Waste vegetable oil (WVO), is used cooking oil that is often discarded by restaurants and businesses that deep-fry a lot of food. It can be used to fuel diesel vehicles and for a variety of other purposes.
In vehicles
Diesel vehicles can be converted to run on vegetable oil. However, vegetable oil is thicker than diesel fuel and may not burn efficiently in the engine, leaving burnt residue in the engine that will eventually break it. In order for vegetable oil to burn efficiently, it must be hot when it enters the fuel injectors. Conversions employ various strategies to ensure this.
Filtration
Particles (e.g. of food that was cooked in the oil) and water in the WVO will damage the engine. Therefore WVO must be filtered and water must be removed before it can be used as fuel. Some people allow the oil to stand (e.g. in barrels) for about a week to allow water to separate from the oil; it can then be drained off through a spout at the bottom of the barrel. Standing also allows particles to separate out from the oil and drift to the bottom of the barrel; this is a first step in filtration.
After standing, the oil is filtered, first through a large filter and then through filters with progressively smaller holes in order to get out small particles that would collect in the engine and damage it. The oil can be filtered multiple times.