Ballasted Flocculation Process for High-Rate Clarification of Combined Sewer Overflow Stormwater

Abstract

Combined sewer overflow stormwater generated during intense rainfall events contains high concentrations of suspended solids, organic pollutants, nutrients, and microbial contaminants that pose significant risks to receiving water bodies and urban aquatic ecosystems. Rapid and efficient clarification technologies are therefore essential for minimizing pollutant discharge and improving stormwater management performance. The present study investigates the application of a ballasted flocculation process for high-rate clarification of combined sewer overflow stormwater under varying hydraulic and operational conditions. The research focuses on evaluating floc formation behavior, settling efficiency, pollutant removal performance, and process optimization associated with ballasted clarification systems. Experimental analysis was conducted to examine the influence of coagulant dosage, ballast material concentration, mixing intensity, hydraulic loading rate, and settling conditions on turbidity reduction, suspended solids removal, and organic contaminant reduction. Results demonstrate that the ballasted flocculation process significantly enhances floc density and settling velocity through the incorporation of microsand ballast particles, resulting in rapid solid–liquid separation and improved clarification efficiency. The study further reveals that optimized coagulation and ballast addition conditions substantially improve pollutant removal performance while reducing hydraulic retention time and treatment footprint requirements. Increased ballast concentration contributed to the formation of larger and more compact flocs capable of stable sedimentation under fluctuating stormwater flow conditions. Comparative assessment with conventional clarification systems confirms the advantages of ballasted flocculation technology, including faster treatment rates, improved operational flexibility, and enhanced performance during peak hydraulic loading events.