Quick Answer:
If you drain one basin and dirty water appears in the other, stop using the sink and avoid food prep in that basin. This usually means a partial blockage or a plumbing connection problem that is letting water flow between bowls instead of out the drain. You can try a few simple checks yourself, but clean the trap and inspect for buildup at the junction as an early step.
Why This Happens
- Most double-bowl sinks share a common drain or a crossover fitting between the two basins. If that path is partially blocked, water can be forced from the used bowl into the other basin.
- Issues after work on the plumbing or when solids collect near the shared junction can reroute flow. This can happen with food particles, grease, or mineral buildup causing a partial seal.
- Garbage disposals and improper connections can create backflow situations, for example **Cross-flow when grinding food** or **Cross-flow after plumbing repair**.
Step-by-Step What to Do
1. Stop using the sink and isolate the area
Turn off any water going to the sink and remove dishes. Place towels or a shallow pan under the trap to catch spills. Don’t let water run until you’ve checked the drain.
2. Clean the trap and inspect for buildup at the junction
Put on gloves and a pair of safety glasses. Place a bucket under the P-trap (the curved pipe under the sink). Loosen the slip nuts by hand or with a wrench, remove the trap, and empty its contents into the bucket. Clean the trap thoroughly and look where the two sink tails meet at the crossing or junction for hair, grease, or trapped food. Reassemble the trap securely and test with a small amount of water.
3. Check the garbage disposal and tailpieces
If you have a disposal, inspect the inlet for trapped food. Remove visible debris from the sink drains and the upper sections of the tailpieces before testing again. Never put your hand inside a disposal unless it is unplugged and powered off.
4. Try a gentle mechanical clear
If water still backs up, use a sink plunger (cover the other bowl with a damp towel first) to attempt to dislodge the blockage. Avoid harsh chemical drain cleaners — they can damage pipes and are hazardous to handle.
5. Re-test and monitor
Run a small amount of water in one bowl and watch the other. If water still flows into the opposite basin or the drain remains slow, you’ve either missed the clog point or it’s deeper in the line and may need professional attention.
What Not to Do
- Do not use the clean bowl for food prep until resolved.
- Do not pour reactive chemical drain cleaners into a clogged sink — they can burn skin, create dangerous fumes, and harm pipes.
- Do not put your hand into a garbage disposal unless the unit is unplugged and locked out.
When to Call a Professional
- Problem continues after cleaning the trap and trying a plunger.
- Multiple fixtures are slow or backing up (sinks, tubs, toilets) — this may indicate a sewer or main line issue.
- Repeated backups occur after repairs, or you suspect damaged or misaligned piping.
- There is a foul sewer smell that you can’t locate or eliminate.
Safety Notes
- Wear gloves and eye protection when working under the sink.
- Keep a bucket and rags handy to catch water; work on a dry, stable surface to avoid slips.
- Turn off electrical power to a garbage disposal before working on it.
- If you use any cleaners, follow the label directions and ventilate the area. Prefer mechanical cleaning over chemicals.
Common Homeowner Questions
- Will running the disposal make the backup go away? Not always — it can help if the problem is trapped food near the disposal, but it can make things worse if the clog is farther down or the disposal is jammed.
- Can I disinfect the bowl after this happens? Yes — once the cause is fixed and the basin is thoroughly cleaned, sanitize with a mild bleach solution or a household disinfectant and rinse well. Don’t use the basin for food prep until you’re confident the issue is resolved.
- How long should the sink work normally after I clear the trap? If the trap removal and cleaning removed the obstruction, the sink should drain normally immediately. If it remains slow, the blockage is deeper and may need a plumber.