Uncontrolled Contamination
Eats Earnings

The following article is reprinted from National Petroleum News, October, 1994.

Unless youíve had a fuel filter become so slimed-up that you couldnít pump fuel, you’re probably confident that you don’t have microbial contamination in your fuel tanks. Chances are you’re wrong. The annual impact of low-level microbial contamination in a singe 100,000-bbl. fuel tank can reach $500,000. Even at retail outlets, the cost impact ranges from $1,500 to $5,000.

Plugged filters is just one of the more obvious and irritating symptoms of microbes in your tanks.

Microbes use fuel as food, converting distillate fractions and additives into new chemicals. Only 2% to 5% of their food becomes new bugs (biomass), the rest becomes byproducts (metabolites). Metabolite molecules range from carbon dioxide to high molecular weight polymers ñ slime. Some metabolites act as surfactants, accelerating the rate at which water gets emulsified into fuel. Others facilitate hydrocarbon polymerization, contributing to sludge formation. Organic acids, produced as metabolites, make fuel and associated water bottoms corrosive.

If instead of thinking of microbes as particles that plug filters, you imagine them as tiny machines that convert fuel into nonfuel products, you begin to understand how low-level contamination can be costly.


Tank Capacity:

100,000 bbl. (4,500,000 gal.)

Layer Volume if 1% of total:


No. times drained/yr:


Total volume lost/yr.:


Cost, if fuel price is $0.50:

4 drainings x 45,000 gal./draining x $0.50/gal. = $22,500

Examine a slime sample taken from the layer that forms between fuel and water, near tank bottoms. The proportion of the mass that is actual bugs is amazingly small (about 0.01%). How can so few microbes produce so much slime?

All cells contain molecules, called enzymes. The enzymes act like machines, cutting, reshaping and joining molecules. Each enzyme performs a very specific step in converting a hydrocarbon molecule into either new cell components or metabolites, depending on the size and structure of the molecule and the product.

Not all microbes have the same enzymes. Some microbes can use chemicals and create products that others can’t. It’s like a factory in production depends on the collective skills of the workers. Bugs are the workers; enzymes are their machines.

Microbial contamination measurements aren’t included in fuel specifications. Microbial tests, when run, use inadequate techniques, essentially unchanged since the 1940s. Consequently, microbial contamination problems are often misdiagnosed.

Fuel used my microbes to make new bugs and metabolites isn’t marketable. This may account for 1% of your total annual volume handled. For a jobber distributing 30 million gal., that’s potentially 300,000 gal./yr. of unmarketable fuel.

The maintenance cost impact may be even greater. The cloudy, invert-emulsion (water in fuel) layer forming just above the fuel-water boundary can account for 0.5% to 3% of your total tank volume. Microbial surfactants accelerate cloud layer formation and are responsible for 90% to 95% of the total layer. A 0.5- to 2-in. layer can develop within two to three months.

An example (assumptions shown in box) will illustrate an approach for estimating the cost-impact of cloud-layer formation.

Complying with federal and state hazardous waste-handling regulations means that drained water bottoms must be handled by a licensed waste hauler and treater. Microbes won’t affect the volume of water bottoms, just the layer volume; therefore, add the $180,000 cost of waste removal (or treatment) at $1.00/gal. (regional charges range from $0.50 to $3.00/gal.). The total cost is $202,500/yr. Since non-biological dispersant-additives are responsible for 5% to 10% of the layer volume, we multiply the cost by 0.90 and 0.95 to get $182,000 to $192,000 annual costs from bug activity. Adding the annual cost of fuel ($0.50/gal. X 300,000 gal. = $150,000) gives a total impact of about $400,000.

What is the cost impact of losing customers who don’t return after they receive a load of substandard or contaminated fuel? Reducing contamination costs begins with recognizing and understanding the problem. The next step is the design and execution of a well-conceived monitoring and maintenance program. Biocides, chemicals designed to kill bugs, may be part of the solution, but unless selected and applied wisely, they only contribute to operational costs.

Dr. Frederick J. Passman, ph. D., is president of Biodeterioration Control Associates, Inc., and an editorial board member for the International Journal of Bioremediation & Biodeterioration and associate editor for Lubrication Engineering.

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