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1 1 clarification, therefore reducing the effective dosage rate. Preparation of working solu- tions and thorough dispersal of polymers in water to be treated is also important to estab- lish the appropriate dosage rate. For a given water sample, there is generally an optimum dosage rate that yields the low- est residual turbidity after settling. When dosage rates below this optimum value (under - dosing) are applied, there is an insufficient quantity of coagulant to react with, and therefore destabilize, all of the turbidity present. The result is residual turbidity (after floc- culation and settling) that is higher than with the optimum dose. Overdosing, application of dosage rates greater than the optimum value, can also negatively impact per- formance. Again, the result is higher residual turbidity than that with the optimum dose. Mixing in Coagulation/Flocculation: The G-value, or just "G", is often used as a measure of the mixing intensity applied during coagulation and flocculation. The symbol G stands for "velocity gradient", which is related in part to the degree of turbulence generated dur- ing mixing. High G-values mean high turbulence, and vice versa. High G-values provide the best conditions for coagulant addition. With high G's, turbulence is high and coagu- lants are rapidly dispersed to their appropriate concentrations for effective destabilization of particle suspensions. Low G-values provide the best conditions for flocculation. Here, the goal is to promote formation of dense, compact flocs that will settle readily. Low G's provide low turbulence to promote particle collisions so that flocs can form. Low G's generate sufficient tur- bulence such that collisions are effective in floc formation, but do not break up flocs that have already formed. Design engineers wishing to review more detailed presentations on this subject are referred to the following textbooks. . Fair, G., J. Geyer and D. Okun, Water and Wastewater Engineering, Wiley and Sons, NY, 1968. . American Water Works Association, Water Quality and Treatment, McGraw-Hill, NY, 1990. . Weber, W.J., Physiochemical Processes for Water Quality Control, Wiley and Sons, NY, 1972. Adjustment of the pH and Alkalinity: The pH must be in the proper range for the polymers to be effective, which is 6.5 to 8.5 for Calgon CatFloc 2953, the most commonly used polymer. As polymers tend to lower the pH, it is important that the stormwater have suf- ficient buffering capacity. Buffering capacity is a function of alkalinity. Without sufficient alkalinity, the application of the polymer may lower the pH to below 6.5. A pH below 6.5 not only reduces the effectiveness of the polymer, it may create a toxic condition for aquatic organisms. Stormwater may not be discharged without readjustment of the pH to above 6.5. The target pH should be within 0.2 standard units of the receiving water pH. 2014 Stormwater Management Manual for Western Washington Volume 11 - Appendix B - Page 421