To eliminate the musty/moldy smell coming from a bathroom sink drain without using chemical products like Drano, you can try the following natural and eco-friendly methods:
Baking soda and vinegar: Start by pouring about 1/2 cup of baking soda down the drain. Follow it up with 1 cup of white vinegar. The mixture will create a foaming reaction that can help dislodge buildup and odors. Let it sit for about 30 minutes, then flush the drain with hot water.
Boiling water: Boil a kettle or a pot of water and carefully pour it down the drain in two to three stages. The hot water can help flush away debris and eliminate some odor-causing bacteria.
Salt and hot water: Mix a few tablespoons of table salt with hot water and pour it down the drain. The abrasive nature of salt can help scrub away grime and odors. Let it sit for a while before flushing with hot water.
Lemon juice and hot water: Squeeze the juice of one or two lemons down the drain and follow it with hot water. Lemon juice has natural antibacterial and deodorizing properties.
Manual cleaning: If possible, remove the sink stopper or pop-up assembly and clean them thoroughly. Use an old toothbrush or pipe brush to scrub the inside of the drain pipe.
Regular cleaning routine: Prevent the buildup of mold and mildew in the drain by implementing a regular cleaning routine. Once a week, pour a kettle of hot water down the drain to keep it clear and odor-free.
Use a drain strainer: Install a drain strainer or stopper to catch hair, soap scum, and debris. This will help prevent clogs and the development of unpleasant odors.
Increase ventilation: Ensure proper ventilation in the bathroom to reduce moisture and humidity, which can contribute to mold growth and musty smells.
If you find that these methods don't completely solve the issue, there might be deeper plumbing problems that require professional attention. In such cases, it's best to consult a plumber to assess and address the underlying cause of the odor.