Hazardous Fluoride and Silica Removal from Semiconductor Wastewater in a Waste Control by Waste Principle

Fluoride and silica contamination in water is a worldwide issue due to the wastewater discharge from semiconductor industry. Coagulation-ultrafiltration (UF) process is commonly used to treat semiconductor wastewater, but requires excessive amounts of coagulant/flocculant. In this study, an unconventional coagulation-ultrafiltration process using chemical mechanical polishing (CMP) wastewater as a coagulant to treat fluoride-containing wastewater was firstly proposed in a waste control by waste principle. The fluoride-containing wastewater, CMP wastewater, mixed fluoride-containing wastewater and CMP wastewater, and mixed fluoride-containing wastewater and polyaluminium chloride/polyacrylamide were compared to investigate turbidity removal efficiency, fluoride removal efficiency and membrane fouling resistance. The negatively charged SiO2 particles in CMP wastewater were found to be adsorbed on the positively charged CaF2 particles surface through electrostatic interaction. Results indicated that CMP wastewater provided a superior turbidity removal efficiency and a minimal membrane fouling resistance compared to conventional coagulant. After UF filtration, the fluoride concentration and turbidity were 2.03 mg L-1 and 0 NTU, respectively. The extended Derjaguin-Landau-Verwey-Overbeek theory showed that increasing molar ratio of SiO2 to CaF2 improved the interfacial free energy between membrane and suspended particles, thus mitigating membrane fouling. This waste control by waste principle will broaden the sustainability of coagulation-ultrafiltration process for wastewater treatment