What Should Not Go Down a Garbage Disposal

Grease and cooking oil

This is the big one, and it is not really about the disposal. Grease goes down warm and liquid and cools in the drain line, where it coats the pipe and collects everything that follows it. EPA is unambiguous: never pour cooking oil or grease down the drain.

The disposal grinds it perfectly well. The pipe fifteen feet away is where it stops.

Source: US EPA, How to Care for Your Septic System.

Fibrous vegetables

Celery, artichoke leaves, corn husks and onion skins do not break into particles — they separate into strings, and the strings wrap the impeller. That produces a hum without rotation rather than a blocked pipe.

Coffee grounds

They pass the disposal without trouble and then behave like fine sediment in the trap, packing down into a dense layer that water moves through slowly and then not at all. EPA lists coffee grounds among the things not to put into a septic system for the same reason.

Bones, stones and anything metal

Bones and fruit stones are the classic jam. Bottle caps, screws and teaspoons fall in unnoticed and wedge between the impeller and the grind ring, and metal against metal can damage the ring rather than just stopping it.

Starches in quantity

Pasta, rice and potato peel expand with water and turn to paste. A small amount is nothing; a colander of it is a blocked trap.

If your drains do not run to a public sewer

Everything above matters more, not less. Ground food waste is still solid waste: it settles as sludge and floats as scum, and both are what a tank is emptied to remove. EPA says plainly that using an in-sink disposal can affect how often a tank needs pumping, and that a household using one should expect to pump more frequently. The disposal is not the fault; the emptying interval is the thing to set with it in mind.

And the one that is not a rule

Cold water, not hot. Cold keeps fats solid enough to be ground and carried out rather than melting and re-setting further down the line.