Dispersants are surface active agents that accelerate the natural dispersion of oil by natural agitation. Dispersants facilitate the dissociation/dissemination of oil slicks on the surface into a multitude of droplets spread throughout the water column (a few metres to a few dozen metres deep). The purpose of dispersants is two-fold: first, the dispersion of surface slicks in the water mass means that they are no longer affected by the wind, which is important when the wind is liable to push slicks towards ecologically sensitive sites. Second, fragmentation of the slick into a multitude of droplets facilitates the breakdown of oil by bacteria naturally present in water.
However, the use of dispersants is not a cure all solution. They are relatively inefficient on heavy oils (viscous or weathered). The decision to use dispersants in a given situation must be made rapidly before the pollutant has time to weather: dispersion is only an option during the first few hours, or at most the first few days. Dispersants are generally applied by spraying onto oil slicks.
Dispersants remain stained with a negative image. They have been wrongly accused of being more toxic than oil and of causing the oil to sink to the seafloor where it is said to form a lethal layer. However modern, recognised, concentrate dispersants are generally less toxic than the dispersed oil. Dispersing oil leads to a local, temporary increase in its toxicity, until the dispersed oil is diluted in a vast volume of oil, causing it to become harmless. This effect implies a certain limitation in terms of the use of dispersion near the shoreline and in sensitive areas and/or when the dilution potential is low.
Dispersant use planning
The use of dispersants should have been planned when drafting contingency plans. Most plans describe the anticipated spraying means (by ship or aircraft – plane or helicopter), plan for appropriately distributed stockpiles, and distinguish the areas where dispersion may be safely conducted (free use) from those where its use is restricted or even prohibited.
Dispersants are used in the natural environment. They must therefore undergo tests for their efficiency and/or toxicity and sometimes their biodegradability to ensure that they are efficient and/or harmless. Approval procedures have been introduced in certain countries.
These procedures are used to draw up lists of authorised or recommended products. In France, the test methods are standardised by AFNOR (Association Française de Normalisation). These methods are applicable to marine dispersants. Cedre has been in charge of approving dispersants since 1978. It carries out this responsibility within the framework of a working group, led by the French Ministry of Ecology, composed of representatives of ministries and research institutes. This working group establishes the criteria and reference values applicable to the products tested. The test results for a given product are valid for a 5 year period.
Different types of dispersants
Currently, two categories of dispersants exist:
1. Conventional dispersants (second generation) are older products with a low surface active agent content in petroleum solvents that are not miscible with water. They are applied neat. They are now very rarely used. They are gradually being replaced by concentrates.
2. Concentrate dispersants (third generation) are more recent products with a higher surface active agent content in solvents that are miscible with water. In France, the Navy now only uses this type of dispersants. They can be applied neat or pre-diluted in seawater as they are soluble or easily emulsify. It is nevertheless preferable to use thel neat, as they are more efficient, in particular on weathered or viscous oil, or oil that is difficult to disperse.
Furthermore, marine dispersants are distinguished from freshwater dispersants
In France, marine dispersants are currently subject to a test procedure which tests their efficiency (method NF.T.90-345), their toxicity (method NF.T.90-349) and their biodegradability (method NF.T.90-346).
Testing of a new product begins with the efficiency test. The results of this test determine whether or not the other tests will be carried out. This procedure was set up in 1978 and revised in 1988. Since 2000, test results are valid for 5 years.
Fresh water dispersants
It seemed necessary to develop a specific test procedure for fresh water dispersants, as their efficiency often depends on the salinity of the water. Even the best marine dispersants give relatively poor results in fresh water. The sensitivity of fresh water organisms must also be taken into account. In France, a specific test procedure was designed, including, as with marine dispersants, an efficiency test, a toxicity test and a biodegradability test.
For the efficiency test, the dilution test method (NF.T.90-345) was chosen, although test conditions were adapted (salinity of the water, water movement and type of oil). For the biodegradability test, procedures are identical to those for marine dispersants. The toxicity test is carried out on a fresh water species, the zebra fish.