Chapter 2 Water Defluoridation Processes: A Review. Application: Nanofiltration (NF) for Future Large-Scale Pilot Plants

Abstract Defluoridation of waters using clays as substrates has become popular in many countries to solve problems related to high fluoride concentrations in drinking water in rural areas. But this treatment is limited to low fresh water production. In this work, F − elimination using a nanofiltration (NF) operation will solve problems for-large-scale pilot plants in the future. Results obtained in these fields help-users to facilitate the selection between reverse osmosis (RO) and NF membranes of the most cost-effective membrane for desalination of high fluorinated water. Two sorts of characterization have been developed: (i) physico-chemicals, in terms of hydrophobicity/hydrophilicity, morphology and topography aspects and (ii) mass transfer in terms of pure water and saline solution permeabilities, charged solute rejections and molecular weight cut-off (MWCO). A model inspired by the phenomenological approach proposed by Kedem and Katchalsky (KK) will help to quantify both parts of the mass transfer occurring in NF and RO, i.e. the pure convection and the pure diffusion, separately. This new and original approach will be applied to three membranes, 2 NF and 1 LPRO (low-polarization reverse osmosis), respectively. The study will be limited to low concentration polarization by using diluted solutions (10 −3 –10 −1  M) and high tangential flow rate (4 m s −1 ) under low conversion ratio (5%), operational conditions. Different tools such as contact angle measurements, topography and roughness measurements using atomic force microscopy (AFM), hydraulic permeability and salt solution permeability, will be used to characterize the three membranes. This analytical approach will be coupled with the Spiegler, Kedem and Katchalsky (SKK) phenomenological mass transfer model in order to determine the mass transfer parameters σ and P s for synthetic chloride and sulphate solutions. This novel integer approach makes it possible to determine today an NF most efficient membrane for the elimination of excess F − in a Senegalese water sample taken from the endemic region of Fatick. This membrane denoted NF90 works very well due to its diffusional behaviour for fluoride rejection, its high hydraulic permeability and sufficient observed rejection for F − in comparison to RO.