Defluoridation of brackish northern Sahara groundwater — Activity product calculations in order to optimize pretreatment before reverse osmosis

High fluoride concentrations in water cause problem in terms of public health. Owing to its efficiency, reverse osmosis (RO) was considered for water defluoridation. However, this technique is not selective. The recovery by means of brackish water RO system is therefore limited by salt precipitation. A computer calculation program was developed to determine the activity products a Ca 2 + . a CO 3 2 − , a Ca 2 + . a SO 4 2 − and a Ca 2 + . a F − 2 by taking into account the influence of all present free and associated species (ion pairs). These activity products are then compared with the solubility product of calcite (CaCO 3), gypsum (CaSO 4,2H 2O) and fluorite (CaF 2). The results allowed a theoretical estimation of the brackish water RO recovery which can be achieved without scaling. From the calco-carbonic equilibrium curve, the Legrand–Poirier diagram allowed to optimize the amount of softening substance (sodium carbonate and soda mixtures) to be added to obtain water which can be subsequently concentrated by reverse osmosis. An experimental softening pretreatment of Algerian brackish water was considered according to the optimal calculated softening amount of 15 mmol L − 1 of sodium, added in the form of sodium carbonate and soda (16%). The pretreatment allowed not only an almost total removal of the calcium but also a partial elimination of magnesium and fluoride.