the hole  68  in the female connector  64 . Cement may also be used to secure the modular underdrain blocks to the floor as well as to one another. 
     The construction embodying the invention further includes a filter media retaining cap  70  fixed to the underdrain blocks  16  and used to prevent filter media from penetrating and clogging the underdrain blocks. The filter media retaining cap  70  typically is comprised of a plate of porous material having pores sized to support the filter media particles with little or no media penetration into the underdrain blocks  16 , but also provide for fluid flow through the retaining cap. While the pore size may be smaller than the particular media size, in one preferred form of the invention, the pore size is somewhat larger than the particular media size. A bridging effect by the media prevents the smaller particles from entering the pores of the media retaining cap  70 . For example, the media retaining cap  70  may have a pore size of 250 microns and support a garnet sand media having a typical particle size of 180 microns. In other constructions, the media retaining cap  70  includes pores having an average pore size of 200 microns or less. In still other constructions, the average pore size can be less than 50 microns. The pores within the media retaining cap may be configured in a variety of shapes and directions and may be multi-directional. In a preferred construction, at least 70% of the pores are multidirectional. 
     The filter media retaining cap  70  includes a bottom surface  44  a top surface  46 . Media retaining cap  70  is disposed on the underdrain block  16  so its bottom surface  44  is supported by top wall  28 . 
     The filter media retaining cap  70  is comprised of porous, planar body or plate, sized and shaped to cover the top surface  28  of one or more underdrain blocks  16 . In a preferred embodiment, media retaining cap  70  has a width and length which corresponds to the width and length of the underdrain block  16 . Media retaining cap  70  has a thickness of approximately less than one inch, preferably one half inch and most preferably one quarter inch. The porous media retaining cap  70  is constructed from a material having a pore size of approximately 250 microns or less. Media retaining cap  70  may be formed of ceramics; metals, in particular sintered metals such as nickel, titanium, stainless steel; and polymers, such as high density polyethylene, polypropylene or styrene. In one preferred embodiment, the porous media retaining cap  70  is molded from sintered plastic beads, as may be obtained from Porex® Technologies of Atlanta, Ga., and Porvair plc of Wrexham, U.K. 
     In a preferred form of the invention, the filter media retaining cap  70  is also supported by a hold down grid  50  having longitudinal bar  52  and transverse bars  54  defining orifices  56 . Hold down grid  50  is disposed on filter media retaining cap  70 , and screws  58  can extend through orifices to secure hold down grid  50  and the filter media retaining cap  70  to underdrain block  16 . The use of the hold down grid  50  allows for the use of a thinner filter media retaining cap  70 , thereby reducing the pressure drop of the flow passing through the filter media retaining cap  70 . The hold down grid  50  illustrated in  FIG. 7  is one of many possible hold down grids. For example, additional longitudinal bars  52  or transverse bars  54  could be used to further increase the strength of the hold down grid  50 . Additional bars  52 ,  54  allow for a further reduction in the thickness of the filter media retaining cap  70 . However, the addition of longitudinal bars  52  and transverse bars  54  reduces the overall flow area and can reduce the flow capacity of the underdrain block  16 . Therefore, a person having ordinary skill in the art must weigh the benefit of a thickness reduction of the filter media retaining cap  70  versus the potential reduced flow capacity into the underdrain block  16  when choosing a configuration for the hold down grid  50 . 
     Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.