Biologically Enhanced HRC System Solving Peak Wet Weather Flow Challenges

A pilot test was conducted with a ballasted flocculation/high rate clarification (BF/HRC) unit at the P Street Wastewater Treatment Plant (WWTP) to investigate the potential of activated sludge addition to enhance biochemical oxygen demand (BOD) removal. Primary clarifier effluent and varying concentrations (100 to 800 mg/L) of mixed liquor suspended solids (MLSS) from the WWTP's stabilization basin were mixed (for two different contact times) and fed to the BF/HRC unit. To determine the performance of the BF/HRC, influent and effluent total and soluble BOD, chemical oxygen demand (COD), and total suspended solids (TSS) were measured. The soluble BOD unit removal was higher at the lower MLSS concentrations, and was 7 mg/L per 100 mg/L MLSS at an MLSS concentration of 200-400 mg/L. The longer detention time yielded slightly higher BOD removal efficiency and average TSS removal was 98%. The results of the study indicate that BOD removal may approach 90% in a BF/HRC system with active biosolids addition.