Verification of a Method for Sizing All Proprietary Single Chamber Treatment Devices with Settling as a Unit Process

Methods for sizing single chamber tanks to remove different amounts of total suspended solids (TSS) have been available for some time. One method found in the literature uses the concept of upflow velocity (v) or the critical settling velocity of a particle in a tank to determine what size particles will be removed in the tank. The critical settling velocity depends on the relationship between discharge rate from the tank (Q) and the surface area of the tank (A) as described in the equation v = Q/A. One of the urban runoff models using this relationship to determine the efficiency of settling devices is the Source Loading and Management Model (WinSLAMM). The purpose of this paper is to verify the upflow velocity concept as it is used in WinSLAMM. The model is tested with the results from monitoring the effectiveness of two proprietary stormwater treatment practices – the Stormceptor and Vortechs System. Both of these practices rely on settling as the mechanism for removing solids from the flow stream. The model is tested using the site specific characteristics, such as rainfall, dimensions of each practice, and the average particle size distributions. The reductions of TSS estimated by the model closely matched the measured sum of the loads values (SOL). The TSS measured SOL value for the Stormceptor and the Vortechs is 5 and 19 percent respectively, while the model predicted values of 12 and 19 percent. The model results do not match the efficiencies of the individual storms as well as a group of storms. Information is not provided to the model on the variation in particle size distribution between storms, the variability in influent concentrations, and the identity of storms with re-suspension of trapped sediments. Results of this verification effort indicate the upflow velocity method should provide a reasonable estimate of the longterm TSS reductions expected for single chamber proprietary treatment practice that use settling as the primary unit process.