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1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />must be obtained at each site where polymers use is proposed prior to use. For more <br />guidance on stormwater chemical treatment see BMP C250: Construction Stormwater <br />Chemical Treatment (p.396). <br />Design and Installation Specifications <br />Two types of filtration systems may be applied to construction stormwater treatment: <br />rapid and slow. Rapid sand filters are the typical system used for water and wastewater <br />treatment. They can achieve relatively high hydraulic flow rates, on the order of 2 to 20 <br />gpm/sf, because they have automatic backwash systems to remove accumulated solids. <br />In contrast, slow sand filters have very low hydraulic rates, on the order of 0.02 gpm/sf, <br />because they do not have backwash systems. Slow sand filtration has generally been <br />used to treat stormwater. Slow sand filtration is mechanically simple in comparison to <br />rapid sand filtration but requires a much larger filter area. <br />Filtration Equipment. Sand media filters are available with automatic backwashing fea- <br />tures that can filter to 50 pm particle size. Screen or bag filters can filter down to 5 pm. <br />Fiber wound filters can remove particles down to 0.5 pm. Filters should be sequenced <br />from the largest to the smallest pore opening. Sediment removal efficiency will be related <br />to particle size distribution in the stormwater. <br />Treatment Process Description. Stormwater is collected at interception point(s) on the <br />site and is diverted to an untreated stormwater sediment pond or tank for removal of <br />large sediment and storage of the stormwater before it is treated by the filtration system. <br />The untreated stormwater is pumped from the trap, pond, or tank through the filtration sys- <br />tem in a rapid sand filtration system. Slow sand filtration systems are designed as flow <br />through systems using gravity. <br />Maintenance Standards <br />Rapid sand filters typically have automatic backwash systems that are triggered by a <br />pre-set pressure drop across the filter. If the backwash water volume is not large or sub- <br />stantially more turbid than the untreated stormwater stored in the holding pond or tank, <br />backwash return to the untreated stormwater pond or tank may be appropriate. However, <br />other means of treatment and disposal may be necessary. <br />• Screen, bag, and fiber filters must be cleaned and/or replaced when they become <br />clogged. <br />Sediment shall be removed from the storage and/or treatment ponds as necessary. <br />Typically, sediment removal is required once or twice during a wet season and at <br />the decommissioning of the ponds. <br />2014 Stormwater Management Manual for Western Washington <br />1 <br />Volume 11 - Chapter 4 - Page 405 <br />