Since turbine engine fuels always contain water, microbial  contamination is always a threat. The large tanks of many  turbine engine aircraft have numerous areas where water  can settle, and microbes can flourish. Areas between the fuel  tank and any water that may come to rest in the bottom of the  tanks is where the microbes thrive. These microorganisms  form a bio-film that can clog filters, corrode tank coatings,  and degrade the fuel. They can be controlled somewhat with  the addition of biocides to the fuel.  [Figure 14-11]  Anti-ice  additives are also known to inhibit bacterial growth. Purity issues related to turbine engine fuels are unique.  While AVGAS experiences similar issues of solid particle  contamination and icing concerns, the presence of water and  fuel-consuming microbes is more prominent in jet fuel, which  has different molecular structure and retains water in two  principal ways. Some water is dissolved into the fuel. Other  water also is entrained in the fuel, which is more viscous than  AVGAS. The greater presence of water in jet fuel allows  microbes to assemble, grow, and live on the fuel.  14-9 Turbine Engine Fuel Issues Figure 14-10.  Petroleum products are produced by distillation. Various fractions condense and are collected at different temperatures  that correspond to the height of collection in the distillation tower. As can be seen, there are significant differences between turbine  engine fuel and ordinary AVGAS.  C$_{1}$ to C$_{4}$ gases 20 °C 70 °C C$_{5}$ to C$_{9}$ naphtha 120 °C C$_{5}$ to C$_{10}$ petrol 170 °C C$_{10}$ to C$_{16}$ kerosene 270 °C 600 °C >C$_{70}$ residue C$_{14}$ to C$_{20}$ diesel oils C$_{20}$ to C$_{50}$ lubricating oils C$_{20}$ to C$_{70}$ fuel oils GAS Fractionating column fractions decreasing in  density and boiling point Crude oil Fractions increasing in  density and boiling point Liquefied petroleum gas Chemicals Gasoline for vehicles and avgas Jet fuel, paraffin for lighting and heating Diesel fuels Lubricating oils, waxes, polishes Fuels for ships, factories, and central heating Biturnen for roads and roofing