A new methodology for re-rating and evaluating the performance of primary sedimentation tanks (PSTs) in wastewater treatment plants was developed. The methodology was based on the integration of mathematical (numerical) modeling with field-testing. A high accuracy clarifier model (HACM) was used to simulate the flow field in the PSTs. A new approach for estimating the settling velocity distribution of the influent suspended solids was developed. The distribution was then used as an input to the model. The model was used to estimate the total removal efficiency of the suspended solids under different flow rates.