Dissolved air flotation (DAF) is a water treatment process that removes suspended solids, oil, grease, and other contaminants from wastewater.
It has been used for decades in various industrial applications, including food processing, mining, paper production, and desalination plants.
In this article we will explore DAF technology which is an effective and reliable, making it one of the most popular methods for wastewater and sewage treatment.
Table of Contents
What is Dissolved Air Flotation DAF?
Dissolved air flotation DAF is a process that separates solids and liquids in wastewater by introducing fine bubbles of air into the water. The air bubbles attach to the suspended solids and float them to the surface of the water, where they can be removed.
DAF technology can range from simple circular basins equipped with a configuration of submersible nozzles, which bubble pressurized air up through a near stagnant pool of wastewater, to dynamic systems designed with multiple compartments, which capitalize on different water retention times in each compartment and a combination of settling, flotation, and impingement to remove contaminants.
Dissolved Air Flotation (DAF) System Design
Dissolved Air Flotation (DAF) systems are designed to remove:
Total suspended solids (TSS)
Chemical oxygen demand (COD)
Biochemical oxygen demand (BOD)
Oils and greases (O&G)
Contaminants are removed using a dissolved air-in-water solution produced by injecting air under pressure into a recycle stream of clarified DAF effluent.
This recycle stream is then combined and mixed with incoming wastewater in an internal contact chamber where the dissolved air comes out of the solution in the form of micron-sized bubbles that attach to the contaminants.
The bubbles and contaminants rise to the surface and form a floating bed of material that is removed by a surface skimmer into an internal hopper for further handling.
A first-principles design would involve predicting how much air per kg of water could be dissolved in water at a given temperature and pressure using Henry’s law, then working out the required recycle water flowrate based upon an amount of air per incoming solids load. It is this second factor which cannot be determined from first principles. It can be measured experimentally, or more commonly estimated based on experience.
How does Dissolved Air Flotation DAF Process Work?
The DAF process involves three main steps:
Coagulation
Flocculation
Flotation
In the coagulation stage, a coagulant is added to the wastewater to destabilize the suspended solids and other contaminants.
In the flocculation stage, a flocculant is added to the water to create larger particles that are easier to remove.
Finally, in the flotation stage, fine bubbles of air are injected into the water, causing the flocculated particles to float to the surface of the water. The floating particles are then removed by a skimming device.
In the picture below we describe the DAF unit process design:
(A) The effluent enters the flotation tank clarification chamber
(B) Tiny air bubbles are released from diffuser nozzles into this chamber pre effluent entry
(C) These micro-bubbles attach to the solid contaminants or chemical flocculants. This causes the impurities like oil and grease to float to the surface and these are then trapped and removed using a mechanical skimmer
(D1) The skimmer consists of a series of paddles or flights in the flotation tank. These skim just below the surface of the flotation tank removing the floated sludge containing contaminants into a separate trough for recovery.
The clarified water flows to the bottom of the flotation clarifying chamber and then to an effluent control weir for separation and collection
(D2) The sinking solids are collected in the hopper and these are then removed via a manual or automated valve
(E) The final treated and clarified effluent gets discharged into a sewage drain or for further treatment, as required
Treatment Efficiency of Dissolved Air Flotation (DAF) Units
The treatment efficiency of a Dissolved Air Flotation (DAF) unit can range from 25% to 99%, depending on the specific conditions of the wastewater being treated and the design and operation of the unit.
It is important to carefully evaluate the factors that can affect treatment efficiency when selecting and operating a DAF unit for wastewater treatment.
In the following table we summarize the minimal efficiency of DAF in removing significant pollutants parameters from sewage and wastewater:Parameter
Minimal Removal Efficiency (
Applications of Dissolved Air Flotation DAF
Dissolved Air Flotation technology is widely used in various industries, including:
Desalination plants
Food and beverage production
Paper and pulp production
Mining and mineral processing
Oil and gas production
Wastewater treatment plants
Dissolved air flotation (DAF) is a commonly used pre-treatment technology in desalination plants. The DAF process removes suspended solids, organic matter, and other contaminants from seawater before it enters the reverse osmosis (RO) system for further treatment.
In the food and beverage industry, DAF is used to remove suspended solids, oils, and greases from wastewater generated during food processing.
In the paper and pulp industry, it is used to remove ink and other contaminants from wastewater.
In the mining industry, DAF is used to remove solids from wastewater generated during the extraction and processing of minerals. In the oil and gas industry, it is used to separate oil and water.
For more applications you can get in touch with our team.
Advantages and Benefits of Dissolved Air Flotation DAF
Dissolved Air Flotation (DAF) technology offers several advantages and benefits over other wastewater treatment processes, including:
High removal efficiency: DAF can remove up to 90% of suspended solids and other contaminants from wastewater.
Flexibility: DAF can be used in various applications and can treat a wide range of wastewater types.
Small footprint: DAF systems require less space than other wastewater treatment processes.
No space constraints on industrial sites tend to mean that high-intensity processes like dissolved air flotation (DAF) are favored over low-intensity alternatives such as sedimentation, though design quality is often low in industrial effluent treatment.
More recently, high-intensity DAF has been used to pre-treat seawater to protect against algal blooms prior to its desalination for drinking or industrial water. In this application, surface loadings of 50 m/h are common as the algal cells have densities close to or lower than seawater.
An additional advantage of dissolved air flotation (DAF) is that it yields sludge at 5% dry solids content as opposed to approximately 1% dry solids from a settlement tank, which means that sludge pumping and dewatering are cheaper.
A suitable mixed buffer tank is recommended to avoid problems caused by variable sludge solids content on downstream processes and for degassing DAF sludge.
Disadvantages of Dissolved Air Flotation (DAF)
Despite its many advantages, dissolved air flotation (DAF) technology also has some disadvantages, including:
Limited capacity: DAF systems may not be suitable for large-scale wastewater treatment.
Sensitivity to temperature: DAF efficiency can be affected by changes in water temperature.
Chemical requirements: DAF requires the use of chemicals such as coagulants and flocculants, which can be expensive.
Complexity: DAF systems can be complex and require skilled operators.
For high-rate dissolved air flotation (DAF), sludge treatment, and industrial applications, required recycle rates may be several hundred percent of the throughput. Air solubility in water is temperature-dependent, which may also be a factor in selecting a DAF process, since sludges and industrial effluents may be warm, as may seawater used by those countries which desalinate for drinking water treatment.
More info about DAF units can be found at: Dissolved Air Flotation - an overview
FAQs about Dissolved Air Flotation DAF
What is a dissolved air flotation (DAF) system?
A dissolved air flotation (DAF) system is a wastewater treatment process that uses tiny air bubbles to separate solid and liquid particles from water.
Why do we use dissolved air flotation (DAF)?
DAF is used for many reasons, including removing pollutants, solids, and oils from wastewater, treating industrial and municipal wastewater, and improving the quality of drinking water.
What are the two methods of air flotation?
The two methods of air flotation are dissolved air flotation (DAF) and static air flotation (SAF). DAF is the most commonly used method for wastewater treatment.
What is the difference between SAF and DAF in wastewater?
The main difference between dissolved air flotation and sedimentation is that DAF uses air bubbles to float particles to the surface, while sedimentation relies on gravity to separate solid and liquid particles.
How efficient is dissolved air flotation oil removal?
Dissolved air flotation is highly efficient at removing oil from water, with removal rates of up to 99%.
How much does a dissolved air flotation (DAF) system cost?
The cost of a dissolved air flotation (DAF) system varies depending on its capacity and complexity. Small systems can cost as little as $20,000, while larger systems can cost several million dollars.
How long does a dissolved air flotation (DAF) unit last?
Dissolved air flotation (DAF) unit can last for several years with proper maintenance and care. Regular cleaning and inspection can extend the life of such systems.
Can Dissolved Air Flotation (DAF) be used for industrial wastewater treatment?
Yes, DAF technology is widely used in various industries for the treatment of industrial wastewater. It is effective in removing suspended solids, oil, and grease from wastewater generated during industrial processes.
Is Dissolved Air Flotation (DAF) suitable for treating sewage?
Dissolved Air Flotation (DAF) units can be used for treating sewage, but it may not be the most suitable method for large-scale municipal wastewater treatment. However, it can be used as a pre-treatment step to remove solids and other contaminants before further treatment.
Is DAF used to remove BOD from highly polluted waste water?
Yes, DAF is often used to remove BOD (biochemical oxygen demand) from highly polluted wastewater, as it is effective at removing organic matter and other pollutants.
What is the difference between dissolved air flotation and sedimentation?
DAF is an alternative clarification process that uses micro air bubbles to attach and float flocculated particles and suspended solids to the water surface for removal. In contrast, sedimentation removes settled solids from the bottom.
What is the pressure for dissolved air flotation?
The pressure for dissolved air flotation can vary depending on the specific system and application, but it typically ranges from 20 to 60 psi.
What is dissolved air flotation (DF) process in short?
In a dissolved air flotation (DAF) process, wastewater is first mixed with a chemical coagulant to create larger particles that are easier to remove. Then, air is added to the water to create tiny bubbles that attach to the particles and float them to the surface, where they can be skimmed off and removed.
What is the bubble size in dissolved air flotation (DAF) units?
The bubble size range that is generated under a pressure of 4–6 atmospheres is generally reported to be 10–100 µm, with the average diameter of approximately 40 µm. The size of the air bubbles produced in DAF is most strongly affected by the pressure difference across the nozzle system.
Conclusion
Dissolved air flotation (DAF) is an effective and reliable method for wastewater treatment. It can remove suspended solids, oil, grease, and other contaminants from wastewater generated in various industrial applications.
DAF technology offers several advantages over other wastewater treatment processes, including high removal efficiency, low operating costs, flexibility, and a small footprint. However, it also has some disadvantages, such as the use of chemicals.
What Is The Best Dissolved Air Flotation (DAF) Unit for Wastewater Treatment?
The best dissolved air flotation (DAF) unit for wastewater treatment depends on various factors such as the volume and quality of the wastewater to be treated, the available space, and the budget.
Also, it is important to consider factors such as the type and quality of materials used in construction, the efficiency of the unit in removing pollutants, and the maintenance requirements before choosing a DAF unit for your wastewater treatment needs.
Ultimately, the best DAF unit for wastewater treatment will depend on the specific needs and requirements of the wastewater treatment plant.
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