Solids separation
C-320Dissolved Air Flotation
Flotation is what you reach for when gravity will not do the job — when the solids are lighter than water, or so close to it that a settling tank would need a footprint nobody will pay for. Air is dissolved into a recycle stream under pressure, released as microbubbles, and used to carry the solids up.
Cutaway — Dissolved Air Flotation
LiquidAir / gasSolidsProductWaste
How it works
- why flotation — Gravity settling stalls when the solids are barely denser than water — algae, oil and grease, light chemical flocs. Watch: with no air, the particles drift the length of the cell and leave with the clarified water. That is carryover, and it is the failure a clarifier cannot design its way out of.
- saturation & release — A slipstream of clarified effluent is pumped to the saturator and held against compressed air. Solubility follows Henry's law — proportional to absolute pressure, falling as the water warms. Releasing that stream through nozzles at the cell floor drops it to atmospheric, and the excess air leaves solution as microbubbles 10–100 µm across.
- contact zone — attachment — Feed and white water rise together through the narrow contact zone for one to two minutes. Bubbles collide with flocs and adhere. Attachment depends on floc size and surface charge, which is why upstream coagulation is not optional — bare colloids will not hold a bubble.
- separation & float — Over the baffle, the aggregates now rise faster than the downward hydraulic velocity and surface as a float layer at 2–5 % dry solids. A skimmer moves it to the beach. Clarified water leaves at the floor; the few heavy particles that never attached fall out as bottom sludge.
- what governs the design — The binding constraint is the air-to-solids ratio — air made available per unit mass of solids. Push the recycle ratio or saturator pressure up and the bubble cloud thickens; raise the influent SS and the same air has more work to do. Whichever check turns amber first is the one sizing your cell.
Design parameters
The panel opposite runs the same correlations as the C-320 design sheet, so the animation and the calculator cannot disagree. Drag any of them and the picture responds.
- Feed flow — m³/h
- Influent SS — mg/L
- Recycle ratio — %
- Saturator pressure — bar g
- Water temperature — °C
- Cell length — m
Open the full C-320 design sheet for the governing equations, the accepted design envelopes and the worked calculation.