Microbial problems were prominent during the early eighties. Over a period of time incidents due to microbial problems have reduced but they do still exist. The effects were mainly seen in distillate fuels and lubricants, but these effects are even seen in drinking water and ballast water. Failure to eradicate completely is due to 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 ready availability of water, nutrients, heat, oxygen (or sometimes lack of it) within an otherwise acceptable environment. And marine environment can be considered as best for growth of such microbes
The main requirement for microbial activity is water. This is the available water and not the water content of the oil. A typical minimum value is 1%. If the tanks are not sufficiently drained then the water content will increase. The presence of free water leads to rapid microbial growth after 1 week at 30’C. In the case of dispersed water or water droplets, the microbial growth is limited to the small region having the water sheath. Modern lead-free gasoline contains water-soluble oxygenates such as methyl and ethyl alcohol, methyl tertiary butyl ether these along with antifreeze glycol when migrated to the water phase causes a reduction in microbial activity. Glycol level must be in excess than the minimum required, if the glycol content is less than that of the minimum required then glycol will actually assist in microbial growth. 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 seawater. Seawater, in addition, promotes the growth of sulphate-reducing bacteria. Cargo residues, particularly for ships carrying cargoes like fertilisers are also a good source of nutrients for microbes. The presence of rust and other particulates promotes growth. Note that clean dry fuel kept at reasonable temperature will never permit any significant growth
Warm engine rooms ( 15 – 35’C) provide the ideal breeding ground for microbial growth. Microbial activity reduces at too hot (70’C) or too cold(5’C) environments.
Most corrosive forms of bacteria prefer a stable environment and dislike agitation. Thus ships in lay-up condition or ships that spend long inactive periods are more susceptible to microbial infestation. Water leakage or condensation will tend to provide a nurturing environment for microbes. The microbes thrive in water utilising the nutrients from the oil. Hence the oil and water separation phase will observe aggressive microbial degradation.
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 pipelines and filters.
Microbial infestation degrades the oil and leads to the formation of acids and sludge, metal staining, deposits and serious corrosion. When the microbes reproduce they tend to generate slime and eggs, which when rot gives out the foul smell in the form of hydrogen sulphide. This smell is an indication that oil is contaminated by microbial degradation.
Types of Microbes
There are three basic types of micro-organisms that cause problems in the marine industry, they are listed below
Bacteria can be subdivided into following
- Aerobic Bacteria Require oxygen to survive.
- Anaerobic Bacteria Live in the absence of oxygen.
- Facultative Bacteria Live with or without oxygen.
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 designed to reproduce rapidly when the time is right 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 extracellular 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 oval 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 environments for their growth.
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 their 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 seawater 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 seawater.
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 seawater or hydrocarbons, from a source already onboard or by poor onboard practices
The oceans contain a very small density of microbes, this 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 including hydrogen degraders and large numbers of SRB
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.
Locations of Contamination
Onboard a ship the typical location of contamination can be found in the following areas.
- Fuel Oil tanks.
- Lub Oil tanks.
Bilges Polluted water and the presence of hydrocarbons in wells that are not pumped fully dry can lead to infestation with SRB, localised pitting and eventually perforation. It is recommended that where it is not possible to remove all the contents from a well then the contents should be agitated or freshen by the introduction of water to prevent stagnation.
Fuel it is inevitable that some contamination will always be brought on board with this. It is then essential to ensure that the amount of available water in the fuel be kept to an absolute minimum. In addition, warmth promotes the growth and tanks such as service tanks that receive the heated recirculated fuel form the engine are particularly susceptible.
Lubricating Oils It is unusual to find microbial growth in normal use of lube oil systems due to the operating temperatures. However, it can occur after a period of inactivity. The worst case I have seen was on a steam turbine vessel after a period in the 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 dehumidifier for the gearbox to ensure it is working correctly.
Hydraulic oils are more susceptible especially were air ingress occurs and the oxygen diffuses into it. Microbes can increase cavitation damage by acting as a nucleus for the bubble formations. Thus it is common to have biocides encompassed into the oil
|Changes||Fuel Oil||Lubricating Oil||Bilge & Ballast Water|
|Visual Changes||Discolouration, turbid and fouled Fuel oil due to microbial degradation.|
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.
|Slimy appearance of the oil; the slime tends to cling to the crankcase doors.|
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 the 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 steelwork, pipes and tank bottoms.
Rapid corrosion of plating.
|Operational Changes||Bacterial polymers may completely choke filters and orifices within a few hours.|
Filters, pumps and injectors will foul and failover continuous operations.
Non-uniform fuel flow and variations in combustion may accelerate piston rings and cylinder liner wear rates and affect cam-shaft torque.
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.|
Loss of suction in pipelines.
Corrosion inside piping system
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
It is possible to make a quick judgement on the degree of contamination in distillates
Sterilise a clear bottle and take a sample
Any contamination will be apparent as a haze caused by the presence of sludge. This sludge should readily disperse by agitation. It will tend to settle out and stick to the sides. A black coloured sludge indicates the presence of SRB
Repeat the process for each fuel location to find the cleanest fuel. If only heavily contaminated is available this should be left to settle as long as possible. Were possible fuel should be drawn only via a filter, coalescer or purifier from the higher levels in the tanks. The use of a biocide at this point is inadvisable as the dislodges biofilms will tend to block all filters.
Take a sample from the bottom of the tank and send to the laboratory for ‘fingerprinting’ against the bunker supplier
When Lub oil is in use the chances of formation of microbes in it are very less while in a lay-up kind of situation where machinery is not used for a long time and in presence of the above-mentioned growth conditions microbial growth will occur.
Bilge & ballast Water
Problems are normally associated with the presence of SRB pitting corrosion and is indicated by a sulphurous smell. Preventative action should be taken as soon as possible. This type of pitting corrosion will lead to damage of the whole bilge & ballast water pipelines.
Systems Affected by Microbiol attack
Microbiological contamination of distillates ( rather than residual) fuels has 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
An 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 the 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 metabolise 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 affects copper, aluminium and there alloys such as bronze. The depolarisation of steel leads to pitting
The most obvious effect of the 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
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 then only on performance basis.
Black stains may be seen and a rancid odour noticeable. If SRB are 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 its effectiveness will be reduced as well as increased acidity and emulsification. Typical sources of contamination are seawater ( from coolers), bilge water, fuel and cooling water. The latter has increased in severity due to the banning of the use of chromates for cooling water treatments which had good biocide properties. The use of increase alkaline lube oils has seen a reduction of microbial attacks
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
This can contain complex groups of bacteria, the 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 reoxygenation 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 seawater produces corrosive sulphides. Sulphur containing hydrocarbons tend to lead to hydrogen sulphide formation. Any detection of SRB in the bilge water will generally indicate a severe infestation as the majority of the bacteria will be found in slime at the steel plate surface.
Microorganism action can have the effect of altering the electro-potential of the water and accelerate the electrochemical corrosion process. The process may be described as follows
- 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.
- 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.
- Simultaneously, SRB depolarises the surface of the 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 has 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 affecting microbiological attack in bilge water include;
- Ingress into the bilge of polluted water
- Nutrients contained in water ingress
- Some microbes such as SRB are very temperature sensitive, Bilges tend to be at the ideal of 15 to 35’C. A reduction to 5’C will see a significant reduction in microbe growth. Where warm water continuously enters bilge ( say condensate drain) then this area may see significantly more activity.
- Regular pumping of bilges not only removes the nutrients for SRB (remembering that they require other bacteria to break down hydrocarbons to simpler compounds) but will also remove the aerobic bacteria themselves and lowers the oxygen depletion layer to a position it can affect the SRB on the plates.
- Ingress of nutrients through shipboard sources. In addition, detergents emulsify the oil and tend to make it more available for the microbes to use.
Corrosion follows a 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
There are many types of microbes that can use hydrocarbons and these can form the basis of differing symbiotic groups. These different groups allow fingerprinting 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;
- Infestation, this is nearly impossible to prevent.
- The size of the initial infestation.
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 the 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 the flow is minimal will tend to see an 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 downside of this is an increase in boil-off of lighter fractions in residual fuels which have led to the use of vapour recovery systems
It should be noted that modern microbes are capable of enclosing themselves 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 an increased attack in bilge water as it allows the microbes to move more freely into the oil phase. The common 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.
It should be recognised that cooling water is not only affected by a microbial attack it is also a common cause of the infestation in other systems.
The following recommendations are made;
- Ensure correct treatment levels.
- Monitor alkalinity and ensure a pH greater than 8.
- Minimise the amount of salts in the system that can act as nutrients.
- Test for microbial contamination regularly. A polished mild steel bar placed in the bottom of the tank can act as an indicator of the presence of SRB.
The present restrictions with regard to the pumping of bilges are the main reason for the increasing occurrence of microbial related failures. It should be noted that once an 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;
- Pump regularly and prevent stagnation. This will help remove the hydrocarbon food and re-oxygenate the water.
- Apply a coating. This must be complete or holidays will act as foci for the attack.
- Use cathodic protection to remove the electro potential that is generated by feeding microbes.
Problems are usually attributed to SRB
The following recommendations are made;
- Inspect tank coatings for failures.
- Regularly remove mud and slime.
- Where tanks are not in use they should be kept as dry as possible.
- Where possible restrict using poor sources of water and test regularly.
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 soluble 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;
- Combustible and clean-burning without ash.
- Not surface active.
- Compatible with fuel additives and system components.
- Not affect flashpoint.
- Not promote corrosion.
- Safe to use in normal use concentrations.
- Destroy a wide range of microbes.
- Able to penetrate and disperse biofilm.
- Not affect the quality of fuel ( or lubricating qualities when added to lubricating oils).
- Mainly they should be water-soluble.
- They should contain dispersants to aid with the removal of debris.
- Environmentally friendly.
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.
Microorganisms do not die naturally they must be killed. Once the microbial infection is established on board 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
- Settling The microbes can settle out to the bottom of a tank because they have an sg of 1.05.
- Centrifuges. They can be efficiently removed with purifiers.
- Filtration Even though the microbes can be much smaller than the filter mesh it is still possible to remove them by proper staged filtration.
- Heat This is a function of both temperature and time at that temperature. A temperature of over 70’C for 20 minutes is effective in killing the microbes. However, this is difficult to achieve at the plate surfaces and it may be necessary to sterilise the tank first say by the use of steam lances before filling with oil for heat treating.
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 the 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.
- The suggested course of action for bilges suffering a microbial attack.
- Use commercial detergent hypochlorite bleaches to break down biofilms.
- Use broad-spectrum biocide to suppress all growth.
- Use narrow-spectrum biocides to target against SRB.
- Pump bilges regularly and prevent stagnation.
- Add alkaline nitrate cooling water treatments to the area where known SRB attack is occurring to reduce effects of hydrogen sulphide and other acidic by-products.
- It is advantageous to oxygenate the water using chemicals such as hydrogen peroxide.
- For ballast systems, the only effective method of elimination is the removal of sludges, muds and slimes.
Alternatives to biocides
- UV Radiation.
- Gamma and x-ray.
- Continuous pasteurisation and heat control.
- 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 concentration 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.
- 3-50 ppm Offensive odour.
- 50-300 ppm Injuries to eyes, respiratory tract, dizziness.
- 100 ppm Loss of sense of smell.
- 300 ppm Life-threatening.
- 700 ppm Rapidly lethal.
- 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