A combined sewer system is a facility that collects both municipal sewage and surface runoffs. These facilities may overflow (combined sewer overflow or CSO) during large storms which results in serious pollution, i.e. the flows exceed the capacity of the treatment plant. An approach to reduce the number of combined sewer overflows is to store rainfall runoffs temporarily [1]. The Treatment Shaft system is a relatively new but proven patented technology (U.S. Patent [2] and other patents) that includes the necessary CSO control and treatment, with less footprints than existing systems, and at a reduced cost. In this system, wastewater is collected in a large shaft equipped with baffles and partitions designed to ensure a very slow velocity within the system. In this study, the efficiency of the Treatment Shaft system for separation of solid contents without the use of flocculation agents is investigated. Moreover, the effect of geometry modifications on the separation efficiency is evaluated. For this purpose, a Computational Fluid Dynamics (CFD) approach for multiphase flow of particulate wastewater is used to evaluate the performance of various Treatment Shaft designs for a 10-year, 1-hour rainstorm event. It is shown that the Treatment Shaft is an effective technology to separate particles larger than 175μm, and more than 50% of the particles of size 175μm or more are settled. Additionally, several design variations are assessed and a design with a less footprint is specified.

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