To get rid of the nasty black gunk in your drain: 1. Boil a pot of water. 2. Meanwhile pour baking soda down the drain. 3. Once the water has reached a rolling boil pour it down the drain. 4. Let the water and baking soda work at decomposing and sterilizing the grime for 10 minutes. 5. After 10 minutes, pour 1 cup of bleach down the drain to completely kill any remaining mildew or bacteria.