DISSOLVED AIR FLOTATION

Process Description

In DAF, air is dissolved under pressure in the water to be treated, according to Henry’s law of dissolution. After pressurization the pressure is released to standard conditions, thus creating millions of microbubbles. The bubbles surround slow-settling particles and float them to the surface for removal. As the float layer increases, it begins to slightly dewater and thicken.

Types of DAF

The two basic types of DAF processes used in water reclamation applications involve pressurization of the recycle-flow or pressurization of the full-flow. As shown, the principal elements of a DAF system are the pressurization system and the dissolved air flotation tank.
The pressurization system consists of a pressurization pump, air compressor, air pressurization tank, and pressure release valve. The flotation tank can be square, rectangular, or circular and is equipped with surface skimmers for the removal of the float material, and bottom scrapers to remove settled solids. Baffles are provided to retain the float material while the clarified underflow passes over an effluent weir.
The selection of the type of process to be used depends on the characteristics of the feed stream, size of the facility, and the results of pilot flotation studies. It should be noted that there other types of DAF processes such as the stacked DAF with integral media filter, used more commonly in the water treatment field.

Recycle-Flow versus Full-Flow Saturation

Recycle-flow DAF is most appropriate for systems or applications requiring the removal of nearly buoyant or neutrally buoyant floc particles. In this process, the total influent flows either initially through the flocculation tank or directly to the flotation tank if separate flocculation is not required.
A portion of the clarified effluent (ranging from 5 to 20 percent) is recycled and delivered to an air saturation tank. The saturation tank may contain a packing material that breaks down the incoming water into small droplets. The upper atmosphere of the tank contains a pressurized volume of air that is dissolved readily into the water.
The pressurized recycle water is introduced to the flotation tank through a pressure release device and is mixed with the flocculated water. In the pressure release device, the pressure is reduced to atmospheric pressure, releasing the air in the form of a blanket of fine bubbles (30 to 100 μm in diameter). Full-flow DAF is most appropriate for small systems. The process is similar to recycle-flow DAF except that chemicals are injected into the suction of the feed pump and the entire influent flow is pressurized.