03-microbes of microbiological attack

7/27/2019 03-Microbes of Microbiological Attack

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Microbes 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 fgone away. In addition whereas effects

were seen ainly in distillate fuels and lubricants, it is now seen in residual fuels, drinking andballast water. Failure to eradicate completely a re put down to advserse trading, poor training and

housekeeping, environmental restrictions in the use of microbial agents and the restrictions in

bilge pumping placed by MARPOL

MICROBES

Microbiological contamination consisting of bacteria, yeasts and moulds, are easily tolerated atlow 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:

1. Distillate fuel;2. Lubricating oil;3. Cooling water;4. Bilge water;5. Ballast water;6. Distillate cargoes.

Conditions promoting growth

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

Water

The main requirement for microbiol activity is water. This must be available water and not just

water content. bA 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 migrted 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

NutrientsHydrocarbons and chemical additives in the fuel and lubricant act as their food source. Inaddition to this are nutritive matter found in contaminated water either fresh or sea water. Sea


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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

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

EnvironmentMost corrosive forms of bacteria prefer astable environment and dislike adgitation. Thus ships in

lay up or ships that spend long periods inactive are particularly susceptible. Water leakage or

condenstaion will ten provide the living environment. The microbes live in the water phase butfeed on nutrients in the oil phase this the boundary area sees agressive growth.

IndicationsThe 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 characteristicsludge formed from accumulated cellular material which may restrict fuel and lubricant pipelines 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

Bacteria can be subdivided into

1. Aerobic Bacteria Require oxygen to survive.2. Anaerobic Bacteria Live in the absence of oxygen3. 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 timeof 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


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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 promoteSuulphate Reducing Bacteria which are particularly agressive.

Yeasts

These are unicellular, being ovoid or spherical in shape some may also produce rudimentary

filaments. They reproduce by budding and growing off the parent untill large enough to seperate.This process may take several hours.They prefer slightly acidity

Moulds

Multicellular with hard chitinous cell walls.They are ususally found as branched hyphaeforming

a thick, tough intertwined mat occurring most commonly at oil/water interfaces. They reproduceby 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 byproducts suitable for other microbes to feed and an atmosphre 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 prescence of other microbes. They willproduce hydrogen Sulphide in the prescence of sulhure containing compounds such as sulphatesfound in sea water.

Desulfotomaculum has the added ability to produce extremely hardy spores able to resistexposure to air, heat and most biocide chemicals. Both this and Desulfovibrio are very insiduous

and able to rapidly cause corrosion in ships hull and machinery

03-microbes of microbiological attack

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