The configuration of contact tanks can result in appreciable differences between actual and theoretical contact times. Contact times and germicidal efficiency depend on a number of parameters, the most important being the mixing characteristics of the basin. Proper designs must account for possible flow pattern elimination via short circuiting, acceptable dosage rates, optimum pH range, and upstream removal
Rapid dispersement of chlorine at the addition point increases chlorine contact and improves disinfection efficiency. Baffles can be designed to generate turbulence at the chlorine addition point and improve mixing. Baffled systems have the advantage of not requiring mechanical equipment. Mechanical mixing or air agitation can be used where plant hydraulics will not allow the use of baffles, or where a portion of the existing basin can be converted to a mixing chamber and the remainder of the basin and/or a long outfall sewer can be used to provide the needed contact time.
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