Chlorine disinfection requires 15 to 30 minutes contact time to provide adequate pathogen kill. Reuse applications typically require additional time. Dechlorination requires only 30 seconds of mixing and contact time to quench residual chlorine. The logical question for plants that have to add dechlorination is "Can I retrofit my chlorination system to accomplish dechlorination without building new tanks or reducing effectiveness of my chlorination system?" Traditional chlorine contact tank (CCT) design tries to minimize short circuiting by using high length to width ratios or serpentine flow paths. These design practices do not always produce ideal plug flow conditions and the contact tank configuration does not alone control chlorine mixing, distribution and contact time. Distribution chambers and feed channels also have to be considered when trying to understand the total performance of a chlorination system. Computational Fluid Dynamics (CFD) modeling, coupled with field investigations, were successfully used in this study to investigate and optimize the chlorination process and aid in design of a new dechlorination system that is able to be retrofitted within the existing chlorine contact tanks at New York City's North River WPCP.
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