This information is taken from Michigan State University Extension Bulletin 'E-2555', dated March 1995, which is part of the Household Hazardous Waste Series.
NEVER DISPOSE OF GASOLINE BY POURING IT ON THE GROUND, POURING DOWN THE DRAIN OR THROWING IT IN THE TRASH. NEVER MIX GASOLINE WITH OTHER PRODUCTS
HOW DOES GASOLINE BECOME A WASTE?
Any time gasoline is not used for its intended purpoese, it becomes a hazardous waste. Usually this happens when gasoline has been stored for a very long period of time or has been contaminated with rust, dirt or water.
WHAT HAPPENS TO GASOLINE WHEN IT IS STORED?
As gasoline ages, some of its components evaporate, leaving behind a thickened material like a lacquer or varnish, which is difficult for an engine to burn. To avoid this problem, store gasoline in an air-tight container. This will prolong the life of the gasoline. If it is less than six months old, it probably hasn't evaporated much and can be used.
HOW DO I USE OLD GASOLINE?
Old gasoline can be restored by diluting it with at least three times as much fresh gas. For example, one quart of old gasoline should be mixed with three quarts of fresh. Generally, the more the old gas is diluted, the better the engine will run. This may be done right in the gas tank if it is large enough to hold the necessary quantities. Otherwise, combine the gas in an approved gasoline container.
Work outside away from flames and heat. DO NOT SMOKE WHILE WORKING.
If you wish to use the old gas in your car, add approximately one gallon of old gasoline each time you fill the tank, until the old gas is gone.
If you have usable gasoline that you do not wish to use, give it to someone who will. Some large trucks, buses, and farm machinery can burn old fuel.
WHAT IF I HAVE 2-CYCLE GASOLINE?
Since 2-cycle gas is actually a mixture of gasoline and oil, it can be used only in engines that specifically call for this kind of mix. As with plain gasoline, old 2-cycle gas can be diluted and used, but more 2-cycle gas must be added to keep the relative proportions of gas and oil the same. Do not dilute 2-cycle gas with straight gasoline or use it in engines for which it is not intended.
WHAT IF MY GASOLINE IS CONTAMINATED?
Dirt, rust, or other solids can be removed form gasoline by filtering it through two layers of thin cloth or a coffee filter.
The cloth or paper filters can be allowed to dry in a well-ventilated area away from children or pets, and disposed of in your regular trash.
GAS AND WATER DO NOT MIX. Water in a gas can sinks to the bottom. Gasoline that has been contaminated with small amounts of water can be restored by adding a fuel dryer. This is available at hardware, auto parts or lawn mower/equipment stores.
Gasoline with larger amounts of water can usually be salvaged by allowing the gas to sit undisturbed for one to two weeks until all the water settles to the bottom. Then carefully pour or siphon off as much gas as possible, leaving mostly water in the bottom of the original container. The remaining material should be handled as a Household Hazardous Waste.
WHAT IF I HAVE GASOLINE I CANNOT USE?
If you have gasoline you cannot use, it should be taken to a Household Hazardous Waste Collection location. It must be brought in an approved gasoline container. Contact your local health department, city hall, fire department or MSU Extension conty office for information and assistance in disposing of your unwanted gasoline.