ORP BASED DISINFECTION/DECHLORINATION CONTROL AT THE METROPOLITAN WATER RECLAMATION DISTRICT OF GREATER CHICAGO

The John E. Egan Water Reclamation Plant is a 30 MGD advanced activated sludge facility with tertiary filtration. Inconsistent performance of the disinfection/dechlorination system utilized since 1991 prompted the installation of a new control system in the fall of 1998. The previous system required frequent monitoring of chlorine residuals and manual adjustment of chemical pump rates to maintain target residuals needed to meet disinfection standards. This system was not flow paced and typically resulted in cycles of overdosing and underdosing until the desired residual values were reached.The plant is required to meet a disinfection standard for fecal coliform of 200 counts/100ml (monthly geometric mean) during the months of May through October. In any given month, no more than 10 percent of the samples may exceed 400 counts/100ml for fecal coliform. Concurrently, the plant is required to maintain a maximum effluent chlorine residual of 0.05 mg/l whenever chlorination is taking place. The previous control system was unable to reliably ensure 100 percent compliance with the above limits.An Oxidation-Reduction Potential (ORP) based system capable of automatically adjusting the chemical feed rates was installed to correct this problem. The paper will present details of the new system, which monitors effluent ORP values and controls sodium hypochlorite and sodium bisulfite dosing pumps to meet disinfection and dechlorination requirements. The control system was placed in automatic operation at the beginning of the 1999 disinfection season. Careful attention was paid to the system's performance to ensure that chlorine residual concentrations and fecal coliform counts remained within permit limits.The paper discusses how occasional variations in effluent quality, particularly with regard to ammonia nitrogen levels, have resulted in periods of excessive sodium hypochlorite usage, and how the system can be adjusted to respond to these conditions. Engineering solutions for difficulties presented by the facility's physical layout (e.g. lack of a chlorine contact chamber) are described.