Characterisation of Grit to Develop an Integrated Plant Solution

The Hampton Roads Sanitation District (HRSD) desired to improve the performance of two identical forced vortex grit systems at two separate wastewater treatment plants (WWTP), the Chesapeake Elizabeth and the Nansemond Treatment Plants. The goal of the study was to use computational fluid dynamics (CFD) to identify the root inefficiencies of the vortex systems and find a baffle design to improve the grit removal efficiency. Nine baffle configurations were developed and modeled in an attempt to improve grit removal performance. The study concluded that CFD modeling was appropriate for this application, specific gravity is an important factor in removal efficiency, and considerable hydraulic and grit short circuiting is occurring in the vortex grit tank under all tested conditions. The modeling demonstrated that only marginal benefit can be expected from applying an optimized full entrance and exit baffle as defined by this study. The modeling revealed that grit is removed by Type 1 discrete particle settling velocity and not the vortex mechanism; therefore, surface overflow rate is the most important factor for grit removal.