Wet Weather Flow Treatment with a Difference: Novel Ideas for Applying Chemically Enhanced Primary Treatment with High Rate Disinfection

The Northeast Ohio Regional Sewer District has initiated an investigation into the feasibility of implementing chemically enhanced primary treatment (CEPT) and high rate disinfection (HRD) of its wet weather flows at its three wastewater treatment plants in the greater Cleveland Metropolitan area. The first two parts of this project have been completed, namely, the jar testing of CEPT–HRD and the design of facilities to implement pilot- and field-scale demonstrations of these technologies at the three plants. Of the five coagulants tested, ferric chloride was the most consistent in performance requiring on the order of 30 mg/L in concert with 1-2 mg/L of very high molecular weight anionic polymer to achieve the size and strength of floc required in these applications. Removals of approximately 80 percent of TSS and BOD were achieved with a residual TSS concentration of approximately 10 mg/L, well within the targeted value of 40 mg/L. Chlorine disinfection jar tests with raw sewage and CEPT supernatant showed that E. coli concentrations of <126 CFU/100 ml were achieved at chlorine doses of 20 mg/L at contact times of 10 minutes in the case of raw sewage, and 5 mg/L in the case of CEPT supernatant at contact times of 3 minutes.