Modeling and application of direct contact membrane distillation for fluoride removal from aqueous solutions

© 2017 Desalination Publications. All rights reserved. The practicability of direct contact membrane distillation (DCMD) process for the removal of fluoride was investigated under different feed water, operational and membrane characteristics. Commercially available hydrophobic 0.22 mm porous polytetrafluoroethylene (PTFE) and polyvinylidenefluoride (PVDF) membrane applied on fluoridated ultrapure water exhibit over 99% rejection of fluoride, yielding fluxes of up to 39.3 and 26.4 kg/m2 h at 60°C, respectively. The dusty gas mathematical model and energy balance equations were employed to study the mass and heat transfer mechanisms. In addition to the good agreement between the theoretical and experimental comparison, the overall mass transfer analysis revealed that Knudsen–molecular–Poiseuille transition diffusion and Knudsen–Poiseuille diffusion are the dominant mass transfer mechanisms across the 0.22 mm flat sheet PVDF and PTFE membranes, respectively. The effects of different parameters, such as temperature, initial fluoride concentration, feed flow rate, and membrane properties, on the flux and removal efficiencies were also evaluated, and feed temperature was found to be the most important operating parameter since higher temperatures induce the lowest temperature polarization coefficient (TPC) and a higher thermal efficiency (TE). Moreover, a wetting rate analysis in actual industrial wastewater sample indicated that a solution with higher organic matter, an membrane distillation (MD) system with PTFE membrane, and a sample with bigger initial fluoride concentration induce bigger wetting rate. Therefore, the DCMD process can be applied on fluoride affected water sources toward producing high-quality water suitable for a potable water supply. Exploiting renewable source potentials or industrial waste (free) energy will bring better economic advantage on the application