Any reference herein to a diesel fuel must be interpreted to mean a diesel fuel including a diesel oil as a major portion thereof. It is well known that diesel fuel decomposes with age. This is characterized by the fuel turning brown in colour and depositing a film of brown resin on all surfaces in contact with the fuel. Diesel fuel, being hygroscopic, absorbs moisture from the air. Water is at the root of the process by which diesel decomposes and is responsible for a number of other processes that are extremely detrimental to both the fuel and diesel engines which use the fuel. Other than the obvious problems of being non-combustible and the potential for freezing, water in diesel fuel causes corrosion of fuel system components, reduces the lubricity of the fuel and, provides habitat for microbial contamination.

Water which collects in diesel fuel exists in the following forms:

• "Free water" which typically settles at the bottom of a storage tank in which diesel fuel is stored and comprises water which has separated from the diesel fuel due to condensation.
• "Suspended water" which is made up of microscopic “particles” of water that are suspended in the diesel fuel. Suspended water can form as molecules which drift together in fuel or when agitation causes free water to break up into minute droplets small enough to become suspended in the fuel.

Both forms of water produce undesirable effects in diesel fuel and, particularly, in diesel engines. The “suspended water” molecules are neutrally buoyant and remain suspended in the diesel. Due to their microscopic size, water molecules pass, unaffected, through water separators and filters into the fuel injection system of diesel engines, where the water molecules are corrosive and abrasive. Injector nozzles become enlarged by water abrasion thereby altering the volumetric efficiency of the injector nozzles. Corrosion due to water also affects fuel lines and fuel pumps, all of which require expensive remedial maintenance as a result. Corroded fuel pumps and fuel injectors have an adverse effect on the fuel efficiency of diesel engines.

Oxides of sulphur and nitrogen are produced in the combustion chambers of diesel engines. Sulphur which forms a constituent part of diesel fuel is a good lubricity agent and anti-oxidant for diesel fuel. Nitrogen comprises approximately 78% of intake air of a diesel engine. On their own, these oxides are gases and they are harmless to engine oil and diesel engine components and would normally be blown out through the exhaust system and crankcase breather. When water is present in diesel fuel however, sulphuric acid and nitric acid is formed from sulphur and nitrogen, respectively. Much of the acid is carried into the exhaust system and the environment. Some of the acid is carried past the piston rings via the blow-by into the engine oil, causing decomposition of the oil and a reduction in the lubrication properties of the oil, resulting in the formation of sludge in the fuel. Sulphuric and nitric acid is circulated throughout the engine causing corrosion of vital engine components such as bearings, piston rings, cylinder liners, cam-lobes, cam-followers and tappets as well as causing the hardening of oilseals. Sulphuric acid also causes oxidation of diesel fuel resulting in the precipitation of resin which is deposited on surfaces in contact with the diesel fuel. This is commonly known as “gumming”. Gumming in the fuel pumps and fuel injectors of diesel engines has an adverse effect on fuel atomization and spray-pattern and also on the volumetric efficiency of the fuel injectors. This typically leads to an oversupply of fuel to the combustion chamber, characterised by the emission of black smoke (incompletely burned fuel) from the exhaust. The logical outcome is loss of fuel efficiency and increased fuel consumption.

Sludging of engine oil in the crankcase of a diesel engine, is a further well known problem. Sludge is formed by a combination of resin, released by the decomposition of engine oil by acid, and soot caused by incomplete combustion of fuel. Sludge increases the viscosity of engine oil, impairing free flow through the engine, blocking oil filters and, in severe cases, blocking oil passages and galleries, leading to engine failure. As such, acid breakdown of engine oil leads to decreased lubricity and accelerated engine wear. The problem of acid contamination therefore remains serious. All diesel fuel contains some water. Additional water accumulates through water vapour and condensation droplets. Microbes depend on water for growth and on the organic and inorganic molecules in diesel for nutrition. Some species attack the fuel directly, consuming hydrocarbons and other fuel components. Growth is generally most abundant at the fuel-water interface. The biodegradation of diesel fuel because of microbial growth, is directly related to the contamination of diesel fuel by water. Colour, heat of combustion, pour point, cloud point, detergent and anti-corrosive properties change as microbes selectively attack fuel components. Sulphur-containing molecules are metabolized by a series of microbe species, leading ultimately to the production of high concentrations of hydrogen sulphide. A variety of organic acids are also produced as by-products of bacterial and fungal growth. As the acids accumulate, they cause a number of indirect adverse effects on diesel fuel.

As bacteria and fungi reproduce, they form biomass, which accumulates at the fuel-water interface, on tank surfaces and on filters. There are several important indirect adverse effects of biomass and slime production on diesel fuel. As biomass turns over and as metabolic waste and dead cells accumulate, they settle out as sludge, which accumulates on tank bottoms. Microbial influenced corrosion, sulphide production and organic acid accumulation occurs beneath the sludge. The types of microbes dominating a particular sludge appear to depend on the physical-chemical conditions of the sludge. Once sufficient sludge builds up, sludge particles will be drawn out with the diesel fuel. Often, filter and line blockages result from bio-film formation on fuel-line walls and filter-matrix surfaces, causing reduced filter life and fuel-starvation resulting in unscheduled maintenance and downtime.

The organic acid produced chemically breaks down the fuel, releasing resin molecules that drift in the fuel and adhere to solid surfaces. Filter elements become clogged with sticky resin to which other impurities (e.g. rust, dust and microbes) adhere. Without resin, solid impurities would fall away from the filter element and settle at the bottom of the filter housing. As described hereinabove, water in diesel fuel becomes acidic, which not only breaks down the diesel fuel, but also accelerates the corrosion of metals which come into contact with the fuel. Most diesel fuel contains rust in dust-sized particles in suspension, which originates from the bottoms of metal bulk storage tanks that become dislodged during the filling of the tank. Much of this rust is trapped by the fuel filters of diesel engines but some of the rust particles are often small enough to pass through the fuel filter into the fuel-injection system, where it is highly abrasive.

Contamination of diesel fuel can occur during transportation, storage, dispensing and during manufacturing or induced in the field during maintenance repair and overhauls. The most effective maintenance practice is to minimise the exposure of diesel fuel to water. In some systems (for example, seawater-ballasted marine fuel tanks) this is impractical. However, tank insulation, recycling through water separators and routine discharge of water from tank bottoms, all minimise water accumulation in vehicles/vessels and fuel storage tanks.

As fuel is drawn from a tank, air or water is drawn in to compensate for the vacuum that would otherwise be caused by the removal of the fuel. This is the most common means by which contaminants are introduced into both bulk storage and service tanks. Bacteria and fungi are carried through the air either attached to dust particles, entrapped in water droplets, or as discrete aeroflora. Storage tanks also become contaminated when transferring contaminated fuel into uncontaminated tanks.