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Flotation is a unit operation, which removes solid or liquid particles from a liquid (such as oil droplets or suspended solids from OMWW). Adding a gas (usually air) to the system facilitates separation. Rising gas bubbles either adhere to or are trapped in the particle structure of the suspended solid, thereby decreasing its specific gravity relative to liquid phase and affecting separation of the suspended particles.

When OMWW is stored for some time (a couple of days) a crust is formed on the surface. If samples of this crust are studied under a microscope, small drops of oil are observed. The oil content of samples taken from the surface with a crust and from the bottom with sludge of a 1 l OMWW having been stored in a basin was found to be 0.2 and 0.03%, respectively. The results show that in order to obtain the greatest quantity of oil from OMWW flotation is preferred over sedimentation (WO9211206, 1992).

Methods of flotation include dispersed- and dissolved-gas flotation. Dispersed-gas flotation, commonly referred to as froth flotation, is not widely used in waste-water treatment. Experiments on formation of solids by flotation were done by Escolano Bueno A. (1975). Curi K. et al. (1980) used gravity separation and dispersed air flotation to evaluate the feasibility of oil recovery from OMWW without much success.

The dissolved-air flotation (DAF) method is also referred to as pressure flotation in which air dissolved in water under pressure is released in the form of small air bubbles by discharge to the atmospheric pressure.

DAF has been evaluated, as potential pretreatment technique, for the removal of suspended solids of OMWW (Mitrakas M. et al., 1996). The pilot unit of Fig. 5.2 was used to investigate the influence of retention time, operating pressure, and chemical addition on the method's efficiency to reduce organic loading and total solids of OMWW as well as the efficiency of flotation to separate oils as foams (Mitrakas M. et al., 1996).

vessel V= 125 L

Fig. 5.2. Schematic diagram of the pilot DAF unit (Mitrakas M. et al., 1996).

vessel V= 125 L

Fig. 5.2. Schematic diagram of the pilot DAF unit (Mitrakas M. et al., 1996).

The high content of OMWW in suspended solids made the DAF technique quite inefficient, since the ratio air to solids was out of the typical working range of 0.005-0.06. DAF can remove COD as well as oils but not as efficiently as centrifugation. Thus, acidified OMWW gave maximum COD reduction of 30% and oil recovery of 30%. DAF performance with raw OMWW was one-half to one-third of these values. An additional disadvantage of dissolved air flotation is that recovered oils (or hydrolyzed oils) should be extracted from the foam of the DAF unit. DAF application in practice was not feasible, despite the relatively high, almost 30%, oil recovery by this process.

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