Pilot-scale fluoride-containing wastewater treatment by the ballasted flocculation process

A pilot-scale ballasted flocculation system was used to remove fluoride from one type of industrial wastewater. The system included the formation of calcium fluoride (CaF₂) using calcium hydroxide followed by coagulation sedimentation. Calcium fluoride was recycled as nuclei for enhancing CaF₂ precipitation and as a ballasting agent for improving fluoride removal and flocculation efficiency. Factors affecting fluoride and turbidity removal efficiencies, including pH in the CaF₂-reacting tank and coagulation-mixing tank, sludge recycling ratio, and dosages of FeCl₃ and polyacrylamide (PAM), were investigated in the pilot-scale system. The recycled CaF₂ precipitates improved CaF₂ formation kinetics, enhanced fluoride removal and flocculation performance. Under the optimized condition, the ballast flocculation process reduced fluoride concentration from 288.9 to 10.67 mg/L and the turbidity from 129.6 NTU to below 2.5 NTU.