This study explored saltless water softening with inorganic Nanofiltration (NF) membranes prepared by Atomic Layer Deposition (ALD) and a Tetraorthosilicate (TEOS) sol–gel process applied by the slip cast method. The experimental investigations were performed on different material–based inorganic NF membranes, including titanium dioxide (TiO2), alumina (Al2O3), silver (Ag) and iron oxide (Fe2O3). The experimental results demonstrated that saltless water softening can be achieved with high flux through the viable rejection of Ca2+ with an initial rate between 7.1% and 34.1% in a single pass flow through filter in a solution of 0.5 g L−1 CaCl2 at neutral pH. A maximum rejection of 28.7% and 29.5% was achieved for Ca2+ at pH 4 and pH 10, respectively. The highest hardness rejection was achieved with a 25% waste water volume. In the recirculating loop tests with the same solution, a rejection rate of 9% was achieved within 15 min at a flow rate of 0.3 L min−1.
Keywords: inorganic nanofiltration, atomic layer deposition, sol–gel, saltless water, water softening, charge–driven separation, Donnan exclusion, nanofiltration membranes, nanotechnology, titanium dioxide, alumina, silver, iron oxide, wastewater