Dissolved Air Flotación (D.A.F.)
Equipment commonly known as DAF (Dissolved Air Flotation) are highly used in the industry for their practicality and high efficiency for the removal of grease and oils that could or not be mixed with the effluent. They are also frecuently used for floating secondary sludge instead of settlers, or for specific processes like MBBR (Moving Bed BioReactor) where they are used as secondary flotation device.
Coalescers are also used for floating free oils, but they are not frecuently used in the industry for risk of clogging, or saturation due to high percentages of FOG (Fat, Oil and Grease).
Flotation equipment can be made of RFP, carbon steel or stainless steel (more common). Likewise, its shape can be rectangular o circular according to flow or available space. Removal of floating material is by surface scrapper, and settled solids removal by hopper or screw conveyor (optional).
Equipment can be requested with previous chemical dosage, to increase up to 60% BOD removal and up to 95% oil and grease removal. The DAF is delivered with a control panel, instruments and start-up service.
DAF equipment achieves oil and grease separation by introducing fine air bubbles in the liquid phase. The bubbles attach to the particles and the ascensional force experimented by the particle-bubble group makes it go up to the surface.
The working principle is the mixing of pressurized air and water for a later depressurisation in the effluent to be treated. This depressurisation generates very fine bubbles that attach to particles of grease and oils increasing their speed to the surface where a scrapper removes them of the equipment.
Pressurisation is commonly made over recirculating flow (20-50 % of design inlet flow rate) and it can be made by tank, phase pump or both. The use of either system will be decided upon each case according to industrial application.
Design considerations to take into account are air/solids ratio; recycle pressure and flow, superficial rate and hydraulic residence time.
Chemical dosage commonly precedes flotation units to reach a higher performance when removing grease, oils, BOD, COD and phosphorus. However, it increases operating costs, and the solids removed shall not qualify for reuse as animal feed or other uses.
Main components of the system are: