Brussels - Oily sludge that has accumulated around petroleum refineries can pollute soil and groundwater. Safe disposal of this contamination can be a problem. A recent study has found a new strain of yeast which can break down the sludge under highly acidic conditions, allowing oil-polluted sites with similar conditions to be bioremediated.

Bioremediation is one method to restore oil-contaminated environments. Microbes, such as bacteria or fungi, including yeasts, are used to degrade, or break down, hydrocarbons (carbon-based compounds) in oil into less harmful products.

Oil refineries produce large quantities of oily sludge from the refining process. For example, in India, where the study was conducted, the 18 refineries process around 128 million metric tonnes of crude oil each year, generating 0.002-0.1 per cent of this amount by weight as sludge. The sludge can be very acidic from the old processing operations; commonly used bioremediation microbes are unable to survive these harsh conditions.

Soil samples contaminated with the oily sludge were cultured in the laboratory to isolate the microbiological strains already existing in the sludge. The procedure ensured that the microbes selected were able to actively degrade the oily sludge and not just tolerate the acidic conditions.

These selected microbes were further screened to choose those that performed best at breaking down the different components of the sludge. Of these, the yeast, Candida digboiensis TERI ASN6 demonstrated superior potential to degrade a wide range of hydrocarbons in acidic conditions. In addition, C. digboiensis was capable of using the carbon in the hydrocarbons as its sole source of carbon for growth and did not therefore require further supplements of carbon nutrients to be effective.

When this cultured C. digboiensis was grown under acidic conditions in the laboratory, its form of growth included fine strands or filaments. C. digboiensis is sensitive to the stress caused by acidic conditions and responds by developing filaments. These strands enable the yeast to penetrate the oily sludge and 'forage' more effectively for nutrients, in particular the carbon. Tests also revealed that C. digboiensis appears to tolerate small fluctuations in temperature. This suggests the yeast should work well as a bioremediation agent in the field.

Overall, the yeast strain C. digboiensis TERI ASN6 was able to use 73.64 per cent of the total petroleum hydrocarbon in the oily sludge within a week. Although the microbe was able to biodegrade a considerable amount of one class of hydrocarbons, the aromatic component, TERI ASN6 demonstrated a preference for another class, the alkanes. The TERI ASN6 strain was also able to biodegrade the sulfur components found in the sludge.

Researchers are currently undertaking tests to assess the effectiveness of C. digboiensis to degrade of petroleum hydrocarbons in acidic oily sludge under real-life field conditions.

Original Source: Sood, N., Lal, B. (2009). Isolation of a novel yeast strain Candida digboiensis TERI ASN6 capable of degrading petroleum hydrocarbons in acidic conditions. Journal of Environmental Management. 90:1728-1736.

Quelle: EU commission

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