Dissolved oily wastewater

Flotation system this process is functioned by increased differences in specific gravity between the oil and water by blowing fine air bubbles through the oily wastewater. The most common floatation system is dissolved air flotation (DAE) process. 

Other plant-scale applications to pollution control include the flotation of suspended sewage particles by depressurizing so as to release dissolved air [Jenkins, Scherfig, and Eckhoff, Applications of Adsorptive Bubble Separation Techniques to Wastewater Treatment, in Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972, chap. 14 and Richter, Internat. Chem. Eng, 16,614 (1976)]. Dissolved-air flotation is also employed in treating waste-water from pulp and paper mills [Coertze, Prog. Water TechnoL, 10, 449(1978) and Severeid, TAPPl 62(2), 61, 1979]. In addition, there is the flotation, with electrolytically released bubbles [Chambers and Cottrell, Chem. Eng, 83(16), 95 (1976)], of oily iron dust [Ellwood, Chem. Eng, 75(16), 82 (1968)] and of a variety of wastes from surface-treatment processes at the maintenance and overhaul base of an airline [Roth and Ferguson, Desalination, 23, 49 (1977)]. 

In addition, oily sludge from a wastewater treatment facility that results from treating sour wastewaters may be a hazardous waste (unless recycled in the refining process). These include API separator sludge, primary treatment sludge, sludge from various gravitational separation units, and float from dissolved air flotation units. 

Dissolved air flotation (DAF) is a process commonly used in refineries to enhance oil and suspended solids from gravity-separator effluent. In some refineries it is used as a secondary clarifier for activated sludge systems and as a sludge thickener. The process involves pressurizing the influent or recycled wastewater at 3-5 atm (40-70 psig) then releasing the pressure, which creates minute bubbles that float the suspended and oily particulates to the surface. The float solids are removed by a mechanical surface collector. 

A schematic of conventional DAF unit is shown in Fig. 9. In this unit a recycle pump combined with a saturation vessel and air compressor are used to dissolve air into the water. The dissolved air comes out of solution, producing microscopic bubbles. These bubbles attach to the solids and float them to the surface where they are mechanically skimmed and removed from the tank. This application can be modified and used for various oily industrial wastewaters (51). 

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