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City of Everett <br /> EVERETT Reviewed for Permit 10/20/2021 <br /> wAsG.o <br /> top of 6" to 8" of 3/4" clean crushed rock. A gate valve on the discharge side of the 6" <br /> pump will control the overall filter system flow rate and a pressure compensating valve <br /> on the outlet side on each of the sand filters will control the overall filter system pressure <br /> across the sand filter bed. <br /> As the sand filter accumulates sediment its backpressure increases due to the <br /> incremental blockage of the sand media. At a pre-set, maximum differential pressure <br /> (the difference between the hydraulic pressure of the water at the input and output of <br /> the pods), or time interval, the filter system independently and automatically initiates a <br /> back wash cycle. During the back wash cycle a portion of the clean filtrate from the <br /> other two filter pods is used to flush out the captured sediment particles in a reverse <br /> hydraulic flow. The back-wash flow rate is approximately 220 gpm (for each 60" pod), <br /> and the cycle duration can vary from 2 to 10 minutes depending on the degree of mass <br /> loading of the filter (directly proportional to the turbidity of the storm water and the <br /> effectiveness of any pre-treatment system). The sight glass on the back-wash pipe is <br /> used to monitor the success of the flush cycle as well as verify no media is present in <br /> the flush water. The back wash water will be directed back to the inlet end of the <br /> storage pond (which is sectioned off with a floating weir or temporary wall), allowing the <br /> solids to settle and be isolated for periodic maintenance. <br /> 2.1.4 CESF Discharge Point <br /> The 6" discharge line from the Water Quality Monitoring Box will be under pressure and <br /> allow temporary piping of up to 350' to a designated discharge point. Discharge can be <br /> directly into waters of the state (using a flow diffuser), or an existing catch basin or pond <br /> overflow structure. Depending on the areas of the site being treated, two discharge <br /> points may be used to avoid any drainage diversion. The CESF discharge points will be <br /> added as an additional point in the monthly Discharge Monitoring Report form (for this <br /> project we will discharge down the asphalt side of the north storage pond). <br /> 9 <br />