Most cooking oils shouldn’t be composted – but a few can be composted in small amounts.
The benefit of adding waste cooking oil to a compost heap are vastly outweighed by the potential problems. Not only can they attract rodents and other undesirable creatures to your compost heap, they can cause problems for the composting process: the oils can form water-resistant barriers around other material, displace water and reduce air flow in a heap, thus slowing down the composting process.
However, vegetable oils such as rapeseed oil, sunflower oil, corn oil or olive oil can be composted in very small quantities as long as they haven’t come into contact with any forbidden food stuffs (like meat, cheese or fish). If you’ve soaked up a bit of oil with some kitchen roll, that’ll be ok to add but don’t pour the contents of a deep fat fryer into your compost bin.
Large quantities of used vegetable oil can be recycled – your local council should be able to advise if it is possible in your area.
Some people use leftover cooking oils that are solid at room temperature to make bird feeders (fat balls) for the garden.
Waste cooking oil should NOT be poured down the sink – it can clog pipes and sewers.