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Source: Adapted from W. Viessman, Jr. and M.J. Hammer, 1993, Water supply and pollution control, 5th ed. (New York: Harper Collins).

Source: Adapted from W. Viessman, Jr. and M.J. Hammer, 1993, Water supply and pollution control, 5th ed. (New York: Harper Collins).

mixing. Mechanical mixing devices insure rapid and complete chlorine mixing with effluent.

Because of the importance of contact time, wastewater treatment facilities usually use a PF chlorination tank with a back-and-forth-flow pattern similar to that shown in Figure 7.25.2 to insure that at least 80 to 90% of the effluent is retained in the tank for the specified contact time. A minimum contact time of 30 min at the peak hourly flow is recommended. A chlorine dosage of 8-15 mg/l is adequate for a well-designed tank for chlorinating secondary effluents.

Wastewater treatment facilities should keep the maximum chlorine residuals in undiluted effluents at 0.1-0.5 mg/l to protect receiving surface water systems. Consequently, dechlorination can be required to reduce chlorine residual toxicity. Sulfur dioxide added at the end of the chlorination tank oxidizes both free chlorine and chlo-ramines to chloride. Activated-carbon adsorption of free-and combined-chlorine residuals is effective but expensive.

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