Quick Answer:
If dark, gelatinous buildup shows up after the sink stops draining, the most likely place for it to come from is the trap or the drain line just beyond the trap. Inspect the trap for black or gelatinous material, clear it, then test the drain flow. If the problem keeps returning or you smell sewage, call a plumber.
Why This Happens
- Soap scum, hair, grease and organic matter collect inside the P-trap and drain pipe and form a slimy, black or gelatinous mass that can break loose after water carries bits through.
- Bacterial biofilms can form in drains, becoming a dark, jelly-like coating that can slough off when flow changes.
- Poor venting or partial blockages farther down the line can change flow patterns so material that was stuck gets pushed into the trap and then settles again when flow stops.
- If you notice pressure changes or frequent flow problems, see Sink drains then pressure builds or check the guidance on Drain stops after running water for related issues.
Step-by-Step What to Do
Step 1 — Prepare and protect the area
- Put on rubber gloves and eye protection.
- Place a bucket or large bowl under the trap to catch water and debris.
- Have rags and a small brush or old toothbrush ready for cleanup.
Step 2 — Inspect the trap
- Carefully loosen the slip nuts on both ends of the P-trap (hand-tighten is common; use pliers if needed) and remove the trap.
- Look inside for gelatinous or black buildup and any foreign objects (hair, food particles, etc.).
- If you find a lot of buildup, empty it into the bucket—do not push it further down the drain.
Step 3 — Clean the trap and nearby pipes
- Scrub the trap interior with a brush and rinse with hot water. For stubborn residue, use a mixture of hot water and mild dish soap or a diluted vinegar solution.
- Inspect and clean the trap arm and tailpiece by running a brush through them or using a small hand auger if reachable.
- Reassemble the trap and check all slip nuts for a snug, leak-free fit.
Step 4 — Test the drain and monitor
- Run hot water for a minute and observe drainage. Flush with a bit of cold water afterward to see if any material reappears.
- If the drain is still slow or material appears again, try a plunger or a hand-held drain auger for further clearing.
- Consider an enzymatic drain cleaner as a follow-up maintenance step; avoid caustic chemical cleaners if you plan to work in the trap again.
What Not to Do
- Do not scrape sludge into the drain without clearing the trap first.
- Don’t pour strong chemical drain cleaners into a trap you will open soon—chemicals can splash or react when you remove the trap.
- Avoid reaching into the trap with bare hands; use gloves and tools to remove material.
When to Call a Professional
- Repeated return of dark, gelatinous buildup after cleaning.
- Persistent or worsening foul odors that suggest a sewer issue.
- Slow drains in multiple fixtures or sewage backups—these may indicate a main sewer line problem.
- Leaks from trap connections you can’t stop or damage to pipes you can’t safely repair.
Safety Notes
- Wear gloves and eye protection when opening traps or handling drain debris.
- Ventilate the work area—steam and odors can be strong when cleaning drains.
- If you’ve used chemical cleaners, flush thoroughly and don’t mix different chemicals (dangerous reactions can occur).
- If you’re uncertain about sewer line issues or working under sinks in tight spaces, hire a licensed plumber.
Common Homeowner Questions
- Why did the buildup suddenly show up? Natural deposits can loosen when water flow or pressure changes, allowing chunks to move into the trap.
- Will boiling water fix it? Boiling water can help dissolve grease and soap scum but won’t remove all gelatinous or biological buildup—inspect and clean the trap if it persists.
- Are chemical cleaners safe to use? Use them cautiously; enzymatic cleaners are safer for routine use, while harsh chemicals can be hazardous and damage pipes.