An integrated numerical model for examining the technical performance and estimating associated costs of low-pressure membrane filtration processes is presented. A model for removal of contaminants predicts permeate quality with respect to colloidal and organic material. A shear-induced diffusion model for estimating permeate flux is modified to also include Brownian diffusion as a particle transport mechanism. These two diffusion mechanisms lead to a minimum in permeate flux through a typical ultrafiltration membrane for particles in the size range of 0.1 μm in diameter. Lower shear rates are predicted to move this unfavorable size towards larger particle diameters. The effect of raw water quality and operating parameters on the performance and cost of typical ultrafiltration and microfiltration facilities is investigated. Particle size appears to be an important variable in determining both performance and cost. A product of this research is a software application package, MeMsys, developed for the PC-DOS environment.