Oil and grease are common industrial contaminants. Several techniques are currently used to treat oily wastewaters, including (1) oil/water separators, (2) dissolved air flotation, and (3) supercritical extraction. Oil/water separators rely upon gravity separation to separate
oily materials (density) from water. This allows floating matter to rise and remain on the surface until it can be skimmed off or removed, while the liquid flows continuously through deep outlets or under partitions, curtain walls, or deep scum boards. Materials collected on the surface of
skimming tanks include oil, grease, pieces of cork and wood, and vegetable debris and fruit skins in households and in industry. Dissolved air flotation relies upon interfacial phenomena to attract/coalesce oily materials from water. Use of surfactants facilitates the removal of oily components
from solution. This technique accommodates high surface-loading rates, and can achieve high removals of grease and floatable material. Supercritical extraction involves fluid extraction performed at supercritical thermodynamic conditions. This is an attractive alternative to solvent extraction,
due to better mass transfer properties of supercritical fluids, shorter extraction times, and ease of extract separation. Sonication involves transmittal of sound waves through a fluid in a series of alternating cavitation cycles. Compression cycles exert a positive pressure on the liquid,
pushing molecules together, while expansion cycles exert a negative pressure, pulling molecules away from each other. In addition to microscale effects (cavitation), it is known that sonication also effects macroscopic mixing. During cavitation, the liquid structure is disrupted, forming tiny
microbubbles. These microbubbles grow to a critical size through a process known as rectified diffusion and finally implode, releasing a large amount of energy. Temperatures on the order of 5,000°K and pressures up to 500 to 1000 atmospheres have been observed in microbubble implosions,
while the bulk solution stays near ambient conditions. Preliminary results indicate that the combination of sonication and air flotation may result in improved removal of oily materials from industrial wastewaters. The results indicate that the removal of benzene, toluene, and ethylbenzene
is enhanced through the combination of sonication and air flotation over the case of mixing alone, sonication alone, and air flotation alone.
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