Industrial Equipment DAF Dissolved

Clarifying Treatment:Dissolved Air Flotation Provides Alternative for Treating Raw Water with Light Particles
Dissolved air flotation is an alternative clarification process ideal for treating raw water with light particles,such as algae or color-causing organics,and at low temperatures when sedimentation is not as effective.

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(see Fig.1).In contrast,sedimentation removes settled solids from the bottom.
Figure1.Generic DAF Process Flow Diagram

Before the flocculated water enters the clarification chamber,millions of tiny air bubbles are released from the diffuser nozzles to the water,which attach and float the floc particles to the water surface.The air bubbles are generated in a pumped recycle stream by an air compressor and are dissolved in water through a packed-tower-type of saturator.The floated sludge is removed periodically to a desludging trough,and the clarified water flows to the bottom of the clarifying chamber and then to an effluent control weir for collection.The air saturator is used to dissolve air in the water and then generate bubbles to float the floc particles.
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 0.5NTU.It is not as sensitive to temperature,especially cold temperatures as is common in settling,and the startup time is very short,approximately 30minutes.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 20gpm/ft2.
With mechanical sludge removal,the DAF sludge can be quite thick(2to 5percent solids),which eliminates the need for thickening before dewatering.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 100NTU,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.

Conclusion
Dissolved air flotation is an alternative clarification process ideal for treating raw water with light particles,such as algae or color-causing organics,and at low temperatures when sedimentation is not as effective.DAF has become a mainstream clarification process for drinking water treatment plants,especially where algae is prevalent,as well as a popular pretreatment process for seawater desalination systems to combat red tide or algae bloom occurrences throughout the world.

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