We have a floor drain with a meatal 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 pipe every one or two weeks. If the water runs dry an odour emanates from this pipe. 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. It returns in a matter of hours or days. What can I do to stop this from occurring?? Is there something that I can add that will not evaporate and stop the smell from escaping, or is it a job for a plumber??