High Rate Microcarrier-Weighted Coagulation for Treating Wet Weather Flow

In wet-weather flow (WWF) a significant amount of toxic pollutants are associated with colloidal solids. Sedimentation with coagulation is considered to be an effective method of removing large quantities of suspended solids from water and wastewater. A new high-rate settling process, using microsand or a microcarrier (MC) as a settling carrier of colloids, has been developed and applied for treating both wet- and dry-weather flows (WWF/DWF). The addition and recirculation of the MC results in higher settling velocities and allowable tank overflow rates. The MC plays a crucial role in enhancing settling properties, and in particular, the removal of colloidal particles and associated contaminants. A detailed testing procedure and a method of experimental analysis using a modified jar test for the MC process have been developed. A series of MC weighted jar tests were undertaken on parking lot storm runoff, synthetic samples, and combined sewer overflow (CSO) mixed with a MC, coagulant and coagulant aid. Two particle analyzers with a range of 0.002 to 5 micrometers (µm) and 0.1 to 2,000 µm, respectively, were used to determine the full range of particle size distribution. Different materials were used as the MC in this study. The operational parameters evaluated include coagulant dosage, coagulant aid type and dosage, mixing- and flocculation-induced hydraulic shear or velocity gradients and duration, and characteristics of the MC. The pH, turbidity, particle size distribution, total solids, total volatile solids, suspended solids, and zeta potential were determined. The experimental results revealed that MC weighted coagulation dramatically reduced coagulation-flocculation duration (< 3min) and settling time (< 8 min) producing flocs with high settling velocity and high quality supernatant. Reductions of turbidity from > 80 NTU (raw samples) to < 2 NTU (supernatant samples) were achieved.