Effects of suction are included in the mass transfer coefficient in the performance prediction of spiral wound and tubular membrane module. The analysis is valid for the laminar flow regime and osmotic pressure controlled filtration. Sodium chloride and dextran are chosen as the solutes. The design equations are solved numerically and the profiles of the bulk concentration, bulk velocity, feed channel pressure, permeate flux, permeate concentration, etc., are obtained along the length of the membrane module. The results are compared with the standard method of module design using film theory and mass transfer coefficient without suction. The results show that the design method without the inclusion of the effects of suction, leads to gross underprediction (about 2-2.5 times) of the average permeate flux and overprediction of the permeate concentration (about 2-14%) for various operating conditions in ultrafiltration. Use of standard film theory also results in an underprediction of the permeate flux of about 10-15% (over a range of operating conditions) compared to the exact mass transfer coefficient with suction. For reverse osmosis, the effects of suction are insignificant. (C) 2005 Elsevier B.V. All rights reserved.