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Stormwater Pollution Prevention Plan <br /> •1 <br /> Treatment chemicals that have already passed the above testing protocol <br /> ' do not need to be reevaluated. Contact the Department of Ecology <br /> Regional Office for a list of treatment chemicals that have been evaluated <br /> and are currently approved for use. <br /> Treatment System Design Considerations <br /> ' The design and operation of a chemical treatment system should take into consideration the <br /> factors that determine optimum, cost-effective performance. It may not be possible to fully <br /> incorporate all of the classic concepts into the design because of practical limitations at <br /> construction sites. Nonetheless, it is important to recognize the following: <br /> • The right chemical must be used at the right dosage. A dosage that is <br /> either too low or too high will not produce the lowest turbidity. There is an <br /> optimum dosage rate. This is a situation where the adage "adding more <br /> is always better" is not the case. <br /> ■ The coagulant must be mixed rapidly into the water to insure proper <br /> dispersion. <br /> ■ A flocculation step important im ortant to increase the rate of settling, to produce <br /> ' the lowest turbidity, and to keep the dosage rate as low as possible. <br /> • Too little energy input into the water during the flocculation phase results <br /> in flocs that are too small and/or insufficiently dense. Too much energy <br /> can rapidly destroy floc as it is formed. <br /> 'I . Since the volume of the basin is a determinant in the amount of energy <br /> per unit volume, the size of the energy input system can be too small <br /> relative to the volume of the basin. <br /> ■ Care must be taken in the design of the withdrawal system to minimize <br /> outflow velocities and to prevent floc discharge. The discharge should be <br /> directed through a physical filter such as a vegetated swale that would <br /> catch any unintended floc discharge. <br /> ' Treatment System Design <br /> Chemical treatment systems shall be designed as batch treatment systems using either ponds <br /> Ior portable trailer-mounted tanks. Flow-through continuous treatment systems are not allowed <br /> at this time. <br /> A chemical treatment system consists of the stormwater collection system (either temporary <br /> diversion or the permanent site drainage system),a storage pond, pumps, a chemical feed <br /> system, treatment cells, and interconnecting piping. <br /> 14 16580.006.doc <br />