In most countries vast amounts of waste vegetable oil (WVO) are discarded daily after it has been used for cooking. To give some idea of the waste, Ireland disposes of over 10,000 tonnes of WVO per year, and six times that amount of animal-based cooking fats (tallow).

Disposal of this waste can be a problem – pouring it down drains (illegal in most countries) eventually results in solid fats forming in sewers, which is expensive to remove, and is likely to become a public health hazard. Solutions have included pouring WVO into landfill sites where it will eventually biodegrade, or filtering it and using it as low-grade fuel for some types of power stations.

However, WVO has a hidden secret; using some simple techniques of adding something and shaking, it can be broken down into two very useful products: diesel fuel and glycerine.

The magic 'something' is easily available, it can even be made at home using wood ash and water. It has a number of names – sodium hydroxide, caustic soda, Lye and NaOH are all the same thing. Warning – this stuff is also used for cleaning drains by dissolving organic matter. It doesn’t care whether the organic matter is dead or alive, so wear protective clothing especially goggles

The basic steps are:

1. Test your WVO to decide how much sodium hydroxide to use. This is known as titration, and involves adding a chemical to a sample of the oil until a colour change takes place. It's not essential but allows more efficient use of the sodium hydroxide.

2. Mix up the sodium hydroxide, slowly add the WVO, keep it agitated, and keep it reasonably warm. The gentle heat speeds up the process.

3. Let it settle.

4. Separate the fuel. The remaining glycerine can be used for making soap and hand cleaners. It is also biodegradable.

The resultant diesel is ready for use, and in this form it can be mixed with regular diesel for use in vehicles. To use 100% bio-diesel, it will need to be washed with water, then the water driven off with gentle heat.

On 26 July 2002, a new duty rate for biodiesel was introduced at 20 pence per litre below the rate for ultra low sulphur diesel (ULSD). Until then, biodiesel for road use was taxed at the same rate as ULSD.

The only reported downside of using bio-diesel is that everyone who smells the exhaust feels hungry...