Design Manual › Sludge › DAF Thickening
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DAF Sludge Thickener
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).
Request a quote for this duty ← Sludge Thickening & Dewatering Anaerobic Digester →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.
Questions about a duty, a specification, or transparent sourcing? Contact our application engineers →
Engineer's FAQ
Frequently asked about DAF thickeners
What is a DAF saturator and why does it matter?
The saturator dissolves air into the recycle stream at 3–5 bar. When pressure is released, 20–50 µm micro-bubbles form, attach to floc and float the sludge blanket. Saturator efficiency sets the air-to-solids ratio — the single biggest lever on DAF performance.
What is the difference between DAF and sludge thickening?
DAF (dissolved air flotation) separates solids by buoyancy for high-rate clarification, while sludge thickening concentrates waste activated sludge. In most plants the DAF unit is the thickener — see the sludge thickening & dewatering guide for the full train.
How much sludge can a DAF thickener handle?
Typical hydraulic loading is 4–10 m/h with solids loading of 3–8 kg/m²·h, depending on flocculation and feed consistency. Size against your peak WAS flow, not the average.
Related calculators
Calculators in this treatment train
- Sludge Thickening & DewateringWAS thickening, cake solids, polymer dose
- Fine Drum ScreenPre-DAF screening, solids recovery
- Anaerobic Digestion VolumeBiogas potential, retention, heating
- Headworks & Grit DesignBar screen, grit removal, bypass