This paper presents a new model for estimating capital costs of membrane facilities that includes separate correlations for major capital cost components as a function of membrane area. This approach accounts for the individual economies of scale associated with different equipment and facilities and considers an overall economy of scale for the entire membrane system that is a function of the design mix. This approach is geared for estimating costs of larger facilities for which cost and design data are currently unavailable. Total treatment costs are obtained by combining the new capital cost model with equations for operating cost developed in a previous cost model. The newly formulated cost model is used to predict the impacts of raw water quality and plant capacity on treatment costs.