Overview of Microbiological Attack

Microbial problems were seen at their height in the early eighties. There has been a reduction in the number of incidents although the problem has never gone away. In addition whereas effects were seen mainly in distillate fuels and lubricants, it is now seen in residual fuels, drinking and ballast water. Failure to eradicate completely a re put down to adverse trading, poor training and housekeeping, environmental restrictions in the use of microbial agents and the restrictions in bilge pumping placed by MARPOL


Microbiological contamination consisting of bacteria, yeasts and moulds, are easily tolerated at low contamination levels. It is only when their numbers are not controlled that rapid infestation occurs

From a marine point of view there are six main areas of concern for microbiological infestation. These are:

Distillate fuel;

Lubricating oil;

Cooling water;

Bilge water;

Ballast water;

Distillate cargoes.

Conditions promoting growth

In each case, it is to be remembered that microbes are living organisms and their growth depends upon the readily availability of water, nutrients, heat, oxygen (or sometimes lack of it) within an otherwise acceptable environment.


The main requirement for microbiol activity is water. This must be available water and not just water content. A typical minimum value is 1%. This can happern due to insufficient draining. The presence of free water can lead to rapid micorbiol growth after 1 week at 30'C. Where water is dispersed then growth is limited the microbes existing in water droplets or surrounded with a water sheath.
Modern lead-free gasolines contain water soluble oxygenates such as methyl and ethyl alcohol, methyl tertiary butyl ether these along with antifreeze glyciol when migrated to the water phase cause a depression in microbiol activity. The level of glycol must be above a minimum as below this the glycol can actually promote growth

Hydrocarbons and chemical additives in the fuel and lubricant act as their food source. In addition to this are nutritive matter found in contaminated water either fresh or sea water. Sea water in addition promotes the growth of sulphate reducing bacteria. Cargo residues , particularly for ships carrying such thinks like fertilisers are also sources. The presence of rust and other particulates can promote growth.

Note that clean dry fuel kept at reasonable temperature will never permit any significant growth

Warm enginerooms ( 15 - 35'C) provide the ideal breeding ground for microbiol growth. Too hot (70'C) or too cold (5'C) will retard growth

Most corrosive forms of bacteria prefer stable environment and dislike adgitation. Thus ships in lay up or ships that spend long periods inactive are particularly susceptible. Water leakage or condensation will ten provide the living environment. The microbes live in the water phase but feed on nutrients in the oil phase this the boundary area sees aggressive growth.

The unpleasant by-products of their digestion, after hydrocarbons have been oxidised into acids, include toxic and pungent hydrogen sulphide. This is produced from any sulphurous compounds within the fuel, lubricant, seawater or waste product. Microbial growth is seen as a characteristic sludge formed from accumulated cellular material which may restrict fuel and lubricant pipe lines and filters.

Types of Microbes

There are three basic types of micro organsims that cause problems in the marine industry, these are bacteria, yeasts and moulds.


Bacteria can be subdivided into

Bacteria is a highly diverse group of single celled organisms with rigid cell walls. They may be rod like, spherical or spiral and many are actively mobile with a whip like appendage (flagellum). They can reproduce asexually and rapidly using binary fission with a doubling time of as low as 20 minutes. They are design to reproduce rapidly when the time is rigth and some are able to produce extremely resistant spores able to withstand high temperatures and disinfectants.

Although in the main they prefer neutral or slightly alkaline environments some can exist in the extremes of acid. They can excrete partial breakdown products on which other forms of bacteria can feed. In addition they can produce large amounts of extra-cellular slime which coats and stabilises the living environment. This slime can protect against or deactivate biocides. This slime can prevent the diffusion of oxygen to the base of the growth and thereby promote Sulphate Reducing Bacteria which are particularly aggressive.


These are unicellular, being ovoid or spherical in shape some may also produce rudimentary filaments. They reproduce by budding and growing off the parent until large enough to separate. This process may take several hours.They prefer slightly acidity


Multicellular with hard chitinous cell walls.They are usually found as branched hyphae forming a thick, tough intertwined mat occurring most commonly at oil/water interfaces. They reproduce by branching and can double there length in a few hours. They can also produce spores.

They prefer slightly acidic conditions, using oxygen in their feeding process they produce by products suitable for other microbes to feed and an atmosphere suitable for Sulphate Reducing bacteria.

They reduce complex hydrocarbons to simpler carbon compounds. Intensive corrosion can occur under the mat. They can be both sea water and temperature tolerant

Sulphate Reducing Bacteria (SRB)

These are a specific group of anaerobic bacteria with special growth requirements. They can only use simple carbon compounds therefore they require the presence of other microbes. They will produce hydrogen Sulphide in the presence of sulphur containing compounds such as sulphates found in sea water.

Desulfotomaculum has the added ability to produce extremely hardy spores able to resist exposure to air, heat and most biocide chemicals. Both this and Desulfovibrio are very insidious and able to rapidly cause corrosion in ships hull and machinery

Sources of contamination

Infestation can come from contaminated sea water or hydrocarbons, from a source already onboard or by poor onboard practices

Sea Waters

The oceans contain a very small density of microbes, htis was also partially true for harbours until such things like contamination by oil spills and fertiliser wash off from arable land as well as chemicals such as corrosion inhibitors changed the constituents of the water. Harbours can thus be rich in microbes includig hydrogen degraders and large numbers of SRB

Refinery Practices

Recent years have seen a dramatic increase in the supply of contaminated fuels to vessels with an underlying cause of bad housekeeping. This has been particularly prevalent in Eastern Europe where protection by detection, heating, filtering and biocides are recommended. On particular cause is the washing of tanks using contaminated river water that not only introduces the microbes but also sources of nutrients particularly Nitrogen and Phosphorus
It should be noted that generally microbes have and sg of about 1.05 so will tend to settle to the bottom of the tank. It is therefore possible to limit the delivery of contamination by settling and floating suctions


the worst case I have seen was on a steam turbine vessel after a period in dock. This occurred early on in my career so I cannot remember all the facts but in a similar circumstance I would look at the condition of the dehumdifier for the gearbox to ensure it is working correctly.

Hydraulic oil s are more susceptible especially were air ingress occurs and the oxygen diffuses into it. Microbes can increase cavitation damage by acting as nucleus for the bubble formations. Thus it is common to have biocides encompassed into the oil







Aggregation of microbes into a biomass, observed as discolouration, turbidity and fouling.

Biosurfactants produced by bacteria promote stable water hazes and encourage particulate dispersion.

Purifiers and coalescers which rely on a clean fuel/water interface, may malfunction.

Tank pitting.

Slimy appearance of the oil; the slime tends to cling to the crankcase doors.

Rust films.

Honey-coloured films on the journals, later associated with corrosion pitting.

Black stains on white metal bearings, pins and journals.

Brown or grey/black deposits on metallic parts.

Corrosion of the purifier bowl and newly machined surface.

Sludge accumulation in crankcase and excessive sludge at the purifier discharge.

Paint stripping in the crankcase.

The formation of slimes and sludges which are black themselves or are black when scraped.

Pitting of steel work, pipes and tank bottoms.

Rapid corrosion of plating.


Bacterial polymers may completely plug filters and orifices within a few hours.

Filters, pumps and injectors will foul and fail.

Non uniform fuel flow and variations in combustion may accelerate piston rings and cylinder liner wear rates and affect cam-shaft torque.

Additive depletion.

Rancid or sulphitic smells.

Increase in oil acidity or sudden loss of alkalinity. (BN)

Stable water content in the oil which is not resolved by the purifier.

Filter plugging in heavy weather.

Persistent demulsification problems.

Reduction of heat transfer in coolers.

Unusual foul or sulphitic smells.

Structural damage.

Loss of suction in pipelines.


When heavily contaminated fuel is brought onboard some or all of the problems listed above will be encountered within a short period of time. Particularly filter blocking and purifier malfunction. More long term will see injector and pump failures

Quick Appraisal of distillates

Lubricating Oil

When operated normally there are few microbes able to live succesfully in the nutrient and environmentally deficient lubricating oil

Bilge & ballast Water

Problems are normally associated with the presence of SRB pitting corrosion and is indicated by a sulphorus smell. Preventative action should be taken as soon as possible

Systems Affected by Microbiol attack


Microbiological contamination of distillates ( rather than residual) fuels have been a well known phenomenon for some time. The changing chemistry of the fuels and the increasing use of fuel additives have exasperated this.

Whilst being rich in carbon sources the fuels are often poor in inorganic nutrients such as Nitrogen, Phosphorous and Potassium and this by themselves do not promote rapid growth. These may be supplied by contaminated water or fuel additives entering the fuel

Initial infestation will break down such components as n-alkenes to form alcohols and fatty acids. These are in turn used by other microbes and thus a self replenishing system is created in the free water

Evolution has led to new species of bacteria in distillate fuels that produce sticky polysaccharide polymers similar to cling film'. These clog filters and other apertures by trapping rust. Thus the microbial contamination appears as a grey/brown sludge at the water/oil interface.

Stagnancy can lead to severe microbial activity in long term fuel storage tanks. The effect of this is to reduce the chain length of the hydrocarbons reducing the overall calorific value. In addition souring may occur as the microbes metabolises hydrogen sulphide. Altering the fuels chemical structure can have the effect of changing its pour point, cloud point and its thermal stability.

The formation of stable growth at the water interface can lead to mal-operation of purifiers and coalescers

Attack by SRB and moulds can infuse hydrogen sulphide and other acidic products into the fuel leading to direct acidic attack. The lower pH particularly effects copper, aluminium and there alloys such as bronze. The depolarisation of steel leads to pitting

The most obvious effect of microbial attack is filter and component blocking. In addition the fuel can become non-homogenous leading to variations in combustion and cylinder pressures. Increase liner and piston ring wear rates can result

It should be noted that the higher temperatures of residual fuels dissuade the growth of microbes although not completely

This shows a gas oil service tank opened up for inspection after premature fuel filter blockages. The overall depth of the sludge seen in the bottom is about 5cm. The black growth occurred in several patches to a height of a few centimeters and was foamy in texture. The tank was cleaned and refilled with gas oil. Biocide was dosed to normal preventative levels and instructions given to watchkeepers for better drainage of this tank. No further problems was experienced

Lubricating Oil

Generally associated with engines with water cooled pistons were the chance of water ingress is higher. Infestations, including those found in hydraulic oils are indicated by a slimy deposit and blocked filters

I have seen this in a CPP system which had blade seal leakage. Before the system was overhauled it was necessary to change the pressure filters every two months. After overhaul and the removal of water this dropped to 1 year and event then only on running hours

Black stains may be seen and a rancid odour noticeable. If SRB as present, this is normally only the case in laid up ships, then severe pitting on ferrous and non-ferrous components may result

As the microbes tend to feed on the constituents and additives of the lube oil it effectiveness will be reduced as well as increased acidity and emulsification.

Typical sources of contamination are sea water ( from coolers),bilge water, fuel and cooling water. The latter has increased in severity due to the banning of the use of chromate's for cooling water treatments which had good biocide properties.

The use of increase alkaline lube oils has seen a reduction of microbial attacks

Cooling water

The first indication is often destruction of the treatment reserves and the water will gradually become acidic. The coolant may be discoloured and have a strong odour and deposit scum's or slimes. Oil emulsion coolants will tend to stratify.

The initial infestation will be by aerobic bacteria which, when they have depleted the dissolved oxygen the can then get the oxygen by reducing chemicals such as Nitrates producing ammonia or nitrogen. Eventually the water becomes so oxygen depleted that anaerobic microbes such as SRB will grow. this progression can occur in a matter of days

Bilge Water

This can contain complex groups of bacteria, yeast of moulds. These groups can contain varieties of species not only at a ships level but even in the same system. Thus it is difficult to identify exactly what individual components are required to lead to corrosion it is more useful to identify what groups will.

Hydrocarbons and other organic matter enter the bilge water and are degraded by specialised microorganisms call 'hydrocarbonclastic'. This requires the presence of dissolved oxygen.
The degraded carbon compounds can then act as food for SRB which extract and use the oxygen in sulphates ( but cannot tolerate molecular or dissolved oxygen). Thus there are two distinct environments in the bilge water. The position of the boundary depends upon the level of re oxygenation of the water surface. This in itself is dependent on such things as surface area, agitation etc but is unlikely to be much above the base of the bilge and more likely to be found in any mud there.

The reduction of the sulphates found in sea water produces corrosive sulphides. Sulphur containing hydrocarbons tend to lead to hydrogen sulphide

Any detection of SRB in the bilge water will generally indicate a severe infestation as the majority of the bacteria will be found in a slime at the steel plate surface.

Microorganism action can have the affect of altering the electro-potential of the water and accelerate the electrochemical corrosion process. The process may be described as follows

  1. Aerobic microorganisms aggregating in slimes, muds or crevices use up the available oxygen in their immediate vicinity and create an oxygen deficient area. In electrochemical terms, such an area will be anodic in relation to relatively oxygen rich zones with fewer microbes. This oxygen gradient may be regarded as an electrochemical cell, precipitating the electron flux from the cathode to the anode, allowing deep anodic corrosion pits to develop. In addition, the microbial by-product which is a very corrosive acid, also acts as an electrolyte within the cell.
  2. The formation of pits is not entirely an electron process based upon aerobic bacteria. These oxygen deficient areas are colonised by the anaerobic SRB, which produces HS and S2- ions and hydrogen sulphide. These ions are highly aggressive towards steel and yellow metals, and form the characteristic craters. In carbon steel, a carbon skeleton remains visible as a graphite black colour and the bottom of each pit is usually black ferrous sulphide.
  3. Simultaneously, SRB depolarise the surface steel. The steel becomes progressively more porous, susceptible to hydrogen ingress and hydrogen embrittlement. When ferrous sulphide forms, it is itself cathodic and thus continues to drive the electron flow and anodic pitting, even after the SRB have died or become less active. Corrosion driven by ferrous sulphide is thought to be most pronounced during intermittent aeration or in the presence of oxygen gradients.

These effects can occur in isolation or together and can have the effect of increasing natural corrosion rates of 0.05mm per year to 10mm per year.

Factors effecting microbiological attack in bilge water include;

Ballast Water

Corrosion follows as similar process as seen in bilge water. In addition it may contain microorganisms that are also harmful to health such as cholera and botulism.
The ballast water can act as a transport for microbes distributing them into areas where they can act as parasites and pathogens. Recent legislation requiring the freshening of ballast mid ocean has only partly solved this with microbes able to remain in the mud and silt on the bottom of the tank

Distillate Cargoes

There are many types of microbes that can use hydrocarbons and these can form the basis of differing symbiotic groups. These differing groups allow finger printing of the cargo and the source of contamination ( say from previous loadings ) can be tracked.

Prevention and elimination of Microbial Contamination


There are three generally accepted and commercially viable methods of prevention. These are good housekeeping, physical cleaning and biocides.

Factors controlling the rate of microbiological problems are;

Physical prevention

Without water it is not possible to have microbial growth. Thus the first line in prevention is the removal of water, generally the more water the greater will be the problem. It is inevitable that there will always be some water with the oil, whether brought in when loading , through leaks or through condensation. Thus the need to constantly purify a system. This is seen on fuel systems where oil is taken from a settling tank to a service tank where it overflows back to the settling tank.( it should be noted that purifiers can act as a source of cross contamination and sterilisation after use on a system is recommended. Tanks should be fitted with drain cocks at there lowest points and should be drained regularly.

It should be noted that dead legs and other area where flow is minimal will tend to see increased attack therefore these should be designed out of the system. Rust and mud should not be allowed to accumulate as these can lead to growth.

Where possible tanks temperatures should be outside the 15-35'C optimum growth range and preferably be as high as practical which ensures sterilisation. The down side of this is an increase in boil off of lighter fractions in residual fuels which has led to the use of vapour recovery systems

It should be noted that modern microbes are capable of enclosing them selves in protective coatings against water removal. Biofilms on the plate surface are unlikely to be removed by water draining alone. Water draining should be carried out regularly. At each occasion the vale should be operated in small bursts to allow water to move to the cock. The used of surfactants for cleaning can cause increased attack in bilge water as it allows the microbes to move more freely into the oil phase. The commonest source of water contamination in Lube oil is coolers and piston cooling water. Every effort should be made to keep leakage to a minimum and water content should not be allowed to increase to greater than 0.5% per vol. The purifier should be set to a minimum temperature of 70'C and preferably higher and a flow rate ensuring complete charge circulation every 8 to 10 hours.

Cooling Water

It should be recognised that cooling water is not only affected by microbial attack it is also a common cause of infestation in other systems.

The following recommendations are made;

Bilge Water

The present restrictions with regard to the pumping of bilges is the main reason for the increase occurrence of microbial related failures. It should be noted that once infestation has occurred dosing with biocides will not remove it by itself as it will not penetrate the biofilm at normal, safe dosages. Cleaning is essential not only to remove the microbes but also to remove the mud & slime environments. This can have the added advantage of removing the ferrous sulphide formed by the SRB which acts as a cathode to the steel of the hull.

The following recommendations are made;

Ballast Water

Problems are usually attributed to SRB

The following recommendations are made;

Chemical Prevention

Fuel preservatives or biocides are not designed to cope with large infestations. Instead they should be used as a preventative. The biocide may be water soluble or fuel solubel depending on the longevity required of protection. For tanks requiring long term protection water soluble agents are generally used. The agent remaining in the tank during fuel changes.

Typical properties of duel preservatives are;

Lube oil preservatives or biocides may be useful as a preventative but tend to break down rapidly under normal operating temperatures

Water preservatives or biocides are water soluble as in water soluble fuel treatment. They must adhere to requisite safety standards especially with jacket water heated evaporators.


Physical Decontamination

Microorganisms do not die naturally – they must be killed. Once microbial infection is established onboard it may be combated by physical treatment methods e.g. heat and/or by the use of biocides. The dead microbes can still block filters.

Physical removal can be one of the following methods

Chemical Decontamination

Killing microbes using microbes is easy and effective, however the selection of chemicals appropriate for the system application and should be done with care. Such things as compatibility and hazards should be taken into account.

It is arguable that elimination after infestation is cheaper than continuous preventative dosing. Where contamination is heavy it may be necessary to add such high concentrations of biocide to make the fuel unusable. It would then have to be discharged and the system mechanically cleaned.

similarly for lube oils heavy contamination will lead to loss of the lube oil.

For cooling water care has to be taken when choosing the biocide to ensure it is temperature stable.

Whilst biocide treatment of bilge water is commercially viable ( taking into account the cost of steel replacement), it is difficult to select and effective solution. This is particularly the case for SRB which is able to produce extremely resistant spores.

For ballast systems the only effective method of elimination is the removal of sludges, muds and slimes

Alternatives to biocides

health considerations

Normal disease producing microbes are not usually found in fuel or lubricating oils. However there are some aerosol born bacteria that can cause flu like symptoms

Of more concern is hydrogen Sulphide produced by Sulphate Reducing bacteria (SRB). This is very toxic in even mild doses. It initially produces a distinctive 'rotting egg' smell. However a small increase in concentrating is enough to allow it to neutralise the sense of smell therefore it is possible to believe the source has disappeared when in fact it is increasing. It will eventually lead to death

Biocide chemicals are themselves toxic and care should be taken in their handling and dosage.

The traditional method of using chlorine against such bacteria found in air conditioning etc can be limited especially against the biofilm in which the multi-species microbe colonies are able to exist stably.

The above is based for the main part by an article by R.A. Stuart