DAF Float Sludge Handling: Skimmings, Solids and Volumes

What is DAF float sludge and why is it thicker?

In dissolved-air flotation, micro-bubbles released from a pressurised recycle stream attach to conditioned flocs and carry them upward. The buoyant flocs accumulate as a float blanket at the surface, which a mechanical or hydraulic skimmer removes as float sludge (also called skimmings or float cake). Because flotation concentrates solids against gravity and the blanket continues to drain and thicken while it sits, the removed float is markedly drier than the underflow from a gravity clarifier.

Typical gravity primary sludge leaves a clarifier at roughly 2–4% dry solids (DS) and waste-activated sludge (WAS) at a dilute 0.5–1.5% DS. A DAF float blanket, by contrast, commonly reaches 2–6% DS, and with good polymer conditioning and a patient skimming regime it can exceed 8%. This higher concentration is the central handling advantage of DAF: the float is already at a solids content that a gravity thickener would have to work to achieve, so thickening is usually unnecessary and the float can feed a dewatering step directly.

The relevant DAF sizing that fixes how much float is produced is covered in our companion guide on sizing a DAF flotation cell; this article picks up at the beach and follows the solids downstream.

How do you do the DAF float mass balance?

The dry mass of float sludge is fixed by conservation of solids, not by the flotation mechanism. Everything the DAF removes — captured influent TSS plus the chemical solids added as coagulant and precipitate — reports either to the float or to the treated effluent. With a solids-capture efficiency E, the float dry-solids rate is:

The chemical contribution is easy to underestimate. Dosing ferric or aluminium coagulant precipitates metal hydroxide: each 1 mg/L of Fe added as Fe³⁺ forms about 1.9 mg/L of Fe(OH)₃, and each 1 mg/L of Al forms about 2.9 mg/L of Al(OH)₃ (mass ratio of hydroxide molar mass to metal molar mass). On a high-FOG or highly coagulated effluent, chemical solids can add 20–40% to the dry float mass, so they must sit inside the balance rather than being treated as a rounding error.

Once the dry mass is known, the wet float volume follows from the float solids concentration and density, which is the quantity that actually sizes pumps, storage and the dewatering feed.

Worked example 1: float mass and volume

Take a food-and-beverage effluent treated by DAF:

• Feed flow Q = 60 m³/h; influent TSS = 1,200 mg/L = 1.2 kg/m³.
• Ferric dose = 80 mg/L as Fe, essentially all precipitated as Fe(OH)₃.
• Solids capture E = 0.95; float concentration = 4.0% DS; float density ≈ 1,010 kg/m³.

Step 1 — influent solids load: Q · TSS_in = 60 × 1.2 = 72 kg/h.

Step 2 — chemical solids: Fe(OH)₃ = 80 mg/L × 1.9 = 152 mg/L = 0.152 kg/m³; × 60 m³/h = 9.1 kg/h.

Step 3 — dry float mass: M_float = 0.95 × (72 + 9.1) = 0.95 × 81.1 = 77.0 kg DS/h.

Step 4 — wet float volume: at 4.0% DS, the wet mass = 77.0 / 0.040 = 1,925 kg/h; dividing by density 1,010 kg/m³ gives ≈ 1.91 m³/h of float sludge (about 46 m³/day). That is only ~3.2% of the feed flow reporting to sludge — the compactness that makes DAF float attractive to handle.

How much does float %DS change the downstream volume?

Because volume varies as 1/c, small gains in float dryness produce large reductions in the volume that has to be pumped, stored, tankered or fed to a dewatering unit. Holding the dry mass at 77 kg DS/h from Example 1 (≈ 1,848 kg DS/day) and varying only the float concentration:

Worked example 2: moving the blanket from 3% to 5% DS — achievable with better polymer conditioning and a longer float residence — cuts the daily volume from 61.6 to 37.0 m³, a 40% reduction, for exactly the same solids captured. At a tankering cost of, say, £25/m³ that is roughly £225/day or ~£80,000/year saved on disposal logistics alone, before counting the smaller dewatering feed. This is why float dryness, not just float mass, is the number to optimise.

How is the float blanket skimmed?

Two families of skimmer remove the blanket, and the choice sets the achievable float dryness:

• Mechanical (beach) skimmers — flights or a rotating scraper push the blanket up an inclined ramp (the beach) so it drains before dropping over a weir into the float launder. Draining on the beach is what lifts the float to the upper end of the 2–6% band; a steeper or longer beach and slower flight speed give a drier cake.
• Hydraulic (flooding) skimmers — the tank level is briefly raised so the float floods over a fixed weir. This is simpler and gentler on fragile flocs but gives a wetter, more dilute float (often 1.5–3% DS) because the blanket is carried off in a wash of water rather than drained.

The beach geometry, weir height and flight speed are the mechanical handles; polymer dose is the chemical one. A modest polymer addition (typically 2–6 kg active/tonne DS) strengthens the float so it drains without shearing back into the water, and is the usual route to pushing a mechanically skimmed blanket above 5% DS.

How is skimming frequency and duty set?

Skimming frequency governs blanket thickness, and blanket thickness sets the trade-off between float dryness and the risk of re-entrainment. A thicker, longer-residence blanket drains more and skims drier — but if it is left too long it can waterlog, roll, and shed solids back into the treated effluent, dropping the capture efficiency E in the mass balance above. The design target is the thickest stable blanket the cell will hold without carry-under.

Worked example 3 — skimmer sizing: the Example 1 float is 77 kg DS/h at 4% DS (ρ = 1,010 kg/m³). A cell 2.0 m wide runs a beach skimmer that carries a blanket ~30 mm (0.03 m) deep. The volumetric float rate is V = M/(ρc) = 77/(1,010 × 0.04) = 1.91 m³/h. The required flight speed is v = V / (W · d) = 1.91 / (2.0 × 0.03) = 31.8 m/h ≈ 0.53 m/min. Running much faster shaves the blanket too thin and skims wet; much slower lets it build until it re-entrains — so the skimmer is set on a timer or blanket sensor around this duty and trimmed on observed float dryness.

What happens to DAF float downstream?

Because the float already leaves at 2–6% DS, the gravity- or mechanical-thickening step that dilute WAS needs is normally skipped — the float is stored briefly (kept mixed to stop it going septic and to avoid stratification) and then fed straight to dewatering. On a screw press, belt press or centrifuge, a well-conditioned DAF float dewaters to a spadeable cake, and the relatively high feed concentration keeps the dewatering unit small for a given solids throughput. Our guide to downstream sludge dewatering equipment covers matching the machine to the float.

Two cautions. First, DAF float can be high in FOG (fats, oils and greases), which blinds filter media and belt cloths and can soften cake — polymer selection and sometimes conditioning chemistry differ from a mineral sludge. Second, chemical float loaded with metal hydroxides dewaters well but adds inert mass to the cake, raising disposal tonnage; that is the cost side of the chemical solids counted in the mass balance. For the equipment comparison, see our articles on sludge dewatering equipment and screw press vs belt press.

DAF float vs primary and WAS sludge

The table contrasts DAF float with the other common raw sludges to show why its handling route is different:

The message: DAF float arrives at a concentration the other streams have to be thickened up to, which is why it typically bypasses thickening and why float dryness is worth engineering hard.