Starting from stress simulations suitable shapes of ceramic capillaries (outer diameter, OD; inner diameter, ID) were calculated. Capillaries were prepared by conventional extrusion down to an OD/ID 1.1 mm/0.7 mm. Capillaries with an 2.9 mm/1.9 mm were selected as the most favorite shape. A special coating technique was developed to prepare NF-membranes inside these types of capillaries. Single capillaries were tested and compared with monochannel tubes with OD/ID 10 mm/7 mm. The same cut-off of 450 g/mol measured with PEG standards and a similar water flux of 17 l/(m(2)hbar) compared with 20 1/(m(2)hbar) were achieved by dilution of the polymeric TiO2-sol. A new quality of ceramic NF-membrane with a cut-off of 250 g/mol and a water flux of 10 1/(m(2)hbar) was developed by a two step coating process. Test modules with approximately 50 capillaries were built in order to prove the module performance. (C) 2003 Elsevier Science B.V. All rights reserved.