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Stormwater Pollution Prevention Plan <br /> The treatment system shall use a minimum of two lined treatment cells. Multiple treatment cells <br /> allow for clarification of treated water while other cells are being filled or emptied. Treatment <br /> cells may be ponds or tanks. Ponds with constructed earthen embankments greater than six <br /> feet high require special engineering analyses. Portable tanks may also be suitable for some <br /> Isites. <br /> The following equipment should be located in an operations shed: <br /> ■ the chemical injector; <br /> • secondary containment for acid, caustic, buffering compound, and <br /> treatment chemical; <br /> ' • emergency shower and eyewash, and <br /> ■ monitoring equipment which consists of a pH meter and a turbidimeter. <br /> Sizing Criteria <br /> The combination of the storage pond or other holding area and treatment capacity should be <br /> large enough to treat stormwater during multiple day storm events. It is recommended that at a <br /> minimum the storage pond or other holding area should be sized to hold 1.5 times the runoff <br /> volume of the 10-year, 24-hour storm event. Bypass should be provided around the chemical <br /> treatment system to accommodate extreme storm events. Runoff volume shall be calculated <br /> using the methods presented in Volume 3, Chapter 2. if no hydrologic analysis is required for <br /> the site, the Rational Method may be used. <br /> Primary settling should be encouraged in the storage pond. A forebay with access for <br /> maintenance may be beneficial. <br /> There are two opposing considerations in sizing the treatment cells. A larger cell is able to treat <br /> a larger volume of water each time a batch is processed. However, the larger the cell the longer <br /> the time required to empty the cell. A larger cell may also be less effective at flocculation and <br /> therefore require a longer settling time. The simplest approach to sizing the treatment cell is to <br /> multiply the allowable discharge flow rate times the desired drawdown time. A 4-hour <br /> drawdown time allows one batch per cell per 8-hour work period, given 1 hour of flocculation <br /> followed by two hours of settling. <br /> The permissible discharge rate governed by potential downstream effect can be used to <br /> ' calculate the recommended size of the treatment cells. The following discharge flow rate limits <br /> shall apply: <br /> ■ If the discharge is directly or indirectly to a stream, the discharge flow rate <br /> shall not exceed 50 percent of the peak flow rate of the 2-year, 24-hour <br /> event for all storm events up to the 10-year, 24-hour event. <br /> 15 16580A06.doc <br />