Abstract:
A method of distributing gas and liquid through an underdrain system includes: a) positioning an underdrain system on a filter floor, the underdrain system comprising at least one underdrain block and filter media positioned on top of the underdrain block; b) distributing gas sufficiently evenly through the underdrain system at a rate of less than 2 scfm/ft 2  by distributing a liquid at a rate of less than 2 gpm/ft 2  concurrently with the gas; c) increasing the distribution rate of the liquid, or increasing the distribution rate of the liquid and the distribution rate of the gas; d) stopping the distribution of gas; e) further increasing the distribution rate of the liquid; and f) decreasing the distribution rate of the liquid. A method of distributing gas and liquid during an upflow filtration mode is also included.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/738,319 filed Dec. 17, 2012, the entire content of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to water or waste water filters using underdrains and, in particular, to a method of distributing gas and liquid through such underdrains. 
         [0004]    2. Description of Related Art 
         [0005]    Gravity filters are used to separate suspended solids from water. The main components of gravity filters are filtering media and underdrain blocks. Underdrain blocks are generally square, rectangular, or triangular in cross section and have a long longitudinal axis as compared to their cross section. They are attached end-to-end to form long sections called laterals, or are extruded to length, or are fabricated to length. Filter underdrain laterals are laid on the floor of the filter tank one next to the other in parallel rows to define gas and liquid flow conduits below a bed of filtering media. The filtering media is used to capture dirt or contaminant particles from a liquid being filtered as the liquid passes through the media. The filter media is supported by a media retainer so that it will not pass into the underdrain block or collection system. 
         [0006]    The gas and liquid flow conduits make possible the collection of filtered liquid during filtration and the distribution of gas and liquid for backwash. Typically, a common main conduit (known as a “flume”) is located immediately next to or under the filter tank to collect the filtered liquid from the underdrain laterals during filtration and to distribute gas and/or liquid to the underdrain laterals during gas (typically air) or liquid (typically water) backwash. The common main conduit also distributes gas and liquid separately to each of the underdrain laterals during backwash. Oftentimes, to prepare the granular media for water backwash, the submerged granular media is first scoured with a gas-only backwash, where gas is pumped upward through underdrain block. The gas-only backwash loosens the dirt and other impurities so that they will be more easily carried off by a subsequent liquid and/or concurrent gas and liquid backwash phase. 
         [0007]    It is sometimes desired to scour the filter media at a lower than customary gas rate, while also being able to distribute gas at traditional rates during other steps and processes. However, lateral-style filter underdrains do not typically distribute air sufficiently evenly across the area of a filter at these low rates. A need, therefore, exists for a method of distributing gas at a low scour rate evenly and efficiently across the area of a filter system while also still being able to distribute gas at traditional rates. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with one embodiment of the present invention, a method of distributing gas and liquid through an underdrain system includes: a) positioning an underdrain system on a filter floor, the underdrain system comprising at least one underdrain block and filter media positioned on top of the underdrain block; b) distributing gas sufficiently evenly through the underdrain system at a rate of less than 2 scfm/ft 2  by distributing a liquid at a rate of less than 2 gpm/ft 2  concurrently with the gas; c) increasing the distribution rate of the liquid, or increasing the distribution rate of the liquid and the distribution rate of the gas; d) stopping the distribution of gas; e) further increasing the distribution rate of the liquid; and f) decreasing the distribution rate of the liquid. In certain embodiments, the gas can be distributed through the underdrain system at a rate of less than 1 scfm/ft 2  in step b), and the liquid can be distributed through the underdrain system at a rate of 0.25 to 2 gpm/ft 2  in step b). The distribution rate of the liquid can be increased to a rate of 4 to 8 gpm/ft 2  in step c), and further increased to a rate of 15 to 30 gpm/ft 2  in step e). 
         [0009]    In certain embodiments, the gas and liquid are distributed through the underdrain system during a backwashing process. In such embodiments, liquid can first be filtered through the underdrain system so that impurities are trapped within the filter media. These impurities can be removed from the filter media during step e). 
         [0010]    In accordance with another embodiment of the present invention, a method of distributing gas and liquid through an underdrain system during an upflow filtration process includes: positioning an underdrain system on a filter floor, the underdrain system comprising at least one underdrain block and filter media positioned on top of the underdrain block; and distributing gas sufficiently evenly through the underdrain system at a rate of less than 2 scfm/ft 2  by distributing a liquid at a rate of less than 5 gpm/ft 2  concurrently with the gas during a filtration production cycle. 
         [0011]    The present invention is also directed to a filtration system for distributing liquid and gas through filter media. The filtration system includes: an underdrain block having a top wall, a bottom wall, and two side walls connecting the top wall to the bottom wall to define an underdrain block interior; a gas inlet for supplying gas under pressure to the underdrain block; and a liquid inlet for supplying liquid under pressure to the underdrain block. The filtration system is adapted to distribute gas evenly across the underdrain block at a rate of less than 2 scfm/ft 2  by distributing a liquid at a rate of less than 2 gpm/ft 2  concurrently with the gas. The filtration system is further adapted to distribute gas evenly across the underdrain block at a rate of 2 scfm/ft 2  or greater. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a filter system in accordance with one embodiment of the present invention; 
           [0013]      FIG. 2  is a front view of an underdrain block in accordance with one embodiment of the present invention; and 
           [0014]      FIG. 3  is a flow chart of the general process of distributing gas and liquid through a filter system in accordance with an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0015]    For purposes of the description hereinafter, the terms “upper,” “lower,” “right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” “lateral,” “longitudinal,” and derivatives thereof shall relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting. 
         [0016]    Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. 
         [0017]    In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances. 
         [0018]    Referring to  FIG. 1 , an example of a filter underdrain system  10  that can be used with the present invention can include a bottom wall  12  and two side walls  14 . A plurality of underdrain blocks  16  can be placed end-to-end in parallel rows across bottom wall  12 . The type of underdrain blocks  16  that can be used with the present invention is not limited to a particular type. In one embodiment, referring to  FIG. 2 , the underdrain blocks  16  each include a top wall  28 , bottom wall  30 , and a pair of opposed side walls  32  extending between top wall  28  and bottom wall  30 . A plurality of internal walls  34  divide the hollow interior of the blocks  16  into a primary horizontal conduit  36  and two secondary horizontal conduits  38 , positioned on opposed sides of the primary horizontal conduit  36 . A trough  42  can be centrally located in top wall  28 . Primary conduits  36  are connected to a collection flume  66 , which in turn conveys the filtered liquid to a clearwell (not shown) for distribution to the consumer. 
         [0019]    Referring again to  FIG. 1 , a media retainer  18  can be secured to the top of each underdrain block  16  in accordance with the present invention. A layer of filter media  20 , such as sand, anthracite or granular activated carbon (“GAC”) for example, is placed directly on top of the media retainer  18 . The filter media  20  acts to remove and/or capture undesirable particles from the contaminated liquid flowing through filter underdrain system  10 . Further details about the configuration of underdrain blocks  16  and its placement in gravity filters  10  may be found in U.S. Pat. No. 4,065,391 to Farabaugh and U.S. Pat. No. 5,639,384 to Brown et al., which are incorporated herein by reference. 
         [0020]    To ensure maximum filter performance, the granular filter media must be washed regularly to remove the particles from the filter media  20 . This is accomplished by distributing gas and liquid back through the underdrain system  10  and upward through the granular media to dislodge the foreign particles from the granular media. Referring to  FIG. 1 , gas can be introduced through a gas inlet  68  and liquid can be introduced through a liquid inlet  70 . Typically, the filter media  20  is first scoured with a gas-only backwash before a concurrent liquid and gas backwash or a liquid-only backwash mode. The gas scour loosens the dirt and impurities within the filter media  20  for a subsequent liquid only and/or simultaneous gas/liquid backwash cycle. It is sometimes desired to use a lower-than-typical gas distribution rate during the gas scouring backwash while still being able to use traditional gas scour rates during the same or different backwashing process. 
         [0021]    In accordance with one embodiment of the present invention, gas can be sufficiently evenly distributed through the underdrain system  10  at traditional scour rates as well as scour rates of less than 2 or 1 standard cubic feet per minute per square foot of filter area (scfm/ft 2 ). Sufficiently evenly distributed refers to the process in which all areas of the filter are cleaned during backwashing. As such, sufficiently even distribution of the gas through the underdrain system  10  means that the gas is able to be distributed to all areas of the filter. In contrast, uneven distribution would leave some areas of the filter contaminated with dirt and/or other substances and would require further cleaning of the underdrain system  10 . Further, as used herein, “traditional gas scour rates” include, but are not limited to, gas scour rates of greater than 2 scfm/ft 2 , such as from 2 to 6 scfm/ft 2 . As such, in certain embodiments according to the present invention, filter media  20  is first scoured with gas. While gas is being pumped through the underdrain system  10  to loosen the impurities within the filter media  20  as described above, a small amount of liquid is also distributed through the underdrain system  10 . By using a small amount of liquid during the initial gas scour, gas can be distributed evenly through the underdrain system  10  at a scour rate of less than 2 scfm/ft 2  or less than 1 scfm/ft 2 . In some of these embodiments, gas is distributed through the underdrain system  10  at a scour rate of 0.25 to 0.75 scfm/ft 2 . Further, while gas is being pumped through the underdrain system  10  at scour rates of less than 2 or 1 scfm/ft 2 , the liquid can be pumped through the underdrain system  10  at a flow rate of less than  2  gallons per minute per square foot of filter media (gpm/ft 2 ). In certain embodiments, liquid is pumped through the underdrain system  10  at a flow rate of 0.5 to 2 gpm/ft 2 . Because the liquid is supplied at a low rate, the liquid will not transport impurities out of the underdrain system  10  or blow filter media  20  out of the media retainer  18 . 
         [0022]    The initial gas scour and liquid distribution phase described above can be held for a period of time that allows the gas to agitate and break-up the impurities contained within the filter media  20  on its ascent to the surface. For instance, in certain embodiments, the concurrent gas scour and liquid backwash are pumped through the underdrain system  10  for a period of 1 to 6 minutes. However, the time period can be adjusted as understood by person(s) having ordinary skill in the art so that all the impurities trapped within the filter media  20  will become loose and capable of being transported out of the underdrain system  10  during a subsequent cycle as will be described in further detail below. 
         [0023]    Once the impurities are agitated loose, the liquid or liquid and gas can be increased to a higher distribution rate. For instance, in one embodiment, after agitating the impurities within the filter media  20  loose, the liquid distribution rate can be increased to a flow rate of 4 to 8 gpm/ft 2 . While the liquid is increasing, the gas is maintained at the same distribution rate. Alternatively, in some embodiments, the distribution rate of the gas can be increased to traditional gas rates such as a rate of greater than 2 scfm/ft 2 , for example. The gas and liquid are sufficiently evenly distributed through the underdrain system  10  during this step. Thus, unlike other systems, gas can be evenly distributed through the underdrain system  10  at both traditional and lower than traditional gas distribution rates. 
         [0024]    In certain embodiments, after increasing the liquid or liquid and gas, the gas can be stopped and only liquid can be distributed through the underdrain system  10 . The liquid can be distributed at a flow rate of 4 to 8 gpm/ft 2 . The liquid flow rate can then be gradually increased even further to a rate of 15 to 30 gpm/ft 2 . The liquid distribution rate is held at the increased flow rate for a period of time necessary to transport the impurities out of the underdrain system  10 . After the impurities are transported out of the underdrain system  10 , the distribution flow rate of the liquid is gradually reduced to zero and a subsequent filtration process can take place.  FIG. 3  shows a flow chart of the general process of distributing gas and liquid through a filter system  10  in accordance with an embodiment of the present invention. As noted above, using a small amount of liquid during the gas-only scour cycle resulted in better air distribution during the initial backwash step when a lower-than-typical gas rate was used. Thus, the use of a small amount of liquid such as water allows the underdrain system  10  described herein to evenly distribute gas at lower-than-typical gas rates in filter underdrain systems  10  that can also evenly distribute gas at traditional gas rates. 
         [0025]    The present method of distributing gas and liquid through an underdrain system  10  can be used with one underdrain block  16 . Alternatively, the present method can also be used for distributing gas and liquid through an underdrain system  10  that contains multiple laterals formed from multiple underdrain blocks  16 . The laterals can have various lengths and sizes. 
         [0026]    Further, filter systems must be designed on the basis of a certain minimum hydraulic headloss to establish optimum backwash distribution. For fluidized media applications, such headloss must generally be greater than the clean filter media headloss at its incipient fluidization point, where full media fluidization is expected. On the other hand, the backwash headloss must not be so high as to require undue energy to achieve appropriate backwash flow. Filter systems that use the present method experience an increase in headloss as compared to current methods that use a lower-rate backwash gas with underdrain systems  10 . 
         [0027]    In addition to gravity filtration, gas can be distributed through an underdrain system  10  at a low distribution rate during an upflow filtration process. In an upflow filtration process, feed water is introduced through a flume and then up through the underdrain blocks  16 . The water then exits through the top of the underdrain blocks  16  and filter media  20 . In one embodiment, during the filtration production cycle of an upflow filtration process where water is filtered, gas is distributed through the underdrain system  10  at a rate of less than 2 or 1 scfm/ft 2 , such as a rate of 0.25 to 0.75 scfm/ft 2 , as feed water is traveling up through the underdrain system  10 . In certain embodiments, during the filtration production cycle of an upflow filtration process, feed water is distributed through the underdrain system  10  at a flow rate of less than 5 gpm/ft 2 , such as less than 4 gpm/ft 2 . After the filtration process is completed, a backwashing method can be commenced. 
         [0028]    While various embodiments were provided in the foregoing description, those skilled in the art may make modifications and alterations to these embodiments without departing from the scope and spirit of the invention. For example, it is to be understood that this disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. Accordingly, the foregoing description is intended to be illustrative rather than restrictive. The invention described hereinabove is defined by the appended claims and all changes to the invention that fall within the meaning and the range of equivalency of the claims are to be embraced within their scope.