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1 particularly important during water treatment. Water temperature is important during set- tling. Both the density and viscosity of water are affected by temperature; these in tum affect settling. Cold temperatures increase viscosity and density, thus slowing down the rate at which the particles settle. The conditions under which clarification is achieved can affect performance. Currents can affect settling. Currents can be produced by wind, by differences between the tem- perature of the incoming water and the water in the clarifier, and by flow conditions near the inlets and outlets. Quiescent water such as that which occurs during batch cla- rification provides a good environment for effective performance as many of these factors become less important in comparison to typical sedimentation basins. One source of cur- rents that is likely important in batch systems is movement of the water leaving the cla- rifier unit. Given that flocs are relatively small and light the exit velocity of the water must be as low as possible. Sediment on the bottom of the basin can be resuspended and removed by fairly modest velocities. Coagulants: Polymers are large organic molecules that are made up of subunits linked together in a chain -like structure. Attached to these chain -like structures are other groups that carry positive or negative charges, or have no charge. Polymers that cant' groups with positive charges are called cationic, those with negative charges are called anionic, and those with no charge (neutral) are called nonionic. Cationic polymers can be used as coagulants to destabilize negatively charged turbidity particles present in natural waters, wastewater and stormwater. Aluminum sulfate (alum) can also be used as this chemical becomes positively charged when dispersed in water. In practice, the only way to determine whether a polymer is effective for a specific applic- ation is to perform preliminary or on -site testing. Polymers are available as powders, concentrated liquids, and emulsions (which appear as milky liquids). The latter are petroleum based, which are not allowed for construction stormwater treatment. Polymer effectiveness can degrade with time and also from other influences. Thus, manufacturers' recommendations for storage should be followed. Man- ufacturer's recommendations usually do not provide assurance of water quality pro- tection or safety to aquatic organisms. Consideration of water quality protection is necessary in the selection and use of all polymers. Application Considerations: Application of coagulants at the appropriate concentration or dosage rate for optimum turbidity removal is important for management of chemical cost, for effective performance, and to avoid aquatic toxicity. The optimum dose in a given application depends on several site -specific features. Turbidity of untreated water can be important with turbidities greater than 5,000 NTU. The surface charge of particles to be removed is also important. Environmental factors that can influence dosage rate are water temperature, pH, and the presence of constituents that consume or otherwise affect polymer effectiveness. Laboratory experiments indicate that mixing previously settled sediment (floc sludge) with the untreated stormwater significantly improves 2014 Stormwater Management Manual for Westem Washington 1 1 1 1 1 1 1 1 1 1 1 (I 1 1 1 LI 1 Volume Il - Appendix 8 - Page 420 11