What is the primary function of a marine fuel water separator / decanting filter (filtro decantador de agua / prefiltro) installed between the fuel tank and the marine diesel engine?

1. To separate free water and heavy solid sediments from the diesel fuel before it reaches the fine engine fuel filter and high-pressure injection pump
2. To mix sea water with diesel fuel to improve exhaust emissions
3. To heat the fuel to 100°C to evaporate ethanol
4. To measure the speed of the vessel through the fuel lines