A softening process of the geothermal waters of Chott El Fejjej (Tunisia) performed during air bubbling in an opened double partition reactor (ODPR) by seeding of calcium carbonate recovered on site constitutes a first and an interesting example of simple and clean technology. The water is treated in continuous flow without chemical additions, so the increase of pH is minimised. Consequently, there is no homogeneous nucleation and Mg(OH)(2) precipitation that would disturb the CaCO3 crystallisation. Thus, large production of waste sludge is avoided. A secondary set-off of precipitation takes place on seed grains in the ODPR. The conical part, where the sludge is collected by sedimentation and purged continuously, makes ODPR a simple technology. Our attention is focused on the effect of water flow rate, amount of added seed and first tank temperature of the reactor on CaCO3 crystallisation.