Quick Answer:
A slippery black film around the drain is usually organic buildup (food, grease, soap scum) or mold/mildew fed by trapped moisture. Start with simple cleaning, then run targeted diagnostics to check the disposal, dishwasher connection, venting behavior, and sink-specific parts. If the film returns quickly, or if you see slow drainage, persistent odor, or leaks, call a plumber.
Why This Happens
- Food and grease collect near the drain and form a slimy biofilm that can go dark with trapped debris.
- Poor drainage or a partial clog keeps moisture in the tailpiece and P‑trap, encouraging mold and bacterial growth.
- Appliance connections (dishwasher or garbage disposal) can push food particles into the drain line.
- Blocked or poorly functioning venting can slow drainage and produce gurgles that indicate trapped air and lingering moisture.
- Damaged gaskets, loose sink flange, or cracked tailpiece let water sit where it shouldn’t and promote film growth.
Step-by-Step What to Do
1. Observe and document
- Note when the film appears (after cooking, after dishwasher runs, continuously).
- Smell the area and check for other symptoms: slow drain, gurgling, leaks, or visible mold under the sink.
- Try to remove a small sample with a paper towel and note color, smell, and texture (do this with gloves).
2. Clean the visible film safely
- Wear gloves. Scrub the drain area and stopper with a soft brush, mild dish soap, and warm water.
- For stubborn residue, use baking soda and white vinegar: sprinkle baking soda, pour vinegar, wait a few minutes, then flush with hot water.
- Run the disposal (if present) with plenty of cold water while grinding small citrus peels or ice to help dislodge buildup — only if the unit is working normally and there are no hard obstructions.
3. Run targeted diagnostics on connected appliances
- Dishwasher: Run a short cycle and watch the drain connection under the sink for leaks or discharge. Check that the dishwasher drain hose has a high loop or air gap to prevent backflow.
- Garbage disposal: Inspect inside the disposal for trapped food and clean around the splash guard. If the disposal is noisy, clogged, or not grinding well, it can push material into the tailpiece.
4. Check drainage and venting behavior
- Fill the sink halfway and release. Note how quickly the water drains and whether there are gurgling sounds from the drain or other fixtures. Slow drain or gurgling suggests a partial clog or vent issue.
- To test venting briefly: run water in several fixtures at once (kitchen plus bathroom) to see if air noises persist. Persistent gurgling or slow flow may indicate a blocked roof vent or failing air admittance valve.
5. Inspect fixture-specific components
- Under the sink: look at the tailpiece, P‑trap, slip nuts, and sink flange for corrosion, film, or slow leaks. Remove and inspect the P‑trap if you’re comfortable — place a bucket below first.
- Check rubber gaskets, the sink stopper mechanism, and splash guard. Food and grease collect in these parts and should be cleaned or replaced if damaged.
- If you remove the P‑trap, flush it and the tailpiece with hot water and use a bottle brush to clear remaining film.
6. Test and monitor
- After cleaning and minor repairs, run hot water and then cold water. Watch for reappearance of the film over the next few days.
- If the film returns quickly, narrow the cause by isolating appliances: don’t run the dishwasher or disposal for a few cycles and see if the film reduces.
What Not to Do
- Avoid forcing fittings or overtightening slip nuts; that can crack plastic parts or damage threads.
- Do not use caustic drain chemicals (lye or strong acids) to try to dissolve the film — they can damage pipes, gaskets, and disposals, and are hazardous to handle.
- Don’t ignore early warning signs like persistent odor, recurring film, slow drainage, or gurgling — small problems often get worse and become more expensive to fix.
When to Call a Professional
- If you find a leak you can’t stop, or water that won’t drain despite clearing the trap.
- If the garbage disposal is jammed, damaged, or emits burning smells; or if the dishwasher drain keeps backing up.
- If the film and odor return quickly after cleaning, suggesting a deeper clog, damaged venting, or sewer line issue.
- If you see extensive mold growth, or if anyone in the home has respiratory problems related to the odor — consider a plumber or mold remediation specialist.
If you want more detail on mechanical sink problems or odors, see **won’t hold water** and **smells like grease** for related troubleshooting steps.
Safety Notes
- Always wear gloves and eye protection when inspecting drains and under-sink areas.
- Turn off electrical power to the garbage disposal at the switch or breaker before placing hands or tools inside it.
- Do not mix cleaning chemicals. Never pour hot water into a drain immediately after a chemical drain opener — that can cause splashing and harmful fumes.
- When removing the P‑trap, put a bucket under the trap to catch water and dispose of waste safely.
Common Homeowner Questions
- What causes the dark color? Organic matter and oxidized grease mixed with bacteria and dirt create a dark, slippery film.
- Will bleach remove it? Bleach can disinfect surfaces but won’t remove grease; mechanical cleaning plus degreasing is more effective. Use bleach sparingly and never with other chemicals.
- Can I prevent it? Regularly rinse the drain with hot water after use, avoid pouring grease down the sink, and clean the stopper and splash guard weekly.