It is the single most common fuel-quality problem on Australian sites, and the most misunderstood. Operators tend to treat it as a fuel problem and reach for a biocide. It is actually a tank problem: the fuel is just the food supply. Kill the colony without removing the water and the nutrients, and you will be doing it again in three months — this time with a filter bill and a day of downtime attached.

This guide covers what diesel bug actually is, how to confirm you have it rather than guessing, how to treat an active infestation properly, and the tank setup that stops it recurring.

What Diesel Bug Actually Is

“Diesel bug” is not one organism. It is the collective name for a mixed community of microbes that can live in fuel systems:

• Bacteria — including Pseudomonas species and sulfate-reducing bacteria (SRB). These reproduce fastest and are usually what turns a clean tank bad in a matter of weeks.
• Fungi and moulds — most famously Hormoconis resinae (long known in the industry as Cladosporium resinae, the “kerosene fungus”). Fungi grow as mats rather than free cells, and those mats are what physically block a filter element.
• Yeasts — including Candida species, often present alongside the others.

They all live in the same place: the fuel–water interface, the thin boundary at the bottom of the tank where the settled water layer meets the diesel. The microbes sit in the water, because they cannot live in dry hydrocarbon, and they consume the fuel as their carbon source.

To get established they need four things, and every one of them is present in a poorly managed tank:

Remove the water and the colony has nowhere to live. Everything else in this guide follows from that one fact.

Why It's Worse Now Than It Used to Be

Operators who have run diesel plant for decades often say the bug is a modern problem. They are right, and there are three reasons.

Low-sulfur diesel

Australian automotive diesel is ultra-low sulfur (10 ppm). Sulfur compounds were mildly biostatic — the fuel used to carry a small amount of its own protection. Hydrotreating removed that along with the sulfur.

Biodiesel blends

Under the Fuel Quality Standards Act, diesel sold in Australia can contain up to 5% biodiesel (B5) without being labelled as a blend, and higher blends are common in some fleets. Biodiesel is hygroscopic — it pulls moisture out of the air and holds it in suspension — and it is biodegradable by design. Both properties are good for the environment and bad for a storage tank.

Tighter engine tolerances

Modern common-rail injection systems run clearances measured in microns at pressures above 2,000 bar. Fuel contamination that a mechanical-injection engine from the 1990s would have shrugged off will now take out an injector set.

The result is that the same tank, in the same paddock, holding the same nominal product, is considerably more vulnerable in 2026 than it was in 1996.

How to Detect Diesel Bug

Guessing is expensive in both directions — untreated infestations wreck equipment, and unnecessary biocide dosing wastes money and breeds resistant colonies. Work through the checks in order.

Signs to look for

• Filters blocking far earlier than usual — the classic first symptom. A dispensing filter that used to last twelve months now needs changing every few weeks.
• Dark brown or black slime on filter elements, distinctly different from the fine grey silt of ordinary dirt ingress.
• A sour, sulfurous or rotten-egg smell from the tank or the fuel, instead of the normal solvent odour. That smell is hydrogen sulfide, produced by sulfate-reducing bacteria.
• A visible dark, stringy or gelatinous layer at the fuel/water boundary when you draw a bottom sample.
• Hazy or cloudy fuel that does not clear when it settles.
• Corrosion or pitting on internal tank surfaces, or unexplained rust flakes in the fuel.
• Equipment symptoms — hard starting, power loss, smoke, or repeated fuel-system faults across multiple machines drawing from the same tank.

Taking a bottom sample

Everything starts with a sample from the bottom of the tank, because that is where both the water and the colony are. A sample drawn from the dispensing nozzle tells you very little.

Use a weighted bacon-bomb sampler or the tank's bottom drain, and take the sample into a clean, clear glass jar. Let it stand for ten minutes. What you are looking for:

• A clean sample is bright, clear and a uniform straw colour, with no separate layer at the bottom.
• Free water shows as a distinct clear layer beneath the fuel.
• An active infestation typically shows a dark, cloudy or stringy interface between the two — sometimes described as looking like weak coffee, or like strands of dark jelly.

Confirming it with a test

Visual inspection tells you something is wrong. It does not tell you how bad, and it will not catch an early colony. Four levels of test are available:

For most operators the practical combination is water-finding paste as a routine habit and an immunoassay kit kept on the shelf for when something looks wrong. Standards such as ASTM D6469 provide industry guidance on microbial contamination monitoring in fuel systems if you need a documented procedure for a compliance file.

A word of caution on culture tests: they only grow the organisms that will grow on that particular medium, and they take days. A negative dip slide does not prove a clean tank.

What an Infestation Actually Costs

The biocide is the cheap part. What makes diesel bug expensive is everything downstream of it.

• Filter consumption — a heavy infestation can block a new dispensing filter in a single afternoon.
• Injector and pump damage — abrasive biomass and the acids produced by SRB attack the highest-precision, highest-cost components in a modern engine.
• Microbially influenced corrosion (MIC) — sulfate-reducing bacteria generate hydrogen sulfide, which forms acids that pit steel from the inside. MIC damage to a tank floor is not repairable by cleaning; on an older single-skin tank it can end the tank's service life.
• Downtime — the failure never happens on a quiet Tuesday. It happens mid-harvest, or when the standby generator is called on.
• Fuel write-off — heavily degraded fuel may not be recoverable by polishing and has to be removed and disposed of as regulated waste.
• Warranty exposure — engine manufacturers routinely decline fuel-contamination claims.

Against that, a desiccant breather and a monthly water check are trivially cheap.

Which Operations Are Most at Risk

Diesel bug does not affect every tank equally. Four situations account for most of the infestations on Australian sites.

Standby generator fuel

The worst case by a distance. The fuel sits for a year or more at ambient temperature, the tank rarely gets opened, nobody is watching the filters because nothing is running — and the one time performance actually matters is an emergency. Generator tanks belong on a formal sampling schedule, six-monthly at minimum, regardless of how good the installation looks.

On-farm storage

Tanks are often older, frequently single-skin, sited in the open, and filled to cover a season rather than a fortnight. Harvest is exactly when a blocked filter costs the most. The seasonal fill-and-draw cycle also means a lot of headspace breathing moist air through summer.

Remote and mine sites

Long supply chains mean fuel may already be weeks old and may already carry a microbial load on delivery. Large volumes and infrequent turnover compound it, and the nearest fuel-polishing contractor may be a day's travel away.

Marine and coastal storage

High humidity and salt air drive condensation hard, and tanks in these environments are notoriously difficult to drain completely.

If your operation is on that list, treat monitoring as a scheduled maintenance task rather than something you do once a filter has already blocked.

Build the Routine Into Your Maintenance Schedule

None of the individual actions in this guide are difficult. What separates a tank that never has a problem from one that does is whether they happen on a schedule instead of after a failure.

A workable baseline:

• Weekly — walk-around. Check the fill cap, the vent and the inspection hatch for damage or pooled water. Note the level.
• Monthly — dip the tank with water-finding paste and draw a bottom sample. Drain any water immediately. Record both, so you are reading a trend rather than a snapshot.
• Quarterly — inspect and, where needed, replace seals, gaskets and breather elements. Check dispensing filters and record the change interval.
• Six-monthly — run a microbial field test on any tank holding fuel for longer than a few months, and on every standby generator tank without exception.
• Annually — review the whole setup: is the tank sized to what you actually use, is the fuel turning over inside its shelf life, and is the filtration still right for the equipment it feeds?

The record-keeping matters as much as the checks themselves. A logbook showing water drained monthly and a clean microbial test three months ago is also the document that settles a warranty dispute in your favour when an injector fails.

How to Treat an Active Infestation

Treatment has an order, and skipping steps is why so many tanks get re-infected. Work through these in sequence.

How to Prevent It Coming Back

Prevention is almost entirely about water and tank design.

Keep water out

• Fit a desiccant breather or a quality vent cap so the tank draws dry air. Every temperature swing makes a tank breathe; in a Queensland summer that is a lot of moist air per day.
• Keep the tank as full as practical. Less headspace means less air, and less air means less condensation on the internal walls overnight.
• Check and replace seals. A perished fill-cap gasket or a cracked vent turns a sealed tank into an open one, and rain does the rest.
• Site the tank to shed water, and make sure fill points and inspection hatches are not sitting in a puddle after rain.

Get water out on a schedule

Draw a bottom sample and check for water monthly as a baseline — weekly in the wet season or on a tank you depend on. Drain any water you find immediately, and record it. A tank that starts producing water where it never did before is telling you a seal has failed.

Filter on the way in and the way out

Fit filtration and water separation on the dispensing line so that whatever is in the tank never reaches your equipment, and filter on delivery if your supply chain is long or your supplier's own storage is an unknown. A-FLO's diesel fuel filters and water separators cover the dispensing side, and the wider diesel accessories range has the fittings to plumb them in.

Use a tank designed for the job

The tank is the largest single variable. A sealed, purpose-built diesel tank with proper venting, a bottom drain point and accessible inspection keeps fuel usable for years longer than a repurposed drum or an ageing single-skin tank with a loose lid. A-FLO's FUELCUBE self-bunded tanks and larger FUELTAINER self-bunded tanks are sealed systems with the containment AS 1940 requires built in, and for on-farm storage a dedicated farm refueller tank beats drums and totes on every measure that affects fuel life.

Rotate stock and monitor it

Size storage to actual consumption so the fuel turns over inside its shelf life — a tank refilled every six weeks will rarely develop a bug. Where fuel has to sit, automatic tank gauging and a fuel management system give you the level, usage and delivery data to keep deliveries sized correctly and to spot the slow water build-up that precedes an infestation.

Diesel Bug, Water and Oxidation Are Three Different Problems

They get conflated constantly, and the treatment for one does nothing for the others. Use this to work out which you actually have:

All three often turn up together, because the conditions that cause one tend to cause the others. If you are working through fuel age and shelf life more broadly, our guide to how long you can store diesel covers the temperature and turnover side in detail.

Frequently Asked Questions