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Pollution Plan <br /> Stormwater o!ution Prevention 1a <br /> Sediment Removal and Disposal S ment <br /> ' ■ Sediment shall be removed from the storage or treatment cells as <br /> necessary. Typically, sediment removal is required at least once during a <br /> wet season and at the decommissioning of the cells. Sediment remaining <br /> in the cells between batches may enhance the settling process and <br /> reduce the required chemical dosage. <br /> ■ Sediment may be incorporated into the site away from drainages. <br /> 3.1.14 Element #14 — Construction Stormwater Filtration <br /> Filtration removes sediment from runoff originating from disturbed areas of the site. <br /> ' Traditional BMPs used to control soil erosion and sediment loss from sites under development <br /> may not be adequate to ensure compliance with the water quality standard for turbidity in the <br /> receiving water. Filtration may be used in conjunction with gravity settling to remove sediment <br /> as small as fine silt(0.5 pm). The reduction in turbidity will be dependent on the particle size <br /> distribution of the sediment in the stormwater. In some circumstances, sedimentation and <br /> filtration may achieve compliance with the water quality standard for turbidity. <br /> Unlike chemical treatment, the use of construction stormwater filtration does not require <br /> approval from Ecology. <br /> Filtration may also be used in conjunction with polymer treatment in a portable system to assure <br /> ' capture of the flocculated solids. <br /> Design and Installation Specifications— Background Information <br /> Filtration with sand media has been used for over a century to treat water and wastewater. The <br /> use of sand filtration for treatment of stormwater has developed recently, generally to treat <br /> runoff from streets, parking lots, and residential areas. The application of filtration to <br /> construction stormwater treatment is currently under development. <br /> Two types of filtration systems may be applied to construction stormwater treatment: rapid and <br /> slow. Rapid sand filters are the typical system used for water and wastewater treatment. They <br /> can achieve relatively high hydraulic flow rates, on the order of 2 to 20 gpm/sf, because they <br /> have automatic backwash systems to remove accumulated solids. In contrast,slow sand filters <br /> have very low hydraulic rates, on the order of 0.02 gpm/sf, because they do not have backwash <br /> systems. To date, slow sand filtration has generally been used to treat stormwater. Slow sand <br /> filtration is mechanically simple in comparison to rapid sand filtration but requires a much larger <br /> filter area. <br /> Filtration Equipment <br /> Sand media filters are available with automatic backwashing features that can filter to 50 pm <br /> particle size. Screen or bag filters can filter down to 5 pm. Fiber wound filters can remove <br /> ' 18 16580.006.doc <br />