Measurement and Automatic Control of Chlorination

by Robert Hill, (M.ASCE),
James Martin,



Document Type: Proceeding Paper

Part of: Critical Issues in Water and Wastewater Treatment

Abstract: Chlorination is perhaps the most commonly utilized process for wastewater effluent disinfection. In Houston's plants sodium hypochlorite solution (bleach) is mixed with secondary effluent for disinfection. The chlorinated effluent is then held in tanks for a minimum of 20 minutes (at maximum flow) to assure nearly complete disinfection. Most of Houston's plants also require dechlorination before discharging their effluent. At Houston's 69th Street wastewater treatment plant, a chlorination control system using a polarigraphic type total chlorine analyzer was implemented. Several deficiencies of the instrument were corrected before good control was achieved. Dual analyzers were used to improve reliability and alarm instrument failures. The chlorine measuring point was located approximately 1 minute from the point of chlorine addition. A relatively simple cascaded control strategy using one residual chlorine loop and one flow control loop was implemented. A process and instrument diagram for the process is presented. Savings from using this control strategy at the 69th Street WWTP are estimated to be greater than $125,000 per year. Savings in the first three months of operation have exceeded the estimate.

Subject Headings: Chlorine | Wastewater treatment plants | Effluents | Water treatment plants | Control systems | Water tanks | Disinfection | Sodium

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