My tenant e-mailed me that the kitchen sink drain, non-disposal side, smells.
I went there and confirmed it.
He had previously tried to find the source of the odor by removing the pipes down to the P-Trap. He had run various cleaning liquids down the drain which worked for a while but then the odor came back.
The odor is not from the connecting pipe to the disposal. It is from before the P-Trap. It is not in the pipe from the drain to the P-Trap.
He said that maybe there's something growing between the drain opening in the sink and the strainer body, or in the strainer body.
There is also a drain hose from a reverse-osmosis filter but I sniffed that and there was no odor.
Any thoughts?