I would go about it two ways!

1.Vinegar and Baking soda down the hatch and when the foam stops hot water rinse.
2. Cut a lemon in half and disposal it completely

Keep in mind too that when it comes to a sink's garbage disposal- pouring frozen ice cubes down it weekly (while its running) helps keep the blades sharp.

If your elbow is intack it shouldn't be any smell from the sewer or septic tank. So your smell might be coming from the elbow itself. Does it drain very slow?