Our house was built in 1995. We have a floor drain with a metal screen cap. We are told by our plumber that it is a "P" Trap or drain. We are supposed to pour some water into this drain every one or two weeks. If the drain goes dry an odour emanates from this drain. The smell is sewer-like and I would like to stop it from coming into our basement. Presently, I pour some water with a little Javex into the drain to remove the odour and to keep the drain wet. The smell returns in a matter of hours or in just a few days. What can I do to stop this from occurring?? Is there something that I can add to the drain that will not evaporate and stop the smell from escaping and/or the water from evaporating?? I am afraid that if I put vegetable oil into the drain it will become rancid and cause other problems.