Gravity: The Simplest Separation
Sedimentation exploits the density difference between suspended particles and water. Under gravity, denser particles sink at a rate governed by their size, shape, and density contrast with the surrounding fluid. For the small, roughly spherical particles in water treatment, Stokes' law provides an accurate prediction of settling velocity. The simulation shows how dramatically particle size affects this velocity — a 100 μm particle settles 100× faster than a 10 μm particle.
Clarifier Design
A sedimentation basin (clarifier) is sized so that its overflow rate — the upward velocity of water through the basin — is less than the settling velocity of the target particles. Any particle settling faster than the overflow rate will reach the bottom before being carried out. The detention time, depth, and surface area are interconnected design parameters that the simulation lets you explore.
The Colloidal Challenge
Particles smaller than about 10 μm settle so slowly that practical sedimentation is impossible. Colloidal particles (0.001-1 μm) also carry electrical surface charges that prevent aggregation. This is why coagulation — adding aluminum or iron salts to neutralize charges and bind particles into larger floc — always precedes sedimentation in conventional treatment plants.
Advanced Settling Systems
Lamella (inclined plate) settlers dramatically increase effective settling area by providing angled surfaces every few centimeters. Particles need only settle a few centimeters onto the nearest plate rather than meters to the basin floor. This allows 3-5× the flow rate in the same footprint. Dissolved air flotation reverses the process entirely, using microbubbles to float floc to the surface for skimming.