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.
DAF technology is a reliable and effective method for wastewater and sewage treatment. The DAF process for industrial wastewater treatment uses micro-bubbles to remove impurities and solids. It is ideal for removing light impurities and solids that tend to float and suspend in liquid, in contrast to water treatments focusing on sedimentation to remove settled solids. However, most DAF systems also account for some sedimentation and settled sludge removal.
DAF systems separate solids, oils, greases, and other impurities from water, enabling the clean water to be reused or discharged. DAF is used for various purposes, including:
- Meeting discharge regulations
- TSS (Total Suspended Solids) removal
- Water recovery and reuse
- Pretreatment to reduce loading on downstream biological treatment systems
- COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand) removal
- Polishing of biologically treated effluent
- Thickening of slurry
- Pretreatment for desalination plants utilising reverse osmosis
- In this article, we will explore DAF technology, highlighting its popularity and effectiveness in wastewater and sewage treatment.
How Does Dissolved Air Flotation Work?
The DAF process involves three main steps: coagulation, flocculation, and flotation.
- Coagulation: In this stage, a coagulant is added to the wastewater to destabilise the suspended solids and other contaminants.
- Flocculation: Next, a flocculant is added to the water to create larger particles, making them easier to remove.
- Flotation: Finally, fine bubbles of air are injected into the water. These bubbles attach to the flocculated particles, causing them to float to the surface. The floating particles are then removed by a skimming device.
DAF uses micro air bubbles (30-50 microns) that attach to impurities and flocculated particles, floating them to the water’s surface. A skimming system then removes the sludge to a desludging trough, and clean water is recovered.
The clean water can be reused for processes or discharged, with some of it recycled back into the DAF process.
Clarification Steps for a DAF System
First, clean water must be accessible for a DAF system to start up. Clean water is essential because the system pressurises the water and mixes it with pressurised air. Under pressure, the air dissolves in the water, and when depressurised, micro-bubbles form, which is necessary for the cleaning process.
Begin by filling the DAF system with clean water. Filling the unit with wastewater could lead to prolonged wastewater discharge and potential damage to the recirculation pump and air saturation system.
With clean water in the system, a recirculation pump pulls some of the clarified water into a pressurisation-saturation system. The water is then pressurised by a pump and mixed with pressurised air, causing the air to dissolve in the water under pressure.
The saturated water flows under pressure to the clarification tank, where the pressure is released, forming micro-bubbles. These micro-bubbles attach to the impurities in the incoming wastewater stream, lifting them to the surface and creating a floating sludge blanket in the clarifier.
A skimming system then gently removes the sludge from the top of the water, completing the clarification process.
Applications of Dissolved Air Flotation (DAF)
Dissolved Air Flotation technology is widely used across various industries, including:
- Desalination plants
- Food and beverage production
- Paper and pulp production
- Mining and mineral processing
- Oil and gas production
- Wastewater treatment plants
In desalination plants, DAF is commonly used as a pre-treatment technology. It 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 employed 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 separate solids from wastewater generated during mineral extraction and processing. In the oil and gas industry, it is used to separate oil and water.
Advantages and Benefits of Dissolved Air Flotation (DAF)
Dissolved Air Flotation (DAF) technology offers several advantages over other wastewater treatment processes:
- High Removal Efficiency: DAF can remove up to 90% of suspended solids and other contaminants from wastewater.
- Flexibility: DAF is suitable for various applications and can treat a wide range of wastewater types.
- Small Footprint: DAF systems require less space than other wastewater treatment processes.
- Industrial Site Suitability: High-intensity processes like DAF are favoured over low-intensity alternatives such as sedimentation on industrial sites due to the lack of space constraints, though design quality can be low in industrial effluent treatment.
High-intensity DAF has been increasingly used to pre-treat seawater to protect against algal blooms before 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.
Another advantage of DAF is its ability to produce sludge with 5% dry solids content, compared to approximately 1% from a settlement tank, resulting in cheaper sludge pumping and dewatering costs. 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, DAF technology also has some disadvantages:
- Limited Capacity: DAF systems may not be suitable for large-scale wastewater treatment.
- Temperature Sensitivity: 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 DAF, sludge treatment, and industrial applications, required recycle rates may be several hundred per cent of the throughput. Air solubility in water is temperature-dependent, which may also influence the selection of a DAF process, especially since sludges and industrial effluents may be warm, as may seawater used by countries that desalinate for drinking water treatment.