Explain the primary hydrodynamic principle that allows an aerated grit chamber to selectively remove grit while keeping most organic solids in suspension.

The primary hydrodynamic principle that allows an aerated grit chamber to selectively remove grit while keeping most organic solids in suspension is controlled velocity differential settling. This principle leverages the distinct physical characteristics of grit versus organic solids to achieve separation under precisely engineered flow conditions. Differential settling refers to the phenomenon where particles suspended in a fluid settle at different rates based on their settling velocity, which is primarily determined by their specific gravity (density relative to water), size, and shape, within a given fluid velocity field.