Dissolved Air Flotation

DAF is usually designed by pumping compressed air bubbles to remove fine SS and FOG. The wastewater is first stored in an air-pressured closed tank and then enters the flotation tank through pressure-reduction valves. Due to a sudden reduction in pressure, air bubbles form and rise to the surface in the tank. SS and FOG adhere to the fine air bubbles and are carried upward. Both dosages of coagulant and pH are important in the removal of BOD5, COD, FOG, and SS. Other important impact factors include the solids content and air/solids ratio. Optimal operation conditions should be determined through pilot-scale experiments. Manjunath et al.16 used a DAF process to decrease waste strength by about 50%, which provides both float and subnatant with higher degradability than raw waste in a slaughterhouse. Furthermore, a DAF unit was adopted by Liu and Lien17 to treat wastewater from a large-scale bakery. The wastewater was preconditioned by alum and ferric chloride. 48.6% of COD and 69.8% of SS were removed in 10 min at a pressure of 4 kg/cm2 and pH 6.0. de Nardi et al.18 found that the removal efficiencies of SS and O&G achieved 74% and 99%, respectively, with 24 mg Al3+/L polyaluminum chloride (PAC) and 1.5 mg/L anionic polymer as well as under a pressurization of 40% recycled DAF-effluent at 450 kPa.

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