AUTOMATIC CONTROL OF DISINFECTION: FIELD EXPERIENCE

The State of California is considering a new Total Residual Chlorine and Chlorine-Produced Oxidants Policy. If approved, the monitoring limits will be lowered considerably. The City of Roseville has looked at the effects it will have for its wastewater effluent. All of the Pleasant Grove wastewater treatment plant effluent is treated to California Title 22 reclaimed water effluent standards.To meet those requirements, absolute control of the chlorine and bisulphite dosages is required. Control based on proportional flow is not acceptable anymore as the chlorine demand changes with time. The city decided to make use of online instruments and PLC algorithms to optimize the dosage of those chemicals.Several technologies are available to measure chlorine in wastewater. Only recently the membrane amperometric technology has been modified to measure total chlorine. The sensor can be mounted directly in the chlorine contact tank eliminating the need for a sampling line.The City of Roseville tested several instruments. In the selection of the instruments the most important criteria where:Stability of the measurement over timeAccuracySensitivity at the very low end of the rangeEase of installationResponse timeMaintenance CostUser Friendly O&MAt the Pleasant Grove Wastewater Treatment Plant the chlorine sensors are integrated in the SCADA system to provide automatic control. The operator has a choice of automatic dosage from flow proportional, residual only, to full flow/analyzer PID based control.