Biodegradation of oil refers to the transformation of carbonated chains by the action of micro-organisms. Over time, the hydrocarbons can be broken down into lighter compounds to the point of complete conversion, i.e. into carbonic gas and water. This is known as mineralisation, which is the ideal purification process, as all the organic carbon is transformed into mineral carbon. However part of the carbon is used by the micro-organisms to mulitply and form biomass, which also requires nitrogen and phosphorus. To give an indication, complete biodegradation of 1 kg of oil consumes 2.6 kg of oxygen and 70 g of nitrogen to produce 1.6 kg of carbonic gas, 1 kg of water and 1 kg of biomass.
The purifying capacity of micro-organisms is limited by a number of factors, as outlined below.
- The nature of the hydrocarbons:
Linear paraffins and light aromatics are easily broken down, however branched hydrocarbons, heavy poly aromatics, resins and asphaltenes are much more difficult to break down.
- The availability of nutrients:
Complete degradation of 1 kg of hydrocarbon consumes the oxygen dissolved in 300 m³ of water and the nitrogen from nitrates contained in 1000 m³ of water, with reference to the average concentration of seawater.
Biodegradation of oil is therefore a slow process, taking several weeks to several months or even years in the least favourable conditions (little oxygenated sediments), and often remains incomplete, in particular with heavy oil.
Treatment implemented to promote, accelerate or provoke the biodegradation of oil is also known as biorestoration.
There are two main types of bioremediation techniques:
- biostimulation: these treatments involve reducing or eliminating the environmental factors which limit bacterial action. For this, nutritive elements (nitrogen, phosphorus) can be added with fertilisers, oxygen deficiencies can be eliminated (harrowing...) and the bioavailability of the pollutant improved (e.g. addition of surface active agents). The use of fertilisers is the most common technique.
- bioaugmentation: these treatments involve adding micro-organisms to the environment or waste to be treated (e.g. introduction of bacterial consortiums). It is possible to combine bioaugmentation actions with biostimulation treatments (e.g. introduction of bacteria with addition of fertilisers).
These products are designed to restore polluted shores. They are currently under development and encompass a large amount of diversity. No standardised and recognised laboratory evaluation method yet exists.