Biodeterioration of fuels and lubricants

Fuels and lubricants are derived for the most part from naturally occurring petroleum deposits and are directly utilized by a range of oxygenrequiring microorganisms (upwards of 1000 species). Interest has recently been focused on the fate of waste oil in the environment, and a number of oil-degrading strains of bacteria have been isolated which may have biotechnological uses. Being essentially hydrophobic materials, fuels and lubricants are at risk of microbial degradation only when they are in close contact with water. The water may be present as a discrete layer beneath the hydrocarbon, as large dispersed droplets in an agitated system or as an oil-in-water or water-in-oil emulsion, where the water droplet sizes are in the range of 1–10 μm in diameter. The presence of only 10 parts per million (ppm) of water may be sufficient for bacteria or fungi to grow at the interface between the oil, or fuel, and the water, and fuels containing as much as 0.1% water will normally meet the quality standards for use. Hence even apparently water-free hydrocarbons may suffer from microbial contamination.

Some microorganisms excrete surface-active materials (biosurfactants), which allow the cells to contact the non-polar hydrocarbon molecules before the uptake and/or degradation of the latter. Figure 3.1 shows the emulsion produced between an aqueous medium and diesel oil by supernatants fromcultures of Pseudomonas isolates, demonstrating that the emulsifying agents are extracellular.

Extracellular surfactants can cause significant problems in fuel storage tanks, causing emulsion (fuel-in-water) and invert emulsion (water-infuel) formation with the water bottom.