Particulate Matter Removal by Filtration and Sedimentation

Sedimentation is the step in conventional water treatment systems that follows flocculation and precedes filtration. Its purpose is to enhance the filtration process by removing particulate matter. Sedimentation requires that the water flow through a basin at a slow enough velocity to permit the particulate matter to settle to the bottom of the basin before the water exits the basin. The equipment required for this process includes a rectangular, square, or circular settling basin. The basin includes provisions for inlet and outlet structures and a sludge collection system. In addition, sedimentation systems are optionally equipped with tube or plate settlers to improve performance. The settling velocity of the particles or particulate matter is governed by the particle size, shape, density, and water viscosity (which varies with temperature). Stokes' law describes the terminal velocity of discrete spherical particles in a laminar flow regime. In an ideal up-flow rectangular settling tank, all particles that have a settling velocity greater than the liquid up-flow velocity will be captured. This up-flow velocity is more commonly called the hydraulic overflow rate or surface loading rate. The surface overflow rate is the primary design parameter for sizing sedimentation basins. This rate is defined as the rate of inflow (Q) divided by the tank surface or floor area (A). Units are typically rated in gallons per day per square foot, gallons per minute per square foot, or cubic meters per hour per square meter. Design hydraulic overflow rates vary with the nature of the settling solids, the water temperature, and the hydraulic characteristics of the sedimentation basin. Keywords: sedimentation; surface overflow rate; weir rate; plate settlers; tube settlers; particulate matter; discrete settling; flocculant settling; hindered settling; compression settling; filtration; granular media filtration; precoat filtration; backwashing; slow sand filter