Found naturally in lemons, oranges and citrus fruit, citric acid is responsible for that tart flavour in many fruits and food products. Scientists use citric acid in laboratory settings while professional cooks use it for flavouring. If you need to dispose of old citric acid, find out whether your community has local regulations before you toss it in the trash to avoid accidentally breaking local laws. As long as you cover up with rubber gloves to avoid getting it on your skin, there's no danger in throwing out this substance.
Call your town's hazardous waste department to ask about proper citric acid disposal for your community. While citric acid is not hazardous, it can irritate the eyes and skin. Some towns have local laws regarding its disposal, notes JT Baker. If your town does not have specific regulations, proceed to Step 2.
Place leftover citric acid in a plastic container and seal the container. If you have the bottle with the citric acid label, use this instead of a plastic container.
Put the container of citric acid into a sealable plastic bag and close it tight. Then put the package into a second plastic bag, advises Cornell University. Even though the citric acid itself is not hazardous, you don't want anyone getting into it. Cornell University advises that often janitors working at chemical facilities don't know the different between hazardous and non-hazardous waste so careful packaging is essential.
Write "non-hazardous" on the outside of the bag using permanent marker.
Place the bag in your garbage bin. Dispose of the garbage bag on trash day by placing it out for kerbside collection or bringing it to a landfill facility, per your town's preferred trash disposal policy.
Oxford University notes that small amounts of citric acid can be flushed down a sink drain so long as your community does not prohibit this. If you prefer, flush citric acid down a sink drain after confirming this is an acceptable practice for your area.