Biodiesel and other fuels have a certain amount of water present, but the amount needs to be carefully controlled.  Water present in FAME, both free and bonded, can lead to the formation of free fatty acids and corrosion of engines and other fuel storage equipment.  Unlike petroleum based diesel, biodiesel is also hydrophilic, leading to a greater chance of microbial growth and degradation.  Appreciable amounts of sediment can plug fuel filters, and cause abrasive damage to fuel injection components if small enough to pass through the filters.  Both water and sediment can also cause fouling of fuel-handling facilities and accelerate oxidation.

Significance

The referee test method for water and sediment detection in biodiesel is ASTM D2709.  A 100 mL sample is placed in two identical tubes and centrifuged at a relative centrifugal force of 800 for 10 minutes at room temperature.  After the 10 minutes, the specially designed tubes are removed and the amount of water and sediment at the bottom of the tube is read off directly (down to 0.005 mL).  ASTM test method D1796 is another method that uses a centrifuge and is an alternative acceptable method.   ASTM D6751 allows for a maximum of 0.05% (by volume) of combined water and sediment in a biodiesel sample.