Defluoridation of water by nanofiltration

Fluoride contained in ground waters as a potable water source can have harmful effects on human health such as dental flurosis. The selectivity desalination of fluorinated brackish water is hence necessary. Drinking water can be produced directly using nanofiltration at a much lower cost than using reverse osmosis by optimizing the pressure for the type of water treated. Fluoride removal operations have been conducted first on a "model" water solution (i.e. distilled) in which the fluoride concentration has been varied and, second, on underground water (Louza 2) using a nanofiltration pilot. The performances of two commercial NF membranes NF5 and NF9 which have different rejection salt rates were evaluated. The influence of initial fluoride content was studied as well as the effect of chloride, sulfate and calcium since these ions usually co-exist in groundwater. The retention of ions by NF membranes may be explained by the combination of theories such as charge shielding, Donnan effect and hydration degree. The experiments indicate that NF membranes repulse divalent anions very strongly. In addition, the smaller the ion, the better it is retained. This is derived from the solvatation energy of the ions by water. Since fluoride ions are more solvated, they are better retained than chloride. After the treatment of Louza 2 water using the NF5 membrane, only chloride concentration is slightly higher than the Word Health Organization standards. With NF5 and NF9, the fluoride concentration in the mean permeate was 1.45 (Fluoride retention R = 57%) and 0.38 mg L-1 (R = 88%) respectively. Fluoride and the other ions are rejected by NF9 membrane which has properties close to a RU membrane. The total dissolved salt (TDS) of the water produced is very low (0.45 g L-1). Mixing with groundwater or remineralization is obligatory to produce water with a satisfactory composition. (C) 2013 Elsevier B.V. All rights reserved.