The water resources in Tunisia are very limited and not fairly distributed: 80 % of them are located in the north and only 20 % in the south. In addition, 50 % of them are characterized by a bad quality since they have salinity greater than 1.5 g L −1. The bad quality is not only due to the high salinity, but also to the presence of natural pollutants such as fluoride. In fact, according to physicochemical analyses of samples of tap waters collected by the Regional Service of Environmental Health from different monitoring sites in mining area (south of Tunisia), in February 2014, it was shown that waters were contaminated by fluoride and contained high concentrations of sulphate and chloride anions. Fluoride concentrations varied between 0.8 and 4 mg L −1 , and then, greatly exceeded the World Health Organization (WHO) standards in some monitoring sites. In this study, the electrodialysis (ED) technique was applied to remove fluoride from these waters. Experiments were carried out using a pilot unit as a conventional ED in batch recirculation mode. It was shown that ED is an efficient technique to remove fluoride and to reduce salinity of water. Removal rate of fluoride tended to 92 % and the concentrations of different species in the treated water were below the amounts recommended by WHO for drinking water. Considering the effect of chloride and sulphate on fluoride removal through the anion exchange membrane, the performance of two types of ion exchange membrane (PC Cell and Neosepta) was compared. Experiments were carried out with synthetic solutions containing 0.0357 mol L −1 of NaHCO 3 doped with 15 mg L −1 of fluoride. Sodium hydrogenocarbonate was replaced by sodium chloride or sodium

The aim of this work is to study the desalination of brackish water by electrodialysis (ED). A two levelthree factor (2 3 ) full factorial design methodology was used to investigate the influence of different physicochemical parameters on the demineralization rate (DR) and the specific power consumption (SPC). Statistical design determines factors which have the important effects on ED performance and studies all interactions between the considered parameters. Three significant factors were used including applied potential, salt concentration and flow rate. The experimental results and statistical analysis show that applied potential and salt concentration are the main effect for DR as well as for SPC. The effect of interaction between applied potential and salt concentration was observed for SPC. A maximum value of 82.24% was obtained for DR under optimum conditions and the best value of SPC obtained was 5.64 Wh L -1 . Empirical regression models were also obtained and used to predict the DR and the SPC profiles with satisfactory results. The process was applied for the treatment of real brackish water using the optimal parameters.

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The composition of feed water used in electrodialysis (ED) desalination causes the risk of CaCO 3 scale crystallization in ED equipments, which leads to the decrease of process efficiency. To control scaling in water systems, several scaling indices and tests, permitting to predict the scaling occurrence, have been devised. In this study, an accelerated scaling test allowing to follow CaCO 3 formation in water desalination plant using ED process was proposed. Tests were performed using a pilot unit as a conventional ED in batch recirculation mode. By simple in situ measurements of pH and flow rate of the concentrate, the crystallization onset and growth of CaCO 3 were followed during the pilot operation. This leads to precise determination of the number of concentrate recirculation times and therefore to the highest recovery rate without scaling risk as a function of the inlet water quality and the antiscale pretreatment.

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