DAF / Dissolved Air Flotation Systems

Dissolved air flotation is one of the most efficient physicochemical methods of industrial wastewater treatment

DAF systems are used for cleaning wastewater from suspended solids including fats, oil products, and phosphates while reducing COD and BOD in wastewater. Industries and use include food (meat, dairy, fish, canning, etc.), textile and leather, oil and fat plants, iron and steel, and sludge thickening.

The initial wastewater is pumped to the mixer inlet, i.e. to the DAF unit, where the effluents are treated with reagents and then enter the mixing chamber with the simultaneous supply of a water-air mixture. Air and suspended solids float to the surface in the form of coagulated particles. Larger flotation complexes reach the surface at the mixing chamber stage. The rest goes with the wastewater to the flotation chamber. The resulting foam is removed from the surface of the water by the scraper mechanism and discharged into the collection tray. Heavy contaminants settle on the bottom of the unit and are removed.

The clarified wastewater is discharged from the installation. Part of the purified water is taken by the pump to saturate it with air. After this, the air-water mixture enters along with the mainstream into the mixing chamber of the flotator. The water level in the flotator is controlled by changing the overflow height.

What is Dissolved Air Flotation (DAF)?

Dissolved Air Flotation (DAF) is a water treatment process that clarifies wastewater and potable water by removing suspended solids, oils, and other contaminants. The process works by dissolving air in water under pressure and then releasing the air at atmospheric pressure in a flotation tank. The released air forms tiny bubbles that adhere to the suspended matter, causing it to float to the surface, where it can be removed by a skimming device.

The Theory of DAF

To achieve effective solid–liquid separation using DAF, especially for influents containing particles and natural color, coagulation or flocculation is necessary before introducing microbubbles. This helps form bubble–floc aggregates, which are essential for efficient flotation.

The main concept in DAF is to float particles whose specific gravity is close to that of water. Low-density gas bubbles, usually air, are introduced to achieve this. The air bubbles adhere to the particles, reducing their specific gravity to below 1.0. This causes the particles to aggregate and float to the surface of the flotation tank.

There are three key processes involved in DAF:

Bubble Formation: The generation of tiny air bubbles by releasing dissolved air at atmospheric pressure.
Bubble–Particle Attachment: The adherence of air bubbles to suspended particles, forming bubble–floc aggregates.
Flotation of Bubble–Particle Agglomerates: The buoyant rise of these aggregates to the surface, where they can be removed efficiently.

Advantages of DAF Application in Wastewater Treatment

The DAF technique offers several advantages over conventional gravity settling methods, particularly for removing low-density particles that tend to float. Key advantages include:

Efficient Particle and Turbidity Removal: Results in more economical filter designs and allows short detention times of about 5–10 minutes in flocculation tanks.
Higher Hydraulic Loading Rates: Can be used at rates higher than most settling processes.
Superior Removal of Low-Density Floc: More efficient than sedimentation in removing floc formed from coagulation of Total Organic Compound (TOC).
Reduced Coagulant Dosage: Leads to smaller chemical storage requirements and lesser sludge production.
Compact Footprint: Smaller plant size with stacked flotation-over-filtration arrangements.
Improved Algae Removal and Cold Water Performance: Particularly beneficial for potable water treatment.
Less Sensitivity to Flow Variations: Ensures stable performance under fluctuating flow conditions.
Process Flexibility: Adjustable air loading to suit specific treatment needs.

Application of DAF Process in Wastewater Treatment

DAF is a widely adopted physical process in wastewater treatment, often combined with coagulation. Industries implementing DAF include:

Paper Mills: For fiber recovery and water recycling.
Chemical-Mechanical Polishing (CMP) Wastewater: Removal of suspended solids.
Meat and Seafood Industries: For efficient removal of organic materials and fats.
Personal Care Products Industry: Treatment of complex wastewater streams.
Oil Refineries: Effective for treating oily wastewater.

Case Study: Zoubolis and Avranas applied a combined coagulation and DAF process to treat simulated oily wastewater (500 mg/L octane concentration). Different coagulants were tested, with FeCl₃ demonstrating the best results by improving droplet–bubble adhesion. The optimal removal rate occurred at 100 mg/L Fe³⁺ with 50 mg/L sodium oleate (NaOl) at pH 6 and a recycle ratio of 30%. At this pH, the zeta potential of hydrocarbon particles is nearly zero, reducing the electrostatic barrier and improving droplet–bubble attachment.

Applications of DAF Technology

DAF technology is versatile and widely used in various industries, including:

Wastewater Clarification: Efficiently removes solids, oils, and grease from municipal and industrial wastewater.
Potable Water Treatment: Used extensively in Europe since the 1960s for producing clean, safe drinking water.
Mineral Processing: Separates valuable minerals from ores, a practice dating back to ancient times.
Artificial Recharge: Enhances groundwater replenishment by treating water before infiltration.
Paper and Pulp Industry: Recovers fibers and recycles process water, improving operational efficiency.

Why Choose DAF Technology?

Rapid Separation: Faster than traditional sedimentation processes.
Compact Design: Requires less space than conventional clarification systems.
High Efficiency: Effectively removes low-density particles, oils, and greases.
Versatility: Suitable for diverse applications across multiple industries.
Operational Flexibility: Adaptable air loading to meet specific treatment requirements.

Dissolved Air Flotation (DAF) technology offers exceptional performance in diverse applications, from municipal water treatment to industrial wastewater management. The process ensures rapid separation of contaminants, occupies a compact footprint, and provides operational flexibility. With high efficiency in removing low-density particles, oils, and greases, DAF remains a top choice for industries seeking reliable, sustainable, and cost-effective water treatment solutions.

Conclusion

Dissolved Air Flotation (DAF) has proven to be a critical technology in modern water treatment and industrial applications. With its ability to efficiently separate solids and contaminants from liquids, DAF ensures cleaner water and more sustainable industrial processes. As global demand for efficient water treatment solutions grows, DAF stands out as a reliable, time-tested, and innovative solution for a wide range of applications.