Abstract:
A modular biofiltration support system is quickly erectable in virtually any size and shape using plastic modules that are interconnected at the site. Included in the system are bed platform modules which assemble together to form a bed platform to support the weight of the biofilter medium within the area defined by the frame, and a multiplicity of bed support legs which are assembled into the bed platform modules to hold the bed platform at a raised position above the ground, defining a plenum for entering gases, between the ground and the bed platform. In one embodiment, at least some of the legs include air directing vanes which can be oriented in positions to cause generally even distribution of the gases within the plenum. The bed platform modules have openings to direct the gases to flow upwardly through the biofilter medium. The periphery of the plenum within the bed platform preferably is substantially closed, and this can be by taller peripheral modules in one embodiment, or by angled edge platform modules in another embodiment, that form a slope from the bed platform down to the ground or base.

Description:
This is a continuation of application Ser. No. 09/248,775 filed Feb. 12, 1999, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention concerns removal of noxious odors and components from gases given off by a sewage treatment facility. More specifically, the invention concerns a modular support system for erecting a biofiltration support bed in virtually any size and shape desired. 
     Biological air pollution control has been used on an increasing basis in recent years to cleanse noxious and odorous gases, such as those emanating from a sewage treatment plant. The biological filtration systems, or biofilters utilize a bed of stacked biofilter material such as compost, tree bark, peat, heather or soil, generally about three feet or more deep. The containment gas from the sewage treatment plant or other sources is blown through the biofilter material in an upward direction. The material in the biofiltration bed provides an environment for a diverse culture of microorganisms, which degrade the gaseous pollutants as they pass through the biofilter. 
     Such biofiltration or biological treatment of the gases is an inexpensive treatment method, but the raw gas stream must generally meet certain conditions: The pollutants should be water soluble and biodegradable, and free of toxic components. Also, there must be sufficient concentration of oxygen. Further, the gas stream should nearly saturated with water (relative humidity at least about 95%), and at temperature between about 40° F. and 140° F. (more preferably between 50° F. and 100° F.). The odorous gas must also contain no more than small amounts of dust and grease. 
     These requirements usually dictate that the raw gas be preconditioned before entering into the biofilter bed. Preconditioning includes humidification, temperature control and removal of particulates. 
     In the biofilter itself, a biofilm covers the substrate of tree bark, peat, heather, etc. Water soluble air pollutants are absorbed into the biofilm and decomposed into carbon dioxide and water by the microbes. The filter beds are often built on a single level, but where area is limited, multiple level biofilters have been constructed, with the raw gas stream divided and fed in parallel to the various levels of the biofilter system. 
     Regarding sizing of the biofilter beds, the height of the filter bed, i.e. the stack of organic material as the medium, is generally in the range of about 1½ feet to 5 feet. Areas are up to tens of thousands of square feet. One rule of thumb regarding the calculation of needed area for a given waste air flow is that the “area load” or ratio between the waste gas flow rate and the filter area should be in the range of about 2.5 to 3.0 CFM/SF. The flow distribution and humidity of the inlet gas have to be carefully controlled, otherwise the filter bed will eventually clog and cease to function. Distribution throughout the area of the filter bed should be relatively even. 
     In addition to sewage treatment plants, biofilters are useful for treating the effluent gases from many other processes. These include factory farming, rendering plants, coffee roasters, foundries, composting plants, kraft paper drying and paint shops. The odorous emissions treatable include ammonia, hydrogen sulfide, amines, aldehydes, mercaptains, alcohols, disulfides, esters, fatty acids, unsaturated hydrocarbons, ketones, hexane, dichloromethane, formaldehyde, phenol, organic acids, acetone, toluene and others. 
     These biofilter systems are being used increasingly in treating the gases from sewage treatment processes and also some of the other processes listed above. There is a need for a relatively simple, inexpensive and modular approach for constructing a filter bed support for such biofilters. 
     Hallsten U.S. Pat. No. 5,454,195 discloses a modular containment system for preventing hazardous materials from leaching into the ground or otherwise escaping a defined containment area. That system includes peripheral modules and a liner to define the containment area, the modules being fillable by water or granular material, and the system has some relevance to the present invention described below. 
     SUMMARY OF THE INVENTION 
     The invention provides a very efficient and relatively inexpensive solution to erection of a biofilter support bed of desired area, on a single level or, in another embodiment, on multiple stacked levels. The system consists of modular components, quickly assembled on a flat site which may be unpaved ground, and with capability to produce many different sizes and rectangular shapes, as well as areas, of a filter bed support to retain and confine a biofilter medium. 
     The system of modules for construction of the support apparatus includes a series of peripheral modules or berm modules with means for assembling the modules end-to-end to form a free-standing peripheral frame surrounding a filtering area. A multiplicity of polygonal bed platform modules with generally planar top surfaces are shaped to be arranged side-by-side contiguously to cover the entire filtering area within the peripheral frame. In the bottom side of each bed platform module are leg-receiving recesses. At least some of the bed platform modules are perforated so as to allow gas to pass vertically through. 
     On the interior sides of the peripheral modules are module support means for engaging with and supporting edges of the bed platform modules to thereby provide partial support for those platform modules positioned at the periphery of the filtering area and adjacent to the peripheral frame. The system includes bed support legs having upper ends sized and shaped to be closely received in the leg-receiving recesses in the bottom sides of the bed platform modules, and of length sufficient to support and elevate the bed platform modules above a base surface, thus forming a plenum under the bed platform modules. The legs are assembled on-site into the platform modules. 
     The peripheral modules are substantially taller than the bed support legs, so that the assembled modular support apparatus forms a large open-topped tray for supporting the bed of biofilter medium, with the peripheral frame extending above the support surface to contain the bed and with the plenum below the support surface and also bordered by the peripheral frame. 
     In a preferred embodiment the module support means on the peripheral modules comprises a horizontally extending recess or slot in the interior side of each peripheral module, of a size to receive an edge of a bed platform module. Accordingly, no support legs are needed near the outer edges of the bed platform modules which are adjacent to the peripheral frame. 
     Since distribution of the gases in a relatively even manner throughout the area of the biofilter medium is important to efficiency of the process, the invention in one specific embodiment has vanes secured to at least some of the bed support legs within the plenum under the support bed, and these vanes are adjustable in orientation such that they can be oriented to direct gases to achieve a generally even distribution. To avoid collection of gas toward the periphery of the plenum, and to prevent gas flow up along the peripheral module walls (avoiding treatment), the outermost bed platform modules, those adjacent to and in contact with the peripheral frame, may be provided without perforations, or with reduced perforations. 
     To seal the gas plenum against the ground and against leakage at the peripheral frame, the system preferably includes a substantially gas-impermeable liner which lies on the ground or slab and on which the legs rest supporting the bed platform modules. The peripheral modules also rest on the liner, and in a preferred embodiment, these modules have a vertical slot extending up from the bottom side in such a way as to form a substantially continuous slot around the periphery, the liner being forced up into this slot or recess by a liner retainer block which is press fit against the liner and into the slot or recess. The peripheral modules are hollow and fillable with water or granular material, and when filled, they improve the seal with the liner. 
     One situation where the modular system of the invention is particularly advantageous is when a relatively large biofiltration system is to be erected to treat a particular effluent. Generally in prior biofiltration system installation, a small pilot biofilter has been erected first, to treat the effluent of the noxious or odorous gas and determine frequent effectiveness. With the system of the invention, a very small biofiltration unit can be first erected to test the efficacy of the system; then the same modules and additional modules can be put together to build the full system as needed. The smaller initial system can also help confirm design size needed. 
     The system of the invention can also be used for composting. The modular components for this purpose are the same. A bed of organic material to be composted is placed on the support platform formed by the platform modules. Air is drawn out of the plenum, thus drawing air down through the compost bed, at a slow rate. 
     In another embodiment the peripheral modules are not employed. The bed platform modules can simply be assembled into a filter bed platform, again forming a plenum beneath, into which noxious or odorous gases can be delivered for passage up through the bed platform and through a biofiltration medium. At edges of the plenum, the biofilter medium can simply be piled over the sides of the platform, sloping down to the ground or base and thus substantially closing the plenum and allowing gases to pass up through the filter medium at the edges. Alternately, the filter platform can be erected in a recessed space, such as a shallow excavation in the ground or on a flat surface but with a dirt berm or other material surrounding the filter bed platform at its edges. Again, the biofilter medium can be stacked on the platform and can slope down to the ground or the berm beyond the edges. 
     Preferably the filter bed platform has some form of substantial closure device around the periphery. This can be comprised of sloping edge platform modules, which interconnect with the edges of the bed platform modules at the periphery, and preferably around all sides. These edge platform modules, like the regular bed platform modules, can include perforations for passage of gases up through the filtering medium. 
     The invention thus achieves an inexpensive, rapidly erected modular support system for biofiltration or composting, that can be erected quickly and adjusted to specifications. These and other objects, advantages and features of the invention will apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a portion of an erected modular support system for the filter medium of a biofiltration system, in accordance with the invention. 
     FIG. 2 is a sectional elevation view showing a portion of the support system and paned filter medium, including a peripheral module at the edge of the support apparatus. 
     FIG. 3 is an enlarged, more detailed sectional view showing a portion of a peripheral module, a liner and other bed support apparatus. 
     FIG. 4 is a plan view showing a bed platform module. 
     FIG. 5 is a schematic sectional elevation view showing a multi-level modular biofiltration system. 
     FIG. 6 is a schematic view, again in sectional elevation, showing a modular biofiltration system positioned on a cover of process tank which generates gases to be filtered. 
     FIG. 7 is a view similar to FIG. 1, but showing a different form of the modular support system. 
     FIG. 7A is a view similar to FIG. 7 but showing a slight variation. 
     FIG. 8 is a schematic sectional elevation view showing the system of FIG. 7, supporting a bed of biofiltration medium. 
     FIG. 8A is a view similar to FIG. 8 but showing a slight variation. 
     FIG. 9 is a more detailed cross-sectional view showing a portion of the erected system of FIGS. 7 and 8, at the edge of the filter bed platform. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a portion of a modular support system  10  for a biofiltration system used to filter and treat noxious or odorous gases. The modular system includes peripheral modules  12 , including corner peripheral modules  14 , which are field-erected and secured together by interlocking devices  16  that include a generally vertically oriented socket  18  on one end of a module and a mating dowel  20  at first and second ends  22  and  24  of the modules, respectively, interfitted together by vertically arranging and lowering one module down against another to seat the dowel  20  in the socket  18 . In a preferred embodiment the dowels and sockets are generally cylindrical. The dowels and sockets are closely sized so as to prevent rotation between the connected modules about a vertical axis and to provide a stable and close connection between adjacent modules, so that the assembled modules act as a unified peripheral frame. 
     Each of the peripheral modules  12  or  14  preferably is hollow, and may be formed by rotocasting polyethylene or polypropylene, polypropylene generally being more heat resistant (the biofiltration process generates heat). The modules are thus fillable with water, other liquid or pourable particulate material such as sand. Access holes with caps  25  are provided for this purpose. 
     FIG. 2 shows a peripheral module  12  in cross section, showing a liquid  26  such as water filling the module. 
     The peripheral frame  28  formed by the assembled peripheral modules  12  and  14  (only a portion of the frame being shown in FIG. 1) may rest on a poured slab or other pavement, or on relatively level ground. The interior space formed by the peripheral frame should be isolated from the ground, and this is achieved by a liner  30  as shown in FIG.  1 . The liner may be approximately 30-mil thick polyethylene and lies on the ground and under the peripheral frame  28 . It is preferably sealed substantially gas tight against the peripheral frame, and this may be achieved by a deep slot or recess  32  extending up into the bottom of each peripheral module  12  and  14 , into which the liner is placed. A sealing block  34  is press-fit up into the liner in the slot to form the substantially gas-tight connection. When the frame modules  12  are filled, the weight puts increased pressure against the top and sides of the sealing block  34 , improving the seal. 
     FIG. 1 also shows a series of bed platform modules  36  and  38  assembled preferably in a grid like manner as shown to form a planar support bed  40  for a filter medium to be contained on the bed and within the confines of the peripheral frame  28 . These bed platform modules  36 ,  38  are closely adjacent or interconnected and define below the support bed a gas plenum  42 , confined by the liner  30  and the interior walls  44  of the peripheral modules  12 . As shown, the modules  36  have openings  46  for delivery of the noxious or odorous gas up from the plenum  42  and through the bed of filter medium for biofiltration treatment. 
     FIG. 1 also shows bed support legs  48  supporting the platform modules  36  and  38 . At the outermost bed platform modules  38 , which are adjacent to the peripheral frame  28 , these bed platform modules  38  preferably interconnect with the peripheral modules  12  for support, as the figure illustrates. Support is provided by the edges of the modules  38  extending into a horizontally extending recess  50  formed on the inner side of all peripheral modules  12 . In the system as shown, the corner peripheral module  14  does not have a face extending to the interior filter medium containment area, and in the form shown, the corner module  14  need not have a horizontal recess  50 . If corner modules are made larger so as to have faces in the interior of the filtering area, then such corner modules would also have recesses similar to the recesses  50 . The bed platform modules  38  are received relatively closely within the recess  50  so as to form a solid connection between the support bed  40  and the peripheral frame  28 . 
     The platform modules  38  are shown without any gas openings  46 . This is for two reasons: to aid in the even distribution of the gas within the plenum  42 , with the goal of a balanced distribution of gas flow through the medium; and to prevent gas from flowing up along the interior walls  44  of the peripheral frame  28 , between the filter medium and the frame, thus escaping biofiltration treatment. 
     FIG. 1 also shows two different arrangements for introduction of gas into the plenum  42  formed underneath the platform bed  40 . In one arrangement, a conduit  54  of appropriate diameter for the design flow rate of gas enters from above the top of the peripheral frame  28 , piping the gas down through a vertical portion  56  and through one of the non-perforated bed platform modules  38 , through a hole (not shown) the size of the conduit. The conduit is large enough so as not to require excessive flow velocity on entry of the plenum  42  which might interfere with achieving the desired generally even flow distribution within the plenum  42 . Another arrangement is shown in dashed lines  58  in the drawing, where a conduit is placed underground (or under a slab), emerging through the liner  30 , as shown at  60 , the end of the conduit being appropriately sealed against the liner, as by a band  62  tightening the liner against the outside surface of the conduit. 
     As noted above, the modular support system can be used for composting, with air flow in the reverse direction. Organic material for composting is loaded onto the bed platform  40  and air is withdrawn at a low flow rate via the conduit  54  or  58 . Exhaust air can be put through a scrubber if desired. 
     FIG. 2 shows a portion of the erected system  10  in cross section, and also showing a bed of stacked filter medium  64 , which may be bark or other organic material. As indicated, the medium  64  can be piled virtually to the top of the inner wall  44  of the peripheral modules  12 . In this exemplary embodiment, the peripheral modules are about four feet tall, with the tops of the bed platform modules  36 ,  38  being at about one foot. This leaves a depth of three feet for the biofilter material  64 . Each peripheral module  12  may be about four feet in length, and  18  inches wide at the base  66 , seen in FIG.  2 . The module may be tapered as shown, for increased stability. 
     FIG. 2 also shows that the bed platform modules  36  and  38  are interconnected side by side, a feature seen in better detail in FIG.  3 . In a preferred embodiment the edges of each bed platform module have a hooked flange, oriented either upwardly or downwardly as shown, each bed platform module having two adjacent edges which hook downwardly and another two adjacent edges, opposite the first two adjacent edges, hooked upwardly. In the sectional views of FIGS. 2 and 3, the downwardly hooked flanges  68  are shown on left sides of the modules or the upwardly hooked flanges  70  are shown on right sides of the modules. This interlocks the adjacent modules and adds to the integrity of the system. 
     FIGS. 2 and 3 also show in greater detail the extension of the bed platform modules  38  into the horizontal slot or recess  50  in the interior wall of the peripheral frame, i.e. of each peripheral module  12 . The depth (height dimension) of the bed platform modules  38  is preferably selected so as to fit closely within the horizontal recess  50 . It may be about four inches. 
     FIGS. 2 and 3 also show the legs  48 , which fit closely into leg receiving sockets or recesses  71  formed in the bottoms of the bed support modules  36 ,  38 . 
     It is important that the support bed  40  of the system can support considerable weight. In large-area biofilter systems, small earth-moving equipment is often used to place the filter material  64  on the support bed. The bed platform modules  36  and  38  may be each about two feet by two feet in size, and molded of polyethylene, approximately 0.15 inch thick. Preferably an egg-crate type design is used, as indicated in FIGS. 2 and 3, with support arches  72  bridging the space between support legs  48 , as shown. The system depicted in FIGS. 2 and 3 can support about 10,000 lbs. over a four square foot area. 
     FIGS. 2 and 3 additionally show directional vanes  74  connected to the support legs  48 . The vanes, which can be oriented appropriately to generally evenly distribute the gas within the plenum under the support bed  40 , may be connected to the legs by holding integrally with the legs (as shown), or they can be attached, as by having a collar which fits over the leg. Pairs of vanes on a leg are preferably at 180°-opposed locations, as shown. 
     FIG. 4 shows in plan view one of the bed platform modules  36 . The leg supporting recesses  71  are indicated in dashed lines beneath the surface, extending toward the bottom of the module, and the flanges  68  and  70  for interlocking with adjacent modules are also indicated. 
     The gas openings  46  are shown in X shape in accordance with a preferred embodiment, distributed on a two foot square module  36 . It has been found that openings of this X-shape, formed of slots about ¼ inch wide for a typical system having bark as a filter medium, tend not to clog during use. The size and frequency of these X-shaped gas openings  46  is selected so as to add virtually no additional pressure head loss to the gas flow, beyond that already introduced by the filter medium  64  itself. Thus, in a preferred embodiment these openings add almost nothing to the head loss of the gas, beyond that introduced by the required movement of the gases throughout the plenum and up through the filter medium. As one example of a system of the invention, 5,000 cubic feet per minute (CFM) gas is introduced via an 18-inch duct  54 . The gas is ducted into a 40 foot by 50 foot plenum  42 , approximately 8 inches in height. With the illustrated arrangement and the gas openings  46  as shown in FIG. 4, the support bed  40  has about 5088 vents, and with equal flow, this would be 0.983 CFM per vent  46 . If each vent opening has an area of 0.0082 square feet, the flow rate through each vent would be only about 119 feet per minute. This compares to the flow through the 18 inch duct of about 2830 FPM. Thus, the velocity pressure across the vent penetrations is so low as to be nearly negligible in calculation of head loss. 
     FIG. 5 shows a stackable modular system  80  according to the invention. In the illustrated version two layers of biofilter medium  64  are shown. Additional layers are possible. 
     In the illustrated system, peripheral modules  12   a  are in a modified form, having a vertical flange  82  at top, to fit closely into the slot like opening  32  formed in the bottom of a module  12   a  above. In this case the bed platform modules are also somewhat modified, in order to receive support legs  48   a  as shown. The legs  48   a  are considerably longer than the first-level legs  48 , since they define a height for the entire first bed of biofilter material  64 . A multiplicity of legs is shown in this embodiment, relatively closely spaced, for the case in which heavy equipment is to be supported on the upper support bed  40   a.    
     A cover  84  is shown over the top level of biofilter material, for containment of gases, and a gas vent  86  is shown schematically. The cover  84  rests on the peripheral modules  12   a , as well as being supported by a relatively small number of support legs  48   b . Such a cover can be included on the first embodiment of FIGS. 1-4, if desired. 
     As can be seen from FIG. 5, the modified bed platform modules  36   a  have leg sockets  71   a  at both top and bottom, with a layer of structural material of the module extending between the top and bottom sockets. 
     In the stackable peripheral modules  12   a , the tops of these modules in many cases are covered by the next-level modules. Access for filling the modules may be via a relocated fill opening and cap  25   a , at a high location on the outer side of the module  12   a . Alternatively, the fill openings can still be on tops of the modules, as shown at  25   b  but of low profile, and the lower-level modules  12   a  can be filled when the first level has been erected, followed by stacking the second level on the first. The fittings  25   b  can be flush fittings, spin welded into an opening in the top of the flange  82  after the unit has been roto-molded. 
     Gas can be introduced to the plenum  42  below the bottom level in the same manner as shown in FIG. 1, i.e. via a conduit similar to the conduit  58 . Flow through the two layers of biofiltration medium  64  can be in series, with initial treatment by the lower level and secondary treatment by the upper level, the treated gas then exhausting through the exit vent  86 . The gas can be moved at higher velocity through this multiple-stage biofiltration arrangement, then through a single-level biofilter covering the same ground area. However, gas can alternatively be introduced in parallel, with a gas entry to each level. In that case an additional plenum preferably is formed below the upper level, so that lower-level gas can be vented just above the lower-level biofilter material  64 , and gas can be introduced just below the support bed  40   a  of the top level. 
     FIG. 6 shows a further embodiment of the invention, in which a modular biofiltration system  10   a  is positioned and supported on a tank cover  90  covering a process tank  92 . The tank  92  can be, for example, a digester tank, clarifier tank or part of a pumping station in a sewage treatment system or in sewage handling, or it can be a process tank for any of the processes mentioned above, producing any of the effluent gases listed above or any other gas treatable by biofiltration. 
     In the illustrated system the tank cover  90  is sufficiently strong to support the load of the biofiltration assembly  10   a . The peripheral modules  12  may be filled to a lesser extent if desired, to reduce total weight on the cover assembly  90 . 
     The biofiltration process in this embodiment occurs directly above the source of the contaminant gas. A simplified ducting arrangement is shown in FIG. 6, comprising a duct  94  (which can include a blower  96 ) leading directly from the interior of the tank  92  up into the plenum  42  of the modular assembly, the gas then to be distributed through the openings in the support bed  40 , to pass upwardly through the bed of biofilter medium  64 . Although a liner can be used (similar to the liner  30  shown in FIGS.  1 - 3 ), in this case there is no need for a barrier between the plenum  42  and the tank below, and the peripheral frame, comprising the modules  12 , can simply be sealed against the surface of the tank cover assembly  90 . 
     In FIG. 7 is shown a modified form of modular filtration support apparatus  100 . The system of FIG. 7 is similar to that of FIG. 1 in many respects, the primary difference being the lack of tall peripheral modules such as the modules  12  shown in FIG. 1 that extend a substantial distance above the top of the bed platform modules  36  and  38  in that system. Instead, the system  100  preferably has edge platform modules  102 , including corner edge platform modules  102   a , to substantially close off the periphery of the plenum  42  defined beneath the bed platform modules  36 . The assembly fragment shown in FIG. 7 is only a portion of the entire assembly. 
     Thus, a filter medium support bed  104  is capable of supporting a stack of biofiltration medium or other filter medium  106  in the manner shown in FIG. 8, stacked on the bed platform modules  36  and extending over the side edges or periphery of the support bed  104 . The pile of filter medium  106  forms a natural slope  108 , sloping back down to the base surface  110  on which the medium rests, and this base surface  110  can comprise a liner  30  over the ground or other surface as in FIG.  1 . 
     The introduction of a withdrawal of gas into the plenum  42  formed underneath the platform bed  104  can be the same as described above, with a conduit  54  entering the plenum down through a non-perforated bed platform module  36   a , or, as an alternative, through an underground or under-slab conduit which emerges through and is sealed to the liner  30  as shown at  60 . Either of these conduits can alternatively be air withdrawal conduits, in the event the support bed  104  is used to support a compost pile, through which air is to be drawn downwardly and into the plenum  42 , prior to exiting via the conduit  54  or  60 . 
     It should be understood that the bed platform modules  36 , which preferably assemble together in an interlocked fashion as described previously and shown in FIG. 2, can alone form the support bed  104 , without any edge platform modules  102  closing the periphery of the plenum. This can be envisioned from FIG. 7, by simply removing the edge platform modules  102  and  102   a . This arrangement, though not preferred, would rely on the filtration medium  106  as substantially closing the periphery of the plenum  42 , simply by being piled over the edges of the support bed and sloping down to the base level  110  as in FIG.  8 . FIG. 8 can be envisioned with the edge platform modules  102  removed. This arrangement, though effective, is not preferred because of the desire for generally even gas distribution. The gas will take the path of least resistance, and thus will be somewhat unevenly distributed in favor of the periphery of the plenum. 
     FIG. 7A shows the same system as in FIG. 7, but with edge platform modules  102   b  having perforations  46 , as in the bed platform modules  36 . This can include the corner edge platform modules  102   c , as shown. As in the previous embodiment, modules near the periphery of the erected system can have reduced gas release area for purposes of achieving a generally even distribution. Thus, the non-perforated edge platform modules  102 ,  102   a  of FIG. 7 may be desirable in some circumstances, or the perforated modules  102   b  and  102   c  of FIG. 7A can have a smaller number or less total area of perforations per square foot in furtherance of achieving even distribution. Generally even distribution is desirable so as to use the biofiltration medium  106  to the maximum extent possible. 
     The schematic sectional view of FIG. 8A shows a form of edge platform module  102   b  having perforations  46 . 
     FIG. 9 is a more detailed cross-sectional view showing a portion of the bed platform  104  at an edge, and showing an edge platform module  102  having a perforation  46 . The supporting legs  48 , sockets  71  for the legs, support arches  72  in the bed platform modules  36 , and hook-type interconnections between modules, which can comprise downwardly hooked phalanges  68  and upwardly hooked phalanges  70  that engage as shown, preferably are the same as in the earlier described form of the invention. Although a leg  48  is shown supported in the edge platform module  102 , such support could be eliminated, at least on two sides of the completed support bed, if the interlocking phalanges  68  and  70  are reversed in orientation. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.