Can You Compost Vegetable Oil?

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Vegetable oils are used frequently in cooking. There are 12 major cooking oils derived from vegetables, including olive oil, rapeseed oil, sunflower oil, peanut oil, coconut oil, and sesame oil. 

Can you compost vegetable oil? You can compost used or unused cooking oils in very small amounts, but we advise against adding too much because it can slow down the composting process. Oils take a long time to decompose fully and also reduce airflow. Less airflow eventually leads to anaerobic decomposition, which is slow and smelly.

Why is too much vegetable oil bad for compost?

Vegetable oil is organic, so it will break down in compost. But adding too much vegetable oil will slow down the composting process.

You’ll know from washing up oily pans that oil and water don’t mix. The same is true of oil and water in compost. In a compost bin, vegetable oil coats the other materials creating a water and oxygen resistant barrier.  

oil in compost
Oil will coat other materials in a compost bin

If you add to much oil, the reduction in airflow will eventually turn your compost anaerobic. Anaerobic composting is slow, doesn’t smell good, and can leave you with a pest problem.

If the vegetable oil has been used for cooking meat or fish, you need to be even more careful with composting it because rotting meat is very attractive to rodents.

Also, the chemical structure of oils are very complex and take the microbes a long time to break down. Therefore, vegetable oil will stick around in your compost for quite a while, which makes the above issues even more problematic.

How to compost cooking oil

You can compost cooking oils in small amounts, as long as they’re a vegetable oil. Never accidentally compost a synthetic oil. Olive oil, coconut oil, rapeseed oil, sunflower oil, peanut oil, and sesame oil are all derived from plants, so they’re suitable for composting.

How much vegetable oil you can add to your compost depends on the size and health of your pile. If you have a big healthy compost pile that’s going to reach temperatures above 120 degrees Fahrenheit, then you’ll be able to get away with adding more oil than somebody with a small pile that’s already turning anaerobic. 

how much oil can I compost
We advise only composting small amounts of oil

In general, adding a paper plate with some grease on, or a paper towel you used to mop up some spilled oil should be fine. But throwing in the oil you used to deep fry your chips with isn’t a good idea. Always make sure oil only makes up a tiny percentage of the total waste you’re putting in. 

If you’re adding oil, you should be aspiring to hot compost as this will break the fat down faster. Hot composting is the same as traditional composting but requires you to pay more attention to things like the ratio of browns:greens in your pile and how often you turn the compost. 

Can you add vegetable oil to a worm bin?

Cooking oils should not be added to a worm bin. Vegetable oils go rancid quickly once they’ve been exposed to air, and the smell will put both you and the worms off. Worm bins tend to be smaller systems so any bad smells are exaggerated.

Did you know worms breathe through their skin? If there’s an excess of oil in the bin, this can coat the worms skin and make it hard for them to breathe.

Always rinse any oily food before putting it in your worm bin and try to get rid of as much of the oil as possible. If the food is absolutely dripping in oil, then consider disposing of it in a different way.

Other ways to dispose of cooking oil

Thankfully, there are lots of alternative ways of getting rid of both used and unused vegetable oil, so you shouldn’t find yourself with too much to compost. 

how to dispose of cooking oil
There are multiple ways to reduce and reuse cooking oil

The 3 R’s of waste disposal are reduce, reuse, and recycle (in that order). The best thing you can do is to reduce the amount of oil you use in the first place. There are a few ways to do this, including:

  • Using a different method of cooking such as steaming or baking
  • Using an oil spray instead of a pouring oil
  • Using non-stick cookware 

You can reuse cooking oil to cook something else as long as you haven’t heated it past its smoking point. To reuse it, let the oil cool and then strain it through a cheesecloth to get rid of any unwanted debris. Put the filtered oil in an airtight container and store it in the fridge to maximize its shelf life.

If you don’t want to use old oil in your cooking, then you can feed it to birds. Cooled down vegetable oil makes a great glue to stick all the seeds together. Apparently, you can also use old cooking oil to make soap, although we haven’t tried this ourselves yet.

And lastly, if all the other options are exhausted, you can use Earth 911 to find somewhere local to you that will take oil for recycling. Lots of places use vegetable oils to make bio-fuel.

About Veronica Fletcher

Veronica has a passion for all things eco-friendly. After growing up on a farm in Ireland, she went on to study Chemistry and Environmental Sciences. Veronica has also volunteered in many sustainability roles, including conservation efforts in Bangladesh and teaching Environmental Sciences to schoolchildren in Kenya.