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1 <br />1 <br />from settling on the cell bottom. This reduces the possibility of picking up sediment -floc <br />from the bottom of the pond. The struts are usually set at a minimum clearance of about <br />12 inches; that is, the float will come within 12 inches of the bottom of the cell. Other sys- <br />tems have used vertical guides or cables which constrain the float, allowing it to drift up <br />and down with the water level. More recent designs have an H-shaped array of pipes, <br />set on the horizontal. <br />This scheme provides for withdrawal from four points rather than one. This configuration <br />reduces the likelihood of sucking settled solids from the bottom. It also reduces the tend- <br />ency for a vortex to form. Inlet diffusers, a long floating or fixed pipe with many small <br />holes in it, are also an option. <br />Safety is a primary concern. Design should consider the hazards associated with oper- <br />ations, such as sampling. Facilities should be designed to reduce slip hazards and <br />drowning. Tanks and ponds should have life rings, ladders, or steps extending from the <br />bottom to the top. <br />Polymer Batch Treatment Process Description: <br />At a minimum, a flow -through chemical treatment system consists of the stormwater col- <br />lection system (either temporary diversion or the permanent site drainage system), an <br />untreated stormwater storage pond, and the chemically enhanced sand filtration system. <br />Stormwater is collected at interception point(s) on the site and is diverted by gravity or by <br />pumping to an untreated stormwater storage pond or other untreated stormwater holding <br />area. The stormwater is stored until treatment occurs. It is important that the holding pond <br />be large enough to provide adequate storage. <br />Stormwater is then pumped from the untreated stormwater storage pond to the chem- <br />ically enhanced sand filtration system where polymer is added. Adjustments to pH may <br />be necessary before chemical addition. The sand filtration system continually monitors <br />the stormwater for turbidity and pH. If the discharge water is ever out of an acceptable <br />range for turbidity or pH, the water is recycled to the untreated stormwater pond where it <br />can be retreated. <br />For batch treatment and flow -through treatment, the following equipment should be <br />located in a lockable shed: <br />. The chemical injector. <br />• Secondary containment for acid, caustic, buffering compound, and treatment chem- <br />ical. <br />Emergency shower and eyewash. <br />. Monitoring equipment which consists of a pH meter and a turbidimeter. <br />2014 Stormwater Management Manual for Western Washington <br />Volume 11 - Chapter 4 - Page 399 <br />