I have large amounts of sewer gas escaping from basement floor drain. The trap has water in it. I have flushed it out for 5 minutes with a hose ( while this is happening a large amount of gas escapes) after 2 days the gas is escaping again(from the trap) and the water in the trap looks nasty again. The gas odor is strong and undoubtably coming from the drain. I have repeated this 4 times in the last 2 weeks. 3 days ago the local sewer district came out ( after I already poured water down the drain and the gas was not as strong) and said because it is not overflowing or no other pluming in the house is backed up that the problem must be in the house and not with the city sewers. I wanted to see if anyone had an Idea on what this could be before I called a plumber so I would know what I am getting into? Does this sound familiar? Any Ideas?

There are several ways this could happen.
1) The trap seal in the floor drain could be lowered allowing sewer gas to escape. Stick a rod down in the trap and tell me how much water is on the end.
2) There's a partial blockage downstream of the floor drain and every time there's a flush or a drain the discharge hits the blockage and bounces back sending a bubble of sewer gas ahead of it and then drains away. Do you hear any bubbles from the floor drain when you flush or drain something?
Let me know, Tom

1) I stuck a stick in the trap and there was about 6 " of water in the trap plus a inch of black sediment that the stick sunk into. It smell real bad. (understatement)

2) I hear no bubbles from the floor drain when any toilet in the house or sink is flushed ordraining

I flushed the floor drain with water while using a stick to loosen and hopefully suspend the black sediment and rinse it out of the drain. Some of it rinsed away but there is still some that I can't seem to rinse away.

Any advise? What do you think?

OK! I believe the gunk may be the source of your problem so let's clear that stinky stuff out of the trap. Try this, take s hose and shove it in the trap while stirring the mess. After you get it flushed out pour 1/2 gallon of bleach in the drain and let it set all night. Next morning ( and this is important ) flush it out with a few pans of boiling water. Floor drain smell better now? Good luck, Tom

I will report back to you tomorrow night and let you know the results. Thank you very much for your help.

Speedball1, Sorry It took so long to get back to you. I did as you instructed and so far so good. I think I got the majority of the gunk out. Thank you for your help.

After you clean all the gunk out pour a quart of bleach down the floor drain and let it set over night.
Next morning flush it out with a couple of pans of boiling water. Bet it smells better now. Good luck, Tom