This study has been carried out to evaluate the applicability of the pilot scale the hybrid type of stormwater runoff treatment systems (SFS) for treatment of combined sewer overflow. And, to determine the optimum operation parameter such as coagulation dosage concentration, effectiveness of coagulant usage, surface loading rate and backwashing conditions. The pilot scale stormwater filtration system (SFS) was installed at the municipal wastewater plant serving the city of Cheongju (CWTP) Korea. CWTP has capacity of 280,000 m3/day. The SFS system is consisted of hydrocyclone coagulation/flocculation with polyaluminium chloride silicate (PACS) and upflow filter to treat combined sewer overflows. There are two modes (without PACS use and with PACS use) of operation for SFS system. In case of without coagulant use, the range of SS, turbidity removal efficiency were 72.0–86.6% (mean 80.0%), 30.9–71.1% (mean 49.3%), respectively. And, the recovery rate of filter was 79.2–83.6% (mean 81.2%) the rate of remained solid loading in filter media was 16.4–20.8% (mean 18.8%) after backwashing. The influent turbidity, SS concentrations were 59.0–90.7 NTU (mean 72.0 NTU), 194.0–320.0mg/L (mean 246.7mg/L), respectively. The range of PACS dosage concentration was 6.0–7.1mg/L (mean 6.7mg/L). The range of SS, turbidity removal efficiency was 84.9–98.2 (mean 91.4%), 70.7–96.3 (mean 84.0%), respectively. It was found that removal efficiency was enhanced with PACS dosage. The recovery rate of filter was 92.0–92.5% (mean 92.3%) the rate of remained solid loading in filter media was 6.1–8.2% (mean 7.2%) after backwashing. In case of coagulant use, the particle size of effluent is bigger than influent particle size. The results showed that SFS with PACS use more effective than without PACS use in SS and turbidity removal efficiency and recovery rate of filter.

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