There are many differing opinions on this. The simple solution is not to do it since you can fill your septic with things that shouldn't be there. That said, many people think a waste disposal unit and septic are a find combination so long as you use a disposal unit that is designed for a septic, like the InSinkErator Evolution Septic Assist Garbage Disposal, and you make sure that you only put biodegradable, mineral-light items in the disposal. So vegetable matter is fine, but grease and egg shells definitely are not.