The growing demand of reliable high-performance membrane materials for separation processes requires new simple, straightforward, environmental friendly, sustainable approaches for membrane fabrication. In this study, we present an environmentally friendly gel-casting, one-pot process based on ionotropic-gelation for obtaining alumina membranes. A slurry of alumina particles and the biopolymer alginate, which acts in combination with calcium iodate like a resin, was gelled in a controllable temperature dependent manner. Alumina membranes are obtained by three different shaping routes (extrusion, free-forming, casting). The suitability of extruded capillaries in a polymer-ceramic hybrid state (green body) and after sintering (1150 degrees C for 2h) for potential application in micro- and ultrafiltration is evaluated by monitoring the chemical and mechanical stability, permeability and separation behavior. Varying the initial alumina particle size from 200 to 900nm, membranes with a narrow pore size distribution, predictable and tunable average pore diameters from 70 up to 480nm and a constant open porosity of similar to 40%, are obtained. The permeability behavior is tested with fluorescence labeled submicron- and nano-particles. Our novel colloidal processing route represents a very versatile tool for designing and manufacturing ceramic membranes with complex shapes for micro- (>0.1m) and ultrafiltration (0.1-0.01m).