Nanofiltration, a recent membrane technique, was used for the softening of very hard water. Our study was based on the elimination of one part of the temporary hardness of Hamma water which provides Constantine City with drinking water. Two techniques were used in order to obtain a partial softening. The first is related to weak pressures (0.5, 1, 2 bar) whereas the second is based on relatively high pressures (4-16 bar) and which is applied on feed water. However, we should mention that at the end of this experience, 50% of filtered water was mixed with 50% of hard water. The results show that this membrane technique is of great interest in the process of eliminating temporary hardness of water, which depends mainly on the applied pressure and the tangential flow rate.