Historical operating data from four full-scale wastewater treatment plants were used to evaluate an empirical expression describing primary sedimentation tank performance as a function of surface overflow rate and influent total suspended solids concentration. The influent total suspended solids concentration was generally found to be a more important variable impacting primary sedimentation tank performance than surface overflow rate, which is typically thought of as the most important variable affecting primary sedimentation tank performance. The resulting expression was used as an example in assessing the impact of primary sedimentation tank performance on the downstream secondary process. The settleable and non-settleable suspended solids concentrations were measured during a storm event at a combined wastewater treatment plant providing primary treatment. Despite the fact that the amounts of influent settleable and non-settleable suspended solids concentrations varied throughout the storm, the primary sedimentation tanks were 100 percent efficient at removing the settleable suspended solids they were designed to remove.