Grit Particle Settling – Refining the Approach
Grit settling velocity is significantly impacted by variability in size, density and shape. The conventional assumption of spheres with a specific gravity of 2.65 is inadequate for characterizing grit. Grit particles larger than 110 micron have a Reynolds number >1, therefore Stokes’ Law is an oversimplification and should not be used in grit removal system design. Targeting 75-150 micron particle size for grit removal system design minimizes the impact of errors due to assumptions of ideal characteristics (e.g. spheres). Characterizing native grit physical particle size and settling velocity is the best means to determine grit removal system design requirements. In the absence of site specific characterization, regional grit gradation and settling velocity data should be used.