Waste Vegetable Oil (WVO) is vegetable oil that has become unfit for food preparation.

The common causes of this degradation are:
Chemical degradation, including
Oxidation.
Hydrogenation.
Accumulation of contaminants.

Waste Vegetable Oil's uses include:
Animal feed additive.
Fuel for waste-to-energy plants.
Use as a biofuel, especially:
As feedstock for the production of biodiesel by transesterification.
As fuel for diesel engines modified to accept unprocessed vegetable oil as straight vegetable oil (SVO) fuel.
As a replacement for home heating oil.
As feedstock for the production of soap.
 

Contents:
1 Animal feed
2 Use in diesel engines
3 The quantities involved
 

Animal feed

As of 2003, the use of Waste Vegetable Oil (WVO) in animal feed is no longer permitted in the European Union, but continues in the United States.

Use in diesel engines

One of the first demonstration diesel engines ran on peanut oil. However, modern diesel engines are designed to run on petroleum diesel. They run poorly on unmodified vegetable oil, with a risk of damage - the fine ports of injectors can be clogged by carbon which forms from the slow or incomplete combustion of heavier fractions of vegetable oil, while the injection pump may suffer premature wear due to the fuel's relatively poor lubricating properties.

There are two solutions to this, both of them technically successful and growing in popularity:
Convert the oil to biodiesel.
Convert the engine to run on straight vegetable oil.

The quantities involved

As of 2000, the United States was producing in excess of 2.9 billion gallons of Waste Vegetable Oil (WVO) annually, mainly from industrial deep fryers in potato processing plants, snack food factories and fast food restaurants.

Waste Vegetable Oil (WVO) has a stable market value of approximately $0.40 per US gallon or $120 per metric ton as of 2003, enough to make collection economically viable.

If all those 2.9 billion gallons could be collected and used to replace the energetically equivalent amount of petroleum (a rather utopian case), almost 1% of US oil consumption could be offset.