What Is Dissolved Air Flotation and How Does It Work?
Dissolved Air Flotation (DAF) is an advanced water and wastewater treatment process designed to remove suspended solids, oils, greases, and other contaminants. It is widely used in municipal and industrial applications, including food processing, oil refineries, chemical plants, and wastewater treatment facilities. The primary goal of DAF is to clarify water by separating out unwanted materials through flotation, rather than sedimentation, which is used in traditional treatment systems.
Understanding Dissolved Air Flotation (DAF)
DAF is a physical-chemical separation process that removes fine suspended solids by using microbubbles of air. These air bubbles attach to particles in the water, increasing their buoyancy and allowing them to float to the surface. Once the particles have floated to the surface, they are skimmed off and removed, leaving behind clarified water.
This method is especially effective in removing particles that are difficult to settle by gravity due to their low density or hydrophobic properties (e.g., oils and fats).
Key Components of a DAF System
A typical Dissolved Air Flotation system consists of the following components:
1. Flocculation Tank
Wastewater is mixed with coagulants and flocculants in this tank. These chemicals help bind the particles together into larger aggregates called flocs, making them easier to remove.
2. Air Saturation System
Clean water (often recycled effluent) is saturated with compressed air in a pressure vessel. This creates a supersaturated solution of air in water.
3. Release Mechanism
The pressurized air-saturated water is released into the flotation tank at atmospheric pressure. The drop in pressure causes the dissolved air to form microbubbles (typically 30–100 microns in diameter).
4. Flotation Tank
The microbubbles rise through the wastewater, attaching to flocs and other particles. This increases the buoyancy of the particles, allowing them to float to the surface.
5. Skimming System
A mechanical skimmer or scraper removes the floated sludge (called float) from the surface of the tank.
6. Clarified Water Outlet
The treated, clarified water exits from the bottom of the flotation tank for either further treatment or discharge.
How Does Dissolved Air Flotation Work?
The operation of a DAF system involves several carefully controlled steps:
1. Pre-Treatment and Chemical Addition
Before entering the DAF unit, the wastewater is often pre-treated to remove coarse solids. Coagulants (such as alum or ferric chloride) and flocculants (like polyacrylamides) are added to promote the aggregation of fine particles and emulsified oils into flocs.
2. Air Dissolution
In the air saturation system, a portion of the clarified effluent is recycled and mixed with compressed air in a saturation tank under pressure (typically 4–6 bar). The water becomes supersaturated with air.
3. Pressure Release and Bubble Formation
When the pressurized water is released into the DAF flotation tank, the pressure drops to atmospheric levels. This results in the release of dissolved air as fine microbubbles.
4. Particle-Bubble Attachment
These microbubbles attach to suspended solids and flocs in the incoming wastewater. Due to the added buoyancy from the bubbles, the particles begin to rise to the surface.
5. Float Removal
A layer of sludge forms at the surface, composed of floated solids, oils, and greases. A skimmer slowly moves across the surface, pushing the sludge into a collection trough for further treatment or disposal
6. Effluent Discharge
The clarified water, now largely free of solids, exits from the bottom of the tank. A portion of this clean water is recycled back into the air saturation system to maintain a continuous cycle.
Advantages of Dissolved Air Flotation
DAF systems offer numerous benefits over other solid-liquid separation methods:
- High Efficiency: Capable of removing 90–95% of suspended solids, fats, oils, and greases.
- Compact Design: DAF units require less space compared to sedimentation systems.
- Fast Operation: Clarification occurs in minutes rather than hours.
- Effective for Low-Density Particles: Ideal for treating particles that are difficult to settle due to their density or surface characteristics.
- Chemical Optimization: The use of coagulants and flocculants enhances performance.
- Flexibility: Suitable for a wide variety of industries and effluent types.
Applications of DAF Systems
DAF technology is used in many sectors where water clarification is critical:
- Municipal Wastewater Treatment: To reduce BOD, suspended solids, and phosphorus before biological treatment or discharge.
- Food and Beverage Industry: Removal of grease, oils, and solids from slaughterhouses, dairies, and breweries.
- Oil and Gas: Separation of emulsified oil and hydrocarbons from process water.
- Pulp and Paper: Removal of fibres and inks from wastewater in paper mills.
- Chemical Manufacturing: Clarification of wastewater containing suspended solids and emulsions.
Limitations of DAF
Despite its advantages, DAF has certain limitations:
- Operational Costs: Requires energy for air compression and chemical dosing.
- Sludge Management: The floated sludge must be properly thickened and disposed of.
- Chemical Dependency: Performance heavily relies on optimal chemical treatment.
Innovations in DAF Technology
Recent advancements have improved the performance and sustainability of DAF systems:
- High-Rate DAF Units: Offer higher flow capacities with smaller footprints.
- Lamella Plates: Increase surface area for better clarification.
- DAF with Integrated Biological Treatment: Used in compact treatment plants.
- Automated Controls: Improve chemical dosing, skimming, and air saturation efficiency.
Conclusion
Dissolved Air Flotation is a vital technology in the field of water and wastewater treatment. Its ability to remove fine suspended solids, oils, and other contaminants makes it an ideal solution for a wide range of industrial and municipal applications. By understanding its working principles, components, and best practices, operators and engineers can ensure efficient and reliable treatment outcomes.
As water quality regulations become stricter and sustainability becomes more critical, DAF systems will continue to play an essential role in ensuring that treated water meets environmental and operational standards.