Whether to grind eggshells in a garbage disposal is a topic of debate among plumbing experts. The shells do not actually sharpen the blades. Instead, the matter centers around whether the shells keep the grinders and plumbing system clean.
What Proponents Say
Eggshells are coarse and mildly abrasive. As a result, some plumbers advocate tossing a few down the disposal and grinding them. They say the rough texture of the shells helps clean the grinding mechanisms inside the disposal. While the shells are not tough enough to actually hone the steel grinders, removing food buildup helps the grinders tear through food waste more efficiently.
Why Opponents Do Not Recommend
Other plumbers take a different point of view, expressing two fatal flaws attached to tossing eggshells down the disposal. The first is the membrane on the inside of the shell. Plumbers in the "do not recommend" camp say the membrane sticks to grinders, lessening how well they cut food waste, especially over time. The other problem they see is that finely ground eggshells have a tendency to build up inside the pipes, clogging them with shell debris if you toss enough shells down the drain.
One option for keeping those grinders clean while eliminating any possibility of added buildup or clogged pipes is ice. Drop a handful of ice cubes into the disposal and turn it on. The cubes challenge the grinders enough to clean them of debris, and they melt so no remnants clog the pipes.
Other Dos and Don’ts
Whether you decide to try using eggshells in the disposal, one recommendation from many plumbers is that you rinse them down the unit with cold water. Warm water has a tendency to separate the fats from some food waste, and those end up coating the disposal's grinders and walls. Avoid grinding bones, which can jam the unit, as well rice or pasta, which swells with water and could pack against pipe walls. Grease or fats from cooking eventually solidify, probably inside the drain pipes, so you do not send them down the disposal either. Stringy vegetables such as celery or lettuce grind when disposed in small quantities, but large doses can leave a mess inside the disposal. Soft foods, discarded ends after trimming vegetables and a few orange peels should go through just fine. The latter has the added benefit of freshening the disposal with a clean scent.