Recovery Means
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RECOVERY BY SKIMMING
Only floating substances can be collected using this technique.
Recovery aims to collect pollutants spilt at sea in the most optimal way possible before they reach the shoreline. Recovery systems are primarily designed to recover oil slicks. Recovery at sea of other pollutants must be examined on a case by case basis.
The three main criteria which characterise all skimmers are as follows:
- Their pollution contact capacity, directly related to the size and shape of the skimmer head. This corresponds to the surface swept per time unit.
- Their recovery rate, which will depend on the pollutant transfer system to a storage tank. This corresponds to the quantity of pollutant recovered in relation to the quantity with which the skimmer comes into contact.
- Their selectivity, the percentage of pollutant present in the pollutant-water mixture collected.
Skimmers are selected according to these three criteria, in addition to the extent of the pollution, the degree of containment and the storage means available.
If the storage means are limited, the skimmer’s selectivity must be a priority. In the case of a large spill, an increase in the contact capacity generally leads to a decrease in the recovery rate or the selectivity.
In the case of oil, skimmers can be divided into two distinct families:
Mechanical skimmers utilise the difference in density between the pollutant and the seawater.
Oleophilic skimmers draw upon the affinity of oil for certain materials to extract it from the water surface.
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| Recovery means at sea can be divided into two main categories |
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| Mechanical skimmers |
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These skimmers combine a skimming mechanism at the water surface with a transfer pumping method towards a storage tank. Floaters ensure that the skimmer stays at the surface and control the skimming levels.
These skimmers provide an excellent recovery rate (500 m3/h for the most efficient) but moderate selectivity (40 to 50% water recovered with the pollutant). |
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Oleophilic skimmers
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Used exclusively on oil, one or several oleophilic elements (presenting an affinity for fats), maintained at the water surface, trap the oil before depositing it in a container from which it is pumped to a storage tank.
These skimmers provide a moderate recovery rate (40 m3/h for the most efficient) but excellent selectivity (less than 10% water recovered with the oil). |
Oleophilic elements can be:
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RECOVERY BY TRAWLING
When recovery by skimming and pumping becomes inefficient due to the pollutant’s high viscosity, surface trawling may be carried out. Trawlnets developed from fishing equipment involve a very fine mesh net and a buoyancy system to keep it at the surface.
When recovery operations begin at sea, a collection chain for the recovered pollutant and oiled materials must be set up on land. |
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| Trawlnets are a means of dynamic recovery of solid, extremely viscous and heavily emulsified products, as well as solid waste, at sea. Specifically developed from fishing gear (pelagic trawl), they combine two funnel-shaped containment cones and a removable bag known as a “cod-end”. Trawlnets are generally deployed by two boats using a technique known as “pair trawling”. |
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Last Updated on Monday, 02 February 2009 20:59 |
