Dissolved Air Flotation (DAF): Applications in Water and Wastewater

Dissolved air flotation (DAF) is a clarification process that removes suspended solids, oils, grease, and other low-density contaminants by attaching them to microbubbles that float material to the surface for removal. DAF treatment systems serve a critical role across municipal water treatment, industrial pretreatment, and wastewater solids thickening applications.

How Dissolved Air Flotation Works

A DAF system operates on a straightforward principle: air is dissolved into water under pressure (60–90 psi), then released at atmospheric pressure to form a cloud of microbubbles (30–100 microns in diameter). These bubbles attach to suspended particles, reducing their effective density and floating them to the surface as a sludge blanket.

The process consists of these primary components:

Pressurization system: A recycle stream (typically 10–30% of influent flow) is saturated with air in a pressure vessel (saturator or air dissolution tank). The saturated water is released through needle valves or proprietary nozzle plates at the DAF tank inlet.

Flotation zone: As pressurized recycle enters the flotation zone, microbubbles form and attach to floc particles. The bubble-particle aggregates rise to the surface, typically within 3–5 minutes. Surface loading rates range from 2–8 gpm/ft² depending on the application.

Skimming system: A mechanical skimmer (chain-and-flight, scoop, or spiral type) removes the accumulated float sludge from the surface. Float solids concentrations typically range from 3–6% for municipal applications.

Effluent collection: Clarified water exits through a submerged collection header or perforated pipe near the bottom of the tank.

Chemical Pretreatment

DAF performance depends heavily on upstream chemical conditioning. Coagulation and flocculation convert dissolved and colloidal contaminants into particles that can be captured by microbubbles.

Typical chemical programs include:

• Coagulant: Aluminum sulfate (alum), polyaluminum chloride (PACl), or ferric chloride at doses of 10–50 mg/L. PACl is increasingly preferred for its performance at lower temperatures and broader pH range.
• Flocculant: Low-dose anionic or nonionic polymer (0.1–0.5 mg/L) to build stronger, more buoyant floc. SNF / Polydyne supplies coagulant/flocculant polymers (FLOPAM) for this duty.
• pH adjustment: Coagulation optimum is typically pH 6.0–7.0 for alum, 5.5–6.5 for ferric chloride.

Proper rapid mix (G value 300–600 s⁻¹) and flocculation (G value 20–70 s⁻¹ for 10–20 minutes) are essential for forming floc that attaches well to microbubbles.

DAF vs. Conventional Sedimentation

DAF excels where conventional settling struggles: low-density particles, algae-laden source water, oil and grease removal, and high-rate applications where footprint is limited. The higher float solids concentration compared to gravity sedimentation sludge is a significant advantage for downstream solids handling.

Municipal Water Treatment Applications

In drinking water treatment, DAF is used as an alternative to sedimentation for raw water clarification. It is particularly effective for:

• Low-turbidity, high-color waters: Common in New England and the Pacific Northwest, where conventional settling is ineffective.
• Algae-laden reservoirs: DAF is the preferred technology for removing algae, which tend to float rather than settle. Critical during cyanobacteria (blue-green algae) bloom events.
• Retrofit/expansion: DAF's high surface loading rate (4–8x higher than sedimentation) allows capacity expansion within existing building footprints.

Wastewater Applications

In wastewater treatment, DAF serves multiple functions:

• Primary treatment: DAF can replace primary clarifiers at facilities treating wastewater with high oil and grease content or where space is limited.
• WAS thickening: DAF thickeners concentrate waste activated sludge from 0.5–1.0% to 3–6%, reducing downstream digester or dewatering volume. Often more reliable than gravity thickeners for WAS.
• Industrial pretreatment: Food processing (meat, dairy, poultry), petroleum refineries, and automotive plants use DAF to meet pretreatment limits for FOG and TSS.
• Tertiary treatment: Some facilities use DAF for phosphorus removal following chemical precipitation, achieving effluent TP below 0.1 mg/L.

Design Considerations

Key design parameters for DAF systems include:

• Air-to-solids ratio (A/S): Typically 0.02–0.06 lb air/lb solids. Higher ratios improve float but increase energy cost.
• Recycle ratio: 10–30% of influent flow. Higher recycle increases bubble density.
• Hydraulic loading rate: 2–4 gpm/ft² for conventional systems; up to 8 gpm/ft² for high-rate DAF.
• Saturator pressure: 60–90 psi. Higher pressure dissolves more air per unit volume.
• Float removal frequency: Continuous skimming prevents float blanket from becoming too thick and breaking through.

Emerging Trends

Recent DAF developments include counter-current DAF designs that improve bubble-particle contact, plate-enhanced DAF systems that increase effective settling area, and DAF systems integrated with membrane filtration for small-footprint treatment trains. Advanced controls using streaming current monitors and turbidity-based coagulant dosing are improving chemical efficiency and reducing operator intervention.