Evaluation of Three Full-scale Grit Removal Processes using CFD Modeling

The Hampton Roads Sanitation District (HRSD) operates thirteen wastewater treatment plants (WWTP) in Southeastern Virginia with various grit removal technologies. HRSD has been confronted with poor grit removal resulting in abrasive wear of equipment, reduction of volume in bioreactors, digesters, and incinerators. Each of these consequences has increased operational cost. Computational fluid dynamics (CFD) modeling was used to study and evaluate grit removal for: forced vortex, detritor, and stacked structured vortex units. The goal was to compare and observe the operating dynamics of the three grit removal technologies, modeled at the rated design flow rates of each unit. The study revealed that CFD modeling: is appropriate for this application; correlated with field test data and observations; can be used to evaluate unit process improvements; can be used to evaluate the effect of process variables such as flow and specific gravity; and the CFD models predicted that a detritor had the highest grit removal efficiency of the units investigated, followed by the stacked structured vortex unit. For the forced vortex unit, the modeling indicated that grit is not substantially removed according to the vortex mechanism, and rather that Type I discrete particle settling velocity and thus surface overflow rate are the most important factors leading to removal of grit.