Dissolved Air Flotation (DAF) A/S Calculator
Size DAF recycle pressurization systems and calculate the Air-to-Solids (A/S) ratio, microscopic bubble air release, and hydraulic loading rates.
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Dissolved Air Flotation (DAF) Physics & Air-to-Solids Sizing
Dissolved Air Flotation (DAF) is an advanced clarification process used widely in food processing, oil refineries, and municipal sludge thickening. Unlike conventional clarifiers where solids settle downward by gravity, DAF dissolves compressed air into recycled clean effluent at 50 to 75 psig. When injected into the wastewater stream through pressure-reducing nozzles, billions of microscopic air bubbles (30 to 50 microns in diameter) nucleate out of solution, attaching to suspended solids and floating them to the surface as a thickened skim.
The DAF Air-to-Solids (A/S) Governing Equation
Where:
- A/S = Air to Solids Ratio (lb air / lb solids)
- sa = Air solubility at wastewater temperature (mL air / liter of water ≈ 18.7 mL/L @ 68°F)
- f = Fraction of theoretical air saturation achieved in saturator tube (≈ 0.80 - 0.85)
- P = Absolute saturator pressure (psia = psig + 14.7)
- R = Pressurized recycle flow rate (GPM)
- Q = Influent wastewater flow rate (GPM)
- S = Influent TSS/FOG concentration (mg/L)
Optimal flotation typically requires an A/S ratio of 0.015 to 0.050 lb air / lb solids.
Frequently Asked Questions
Why must DAF bubbles be microscopic (30-50 microns)?
Large bubbles (such as those from aeration diffusers) rise too rapidly with high turbulent wake, shearing delicate chemical flocs and dislodging grease particles. Microscopic 30-50 micron bubbles rise smoothly in laminar flow, attaching firmly to hydrophobic grease and polymer flocs without destroying them.
What happens if the A/S ratio is too low (<0.010)?
If insufficient air is dissolved, there are not enough micro-bubbles to overcome the negative buoyancy of the solids. Heavy flocs sink to the bottom hopper rather than floating, blinding the drain and allowing oily solids into the treated effluent.
What is a typical DAF hydraulic surface loading rate?
Without plate packs, standard open DAF tanks operate at 1.5 to 3.5 GPM/sq ft. High-rate DAF units equipped with inclined lamella plate packs can achieve 6.0 to 10.0 GPM/sq ft by dramatically reducing bubble travel distances.
What chemicals are typically injected ahead of a DAF?
Coagulants (polyaluminum chloride or ferric chloride) neutralize colloidal surface charges, followed by high-molecular-weight cationic or anionic polymers to agglomerate micro-flocs into buoyant pinhead-sized structures ideal for micro-bubble capture.