Patent Publication Number: US-2002008061-A1

Title: Disposable rotary drum filter

Description:
RELATED APPLICATIONS  
     [0001] This application is based on prior copending U.S. provisional patent application Serial No. 60/138,863, filed Jun. 11, 1999, the benefit of the filing date of which is hereby claimed under 35 U.S.C.§119(e). 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention is generally directed to rotary drum filters, and more specifically, to disposable rotary drum filters for batch processing.  
       BACKGROUND OF THE INVENTION  
       [0003] In the pharmaceutical, fine chemical, hazardous material, beverage, juice, wine, and beer industries, most of the production processes are inherently batch processes. Batch processing, rather than continuous flow processing, is advantageous when a limited volume of material is to be processed, when there is a need for traceable “lots” of product, and/or when the intrinsic value of the product is high. A common process step in both batch and continuous flow processing is the separation of solids from liquids.  
       [0004] Traditionally, solid-liquid separation for batch processing has been carried out using centrifuges, gravity filtration, vacuum filtration, and/or pressure filtration equipment. Although such equipment has been used for years, there are inherent disadvantages to its use, especially when employed in laboratory, pilot plant, or other small volume applications where time and resources are limited, and for processes in which it is critical that the materials be isolated from the outside environment.  
       [0005] For example, centrifuges are often relatively expensive, require significant maintenance, and are labor intensive to operate under aseptic or “clean” conditions. Other types of conventional batch type filtration equipment can also be expensive and similarly require a high degree of operator attendance. A further inherent problem with prior art batch filtration units is that the designs of small batch volume filtration units are often radically different than the designs of larger batch volume equipment, making the results of scaling up from laboratory or pilot plant volume production to industrial scale production less predictable.  
       [0006] An additional drawback with the use of centrifuges and other batch filtration units in the laboratory and pilot plant environments is that cleaning these units between batches requires considerable time and labor. Frequently, time and technical manpower resources in laboratory and pilot plant environments are at a premium, and such cleaning operations can become an unacceptable bottleneck. These cleaning operations also introduce an element of risk, both in terms of exposing personnel to potentially hazardous materials, and in terms of introducing contaminates into the processing equipment that may degrade the quality of, or even completely ruin, the next batch to be processed.  
       [0007] Thus, it would be desirable to provide efficient means to separate solids and liquids that is usable in batch processing, and which does not require time consuming cleaning procedures between processing batches. To eliminate the requirement for cleaning, it would be preferable if such means were sufficiently low cost as to be disposable, so that a new unit could be used for each batch of material processed.  
       [0008] A common non-disposable device used for solid-liquid phase separation for continuous flow processing is the rotary drum filtration unit. In fact, the rotary drum vacuum filter is considered to be the workhorse of the chemical process industry. In this type of filter, a drum that is partially submerged in a trough of suspended solids rotates slowly at only a few revolutions per minute (RPM). Solids adhere to the outer surface of the drum, which may be coated to aid in the collection of the solids. The slow rotational speed of the drum, as well as air flow over the portion of the drum that is not submerged, help to dry the solids adhering to the drum. This solid filtrate is then removed from the rotating drum, often by a blade that scrapes the collected solids from the drum. While efficient, such units are generally quite large and expensive. Consequently, they have been used primarily in association with large-scale continuous flow processing, such as for wastewater treatment. In view of the operating efficiencies of this type of device, it would be desirable to provide a much smaller and substantially lower cost disposable filtration unit that operates according to the principles of rotary drum filtration, but which can be used for batch processing in place of the traditional centrifuges and other batch filtering units.  
       [0009] Preferably, such a batch filtration disposable rotary drum filter should be capable of running continuously, in a steady state fashion for a period of time sufficient to process a batch of product, enabling the device to be very small as compared to conventional types of batch equipment that would normally be employed to handle the same amount of material. It would also be desirable to provide a unit that is enclosed, to minimize the risk of product and personnel contamination. It would further be desirable to fabricate such a unit from inexpensive plastic materials that can be sterilized if a process requires that sterile equipment be used. Disposability offers the advantages of eliminating clean up, containing any hazardous materials used within the disposed device, reducing the chances of product contamination, and enhancing reliability and reproducibility.  
       [0010] Typical conventional rotary drum filters do not provide these desirable features. Prior art rotary drum filters are relatively complicated mechanical constructions and not readily adaptable to be mass produced in quantity and at low cost for use as a disposable unit. Specifically, prior art rotary drum filters often include complicated float valves to control the level of slurry in the trough in which the drum rotates, as well as complicated valves to control pressure in the system to ensure the proper flow of fluid into the filter unit. Because of the relatively large size of prior art rotary drum filters, and the fact that such units are designed to have very substantial service lives, such units have often incorporated relatively expensive bearing and seal components. In contrast, it would be desirable to provide a disposable rotary drum filter that replaces these mechanically complicated float control and pressure control schemes in the prior art with control means that are similarly effective, yet much simpler, lower cost, and more appropriate for a disposable unit. It would further be desirable to provide a disposable rotary drum filter that does not require expensive bearing and seal components.  
       [0011] Examples of prior art designs of a typical rotary drum filter include those described in U.S. Pat. Nos. 3,794,178; 3,837,499; 4,157,301; 4,346,008; 5,055,205; 5,244,572; 5,262,069; and 5,308,488. The configurations shown in these patents typically were created to optimize high production rates, to provide long service life, and with minimal concern about product loss due to retention of the product in the filters. While plastic and fiberglass have been used in the design of rotary drum filters (see U.S. Pat. Nos. 3,894,899; 4,419,165; and 4,515,693) to enable the processing of corrosive products, such rotary drum filters are not suitable for use in a single batch process as a disposable unit.  
       [0012] It would be therefore desirable to provide a rotary drum filter unit designed to process a batch of material, and then be replaced with a new unit for the next batch. Preferably, such a rotary drum filter would be engineered such that its fabrication costs are low and its operation is very simple. It would be desirable to provide a rotary drum filter made from plastic and encased in a plastic housing. Preferably, the unit should be relatively small, especially as compared with traditional rotary drum filters, and made from relatively inexpensive materials, such that it is practical for a single-use, and can be economically considered a disposable unit.  
       [0013] A desirable feature of such a single-use device would be the elimination of traditional seals and other isolation mechanisms. To further simplify the disposable unit, it would be desirable to eliminate internal prime movers and to instead rotate the inner drum using an external prime mover that is not part of the disposable component. In addition, seals associated with a drive shaft that penetrates the filter&#39;s outer housing should preferably be eliminated by magnetic coupling of the external prime mover to an internal drive shaft. Prior art rotary filter units generally cannot employ a magnetic coupled drive, because the size of such units it too great and the magnetic coupling is incapable of providing the required torque. In contrast, a small, disposable rotary drum filter can make use of a magnetic drive because of the relatively low torque required to rotate the smaller sized device. It would further be desirable to replace the often complicated, adjustable-position metallic sharp edge of prior art rotary drum filters with a simple fixed plastic “sharp” edge that is formed integral to the housing, to remove solids from the drum. The prior art does not disclose or suggest such an apparatus.  
       SUMMARY OF THE INVENTION  
       [0014] In accord with the present invention, a disposable rotary drum filter is defined, which is useful for separating a slurry into a liquid filtrate and a solid filter cake. The disposable rotary drum filter includes a housing defining a slurry reservoir. The housing includes a slurry inlet adapted to receive a slurry into the slurry reservoir, a gas inlet adapted to receive gas into the housing, a filtrate outlet adapted to convey filtrate from the housing after the filtrate has been separated from the filter cake, a filter cake outlet adapted to convey the filter cake from the housing, and an overflow outlet. The overflow outlet is disposed within the housing and has an opening disposed at an elevation selected to determine a maximum level of a slurry within the slurry reservoir. An inner drum is rotatably mounted within the housing and includes a generally cylindrical frame. In addition, the inner drum includes a filter extending around the frame and is disposed within the housing such that the filter is partially below the level determined by the opening of the overflow outlet. The filter is adapted to receive a filter cake that is deposited on an outer surface of the filter as filtrate separated from the filter cake is drawn through the filter and the filtrate outlet. A portion of the frame provides fluid communication between an interior surface of the filter and the filtrate outlet and is thereby adapted to convey the filtrate from the inner drum after the filtrate has passed though said filter. A driven member is connected to the frame and is adapted to be drivingly coupled to an external prime mover to rotate the inner drum, exposing the filter cake deposited on the filter to gas that is drawn through the filter into the inner drum from inside the housing. This flow of gas through the filter cake at least partially dries it. Also included within the device are means for removing at least a portion of the filter cake deposited on said filter after the filter cake has been at least partially dried by the gas. The filter cake that is thus removed passes through said filter cake outlet.  
       [0015] The slurry inlet is preferably disposed at a bottom of the housing and adapted to couple to a fluid line that conveys the slurry from an outlet of a slurry pump. Also, the slurry reservoir is preferably disposed in a lower portion of the housing, such that said inner drum is partially disposed within the slurry reservoir.  
       [0016] Also included in the housing is a journal that supports the inner drum so as to enable the internal drum to freely rotate about its longitudinal axis. A spring is provided to exert a biasing force to position the inner drum at a desired location along its longitudinal axis, within the housing. This spring is preferably disposed between the driven member and the housing. In one embodiment, the driven member is magnetically coupled to a driving member, and a magnetic force is used to provide a biasing force to similarly position the inner drum at a desired location along its longitudinal axis, within the housing, thus eliminating the need for a spring.  
       [0017] In one preferred embodiment, the filter cake outlet is oriented vertically, such that a gravitational force facilitates the removal of the filter cake from the housing. In this embodiment, the filter cake outlet is elongate and is disposed along one side of the housing, extending generally in alignment with the outer surface of the filter.  
       [0018] The means for removing at least the portion of the filter cake preferably comprises an elongate edge formed in the housing. This edge extends adjacent to the outer surface of the filter.  
       [0019] The filtrate outlet is preferably adapted to couple to a fluid line connected with a vacuum source.  
       [0020] The filter may comprise a plastic mesh screen having a plurality of openings sized to enable the filtrate to pass therethrough. However, the openings are sized to block passage of the solids comprising the filter cake. In one preferred embodiment, the filter is fabricated from a porous plastic material, whose openings are similarly sized to block passage of the solids.  
       [0021] In a preferred embodiment, the gas inlet is disposed in an upper part of the housing. Pressure control means are included for equalizing a pressure drop within the housing as the gas within the housing is drawn into the inner drum through the filter. The pressure control means include an orifice through which gas flowing through the filtrate outlet is drawn. This orifice is preferably formed in a plate disposed adjacent to the filtrate outlet. A fluid flow through the filter is optimized by selecting an appropriate size of the orifice in the plate.  
       [0022] A plurality of fluid channels in fluid communication with said filtrate outlet are provided on the inner drum. Also included are a plurality of filtrate tubes in fluid communication with said filtrate outlet. The plurality of filtrate tubes connect the longitudinal fluid channels in fluid communication with the cavity, thereby establishing a fluid path between the longitudinal channels and the filtrate outlet.  
       [0023] A plate separates the cavity into an upper portion and a lower portion. The plate is fixedly attached to the housing, with the inner drum being free to rotate about the plate. An orifice controls a gas flow between the upper portion and the lower portion of the cavity and thereby controls gas flow through the filter from inside the housing.  
       [0024] The driven member comprises a shaft that is adapted to magnetically couple to an external prime mover. At least one permanent magnet is included on the shaft, and more preferably, a plurality of permanent magnets are disposed in a spaced-apart array around an end of the shaft. In one embodiment a magnetic material (such as iron), rather than a magnet, is incorporated on the shaft.  
       [0025] The device can include an external coupling hub adapted to be drivingly connected to a prime mover. In this case, the driven member is magnetically coupled to the external hub, which preferably includes at least one magnet or at least one electromagnet.  
       [0026] The means to remove the filter cake preferably comprises a generally sharp edge and has a length that is substantially equal to that of said inner drum, being formed as an integral part of the housing. Other means to remove the filter cake from the rotating drum include serpentine belts, bursts of compressed gas or air, or an additional, smaller rotating drum (roller discharge).  
       [0027] In some applications, the device must be sterile. For such applications, the housing is fabricated from a material that can be readily sterilized, e.g., using ethylene oxide, steam, gamma radiation, electron beam radiation, or light.  
       [0028] Another aspect of the present invention is directed to a method for using a disposable rotary drum filter to process a batch of slurry. The method includes steps generally consistent with the functions of the elements included in the apparatus described above. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS FIGURES  
     [0029] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
     [0030]FIG. 1 is a schematic view of a disposable rotary drum filter in accord with one preferred embodiment of the present invention, a portion of the housing and of an internal filter being cut away to more clearly show interior details of the device;  
     [0031]FIG. 2 is an exploded isometric view of the disposable rotary drum filter of FIG. 1;  
     [0032]FIG. 3 is a front elevation of the disposable rotary drum filter of FIGS. 1 and 2;  
     [0033]FIG. 4 is a right side elevation of the disposable rotary drum filter of FIGS. 1 and 2;  
     [0034]FIG. 5 is a cross-sectional view taken along section line V-V of FIG. 4, showing mounting details for an inner drum used in the disposable rotary drum filter of FIGS. 1 and 2;  
     [0035]FIG. 6 is a cross-sectional view taken along line section VI-VI of FIG. 3, showing a sharp edge used for removal of solids from the inner drum, a discharge chute for the solids, and an overflow to remove excess slurry;  
     [0036]FIG. 7 is a cross-sectional view taken along section lines VII-VII of FIG. 5, showing an interior detail of the inner drum; and  
     [0037]FIG. 8 is a schematic process diagram illustrating how the disposable rotary drum filter of the preceding Figures can be used to separate a slurry into a filter cake fraction and a filtrate fraction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0038] Overview of the Present Invention  
     [0039] The present invention employs filtering concepts developed for use in continuous processing environments and adapts them for use in a batch processing environment. As noted above in the Background of the Invention, rotary drum filters are devices very commonly used in the chemical process industry for continuous processing, when it is necessary to separate slurries into a filter cake fraction and a filtrate fraction. Rotary drum filters are traditionally massive units fabricated from cast iron and other heavy structural materials, and are traditionally employed in high volume applications. The present invention employs the basic operating principles of rotary drum filters in a compact unit made from non-traditional lightweight materials, such as injection molded plastic, to provide a disposable rotary drum filter that is ideally suited for use in batch processing environments.  
     [0040] As used herein in the specification and in the claims that follow, the term “continuous processing” refers to a processing environment in which a continuous stream of material is processed without interruption, generally over a relatively long period of time, measured, for example, in terms of days or weeks. The continuous process is generally interrupted only for maintaining the processing equipment, and not because the supply of material being treated has been exhausted. In contrast, the term “batch processing” as used herein refers to a processing environment in which a finite volume of material is processed without interruption, but only until the supply of material is exhausted, in a period of time that is relatively short, and generally measurable in terms of minutes or hours. Furthermore, the term “magnetic material” as used herein and in the claims that follow is intended to mean a material, such as a ferrous material, that is characterized by its ability to be attracted by a magnet. Also, it should be noted that in reference to the Figures, when appropriate, lines with arrows have been used to indicate the direction of a material flow, or to indicate a direction of rotation.  
     [0041] Details of a Preferred Embodiment  
     [0042]FIG. 1 illustrates one preferred embodiment of the present invention in which an enclosed disposable drum filter  10  has a rotatably mounted inner drum  18  encased within an outer plastic housing  11 . Housing  11  includes an upper housing  12  and a lower housing  14 , which are preferably fabricated from injection molded plastic, such that the upper and lower housing can be easily assembled and joined using a suitable adhesive, plastic welding, fasteners, or other techniques commonly employed for joining plastic components in a sealed relationship. Those of ordinary skill in the art will readily appreciate that a number of well-known plastic molding and assembly techniques can be used to fabricate and join upper housing  12  to lower housing  14 . It should also be noted that housing  11  can be fabricated from more than two sections, or rather than being fabricated from an upper and lower section, housing  11  can be fabricated from a right hand and left hand section. In the preferred embodiment, two sections are used to reduce the complexity, cost, and labor to assemble the unit, since an important object of the present invention is to provide an economical, simple and disposable rotary drum filter. Furthermore, using an upper and a lower section, rather than a right hand and a left hand section, ensures that the lower portion of housing  11 , in which a level of slurry is accumulated sufficient to at least partially bathe inner drum  18 , will not include a seam or joint (that could leak).  
     [0043] Many different types of plastic materials can be used to fabricate housing  11 . Because the rotary drum filter  10  is intended to be small, in comparison to the traditional industrial scale, rotary drum filters of the prior art designed for continuous processing, a wide range of plastic materials will provide sufficient structural integrity. While high grade plastic materials such as fiber reinforced products can be used, it may be preferable to use lower cost plastics to minimize the cost of rotary drum filter  10 . The rotary drum filter in accord with the present invention is intended to process a batch of slurry and then be discarded, so relatively inexpensive plastic can be used in fabricating it.  
     [0044] In one embodiment, inner drum  18  is approximately the size and shape of a common 12 oz. soft drink beverage can. At such a scale, the structural demands placed on the material are very modest, and it is envisioned that cost will be a primary consideration in choosing the material. It is expected that the thickness of the plastic components used in constructing rotary drum filter  10  will likely far exceed that required for the structural integrity of the device, when operating under its normal intended processing conditions.  
     [0045] It should be noted that plastics exhibit very good resistance to a wide variety of chemicals. For certain applications, it may be necessary to select a particular type of plastic based upon its resistance to a particular component in a given slurry that will be processed. Those of ordinary skill in the art will readily recognize that plastic materials suitable for fabrication of rotary drum filter  10  can be selected to exhibit good resistance to a particular class of chemicals.  
     [0046] In a typical application of the present invention, a slurry is pumped into rotary drum filter  10 , for example, using a peristaltic pump, through a slurry inlet  16 . The slurry substantially fills lower housing  14 , at least to a maximum level determined by the elevation of the lower edge of the opening into an overflow outlet  40  (shown in FIG. 2), so that inner drum  18  is partially immersed in the slurry. Any excess of the slurry simply flows out of the housing through overflow outlet  40  and is typically returned to the source of the slurry by gravity flow.  
     [0047] Because rotary filter drum  10  is designed to be disposable, no prime mover is included within the unit. Instead, an external drive coupling hub  20  enables a prime mover (not shown) to be magnetically drivingly coupled to inner drum  18 . Preferably drive coupling hub  20  includes a plurality of circumferentially spaced-apart permanent magnets  23  (shown in FIG. 2) that are magnetically coupled to corresponding magnets  43  (also shown in FIG. 2) disposed on a shaft  42  of inner drum  18 . Further details of drive coupling hub  20  are provided below.  
     [0048] The outer surface of inner drum  18  is a filter  22 , which is fabricated from a porous plastic fabric, plastic mesh, or other porous membrane. Those of ordinary skill in the art will readily recognize that a wide variety of different porous materials can be used to fabricate filter  22 , and the choice will be made primarily based upon the size and quantity of the porous openings in the material, which must permit the liquid fraction of the slurry to pass through, while blocking the solid fraction. Again, because rotary drum filter  10  is intended to be a low cost, single use, disposable unit, cost is also expected to drive the selection of material. Note that filter  22  does not need to be made of a long-lasting material. Instead, it is sufficient that the material selected have a service life at least exceeding that required to process a batch of slurry. Filter  22  forms a base upon which a filter cake is formed when disposable rotary drum filter  10  is in operation.  
     [0049] Filter  22  preferably is fabricated from a material that can withstand sterilization procedures, so that disposable rotary drum filter  10  can be sterilized before use if a user so desires. It is also important for the filter material to be chemically compatible with the slurry to be filtered. With respect to plastic based filter materials, those of ordinary skill in the art will recognize that plastic materials are naturally hydrophobic (water repelling), and can be treated with an appropriate material, if a hydrophilic (water absorbing) characteristic is required.  
     [0050] In one embodiment, a plastic mesh fabricated with the required mesh opening size will likely provide the required characteristics of low cost, appropriate service life, and appropriate porosity. In another embodiment the filter is preferably fabricated from a porous plastic material. Such materials are readily available in several different forms, including High-Density Polyethylene (HDPE), Ultra-High Molecular Weight Polyethylene (UHMW), Polypropylene (PP), Polyvinylidene Fluoride (PVDF), Polytetrafluoroethylene (PTFE), Nylon 6 (N6), Polyethersulfone (PES), and Ethyl Vinyl Acetate (EVA), from Porex Technologies of Fairburn, Ga. (http://www.porex.com/Technologies/index.html). Porous plastics are readily available in different pore sizes, so that the pore size of a particular filter can be matched with the size of the solids in a slurry to ensure that good filtration of the solids is achieved. Empirical testing has indicated that a porous plastic material having a pore size of 20 microns is well suited to filtering a diatomaceous earth slurry.  
     [0051] One advantage of a porous plastic over a fine plastic mesh is that the porous plastic is a self supporting material. Prior art rotary drum filters have typically required complicated support structures for fabric or mesh filters that do not have sufficient structural rigidity to be self-supporting. Porous plastics are available in sheet form, from which a generally tubular filter can be fabricated by welding opposite ends of the sheet together to form a tube. Porous plastics are also available in tubes of various sizes. A generally tubular porous plastic filter of the appropriate size is well adapted to provide a self supporting filter  22  when placed over the outer surface of inner drum  18 . Liquid passing through filter  22  enters a plurality of fluid channels  24  formed on the surface of inner drum  18  and extending longitudinally just behind the filter material. The filtered liquid (i.e., the filtrate) that is inside fluid channels  24  flows into orifices  26 , one orifice being provided proximate one end of each fluid channel. Orifices  26  are in fluid communication with a filtrate discharge  28 , through which the filtrate is discharged, as will be described in further detail below in connection with the discussion of FIGS. 5 and 7. A gas inlet  34  is included at the top of upper housing  12  to enable sufficient air to enter disposable rotary drum filter  10  to make up for air exiting with the filtrate through filtrate discharge  28 . It should be noted that while the gas entering through gas inlet  34  will often be ambient or filtered air, other gases, such as nitrogen from a compressed gas supply, can alternatively be used. An orifice  36  functions as a pressure control, ensuring that pressure levels within the unit maintain a required flow of filtrate from the device. Further details regarding the operation of disposable rotary drum filter  10  and the flow of fluids through the device are provided below.  
     [0052] The filter cake that is formed on filter  22  as the liquid fraction passes through filter  22  is removed by an edge  30 , which is preferably formed extending longitudinally along the inner side of a filter cake discharge  32 , which is integrally formed on lower housing  14 . In the preferred embodiment, edge  30  is preferably not adjustable to set the distance it is spaced apart from the filter, as any mechanism required to provide a positionable edge would slightly increase the complexity and cost of the design. Edge  30  is disposed sufficiently close to inner drum  18  to ensure that the mass of filter cake deposited on filter  22  does not negatively impact the rotation of inner drum  18 . It is contemplated that the disposition or spacing of edge  30  relative to inner drum  18  will be varied in different production models of the present invention. For instance, a rotary drum filter that is optimized to filter a suspended solid that is characterized as being extremely dense will preferably have a separation distance between edge  30  and inner drum  18  that is less than that for a rotary drum filter optimized to filter a suspended solid that is characterized as having a low density. In this embodiment in which inner drum  18  is approximately the size of a 12 oz. beverage can, empirical data indicates that a separation distance of approximately ⅛ of an inch is effective for a wide variety of different slurries. Filter cake that is removed from filter  22  by edge  30  exits rotary drum filter  10  through filter cake discharge  32 . Note that instead of being provided as an integral part of lower housing  14 , edge  30  could be fabricated as a separate component that is secured to the housing with a suitable adhesive or with suitable fasteners. However, fabricating edge  30  as an integral part of lower housing  14  will help to achieve lower production costs, ensuring that a cost effective disposable unit is achieved.  
     [0053] Edge  30  is not required to be particularly “sharp.” Prior art rotary drum filters often employ an edge element to remove filter cake from the surface of the filter, and such an element is commonly referred to as a “knife blade.” Because of the relatively large sizes of prior art rotary drum filters, the mass of filter cake being removed from the surface of the filter by such “knife edges” can be relatively large, requiring that such “knife edges” be fabricated from a strong structural material, such as metal. Including a relatively sharp surface on such “knife edges” aids in the removal of a large mass of filter cake from the filters.  
     [0054] Because inner drum  18  is of a relatively small size, the mass of the filter of filter cake being removed as inner drum  18  rotates past edge  30  is significantly less than the mass that would be removed in a much larger prior art rotary drum filter. As long as edge  30  is capable of removing a portion of the filter cake deposited on filter  22  as inner drum  18  rotates, the relative “sharpness” of edge  30  is not critical. A prototype device fabricated from a plastic material has successfully proven that edge  30 , fabricated without any attention given to the relative “sharpness” of the edge, is quite effective in removing filter cake from the surface of filter  22 . Preferably edge  30  will remove from approximately {fraction (1/16)} th  inch to approximately ⅛ th  inch of filter cake from the surface of filter  22 . Removing more than approximately ⅛ th  inch of filter cake will likely result in a filter cake that is not as dry as would be desired.  
     [0055] In prior art rotary drum filters, means other than a edge have been used to remove filter cake from a rotating drum filter. For example, serpentine belts, bursts of compressed gas or air, and an additional, smaller rotating drum (roller discharge) have been used for this purpose. While any of these means could be incorporated into a disposable rotary drum filter of the present invention, each introduces additional complexity into a design that is intentionally simple. Thus, edge  30  is used in the disclosed preferred embodiment of the present invention for removing filter cake from inner drum  18 .  
     [0056]FIG. 2 is an exploded view of disposable rotary drum filter  10 . In this perspective, it can be clearly seen how lower housing  14  forms a trough or reservoir for the slurry in which inner drum  18  is partially immersed. Note that lower housing  14  includes overflow outlet  40  with a lower edge of its opening positioned at an elevation selected to ensure that a desired maximum level of slurry in rotary drum filter  10  is maintained. If the level of slurry is not controlled, excess unfiltered slurry entering the housing would flow over edge  30 , and exit the rotary drum filter through filter cake discharge  32 . Typically, overflow outlet  40  will be positioned such that about 30-35% of inner drum  18  is bathed in slurry.  
     [0057]FIG. 2 also illustrates details showing how external coupling hub  20  is drivingly coupled to inner drum  18 . One end of inner drum  18  has a shaft  42  which extends outward from inner drum  18  along its longitudinal axis. Upper and lower housing  12  and  14  are shaped to accommodate shaft  42 , so that shaft  42  is fully enclosed when upper housing  12  and lower housing  14  are joined. External coupling hub  20  nests over shaft  42 , and is drivingly connected to a prime mover (not shown). Because disposable rotary drum filter  10  is a relatively small unit, the mass of inner drum  18  is quite small. Thus, the prime mover used to drive external coupling hub  20  can be a small fractional horsepower alternating current (AC) or direct current (DC) electric motor. Preferably inner drum  18  will be rotated at a speed of one to ten revolutions per minute (RPM), although it is anticipated that a broader range of rotational speeds can be beneficially employed. It should be noted that these rotational speeds are simply exemplary, and are not intended to be limiting on the scope of the present invention. The optimal rotational speed is a function of the concentration of the solids in the slurry, the porosity of the filter, and the strength of the vacuum. Thus it is preferable for the prime mover to have a variable output such that the RPM of inner drum  18  can be selectively varied. However, a prime mover having a fixed output of one to ten RPM could also be used. Preferably external coupling hub  20  is drivingly coupled to shaft  42  magnetically, through the coupling between a plurality of permanent magnets  23  on the coupling hub and a corresponding plurality of permanent magnets  43  on the shaft, as will be described in more detail in relation to the description of FIG. 5. The prototype unit discussed below was successfully operated using a {fraction (1/30)} horsepower motor, with a torque rating of 50 inch/pounds. For embodiments employing a magnetically coupled external prime mover, it is anticipated that magnetic coupling will be successful if the torque applied by the external prime mover is limited to less than 12 inch/pounds.  
     [0058] With respect to the relative size of disposable rotary drum filter  10 , a prototype unit in which inner drum  18  is approximately 3 inches in diameter by 4.5 inches in length has been successfully tested. While it is likely that substantially larger rotary drum filters will be built in accord with the present invention, the relatively small unit size enables the device to be sufficiently low in cost to be considered disposable after use in processing a single batch of slurry.  
     [0059] Tests of this initial prototype disposable rotary drum filter have determined that it can process a typical slurry at a rate of about 1-3 liters/minute. Based upon this determination, it appears that this prototype of the present invention is usable for processing batches ranging, for example, from about 10-1800 liters in volume. It is expected that a disposable rotary drum filter of this size would be ideal for use in applications such as the animal studies required for drug testing, clinical trial production, and other moderate sized batch applications. A smaller sized unit with an inner drum 3″ long by 1.5″ in diameter should operate at rates of about 300-650 ml/minute, making it ideal for processing batch sizes of up to 400 liters. This smaller size disposable rotary drum filter would thus be suitable for use in processing small laboratory batches typically seen in the early stages of drug development, and other small scale batch applications. Finally, a larger unit with an inner drum 20″ long and 10″ in diameter should be able to process about 14-40 liters/minute, making it applicable for processing batches from 200-24,000 liters in volume. This larger size disposable rotary drum filter would thus be suitable for most batch processing operations involving large scale production, as even in large scale production facilities individual batch sizes are often less than 24,000 liters in volume. It should be noted that these examples of size and processing capability are simply exemplary and are not intended to be limiting on the scope of the present invention. However, it is also recognized that the present invention is not infinitely scaleable, if the intention is maintained of keeping the costs of the device relatively low.  
     [0060] Referring again to FIG. 2, it will be noted that lower housing  14  includes one half of a journal  60  that supports shaft  42 , and one half of a journal  62  that supports a collar  45 , which is formed around the central opening into inner drum  18  at one end of the inner drum. Upper housing  12  and lower housing  14  extend outwardly beyond journal  60  to fully enclose shaft  42 . While not shown in this Figure, it should be understood that upper housing  12  includes the other halves, respectively, of journals  60  and  62 . Journals  60  and  62  are sized such that shaft  42  and collar  45  are supported by the journals, yet so that the shaft and collar can rotate freely about the longitudinal axis of inner drum  18  within these journals. Since no bearings are included in journals  60  and  62 , eventually long-term wear at these journals might cause failure of the rotary drum filter. But because rotary drum filter  10  is designed to be disposable, short-term wear is acceptable, as even inexpensive plastic materials are expected to be sufficiently wear resistant to achieve a service life in excess of the time for processing a single batch volume. It should also be noted that the relatively low mass of inner drum  18 , in addition to the self lubricating characteristics of plastic materials, provide a service life sufficient to process a batch volume without the use of expensive bearings. In at least one embodiment, inner drum  18  is hollow, and the buoyancy of the slurry reduces the load on the lower portions of journals  60  and  62 , further reducing wear at those locations.  
     [0061] A sealing surface  44  on the central end of inner drum  18  minimizes the leakage of slurry in bypass of filter  22 , preventing the slurry from exiting the unit through filtrate discharge  28 . This end of the inner drum is biased against the interior surface of the housing around journal  62 , as explained below in the discussion of FIG. 5.  
     [0062] An additional structural element that is shown in FIG. 2 is a plate  56 , which is fixedly attached to the interior of housing  11 . Plate  56  is elongate and extends into inner drum  18  to a depth beyond the radial disposition of orifices  26 . Inner drum  18  rotates freely about plate  56 . As will be described in more detail below, plate  56  effectively separates the interior of inner drum  18  into an upper portion and a lower portion, and as the inner drum rotates, the orifices in the inner drum that are above the plate (in fluid communication with the then upper portion) and those that are below the plate (in fluid communication with the then lower portion) changes. An orifice  36  formed in plate  56  is sized to ensure that a vacuum source (not shown in this Figure) that is in fluid communication with filtrate discharge  28  draws filtrate from the filtrate discharge at a desired rate and is not defeated by being subject to full ambient air pressure (or the pressure of any gas being introduced via gas inlet  34 ).  
     [0063] In FIGS. 3 and 4, gas inlet  34  can be seen at the top of upper housing  12  in each Figure. FIG. 3 clearly illustrates how the upper and lower housings extend outwardly to enclose shaft  42 . FIG. 4 clearly shows filter cake discharge  32 , slurry inlet  16 , filtrate discharge  28 , and slurry overflow  40 . It is important to note that while FIG. 4 might lead one to conclude that filter cake discharge  32  and overflow  40  (and to some extent slurry inlet  16 ) are acting as legs and thereby providing support for disposable rotary drum filter  10 , that is not the case. Filter cake discharge  32 , slurry overflow  40  and slurry inlet  16  cannot be used to support disposable rotary drum filter  10 , because these elements must be accessible so that they may be connected to the appropriate vessels (a filter cake receptacle and a slurry supply source), or connected to appropriate fluid lines.  
     [0064] Accordingly, disposable rotary drum filter  10  will likely be secured in a cradle (not shown) that supports the arcuate portion of lower housing  14 . Alternatively, if desired, downwardly depending legs can be formed on lower housing  14 . However, because inner drum  18  is rotating (albeit at a modest 1-10 RPM), disposable rotary drum filter  10  is subject to some vibration, and it is preferable that the unit be secured to prevent movement, particularly when disposable rotary drum filter  10  is being used in a pharmaceutical or fine chemical application in which the value of the material being processed can be extremely high. Those of ordinary skill in the art will readily understand that a variety of different support structures can be used to provide support for disposable rotary drum filter  10 .  
     [0065] It should also be noted that the idea of using an inexpensive and disposable piece of equipment in a process involving very high value material is not as unusual as it might initially appear. One might normally expect that only expensive equipment would be used to process high value material. However, the disposable nature of the unit means that the requirement of cleaning a filtering unit in between batches is eliminated, since a new clean filtering unit will be used for each successive batch processed. As noted previously, use of a disposable rotary drum filter in accord with the present invention effects labor cost savings related to the equipment cleaning and pre-batch processing setup, and perhaps more importantly, minimizes the potential for introducing contaminants, possibly missed in a cleaning process, into a next batch of high value material. Such contaminants could conceivably destroy the value of the entire batch. As the value of the batch increases, so does the need to eliminate potential sources of contamination. So while a disposable rotary drum filter in accord with the present invention is an inexpensive piece of equipment, it offers significant advantages in the processing of high value material.  
     [0066]FIG. 5 shows further details for the support of shaft  42 . Shaft  42  can either be formed integral to inner drum  18 , or can be a separate component that is fixedly attached to inner drum  18 . Preferably, shaft  42  is integral to inner drum  18 , so that fewer production steps are required to produce disposable rotary drum filter  10 . On the opposite end of disposable rotary drum filter  10 , collar  45  is supported by journal  62 , cooperatively formed in the upper and lower housings (as is clearly shown in FIG. 2). As noted above, the fit between journal  60 /shaft  42  and journal  62 /collar  45  should be sufficiently tight to provide the required support, yet loose enough so as to enable inner drum  18  to freely rotate.  
     [0067] Shaft  42  incorporates a magnetic portion  48  in which are disposed permanent magnets  43  to enable inner drum  18  to be drivingly rotated without requiring a direct drive coupling that penetrates housing  11 . Because disposable rotary drum filter  10  is expected to find application in the pharmaceutical and food industries, it is important that the interior of the housing be isolated from contaminants in the environment. Eliminating a drive coupling that requires an additional opening in housing  11  enhances the sealed state of the interior of the housing to isolate the material being processed from environmental contaminates. Magnetic portion  48  is magnetically coupled to a magnetic portion  52  in external coupling hub  20  in which are disposed permanent magnets  23 . External coupling hub  20  incorporates a cavity having a size and shape corresponding to shaft  42 . Note that the strength of the magnets must be such that magnetic coupling across the thickness of housing  11  is readily achievable.  
     [0068] Those of ordinary skill in the art will readily understand that a variety of modifications are possible with respect to magnetically coupled drives. Magnetic portion  48  could include a plurality of strips of magnetic material rather than permanent magnets  43 . Similarly, magnetic portion  52  could include a plurality of circumferentially spaced-apart strips of magnetic material, rather than permanent magnets  23 . As a further alternative, magnetic portion  52  could include a plurality of spaced-apart electromagnets rather than permanent magnets. Because rotary drum filter  10  is designed to be disposable, the lowest cost option capable of providing the rotational driving force to shaft  42  is generally preferred. Note, however, that external coupling hub  20  is not intended to be disposable, but instead, will likely be reusable and can be left permanently attached with a set screw or key to a drive shaft of an electric motor or other prime mover (not shown). Thus, it is envisioned that an economical design will likely include relatively powerful and more expensive permanent magnets  23  in external coupling hub  20 , and relatively inexpensive magnets  43  or strips of magnetic material (such as iron) on shaft  42 . In such an embodiment the magnets in external hub  20  can be either permanent magnets, or electromagnets.  
     [0069] Note that it is possible to fabricate magnetic portion  48  as a reusable component that is removably attached to drive shaft  42 , to be removed and reused in subsequent disposable rotary drum filters, thus enabling more expensive, more powerful magnets to be used without adversely impacting the cost of the devices. However, such a component would increase the number of steps required to set up and discard disposable rotary drum filter  10 , adding complexity to a device that by design is elegantly simple, yet extremely effective and easy to operate.  
     [0070] It should also be noted that at least a portion of shaft  42  could be in contact with the slurry in disposable rotary drum filter  10 , thus preferably, the permanent magnets or strips of magnetic material that are included on shaft  42  must be inert and unaffected by chemicals in the slurry during the time of its use. Polymer coated magnets, gold coated magnets (e.g., rare earth magnets used for magnetic therapy are frequently coated in gold, and are readily available), and polymer coated magnetic material can be used in the present invention. The use of such an inert material coating is especially important when disposable rotary drum filter  10  is to be employed in a pharmaceutical application, or in any other application where components that are not inert might introduce some contamination (such as rust from uncoated permanent magnets or magnetic material) into the slurry. In some applications, the introduction of a contaminant such as rust might not be a problem, and in such cases, the use of coated magnets (or strips of magnetic material) would not be required.  
     [0071] A helical spring  50  is disposed between the end of shaft  42  and the inner surface of the portion of the housing that surround the shaft. This spring biases inner drum  18  toward the end of disposable rotary drum filter  10 , ensuring that sealing surface  44  is sealingly abutted against housing  11 . Note that helical spring  50  must not provide so much tension that friction between sealing surface  44  and the inside surface of the housing against which it abuts prevents the inner drum from rotating. In a device designed to be durable over a long-term use, a rotating seal like that used in the present invention would represent an unacceptable source of wear. However, since disposable rotary drum filter  10  is designed to be discarded after treating a batch of material, the design of this seal is completely appropriate here. Helical spring  50  also helps to compensate for expansion and contraction of the housing due to any temperature changes that may occur during use. In at least one embodiment, helical spring  50  is fabricated from a plastic material.  
     [0072] It is anticipated that in one embodiment, the magnetic force described above (coupling shaft  42  to external coupling hub  20 ) will also provide a biasing force to position inner drum  18  toward the end of disposable rotary drum filter  10 , similarly ensuring that sealing surface  44  is sealingly abutted against housing  11 . In such an embodiment, helical spring  50  is not required. FIG. 5 shows a distributor  54 , which distributes slurry entering via slurry inlet  16 , thereby deterring the accumulation of solids in the bottom of the housing  11 . Preferably distributor  54  is a flat circular plate that distributes the incoming slurry flow outwardly from the longitudinal center of the slurry inlet, although shapes other than a circular plate can be used. The distance between distributor  54  and housing  11  is such that the diverted flow retains a sufficient velocity to prevent the solids suspended in the incoming slurry from accumulating in the bottom of housing  11 .  
     [0073] Also shown in FIG. 5 are filtrate tubes  38 , which provide fluid communication between fluid channels  24  and filtrate discharge  28 . As is clearly illustrated in FIG. 7, filtrate tubes  38  are arrayed in a radial pattern, with each fluid channel  24  being provided a filtrate tube  38  that conveys filtrate from one of the fluid channels toward the center at the end of inner drum  18  that is adjacent to filtrate discharge  28 . Filtrate tubes  38  terminate in a cavity  46  that is disposed along the longitudinal axis of inner drum  18 , and which is in fluid communication with filtrate tubes  38  and filtrate discharge  28 .  
     [0074] Disposed within cavity  46  is plate  56 . As noted above, plate  56  effectively separates the interior of inner drum  18  into which the plate extends, into an upper portion and a lower portion. The purpose of plate  56  and orifice  36  is to provide a mechanism to equalize pressure within disposable rotary drum filter  10 . Orifice  36  is sized so that a pressure within fluid channels  24 , orifices  26 , and filtrate tubes  38  in the current upper portion of inner drum  18  is substantially equal to a pressure within fluid channels  24 , orifices  26 , and filtrate tubes  38  in the current lower portion of inner drum  18 . The size of orifice  36  determines the flow of fluids through filtrate discharge, because the size of orifice  36  regulates the pressure differentials within housing  11 , thereby effecting a filtrate flow rate through disposable rotary drum filter  10 .  
     [0075] Flow resistance for air (or gas) within fluid channels  24   a  defined by the frame comprising the upper portion of the inner drum is much smaller than flow resistance for liquid filtrate within fluid channels  24   b  defined by the frame comprising the lower portion of the inner drum. Without a mechanism to equalize the pressure drops through these two different sets of fluid channels and their corresponding filtrate tubes, a relatively weak vacuum source coupled to the filtrate discharge would only draw air from the disposable rotary filter. In traditional rotary drum filters, this problem is solved by the use of a complicated filter valve (see U.S. Pat. Nos. 4,419,165 and 4,515,693, for example). The use of such complicated valves is not appropriate in a device designed for low cost, simplicity, and disposability.  
     [0076] By providing orifice  36  in the plate  56 , the applied vacuum can only draw a limited amount of gas through the filtrate discharge. Further, the size of orifice  36  is selected to achieve the desired ratio of gas flow to filtrate flow from the disposable rotary filter. Note that air (or some other supplied gas) enters disposable rotary drum filter  10  though gas inlet  34  to compensate for the volume of air (or gas) that flows out of disposable rotary drum filter  10  via filtrate discharge  28 . For applications in which it is beneficial to isolate the material being processed from external contaminants that could be found in ambient air, an air filter (not shown) can be attached to gas inlet  34 . Alternately, gas inlet  34  can be placed in fluid communication with a source of pre-filtered air or gas, such as a tank of compressed nitrogen. Appropriate air filters and clean air sources are well known in the art.  
     [0077] It should be noted that when the relative size of disposable rotary drum filter  10  is small, and if the vacuum source that is in fluid communication with filtrate discharge  28  is relatively strong, control of the pressure differentials within housing  11  is not required (because under such circumstances the vacuum source is so much stronger than a minimal vacuum level required for the unit to draw the slurry into the filter). As used herein to describe a vacuum source, the terms “weak” and “strong” refer to the vacuum level achievable by the vacuum source, i.e., a “strong” vacuum source draws a substantially lower vacuum level than a “weak” vacuum source. When a relatively weak vacuum source is used, control of the pressure differentials within housing  11  can be a concern. Note that because the strength of the vacuum source required is directly related to the size of the rotary drum filter, it is only when disposable rotary drum filter  10  is relatively small that a standard industrial vacuum source is likely to provide a significantly lower vacuum level than would minimally be required. Because of the significantly larger sizes of prior art rotary drum filters, the strength of the vacuum sources typically used is not significantly greater than that capable of achieve a minimally required vacuum level, necessitating the control of pressure differentials within the prior art units by the use of complicated valve mechanisms. It is anticipated that a vacuum level of 0.25 to 0.50 atmospheres will be sufficient to operate disposable rotary drum filter  10 .  
     [0078]FIG. 6 shows further details of edge  30 , which is provided for removing filter cake from an outer surface of inner drum  18 , and details of filter cake discharge  32 , and slurry overflow  40 . Note that the arcuate shaped portion of the bottom of housing  11  serves as the slurry trough or reservoir (see also lower housing  14  in FIG. 2). As the slurry fills the bottom of housing  11 , it reaches the level of the lower edge of the opening into slurry overflow  40 , and exits disposable rotary drum filter  10 . The level of this opening thus determines a maximum slurry level (generally as indicated by the dash lines). As noted above, preferably 30-35% of inner drum  18  is bathed in the slurry. The triangular cross-sectional shape of fluid channels  24  formed by the frame of the inner drum can also be clearly seen in this sectional view. This shape adds structural rigidity to inner drum  18 , though other shapes can be used to provide similar results.  
     [0079] The directions of fluid flow and inner drum rotation are clearly illustrated in FIG. 6. Raw slurry enters disposable rotary drum filter  10  via slurry inlet  16 , and make up air or gas (to replace air or gas exiting the unit with the filtrate) enters via gas inlet  34 . Filter cake shaved from the filter drops through disposable rotary drum filter  10  through filter cake discharge  32 , and filtrate exits through filtrate discharge  28 . As illustrated, inner drum  18  must rotate in the direction indicated (shown here as turning in a counterclockwise direction) for edge  30  to be effective in shaving filter cake from the filter on inner drum  18 . Note that a mirror image device in which the inner drum rotates in a clockwise direction would function equally as well.  
     [0080] Note that the specific dimensional details for slurry inlet  16 , filter cake discharge  32  and filtrate discharge  28  can be chosen as appropriate to couple with tubing, sanitary piping, ASME-type piping, or any other pipe, tube, hose, or delivery/receiving passage desired. Alternatively, the specific dimensional details for slurry inlet  16 , filter cake discharge  32  and filtrate discharge  28  can be set to a standardized dimension, based on the most likely required size, and then appropriately-sized couplings can be used to accommodate the tubing, piping or hoses of different size that will be coupled thereto. The inlets and outlets for disposable rotary drum filter  10  can be integral to housing  11  as shown in the Figures, or attached to housing  11  as separate components using a suitable adhesive or fasteners.  
     [0081]FIG. 7 clearly shows the radial configuration of filtrate tubes  38  and plate  56  that is used for separating the volume within housing  11  into an upper region and a lower region (as described above) to control the flow of filtrate through disposable rotary drum filter  10 .  
     [0082]FIG. 8 is a processing diagram that illustrates the additional components used in conjunction with rotary drum filter  10 . Raw slurry to be processed is provided from a slurry supply  70 . It should be noted that slurries with extremely high levels of suspended solids (greater than 60%) generally are processed with some sort of gravity-based separation techniques. It is anticipated that the present invention will be able to process a wide variety of different types of slurries containing lower levels of suspended solids. Note that the use of plastic to fabricate rotary drum filter  10  assures compatibility with a wide variety of slurry chemicals.  
     [0083] A pump  72  is used to delivery slurry to slurry inlet  16  at the bottom the lower housing of rotary drum filter  10 . It is anticipated that a peristaltic type pump will be particularly useful in providing the necessary pumping action, although other types of pumps can alternatively be used. A prime mover (an electric motor)  84  is preferably used to power both pump  72  and rotary drum filter  10 , although a separate electric motor or other type of prime mover can be used to power pump  72 . Note that to reduce the number of components used in rotary drum filter  10  and to minimize costs that enable the unit to be used once and then disposed, rotary drum filter  10  does not contain an electric motor. As explained above, disposable rotary drum filter  10  is preferably magnetically coupled via external coupling hub  20  to prime mover  84 . However, prime mover could be coupled directly to shaft  42  of inner drum  18  if the shaft extended outside housing  11  through an opening that were fitted with a seal (the opening and seal are not shown). Such an arrangement would be acceptable if the material being processed were not subject to quality concerns due to the introduction of foreign matter entering through the opening for the shaft. Also, forming a leak-proof seal around the opening at which the shaft exits the housing would increase the costs and complexity of the device.  
     [0084] Also provided in the system employed to operate the disposable rotary filter is a vacuum source  82 , which is in fluid communication with filtrate discharge  28 . While prime mover  84  provides the mechanical rotational force required to rotate inner drum  18  of disposable rotary drum filter  10 , vacuum source  82  provides the negative pressure that draws filtrate from disposable rotary drum filter  10  through fluid channels  24  and filtrate tubes  38 , and out through filtrate discharge  28 . Because of the relatively small size of disposable rotary drum filter  10 , a strong vacuum source is not required. In fact, a standard vacuum pump or the in-house vacuum system that is typically readily available in industrial or commercial laboratory settings is expected to provide a much higher vacuum than required, and an appropriate throttling mechanism may be used to control the vacuum to achieve a desired level. The prototype rotary drum filter discussed above has been successfully operated using a consumer vacuum cleaner to provide the vacuum. As noted above, it is anticipated that a vacuum of 0.25 to 0.50 atmospheres will be sufficient to operate disposable rotary drum filter  10 .  
     [0085] With prime mover  84 , pump  72 , and vacuum source  82  all properly connected and operating, raw slurry from slurry supply  70  is drawn into disposable rotary drum filter  10  through slurry inlet  16 . The vacuum produced from the external source and applied to disposable rotary drum filter  10  through filtrate discharge  28  draws the filtrate fraction of the slurry through filter  22  as inner drum  18  is rotated through the slurry filling the bottom of lower housing  14  (see FIG. 6), depositing the filter cake on the outer surface of the filter. As inner drum  18  rotates out of the slurry, air or gas drawn through filter  22 , at least partially dries the filter cake by removing additional fluid. The air that is drawn into filter  22  does create a pressure drop, which could necessitate the inclusion of plate  56  and orifice  36  as described in conjunction with FIG. 5.  
     [0086] The filtrate passing through filter  22  is directed into fluid channels  24  that extend longitudinally along the frame of the inner drum under filter  22 . These fluid channels extend along the length of inner drum  18  and are distributed in spaced-apart array evenly around the circumference on its frame. The vacuum from vacuum source  82  and filtrate discharge  28  draws the filtrate through fluid channels  24 , and into orifices  26  that are disposed in the then current lower portion of the inner drum. The filtrate is then drawn through filtrate tubes  38  into cavity  46 , and exits through filtrate discharge  28 . The air (or other gas) drawn through filter  22 , similarly passes through fluid channels  24 , orifices  26 , filtrate tubes  38  and cavity  46 , and then exits disposable rotary drum filter  10  via filtrate discharge  28 . Additional air (or gas) must be introduced into disposable rotary drum filter  10  to make up for the air (or gas) that exits. This makeup air (or gas) enters disposable rotary drum filter  10  through gas inlet  34 . A valve  78  can be included to control the volume of makeup air (or gas) introduced into disposable rotary drum filter  10  via gas inlet  34 . Note that gas inlet  34  can be in fluid communication either with the ambient atmosphere, if air is the selected gas, or in fluid communication with a different gas source, such as a compressed gas tank. The advantage of using a compressed gas supply is that an inert gas, such as nitrogen, can be used. Even if air is the gas selected, a compressed gas supply source can be used to provide air that is clean and filtered, to reduce the likelihood of introducing any contaminants into disposable rotary drum filter  10 . If ambient air, rather than a compressed gas supply source is used, filters (not shown) can be used to “clean” the ambient air before it is introduced into disposable rotary drum filter  10 . Such filters are well known in the art.  
     [0087] As illustrated in FIG. 8, filtrate exiting disposable rotary drum filter  10  is selectively diverted by a valve  80  to one of two filtrate receivers  76 . A valve  86  selectively connects vacuum source  82  to the selected filtrate receiver so that the vacuum source is applied through the filtrate receive to the filtrate discharge. Note that a single filtrate receiver  76  can be employed if it is sufficiently large to accommodate the desired batch volume. Incorporating a plurality of filtrate receivers  76  and a valve  86  into a system used in conjunction with disposable rotary drum filter  10  ensures that a full filtrate receiver  76  can be isolated from filtrate discharge  28  and replaced with an empty filtrate receiver  76 , while at the same time an additional filtrate receiver  76  is connected to filtrate discharge  28  and vacuum source  82 , enabling disposable rotary drum filter  10  to be operated continuously without interruption. Note that a filter cake receiver  74  is not in fluid communication with vacuum source  82 , and that replacing a full filter cake receiver  74  with an empty unit does not require a similar valve and multiple receiver arrangement to ensure that disposable rotary drum filter  10  can be operated continuously. It should also be understood that more than two filtrate receivers  76  can be employed, as long as filtrate discharge  28  remains in fluid communication with vacuum source  82  while disposable rotary drum filter  10  is in operation.  
     [0088] As inner drum  18  rotates, the filter cake is deposited on the filter out of the slurry, and then the filter cake is dried by the flow of air or gas through it. When inner drum  18  reaches approximately the 9 o&#39;clock position, at least a portion of the built-up filter cake is removed by contacting edge  30 . Note that edge  30  does not remove all of the filter cake from the filter, as a certain clearance (preferably {fraction (1/16)}th to ⅛th inch) is provided between edge  30  and inner drum  10  in order to leave a predetermined thickness of filter cake on the filter throughout the operation of disposable rotary drum filter  10 . The filter cake shaved away by edge  30  falls down filter cake discharge  32  under the influence of gravity, into a filter cake receiver  74 . Preferably filter cake receiver  74  is temporary sealed to filter cake discharge  32  to maintain sealed integrity within disposable rotary drum filter  10 . Either the filter cake and/or the filtrate can then be employed as desired in any further processing steps, depending upon the specific application of the disposable rotary drum filter. After the batch of slurry is processed, the prime mover is de-energized to stop both pump  72  and the rotation of inner drum  18 . The rotary drum filter can then be removed and discarded in an appropriate manner.  
     [0089] Although the present invention has been described in connection with the preferred form of practicing it and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the present invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.