Online Monitoring of Wastewater Effluent Chlorination Using Oxidation Reduction Potential (ORP) vs. Residual Chlorine Measurement

Various control strategies are employed to insure that a sufficient amount of chlorine has been applied to the wastewater effluent to achieve desired disinfection goals. In the commonly used control strategies, such as Feed Back Control and Compound Loop Control, the combination of effluent flow rate, chlorine flow rate and chlorine residual are measured for dosing control with the chlorine residual used as a set point parameter. Recently Oxidation-Reduction Potential (ORP) has been...
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Various control strategies are employed to insure that a sufficient amount of chlorine has been applied to the wastewater effluent to achieve desired disinfection goals. In the commonly used control strategies, such as Feed Back Control and Compound Loop Control, the combination of effluent flow rate, chlorine flow rate and chlorine residual are measured for dosing control with the chlorine residual used as a set point parameter. Recently Oxidation-Reduction Potential (ORP) has been employed as a control measurement and set point parameter for effluent chlorine dosing control as well. Theoretically, use of chlorine residual or ORP set point makes the chlorine dosing possible to simultaneously respond to changes in effluent flow rate and in process (such as fluctuating effluent ammonia levels), which cause variations in chlorine demand and thus the ORP or chlorine residual after the dose point. This would also potentially reduce chlorine feed requirements. Specific ORP or residual chlorine residual levels to ensure effective disinfection could be effectively monitored and maintained under changing effluent quality conditions. In addition, immediate detection of chlorine feed malfunction could be achieved when analyzers are employed at the chlorine dosing point. Despite all these advantages, many wastewater treatment plants have chosen not to use signals from chlorine residual or ORP analyzers or results from manual grab chlorine residual testing for their chlorine dosing control. Instead, chlorine residual or ORP information was only used to monitor the chlorine dosing strategy and serve as an alarm function to the operations staff. Typical reasons of the plants for not using on-line chlorine residual or ORP analyzers in their dose control systems include: Lack of confidence in on-line analyzers to provide an accurate and continuous output of ORP or chlorine residual readings. There was a concern that incorrectly operating on-line analyzers would cause water quality permit requirements not to be met.          Insufficient staff was available to meet the operations and maintenance requirements demanded by on-line analyzers.          Relative effectiveness in meeting regulatory permitting requirements of the current chlorine dose control system, which typically involve flow-paced dose control system with monitoring and control of chlorine mass feed. As a result, an investigation of how effective the current residual chlorine and ORP measuring technologies are as applied in wastewater disinfection becomes imperative to address these concerns. This project examined the three key chlorination process monitoring methodologies employed currently: ORP sensors, membrane probe chlorine residual sensors, and chlorine residual automatic chemistry systems (colorimetric and amperometric). Side-by-side tests of the three monitoring methodologies were conducted at multiple test sites. The test sites were chosen to reflect at least one municipal site that has primarily residential wastewater flow sources and a municipal site that has a significant contribution of complex industrial waste flow. The differences, drawbacks, advantages, and overall strengths and weaknesses of each methodology tested as a monitoring and dosing control measure were identified. In addition, bench scale laboratory experiments were performed to further compare the correlation of ORP and chlorine residual to microbial inactivation. Based on the findings of the project, issues and optimization tips were examined for employing on-line chlorine residual or ORP analyzers in wastewater effluent chlorine dose control.   This publication can be purchased and downloaded via Pay Per View on Water Intelligence