Surface overflow rate (basin 30 m x 10 m)–

Time for floc to settle the full depth–

Time water spends in the basin–

This floc–

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Interactive 3D · 5 min

How a treatment plant settles out dirt

Raw river water carries particles too small to sink on their own. A coagulant neutralizes their electric charge in a fast mixing tank, slow paddles then gently stir them into larger, heavier clumps called floc, and a long quiet basin gives the floc time to settle to the floor. Clear water skims off the top over a weir while the sludge is scraped away.

Controls

Labels

Animation speed

Plant flow

Basin water depth

Floc settling velocity

Bigger, denser floc from good coagulation sinks faster.

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CoagulationA coagulant such as an aluminum or iron salt is mixed in hard and fast for seconds. It cancels the negative charges that keep tiny particles apart.
FlocculationSlow paddles stir for tens of minutes so the destabilized particles bump together and grow into fluffy floc. Too much speed would tear the floc apart.
SettlingIn the long quiet basin the floc sinks while the water drifts slowly toward the far end.
The overflow rate testAny floc that sinks faster than the surface overflow rate reaches the floor before the weir. Push the plant flow up and lighter floc starts escaping.
Sludge removalA slow scraper pushes settled sludge to a hopper so it can be pumped out and dewatered.

Ideal settling basin theory

Imagine a particle entering at the surface. It has to fall the full depth while the water carries it the length of the basin. Settling time = depth / settling velocity, and the water's time in the basin = volume / flow. Set them equal and depth cancels: a particle is captured if its settling velocity is at least flow / surface area. That ratio is the surface overflow rate.

This is the classic ideal basin model, simplified: real basins have currents, wind and temperature effects, so designers add safety margin. Operators often quote overflow rate in gallons per day per square foot; 1 m/h is about 590 gpd/ft².

Try it: hold the floc settling velocity at 1.5 m/h and raise the plant flow from 400 to 900 m³/h. The overflow rate climbs past 1.5 m/h, the readout flips to 'Partly carried over the weir', and the brown floc particles start riding over the far end.

Depth buys time, not capture

Making the basin deeper increases how long water stays, which helps with storage and smoothing out surges, but in the ideal model it does not change which particles are caught. Better coagulation, which makes faster sinking floc, or more surface area are what improve capture.

Sources

Turbidity Through the Treatment Processes (Guidance Manual, Chapter 10)US EPA, archived by University of North Texas Libraries
Settling Tanks (Clarifiers), ENGS 37 lecture notesDartmouth College
Sedimentation (Operator Certification Webinar)Environmental Finance Center Network, University of New Mexico

How to cite this model

Avid Solutions International. (2026, November 11). How a treatment plant settles out dirt [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/water-treatment-clarifier

Last updated November 11, 2026

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