I got a call today from a lady that was complaining of a sewer smell in her bathroom. She said she had called the water company and they advised her to run water down each of the drains in the bathroom. Good advice.
This customer sounded pretty hands-on and said she had taken the drain pipes apart below the sinks and washed them. That sounds pretty gross to me, and it wouldn't have much of an effect. I work around drain pipes and I don't want to touch them if I don't have to, much less wash them.
Over the phone she gave me the details. The bathroom has 2 sinks one of which she doesn't use, a toilet, a shower, and a bathtub which she never uses. A couple of issues jumped out. First, she doesn't use one of the sinks. Second, and perhaps the more likely culprit, she never uses the bathtub. The reason I say more likely is that the tub has a larger drain than the sink, and consequently could let more sewer smell into the room than a smaller diameter sink drain.
Over time if you don't use a fixture, the water in the drain trap (the "U" shaped drain pipe) will evaporate which allows the sewer smell into the room. This water barrier is the only thing preventing this from happening. I also told her to make sure there is water in the drains, i.e. turn on the water for a few seconds.
The customer said that within a few weeks the smell would return. Well, that makes sense...if she is not using the tub, the water will evaporate from the drain and the smell returns. I suggested that after she runs the water, pour a tablespoon of mineral oil into the drain. The oil is lighter than water and will float to the top and not evaporate.
I also told her keep an eye on the level of water in the trap. I suggested that she shine a flashlight down the tub drain every so often to make sure she could still see the water/oil in the trap. If there is no water left, there may be a crack in the drain that is allowing the water in the trap to drain out, leaving a wide open path for the smell to enter the room.