Disposal runs but water backs up into the sink during operation and drains away only slowly afterwards.

Safety first: Turn off power to the disposal.

• Plunge first: Use a sink plunger on the disposal side with the other sink (if present) blocked. Do not use chemical drain cleaners with a disposal.
• Remove and clean trap: Place a bucket under the McAlpine trap, undo the compression nuts, and remove the trap. Clean out all sludge and debris.
• Flush the pipe: Use a flexible brush or small drain snake to clear the horizontal waste pipe leading to the wall.
• Reassemble and test: Refit the trap with intact washers, tighten nuts, then run water and operate the disposal to confirm free flow.