Dispersion

Dispersants are agents which accelerate natural dispersion by wave action, facilitating the break up of slicks on the surface into a multitude of smaller droplets dispersed throughout the top few metres of the water column. This facilitates the breakdown of the hydrocarbons by bacteria which are naturally present in the water and reduces the local toxic effect.

However, the use of dispersants is limited by technical factors. They must be used in precise proportions and conditions. They remain inefficient on viscous or weathered oil. The decision to use dispersants in a particular situation cannot be put on hold, as dispersion is only an option in the first few hours or at the most the first few days. The decision to disperse the oil should be anticipated during the development of the contingency plan, depending on the characteristics of the zone. Most plans distinguish zones of free use of dispersants, zones of use in certain conditions and zones where the use of dispersants is prohibited.

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Dispersant toxicity

Dispersants remain marred by a negative image. They have been accused of being more toxic than oil and of causing oil to sink to the seafloor, forming a deadly carpet of toxins.

These accusations are unfounded. Dispersants fragment oil into a multitude of droplets which spread out in the water mass, they do not drop to the bottom. Dispersing hydrocarbons causes a temporary, local increase in their toxicity, while the dispersed oil spreads and dilutes through a vast volume of water to then become harmless.

This effect implies a certain limitation concerning the use of dispersants near the shoreline and sensitive areas and/or when dilution conditions are low. However, recognised, modern, concentrated dispersants generally prove less toxic than dispersed hydrocarbons.

Testing dispersants


Testing sheet sorbents

 

In certain countries, dispersants undergo tests of their efficiency, toxicity and biodegradability.

In France, these tests have been carried out by Cedre since 1978. The testing of a new product begins with the efficiency test, which determines whether or not the other tests are then carried out.

Spreading dispersants by boat and by plane

Testing pollution response products

Oil spill response often involves the use of chemicals which facilitate the breakdown or recovery of the pollutant. There are several families of response products: dispersants, surface washing agents, sorbents, gelling agents/solidifiers, emulsion breakers, bioremediation agents, film-forming agents…

These agents are used in the natural environment. It is therefore important that they undergo testing to ensure that they are harmless. Procedures which aim to approve or accredit these products have been set up in certain countries for some types of products. Lists of authorised or recommended products can then be drawn up based on the test results.

In France, the majority of test methods have been standardised by AFNOR (the French standardisation association), in particular the test procedures for marine dispersants, sorbents and rock washing agents. Cedre is responsible for testing these types of products, with assistance from a working group led by the French Ministry of Ecology and Sustainable Development and Planning, made up of representatives from ministries and research organisations. This group establishes the criteria and the acceptance levels which apply to the products tested.

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Spreading dispersants by boat and by plane