Natural flotation is generally used for all preliminary oil separation processes. This flotation may be preceded by a coalescing (in which the microdroplets adhere to one another) process to achieve a minimum size promoting separation.

Natural flotation may take place as a result of a release of gas from fermentation. This is the case with the scum in digesters that can contain 20 to 40 % of SS even though its density is only 0.7 or 0.8 kg.l^-1.

The natural flotation is improved by blowing air bubbles into the liquid mass. This Aided flotation is particularly used with the separation of greases (solid particles) that are dispersed in a turbid liquid (sewage).

For more thorough oil separation, air is diffused in fine bubble (0.5 to 1mm) by an underwater mechanical aerator. The air lift effect of the air bubbles carries the particles to the surface, in order to aid their accumulation.

Resource: Water treatment Handbook Degrémont