Essential Oil Disposal
Within this article, all volatile oils including essential oils, absolutes and CO2s are collectively referred to as "essential oils."
Most consumers purchase and store essential oils in small bottles that usually don't exceed 15 ml (1/2 fl. oz) in size. When dealing with such small quantities of essential oils, it can be easy to forget that essential oils are highly concentrated, flammable substances that must be treated like other hazardous materials.
Although it's generally okay to freshen a drain by occasionally mixing 1-2 drops of essential oil with liquid dishsoap and pouring it down the drain as the water runs, this practice should be avoided if your sewer line cannot handle that. If you're not sure, don't take the risk. Never dump larger quantities of essential oil down your drain.
As with pharmaceuticals, paint thinner, household chemicals and gasoline, essential oils (aside from what is mentioned above) should not be disposed of down drains or via other methods that can cause the substances to come into contact with water supplies, vegetation or animals.
Essential oils don't go rancid, but most essential oils do deteriorate with age and should be used up prior to that time or disposed of properly. See the Essential Oil Shelf Life article for more information.
If you have aging essential oils on hand that you no longer want to use in skin care or direct diffusion applications, refer to AromaWeb's How to Use Up Those Aging Essential Oils article for ideas on how to still get some use out of some of your deteriorating oils.
Most likely, your city, county or community has guidelines in place for the proper disposal of hazardous fluid ingredients. Call your vicinity's waste management department for details. Most guidelines aren't hard to comply with once you're familiar with the process.
Your retailer or supplier should also have a Material Safety Data Sheet (MSDS) available for each essential oil that you've purchased. The MSDS provides specific safety, storage, usage and disposal information for each oil.