Treatment of high fluoride-content wastewater by continuous electrocoagulation–flotation system with bipolar aluminum electrodes

A continuous electrocoagulation–flotation (ECF) system was employed to treat synthetic high fluoride-content wastewater following calcium precipitation. The dose of sodium dodecyl sulfate (SDS) in a continuous ECF system was higher than that in the batch system indicating the SDS acted not only as frother, but also as collector in the continuous system. The removal of suspended solids (SS) in the continuous system was less than that in the batch system because the scum was disturbed by the flow of wastewater in the flotation tank. It decreased with the increase in flow rate when the flow rate reaches higher than 800 mL/min, yet it increased with the increase in flow rate when the flow rate falls under 200 mL/min. Two parameters, gas/liquid ratio (ΓG/L) and gas/solid ratio (ΓG/S), were determined to indicate the discontinuity of the flow and flotation ability, respectively, in order to explain the phenomenon. The result indicates that the flotation ability was insufficient when ΓG/S was under 0.1 L/g and the flow of wastewater became discontinuous when ΓG/L was over 0.4. Both of these situations would lead to the increase of SS.