Dissolved Air Flotation Thickener

Dissolved air flotation is the most common sludge flotation thickening method. In this method, a pressurized mixture of air / water is injected at the bottom of flotation tank. Quickly after the mixture is depressurized, tiny air bubbles are released from the mixture and float to the surface of water. As the bubbles pass through the water surface, they collide with the sludge particles and carry them along to the water surface. When the sludge particles reach the surface of water, a skimmer is utilized for removal.


The dissolved air flotation method works best for sludges with specific gravity close to 1.0, as the heavier particles are difficult to carry to the surface. Due to this, the method has been applied to waste sludges from suspended growth systems, like activated sludge.

The dissolved air flotation thickening refers to the method of liquids-solids separation caused by the introduction of fine gas bubbles to the liquid phase. The bubbles bind to the solids, and the resulting buoyancy of the combined solid-gas matrix makes the matrix to rise to the liquid surface where it is collected by a skimming mechanism. The method is generally used for waste activated sludge, aerobically digested solids, and contact stabilized without primary setting. The method is generally used for primary or trickling filter sludges due to the fact that the gravity settling for these solids is more economical than dissolved air flotation.

Design
The main components of a dissolved air flotation thickener are the pressurization system and the dissolved air flotation tank. The pressurization system is designed with a recycle pressurization pump, an air saturation tank, an air compressor, and a pressure release valve. The tank of a dissolved air flotation thickener is designed with both the bottom solid removal and surface skimmer mechanism. The mechanism removes the non-floatable heavier solids, which settle to the bottom of tank.

The tanks of a dissolved air flotation thickener are designed with an overflow weir. Clarified effluent travels under an end baffle (rectangular units) or peripheral baffle (circular units) and over the weir to an effluent number. The weir regulates the level of liquid within the tank with respect to the float collection box and helps in controlling the capacity and performance of unit.

Advantages
Some of the important advantages of a modern day dissolved air flotation thickener are–

• The use of standard components and proven hydraulic design ensures economical construction and efficient performance.
  
  
• Design flexibility: Variety of tank sizes, steel or concrete, new or existing installations, with or without chemical addition.
  
  
• Controlled results: The correct pressurized flow for each application ensures controlled results for each and every task.