Water can be found in fuel in three different forms: dissolved, free or course and emulsified. Fuel water separators are designed to remove them.
Dissolved water - The presence of dissolved water in petroleum-based fuels usually has an upper limit of 100 to 200 part per million (ppm), Braswell says, but biodiesel can contain up to 1,500 to 3,000 ppm of dissolved water. Dissolved water is not known to have any damaging effect on fuel system components.
Free or coarse water - Free or coarse water does not have a universal definition, he observes. However, these terms are typically used to describe water that is not fully dispersed throughout the fuel.
Emulsified water - Emulsified water does not have a universally accepted definition either. Nevertheless, says Braswell, this term is typically used to describe when water and fuel form an emulsion in which the water droplets are so tiny that they remain in suspension for a very long time. In high enough concentrations, it can make the fuel appear milky.
