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fl charge requirements, especially when construction is to proceed through the wet sea- son. Conditions of Use Formal written approval from Ecology is required for the use of chemical treatment regardless of site size. The Local Permitting Authority may also require review and approval. When approved, the chemical treatment systems must be included in the Con- struction Stormwater Pollution Prevention Plan (SWPPP). Design and Installation Specifications See Appendix II-B: Background Information on Chemical Treatment (p.419) for back- ground information on chemical treatment. Criteria for Chemical Treatment Product Use: Chemically treated stormwater dis- charged from construction sites must be nontoxic to aquatic organisms. The Chemical Technology Assessment Protocol (CTAPE) must be used to evaluate chemicals pro- posed for stormwater treatment. Only chemicals approved by Ecology under the CTAPE may be used for stormwater treatment. The approved chemicals, their allowable applic- ation techniques (batch treatment or flow -through treatment), allowable application rates, and conditions of use can be found at the Department of Ecology Emerging Tech- nologies website: http://www.ecy.wa.gov- /programs/wq/stormwater/newtech/technologies.html . Treatment System Design Considerations: The design and operation of a chemical treatment system should take into consideration the factors that determine optimum, cost-effective performance. It is important to recognize the following: • Only Ecology approved chemicals may be used and must follow approved dose rate. . The pH of the stormwater must be in the proper range for the polymers to be effect- ive, which is typically 6.5 to 8.5 . The coagulant must be mixed rapidly into the water to ensure proper dispersion. . A flocculation step is important to increase the rate of settling, to produce the low- est turbidity, and to keep the dosage rate as low as possible. . Too little energy input into the water during the flocculation phase results in flocs that are too small and/or insufficiently dense. Too much energy can rapidly destroy floc as it is formed. • Care must be taken in the design of the withdrawal system to minimize outflow velocities and to prevent floc discharge. Discharge from a batch treatment system should be directed through a physical filter such as a vegetated swale that would catch any unintended floc discharge. Currently, flow -through systems always 2014 Stormwater Management Manual for Western Washington Volume 11 - Chapter 4 - Page 397