The study emphasized a new approach for modeling the activated sludge process with a membrane separation (MASM) for organic carbon removal of tannery wastewaters. The new model, differing from the commonly used activated sludge models 1, relied on a modified chemical oxygen demand (COD) fractionation considering the particle size distribution of the organic matter. A new COD fraction, entrapped COD, was included into the model related to the effective filtration size of the membrane module. Modeling studies were conducted for high rate; conventional and extended aeration activated sludge systems operated at sludge age levels of 4, 8- 12, and 15-18 d, respectively. Model simulations were carried out for parallel systems both with gravity settling and membrane filtration. Comparative evaluation reflected a better effluent quality for the activated sludge systems with membrane separation at all sludge ages when both systems were designed for the same reactor volume. At a high rate operation, the effluent soluble COD level was estimated as 198 mg/L for membrane separation, below the threshold of 200 mg/L and 63% lower than the conventional scheme. Even for conventional and extended type of operation, the effluent COD remained around 170 mg/L, where it fluctuated between 300-350 mg/L including the particulate COD in the supernatant for the conventional scheme.