Using Real Time Measurement of Influent Suspended Solids to Minimize Effluent Total Residual Chlorine at a Retention Treatment Basin

The Hubbell Southfield Combined Sewer Overflow (CSO) Retention and Treatment Basin (RTB) was originally designed and constructed by Detroit Water and Sewerage Department (DWSD) to meet the end-of-pipe bacteria limits established by the Michigan Department of Environmental Quality (MDEQ) to protect the public health and meet state water quality standards for fecal coliform. Post-construction performance testing has confirmed that the CSO control facility has consistently met the disinfection requirements. However, the facility often discharged treated effluent with Total Residual Chlorine (TRC) concentrations above 1.0 mg/l. MDEQ expressed concern over the effluent concentrations and the potential for toxic impacts in the receiving waters. As a result, the MDEQ proposed new National Pollutant Discharge Elimination System (NPDES) permit TRC “Operational Goals”, plus a requirement to implement a TRC Minimization Program to experiment with various chlorine feed rates and operational protocols to optimize the chlorine feed rate. The DWSD completed a 36 month evaluation at the RTB from March 1, 2008 through March 1, 2011. During this period DWSD experimented using real time measurement of total suspended solids (TSS) in the influent as a representative measurement of the influent chlorine demand to develop a minimized NaOCl Injection Dosage Guidance for CSO treatment at the RTB. Twelve years of operator experience in combination with the newly developed NaOCl Injection Dosage Guidance has resulted in the successful achievement of meeting fecal coliform limits while minimizing TRC residuals to values at or near the NPDES permit goals at the Hubbell-Southfield RTB Facility. DWSD is currently designing a SCADA System that will directly implement the NaOCl Injection Dosage Guidance.