The actions of biosurfactants often cause haziness in jet fuel. Biosurfactants are produced by the living cells of organisms, and have the properties of reducing surface tensions and stabilizing emulsions. An increase of biosurfactants will increase the solubility of a solution. Solubility is the amount of a solute that will dissolve in a specific solvent under given conditions---in this case, water into jet fuel. The most common type of jet fuel---Jet Fuel A---has a composition that allows the easy absorption and suspension of water.
Fine water droplets in suspension throughout the fuel will give the fuel a "hazy" appearance; an increased amount of water in solution will result in a cloudy appearance. The production of biosurfactants by organisms causes an increase in the solubility of water in the fuel, as can the addition of surfactants (soap or detergent-like compounds) through the refinery process, transportation, or fuel additives. Typically, this water will settle out over time---water is heavier than jet fuel---or can be vacuumed out by special pumps. Significant amounts of biocide or water dispersant additives present in the jet fuel will increase the time required to settle out the water.
Standard jet fuel contains a number of additives: anti-knock, anti-oxidant, static dissipaters, corrosion inhibitors, metal de-activators, and anti-icing and biocide additives. These last two are especially important because anti-icing additives may have certain biocide properties. Biocides are designed to destroy organisms, and are added to jet fuel to combat microbiological growths.
Not removing water from jet fuel can lead to microbial growth and freezing. Since jet fuel usually contains small amounts of water already, and since water can condense from jet fuel at lower temperatures, fuel piping and tanks are designed to allow water to accumulate at low points for later removal.