Characterization and control of foulants occurring from RO disc-tube-type, membrane treating, fluorine manufacturing, process wastewater

This study was carried out to find a way to limit fouling of disc-tube-type reverse osmosis membranes in the treatment of acid rinse wastewater from the fluorine manufacturing process as well as to pretreat the wastewater before it entered the membrane process. Experiments showed that the scale consisted of Ca²⁺, SO₄⁻² and F⁻. Complex scales were removed in a subsequent procedure where the membrane was rinsed with NaOH followed by citric acid which could consequently recover its flux up to 86%. Cleaning chemicals had to be used regularly for efficient recovery of permeate flux. Ultrasonic cleaning could also improve the recovery of permeate flux up to 83%. Calcium salts were used to remove fluoride ions. CaCl₂ removed fluoride ions up to 11% more than Ca(OH)₂ at 0.5 Ca²⁺]/F⁻]. At acidic pH 4-7 or alkaline pH 7 and above, residual fluoride ions increased as Ca²⁺ reacted more efficiently with Cl⁻, OH⁻ and SO₄⁻² rather than F⁻. On the other hand, fluoride ions were best removed at pH 7. Adding Ca²⁺ salt above pH 7 caused an increase of residual Ca²⁺ salt in the effluent, even if fluoride ions can be ideally removed in the form of CaF₂ at a pH over 11.