DAF System

Overview

Dissolved air flotation (DAF) is a clarification process that removes suspended solids, oils, fats, algae, and colloidal matter by attaching fine gas bubbles to particles, causing them to float to the surface for removal. Unlike conventional settling which relies on particle density to achieve separation, DAF exploits buoyancy, making it ideal for low-density or colloidal materials unsuitable for gravity settling.

The core principle is Henry's Law: compressed air dissolves into water at pressure, releasing as micro-bubbles when pressure drops. A Saturation Vessel maintains 4–6 bar absolute pressure, dissolving 35–50 mg/L air. Saturated water is then released into the main Flotation Tank where rapid decompression nucleates bubbles (20–100 μm diameter). These bubbles attach to flocculated solids, forming aggregates less dense than water that rise to the surface in minutes.

Performance depends on three factors: coagulation (destabilizing colloids), flocculation (growing micro-flocs), and flotation (attaching bubbles). A DAF system typically removes 80–95% of suspended solids and 60–80% of oils/grease when properly operated.

Process Chemistry

Coagulation destabilizes colloids via charge neutralization. Common coagulants:

• Aluminum salts (alum, PAC): 5–30 mg/L, optimal pH 6–8, produces aluminum hydroxide micro-precipitate
• Iron salts (ferric chloride, ferrous sulfate): 10–40 mg/L, optimal pH 7–9

Flocculation follows: polyelectrolytes (anionic or cationic PAM) at 0.1–1 mg/L bridge coagulated particles into larger aggregates (0.1–1 mm). The resulting floc is loosely bonded and easily attached to bubbles.

Henry's Law governs air dissolution: C_air = k_H × P_air

Where k_H = solubility coefficient (~0.0058 mol/(L·atm) at 20°C), P_air = partial pressure. At 4 bar gauge (5 bar absolute): C_air = 0.0058 × 5 = 0.029 mol/L ≈ 45 mg/L

When released to atmospheric pressure (1 bar), the air precipitates as bubbles.

Saturation Vessel Operation

The Saturation Vessel holds recycled effluent at 4–6 bar for 1–5 minutes. Residence time formula: t = V / Q_recycle

For a 1 m³ vessel with 10 m³/h recycle flow: t = (1 × 60) / 10 = 6 minutes

Air injection occurs continuously via a bubble stone, introducing 1–2 m³/min compressed air. Pressure is maintained by a relief valve set to 5 bar.

The saturation tank must be sized generously: excessive pressure drop or short residence time results in incomplete air dissolution, reducing bubble yield in the flotation tank.

Flotation Tank Dynamics

Saturated water enters the Flotation Tank through a depressurization valve. As pressure drops to atmospheric, dissolved air nucleates into 20–100 μm bubbles. The bubble count depends on air concentration and nucleation efficiency; typical yield is 10⁷–10⁹ bubbles per liter.

Bubble attachment to floc particles is governed by particle size, bubble size, and floc density. Optimal performance occurs with particle size 50–500 μm and bubble size 20–100 μm. Floc density should be slightly greater than water (density difference: 0.5–2% of water density) to ensure gentle rise without shearing.

The Baffle Wall divides the flotation tank into inlet and flotation zones, ensuring even distribution and preventing short-circuiting. The flotation zone has upflow velocity of 0.5–1.0 m/h, allowing 15–25 minutes residence. At this rate, floc-bubble aggregates rise at 0.5–2 m/h (depending on density and bubble attachment strength) and reach the surface for removal by the Skimmer.

The Skimmer operates at 0.2–2 rpm, continuously raking foam and float sludge into a discharge hopper. Settled sludge in the Sludge Hopper is withdrawn periodically via a Hopper Discharge Valve.

Recycle Ratio and Energy Balance

The recycle pump recirculates 10–30% of the clarified effluent through the saturation vessel. Higher recycle ratios produce more bubbles but increase energy consumption. A typical balance:

• 10% recycle: Low bubble yield, minimal power (3–5 kW), suitable for low-turbidity inlet
• 20% recycle: Standard operation, moderate power (5–10 kW), removal 85–90%
• 30% recycle: High bubble yield, increased power (10–15 kW), removal 90–95%

The Recycle Pump System is typically a gear pump or positive-displacement centrifugal pump delivering recycle flow at pump discharge pressure minus pressure drop through the saturation vessel (typically 0.5–1.5 bar).

Sludge Characteristics

Float sludge from the DAF is typically 3–5% solids by weight, much finer than gravity-settled sludge. It contains primarily coagulant hydroxide precipitate, flocculated colloids, and attached micro-bubbles. Float sludge often requires thickening (via dissolved-air flotation, gravity thickening, or centrifugal thickening) before dewatering via Belt Filter Press or Sludge Screw Press.

Typical Applications

1. Food and Beverage: Removal of suspended solids, fats, oils from dairy, meat, or vegetable processing wastewater
2. Pulp and Paper: Clarification of white water and paper machine effluent
3. Oily Water: Tank-truck wash, truck stop, or petroleum facility wastewater (removal of oils and grease to <10 mg/L)
4. Algae Removal: Secondary clarifier or polishing stage to remove algal cells
5. Combined Sewer Overflow (CSO): Emergency or stormwater treatment removing grit and floatables

Advantages

• Low Footprint: 0.3–1.0 m² per m³/day, 1.5–2.0 m deep (vs. 1.5–3.0 m² for gravity clarifier)
• Fast Startup: No sludge bed development; full capacity in hours
• Handling Low-Density Solids: Algae, fibers, fats that don't settle in clarifiers
• Robust to Shock Loads: Brief overdoses of oil or grease don't cause clarifier upset

Limitations

• Chemical Dependency: Requires coagulant dosing; suboptimal dosing reduces efficiency
• Air Bubble Loss: Bubble coalescence or escape reduces effective bubble count
• Float Sludge Volume: Higher solids concentration than gravity-settled sludge; requires follow-on thickening
• Power Consumption: Compressor and recycle pump typically 10–30 kW continuous

Operational Checklist

Standards

• AWWA Standard B200: Filtration and Clarification
• EPA Design Manual: Fine Pore Aeration Systems
• EN 12255-8: Flotation equipment