Dissolved air floatation (DAF)
In Dissolved Air Floatation, the raw water particles are flocculated and separated out of the water by floating them to the surface, rather than settling them to the bottom of the basin. The process introduces air bubbles through diffusers at the bottom of the contactor to float the floc. The air bubbles are produced by recycling a portion of the effluent through a tank where air is introduced and the water saturated, then reduced to ambient pressure. This creates the pressurized flow. The floated sludge is removed from the top of the basin by mechanical or hydraulic means. The clarified water is removed by laterals from the bottom of the basin.
Dissolved Air Floatation is particularly effective in removing low-density solids such as turbidity, color, algae, Giardia/Crypto, or precipitated organic and metals. These are all contaminants that do not settle well but tend to float or hover in the water column. It can also remove some taste and odor compounds that can be readily stripped by the dissolved air in the water. It can handle rapid changes in temperature and water quality provided that the coagulation chemistry is optimized.
Dissolved Air Floatation is less costly than conventional sedimentation basins because the flocculation section is half the size, and the surface loading of the solids separation part of the process can be up to 8 gpm/ft2. Since the particle removal is by floatation, rather than sedimentation, both the flocculation and clarification detention times are less than conventional treatment. In addition, the unit will produce a more consistent effluent quality for filter loading, thus maximizing the filter run times between backwashes. It will also produce sludge solids in the float of 2 to 5% which reduces the cost of further handling whether dewatering or hauling away the sludge solids.
The process requires more energy input than conventional treatment as well as more mechanical equipment to run the system.