Dissolved Air Floatation

DAF as an effective process for pretreatment

Dissolved Air Flotation(DAF) is a water treatment process which removes suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank or basin. The released air forms tiny bubbles, which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device.  

Some of the main advantages of DAF include better performance for removing light particles such as algae, which are difficult to settle. DAF can also usually achieve lower effluent turbidity than settling, typically less than 0.5 NTU. It is not as sensitive to temperature, especially cold temperatures as is common in settling, and the startup time is very short, approximately 30 minutes. Further, DAF does not need to generate heavy floc for settling, so lower coagulant dosage and shorter flocculation time can be used. The process can also operate at much higher surface loading rates (SLR) than sedimentation, especially in the high-rate DAF processes, which can be up to 20 gpm/ft2.

Other Advantages of DAF are consist of:

• Small footprint, compact and robust.

• Flexible, can handle a reasonable variation in influent water quality.

• Optimum use of coagulants and flocculants, low operating cost.

• Easy to operate, service and clean.

• Can deal with red tide, algae bloom, oil spills, turbidity removal efficiency 95%.

Because of air attachment, polymer is not required for stable performance, as opposed to high rate settling processes that must use polymer to attach particles to ballast. Using polymer is a major concern if the clarification process is used for membrane filtration pretreatment. However, DAF is not suited for raw water with high-density solids or turbidities higher than 100 NTU, as the pumped recycle percentage required is higher, making the process more energy intensive and less economical than competing technologies. In general, DAF uses more energy than sedimentation because of the recycle water pumping and air compressing requirements, and it usually needs protection from freezing and raining to prevent the floated solids from settling.

Color, organics, iron and/or manganese compounds are contaminants that will create a need for some sort of pretreatment process. Algae and excess particles can lead to higher capital cost due to designing systems at lower flux rates, and higher operational cost as a result of more frequent backwashes and chemical cleanings. The performance of MF/UF membranes can be improved when a pretreatment system is used to reduce the potential contaminants that can foul the membrane.

Considering the source water contaminants which foul membranes, the high rate dissolved air flotation (DAF) clarification process provides one of the best membrane pretreatment alternatives. Color, organics, soluble metals, or colloidal solids that are removed by adding inorganic chemicals to form aluminum or iron hydroxide flocs create low density floc and require excess chemicals and mixing time to create particles large enough to settle. Normally, solids with hundreds of microns in size are required to settle, while particles of tens of microns in size can be floated. It is also recognized and well documented that algae in the source water that will foul membranes is best removed by taking advantage of algae buoyancy and flotation technology.

The floated sludge is removed from the top of the basin by mechanical or hydraulic means, while laterals from the bottom of the basin collect the clarified water. Mechanical desludging will provide a floated sludge with 2% to 5% solids content resulting in significantly less sludge volume to process in the solids handling portion of the treatment plant.

The benefits of providing high rate dissolved air flotation for pretreatment are:

1.) To increase the membrane flux rate,

2.) To reduce backwashing and chemical cleaning of the membranes, and

3.) To prolong the membrane life and increase the time between membrane replacements.