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The present invention generally relates to the collection and removal of trash or floating debris from waterways and, more particularly, to systems designed for use in combined sewer systems or storm drain conduits to trap water borne trash for removal. 
     BACKGROUND OF THE INVENTION 
     Trash and debris floating on the surfaces of waterways or along shorelines and beaches is a highly visible form of water pollution, which is receiving attention for its adverse, polluting effect and for its unaesthetic appearance on the surfaces of lakes and other water bodies. One type of system for the collecting and removing of floating debris has consisted of arrays of disposable mesh nets installed in receiving bodies of water in the flow path of a sewer outlet, particularly in applications referred to as “Combined Sewer Overflows” or “CSOs”. Such systems are described in Vol. 2, No. 3, of Fresh Creek Technologies, Inc. “Shorelines” newsletter. Systems of this type are effective in collecting floatables or trash for removal and are shown in Fresh Creek Technologies, Inc. Netting Trashtrap™ Product Bulletin. Improvements in such devices are described in U.S. Pat. No. 5,562,819, owned by the assignee of the present application, which provides an underground, in-line apparatus for trapping and collecting debris in a sewer or storm flow conduit, a secondary trap which provides continued protection when primary collection traps are full, a system which signals when primary bags or nets are full and servicing is required, and a trapping facility in which bags or nets may be replaced without loss of trapping protection during servicing. 
     More specifically, the device in the patent referred to above includes an enclosure or chamber with an inlet and an outlet each adapted to be connected to a sewer, storm drain conduit or outflow. A debris removing system is disposed within the chamber between the inlet and the outlet for trapping and collecting water borne debris entering at the inlet and thereby providing for an outflow of substantially debris-free water. The enclosure includes an access opening comprising upper doors or hatches or access hatches in the enclosure sized to allow the debris removing system to be removed and replaced. The debris removing system specifically includes a perforated container having an open end facing the inlet of the chamber. The perforated container includes a netting assembly that traps and collects the trash or floating debris. The container is in the form of a netting assembly having a flexible bag-shaped mesh net attached to a frame. The netting assembly is attached to lifting structure having supports or handles for allowing the frame and net to be lifted out when the net is full of captured debris. In some applications, a bypass weir or screen is provided to normally direct flow from the chamber inlet through the open end of the net while allowing flow to bypass the net and flow to the chamber outlet when the net is full of debris. 
     Sensing and signaling elements are typically provided for sensing and signaling the passage of solid debris around the net when the net is full of debris and is in need of servicing. The sensing and signaling elements may include mechanical structure which permits passage of water, but is displaced by impingement of solid debris flowing around the nets. Displacement of such mechanical structure signals when the net is full of debris, for example, by actuating a visible flag above ground or by actuating an electrical switch which activates an aboveground indicator or remote indicator. The sensing and signaling may include an optical sensor for detecting the passage of debris around the netting assembly. Upon detection of debris, the optical sensor emits a signal indicating that the trap is full of debris. The signal may also activate an aboveground indicator or a remote indicator. 
     Multiple trap systems are employed in which the enclosure includes side-by-side trap assemblies. Such systems may be configured such that, upon filling of the first trap, the flow and debris can be diverted over a bypass weir disposed between the inlet ends of the first and second traps so that flow is thereby directed through the second trap and overflow debris is trapped and collected. Closure panels may be provided in a stationary frame structure disposed adjacent the inlet ends of the traps in either the single-trap systems or the multitrap systems to restrain debris from flowing through the chamber during servicing. 
     The reliability of debris removing systems depends on the strength of the mesh nets and on the manner in which the net material is fabricated into the disposable net assemblies. The resultant hydraulic forces are a function of the velocity of the flow of water through the mesh of the nets as well as on the pressure exerted on the debris trapped by the nets. There are many outfalls where extreme forces exist that are too high for standard and commonly available materials or for materials made by normal fabrication practices to last. 
     Furthermore, the operation of such debris removal systems results in the nets filling with floatable materials over time as one or more overflows occur. In the process, large objects such as plastic bottles and sheets of plastic wrapping materials tend to cover and blind openings of the mesh, which reduces the overall effective area of the filter and results in higher hydraulic loading on the mesh, contributing to a higher pressure drop through the system and increased loads, and excessive forces on the nets. 
     Accordingly, a need exists for stronger and more reliable mesh nets in the traps of floatable debris collecting systems, and particularly for net assemblies that can be easily constructed and easily replaced. 
     SUMMARY OF THE INVENTION 
     A primary objective of the present invention is to provide a stronger and more reliable mesh net for the traps of systems for collecting floatable debris than have been provided by the prior art. A further objective of the invention is to provide a reliable net assembly for such systems that can be easily constructed and easily replaced. 
     According to principles of the present invention, disposable mesh nets are provided for debris traps that can withstand higher level of forces than can nets of the prior art. Such nets are, according to a preferred embodiment, made with a high strength and high stretch yarn and may be provided with reinforcing tape on seams and high stress areas of the net material. The flexible, stretchable mesh material allows for an increase in the free area of the mesh as the nets expand under hydraulic loads as the nets fill. High elasticity materials are those that are elastic enough, either due to their composition or the ways in which they are knitted, to allow the nets to deform when clogged with debris and thereby expand to allow flow paths around the trapped debris to minimize pressure. Nylon that has these properties would, for example, be suitable. The knit of the mesh material yarn is selected to produce the desired aperture size and maximize the breaking strength of the finished material and ability to maintain constant aperture. The material used in the manufacturing process enables the flexible mesh to maintain a consistent percentage of free area as the nets fill and expand. The material is fabricated into the form of a bag-shaped mesh net from flat material with seams that are rolled and stitched to give a strength greater than the knitted material itself. 
     Further according to principles of the invention, a netting assembly is provided with structure for holding the mouth of the bag-shaped net in an open position and which can be easily and securely attached to the netting material. In the preferred embodiment, the structure includes a one-piece frame that is provided with a strap configured to hold the netting material in place on the frame. The strap fits in a recessed groove molded into the outer perimeter of a generally rectangular molded plastic frame. Rows of raised buttons integrally molded into the frame extend from the bottom of the groove such that the mesh net will be sandwiched between the strap and the buttons. The frame is sized to provide sufficient strength to counter the hydraulic forces on the net. This particular embodiment of the invention is particularly suited to resist hydraulic forces in the dirty environment wherein the netting assemblies trap floating debris from waterways, sewers or storm drain conduits, as the frame assembly requires no removable locks, pins, clamps, brackets or other devices to hold down the netting material to the frame. The structure has a minimum of parts to collect debris while permitting the netting assembly to be loosened from the system with a pair of gloved hands. 
     In other embodiments, the netting assemblies are provided with a two part molded plastic rectangular frame, the parts of which clamp together with the knitted mesh material around the mouth of the net clamped therebetween, thereby evenly distributing the forces around the mouth of the net and holding the mouth in an open condition. The two part frame uses hole and post members on the respective parts that snap together for easy assembly. In another alternative embodiment, a one part rectangular frame is provided to which four plates having post members clamp into hole members on the frame. These embodiments have limited projections, thereby avoiding the collection thereon of debris with structure that can easily be loosened by gloved hands. 
     In accordance with certain principles of the invention, the traps are provided with net assemblies having a two-stage filter mesh. The nets for such traps are constructed of an inner net and an outer net. The inner net provides a first layer of mesh having larger aperture mesh openings so that the inner net captures only the larger items of debris, allowing the smaller items to pass through to the outer net or second layer of mesh. The outer net has smaller openings that trap smaller items of debris that pass through the openings of the inner net. The openings in the inner net may, for example, be at least two or three times the dimension of the openings in the outer net, or have an area from about four to ten times the area of the openings in the outer net. The outer net may also have a greater volume than the inner net, for example, at least about one fourth larger than that of the inner net. The two stage filter produces a larger effective filtering capacity, in that the trap does not blind as quickly, holds more material and distributes the hydraulic loads between the two layers resulting in greater overall strength. Further, were the first or inner net to fail, the second or outer net retains the ability to trap additional debris. 
     These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the preferred embodiments of the invention, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing the common features of a debris trapping system of the prior art for the removal of trash or floatables from flowing water. 
     FIG. 1A is an underground in-line version of the prior art system of FIG.  1 . 
     FIG. 1B is a floating version of the prior art system of FIG.  1 . 
     FIG. 1C is an end-of-pipe version of the prior art system of FIG.  1 . 
     FIG. 2 is a perspective view of the net assembly of a trap according to certain principles of the invention. 
     FIG. 2A is a cross-sectional view along line  2 A— 2 A of FIG.  2 . 
     FIG. 2B is a cross-sectional view along line  2 B— 2 B of FIG.  2 . 
     FIG. 3 is a perspective view of the net of a trap utilizing a net frame construction alternative to that of FIG.  2 . 
     FIG. 3A is a cross-sectional view along line  3 A— 3 A of FIG.  3 . 
     FIG. 3B is a cross-sectional view along line  3 B— 3 B of FIG.  3 . 
     FIG. 3C is a cross-sectional of an alternative to FIG.  3 B. 
     FIGS. 4A-4B are cross-sectional views illustrating double net construction according to certain embodiments of the present invention. 
     FIG. 5 is a perspective view of the net assembly of a trap according to an alternative embodiment of the invention. 
     FIG. 5A is a cross-sectional view along line  5 A— 5 A of FIG.  5 . 
     FIG. 5B is a cross-sectional view along line  5 B— 5 B of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates the basic components of one system  40  of the prior art described in the background of the invention above. The system  10  includes one or more traps  12 , illustrated as two in number, separately designated as traps  12   a  and  12   b . The traps  12   a ,  12   b  are located within a flow constraining housing or enclosure  11  between inlet  13  and outlet  14  thereof. The inlet  13  and the outlet  14  are each respectively connected in a known manner to conduits  15  and  16 , which may be storm drain or combined sewer conduits or other structures or the terrain of the site. The traps  12   a ,  12   b  each include a netting assembly  19  formed of a bag-shaped mesh net  17  that is attached to a lifting basket  18 . Each of the netting assemblies  19  captures and holds floatable velocity borne debris  20  entering enclosure  11  through inlet  13 . The arrows  25  indicate the direction of water flow. 
     Perforations or openings in nets  17  may vary in size depending on the intended use, with sizes generally in the range of from about 0.1″ to about 2″. Nets  17  are open on the upstream facing end  17   a  thereof, toward inlet  13  of enclosure  11 . Upper support members (not shown in FIG. 1) are attached to lifting baskets  18  for allowing the netting assemblies  19  of traps  12   a ,  12   b  to be lifted out of enclosure  11  for periodic removal of captured debris. The netting assemblies  19  are configured such that the nets  17  provide a large filter area for the size of the mouth, thereby minimizing head loss. For example, 80 square feet of net  17  may be provided for a netting assembly mouth area of 6 ½ square feet, resulting in a pressure drop across a net  17  of three or four pounds. 
     A bypass weir (not shown in FIG. 1) or screen is typically located upstream of traps  12  and on one side of inlet  13  to permit continued flow in the event that the nets  17  of traps  12   a ,  12   b  are filled to capacity with debris. To signal that nets  17  of the netting assemblies  19  of traps  12   a ,  12   b  are in need of replacement or emptying, sensing and signaling mechanisms may be provided. The multiple trap system  10  can be configured to provide continuous and uninterrupted capture of debris through second trap  12   b  after the netting assembly of first trap  12   a  has been filled and during the process of removing and replacing it. While servicing is being performed, movable panels can be positioned in front of each respective trap  12   a  or  12   b  being serviced, as necessary, prior to its removal from enclosure  11 . In this way, the system  10  is protected against passage of floatable debris during net removal and replacement. 
     FIGS. 1A-1C illustrate the basic system  10  of the prior art in three environments. These arrangements are generally described in a publication of the United States Environmental Protection Agency, Office of Water, No. EPA 832-F-99-037, September, 1999, hereby expressly incorporated by reference herein. 
     In particular, in FIG. 1A, an in-line system  10   a  is illustrated in which the two traps  12   a ,  12   b  are contained in an enclosure in the form of an underground or subterranean vault  11   a . The vault  11   a  includes its inlet  13   a  and its outlet  14   a  respectively connected to conduits in the form of buried pipes  15   a ,  16   a , for example, of a storm drain. The in-line traps  12   a ,  12   b  each include a netting assembly  19  with a mesh net  17  installed in and held in place by a respective lifting basket  18 . A lifting bridle (not shown) is attached to upper support members  21  of the lifting basket  18  for allowing the netting assemblies  19  of traps  12   a  and  12   b  to be lifted out of vault  11   a  through doors  22   a  for periodic removal of captured debris. A bypass screen  23   a  is located above the traps  12   a ,  12   b  to allow flow to divert from the inlet  13   a  to permit continued flow in the event that nets  17  of the traps  12   a ,  12   b  are both filled to capacity with debris. 
     In FIG. 1B, a floating system  10   b  is illustrated that is configured to float in a body of water in front of a stream, pipe or other water source from which enters into the body of water a flow of water containing trash or floatables to be removed by the system. The direction of water flow into and through the system  10   b  is also indicated by arrows  19 . The floating system  10   b  also includes two traps  12   a ,  12   b , shown in a floating hull  11   b  that is provided with closed cell foam panels  23  and pontoons to float the hull at the surface  28  of the body of water. The traps  12   a ,  12   b  also each include a mesh net  17  held in place within a lifting support  18   a . Because the system  10   b  is floating and the traps  12   a ,  12   b  are immersed in water, a less extensive support frame  18   a  is substituted for the lifting basket  18  of system  10   a , described above. 
     In the system  10   b,  the hull  11   b  has its inlet  13   b  extending above and below the surface  28  of the water so that trash or floatables at and immediately below the surface enter through it into the interior of the hull  11   b . The hull  11   b  has its outlet  14   b  below the water surface  28  on the back of the hull  11   b . The inlet conduit  15  is formed of a set of curtains  15   b  which hang from below the inlet  13   b  and from floats  24  extending respectively between the hull  11   b  on both sides of the inlet  13   b  to the shore on the opposite sides of the flowing source, connected to buried concrete conduits (not shown) of a storm drain, for example. The curtains  15   b  may extend from the water surface  28  to the bottom  29  of the water body and channel water from the source into the inlet  13   b . The traps  12   a ,  12   b  are supported in the hull  11   b  in a manner similar to the way they are supported in the vault  11   a  described above. They can be lifted out of hull  11   b  through grate doors  22   b  for periodic removal of captured debris from the nets  17  thereof. 
     In FIG. 1C, an end-of-pipe system  10   c  is illustrated in which the two traps  12   a ,  12   b  are shown in an enclosure in the form of a surface mounted three-sided concrete headwall and knee wall enclosed cavity  11   c  having an open end that defines its outlet  14   c . The cavity  11   c  has its inlet  13   c  connected to a pipe  15   c  draining into the cavity  11   c . The traps  12   a ,  12   b  each include a net assembly  19  having a mesh net  17 . A fiberglass drain grating  16   c  is provided beneath the netting assemblies  19  to allow flow to exit each net  17  through its bottom to the outlet  14   c  of the enclosure  11   c . The net  17  of each netting assembly is attached to a lifting structure (not shown), which may be similar to the lifting basket  18  described in FIG. 1A above, or in the form of lifting frame  18   a  described in FIG. 1B above where the traps  12   a ,  12   b  are submerged. Door grates  22   c  are provided above the traps  12   a ,  12   b  to permit them to be raised for periodic removal of captured debris. A bypass weir  23   c  may be located above the traps  12   a ,  12   b  to allow flow to divert from the inlet  13  to permit continued flow in the event that traps  12   a ,  12   b  are both filled to capacity with debris. 
     In FIGS. 2,  2 A and  2 B are illustrated netting assemblies for the traps  12  for use in systems  10  of the various types illustrated in FIGS. 1A-1C described above. According to certain aspects of the invention, the netting assemblies  19  are constructed with a mesh net  17  connected to a frame assembly  30 . The frame assembly  30  includes a rectangular frame body having a pair of horizontal top and bottom members  31  and  32 , respectively, and a pair of side members  33 . The top member  31  is wider than the bottom member  32 , and the side members  33  are tapered from the wider top member toward the narrower bottom member  32 , as illustrated in FIG. 2B, for easy installation and removal from the lifting basket  18  or support frame  18   a . The side members  33  are also inwardly tapered in the downstream direction, as illustrated in FIG. 2A, to lock into the supporting rails as the flow goes through the nets  17 . Flow direction is indicated by the arrows  25 . 
     Each of the members  31 - 33  has a rim  34  on the upstream side thereof and a recessed step  35  on the downstream side thereof. A pattern of holes  36  is formed in the steps  35  of each of the members  31 - 33 . Each of the members  31 - 33  has associated therewith a plate  37  having a plurality of projections in the form of posts  38  arranged in a pattern that corresponds to the pattern of the holes  36  in the respectively associated member  31 - 33  of the frame  30  so that the plates  37  can be connected to the members  31 - 33  by snap fitting the posts  38  into the holes  36 . The plates  37  are so connected with the edge of the mouth of the net  17  between the plate  37  and the respective member  31 - 33  and the posts  38  extending through holes in the mesh of the net  17 , thereby locking the mouth of the net  17  to the frame  30 . When so connected, the plates  37  set into the steps  35  so that the tops thereof are flush with the lip  34  of the members  31 - 33 . When the net  17  is attached to the frame  30 , the net extends around the outside of the members  31 - 33  with the mouth of the net wrapping around the upstream side of the frame  30  to the inside of the frame  30  and between the plates  37  and the members  31 - 33 . 
     The frame  30  may be made of wood and the plates  37  made of metal, but other materials may be used. In one preferred embodiment, the frame  30  is formed of an integral piece of molded plastic material. The plates  37  may also be formed of molded plastic. The frame  30  securely attaches to the nets  17  by being formed of elements that clamp together with the mesh material of the nets  17  between them, with one of the elements having posts or projections thereon against which the other member bears so that the projections serve as hooks that trap the net between the elements while the other element prevents the net from slipping off the projections. 
     An alternative frame structure  18  is illustrated in FIGS. 3,  3 A,  3 B and  3 C, in which mesh net  17  is shown connected to a frame assembly  40 . The frame assembly  40  is a two part rectangular frame that includes an inner frame portion  40   a  having an array of holes  46  on the upstream facing side thereof and an outer frame portion  40   b  having a matching array of posts on the downstream facing side thereof. The two portions  40   a , 40   b  of the frame snap together and clamp the mouth of the net  17  therebetween. The two parts of the frame  40  are preferably formed of an integral piece of molded plastic, but other materials may be used. 
     The frame  40  has a pair of horizontal top and bottom members  41  and  42  and a pair of side members  43 . The side members are tapered inwardly in the downstream direction and fit in correspondingly tapered vertical channels  44  in vertical rails  45  that are part of the lifting basket  18  or support frame  18   a . Further, the top member  41  is thicker in the flow direction (that is, upstream to downstream) than is the bottom member  42 ; and the side members  43  are correspondingly tapered in the downward direction to fit into the channels  44 , which are similarly tapered, as illustrated in FIG.  3 A. As a result of the tapers, the frame  40  of the netting assemblies  19  fit firmly in the channels  44  of the rails  45  when in position, but can be loosened by impact and removed with a minimum of sliding friction. FIG. 3B shows the net  17  wrapped around the outside of the frame  40  with the mouth of the net  17  wrapping around the front of the frame  40  and extending between the portions  40   a ,  40   b  thereof from the inside. Alternatively, FIG. 3C shows the net  17  wrapped around the inside of the frame  40  with the mouth of the net  17  wrapping around the front of the frame  40  and extending between the portions  40   a ,  40   b  thereof from the outside. 
     As a result of the tapers described above, the greater the forces on the traps, the more tightly the mesh nets  17  are gripped and the less likely are the nets to pull out or tear around the posts. 
     FIG. 4A illustrates a two layered net  17  that includes an inner net  17   a  of a course mesh having holes mounted to frame structure  18   c  so as to extend through the inside of the frame and with an outer net  17   b  of a fine mesh mounted to frame structure  18   c  so as to extend around the outside of the frame and thereby enclosing the inner net. The holes in the inner net  17   a  may, for example, be about 1-2 inches in size with the holes in the outer net  17   b  being of about ½ inches in size. The holes of the inner net  17   a  should be at least two to three times larger on a side than those of the outer net, with a cross sectional area of at least about four times the area of the holes of the outer net. As a result, large pieces of debris  48  such as plastic bottles, cans, plastic bags, styrofoam cups, etc. only are trapped by the inner net  17   a  while smaller pieces of debris  49  pass through the larger holes of the inner net  17   a  and are trapped by the outer net  17   b.    
     FIGS. 5,  5 A and  5 B illustrate netting assemblies for the traps  12  that are alternative embodiments of the assemblies of FIGS. 2-2B and FIGS. 3-3C described above. In FIGS. 5-5B, the traps  12  are each constructed with mesh net  17  connected to a frame assembly  50 . The frame assembly  50  includes a rectangular frame body. As with the embodiments above, the frame  50  is preferably formed of an integral piece of molded plastic, but other materials are suitable. The body of frame  50  has a pair of horizontal top and bottom members  51  and  52 , respectively, and a pair of side members  53 , with the top member  51  wider than the bottom member  52  and the side members  53  tapered from top to bottom as was illustrated in the embodiment of FIG.  2 B. The side members  53  are also inwardly tapered in the downstream direction, as illustrated in FIG.  5 A. Each of the members  51 - 53  has an outside surface  54  having a groove  55  extending around the frame  50 . On the bottom surface of the groove  55  is preferably a plurality of projections or posts  56  to help grasp the netting material, particularly where the frame is formed of plastic or other low friction material. A clamping element in the form of a tension band  57  lies in the groove  55  in contact with the tips of the projections  56 . The tension band may be of a natural fiber, metal or plastic. Plastic is particularly suitable for the band  57 . The net  17  extends between the band  57  and the frame members  51 - 53 , so that the mouth of the net  17  is locked to the frame  50 . When the frame  50  is inserted into the rails of the system, the tapered frame is forced against the frame by the forces produced by the flowing water on the net  17  to further clamp the net  17  between the frame  50  and the rail. 
     Other applications of the invention can be made. Those skilled in the art will appreciate that the applications of the present invention herein are varied, and that the invention is described in preferred embodiments. Accordingly, additions and modifications can be made without departing from the principles of the invention.

Summary:
A disposable net assembly is provided for a trap for collecting floatable debris in a waterway or combined sewer system. The net assembly includes a knitted bag-shaped mesh net having a frame surrounding the mouth of the net with the net secured around its rim to the frame. The net may be formed of an inner layer and an outer layer of mesh with the openings of the inner layer being substantially larger than the openings of the outer layer. The frame may be formed of a plastic molded material having side members tapered in the vertical direction to facilitate the changing of the netting assemblies and tapered in the downstream direction to lock into place under the force of the flow. Several embodiments of the frame members have projections thereon which cooperate with a clamping element to hold the net to the frame. Some embodiments of the members have parts that lock together with a post and hole construction while others employ a tension band to clamp the net to the projections on the frame. The net is preferably secured around its rim to the frame, with the mouth of the net extending around the outside and upstream side of the frame and over the surface having the projections. The net is preferably formed of a high strength and high stretch yarn, with rolled sewn seams and having reinforcing on the seams and on high stress areas of the net.