Membrane distn. was used for the prodn. of demineralized H2O from underground water. The effect of feeding water pretreatment by coagulation with FeSO4 and softening with time soln. followed by filtering off the crystals formed on the process effectiveness was evaluated. A high H2O recovery degree (96%) was achieved. No formation of deposits on the membrane surface was obsd. The morphology and compn. of the filter cake was studied by scanning electron microscopy coupled with energy-dispersing spectrometry.