Available online at www.sciencedirect.com Separation and Purification Technology 60 (2008) 1–5 Treatment of high fluoride-content wastewater by continuous electrocoagulation–flotation system with bipolar aluminum electrodes Ching-Yao Hu a , Shang-Lien Lo a,∗ , Wen-Hui Kuan b , Yu-De Lee a a Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, National Taiwan University, No. 71 Chou-shan Road, Taipei 106, Taiwan, ROC b Department of Environmental and Safety Engineering, Ming-Chi University of Technology, Taishan, Taipei County 243, Taiwan, ROC Received 14 November 2006; received in revised form 9 July 2007; accepted 11 July 2007 Abstract 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 (Γ G/L ) and gas/solid ratio (Γ G/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 Γ G/S was under 0.1 L/g and the flow of wastewater became discontinuous when Γ G/L was over 0.4. Both of these situations would lead to the increase of SS. © 2007 Elsevier B.V. All rights reserved. Keywords: Fluoride; Electrocoagulation–flotation (ECF); Calcium precipitation; Sodium dodecyl sulfate (SDS) 1. Introduction Fluoride concentration in drinking water ranging from 0.5 to 0.8 mg/L prevents people from getting dental cavities, but long-term intake of water that contains more than 1.5 mg/L of fluoride may cause bone disease and mottling of the teeth [1–4]. Treating high fluoride-content wastewater efficiently has been an important issue for environmental engineers because of the dramatic development of the semiconductor industry which uses a large amount of hydrofluoric acid in etching and quartz clean- ing operations [5,6]. Several methods such as alum coagulation [7–11], adsorption [12–16], reverse osmosis (RO) [17], electro- dialysis [18], calcite filters [19,20], selective ion exchange [21], and electrochemical methods [22–25] have been employed or tested for defluoridation. The method of fluoride removal from high fluoride-content industrial wastewater generally involves a chemical precipitation process [5–10]. This process produces ∗ Corresponding author. Tel.: +886 2 23625373; fax: +886 2 23928821. E-mail address: sllo@ntu.edu.tw (S.-L. Lo). calcium fluoride (CaF 2 ) particles through the addition of lime or another calcium salt, such as CaCl 2 (Eq. (1)): Ca (aq) 2+ + 2F (aq) - → CaF 2(s) (1) Moreover CaF 2 particles are too tiny to be removed without coagulation. Therefore, alternative process is needed to lower the suspended solids (SS) and fluoride concentration after calcium precipitation. The wastewater after calcium precipitation with addition of a stoichiometric amount of calcium salt contains both SS and sol- uble fluoride ions. Our previous study [26] has shown that the electrocoagulation–flotation (ECF) process with the addition of an anodic surfactant, sodium dodecyl sulfate (SDS), can effec- tively remove the SS and soluble fluoride ions simultaneously in a batch experiment [26]. In ECF cells, Al(III) and hydrogen gas are produced stoichiometrically when electric current passes through the aluminum electrodes (Eqs. (2) and (3)): Al (s) → Al (aq) 3+ + 3e - (2) 2H 2 O (l) + 2e - → 2OH (aq) - + H 2(g) (3) 1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.07.040