Microbial Field Test Kit

Compare the slide with the chart printed on the side of the black container to see how heavily the fuel is contaminated.

Any reaction needs action straight away. Bacteria and fungi form insoluble particles that clog fuel filters, starving the engine of fuel until it stops. They corrode metal, including storage tanks and pumps, and form organic acids that make the fuel unstable. 201 Diesel Fuel Biocide kills them.

Robert Williams, a property manager in north-west New South Wales, noticed the 6,000 litres of diesel in his storage tank slowly getting darker, soon after a delivery from a local bulk supplier. We tested a sample from the drain tap at the very bottom of the tank with the field test kit, and photographed the dip slide under a microscope every 12 hours. It showed several strains of microorganisms.

Rob treated the tank with 201 before the infestation became a problem, and bought more dip slides to check it was completely gone. We also suggested a dose of 201 in all the equipment filled from the tank, so the bug couldn't spread.

His problem is very common. The contamination had most likely been festering, undisturbed and undetected, in water at the bottom of the tank, which happens when the draw-off tap sits well above the bottom. The new delivery stirred it up and spread it through the diesel.

Diesel is an organic fuel, so it gives microscopic fungi, yeast and bacteria everything they need: dissolved water to germinate, carbon for food, oxygen and sulphur to respire, and trace elements to grow. Over twenty species are known to contaminate diesel storage tanks. They can come from the air or moisture, or in during filling, or as tanks expand and contract. They cover themselves in protective slime and can lie dormant in minute crevices in the metal, rubber and polyurethane coatings of tanks and fuel systems until there's water and the temperature is right. Once contaminated diesel is in a fuel system, it's very hard to eradicate.

• Bacteria feed on hydrocarbons and double their numbers every 20 minutes: one spore becomes 262,144 in six hours.
• Sulphate-reducing bacteria produce hydrogen sulphide, a sludge that smells of rotten eggs, and make the environment acidic, which speeds up corrosion. They attach to steel as a film and are particularly hard to deal with.
• Iron-reducing bacteria also eat steel, adding to corrosion.
• Yeasts thrive in the acidic conditions sulphate-reducing bacteria create.
• Fungi grow in fine threads that form thick, tough, tangled mats at the fuel and water interface.

Biofilms on steel cause accelerated corrosion wherever they settle, usually in pits and crevices: an attack on one specific spot rather than general corrosion. For the signs of diesel bug, and how it affects boats in particular, see 201 Diesel Fuel Biocide.

1. Take a diesel sample from the bottom of the fuel tank, or from the fuel filter or water separator.
2. Take the double-sided slide out of the black storage container, holding it by the handle only. Don't touch the slide surfaces.
3. Dip the slide in the fuel sample for ten seconds, making sure both sides are fully coated.
4. Drain the excess fuel by touching the tip of the slide on a clean paper towel.
5. Put the slide back in the black container and tighten the cap. Bacteria, yeast and mould grow best in the dark.
6. Keep the container somewhere warm (30°C to 35°C) for at least 24 hours.

The white side of the slide shows bacteria between 24 and 36 hours after the test; the off-white side shows mould and yeast between 36 and 48 hours. Higher temperatures may hold back bacterial growth. If the result is negative, a further five days of incubation can pick up slow-growing strains. Then compare the slide with the chart on the container.