referring to fig1 a filter device 1 of the present invention may comprise a filtration means 2 , cage means 3 and support means 4 . the filtration means 2 is made from a geotextile material or nylon material or shade cloth , suitable for the filtration of suspended solids from water passing through the material . in fig1 the filter material is formed into an elongate bag - configuration . the upper end 5 of the filtration means 2 is open and affixed or securable to the support means 4 by means of a steel support ring 6 placed in a sewn loop 7 at the top of the bag . in fig3 the steel ring 6 sits on a lip 8 built in to the support means 4 . this allows the filter bag 2 to be removed easily and the steel support ring 6 can also be removed from the filter bag 2 for washing . the bottom 9 of the filtration means 2 extends substantially to the bottom of the cage means 3 . the filtration means 2 may be formed into the required bag shape by sewing or such other like method which provides substantially sealed seams . fig1 shows a filter device 1 of the present invention positioned in a storm water drain having a catchpit 10 , a gate 11 , a curb edge 12 , and overflow opening 13 and an outlet 14 . the support means 4 of a filter device of the present invention is positioned in catchpit 10 beneath grate 11 and held in position by lugs ( not shown ) on the catchpit frame 10 which stop the grate 11 from falling through . where the catchpit has no lugs the cage 3 must extend to the bottom of the catchpit and the support means then abuts on the cage 3 . in this case the cage has a base to prevent the bottom 9 of the filter bag 2 from abutting the bottom of the catchpit . referring now to fig2 to 9 the support means 4 comprises a frame 15 and a sill 16 abutting an upper surface of the frame 15 . the sill 16 is adapted to direct water into the orifice 17 of frame 15 and prevent the passage of water into the catchpit except via the cage means 3 . an opening 18 is adapted to direct water into the orifice 17 from an backflap 19 ( see fig9 ) positioned in a storm water drain via flexible chute 25 . the cage 3 includes projections 30 adapted to maintain the cage 3 away from the side walls of the catchpit . this is particularly important when the cage filter bag is full and may partially protrude through the cage 3 . without the projections 30 the cage 3 could be forced against the outlet causing a blockage . the projections maintain a water flow around the cage . the cage 3 has large spacings 31 to minimize the surface area of the cage and maximize the filter bag area . the backflow 19 may also include boot seal rubber 20 ( see fig9 ), to ensure a snug fit , placed on the ends of the backflap which can be pushed firmly into place and the front of the backflap 19 is designed to clip over the existing cast iron grate of the drain . in the embodiment shown in the drawings , the support means has a rectangular configuration with a sill on each side , including an extended sill on one longitudinal side . however , it will be appreciated that the invention is not restricted to any particular shape of support means , different shapes being required for drains or catchpits having other than a rectangular cross - section . a filter device of the present invention may be quickly and easily retrofitted into existing storm water drains by raising the grate , locating the filtration means into the catchpit and slotting the support means into the top part of the catchpit . where the drain includes a backflow opening the backflap should be positioned in the back opening first . collected sediment may be removed , either by conventional suction means with the filter device retained within the catchpit , or by temporary removal or replacement of the storm water filter device , or at least the filtration means , for cleaning and disposal of the sediment in an appropriate manner . in a pilot study report prepared by city design for auckland city council ( unpublished ) june 1997 two field trials were performed to determine overall sediment removal efficiency and to determine hydraulic performance . the first trial involved 22 catchpits with filter devices of the present invention installed therein . various grades of bidim geotextile were the filter medium . the second trial used 3 pairs of catchpits in similar catchments and a control catchpit , and utilized various filter media such as bidim geotextile , nylon ( 100 micron pore size ) nylon ( 300 micron pore size ) and shade cloth . the removal efficiencies were determined by collecting samples of materials removed by the filter bags and their hydraulic performance was determined by taking field measurements over time . the bidim geotextiles had a greater removal of suspended solids ( 63 %- 77 %) than the nylon mesh , which removed 32 %- 48 %. the control catchpit , without a filter device installed , removed between 5 %- 6 %. a surprising result was that the nylon mesh was partially self - cleaning with water flushing the dry solids adhering to the sides of the bag into the base of the bag . this did not occur for the geotextile bags because of the fibrous nature of the bag . whilst the geotextile bags had the greatest degree of suspended solid removal they also clogged more easily and exhibited reduced hydraulic efficiency than the nylon mesh bags . over time the filter bag clogs and loses permeability . when inflow exceeds outflow , the water level in the bag slowly rises until flow bypasses the filter through the overflow . under overflow conditions the static water level was much higher than normal which resulted in less turbulence . this aids the settling of particles and prevent re - suspension of particles , improving removal efficiency . as some flow was still passing through the filter bag there was a &# 34 ; suction &# 34 ; effect within the bag which drew the suspended particles downwards reducing particle re - suspension . the study also showed that the filter device of the present invention could provide significant removal of heavy metals , ie . cadmium , copper , lead , zinc and mercury whether dissolved or attached to sediment from storm water . heavy traffic on roads caused the most pollution and optimally required that a geotextile bag would have to be replaced on a weekly basis . in respect of accommodating runoff generated from a 1 in 1 year design storm from a catchpit and replacing bags when their permeability drops below this yielded the following life of a bag . ______________________________________bidim geotextile 1 to 7 weeksnylon mesh ( 100 micron ) 3 to 12 weeksnylon mesh ( 300 microns ) 6 to 12 weeksshade cloth 3 mm 12 + weeks______________________________________ an advantage of the nylon mesh over the geotextile is that the nylon mesh can be cleaned and re - used . from these results it is clear that the filter device of the present invention not only provides for source control of pollutants , but also substantially reduces maintenance costs associated with cleaning line blockages through sediment build - up . whilst the invention has been described with reference to a storm water filter device which may be retrofitted in existing storm water drains with a catchpit , it will be appreciated that new or replacement storm water drains or catchpits may be designed with the frame means integrally informed as an upper part of the drain or catchpit . in such embodiments of the invention the filtration means would be secured in the support means in a removable manner , to facilitate cleaning of the filtration means or disposal of damaged filtration means . furthermore , whilst the invention has been described with reference to storm water drains it will be appreciated that a filter device of the present invention may be employed in any type of drain where there is a need for pollution control and , in particular , the removal of suspended solids from a liquid , such an example is a soak hole which has an outlet at the bottom which is a bore down to the acquifer or fractured rock . in soak hole applications it is important that the bottom of the filter bag is held above the bottom of the soak hole . where in the foregoing description reference has been made to specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth . although this invention has been described by way of example and with reference to possible embodiments thereof it is to be understood that modifications and variations may be made thereto without departing from the scope or spirit of the invention .