products to be separated out

Grease removal and oil removal operations consist in separating out products having a density that is slightly lower than that of water. This step is carried out using natural or assisted flotation (see floatability and rising velocity), in a liquid container having a sufficient capacity.

Grease is suspended matter (on condition that a sufficiently low temperature allows it to solidify) that comes mainly from animals (or plants) and that is present in UWW and in some IWW (agri-food) and, in small amounts, in storm water tanks, lagoons, ponds etc. The product takes the form of free particles or rather bound up with various suspended solids (that we have to attempt to detach for flotation purposes). In effect, grease itself is always recovered with some floating matter such as miscellaneous plant (fibres) or animal (abattoir) waste, elastomers and plastics etc.

Therefore, grease removal consists in a compromise between maximum grease retention and minimum fermentable bottom sludge deposits. Due to sampling and analysis difficulties, it is difficult to define the efficiency of this system.

We tend to term «oils» liquid products that will not mix with water; these products range from vegetable oils, mineral oils, to light hydrocarbons; the term oil removal is normally used in connection with the removal of oils present in significant quantities in IWW, especially in the oil industries (but not usually found in UWW because their discharge into the sewers is prohibited).