Waste activated sludge does not settle well — it is light, fluffy and largely water.
Dissolved air flotation gets around that by floating it instead, reaching 4–6 % solids where gravity
thickening tops out around 1–2 %. This tool sizes the float area, works out the air and recycle needed
to lift the load, and shows what the volume reduction is actually worth.
1Sludge duty
Everything downstream scales with the dry solids mass, not with the sludge volume.
Get the feed concentration right — it is the number most often guessed.
2Air, saturator & polymer
The air-to-solids ratio is the heart of a DAF design. Too little air and the solids
never lift; too much and you shear the floc apart and pay for compression you do not need.
Results — float area & thickening
Float area
—m²
Thickened DS
—%
Volume reduction
—%
Item
Value
Basis
3Air & saturator loop
Air dissolves under pressure, then comes out of solution as fine bubbles across the
whole floor of the tank. The recycle rate falls out of the air demand and how much air the saturator can
actually hold at that pressure.
Item
Value
Basis
4Mass balance & volume reduction
The reason to thicken: every cubic metre taken out before digestion or dewatering is a
cubic metre of tankage, heating and haulage you do not pay for.
Item
Value
Basis
DAF vs gravity thickening
Item
DAF
Gravity
5Instrument schedule
DAF is more sensitive to feed consistency than a gravity thickener. Two concentration
measurements — feed and thickened — are what make it controllable.
Point
Instrument
Why here
Quote the DAF instruments at factory price
Feed and thickened sludge solids, flow, saturator pressure and level — all mappable to our catalogue
and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the
device).
Response within 24 hours · Inputs stay in your browser — nothing is uploaded.
Sources & parameter basis
[1]
METCALF & EDDY / EPA — sludge thickening: WAS feed 0.4–1.2 % DS, primary sludge 2–6 %, chemical sludges lower. Feed concentration is the input most often assumed rather than measured.
[2]
DAF thickening loading: solids loading rate 10–30 kg/m²·h (WAS toward the lower end), hydraulic loading 2–10 m³/m²·h including the pressurised recycle.
[3]
Float tank depth 1.5–3 m — markedly shallower than a sedimentation tank, since the separation distance is short and bubbles do the work.
[4]
Air-to-solids ratio 0.01–0.04 kg air per kg dry solids; 0.02 is a common starting point for WAS.
[5]
Saturator: air solubility ≈20.9 mL/L at 1 atm and 20 °C, scaling linearly with absolute pressure; saturator achieves 60–90 % of theoretical; detention 1–3 min. Recycle pump head ≈ pressure in metres; P = Q·H/(367·η) kW.
[6]
Achievable thickened concentration: WAS 4–6 % by DAF (vs 1–2 % by gravity), mixed primary+WAS 4–8 %, chemical sludges 2–4 %.
[7]
Polymer 2–10 kg per tonne of dry solids for DAF thickening; the upper end for difficult or dilute feeds.
Full derivation and worked example: DAF-Thickener-Calculation-Sheet.md. Raise the
saturator pressure from 4 to 6 bar(g) and watch the recycle rate fall — that is the trade between
compression energy and pumping energy, and it is worth running on your own electricity tariff.
