I work at a pub and in the kitchen we got a new grease trap sometimes when I pull the plug with water in it we get a strong sewer smell in the whole building its getting to the point that even if I turn on the water we get the smell, we had the plumber come back out 4x's and he can't figure it out I clean out the grease trap every week

I'm not a plumber :) but since you do have one and he's not found source, I'd check with the local authorities if that strong a smell was coming back into my residence or place of work. Have they done any recent work outside on the mains? Good luck

Did the smell just start since the grease trap was installed or is this a ongoing problem? Any problems with the sink draining? Does it drain a little slower since the trap was installed? Any back pressure out of the drain as it's draining? Let me know, Tom

Tom it only started after the grease trap was installed & the sink drains fine, were puzzled and afraid to use the sink when were busy so we don't send people running for the door that's not a smell you want to smell when your eating and its hard to run a kitchen with out a sink.

The very first thing I would do is snake the kitchen roof vent. Is there any noise from the trap as the sink drains. Fill a tub up to the rim and check. If you were gettinga partial clog that produced a "blowback" of sewer gas The vent usually vents the gas out but if the vent were clogged the gas would exit through the trap and drain. Good luck, Tom