Pilot Study On Treatment Of Fluoride-containing Wastewater By Loading Flocculation Process

Effluents from industrial processes constitute a major source of contamination of fluoride in water, e.g. fluoride discharge in manufacture of iron and steel, non-ferrous metal, chemical fertilizers, organic fluorine chemical products, semiconductors, rocket propellant and so on. Fluoride contamination in certain aquatic systems has caused harmful effects. Flouride-containing wastewater are commonly treated by coagulation and precipitation process with calcium salts. However, this process is not very efficient for fluoride removal, and it costs a great deal and produces a huge amount of sludge that is hard to dispose. Based on the traditional processes, we developed a new loading flocculation method which took recycled sludge as flocculation carrier in order to improve fluoride removal efficiency and reduce running costs.By studies on efficiency and mechanisms of coagulation and precipitation process with calcium salts. We found that fluoride reacts with calcium to form sparingly soluble calcium fluoride colloidal particles and residual fluoride concentration decreased when the molar ratio of [Ca2+] to [F-] increased. The solubility of calcium fluoride increased when the pH <2.5 or at the present of anions such as nitrate and sulfate. Calcium fluoride particles with small diameter(0.226±0.104μm) and strong positive charge, which have poor gravitational settling, were usually removed by coagulation. Iron coagulants could efficiently removed the calcium fluoride, and anionic PAM aids coagulation greatly.Pilot study on flouride-containing wastewater by loading flocculation process indicated that sludge Cycling system could improved the fluoride removal efficiency. Wasterwater flow: 2m3/h, sludge recycle ratio: 50%, adjusted pH to 11 by calcium hydroxide, then coagulated with 0.15 mmol/L ferric chloride and 3 mg/L PAM at pH 8.5. After precipitation, the residual fluoride concentration was 10.67±0.15 mg/L, and residual turbidity was 2.48±0.52 NTU in outflow; also average fluoride removal rate was 96.31% and average turbidity removal rate was 98.00%. Moreover, the system runs stably, costs lower.The mechanism of the loading flocculation process was discussed: the sludge flocculation carrier increased effective collision probability among particles."Colloid"flocs formed in the process have strong adsorption capacity. The effective utilization of calcium hydroxide is improved by the dilution process; meanwhile it is possible that cycled CaF2 sludge induced the growth of CaF2 crystal. The effective utilization of calcium hydroxide and the removal efficiency of fluoride are improved by re-released process of Ca²⁺.