Mechanical recovery is the most commonly used oil spill response technique and it’s accomplished by devices called «skimmer» which recover hydrocarbons and fats floating on the water surface.
Several types of skimmer exist among which weir skimmers and oleophilic skimmers are widespread.
The first ones, thanks to an onboard pump, create an inlet flow through a tared floating ring that floats on the water but not on lighter fluids, as most hydrocarbons are; so the incoming fluid is, in theory, the spilled oil.
The second ones make a more accurate recovery thanks to the adhesion of oil to a rotating surface, to which water does not adhere. Different contact surfaces may be used such as brushes, disc, drum, mop
In all cases, after recovery, oil is pumped into temporary tanks while cleaning operations are developed.
Despite manufacturers’ efforts to improve the effectiveness of their equipments, the reality is that Recovery Efficiency (RE = Recovery Efficiency,%) of skimmers is very lower than 100%. (The recovery efficiency is defined as the ratio between the volume of oil recovered and the volume of total fluid recovered by the skimmer, expressed as a percentage)
There are different reasons that justify variable and imperfect performance of skimmers:
- Type: Weir skimmers RE is lower than on oleophilic skimmers. Among the oleophilic skimmers considerable differences exist depending on the type.
- Construction materials: In the case of oleophilic skimmers different studies show strong differences on skimmer performance depending on the material used in the construction of the contact material (polypropylene, polyethylene, aluminum, pvc, stainless steel, …).
- Operational Speed: Each type of skimmer presents an ideal operational speed (depending on the viscosity of the fluid recovered) beyond which water content in the recovered substance increase, thereby decreasing by the RE.
- Viscosity of spilled oil at the moment of the cleaning operations
- Sea state and weather: The worse the state of the sea and the higher the waves, the lower skimmer RE.
In short, if existing skimmer Recovery Efficiency (RE) in ideal conditions ranges between 50% and 85-90%, we can say that in real conditions it will hardly exceed 50%. Ideal conditions can only be given in cases of calm sea and in the presence of a high concentration of hydrocarbons, as well as in industrial facilities.
The consequence is that the solution to the spill causes a further problem: the presence of equal parts of water in the recovered substance and the need to purify it so that water can be returned to sea with acceptable / legal hydrocarbon concentrations.