The tips provided by Maira in the above response will likely help.
Here is a procedure that I recommend:
Run some hot water through the garbage disposal to clear out any debris that may be in it.
Place a whole lemon or small orange in the disposal (skin still on it).
Without running any water through the disposal, pulse it on and off to agitate the lemon (or orange). A few seconds on and then off for a while should do the trick.
After six or seven repetitions, you will likely smell a strong citrus odor indicated that the fruit is broken down inside the disposal. Let the item sit and soak for about 1/2 hour.
Run some ice cubes through the disposal and finish running the lemon through it. You can use some water to help dispose of the lemon. If the lemon won't break down and go through the disposal, you may have to pull it out after this process.
The ice will help clean and sharpen the disposal blades. The lemon or orange will help eliminate the odor and kit the mildew smell.
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