Diesel bug in fuel tanks: how to identify, test and prevent microbial contamination

Diesel fuel that is stored for extended periods can be affected by microbial contamination. In practice, this is often referred to as diesel bug, diesel bacteria or microbial growth in diesel fuel. This contamination can lead to sludge, blocked fuel filters, corrosion, engine problems and unplanned downtime. That makes diesel bug relevant for organisations operating standby generators, vehicles, machinery, fuel storage facilities and vessels. With effective fuel management, control of water in the tank and routine microbiological screening, potential problems can often be identified at an early stage. Dipslides can play a practical role in this process.

What is diesel bug?

Diesel bug is not the name of one specific bacterium. The term is used to describe microbial growth in diesel fuel and fuel systems. This may involve bacteria, yeasts and moulds. Growth mainly occurs when water is present in a diesel tank. Micro-organisms develop particularly at the interface between water and fuel. Long-term storage, condensation, contaminated fuel or leaking seals can all contribute to water entering or accumulating in a tank. The consequences are not always immediately visible. A tank may appear to be in good condition from the outside, while water, sediment and microbial growth accumulate at the bottom.

Why is microbial growth in diesel a problem?

Micro-organisms can multiply and form slimy deposits, sediment and biofilm. These deposits can spread through fuel lines, filters and other parts of the fuel system.

Possible consequences include:

• Fuel filters becoming blocked more frequently or unexpectedly
• Black, brown or slimy deposits in filters, water separators or tanks
• Loss of power, difficulty starting or engine faults
• Blocked fuel lines and contamination of components
• Corrosion in tanks or fuel systems
• Higher maintenance costs and unplanned downtime
• Reduced reliability of standby power systems or other critical equipment

When sludge or sediment at the bottom of a tank is disturbed, for example, by the movement of a vehicle, vessel or machine – contamination can quickly be carried towards the fuel filter, increasing the risk of blockage.

Where is diesel bug most likely to occur?

Diesel bug can occur in any diesel tank where water and fuel are present together. The risk is particularly relevant when diesel is stored for long periods, when an installation is used infrequently, or when business continuity depends on reliable fuel.

Examples include:

• Healthcare facilities, data centres and commercial properties with standby generators
• Transport companies and organisations operating their own vehicle fleets
• Construction, infrastructure, earthmoving operations and agricultural businesses
• Filling stations, fuel suppliers and maintenance providers
• Organisations with bulk fuel storage, reserve fuel or seasonal equipment
• Vehicle, machinery and generator hire companies
• Inland shipping, fishing vessels, workboats, marinas and leisure craft
• Commercial shipping and passenger vessels, including cruise ships, with fuel tanks for auxiliary engines and generators

The same principle applies across all these applications: the longer diesel is stored, and the harder it is to monitor water and sediment, the more important preventive monitoring becomes.

Testing diesel with dipslides

Dipslides are plastic paddles coated with culture media on which micro-organisms can grow. After sampling and incubation, dipslides provide a practical indication of the microbiological load present. For diesel and fuel applications, dedicated dipslides can be used to monitor bacteria as well as yeasts and moulds. This makes them suitable for preventive screening and trend monitoring.

Dipslides can be used for:

• Periodic monitoring of stored diesel fuel
• Checking standby generators and reserve fuel
• Investigation where fuel filters regularly become blocked
• Screening diesel in machinery, vehicles, tanks and vessels
• Monitoring before and after cleaning or treatment of a fuel system
• Trend monitoring as part of maintenance and quality programmes

A dipslide is not a full fuel analysis or a technical diagnosis. However, it can provide a straightforward and practical first indication of whether microbial growth may be contributing to a problem.

Also read our step-by-step guide to bacterial monitoring with dipslides and view our range of dipslides.

Taking a representative sample

The quality of microbiological monitoring depends greatly on the sampling method. Microbial growth is not always evenly distributed throughout a tank. It is most likely to be found in water, sediment or at the interface between water and diesel. Where it is safe and practical to do so, take a sample from:

• The lowest point of the tank
• A water separator
• A drain point at the bottom of the fuel tank
• Sediment or contamination found in a blocked fuel filter

Always record the tank, location and date of the test. Also note any unusual observations, such as water, sediment, discolouration, odour, visible sludge or a blocked filter. Recording results over time provides a useful trend analysis.

What should you do if the microbial growth is detected?

An abnormal dipslide result indicates that microbial growth may be present. This is a reason to assess the condition of the tank and the fuel system more broadly.

A logical follow-up approach is:

1. Check for free water in the tank or water separator.
2. Remove any water and assess the presence of sludge or sediment.
3. Inspect filters, lines and, where possible, the tank.
4. Determine whether additional fuel analysis or technical investigation is required.
5. Where contamination is confirmed, arrange appropriate treatment, cleaning or filtration.
6. Repeat the test to assess whether the microbiological load has been reduced.

Adding a biocide alone is not always sufficient. In more serious cases, water removal, tank cleaning, filtration and replacement of fuel filters may also be necessary.

Preventing diesel bug

Prevention starts with limiting water in the fuel system and monitoring fuel condition on a structured basis. Practical measures include:

• Check tank caps, seals and vents for potential water ingress.
• Drain water and sediment at regular intervals.
• Inspect fuel filters during routine maintenance.
• Record abnormalities and completed inspections.
• Check stored diesel periodically, particularly after prolonged periods of inactivity.
• Take a representative sample immediately if a fault occurs or sediment becomes visible.
• Following an abnormal result, choose measures that address the cause rather than only the symptoms.

Water management is essential: microbial growth in diesel depends strongly on the presence of water.

Dipslides for diesel monitoring

Would you like to test stored diesel, a fuel tank or a water separator for microbial contamination? CCD recommends the DP01 dipslide, which is suitable for detecting bacteria, yeasts and moulds.

We can also supply a compact, calibrated incubator, allowing you to carry out routine testing easily in-house.