Detection and control of microbial growth in marine fuel

Microbial growth in marine fuel is a common and costly problem, especially in low-sulphur fuels as they hold more water and have fewer natural biocidal properties than older high-sulphur blends. This happens because microbes such as bacteria, yeast, fungi, live in the water phase, not the fuel itself, and feed on hydrocarbons at the fuel-water interface. Every fuel tank accumulates some water from condensation, humid bunkering, or fuel that already contains trace water, so any tank with standing water is a candidate for growth, particularly in warm, humid, or slow-turnover storage conditions.

Fuels containing FAME require particular attention because FAME can attract and retain water and is associated with increased susceptibility to microbial activity. 

Left unchecked, microbial colonies build up as biomass and sludge that migrate through the fuel system. This raises filter differential pressure, restricts fuel flow, and contributes to deposits on tank and system surfaces. Some microbial activity also produces acidic by-products, which are associated with localized corrosion so the risk here isn't just a fuel-quality issue, it can extend to the physical condition of the tank and system components themselves.

Contamination does not always announce itself clearly. The most common indicators are unexplained or repeated filter blockage, appearance of dark or slimy material, persistent hazy fuel, and unusual odour, deposits, or corrosion around tank bottoms and fuel-system components. A failed onboard screening test or laboratory analysis is often the clearest signal of all. Recognizing the warning signs:

• Unexplained or repeated filter blockage
• Dark, stringy or slimy material in tanks, drains or filters
• Persistent water bottoms or hazy fuel
• Unusual deposits, odour or corrosion around tank bottoms and fuel-system components
• A failed onboard screening test or laboratory microbial analysis

Visual appearance is not proof. Clear fuel may still fail microbial screening, while soft or wax-like deposits may have non-microbial causes. Representative sampling and appropriate testing should guide the response.

The commercial case for prevention is straightforward: the later the intervention, the larger and more complex the response may become. Beyond the additive itself, late intervention can bring filter consumption, tank cleaning, fuel handling, laboratory work, maintenance and the risk of operational disruption. 

For the 300 MT fuel, preventive treatment requires approximately 60 litres of MG-Control, compared with 900 litres at the shock-treatment rate. In volume terms, shock treatment therefore requires 15 times more additive than preventive treatment. This comparison is illustrative and based on the stated MG-Control dosage modes. Actual treatment requirements depend on the fuel volume, contamination severity, and system condition. 

The potential exposure extends beyond additive volume: a 2018 report from The Swedish Club cited an average cost of USD 344,000 per fuel-related damage incident on vessels. This industry figure provides context for the cost of late intervention.

Mitigating the risks with preventive treatment of microbial contamination

For heavily contaminated systems, treatment is best planned as a managed operation rather than a single chemical addition: filters need to be monitored closely, water bottoms removed, and persistent biosludge cleaned out physically where required. DieselPower MG-Control is a self-dispersing fuel biocide designed for both preventive treatment and the treatment of existing microbial contamination. It can be dosed directly into the fuel tank, either manually or through a dosing pump. As with any biocide treatment, its effectiveness depends on more than the correct dose; distribution through the fuel volume, adequate contact time, water removal, and the physical condition of the tank and fuel system all play a part. 

As always, the current product and safety data sheets, along with any applicable local requirements, should be checked before use.

Practical lessons for ship operators

A few principles hold regardless of the specific system or fuel grade involved. Appearance alone should not be relied on. Contamination should be tested for whenever it's suspected. Water removal is the first line of defense, and it should continue after chemical treatment, not stop once dosing begins. The dose needs to match the confirmed condition, and the product needs to be properly distributed through the treated fuel volume. Filters should be expected to capture released biomass after treatment, so adequate monitoring and spares should be planned for in advance. Heavily fouled tanks and system components should be physically cleaned, since residues can shelter microorganisms and drive recontamination. And the outcome should always be verified through repeat testing and continued trend monitoring, rather than assumed.

Microbial growth in marine fuel is best controlled through a combined programme of prevention, representative testing, targeted treatment, residue management, and verification. DieselPower MG-Control provides the treatment component of that programme, but it works alongside dry tanks, good housekeeping, correct application, and follow-up monitoring. The cases above show why disciplined diagnosis and operational planning matter most when contamination is extensive, or when FAME-containing fuels are involved.