Dissolved Air Flotation (DAF) is a widely used physical separation technique in water and wastewater treatment. Its primary function is to clarify water by removing suspended matter, including fine solid particles, fats, oils, and grease. DAF is an efficient method for solid-liquid separation, relying on buoyancy to float contaminants to the surface for removal, especially those difficult to separate via gravity settling.
The Core Mechanism of Dissolved Air Flotation
The effectiveness of DAF hinges on creating and attaching microscopic air bubbles to suspended contaminants. The process begins with air saturation, where a portion of the clarified water is pumped into a pressure vessel. Compressed air is introduced and dissolved into the water at elevated pressures, typically between 400 and 600 kilopascals (kPa). This high pressure supersaturates the water, allowing it to hold significantly more dissolved gas than normal atmospheric conditions.
The air-saturated water is then released through a pressure reduction valve into the flotation tank, which is at atmospheric pressure. This sudden pressure drop causes the excess dissolved air to instantly come out of solution, forming millions of minute bubbles. These microbubbles generally measure between 10 and 100 micrometers in diameter.
As these fine bubbles ascend, they collide with and adhere to the suspended solid particles. The attachment of numerous bubbles significantly lowers the overall apparent density of the particle-bubble cluster. This density reduction overcomes gravity, causing the buoyant agglomerates to rapidly float to the surface.
Before entering the DAF tank, the raw water is often treated with chemical coagulants and flocculants. This pretreatment destabilizes the fine particles and forms larger flocs, providing a better surface area for microbubble attachment and maximizing removal efficiency. Once the buoyant layer of contaminants, known as the “float” or “scum,” collects at the surface, a mechanical skimming device scrapes it away for sludge handling. The clarified water exits the tank bottom as treated effluent.
Key Applications of DAF Technology
DAF technology is versatile, employed across municipal and industrial environments to address specific water quality challenges.
Municipal Treatment
In municipal drinking water treatment, DAF is well-suited for source waters with low turbidity, high color, or algal blooms. The fine air bubbles effectively lift light, low-density algae cells and organic color particles that are difficult to remove via conventional sedimentation. This application helps ensure the final drinking water meets quality standards.
Industrial Pretreatment
Within the industrial sector, DAF is a standard pretreatment method for complex wastewater streams. It is extensively used in oil refineries, petrochemical plants, and food processing facilities. Its high efficiency in removing Fats, Oils, and Grease (FOG) makes it a preferred choice for high-strength effluent, such as from meat processing or dairy operations. Removing these contaminants protects downstream biological treatment systems from inhibition.
Sludge Thickening
Beyond water clarification, DAF is widely used for sludge management, specifically for thickening biological sludge. Excess activated sludge from secondary treatment is often very dilute, making subsequent dewatering expensive. DAF effectively concentrates this dilute sludge, achieving a much higher solids content, sometimes up to 6% dry matter. This thickening step significantly reduces the overall volume of sludge requiring disposal.
Operational Advantages of Using DAF
DAF systems offer several practical benefits compared to traditional gravity-based clarification methods, such as sedimentation tanks.
One significant advantage is the system’s compact footprint. Since the flotation process is faster than gravity settling, DAF units require less surface area and hydraulic retention time for the same flow rate. They often need five times less space than a static settler, making DAF ideal for facilities with limited space or for retrofitting.
DAF also achieves a higher quality effluent, especially when dealing with fine, light particles and oil emulsions. The microbubbles capture and float even the smallest suspended solids, resulting in enhanced contaminant removal. Removal efficiency for Total Suspended Solids (TSS) can reach up to 97%. This performance helps ensure compliance with discharge or reuse regulations.
The flotation process results in a higher concentration of solids in the removed sludge. The buoyant float layer is typically thicker and denser than sludge produced by gravity settling. This higher solids concentration improves the efficiency of downstream sludge handling, dewatering, and disposal processes, leading to reduced operational costs.