Dissolved Air flotation (DAF)

Dissolved Air flotation (DAF) has been used in the wastewater field for the removal of oil and grease from industrial waste; for the thickening of waste-activated sludge prior to digestion or further processing; for thickening of backwash water from depth filters, usually in large installations ; for removing suspended particulate matter that is difficult to remove by conventional flocculation and sedimentation  and for the removal of algae from stabilization and storage pond effluents prior to filtration .In water treatment applications when compared to gravity sedimentation ,DAF is a more efficient process for separating low density floc particles. Considerable research has been done in recent years in adapting high – rate DAF for drinking water applications. Much of the same technology can be used for reclaimed water.

In DAF, air is dissolved under pressure in the water to be treated, according to Henrys 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 process 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 materials, and bottom scrapers to remove settled solids..Bafflues are provided to retain the float materials 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.