The rise step — how bubble-particle aggregate density, size and Stokes’ law set the rise velocity that the DAF flotation tank must be sized around.
The Science Behind Dissolved Air Flotation
Once a bubble-particle aggregate forms, its mean density falls below that of water and it rises. The rise velocity — governed by Stokes’ law, the aggregate’s effective (reduced) density and the number of bubbles attached — determines the hydraulic loading the flotation tank can carry, and is the direct link between micro-bubble physics and tank sizing.
vrise = g(ρw−ρagg)dagg²/18µ. Attaching air lowers the aggregate’s effective density ρagg; the more air (higher air-to-solids ratio) and the larger the floc, the faster it rises — which is why coagulation and a correct A/S ratio matter.
From aggregate density to design rise rate
Attached bubbles drop the aggregate density below water so it floats.
Rise velocity scales with the square of aggregate size.
More dissolved air per unit solids gives more lift and faster rise.
Several bubbles per floc lift even dense or heavy particles.
At high solids the rising blanket interacts and slows — a design limit.
The slowest design aggregate sets the allowable hydraulic loading.
| Aggregate rel. density | Behaviour | Note |
|---|---|---|
| >1.0 | Sinks | Insufficient air attached |
| 0.95–1.0 | Slow rise | Marginal A/S; increase recycle/pressure |
| 0.8–0.95 | Good rise | Typical well-conditioned DAF |
| <0.8 | Rapid rise | Strongly aerated, light flocs |
Continue across the DAF science series
The micro-bubble DAF overview and hub.
Read MoreWhy 20-50 µm bubbles win: surface area, Laplace pressure, number density.
Read MoreFloat build-up, solids concentration and surface removal.
Read MoreRise velocities enable 5-15 m/h loading and a compact footprint.
Read MoreSize a DAF and check A/S, loading and recycle.
Read MoreOur process engineers design dissolved air flotation from first principles — buoyant rise velocity and the full bubble physics — for guaranteed removal at minimum lifecycle cost.
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